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of Medical
Physiology
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of Medical
Physiology
E L E V E N T H
E D I T I O N
Arthur C. Guyton, M.D.†
Professor Emeritus
Department of Physiology and Biophysics
University of Mississippi Medical Center
Jackson, Mississippi
†
Deceased
John E. Hall, Ph.D.
Professor and Chairman
Department of Physiology and Biophysics
University of Mississippi Medical Center
Jackson, Mississippi
Elsevier Inc.
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TEXTBOOK OF MEDICAL PHYSIOLOGY
ISBN 0-7216-0240-1
International Edition ISBN 0-8089-2317-X
Copyright © 2006, 2000, 1996, 1991, 1986, 1981, 1976, 1971, 1966, 1961, 1956 by Elsevier Inc.
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NOTICE
Knowledge and best practice in this field are constantly changing. As new research and experience
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Library of Congress Cataloging-in-Publication Data
Guyton, Arthur C.
Textbook of medical physiology / Arthur C. Guyton, John E. Hall.—11th ed.
p. ; cm.
Includes bibliographical references and index.
ISBN 0-7216-0240-1
1. Human physiology. 2. Physiology, Pathological. I. Title: Medical physiology.
John E. (John Edward) III. Title.
[DNLM: 1. Physiological Processes. QT 104 G992t 2006]
QP34.5.G9 2006
612—dc22
II. Hall,
2004051421
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Cover illustration is a detail from Opus 1972 by Virgil Cantini, Ph.D., with permission of the artist and
Mansfield State College, Mansfield, Pennsylvania.
Chapter opener credits: Chapter 43, modified from © Getty Images 21000058038; Chapter 44, modified
from © Getty Images 21000044598; Chapter 84, modified from © Corbis.
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8
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1
To
My Family
For their abundant support, for their patience and
understanding, and for their love
To
Arthur C. Guyton
For his imaginative and innovative research
For his dedication to education
For showing us the excitement and joy of physiology
And for serving as an inspirational role model
Arthur C. Guyton, M.D.
1919–2003
I N
M E M O R I A M
The sudden loss of Dr. Arthur C. Guyton in an automobile accident on April 3,
2003, stunned and saddened all who were privileged to know him. Arthur
Guyton was a giant in the fields of physiology and medicine, a leader among
leaders, a master teacher, and an inspiring role model throughout the world.
Arthur Clifton Guyton was born in Oxford, Mississippi, to Dr. Billy S.
Guyton, a highly respected eye, ear, nose, and throat specialist, who later
became Dean of the University of Mississippi Medical School, and Kate Smallwood Guyton, a mathematics and physics teacher who had been a missionary
in China before marriage. During his formative years, Arthur enjoyed watching
his father work at the Guyton Clinic, playing chess and swapping stories with
William Faulkner, and building sailboats (one of which he later sold to
Faulkner). He also built countless mechanical and electrical devices, which he
continued to do throughout his life. His brilliance shone early as he graduated
top in his class at the University of Mississippi. He later distinguished himself
at Harvard Medical School and began his postgraduate surgical training at
Massachusetts General Hospital.
His medical training was interrupted twice—once to serve in the Navy during
World War II and again in 1946 when he was stricken with poliomyelitis during
his final year of residency training. Suffering paralysis in his right leg, left arm,
and both shoulders, he spent nine months in Warm Springs, Georgia, recuperating and applying his inventive mind to building the first motorized wheelchair
controlled by a “joy stick,” a motorized hoist for lifting patients, special leg
braces, and other devices to aid the handicapped. For those inventions he
received a Presidential Citation.
He returned to Oxford where he devoted himself to teaching and research
at the University of Mississippi School of Medicine and was named Chair of the
Department of Physiology in 1948. In 1951 he was named one of the ten outstanding men in the nation. When the University of Mississippi moved its
Medical School to Jackson in 1955, he rapidly developed one of the world’s
premier cardiovascular research programs. His remarkable life as a scientist,
author, and devoted father is detailed in a biography published on the occasion
of his “retirement” in 1989.1
A Great Physiologist. Arthur Guyton’s research contributions, which include
more than 600 papers and 40 books, are legendary and place him among the
greatest physiologists in history. His research covered virtually all areas of cardiovascular regulation and led to many seminal concepts that are now an integral part of our understanding of cardiovascular disorders, such as hypertension,
heart failure, and edema. It is difficult to discuss cardiovascular physiology
without including his concepts of cardiac output and venous return, negative
interstitial fluid pressure and regulation of tissue fluid volume and edema,
regulation of tissue blood flow and whole body blood flow autoregulation,
renal-pressure natriuresis, and long-term blood pressure regulation. Indeed, his
concepts of cardiovascular regulation are found in virtually every major textbook of physiology. They have become so familiar that their origin is sometimes
forgotten.
One of Dr. Guyton’s most important scientific legacies was his application of
principles of engineering and systems analysis to cardiovascular regulation. He
used mathematical and graphical methods to quantify various aspects of circulatory function before computers were widely available. He built analog computers and pioneered the application of large-scale systems analysis to modeling
the cardiovascular system before the advent of digital computers. As digital
computers became available, his cardiovascular models expanded dramatically
to include the kidneys and body fluids, hormones, and the autonomic nervous
system, as well as cardiac and circulatory functions.2 He also provided the first
comprehensive systems analysis of blood pressure regulation. This unique
approach to physiological research preceded the emergence of biomedical
vii
viii
In Memoriam
engineering—a field that he helped to establish and to
promote in physiology, leading the discipline into a
quantitative rather than a descriptive science.
It is a tribute to Arthur Guyton’s genius that his
concepts of cardiovascular regulation often seemed
heretical when they were first presented, yet stimulated investigators throughout the world to test them
experimentally. They are now widely accepted. In fact,
many of his concepts of cardiovascular regulation
are integral components of what is now taught in
most medical physiology courses. They continue to
be the foundation for generations of cardiovascular
physiologists.
Dr. Guyton received more than 80 major honors
from diverse scientific and civic organizations and universities throughout the world. A few of these that are
especially relevant to cardiovascular research include
the Wiggers Award of the American Physiological
Society, the Ciba Award from the Council for High
Blood Pressure Research, The William Harvey Award
from the American Society of Hypertension, the
Research Achievement Award of the American Heart
Association, and the Merck Sharp & Dohme Award
of the International Society of Hypertension. It was
appropriate that in 1978 he was invited by the Royal
College of Physicians in London to deliver a special
lecture honoring the 400th anniversary of the birth of
William Harvey, who discovered the circulation of the
blood.
Dr. Guyton’s love of physiology was beautifully
articulated in his president’s address to the American
Physiological Society in 1975,3 appropriately entitled
Physiology, a Beauty and a Philosophy. Let me quote
just one sentence from his address: What other person,
whether he be a theologian, a jurist, a doctor of medicine, a physicist, or whatever, knows more than you, a
physiologist, about life? For physiology is indeed an
explanation of life. What other subject matter is more
fascinating, more exciting, more beautiful than the
subject of life?
A Master Teacher. Although Dr. Guyton’s research
accomplishments are legendary, his contributions as an
educator have probably had an even greater impact.
He and his wonderful wife Ruth raised ten children,
all of whom became outstanding physicians—a
remarkable educational achievement. Eight of the
Guyton children graduated from Harvard Medical
School, one from Duke Medical School, and one from
The University of Miami Medical School after receiving a Ph.D. from Harvard. An article published in
Reader’s Digest in 1982 highlighted their extraordinary
family life.4
The success of the Guyton children did not occur by
chance. Dr. Guyton’s philosophy of education was to
“learn by doing.” The children participated in countless family projects that included the design and
construction of their home and its heating system,
the swimming pool, tennis court, sailboats, go-carts
and electrical cars, household gadgets, and electronic
instruments for their Oxford Instruments Company.
Television programs such as Good Morning America
and 20/20 described the remarkable home environment that Arthur and Ruth Guyton created to raise
their family. His devotion to family is beautifully
expressed in the dedication of his Textbook of Medical
Physiology5:
To
My father for his uncompromising principles that
guided my life
My mother for leading her children into intellectual
pursuits
My wife for her magnificent devotion to her family
My children for making everything worthwhile
Dr. Guyton was a master teacher at the University
of Mississippi for over 50 years. Even though he was
always busy with service responsibilities, research,
writing, and teaching, he was never too busy to talk
with a student who was having difficulty. He would
never accept an invitation to give a prestigious lecture
if it conflicted with his teaching schedule.
His contributions to education are also far reaching through generations of physiology graduate
students and postdoctoral fellows. He trained over
150 scientists, at least 29 of whom became chairs of
their own departments and six of whom became presidents of the American Physiological Society. He gave
students confidence in their abilities and emphasized
his belief that “People who are really successful in the
research world are self-taught.” He insisted that his
trainees integrate their experimental findings into a
broad conceptual framework that included other
interacting systems. This approach usually led them
to develop a quantitative analysis and a better
understanding of the particular physiological systems
that they were studying. No one has been more prolific in training leaders of physiology than Arthur
Guyton.
Dr. Guyton’s Textbook of Medical Physiology, first
published in 1956, quickly became the best-selling
medical physiology textbook in the world. He had a
gift for communicating complex ideas in a clear and
interesting manner that made studying physiology fun.
He wrote the book to teach his students, not to impress
his professional colleagues. Its popularity with students has made it the most widely used physiology
textbook in history. This accomplishment alone was
enough to ensure his legacy.
The Textbook of Medical Physiology began as
lecture notes in the early 1950s when Dr. Guyton was
teaching the entire physiology course for medical students at the University of Mississippi. He discovered
that the students were having difficulty with the textbooks that were available and began distributing
copies of his lecture notes. In describing his experience, Dr. Guyton stated that “Many textbooks of
medical physiology had become discursive, written primarily by teachers of physiology for other teachers of
physiology, and written in language understood by
other teachers but not easily understood by the basic
student of medical physiology.”6
Through his Textbook of Medical Physiology, which
is translated into 13 languages, he has probably done
In Memoriam
more to teach physiology to the world than any other
individual in history. Unlike most major textbooks,
which often have 20 or more authors, the first eight
editions were written entirely by Dr. Guyton—a feat
that is unprecedented for any major medical textbook.
For his many contributions to medical education, Dr.
Guyton received the 1996 Abraham Flexner Award
from the Association of American Medical Colleges
(AAMC). According to the AAMC, Arthur Guyton
“. . . for the past 50 years has made an unparalleled
impact on medical education.” He is also honored each
year by The American Physiological Society through
the Arthur C. Guyton Teaching Award.
ix
We celebrate the magnificent life of Arthur Guyton,
recognizing that we owe him an enormous debt. He
gave us an imaginative and innovative approach to
research and many new scientific concepts. He gave
countless students throughout the world a means of
understanding physiology and he gave many of us
exciting research careers. Most of all, he inspired us—
with his devotion to education, his unique ability to
bring out the best in those around him, his warm and
generous spirit, and his courage. We will miss him
tremendously, but he will remain in our memories as
a shining example of the very best in humanity. Arthur
Guyton was a real hero to the world, and his legacy is
everlasting.
An Inspiring Role Model. Dr. Guyton’s accomplish-
ments extended far beyond science, medicine, and education. He was an inspiring role model for life as well
as for science. No one was more inspirational or influential on my scientific career than Dr. Guyton. He
taught his students much more than physiology—
he taught us life, not so much by what he said but by
his unspoken courage and dedication to the highest
standards.
He had a special ability to motivate people through
his indomitable spirit. Although he was severely challenged by polio, those of us who worked with him
never thought of him as being handicapped. We were
too busy trying to keep up with him! His brilliant
mind, his indefatigable devotion to science, education,
and family, and his spirit captivated students and
trainees, professional colleagues, politicians, business
leaders, and virtually everyone who knew him. He
would not succumb to the effects of polio. His courage
challenged and inspired us. He expected the best and
somehow brought out the very best in people.
References
1. Brinson C, Quinn J: Arthur C. Guyton—His Life, His
Family, His Achievements. Jackson, MS, Hederman
Brothers Press, 1989.
2. Guyton AC, Coleman TG, Granger HJ: Circulation:
overall regulation. Ann Rev Physiol 34:13–46, 1972.
3. Guyton AC: Past-President’s Address. Physiology, a
Beauty and a Philosophy. The Physiologist 8:495–501,
1975.
4. Bode R: A Doctor Who’s Dad to Seven Doctors—So Far!
Readers’ Digest, December, 1982, pp. 141–145.
5. Guyton AC: Textbook of Medical Physiology. Philadelphia, Saunders, 1956.
6. Guyton AC: An author’s philosophy of physiology textbook writing. Adv Physiol Ed 19: s1–s5, 1998.
John E. Hall
Jackson, Mississippi
P
R
E
The first edition of the Textbook of Medical Physiology was written by Arthur C. Guyton almost 50
years ago. Unlike many major medical textbooks,
which often have 20 or more authors, the first
eight editions of the Textbook of Medical Physiology were written entirely by Dr. Guyton with
each new edition arriving on schedule for nearly
40 years. Over the years, Dr. Guyton’s textbook
became widely used throughout the world and was translated into 13 languages.
A major reason for the book’s unprecedented success was his uncanny ability
to explain complex physiologic principles in language easily understood by students. His main goal with each edition was to instruct students in physiology,
not to impress his professional colleagues. His writing style always maintained
the tone of a teacher talking to his students.
I had the privilege of working closely with Dr. Guyton for almost 30 years
and the honor of helping him with the 9th and 10th editions. For the 11th
edition, I have the same goal as in previous editions—to explain, in language
easily understood by students, how the different cells, tissues, and organs of the
human body work together to maintain life. This task has been challenging and
exciting because our rapidly increasing knowledge of physiology continues to
unravel new mysteries of body functions. Many new techniques for learning
about molecular and cellular physiology have been developed. We can present
more and more the physiology principles in the terminology of molecular and
physical sciences rather than in merely a series of separate and unexplained biological phenomena. This change is welcomed, but it also makes revision of each
chapter a necessity.
In this edition, I have attempted to maintain the same unified organization
of the text that has been useful to students in the past and to ensure that
the book is comprehensive enough that students will wish to use it in later life
as a basis for their professional careers. I hope that this textbook conveys
the majesty of the human body and its many functions and that it stimulates
students to study physiology throughout their careers. Physiology is the link
between the basic sciences and medicine. The great beauty of physiology is
that it integrates the individual functions of all the body’s different cells, tissues,
and organs into a functional whole, the human body. Indeed, the human
body is much more than the sum of its parts, and life relies upon this total function, not just on the function of individual body parts in isolation from the
others.
This brings us to an important question: How are the separate organs and
systems coordinated to maintain proper function of the entire body? Fortunately, our bodies are endowed with a vast network of feedback controls that
achieve the necessary balances without which we would not be able to live.
Physiologists call this high level of internal bodily control homeostasis. In
disease states, functional balances are often seriously disturbed and homeostasis is impaired. And, when even a single disturbance reaches a limit, the whole
body can no longer live. One of the goals of this text, therefore, is to emphasize
the effectiveness and beauty of the body’s homeostasis mechanisms as well as
to present their abnormal function in disease.
Another objective is to be as accurate as possible. Suggestions and critiques
from many physiologists, students, and clinicians throughout the world have
been sought and then used to check factual accuracy as well as balance in the
text. Even so, because of the likelihood of error in sorting through many thousands of bits of information, I wish to issue still a further request to all readers
to send along notations of error or inaccuracy. Physiologists understand the
importance of feedback for proper function of the human body; so, too, is feedback important for progressive improvement of a textbook of physiology. To
the many persons who have already helped, I send sincere thanks.
xi
F
A
C
E
xii
Preface
A brief explanation is needed about several features
of the 11th edition. Although many of the chapters
have been revised to include new principles of physiology, the text length has been closely monitored
to limit the book size so that it can be used effectively in physiology courses for medical students and
health care professionals. Many of the figures have
also been redrawn and are now in full color. New
references have been chosen primarily for their presentation of physiologic principles, for the quality of
their own references, and for their easy accessibility.
Most of the selected references are from recently
published scientific journals that can be freely
accessed from the PubMed internet site at http://
www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed.
Use of these references, as well as cross-references
from them, can give the student almost complete coverage of the entire field of physiology.
Another feature is that the print is set in two sizes.
The material in small print is of several different kinds:
first, anatomical, chemical, and other information that
is needed for immediate discussion but that most students will learn in more detail in other courses; second,
physiologic information of special importance to
certain fields of clinical medicine; and, third, information that will be of value to those students who may
wish to study particular physiologic mechanisms more
deeply.
The material in large print constitutes the fundamental physiologic information that students will
require in virtually all their medical activities and
studies.
I wish to express my thanks to many other persons
who have helped in preparing this book, including
my colleagues in the Department of Physiology &
Biophysics at the University of Mississippi Medical
Center who provided valuable suggestions. I am also
grateful to Ivadelle Osberg Heidke, Gerry McAlpin,
and Stephanie Lucas for their excellent secretarial
services, and to William Schmitt, Rebecca Gruliow,
Mary Anne Folcher, and the rest of the staff of
Elsevier Saunders for continued editorial and production excellence.
Finally, I owe an enormous debt to Arthur Guyton
for an exciting career in physiology, for his friendship,
for the great privilege of contributing to the Textbook
of Medical Physiology, and for the inspiration that he
provided to all who knew him.
John E. Hall
Jackson, Mississippi
TA B L E O F C O N T E N T S
U N I T
The DNA Code in the Cell Nucleus Is
Transferred to an RNA Code in the
Cell Cytoplasm—The Process
of Transcription
Synthesis of RNA
Assembly of the RNA Chain from Activated
Nucleotides Using the DNA Strand
as a Template—The Process of
“Transcription”
Messenger RNA—The Codons
Transfer RNA—The Anticodons
Ribosomal RNA
Formation of Proteins on the Ribosomes—
The Process of “Translation”
Synthesis of Other Substances in the
Cell
Control of Gene Function and
Biochemical Activity in Cells
Genetic Regulation
Control of Intracellular Function by
Enzyme Regulation
The DNA-Genetic System Also Controls
Cell Reproduction
Cell Reproduction Begins with Replication
of DNA
Chromosomes and Their Replication
Cell Mitosis
Control of Cell Growth and Cell
Reproduction
Cell Differentiation
Apoptosis—Programmed Cell Death
Cancer
I
Introduction to Physiology: The
Cell and General Physiology
C H A P T E R
1
Functional Organization of the
Human Body and Control of the
“Internal Environment”
Cells as the Living Units of the Body
Extracellular Fluid—The “Internal
Environment”
“Homeostatic” Mechanisms of the Major
Functional Systems
Homeostasis
Extracellular Fluid Transport and Mixing
System—The Blood Circulatory System
Origin of Nutrients in the Extracellular Fluid
Removal of Metabolic End Products
Regulation of Body Functions
Reproduction
Control Systems of the Body
Examples of Control Mechanisms
Characteristics of Control Systems
Summary—Automaticity of the Body
C H A P T E R
2
The Cell and Its Functions
Organization of the Cell
Physical Structure of the Cell
Membranous Structures of the Cell
Cytoplasm and Its Organelles
Nucleus
Nuclear Membrane
Nucleoli and Formation of Ribosomes
Comparison of the Animal Cell with
Precellular Forms of Life
Functional Systems of the Cell
Ingestion by the Cell—Endocytosis
Digestion of Pinocytotic and Phagocytic
Foreign Substances Inside the Cell—
Function of the Lysosomes
Synthesis and Formation of Cellular
Structures by Endoplasmic Reticulum
and Golgi Apparatus
Extraction of Energy from Nutrients—
Function of the Mitochondria
Locomotion of Cells
Ameboid Movement
Cilia and Ciliary Movement
C H A P T E R
3
Genetic Control of Protein Synthesis,
Cell Function, and Cell Reproduction
Genes in the Cell Nucleus
Genetic Code
3
3
3
4
4
4
5
5
5
6
6
6
7
9
11
11
12
12
14
17
17
18
U N I T
30
30
31
31
32
33
33
35
35
35
36
37
37
38
38
39
40
40
40
I I
Membrane Physiology, Nerve,
and Muscle
18
19
19
C H A P T E R
4
Transport of Substances Through
the Cell Membrane
20
The Lipid Barrier of the Cell Membrane,
and Cell Membrane Transport
Proteins
Diffusion
Diffusion Through the Cell Membrane
Diffusion Through Protein Channels, and
“Gating” of These Channels
Facilitated Diffusion
Factors That Affect Net Rate of Diffusion
Osmosis Across Selectively Permeable
Membranes—“Net Diffusion” of Water
“Active Transport” of Substances
Through Membranes
Primary Active Transport
Secondary Active Transport—Co-Transport
and Counter-Transport
Active Transport Through Cellular Sheets
20
22
24
24
24
27
27
29
xiii
45
45
46
46
47
49
50
51
52
53
54
55
xiv
C H A P T E R
5
Membrane Potentials and Action
Potentials
Basic Physics of Membrane
Potentials
Membrane Potentials Caused by
Diffusion
Measuring the Membrane Potential
Resting Membrane Potential of Nerves
Origin of the Normal Resting Membrane
Potential
Nerve Action Potential
Voltage-Gated Sodium and Potassium
Channels
Summary of the Events That Cause the
Action Potential
Roles of Other Ions During the Action
Potential
Initiation of the Action Potential
Propagation of the Action Potential
Re-establishing Sodium and Potassium
Ionic Gradients After Action Potentials
Are Completed—Importance of Energy
Metabolism
Plateau in Some Action Potentials
Rhythmicity of Some Excitable Tissues—
Repetitive Discharge
Special Characteristics of Signal
Transmission in Nerve Trunks
Excitation—The Process of Eliciting
the Action Potential
“Refractory Period” After an Action
Potential
Recording Membrane Potentials and
Action Potentials
Inhibition of Excitability—“Stabilizers”
and Local Anesthetics
C H A P T E R
6
Contraction of Skeletal Muscle
Physiologic Anatomy of Skeletal
Muscle
Skeletal Muscle Fiber
General Mechanism of Muscle
Contraction
Molecular Mechanism of Muscle
Contraction
Molecular Characteristics of the
Contractile Filaments
Effect of Amount of Actin and Myosin
Filament Overlap on Tension Developed
by the Contracting Muscle
Relation of Velocity of Contraction to
Load
Energetics of Muscle Contraction
Work Output During Muscle Contraction
Sources of Energy for Muscle Contraction
Characteristics of Whole Muscle
Contraction
Mechanics of Skeletal Muscle Contraction
Remodeling of Muscle to Match Function
Rigor Mortis
Table of Contents
57
57
57
58
59
60
61
62
64
64
65
65
66
66
67
68
69
70
70
70
72
72
72
74
74
C H A P T E R
7
Excitation of Skeletal Muscle:
Neuromuscular Transmission and
Excitation-Contraction Coupling
Transmission of Impulses from Nerve
Endings to Skeletal Muscle Fibers:
The Neuromuscular Junction
Secretion of Acetylcholine by the Nerve
Terminals
Molecular Biology of Acetyline
Formation and Release
Drugs That Enhance or Block
Transmission at the Neuromuscular
Junction
Myasthenia Gravis
Muscle Action Potential
Spread of the Action Potential to the
Interior of the Muscle Fiber by Way of
“Transverse Tubules”
Excitation-Contraction Coupling
Transverse Tubule–Sarcoplasmic Reticulum
System
Release of Calcium Ions by the
Sarcoplasmic Reticulum
C H A P T E R
8
Contraction and Excitation of
Smooth Muscle
Contraction of Smooth Muscle
Types of Smooth Muscle
Contractile Mechanism in Smooth Muscle
Regulation of Contraction by Calcium Ions
Nervous and Hormonal Control of
Smooth Muscle Contraction
Neuromuscular Junctions of Smooth
Muscle
Membrane Potentials and Action Potentials
in Smooth Muscle
Effect of Local Tissue Factors and
Hormones to Cause Smooth Muscle
Contraction Without Action Potentials
Source of Calcium Ions That Cause
Contraction (1 ) Through the Cell
Membrane and (2 ) from the Sarcoplasmic
Reticulum
U N I T
85
85
85
88
88
89
89
89
89
89
90
92
92
92
93
95
95
95
96
98
99
I I I
The Heart
75
77
78
78
78
79
80
81
82
83
C H A P T E R
9
Heart Muscle; The Heart as a Pump
and Function of the Heart Valves
Physiology of Cardiac Muscle
Physiologic Anatomy of Cardiac Muscle
Action Potentials in Cardiac Muscle
The Cardiac Cycle
Diastole and Systole
Relationship of the Electrocardiogram to
the Cardiac Cycle
Function of the Atria as Primer Pumps
Function of the Ventricles as Pumps
103
103
103
104
106
106
107
107
108
Table of Contents
Function of the Valves
Aortic Pressure Curve
Relationship of the Heart Sounds to
Heart Pumping
Work Output of the Heart
Graphical Analysis of Ventricular Pumping
Chemical Energy Required for Cardiac
Contraction: Oxygen Utilization by
the Heart
Regulation of Heart Pumping
Intrinsic Regulation of Heart Pumping—
The Frank-Starling Mechanism
Effect of Potassium and Calcium Ions on
Heart Function
Effect of Temperature on Heart Function
Increasing the Arterial Pressure Load
(up to a Limit) Does Not Decrease the
Cardiac Output
C H A P T E R
1 0
Rhythmical Excitation of the Heart
Specialized Excitatory and Conductive
System of the Heart
Sinus (Sinoatrial) Node
Internodal Pathways and Transmission of
the Cardiac Impulse Through the Atria
Atrioventricular Node, and Delay of Impulse
Conduction from the Atria to the Ventricles
Rapid Transmission in the Ventricular
Purkinje System
Transmission of the Cardiac Impulse in the
Ventricular Muscle
Summary of the Spread of the Cardiac
Impulse Through the Heart
Control of Excitation and Conduction
in the Heart
The Sinus Node as the Pacemaker of the
Heart
Role of the Purkinje System in Causing
Synchronous Contraction of the
Ventricular Muscle
Control of Heart Rhythmicity and Impulse
Conduction by the Cardiac Nerves: The
Sympathetic and Parasympathetic Nerves
C H A P T E R
1 1
The Normal Electrocardiogram
Characteristics of the Normal
Electrocardiogram
Depolarization Waves Versus
Repolarization Waves
Relationship of Atrial and Ventricular
Contraction to the Waves of the
Electrocardiogram
Voltage and Time Calibration of the
Electrocardiogram
Methods for Recording
Electrocardiograms
Pen Recorder
Flow of Current Around the Heart
During the Cardiac Cycle
Recording Electrical Potentials from a
Partially Depolarized Mass of Syncytial
Cardiac Muscle
109
109
109
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110
111
111
111
113
114
114
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118
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119
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120
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126
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126
xv
Flow of Electrical Currents in the Chest
Around the Heart
Electrocardiographic Leads
Three Bipolar Limb Leads
Chest Leads (Precordial Leads)
Augmented Unipolar Limb Leads
126
127
127
129
129
C H A P T E R
1 2
Electrocardiographic Interpretation
of Cardiac Muscle and Coronary
Blood Flow Abnormalities: Vectorial
Analysis
131
Principles of Vectorial Analysis of
Electrocardiograms
Use of Vectors to Represent Electrical
Potentials
Direction of a Vector Is Denoted in Terms
of Degrees
Axis for Each Standard Bipolar Lead and
Each Unipolar Limb Lead
Vectorial Analysis of Potentials Recorded
in Different Leads
Vectorial Analysis of the Normal
Electrocardiogram
Vectors That Occur at Successive Intervals
During Depolarization of the Ventricles—
The QRS Complex
Electrocardiogram During Repolarization—
The T Wave
Depolarization of the Atria—The P Wave
Vectorcardiogram
Mean Electrical Axis of the Ventricular
QRS—And Its Significance
Determining the Electrical Axis from
Standard Lead Electrocardiograms
Abnormal Ventricular Conditions That Cause
Axis Deviation
Conditions That Cause Abnormal
Voltages of the QRS Complex
Increased Voltage in the Standard Bipolar
Limb Leads
Decreased Voltage of the Electrocardiogram
Prolonged and Bizarre Patterns of the
QRS Complex
Prolonged QRS Complex as a Result of
Cardiac Hypertrophy or Dilatation
Prolonged QRS Complex Resulting from
Purkinje System Blocks
Conditions That Cause Bizarre QRS
Complexes
Current of Injury
Effect of Current of Injury on the QRS
Complex
The J Point—The Zero Reference Potential
for Analyzing Current of Injury
Coronary Ischemia as a Cause of Injury
Potential
Abnormalities in the T Wave
Effect of Slow Conduction of the
Depolarization Wave on the
Characteristics of the T Wave
Shortened Depolarization in Portions of
the Ventricular Muscle as a Cause of
T Wave Abnormalities
131
131
131
132
133
134
134
134
136
136
137
137
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140
140
140
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141
141
141
141
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142
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145
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Table of Contents
C H A P T E R
1 3
Cardiac Arrhythmias and Their
Electrocardiographic Interpretation
Abnormal Sinus Rhythms
Tachycardia
Bradycardia
Sinus Arrhythmia
Abnormal Rhythms That Result from
Block of Heart Signals Within the
Intracardiac Conduction Pathways
Sinoatrial Block
Atrioventricular Block
Incomplete Atrioventricular Heart Block
Incomplete Intraventricular Block—
Electrical Alternans
Premature Contractions
Premature Atrial Contractions
A-V Nodal or A-V Bundle Premature
Contractions
Premature Ventricular Contractions
Paroxysmal Tachycardia
Atrial Paroxysmal Tachycardia
Ventricular Paroxysmal Tachycardia
Ventricular Fibrillation
Phenomenon of Re-entry—“Circus
Movements” as the Basis for Ventricular
Fibrillation
Chain Reaction Mechanism of Fibrillation
Electrocardiogram in Ventricular Fibrillation
Electroshock Defibrillation of the Ventricle
Hand Pumping of the Heart
(Cardiopulmonary Resuscitation) as
an Aid to Defibrillation
Atrial Fibrillation
Atrial Flutter
Cardiac Arrest
U N I T
147
147
147
147
148
148
148
148
149
150
150
150
150
151
151
152
152
152
153
153
154
154
155
155
156
156
I V
The Circulation
C H A P T E R
1 4
Overview of the Circulation; Medical
Physics of Pressure, Flow, and
Resistance
Physical Characteristics of the
Circulation
Basic Theory of Circulatory Function
Interrelationships Among Pressure,
Flow, and Resistance
Blood Flow
Blood Pressure
Resistance to Blood Flow
Effects of Pressure on Vascular Resistance
and Tissue Blood Flow
C H A P T E R
1 5
Vascular Distensibility and Functions
of the Arterial and Venous Systems
Vascular Distensibility
Vascular Compliance (or Vascular
Capacitance)
161
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163
164
164
166
167
170
171
171
171
Volume-Pressure Curves of the Arterial
and Venous Circulations
Arterial Pressure Pulsations
Transmission of Pressure Pulses to the
Peripheral Arteries
Clinical Methods for Measuring Systolic
and Diastolic Pressures
Veins and Their Functions
Venous Pressures—Right Atrial Pressure
(Central Venous Pressure) and
Peripheral Venous Pressures
Blood Reservoir Function of the Veins
C H A P T E R
1 6
The Microcirculation and the
Lymphatic System: Capillary Fluid
Exchange, Interstitial Fluid, and
Lymph Flow
Structure of the Microcirculation and
Capillary System
Flow of Blood in the Capillaries—
Vasomotion
Average Function of the Capillary System
Exchange of Water, Nutrients, and
Other Substances Between the Blood
and Interstitial Fluid
Diffusion Through the Capillary Membrane
The Interstitium and Interstitial Fluid
Fluid Filtration Across Capillaries Is
Determined by Hydrostatic and
Colloid Osmotic Pressures, and
Capillary Filtration Coefficient
Capillary Hydrostatic Pressure
Interstitial Fluid Hydrostatic Pressure
Plasma Colloid Osmotic Pressure
Interstitial Fluid Colloid Osmotic Pressure
Exchange of Fluid Volume Through the
Capillary Membrane
Starling Equilibrium for Capillary Exchange
Lymphatic System
Lymph Channels of the Body
Formation of Lymph
Rate of Lymph Flow
Role of the Lymphatic System in Controlling
Interstitial Fluid Protein Concentration,
Interstitial Fluid Volume, and Interstitial
Fluid Pressure
C H A P T E R
1 7
Local and Humoral Control of Blood
Flow by the Tissues
Local Control of Blood Flow in Response
to Tissue Needs
Mechanisms of Blood Flow Control
Acute Control of Local Blood Flow
Long-Term Blood Flow Regulation
Development of Collateral Circulation—A
Phenomenon of Long-Term Local Blood
Flow Regulation
Humoral Control of the Circulation
Vasoconstrictor Agents
Vasodilator Agents
Vascular Control by Ions and Other
Chemical Factors
172
173
174
175
176
176
179
181
181
182
183
183
183
184
185
186
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188
188
189
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192
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195
196
196
200
201
201
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202
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Table of Contents
C H A P T E R
1 8
Nervous Regulation of the Circulation,
and Rapid Control of Arterial Pressure 204
Nervous Regulation of the Circulation
Autonomic Nervous System
Role of the Nervous System in Rapid
Control of Arterial Pressure
Increase in Arterial Pressure During Muscle
Exercise and Other Types of Stress
Reflex Mechanisms for Maintaining Normal
Arterial Pressure
Central Nervous System Ischemic
Response—Control of Arterial Pressure
by the Brain’s Vasomotor Center in
Response to Diminished Brain Blood
Flow
Special Features of Nervous Control
of Arterial Pressure
Role of the Skeletal Nerves and Skeletal
Muscles in Increasing Cardiac Output
and Arterial Pressure
Respiratory Waves in the Arterial Pressure
Arterial Pressure “Vasomotor” Waves—
Oscillation of Pressure Reflex Control
Systems
C H A P T E R
1 9
Dominant Role of the Kidney in LongTerm Regulation of Arterial Pressure
and in Hypertension: The Integrated
System for Pressure Control
Renal–Body Fluid System for Arterial
Pressure Control
Quantitation of Pressure Diuresis as a Basis
for Arterial Pressure Control
Chronic Hypertension (High Blood Pressure)
Is Caused by Impaired Renal Fluid
Excretion
The Renin-Angiotensin System:
Its Role in Pressure Control and in
Hypertension
Components of the Renin-Angiotensin
System
Types of Hypertension in Which Angiotensin
Is Involved: Hypertension Caused by a
Renin-Secreting Tumor or by Infusion
of Angiotensin II
Other Types of Hypertension Caused by
Combinations of Volume Loading and
Vasoconstriction
“Primary (Essential) Hypertension”
Summary of the Integrated,
Multifaceted System for Arterial
Pressure Regulation
C H A P T E R
2 0
Cardiac Output, Venous Return,
and Their Regulation
Normal Values for Cardiac Output at
Rest and During Activity
Control of Cardiac Output by Venous
Return—Role of the Frank-Starling
Mechanism of the Heart
204
204
208
208
209
212
213
213
214
214
216
216
217
220
223
223
226
227
228
230
Cardiac Output Regulation Is the Sum of
Blood Flow Regulation in All the Local
Tissues of the Body—Tissue Metabolism
Regulates Most Local Blood Flow
The Heart Has Limits for the Cardiac Output
That It Can Achieve
What Is the Role of the Nervous System in
Controlling Cardiac Output?
Pathologically High and Pathologically
Low Cardiac Outputs
High Cardiac Output Caused by Reduced
Total Peripheral Resistance
Low Cardiac Output
A More Quantitative Analysis of Cardiac
Output Regulation
Cardiac Output Curves Used in the
Quantitative Analysis
Venous Return Curves
Analysis of Cardiac Output and Right Atrial
Pressure, Using Simultaneous Cardiac
Output and Venous Return Curves
Methods for Measuring Cardiac
Output
Pulsatile Output of the Heart as Measured
by an Electromagnetic or Ultrasonic
Flowmeter
Measurement of Cardiac Output Using the
Oxygen Fick Principle
Indicator Dilution Method for Measuring
Cardiac Output
C H A P T E R
2 1
Muscle Blood Flow and Cardiac
Output During Exercise; the
Coronary Circulation and Ischemic
Heart Disease
Blood Flow in Skeletal Muscle
and Blood Flow Regulation
During Exercise
Rate of Blood Flow Through the Muscles
Control of Blood Flow Through the Skeletal
Muscles
Total Body Circulatory Readjustments
During Exercise
Coronary Circulation
Physiologic Anatomy of the Coronary Blood
Supply
Normal Coronary Blood Flow
Control of Coronary Blood Flow
Special Features of Cardiac Muscle
Metabolism
Ischemic Heart Disease
Causes of Death After Acute Coronary
Occlusion
Stages of Recovery from Acute
Myocardial Infarction
Function of the Heart After Recovery
from Myocardial Infarction
Pain in Coronary Heart Disease
Surgical Treatment of Coronary Disease
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241
243
243
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232
232
C H A P T E R
Cardiac Failure
2 2
Dynamics of the Circulation in
Cardiac Failure
258
258
xviii
Acute Effects of Moderate Cardiac Failure
Chronic Stage of Failure—Fluid Retention
Helps to Compensate Cardiac Output
Summary of the Changes That Occur After
Acute Cardiac Failure—“Compensated
Heart Failure”
Dynamics of Severe Cardiac Failure—
Decompensated Heart Failure
Unilateral Left Heart Failure
Low-Output Cardiac Failure—
Cardiogenic Shock
Edema in Patients with Cardiac Failure
Cardiac Reserve
Quantitative Graphical Method for Analysis
of Cardiac Failure
C H A P T E R
2 3
Heart Valves and Heart Sounds;
Dynamics of Valvular and Congenital
Heart Defects
Heart Sounds
Normal Heart Sounds
Valvular Lesions
Abnormal Circulatory Dynamics in
Valvular Heart Disease
Dynamics of the Circulation in Aortic
Stenosis and Aortic Regurgitation
Dynamics of Mitral Stenosis and Mitral
Regurgitation
Circulatory Dynamics During Exercise in
Patients with Valvular Lesions
Abnormal Circulatory Dynamics in
Congenital Heart Defects
Patent Ductus Arteriosus—A Left-to-Right
Shunt
Tetralogy of Fallot—A Right-to-Left Shunt
Causes of Congenital Anomalies
Use of Extracorporeal Circulation
During Cardiac Surgery
Hypertrophy of the Heart in Valvular
and Congenital Heart Disease
C H A P T E R
2 4
Circulatory Shock and Physiology of
Its Treatment
Physiologic Causes of Shock
Circulatory Shock Caused by Decreased
Cardiac Output
Circulatory Shock That Occurs Without
Diminished Cardiac Output
What Happens to the Arterial Pressure in
Circulatory Shock?
Tissue Deterioration Is the End Result of
Circulatory Shock, Whatever the Cause
Stages of Shock
Shock Caused by Hypovolemia—
Hemorrhagic Shock
Relationship of Bleeding Volume to
Cardiac Output and Arterial Pressure
Progressive and Nonprogressive
Hemorrhagic Shock
Irreversible Shock
Hypovolemic Shock Caused by Plasma
Loss
Hypovolemic Shock Caused by Trauma
Table of Contents
258
259
260
260
262
262
263
264
265
Neurogenic Shock—Increased Vascular
Capacity
Anaphylactic Shock and Histamine
Shock
Septic Shock
Physiology of Treatment in Shock
Replacement Therapy
Treatment of Shock with Sympathomimetic
Drugs—Sometimes Useful, Sometimes
Not
Other Therapy
Circulatory Arrest
Effect of Circulatory Arrest on the Brain
U N I T
285
285
286
286
286
287
287
287
287
V
The Body Fluids and Kidneys
269
269
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271
272
272
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273
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278
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280
284
284
285
C H A P T E R
2 5
The Body Fluid Compartments:
Extracellular and Intracellular Fluids;
Interstitial Fluid and Edema
Fluid Intake and Output Are Balanced
During Steady-State Conditions
Daily Intake of Water
Daily Loss of Body Water
Body Fluid Compartments
Intracellular Fluid Compartment
Extracellular Fluid Compartment
Blood Volume
Constituents of Extracellular and
Intracellular Fluids
Ionic Composition of Plasma and
Interstitial Fluid Is Similar
Important Constituents of the Intracellular
Fluid
Measurement of Fluid Volumes in the
Different Body Fluid Compartments—
The Indicator-Dilution Principle
Determination of Volumes of Specific
Body Fluid Compartments
Regulation of Fluid Exchange and
Osmotic Equilibrium Between
Intracellular and Extracellular Fluid
Basic Principles of Osmosis and
Osmotic Pressure
Osmotic Equilibrium Is Maintained
Between Intracellular and
Extracellular Fluids
Volume and Osmolality of Extracellular
and Intracellular Fluids in Abnormal
States
Effect of Adding Saline Solution to the
Extracellular Fluid
Glucose and Other Solutions
Administered for Nutritive Purposes
Clinical Abnormalities of Fluid Volume
Regulation: Hyponatremia and
Hypernatremia
Causes of Hyponatremia: Excess Water or
Loss of Sodium
Causes of Hypernatremia: Water Loss or
Excess Sodium
Edema: Excess Fluid in the Tissues
Intracellular Edema
Extracellular Edema
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302
302
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Table of Contents
Summary of Causes of Extracellular Edema
Safety Factors That Normally Prevent
Edema
Fluids in the “Potential Spaces” of
the Body
C H A P T E R
2 6
Urine Formation by the Kidneys:
I. Glomerular Filtration, Renal Blood
Flow, and Their Control
Multiple Functions of the Kidneys in
Homeostasis
Physiologic Anatomy of the Kidneys
General Organization of the Kidneys and
Urinary Tract
Renal Blood Supply
The Nephron Is the Functional Unit of the
Kidney
Micturition
Physiologic Anatomy and Nervous
Connections of the Bladder
Transport of Urine from the Kidney
Through the Ureters and into
the Bladder
Innervation of the Bladder
Filling of the Bladder and Bladder Wall
Tone; the Cystometrogram
Micturition Reflex
Facilitation or Inhibition of Micturition
by the Brain
Abnormalities of Micturition
Urine Formation Results from
Glomerular Filtration, Tubular
Reabsorption, and Tubular Secretion
Filtration, Reabsorption, and Secretion of
Different Substances
Glomerular Filtration—The First Step in
Urine Formation
Composition of the Glomerular Filtrate
GFR Is About 20 Per Cent of the Renal
Plasma Flow
Glomerular Capillary Membrane
Determinants of the GFR
Increased Glomerular Capillary Filtration
Coefficient Increases GFR
Increased Bowman’s Capsule Hydrostatic
Pressure Decreases GFR
Increased Glomerular Capillary Colloid
Osmotic Pressure Decreases GFR
Increased Glomerular Capillary Hydrostatic
Pressure Increases GFR
Renal Blood Flow
Renal Blood Flow and Oxygen
Consumption
Determinants of Renal Blood Flow
Blood Flow in the Vasa Recta of the Renal
Medulla Is Very Low Compared with Flow
in the Renal Cortex
Physiologic Control of Glomerular
Filtration and Renal Blood Flow
Sympathetic Nervous System Activation
Decreases GFR
Hormonal and Autacoid Control of Renal
Circulation
Autoregulation of GFR and Renal
Blood Flow
303
304
305
Importance of GFR Autoregulation in
Preventing Extreme Changes in Renal
Excretion
Role of Tubuloglomerular Feedback in
Autoregulation of GFR
Myogenic Autoregulation of Renal Blood
Flow and GFR
Other Factors That Increase Renal Blood
Flow and GFR: High Protein Intake and
Increased Blood Glucose
xix
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323
325
325
307
307
308
308
309
310
311
311
312
312
312
313
313
313
314
315
316
316
316
316
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319
320
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321
321
321
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323
C H A P T E R
2 7
Urine Formation by the Kidneys:
II. Tubular Processing of the
Glomerular Filtrate
Reabsorption and Secretion by the
Renal Tubules
Tubular Reabsorption Is Selective and
Quantitatively Large
Tubular Reabsorption Includes
Passive and Active Mechanisms
Active Transport
Passive Water Reabsorption by Osmosis
Is Coupled Mainly to Sodium
Reabsorption
Reabsorption of Chloride, Urea, and Other
Solutes by Passive Diffusion
Reabsorption and Secretion Along
Different Parts of the Nephron
Proximal Tubular Reabsorption
Solute and Water Transport in the Loop
of Henle
Distal Tubule
Late Distal Tubule and Cortical Collecting
Tubule
Medullary Collecting Duct
Summary of Concentrations of Different
Solutes in the Different Tubular
Segments
Regulation of Tubular Reabsorption
Glomerulotubular Balance—The Ability
of the Tubules to Increase Reabsorption
Rate in Response to Increased Tubular
Load
Peritubular Capillary and Renal Interstitial
Fluid Physical Forces
Effect of Arterial Pressure on Urine
Output—The Pressure-Natriuresis and
Pressure-Diuresis Mechanisms
Hormonal Control of Tubular Reabsorption
Sympathetic Nervous System Activation
Increases Sodium Reabsorption
Use of Clearance Methods to Quantify
Kidney Function
Inulin Clearance Can Be Used to Estimate
GFR
Creatine Clearance and Plasma Creatinine
Clearance Can Be Used to Estimate
GFR
PAH Clearance Can Be Used to Estimate
Renal Plasma Flow
Filtration Fraction Is Calculated from GFR
Divided by Renal Plasma Flow
Calculation of Tubular Reabsorption or
Secretion from Renal Clearance
327
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327
328
328
332
332
333
333
334
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336
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341
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343
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346
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C H A P T E R
2 8
Regulation of Extracellular Fluid
Osmolarity and Sodium
Concentration
The Kidneys Excrete Excess Water
by Forming a Dilute Urine
Antidiuretic Hormone Controls Urine
Concentration
Renal Mechanisms for Excreting a
Dilute Urine
The Kidneys Conserve Water by
Excreting a Concentrated Urine
Obligatory Urine Volume
Requirements for Excreting a Concentrated
Urine—High ADH Levels and Hyperosmotic
Renal Medulla
Countercurrent Mechanism Produces a
Hyperosmotic Renal Medullary Interstitium
Role of Distal Tubule and Collecting Ducts in
Excreting a Concentrated Urine
Urea Contributes to Hyperosmotic Renal
Medullary Interstitium and to a
Concentrated Urine
Countercurrent Exchange in the Vasa Recta
Preserves Hyperosmolarity of the
Renal Medulla
Summary of Urine Concentrating Mechanism
and Changes in Osmolarity in Different
Segments of the Tubules
Quantifying Renal Urine Concentration
and Dilution: “Free Water” and Osmolar
Clearances
Disorders of Urinary Concentrating
Ability
Control of Extracellular Fluid Osmolarity
and Sodium Concentration
Estimating Plasma Osmolarity from Plasma
Sodium Concentration
Osmoreceptor-ADH Feedback System
ADH Synthesis in Supraoptic and
Paraventricular Nuclei of the
Hypothalamus and ADH Release from
the Posterior Pituitary
Cardiovascular Reflex Stimulation of ADH
Release by Decreased Arterial Pressure
and/or Decreased Blood Volume
Quantitative Importance of Cardiovascular
Reflexes and Osmolarity in Stimulating
ADH Secretion
Other Stimuli for ADH Secretion
Role of Thirst in Controlling Extracellular
Fluid Osmolarity and Sodium
Concentration
Central Nervous System Centers for Thirst
Stimuli for Thirst
Threshold for Osmolar Stimulus of Drinking
Integrated Responses of Osmoreceptor-ADH
and Thirst Mechanisms in Controlling
Extracellular Fluid Osmolarity and Sodium
Concentration
Role of Angiotensin II and Aldosterone
in Controlling Extracellular Fluid
Osmolarity and Sodium Concentration
Salt-Appetite Mechanism for
Controlling Extracellular Fluid
Sodium Concentration and Volume
Table of Contents
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348
348
349
350
350
350
351
352
353
354
355
357
357
358
358
358
359
360
360
360
361
361
361
362
362
362
363
C H A P T E R
2 9
Renal Regulation of Potassium,
Calcium, Phosphate, and Magnesium;
Integration of Renal Mechanisms for
Control of Blood Volume and
Extracellular Fluid Volume
Regulation of Potassium Excretion
and Potassium Concentration in
Extracellular Fluid
Regulation of Internal Potassium
Distribution
Overview of Renal Potassium Excretion
Potassium Secretion by Principal Cells of
Late Distal and Cortical Collecting
Tubules
Summary of Factors That Regulate
Potassium Secretion: Plasma Potassium
Concentration, Aldosterone, Tubular Flow
Rate, and Hydrogen Ion Concentration
Control of Renal Calcium Excretion
and Extracellular Calcium Ion
Concentration
Control of Calcium Excretion by the
Kidneys
Regulation of Renal Phosphate Excretion
Control of Renal Magnesium Excretion
and Extracellular Magnesium Ion
Concentration
Integration of Renal Mechanisms for
Control of Extracellular Fluid
Sodium Excretion Is Precisely Matched to
Intake Under Steady-State Conditions
Sodium Excretion Is Controlled by Altering
Glomerular Filtration or Tubular Sodium
Reabsorption Rates
Importance of Pressure Natriuresis and
Pressure Diuresis in Maintaining Body
Sodium and Fluid Balance
Pressure Natriuresis and Diuresis Are Key
Components of a Renal-Body Fluid
Feedback for Regulating Body Fluid
Volumes and Arterial Pressure
Precision of Blood Volume and Extracellular
Fluid Volume Regulation
Distribution of Extracellular Fluid
Between the Interstitial Spaces and
Vascular System
Nervous and Hormonal Factors Increase
the Effectiveness of Renal-Body Fluid
Feedback Control
Sympathetic Nervous System Control of
Renal Excretion: Arterial Baroreceptor and
Low-Pressure Stretch Receptor Reflexes
Role of Angiotensin II In Controlling Renal
Excretion
Role of Aldosterone in Controlling Renal
Excretion
Role of ADH in Controlling Renal Water
Excretion
Role of Atrial Natriuretic Peptide in
Controlling Renal Excretion
Integrated Responses to Changes in
Sodium Intake
Conditions That Cause Large Increases
in Blood Volume and Extracellular
Fluid Volume
365
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366
367
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368
371
372
372
373
373
373
374
374
375
376
376
377
377
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380
Table of Contents
Increased Blood Volume and Extracellular
Fluid Volume Caused by Heart Diseases
Increased Blood Volume Caused by
Increased Capacity of Circulation
Conditions That Cause Large Increases
in Extracellular Fluid Volume but with
Normal Blood Volume
Nephrotic Syndrome—Loss of Plasma
Proteins in Urine and Sodium Retention
by the Kidneys
Liver Cirrhosis—Decreased Synthesis of
Plasma Proteins by the Liver and
Sodium Retention by the Kidneys
C H A P T E R
3 0
Regulation of Acid-Base Balance
Hydrogen Ion Concentration Is
Precisely Regulated
Acids and Bases—Their Definitions
and Meanings
Defenses Against Changes in Hydrogen
Ion Concentration: Buffers, Lungs,
and Kidneys
Buffering of Hydrogen Ions in the Body
Fluids
Bicarbonate Buffer System
Quantitative Dynamics of the Bicarbonate
Buffer System
Phosphate Buffer System
Proteins: Important Intracellular
Buffers
Respiratory Regulation of Acid-Base
Balance
Pulmonary Expiration of CO2 Balances
Metabolic Formation of CO2
Increasing Alveolar Ventilation Decreases
Extracellular Fluid Hydrogen Ion
Concentration and Raises pH
Increased Hydrogen Ion Concentration
Stimulates Alveolar Ventilation
Renal Control of Acid-Base Balance
Secretion of Hydrogen Ions and
Reabsorption of Bicarbonate Ions
by the Renal Tubules
Hydrogen Ions Are Secreted by Secondary
Active Transport in the Early Tubular
Segments
Filtered Bicarbonate Ions Are Reabsorbed
by Interaction with Hydrogen Ions in the
Tubules
Primary Active Secretion of Hydrogen Ions in
the Intercalated Cells of Late Distal and
Collecting Tubules
Combination of Excess Hydrogen Ions
with Phosphate and Ammonia Buffers
in the Tubule—A Mechanism for
Generating “New” Bicarbonate Ions
Phosphate Buffer System Carries Excess
Hydrogen Ions into the Urine and
Generates New Bicarbonate
Excretion of Excess Hydrogen Ions and
Generation of New Bicarbonate by the
Ammonia Buffer System
Quantifying Renal Acid-Base Excretion
Regulation of Renal Tubular Hydrogen Ion
Secretion
380
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385
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395
Renal Correction of Acidosis—Increased
Excretion of Hydrogen Ions and
Addition of Bicarbonate Ions to the
Extracellular Fluid
Acidosis Decreases the Ratio of HCO3-/H+ in
Renal Tubular Fluid
Renal Correction of Alkalosis—Decreased
Tubular Secretion of Hydrogen Ions
and Increased Excretion of
Bicarbonate Ions
Alkalosis Increases the Ratio of HCO3-/H+
in Renal Tubular Fluid
Clinical Causes of Acid-Base Disorders
Respiratory Acidosis Is Caused by
Decreased Ventilation and Increased PCO2
Respiratory Alkalosis Results from Increased
Ventilation and Decreased PCO2
Metabolic Acidosis Results from Decreased
Extracellular Fluid Bicarbonate
Concentration
Treatment of Acidosis or Alkalosis
Clinical Measurements and Analysis of
Acid-Base Disorders
Complex Acid-Base Disorders and Use of
the Acid-Base Nomogram for Diagnosis
Use of Anion Gap to Diagnose Acid-Base
Disorders
C H A P T E R
3 1
Kidney Diseases and Diuretics
Diuretics and Their Mechanisms of
Action
Osmotic Diuretics Decrease Water
Reabsorption by Increasing Osmotic
Pressure of Tubular Fluid
“Loop” Diuretics Decrease Active
Sodium-Chloride-Potassium Reabsorption
in the Thick Ascending Loop of Henle
Thiazide Diuretics Inhibit Sodium-Chloride
Reabsorption in the Early Distal Tubule
Carbonic Anhydrase Inhibitors Block
Sodium-Bicarbonate Reabsorption in the
Proximal Tubules
Competitive Inhibitors of Aldosterone
Decrease Sodium Reabsorption from and
Potassium Secretion into the Cortical
Collecting Tubule
Diuretics That Block Sodium Channels
in the Collecting Tubules Decrease
Sodium Reabsorption
Kidney Diseases
Acute Renal Failure
Prerenal Acute Renal Failure Caused by
Decreased Blood Flow to the Kidney
Intrarenal Acute Renal Failure Caused by
Abnormalities within the Kidney
Postrenal Acute Renal Failure Caused by
Abnormalities of the Lower Urinary
Tract
Physiologic Effects of Acute Renal Failure
Chronic Renal Failure: An Irreversible
Decrease in the Number of Functional
Nephrons
Vicious Circle of Chronic Renal Failure
Leading to End-Stage Renal Disease
Injury to the Renal Vasculature as a Cause
of Chronic Renal Failure
xxi
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Table of Contents
Injury to the Glomeruli as a Cause of
Chronic Renal Failure—
Glomerulonephritis
Injury to the Renal Interstitium as a
Cause of Chronic Renal Failure—
Pyelonephritis
Nephrotic Syndrome—Excretion of Protein
in the Urine Because of Increased
Glomerular Permeability
Nephron Function in Chronic Renal Failure
Effects of Renal Failure on the Body
Fluids—Uremia
Hypertension and Kidney Disease
Specific Tubular Disorders
Treatment of Renal Failure by Dialysis
with an Artificial Kidney
U N I T
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409
409
409
411
412
413
414
V I
Blood Cells, Immunity, and Blood
Clotting
C H A P T E R
3 2
Red Blood Cells, Anemia, and
Polycythemia
Red Blood Cells (Erythrocytes)
Production of Red Blood Cells
Formation of Hemoglobin
Iron Metabolism
Life Span and Destruction of Red Blood
Cells
Anemias
Effects of Anemia on Function of the
Circulatory System
Polycythemia
Effect of Polycythemia on Function of the
Circulatory System
C H A P T E R
3 3
Resistance of the Body to Infection: I.
Leukocytes, Granulocytes, the
Monocyte-Macrophage System, and
Inflammation
Leukocytes (White Blood Cells)
General Characteristics of Leukocytes
Genesis of the White Blood Cells
Life Span of the White Blood Cells
Neutrophils and Macrophages Defend
Against Infections
Phagocytosis
Monocyte-Macrophage Cell System
(Reticuloendothelial System)
Inflammation: Role of Neutrophils and
Macrophages
Inflammation
Macrophage and Neutrophil Responses
During Inflammation
Eosinophils
Basophils
Leukopenia
The Leukemias
Effects of Leukemia on the Body
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427
427
428
429
429
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431
431
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432
C H A P T E R
3 4
Resistance of the Body to Infection: II.
Immunity and Allergy
Innate Immunity
Acquired (Adaptive) Immunity
Basic Types of Acquired Immunity
Both Types of Acquired Immunity Are
Initiated by Antigens
Lymphocytes Are Responsible for
Acquired Immunity
Preprocessing of the T and B Lymphocytes
T Lymphocytes and B-Lymphocyte
Antibodies React Highly Specifically
Against Specific Antigens—Role of
Lymphocyte Clones
Origin of the Many Clones of Lymphocytes
Specific Attributes of the B-Lymphocyte
System—Humoral Immunity and the
Antibodies
Special Attributes of the T-Lymphocyte
System–Activated T Cells and CellMediated Immunity
Several Types of T Cells and Their Different
Functions
Tolerance of the Acquired Immunity
System to One’s Own Tissues—Role
of Preprocessing in the Thymus and
Bone Marrow
Immunization by Injection of Antigens
Passive Immunity
Allergy and Hypersensitivity
Allergy Caused by Activated T Cells:
Delayed-Reaction Allergy
Allergies in the “Allergic” Person, Who Has
Excess IgE Antibodies
C H A P T E R
3 5
Blood Types; Transfusion; Tissue and
Organ Transplantation
Antigenicity Causes Immune Reactions
of Blood
O-A-B Blood Types
A and B Antigens—Agglutinogens
Agglutinins
Agglutination Process In Transfusion
Reactions
Blood Typing
Rh Blood Types
Rh Immune Response
Transfusion Reactions Resulting from
Mismatched Blood Types
Transplantation of Tissues and Organs
Attempts to Overcome Immune Reactions
in Transplanted Tissue
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C H A P T E R
3 6
Hemostasis and Blood Coagulation
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436
437
437
Events in Hemostasis
Vascular Constriction
Formation of the Platelet Plug
Blood Coagulation in the Ruptured
Vessel
Fibrous Organization or Dissolution of the
Blood Clot
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Table of Contents
Mechanism of Blood Coagulation
Conversion of Prothrombin to Thrombin
Conversion of Fibrinogen to Fibrin—
Formation of the Clot
Vicious Circle of Clot Formation
Initiation of Coagulation: Formation of
Prothrombin Activator
Prevention of Blood Clotting in the
Normal Vascular System—Intravascular
Anticoagulants
Lysis of Blood Clots—Plasmin
Conditions That Cause Excessive
Bleeding in Human Beings
Decreased Prothrombin, Factor VII,
Factor IX,and Factor X Caused by
Vitamin K Deficiency
Hemophilia
Thrombocytopenia
Thromboembolic Conditions in the
Human Being
Femoral Venous Thrombosis and Massive
Pulmonary Embolism
Disseminated Intravascular Coagulation
Anticoagulants for Clinical Use
Heparin as an Intravenous Anticoagulant
Coumarins as Anticoagulants
Prevention of Blood Coagulation Outside
the Body
Blood Coagulation Tests
Bleeding Time
Clotting Time
Prothrombin Time
U N I T
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460
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461
463
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464
465
465
465
466
466
466
466
466
466
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467
467
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V I I
Respiration
C H A P T E R
Pulmonary Ventilation
3 7
Mechanics of Pulmonary Ventilation
Muscles That Cause Lung Expansion and
Contraction
Movement of Air In and Out of the Lungs
and the Pressures That Cause the
Movement
Effect of the Thoracic Cage on Lung
Expansibility
Pulmonary Volumes and Capacities
Recording Changes in Pulmonary Volume—
Spirometry
Abbreviations and Symbols Used in
Pulmonary Function Tests
Determination of Functional Residual
Capacity, Residual Volume, and Total
Lung Capacity—Helium Dilution Method
Minute Respiratory Volume Equals
Respiratory Rate Times Tidal Volume
Alveolar Ventilation
“Dead Space” and Its Effect on Alveolar
Ventilation
Rate of Alveolar Ventilation
Functions of the Respiratory
Passageways
Trachea, Bronchi, and Bronchioles
Normal Respiratory Functions of the
Nose
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477
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477
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478
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480
C H A P T E R
3 8
Pulmonary Circulation, Pulmonary
Edema, Pleural Fluid
Physiologic Anatomy of the Pulmonary
Circulatory System
Pressures in the Pulmonary System
Blood Volume of the Lungs
Blood Flow Through the Lungs and
Its Distribution
Effect of Hydrostatic Pressure
Gradients in the Lungs on Regional
Pulmonary Blood Flow
Zones 1, 2, and 3 of Pulmonary Blood Flow
Effect of Increased Cardiac Output on
Pulmonary Blood Flow and Pulmonary
Arterial Pressure During Heavy Exercise
Function of the Pulmonary Circulation
When the Left Atrial Pressure Rises as a
Result of Left-Sided Heart Failure
Pulmonary Capillary Dynamics
Capillary Exchange of Fluid in the Lungs,
and Pulmonary Interstitial Fluid Dynamics
Pulmonary Edema
Fluid in the Pleural Cavity
C H A P T E R
3 9
Physical Principles of Gas Exchange;
Diffusion of Oxygen and Carbon
Dioxide Through the Respiratory
Membrane
Physics of Gas Diffusion and Gas
Partial Pressures
Molecular Basis of Gas Diffusion
Gas Pressures in a Mixture of Gases—
“Partial Pressures” of Individual Gases
Pressures of Gases Dissolved in Water
and Tissues
Vapor Pressure of Water
Diffusion of Gases Through Fluids—
Pressure Difference Causes Net
Diffusion
Diffusion of Gases Through Tissues
Composition of Alveolar Air—Its Relation
to Atmospheric Air
Rate at Which Alveolar Air Is Renewed by
Atmospheric Air
Oxygen Concentration and Partial Pressure
in the Alveoli
CO2 Concentration and Partial Pressure in
the Alveoli
Expired Air
Diffusion of Gases Through the
Respiratory Membrane
Factors That Affect the Rate of Gas
Diffusion Through the Respiratory
Membrane
Diffusing Capacity of the Respiratory
Membrane
Effect of the Ventilation-Perfusion
Ratio on .Alveolar
Gas Concentration
.
PO2-PCO2, VA/Q Diagram
Concept of
. the
. “Physiological Shunt”
(When VA/Q Is Greater Than Normal)
Abnormalities of Ventilation-Perfusion Ratio
xxiii
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C H A P T E R
4 0
Transport of Oxygen and Carbon
Dioxide in Blood and Tissue Fluids
Transport of Oxygen from the Lungs to
the Body Tissues
Diffusion of Oxygen from the Alveoli to the
Pulmonary Capillary Blood
Transport of Oxygen in the Arterial Blood
Diffusion of Oxygen from the Peripheral
Capillaries into the Tissue Fluid
Diffusion of Oxygen from the Peripheral
Capillaries to the Tissue Cells
Diffusion of Carbon Dioxide from the
Peripheral Tissue Cells into the
Capillaries and from the Pulmonary
Capillaries into the Alveoli
Role of Hemoglobin in Oxygen Transport
Reversible Combination of Oxygen with
Hemoglobin
Effect of Hemoglobin to “Buffer” the
Tissue PO2
Factors That Shift the Oxygen-Hemoglobin
Dissociation Curve—Their Importance for
Oxygen Transport
Metabolic Use of Oxygen by the Cells
Transport of Oxygen in the Dissolved State
Combination of Hemoglobin with Carbon
Monoxide—Displacement of Oxygen
Transport of Carbon Dioxide in the Blood
Chemical Forms in Which Carbon Dioxide
Is Transported
Carbon Dioxide Dissociation Curve
When Oxygen Binds with Hemoglobin,
Carbon Dioxide Is Released (the Haldane
Effect) to Increase CO2 Transport
Change in Blood Acidity During Carbon
Dioxide Transport
Respiratory Exchange Ratio
C H A P T E R
4 1
Regulation of Respiration
Respiratory Center
Dorsal Respiratory Group of Neurons—Its
Control of Inspiration and of Respiratory
Rhythm
A Pneumotaxic Center Limits the Duration
of Inspiration and Increases the
Respiratory Rate
Ventral Respiratory Group of Neurons—
Functions in Both Inspiration and
Expiration
Lung Inflation Signals Limit Inspiration—
The Hering-Breuer Inflation Reflex
Control of Overall Respiratory Center
Activity
Chemical Control of Respiration
Direct Chemical Control of Respiratory
Center Activity by Carbon Dioxide and
Hydrogen Ions
Peripheral Chemoreceptor System for
Control of Respiratory Activity—Role
of Oxygen in Respiratory Control
Effect of Low Arterial PO2 to Stimulate
Alveolar Ventilation When Arterial Carbon
Dioxide and Hydrogen Ion Concentrations
Remain Normal
Table of Contents
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512
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514
514
514
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516
516
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519
Chronic Breathing of Low Oxygen Stimulates
Respiration Even More—The Phenomenon
of “Acclimatization”
Composite Effects of PCO2, pH, and PO2 on
Alveolar Ventilation
Regulation of Respiration During
Exercise
Other Factors That Affect Respiration
Sleep Apnea
C H A P T E R
4 2
Respiratory Insufficiency—
Pathophysiology, Diagnosis, Oxygen
Therapy
Useful Methods for Studying Respiratory
Abnormalities
Study of Blood Gases and Blood pH
Measurement of Maximum Expiratory Flow
Forced Expiratory Vital Capacity and Forced
Expiratory Volume
Physiologic Peculiarities of Specific
Pulmonary Abnormalities
Chronic Pulmonary Emphysema
Pneumonia
Atelectasis
Asthma
Tuberculosis
Hypoxia and Oxygen Therapy
Oxygen Therapy in Different Types of
Hypoxia
Cyanosis
Hypercapnia
Dyspnea
Artificial Respiration
U N I T
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525
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526
526
527
528
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530
530
530
531
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532
V I I I
Aviation, Space, and Deep-Sea
Diving Physiology
C H A P T E R
4 3
Aviation, High-Altitude, and Space
Physiology
Effects of Low Oxygen Pressure on the
Body
Alveolar PO2 at Different Elevations
Effect of Breathing Pure Oxygen on Alveolar
PO2 at Different Altitudes
Acute Effects of Hypoxia
Acclimatization to Low PO2
Natural Acclimatization of Native Human
Beings Living at High Altitudes
Reduced Work Capacity at High Altitudes
and Positive Effect of Acclimatization
Acute Mountain Sickness and High-Altitude
Pulmonary Edema
Chronic Mountain Sickness
Effects of Acceleratory Forces on the
Body in Aviation and Space Physiology
Centrifugal Acceleratory Forces
Effects of Linear Acceleratory Forces on the
Body
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Table of Contents
“Artificial Climate” in the Sealed
Spacecraft
Weightlessness in Space
C H A P T E R
4 4
Physiology of Deep-Sea Diving and
Other Hyperbaric Conditions
Effect of High Partial Pressures of
Individual Gases on the Body
Nitrogen Narcosis at High Nitrogen
Pressures
Oxygen Toxicity at High Pressures
Carbon Dioxide Toxicity at Great Depths
in the Sea
Decompression of the Diver After Excess
Exposure to High Pressure
Scuba (Self-Contained Underwater
Breathing Apparatus) Diving
Special Physiologic Problems in
Submarines
Hyperbaric Oxygen Therapy
U N I T
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543
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545
545
546
547
547
549
550
550
I X
The Nervous System: A. General
Principles and Sensory Physiology
C H A P T E R
4 5
Organization of the Nervous System,
Basic Functions of Synapses,
“Transmitter Substances”
General Design of the Nervous System
Central Nervous System Neuron: The Basic
Functional Unit
Sensory Part of the Nervous System—
Sensory Receptors
Motor Part of the Nervous System—
Effectors
Processing of Information—“Integrative”
Function of the Nervous System
Storage of Information—Memory
Major Levels of Central Nervous System
Function
Spinal Cord Level
Lower Brain or Subcortical Level
Higher Brain or Cortical Level
Comparison of the Nervous System
with a Computer
Central Nervous System Synapses
Types of Synapses—Chemical and
Electrical
Physiologic Anatomy of the Synapse
Chemical Substances That Function as
Synaptic Transmitters
Electrical Events During Neuronal Excitation
Electrical Events During Neuronal
Inhibition
Special Functions of Dendrites for Exciting
Neurons
Relation of State of Excitation of the Neuron
to Rate of Firing
Some Special Characteristics of
Synaptic Transmission
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557
558
558
558
559
559
559
562
564
566
568
569
570
C H A P T E R
4 6
Sensory Receptors, Neuronal Circuits
for Processing Information
Types of Sensory Receptors and the
Sensory Stimuli They Detect
Differential Sensitivity of Receptors
Transduction of Sensory Stimuli into
Nerve Impulses
Local Electrical Currents at Nerve Endings—
Receptor Potentials
Adaptation of Receptors
Nerve Fibers That Transmit Different
Types of Signals, and Their
Physiologic Classification
Transmission of Signals of Different
Intensity in Nerve Tracts—Spatial and
Temporal Summation
Transmission and Processing of Signals
in Neuronal Pools
Relaying of Signals Through Neuronal
Pools
Prolongation of a Signal by a Neuronal
Pool—“Afterdischarge”
Instability and Stability of Neuronal
Circuits
Inhibitory Circuits as a Mechanism for
Stabilizing Nervous System Function
Synaptic Fatigue as a Means for Stabilizing
the Nervous System
C H A P T E R
4 7
Somatic Sensations: I. General
Organization, the Tactile and
Position Senses
CLASSIFICATION OF SOMATIC SENSES
Detection and Transmission of Tactile
Sensations
Detection of Vibration
TICKLE AND ITCH
Sensory Pathways for Transmitting
Somatic Signals into the Central
Nervous System
Dorsal Column–Medial Lemniscal System
Anterolateral System
Transmission in the Dorsal Column—
Medial Lemniscal System
Anatomy of the Dorsal Column—Medial
Lemniscal System
Somatosensory Cortex
Somatosensory Association Areas
Overall Characteristics of Signal
Transmission and Analysis in the Dorsal
Column–Medial Lemniscal System
Position Senses
Interpretation of Sensory Stimulus Intensity
Judgment of Stimulus Intensity
Position Senses
Transmission of Less Critical Sensory
Signals in the Anterolateral Pathway
Anatomy of the Anterolateral Pathway
Some Special Aspects of
Somatosensory Function
Function of the Thalamus in Somatic
Sensation
xxv
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Table of Contents
Cortical Control of Sensory Sensitivity—
“Corticofugal” Signals
Segmental Fields of Sensation—The
Dermatomes
C H A P T E R
4 8
Somatic Sensations: II. Pain,
Headache, and Thermal Sensations
Types of Pain and Their Qualities—
Fast Pain and Slow Pain
Pain Receptors and Their Stimulation
Rate of Tissue Damage as a Stimulus
for Pain
Dual Pathways for Transmission of Pain
Signals into the Central Nervous
System
Dual Pain Pathways in the Cord and Brain
Stem—The Neospinothalamic Tract and
the Paleospinothalamic Tract
Pain Suppression (“Analgesia”) System
in the Brain and Spinal Cord
Brain’s Opiate System—Endorphins and
Enkephalins
Inhibition of Pain Transmission by
Simultaneous Tactile Sensory Signals
Treatment of Pain by Electrical Stimulation
Referred Pain
Visceral Pain
Causes of True Visceral Pain
“Parietal Pain” Caused by Visceral Disease
Localization of Visceral Pain—“Visceral”
and the “Parietal” Pain Transmission
Pathways
Some Clinical Abnormalities of Pain
and Other Somatic Sensations
Hyperalgesia
Herpes Zoster (Shingles)
Tic Douloureux
Brown-Séquard Syndrome
Headache
Headache of Intracranial Origin
Thermal Sensations
Thermal Receptors and Their Excitation
Transmission of Thermal Signals in the
Nervous System
U N I T
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X
The Nervous System: B.
The Special Senses
C H A P T E R
4 9
The Eye: I. Optics of Vision
Physical Principles of Optics
Refraction of Light
Application of Refractive Principles to
Lenses
Focal Length of a Lens
Formation of an Image by a Convex Lens
Measurement of the Refractive Power of a
Lens—“Diopter”
Optics of the Eye
The Eye as a Camera
Mechanism of “Accommodation”
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617
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Pupillary Diameter
Errors of Refraction
Visual Acuity
Determination of Distance of an Object
from the Eye—“Depth Perception”
Ophthalmoscope
Fluid System of the Eye—Intraocular
Fluid
Formation of Aqueous Humor by the Ciliary
Body
Outflow of Aqueous Humor from the Eye
Intraocular Pressure
C H A P T E R
5 0
The Eye: II. Receptor and Neural
Function of the Retina
Anatomy and Function of the
Structural Elements of the Retina
Photochemistry of Vision
Rhodopsin-Retinal Visual Cycle, and
Excitation of the Rods
Automatic Regulation of Retinal Sensitivity—
Light and Dark Adaptation
Color Vision
Tricolor Mechanism of Color Detection
Color Blindness
Neural Function of the Retina
Neural Circuitry of the Retina
Ganglion Cells and Optic Nerve Fibers
Excitation of the Ganglion Cells
C H A P T E R
5 1
The Eye: III. Central
Neurophysiology of Vision
Visual Pathways
Function of the Dorsal Lateral Geniculate
Nucleus of the Thalamus
Organization and Function of the Visual
Cortex
Layered Structure of the Primary Visual
Cortex
Two Major Pathways for Analysis of Visual
Information—(1) The Fast “Position” and
“Motion” Pathway; (2) The Accurate Color
Pathway
Neuronal Patterns of Stimulation During
Analysis of the Visual Image
Detection of Color
Effect of Removing the Primary Visual
Cortex
Fields of Vision; Perimetry
Eye Movements and Their Control
Fixation Movements of the Eyes
“Fusion” of the Visual Images from the
Two Eyes
Autonomic Control of Accommodation
and Pupillary Aperture
Control of Accommodation (Focusing the
Eyes)
Control of Pupillary Diameter
C H A P T E R
The Sense of Hearing
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5 2
Tympanic Membrane and the Ossicular
System
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Table of Contents
Conduction of Sound from the Tympanic
Membrane to the Cochlea
Transmission of Sound Through Bone
Cochlea
Functional Anatomy of the Cochlea
Transmission of Sound Waves in the
Cochlea—“Traveling Wave”
Function of the Organ of Corti
Determination of Sound Frequency—The
“Place” Principle
Determination of Loudness
Central Auditory Mechanisms
Auditory Nervous Pathways
Function of the Cerebral Cortex in Hearing
Determination of the Direction from Which
Sound Comes
Centrifugal Signals from the Central
Nervous System to Lower Auditory
Centers
Hearing Abnormalities
Types of Deafness
C H A P T E R
5 3
The Chemical Senses—Taste and
Smell
Sense of Taste
Primary Sensations of Taste
Taste Bud and Its Function
Transmission of Taste Signals into the
Central Nervous System
Taste Preference and Control of the Diet
Sense of Smell
Olfactory Membrane
Stimulation of the Olfactory Cells
Transmission of Smell Signals into the
Central Nervous System
U N I T
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X I
The Nervous System: C. Motor and
Integrative Neurophysiology
C H A P T E R
5 4
Motor Functions of the Spinal Cord;
the Cord Reflexes
Organization of the Spinal Cord for Motor
Functions
Muscle Sensory Receptors—Muscle
Spindles and Golgi Tendon Organs—
And Their Roles in Muscle Control
Receptor Function of the Muscle Spindle
Muscle Stretch Reflex
Role of the Muscle Spindle in Voluntary
Motor Activity
Clinical Applications of the Stretch
Reflex
Golgi Tendon Reflex
Function of the Muscle Spindles and Golgi
Tendon Organs in Conjunction with Motor
Control from Higher Levels of the Brain
Flexor Reflex and the Withdrawal
Reflexes
Crossed Extensor Reflex
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Reciprocal Inhibition and Reciprocal
Innervation
Reflexes of Posture and Locomotion
Postural and Locomotive Reflexes of
the Cord
Scratch Reflex
Spinal Cord Reflexes That Cause
Muscle Spasm
Autonomic Reflexes in the Spinal
Cord
Spinal Cord Transection and Spinal
Shock
C H A P T E R
5 5
Cortical and Brain Stem Control of
Motor Function
MOTOR CORTEX AND CORTICOSPINAL
TRACT
Primary Motor Cortex
Premotor Area
Supplementary Motor Area
Some Specialized Areas of Motor Control
Found in the Human Motor Cortex
Transmission of Signals from the Motor
Cortex to the Muscles
Incoming Fiber Pathways to the Motor
Cortex
Red Nucleus Serves as an Alternative
Pathway for Transmitting Cortical Signals
to the Spinal Cord
“Extrapyramidal” System
Excitation of the Spinal Cord Motor Control
Areas by the Primary Motor Cortex and
Red Nucleus
Role of the Brain Stem in Controlling
Motor Function
Support of the Body Against Gravity—
Roles of the Reticular and Vestibular
Nuclei
Vestibular Sensations and Maintenance
of Equilibrium
Vestibular Apparatus
Function of the Utricle and Saccule in the
Maintenance of Static Equilibrium
Detection of Head Rotation by the
Semicircular Ducts
Vestibular Mechanisms for Stabilizing the
Eyes
Other Factors Concerned with Equilibrium
Functions of Brain Stem Nuclei in
Controlling Subconscious, Stereotyped
Movements
C H A P T E R
5 6
Contributions of the Cerebellum and
Basal Ganglia to Overall Motor
Control
Cerebellum and Its Motor Functions
Anatomical Functional Areas of the
Cerebellum
Neuronal Circuit of the Cerebellum
Function of the Cerebellum in Overall
Motor Control
Clinical Abnormalities of the Cerebellum
xxvii
681
682
682
683
683
683
684
685
685
685
686
686
686
687
688
688
689
689
691
691
692
692
694
695
696
696
697
698
698
699
700
703
706
xxviii
Basal Ganglia—Their Motor Functions
Function of the Basal Ganglia in Executing
Patterns of Motor Activity—The Putamen
Circuit
Role of the Basal Ganglia for Cognitive
Control of Sequences of Motor Patterns—
The Caudate Circuit
Function of the Basal Ganglia to Change
the Timing and to Scale the Intensity of
Movements
Functions of Specific Neurotransmitter
Substances in the Basal Ganglial
System
Integration of the Many Parts of the
Total Motor Control System
Spinal Level
Hindbrain Level
Motor Cortex Level
What Drives Us to Action?
C H A P T E R
5 7
Cerebral Cortex, Intellectual Functions
of the Brain, Learning and Memory
Physiologic Anatomy of the Cerebral
Cortex
Functions of Specific Cortical Areas
Association Areas
Comprehensive Interpretative Function of
the Posterior Superior Temporal Lobe—
“Wernicke’s Area” (a General
Interpretative Area)
Functions of the Parieto-occipitotemporal
Cortex in the Nondominant Hemisphere
Higher Intellectual Functions of the
Prefrontal Association Areas
Function of the Brain in
Communication—Language Input
and Language Output
Function of the Corpus Callosum and
Anterior Commissure to Transfer
Thoughts, Memories, Training, and
Other Information Between the Two
Cerebral Hemispheres
Thoughts, Consciousness, and Memory
Memory—Roles of Synaptic Facilitation and
Synaptic Inhibition
Short-Term Memory
Intermediate Long-Term Memory
Long-Term Memory
Consolidation of Memory
C H A P T E R
5 8
Behavioral and Motivational
Mechanisms of the Brain—The
Limbic System and the
Hypothalamus
Activating-Driving Systems of the Brain
Control of Cerebral Activity by Continuous
Excitatory Signals from the Brain Stem
Neurohormonal Control of Brain Activity
Limbic System
Functional Anatomy of the Limbic
System; Key Position of the
Hypothalamus
Table of Contents
707
708
709
709
710
712
712
712
712
713
714
714
715
716
718
719
719
720
722
723
723
724
724
725
725
728
728
728
730
731
731
Hypothalamus, a Major Control
Headquarters for the Limbic System
Vegetative and Endocrine Control
Functions of the Hypothalamus
Behavioral Functions of the Hypothalamus
and Associated Limbic Structures
“Reward” and “Punishment” Function of
the Limbic System
Importance of Reward or Punishment in
Behavior
Specific Functions of Other Parts of
the Limbic System
Functions of the Hippocampus
Functions of the Amygdala
Function of the Limbic Cortex
C H A P T E R
5 9
States of Brain Activity—Sleep, Brain
Waves, Epilepsy, Psychoses
Sleep
Slow-Wave Sleep
REM Sleep (Paradoxical Sleep,
Desynchronized Sleep)
Basic Theories of Sleep
Physiologic Effects of Sleep
Brain Waves
Origin of Brain Waves
Effect of Varying Levels of Cerebral
Activity on the Frequency of the EEG
Changes in the EEG at Different Stages of
Wakefulness and Sleep
Epilepsy
Grand Mal Epilepsy
Petit Mal Epilepsy
Focal Epilepsy
Psychotic Behavior and Dementia—
Roles of Specific Neurotransmitter
Systems
Depression and Manic-Depressive
Psychoses—Decreased Activity of the
Norepinephrine and Serotonin
Neurotransmitter Systems
Schizophrenia—Possible Exaggerated
Function of Part of the Dopamine
System
Alzheimer’s Disease—Amyloid Plaques
and Depressed Memory
C H A P T E R
6 0
The Autonomic Nervous System and
the Adrenal Medulla
General Organization of the Autonomic
Nervous System
Physiologic Anatomy of the Sympathetic
Nervous System
Preganglionic and Postganglionic
Sympathetic Neurons
Physiologic Anatomy of the
Parasympathetic Nervous System
Basic Characteristics of Sympathetic
and Parasympathetic Function
Cholinergic and Adrenergic Fibers—
Secretion of Acetylcholine or
Norepinephrine
Receptors on the Effector Organs
732
733
734
735
736
736
736
737
738
739
739
739
740
740
741
741
742
743
743
743
743
744
744
745
745
745
746
748
748
748
748
750
750
750
752
Table of Contents
Excitatory and Inhibitory Actions of
Sympathetic and Parasympathetic
Stimulation
Effects of Sympathetic and Parasympathetic
Stimulation on Specific Organs
Function of the Adrenal Medullae
Relation of Stimulus Rate to Degree of
Sympathetic and Parasympathetic Effect
Sympathetic and Parasympathetic “Tone”
Denervation Supersensitivity of Sympathetic
and Parasympathetic Organs after
Denervation
Autonomic Reflexes
Stimulation of Discrete Organs in Some
Instances and Mass Stimulation in
Other Instances by the Sympathetic
and Parasympathetic Systems
“Alarm” or “Stress” Response of the
Sympathetic Nervous System
Medullary, Pontine, and Mesencephalic
Control of the Autonomic Nervous
System
Pharmacology of the Autonomic
Nervous System
Drugs That Act on Adrenergic Effector
Organs—Sympathomimetic Drugs
Drugs That Act on Cholinergic Effector
Organs
Drugs That Stimulate or Block Sympathetic
and Parasympathetic Postganglionic
Neurons
C H A P T E R
6 1
Cerebral Blood Flow, Cerebrospinal
Fluid, and Brain Metabolism
Cerebral Blood Flow
Normal Rate of Cerebral Blood Flow
Regulation of Cerebral Blood Flow
Cerebral Microcirculation
Cerebral Stroke Occurs When Cerebral
Blood Vessels are Blocked
Cerebrospinal Fluid System
Cushioning Function of the Cerebrospinal
Fluid
Formation, Flow, and Absorption of
Cerebrospinal Fluid
Cerebrospinal Fluid Pressure
Obstruction to Flow of Cerebrospinal Fluid
Can Cause Hydrocephalus
Blood–Cerebrospinal Fluid and Blood-Brain
Barriers
Brain Edema
Brain Metabolism
U N I T
753
753
755
756
756
756
757
757
758
758
Physiological Anatomy of the
Gastrointestinal Wall
Neural Control of Gastrointestinal
Function—Enteric Nervous System
Differences Between the Myenteric and
Submucosal Plexuses
Types of Neurotransmitters Secreted by
Enteric Neurons
Hormonal Control of Gastrointestinal
Motility
Functional Types of Movements in the
Gastrointestinal Tract
Propulsive Movements—Peristalsis
Mixing Movements
Gastrointestinal Blood Flow—
“Splanchnic Circulation”
Anatomy of the Gastrointestinal Blood
Supply
Effect of Gut Activity and Metabolic
Factors on Gastrointestinal Blood
Flow
Nervous Control of Gastrointestinal Blood
Flow
xxix
771
773
774
775
776
776
776
777
777
778
778
779
759
759
759
759
761
761
761
761
763
763
763
763
764
765
C H A P T E R
6 3
Propulsion and Mixing of Food in the
Alimentary Tract
Ingestion of Food
Mastication (Chewing)
Swallowing (Deglutition)
Motor Functions of the Stomach
Storage Function of the Stomach
Mixing and Propulsion Of Food in the
Stomach—The Basic Electrical Rhythm
of the Stomach Wall
Stomach Emptying
Regulation of Stomach Emptying
Movements of the Small Intestine
Mixing Contractions (Segmentation
Contractions)
Propulsive Movements
Function of the Ileocecal Valve
Movements of the Colon
Defecation
Other Autonomic Reflexes That Affect
Bowel Activity
781
781
781
782
784
784
784
785
785
786
786
787
788
788
789
790
766
766
766
767
X I I
Gastrointestinal Physiology
C H A P T E R
6 2
General Principles of Gastrointestinal
Function—Motility, Nervous Control,
and Blood Circulation
771
General Principles of Gastrointestinal
Motility
771
C H A P T E R
6 4
Secretory Functions of the Alimentary
Tract
General Principles of Alimentary Tract
Secretion
Anatomical Types of Glands
Basic Mechanisms of Stimulation of the
Alimentary Tract Glands
Basic Mechanism of Secretion by Glandular
Cells
Lubricating and Protective Properties of
Mucus, and Importance of Mucus in the
Gastrointestinal Tract
Secretion of Saliva
Nervous Regulation of Salivary Secretion
Esophageal Secretion
791
791
791
791
791
793
793
794
795
xxx
Gastric Secretion
Characteristics of the Gastric Secretions
Pyloric Glands—Secretion of Mucus and
Gastrin
Surface Mucous Cells
Stimulation of Gastric Acid Secretion
Regulation of Pepsinogen Secretion
Inhibition of Gastric Secretion by Other
Post-Stomach Intestinal Factors
Chemical Composition of Gastrin And Other
Gastrointestinal Hormones
Pancreatic Secretion
Pancreatic Digestive Enzymes
Secretion of Bicarbonate Ions
Regulation of Pancreatic Secretion
Secretion of Bile by the Liver; Functions
of the Biliary Tree
Physiologic Anatomy of Biliary Secretion
Function of Bile Salts in Fat Digestion and
Absorption
Liver Secretion of Cholesterol and
Gallstone Formation
Secretions of the Small Intestine
Secretion of Mucus by Brunner’s Glands in
the Duodenum
Secretion of Intestinal Digestive Juices by
the Crypts of Lieberkühn
Regulation of Small Intestine Secretion—
Local Stimuli
Secretions of the Large Intestine
C H A P T E R
6 5
Digestion and Absorption in the
Gastrointestinal Tract
Digestion of the Various Foods by
Hydrolysis
Digestion of Carbohydrates
Digestion of Proteins
Digestion of Fats
Basic Principles of Gastrointestinal
Absorption
Anatomical Basis of Absorption
Absorption in the Small Intestine
Absorption of Water
Absorption of Ions
Absorption of Nutrients
Absorption in the Large Intestine:
Formation of Feces
C H A P T E R
6 6
Physiology of Gastrointestinal
Disorders
Disorders of Swallowing and of the
Esophagus
Disorders of the Stomach
Peptic Ulcer
Specific Causes of Peptic Ulcer in the
Human Being
Disorders of the Small Intestine
Abnormal Digestion of Food in the Small
Intestine—Pancreatic Failure
Malabsorption by the Small Intestine
Mucosa—Sprue
Disorders of the Large Intestine
Constipation
Table of Contents
794
794
797
797
797
798
Diarrhea
Paralysis of Defecation in Spinal Cord
Injuries
General Disorders of the
Gastrointestinal Tract
Vomiting
Nausea
Gastrointestinal Obstruction
822
823
823
823
824
824
798
799
799
799
800
800
802
802
804
804
805
805
805
806
806
808
808
809
810
811
812
812
813
814
814
815
817
819
819
819
820
821
821
821
822
822
822
U N I T
X I I I
Metabolism and Temperature
Regulation
C H A P T E R
6 7
Metabolism of Carbohydrates,
and Formation of Adenosine
Triphosphate
Release of Energy from Foods, and the
Concept of “Free Energy”
Role of Adenosine Triphosphate in
Metabolism
Central Role of Glucose in
Carbohydrate Metabolism
Transport of Glucose Through the
Cell Membrane
Insulin Increases Facilitated Diffusion of
Glucose
Phosphorylation of Glucose
Glycogen Is Stored in Liver and
Muscle
Glycogenesis—The Process of Glycogen
Formation
Removal of Stored Glycogen—
Glycogenolysis
Release of Energy from the Glucose
Molecule by the Glycolytic Pathway
Summary of ATP Formation During the
Breakdown of Glucose
Control of Energy Release from Stored
Glycogen When the Body Needs Additional
Energy
Anaerobic Release of Energy—“Anaerobic
Glycolysis”
Release of Energy from Glucose by the
Pentose Phosphate Pathway
Glucose Conversion to Glycogen or Fat
Formation of Carbohydrates from
Proteins and Fats—“Gluconeogenesis”
Blood Glucose
C H A P T E R
Lipid Metabolism
829
829
829
830
831
831
831
831
832
832
832
836
836
836
837
838
838
839
6 8
Transport of Lipids in the Body Fluids
Transport of Triglycerides and Other Lipids
from the Gastrointestinal Tract by
Lymph—The Chylomicrons
Removal of the Chylomicrons from the
Blood
“Free Fatty Acids” Are Transported in the
Blood in Combination with Albumin
840
840
840
841
841
Table of Contents
Lipoproteins—Their Special Function in
Transporting Cholesterol and
Phospholipids
Fat Deposits
Adipose Tissue
Liver Lipids
Use of Triglycerides for Energy:
Formation of Adenosine Triphosphate
Formation of Acetoacetic Acid in the
Liver and Its Transport in the Blood
Synthesis of Triglycerides from
Carbohydrates
Synthesis of Triglycerides from Proteins
Regulation of Energy Release from
Triglycerides
Obesity
Phospholipids and Cholesterol
Phospholipids
Cholesterol
Cellular Structural Functions of
Phospholipids and Cholesterol—
Especially for Membranes
Atherosclerosis
Basic Causes of Atherosclerosis—The
Roles of Cholesterol and Lipoproteins
Other Major Risk Factors for
Atherosclerosis
Prevention of Atherosclerosis
C H A P T E R
Protein Metabolism
842
844
844
845
846
846
846
846
847
848
848
850
850
850
6 9
Basic Properties
Amino Acids
Transport and Storage of Amino Acids
Blood Amino Acids
Storage of Amino Acids as Proteins in the
Cells
Functional Roles of the Plasma
Proteins
Essential and Nonessential Amino Acids
Obligatory Degradation of Proteins
Hormonal Regulation of Protein
Metabolism
C H A P T E R
The Liver as an Organ
841
842
842
842
852
852
852
854
854
854
855
855
857
857
7 0
Physiologic Anatomy of the Liver
Hepatic Vascular and Lymph
Systems
Blood Flows Through the Liver from the
Portal Vein and Hepatic Artery
The Liver Functions as a Blood Reservoir
The Liver Has Very High Lymph Flow
Regulation of Liver Mass—Regeneration
Hepatic Macrophage System Serves a
Blood-Cleansing Function
Metabolic Functions of the Liver
Carbohydrate Metabolism
Fat Metabolism
Protein Metabolism
Other Metabolic Functions of the Liver
Measurement of Bilirubin in the Bile
as a Clinical Diagnostic Tool
Jaundice—Excess Bilirubin in the
Extracellular Fluid
859
859
859
860
860
860
860
861
861
861
861
862
862
862
863
C H A P T E R
7 1
Dietary Balances; Regulation of
Feeding; Obesity and Starvation;
Vitamins and Minerals
Energy Intake and Output Are Balanced
Under Steady-State Conditions
Dietary Balances
Energy Available in Foods
Methods for Determining Metabolic
Utilization of Proteins, Carbohydrates,
and Fats
Regulation of Food Intake and Energy
Storage
Neural Centers Regulate Food Intake
Factors That Regulate Quantity of Food
Intake
Obesity
Decreased Physical Activity and
Abnormal Feeding Regulation as
Causes of Obesity
Treatment of Obesity
Inanition, Anorexia, and Cachexia
Starvation
Vitamins
Vitamin A
Thiamine (Vitamin B1)
Niacin
Riboflavin (Vitamin B2)
Vitamin B12
Folic Acid (Pteroylglutamic Acid)
Pyridoxine (Vitamin B6)
Pantothenic Acid
Ascorbic Acid (Vitamin C)
Vitamin D
Vitamin E
Vitamin K
Mineral Metabolism
C H A P T E R
7 2
Energetics and Metabolic Rate
Adenosine Triphosphate (ATP)
Functions as an “Energy Currency”
in Metabolism
Phosphocreatine Functions as an
Accessory Storage Depot for Energy
and as an “ATP Buffer”
Anaerobic Versus Aerobic Energy
Summary of Energy Utilization by the
Cells
Control of Energy Release in the Cell
Metabolic Rate
Measurement of the Whole-Body Metabolic
Rate
Energy Metabolism—Factors That
Influence Energy Output
Overall Energy Requirements for Daily
Activities
Basal Metabolic Rate (BMR)—The
Minimum Energy Expenditure for the
Body to Exist
Energy Used for Physical Activities
Energy Used for Processing Food—
Thermogenic Effect of Food
Energy Used for Nonshivering
Thermogenesis—Role of Sympathetic
Stimulation
xxxi
865
865
865
865
866
865
867
870
872
872
873
874
874
875
875
875
876
876
876
877
877
877
877
878
878
878
878
881
881
882
882
883
884
884
885
885
885
886
887
887
887
xxxii
Table of Contents
C H A P T E R
7 3
Body Temperature, Temperature
Regulation, and Fever
Normal Body Temperatures
Body Temperature Is Controlled by
Balancing Heat Production Against
Heat Loss
Heat Production
Heat Loss
Regulation of Body Temperature—Role
of the Hypothalamus
Neuronal Effector Mechanisms That
Decrease or Increase Body Temperature
Concept of a “Set-Point” for Temperature
Control
Behavioral Control of Body Temperature
Abnormalities of Body Temperature
Regulation
Fever
Exposure of the Body to Extreme Cold
U N I T
896
897
Growth Hormone Promotes Growth of
Many Body Tissues
Growth Hormone Has Several Metabolic
Effects
Growth Hormone Stimulates Cartilage and
Bone Growth
Growth Hormone Exerts Much of Its Effect
Through Intermediate Substances Called
“Somatomedins” (Also Called “Insulin-Like
Growth Factors”)
Regulation of Growth Hormone Secretion
Abnormalities of Growth Hormone Secretion
Posterior Pituitary Gland and Its
Relation to the Hypothalamus
Chemical Structures of ADH and Oxytocin
Physiological Functions of ADH
Oxytocic Hormone
898
898
900
C H A P T E R
7 6
Thyroid Metabolic Hormones
889
889
889
889
890
894
895
X I V
Endocrinology and Reproduction
C H A P T E R
7 4
Introduction to Endocrinology
Coordination of Body Functions by
Chemical Messengers
Chemical Structure and Synthesis of
Hormones
Hormone Secretion, Transport, and
Clearance from the Blood
Feedback Control of Hormone Secretion
Transport of Hormones in the Blood
“Clearance” of Hormones from the Blood
Mechanisms of Action of Hormones
Hormone Receptors and Their Activation
Intracellular Signaling After Hormone
Receptor Activation
Second Messenger Mechanisms for
Mediating Intracellular Hormonal
Functions
Hormones That Act Mainly on the Genetic
Machinery of the Cell
Measurement of Hormone
Concentrations in the Blood
Radioimmunoassay
Enzyme-Linked Immunosorbent Assay
(ELISA)
C H A P T E R
7 5
Pituitary Hormones and Their Control
by the Hypothalamus
Pituitary Gland and Its Relation to the
Hypothalamus
Hypothalamus Controls Pituitary
Secretion
Hypothalamic-Hypophysial Portal Blood
Vessels of the Anterior Pituitary Gland
Physiological Functions of Growth
Hormone
905
905
906
908
909
909
909
910
910
910
912
915
915
915
916
Synthesis and Secretion of the Thyroid
Metabolic Hormones
Iodine Is Required for Formation of
Thyroxine
Iodide Pump (Iodide Trapping)
Thyroglobulin, and Chemistry of Thyroxine
and Triiodothyronine Formation
Release of Thyroxine and Triiodothyronine
from the Thyroid Gland
Transport of Thyroxine and Triiodothyronine
to Tissues
Physiologic Functions of the Thyroid
Hormones
Thyroid Hormones Increase the
Transcription of Large Numbers of Genes
Thyroid Hormones Increase Cellular
Metabolic Activity
Effect of Thyroid Hormone on Growth
Effects of Thyroid Hormone on Specific
Bodily Mechanisms
Regulation of Thyroid Hormone
Secretion
Anterior Pituitary Secretion of TSH Is
Regulated by Thyrotropin-Releasing
Hormone from the Hypothalamus
Feedback Effect of Thyroid Hormone to
Decrease Anterior Pituitary Secretion
of TSH
Diseases of the Thyroid
Hyperthyroidism
Symptoms of Hyperthyroidism
Hypothyroidism
Cretinism
C H A P T E R
7 7
Adrenocortical Hormones
918
918
919
920
921
Synthesis and Secretion of
Adrenocortical Hormones
Functions of the MineralocorticoidsAldosterone
Renal and Circulatory Effects of
Aldosterone
Aldosterone Stimulates Sodium and
Potassium Transport in Sweat Glands,
Salivary Glands, and Intestinal Epithelial
Cells
922
922
923
923
924
926
927
928
928
929
931
931
931
932
932
933
934
934
934
934
936
936
938
938
939
940
940
940
941
942
944
944
947
948
949
Table of Contents
Cellular Mechanism of Aldosterone Action
Possible Nongenomic Actions of
Aldosterone and Other Steroid Hormones
Regulation of Aldosterone Secretion
Functions of the Glucocorticoids
Effects of Cortisol on Carbohydrate
Metabolism
Effects of Cortisol on Protein Metabolism
Effects of Cortisol on Fat Metabolism
Cortisol Is Important in Resisting Stress
and Inflammation
Other Effects of Cortisol
Cellular Mechanism of Cortisol Action
Regulation of Cortisol Secretion by
Adrenocorticotropic Hormone from the
Pituitary Gland
Adrenal Androgens
Abnormalities of Adrenocortical
Secretion
Hypoadrenalism-Addison’s Disease
Hyperadrenalism-Cushing’s Syndrome
Primary Aldosteronism (Conn’s Syndrome)
Adrenogenital Syndrome
C H A P T E R
7 8
Insulin, Glucagon, and Diabetes
Mellitus
Insulin and Its Metabolic Effects
Effect of Insulin on Carbohydrate
Metabolism
Effect of Insulin on Fat Metabolism
Effect of Insulin on Protein Metabolism
and on Growth
Mechanisms of Insulin Secretion
Control of Insulin Secretion
Other Factors That Stimulate Insulin
Secretion
Role of Insulin (and Other Hormones) in
“Switching” Between Carbohydrate and
Lipid Metabolism
Glucagon and Its Functions
Effects on Glucose Metabolism
Regulation of Glucagon Secretion
Somatostatin Inhibits Glucagon and
Insulin Secretion
Summary of Blood Glucose
Regulation
Diabetes Mellitus
Type I Diabetes—Lack of Insulin Production
by Beta Cells of the Pancreas
Type II Diabetes—Resistance to the
Metabolic Effects of Insulin
Physiology of Diagnosis of Diabetes
Mellitus
Treatment of Diabetes
Insulinoma—Hyperinsulinism
950
950
950
950
951
952
952
952
954
954
955
957
957
957
958
959
959
961
961
963
965
966
967
968
969
969
970
970
971
971
971
972
972
974
975
976
976
C H A P T E R
7 9
Parathyroid Hormone, Calcitonin,
Calcium and Phosphate
Metabolism, Vitamin D, Bone,
and Teeth
978
Overview of Calcium and Phosphate
Regulation in the Extracellular Fluid
and Plasma
978
Calcium in the Plasma and Interstitial
Fluid
Inorganic Phosphate in the Extracellular
Fluids
Non-Bone Physiologic Effects of Altered
Calcium and Phosphate Concentrations
in the Body Fluids
Absorption and Excretion of Calcium and
Phosphate
Bone and Its Relation to Extracellular
Calcium and Phosphate
Precipitation and Absorption of Calcium
and Phosphate in Bone—Equilibrium
with the Extracellular Fluids
Calcium Exchange Between Bone and
Extracellular Fluid
Deposition and Absorption of Bone—
Remodeling of Bone
Vitamin D
Actions of Vitamin D
Parathyroid Hormone
Effect of Parathyroid Hormone on Calcium
and Phosphate Concentrations in the
Extracellular Fluid
Control of Parathyroid Secretion by
Calcium Ion Concentration
Calcitonin
Summary of Control of Calcium Ion
Concentration
Pathophysiology of Parathyroid
Hormone, Vitamin D, and Bone
Disease
Primary Hyperparathyroidism
Secondary Parathyroidism
Rickets—Vitamin D Deficiency
Osteoporosis—Decreased Bone Matrix
Physiology of the Teeth
Function of the Different Parts of the
Teeth
Dentition
Mineral Exchange in Teeth
Dental Abnormalities
C H A P T E R
8 0
Reproductive and Hormonal
Functions of the Male (and Function
of the Pineal Gland)
Physiologic Anatomy of the Male
Sexual Organs
Spermatogenesis
Steps of Spermatogenesis
Function of the Seminal Vesicles
Function of the Prostate Gland
Semen
Male Sexual Act
Abnormal Spermatogenesis and Male
Fertility
Neuronal Stimulus for Performance of the
Male Sexual Act
Stages of the Male Sexual Act
Testosterone and Other Male Sex
Hormones
Secretion, Metabolism, and Chemistry of
the Male Sex Hormone
Functions of Testosterone
Basic Intracellular Mechanism of Action of
Testosterone
xxxiii
978
979
979
980
980
981
982
982
983
985
985
986
988
988
989
990
990
991
991
991
992
992
993
993
994
996
996
996
996
999
999
999
1001
1001
1001
1002
1003
1003
1004
1006
xxxiv
Control of Male Sexual Functions by
Hormones from the Hypothalamus and
Anterior Pituitary Gland
Abnormalities of Male Sexual Function
Prostate Gland and Its Abnormalities
Hypogonadism in the Male
Testicular Tumors and Hypergonadism in
the Male
Pineal Gland—Its Function in Controlling
Seasonal Fertility in Some Animals
C H A P T E R
8 1
Female Physiology Before Pregnancy
and Female Hormones
Physiologic Anatomy of the Female
Sexual Organs
Female Hormonal System
Monthly Ovarian Cycle; Function of the
Gonadotropic Hormones
Gonadotropic Hormones and Their Effects
on the Ovaries
Ovarian Follicle Growth—The “Follicular”
Phase of the Ovarian Cycle
Corpus Luteum—“Luteal” Phase of the
Ovarian Cycle
Summary
Functions of the Ovarian Hormones—
Estradiol and Progesterone
Chemistry of the Sex Hormones
Functions of the Estrogens—Their Effects
on the Primary and Secondary Female Sex
Characteristics
Functions of Progesterone
Monthly Endometrial Cycle and Menstruation
Regulation of the Female Monthly
Rhythm—Interplay Between the
Ovarian and Hypothalamic-Pituitary
Hormones
Feedback Oscillation of the HypothalamicPituitary-Ovarian System
Puberty and Menarche
Menopause
Abnormalities of Secretion by the
Ovaries
Female Sexual Act
Female Fertility
C H A P T E R
8 2
Pregnancy and Lactation
Maturation and Fertilization of the Ovum
Transport of the Fertilized Ovum in the
Fallopian Tube
Implantation of the Blastocyst in the Uterus
Early Nutrition of the Embryo
Function of the Placenta
Developmental and Physiologic Anatomy
of the Placenta
Hormonal Factors in Pregnancy
Human Chorionic Gonadotropin and Its
Effect to Cause Persistence of the
Corpus Luteum and to Prevent
Menstruation
Secretion of Estrogens by the Placenta
Secretion of Progesterone by the Placenta
Human Chorionic Somatomammotropin
Other Hormonal Factors in Pregnancy
Table of Contents
1006
1008
1008
1008
1009
1009
1011
1011
1011
1012
1012
1013
1014
1015
1016
1016
1017
1018
1018
1019
1021
1021
1022
1023
1023
1024
1027
1027
1028
1029
1029
1029
1029
1031
1032
1032
1033
1033
1034
Response of the Mother’s Body to
Pregnancy
Changes in the Maternal Circulatory System
During Pregnancy
Parturition
Increased Uterine Excitability Near Term
Onset of Labor—A Positive Feedback
Mechanism for Its Initiation
Abdominal Muscle Contractions During
Labor
Mechanics of Parturition
Separation and Delivery of the Placenta
Labor Pains
Involution of the Uterus After Parturition
Lactation
Development of the Breasts
Initiation of Lactation—Function of
Prolactin
Ejection (or “Let-Down”) Process in Milk
Secretion—Function of Oxytocin
Milk Composition and the Metabolic Drain
on the Mother Caused by Lactation
C H A P T E R
8 3
Fetal and Neonatal Physiology
Growth and Functional Development
of the Fetus
Development of the Organ Systems
Adjustments of the Infant to
Extrauterine Life
Onset of Breathing
Circulatory Readjustments at Birth
Nutrition of the Neonate
Special Functional Problems in the
Neonate
Respiratory System
Circulation
Fluid Balance, Acid-Base Balance, and
Renal Function
Liver Function
Digestion, Absorption, and Metabolism
of Energy Foods; and Nutrition
Immunity
Endocrine Problems
Special Problems of Prematurity
Immature Development of the Premature
Infant
Instability of the Homeostatic Control
Systems in the Premature Infant
Danger of Blindness Caused by Excess
Oxygen Therapy in the Premature Infant
Growth and Development of the Child
Behavioral Growth
U N I T
1034
1035
1036
1036
1037
1037
1037
1038
1038
1038
1038
1038
1039
1040
1041
1042
1042
1042
1044
1044
1045
1047
1047
1047
1047
1048
1048
1048
1049
1049
1050
1050
1050
1051
1051
1052
X V
Sports Physiology
C H A P T E R
Sports Physiology
8 4
Muscles in Exercise
Strength, Power, and Endurance of Muscles
Muscle Metabolic Systems in Exercise
Phosphocreatine-Creatine System
1055
1055
1055
1056
1057
Table of Contents
Nutrients Used During Muscle Activity
Effect of Athletic Training on Muscles and
Muscle Performance
Respiration in Exercise
Cardiovascular System in Exercise
Body Heat in Exercise
1059
1060
1061
1062
1065
xxxv
Body Fluids and Salt in Exercise
Drugs and Athletes
Body Fitness Prolongs Life
1065
1065
1066
Index
1067
U
N
I
Introduction to
Physiology:
The Cell and
General Physiology
1. Functional Organization of the Human
Body and Control of the “Internal Environment”
2. The Cell and Its Functions
3. Genetic Control of Protein Synthesis,
Cell Function, and Cell Reproduction
T
I
C
H
A
P
T
E
R
1
Functional Organization of the
Human Body and Control of the
“Internal Environment”
The goal of physiology is to explain the physical and
chemical factors that are responsible for the origin,
development, and progression of life. Each type of
life, from the simple virus to the largest tree or the
complicated human being, has its own functional
characteristics. Therefore, the vast field of physiology can be divided into viral physiology, bacterial
physiology, cellular physiology, plant physiology,
human physiology, and many more subdivisions.
Human Physiology. In human physiology, we attempt to explain the specific characteristics and mechanisms of the human body that make it a living being. The
very fact that we remain alive is almost beyond our control, for hunger makes
us seek food and fear makes us seek refuge. Sensations of cold make us look
for warmth. Other forces cause us to seek fellowship and to reproduce. Thus,
the human being is actually an automaton, and the fact that we are sensing,
feeling, and knowledgeable beings is part of this automatic sequence of life;
these special attributes allow us to exist under widely varying conditions.
Cells as the Living Units of the Body
The basic living unit of the body is the cell. Each organ is an aggregate of many
different cells held together by intercellular supporting structures.
Each type of cell is specially adapted to perform one or a few particular
functions. For instance, the red blood cells, numbering 25 trillion in each
human being, transport oxygen from the lungs to the tissues. Although the red
cells are the most abundant of any single type of cell in the body, there are
about 75 trillion additional cells of other types that perform functions different
from those of the red cell. The entire body, then, contains about 100 trillion
cells.
Although the many cells of the body often differ markedly from one another,
all of them have certain basic characteristics that are alike. For instance, in all
cells, oxygen reacts with carbohydrate, fat, and protein to release the energy
required for cell function. Further, the general chemical mechanisms for changing nutrients into energy are basically the same in all cells, and all cells deliver
end products of their chemical reactions into the surrounding fluids.
Almost all cells also have the ability to reproduce additional cells of their
own kind. Fortunately, when cells of a particular type are destroyed from one
cause or another, the remaining cells of this type usually generate new cells until
the supply is replenished.
Extracellular Fluid—The “Internal Environment”
About 60 per cent of the adult human body is fluid, mainly a water solution of
ions and other substances. Although most of this fluid is inside the cells and is
called intracellular fluid, about one third is in the spaces outside the cells and
3
4
Unit I
Introduction to Physiology: The Cell and General Physiology
is called extracellular fluid. This extracellular fluid is in
constant motion throughout the body. It is transported
rapidly in the circulating blood and then mixed
between the blood and the tissue fluids by diffusion
through the capillary walls.
In the extracellular fluid are the ions and nutrients
needed by the cells to maintain cell life. Thus, all cells
live in essentially the same environment—the extracellular fluid. For this reason, the extracellular fluid is
also called the internal environment of the body, or the
milieu intérieur, a term introduced more than 100 years
ago by the great 19th-century French physiologist
Claude Bernard.
Cells are capable of living, growing, and performing
their special functions as long as the proper concentrations of oxygen, glucose, different ions, amino acids,
fatty substances, and other constituents are available
in this internal environment.
Extracellular Fluid Transport and
Mixing System—The Blood
Circulatory System
Extracellular fluid is transported through all parts of
the body in two stages. The first stage is movement of
blood through the body in the blood vessels, and the
second is movement of fluid between the blood capillaries and the intercellular spaces between the tissue
cells.
Figure 1–1 shows the overall circulation of blood.
All the blood in the circulation traverses the entire circulatory circuit an average of once each minute when
the body is at rest and as many as six times each minute
when a person is extremely active.
As blood passes through the blood capillaries,
continual exchange of extracellular fluid also occurs
between the plasma portion of the blood and the
Differences Between Extracellular and Intracellular Fluids.
The extracellular fluid contains large amounts of
sodium, chloride, and bicarbonate ions plus nutrients
for the cells, such as oxygen, glucose, fatty acids, and
amino acids. It also contains carbon dioxide that is
being transported from the cells to the lungs to be
excreted, plus other cellular waste products that are
being transported to the kidneys for excretion.
The intracellular fluid differs significantly from
the extracellular fluid; specifically, it contains large
amounts of potassium, magnesium, and phosphate ions
instead of the sodium and chloride ions found in the
extracellular fluid. Special mechanisms for transporting ions through the cell membranes maintain the ion
concentration differences between the extracellular
and intracellular fluids. These transport processes are
discussed in Chapter 4.
Lungs
CO2
O2
Right
heart
pump
Left
heart
pump
Gut
“Homeostatic” Mechanisms of
the Major Functional Systems
Nutrition and excretion
Kidneys
Homeostasis
The term homeostasis is used by physiologists to mean
maintenance of nearly constant conditions in the internal environment. Essentially all organs and tissues of
the body perform functions that help maintain these
constant conditions. For instance, the lungs provide
oxygen to the extracellular fluid to replenish the
oxygen used by the cells, the kidneys maintain constant ion concentrations, and the gastrointestinal
system provides nutrients.
A large segment of this text is concerned with the
manner in which each organ or tissue contributes to
homeostasis. To begin this discussion, the different
functional systems of the body and their contributions
to homeostasis are outlined in this chapter; then we
briefly outline the basic theory of the body’s control
systems that allow the functional systems to operate in
support of one another.
Regulation
of
electrolytes
Excretion
Venous
end
Arterial
end
Capillaries
Figure 1–1
General organization of the circulatory system.
Chapter 1
Functional Organization of the Human Body and Control of the “Internal Environment”
5
the gastrointestinal tract. Here different dissolved
nutrients, including carbohydrates, fatty acids, and
amino acids, are absorbed from the ingested food into
the extracellular fluid of the blood.
Arteriole
Liver and Other Organs That Perform Primarily Metabolic Functions. Not all substances absorbed from the gastroin-
Venule
Figure 1–2
Diffusion of fluid and dissolved constituents through the capillary
walls and through the interstitial spaces.
interstitial fluid that fills the intercellular spaces. This
process is shown in Figure 1–2. The walls of the capillaries are permeable to most molecules in the plasma
of the blood, with the exception of the large plasma
protein molecules. Therefore, large amounts of fluid
and its dissolved constituents diffuse back and forth
between the blood and the tissue spaces, as shown by
the arrows. This process of diffusion is caused by
kinetic motion of the molecules in both the plasma and
the interstitial fluid. That is, the fluid and dissolved
molecules are continually moving and bouncing in all
directions within the plasma and the fluid in the intercellular spaces, and also through the capillary pores.
Few cells are located more than 50 micrometers from
a capillary, which ensures diffusion of almost any substance from the capillary to the cell within a few
seconds. Thus, the extracellular fluid everywhere in the
body—both that of the plasma and that of the interstitial fluid—is continually being mixed, thereby
maintaining almost complete homogeneity of the
extracellular fluid throughout the body.
Origin of Nutrients in the
Extracellular Fluid
Respiratory System. Figure 1–1 shows that each time the
blood passes through the body, it also flows through
the lungs. The blood picks up oxygen in the alveoli,
thus acquiring the oxygen needed by the cells. The
membrane between the alveoli and the lumen of the
pulmonary capillaries, the alveolar membrane, is only
0.4 to 2.0 micrometers thick, and oxygen diffuses by
molecular motion through the pores of this membrane
into the blood in the same manner that water and ions
diffuse through walls of the tissue capillaries.
Gastrointestinal Tract. A large portion of the blood
pumped by the heart also passes through the walls of
testinal tract can be used in their absorbed form by the
cells. The liver changes the chemical compositions of
many of these substances to more usable forms, and
other tissues of the body—fat cells, gastrointestinal
mucosa, kidneys, and endocrine glands—help modify
the absorbed substances or store them until they are
needed.
Musculoskeletal System. Sometimes the question is
asked, How does the musculoskeletal system fit into
the homeostatic functions of the body? The answer is
obvious and simple: Were it not for the muscles, the
body could not move to the appropriate place at the
appropriate time to obtain the foods required for
nutrition. The musculoskeletal system also provides
motility for protection against adverse surroundings,
without which the entire body, along with its homeostatic mechanisms, could be destroyed instantaneously.
Removal of Metabolic End Products
Removal of Carbon Dioxide by the Lungs. At the same time
that blood picks up oxygen in the lungs, carbon dioxide
is released from the blood into the lung alveoli; the respiratory movement of air into and out of the lungs
carries the carbon dioxide to the atmosphere. Carbon
dioxide is the most abundant of all the end products
of metabolism.
Kidneys. Passage of the blood through the kidneys
removes from the plasma most of the other substances
besides carbon dioxide that are not needed by the
cells. These substances include different end products
of cellular metabolism, such as urea and uric acid; they
also include excesses of ions and water from the food
that might have accumulated in the extracellular fluid.
The kidneys perform their function by first filtering
large quantities of plasma through the glomeruli into
the tubules and then reabsorbing into the blood those
substances needed by the body, such as glucose, amino
acids, appropriate amounts of water, and many of the
ions. Most of the other substances that are not needed
by the body, especially the metabolic end products
such as urea, are reabsorbed poorly and pass through
the renal tubules into the urine.
Regulation of Body Functions
Nervous System. The nervous system is composed of
three major parts: the sensory input portion, the central
nervous system (or integrative portion), and the motor
output portion. Sensory receptors detect the state of
the body or the state of the surroundings. For instance,
6
Unit I
Introduction to Physiology: The Cell and General Physiology
receptors in the skin apprise one whenever an object
touches the skin at any point. The eyes are sensory
organs that give one a visual image of the surrounding
area. The ears also are sensory organs. The central
nervous system is composed of the brain and spinal
cord. The brain can store information, generate
thoughts, create ambition, and determine reactions
that the body performs in response to the sensations.
Appropriate signals are then transmitted through the
motor output portion of the nervous system to carry
out one’s desires.
A large segment of the nervous system is called the
autonomic system. It operates at a subconscious level
and controls many functions of the internal organs,
including the level of pumping activity by the heart,
movements of the gastrointestinal tract, and secretion
by many of the body’s glands.
Hormonal System of Regulation. Located in the body are
eight major endocrine glands that secrete chemical
substances called hormones. Hormones are transported in the extracellular fluid to all parts of the body
to help regulate cellular function. For instance, thyroid
hormone increases the rates of most chemical reactions in all cells, thus helping to set the tempo of bodily
activity. Insulin controls glucose metabolism; adrenocortical hormones control sodium ion, potassium ion,
and protein metabolism; and parathyroid hormone
controls bone calcium and phosphate. Thus, the hormones are a system of regulation that complements
the nervous system. The nervous system regulates
mainly muscular and secretory activities of the body,
whereas the hormonal system regulates many metabolic functions.
Reproduction
Sometimes reproduction is not considered a homeostatic function. It does, however, help maintain homeostasis by generating new beings to take the place of
those that are dying. This may sound like a permissive
usage of the term homeostasis, but it illustrates that, in
the final analysis, essentially all body structures are
organized such that they help maintain the automaticity and continuity of life.
Control Systems of the Body
The human body has thousands of control systems in
it. The most intricate of these are the genetic control
systems that operate in all cells to help control intracellular function as well as extracellular function. This
subject is discussed in Chapter 3.
Many other control systems operate within the
organs to control functions of the individual parts
of the organs; others operate throughout the entire
body to control the interrelations between the organs.
For instance, the respiratory system, operating in
association with the nervous system, regulates the
concentration of carbon dioxide in the extracellular
fluid. The liver and pancreas regulate the concentration of glucose in the extracellular fluid, and the
kidneys regulate concentrations of hydrogen, sodium,
potassium, phosphate, and other ions in the extracellular fluid.
Examples of Control Mechanisms
Regulation of Oxygen and Carbon Dioxide Concentrations in the
Extracellular Fluid. Because oxygen is one of the major
substances required for chemical reactions in the cells,
it is fortunate that the body has a special control
mechanism to maintain an almost exact and constant
oxygen concentration in the extracellular fluid. This
mechanism depends principally on the chemical characteristics of hemoglobin, which is present in all red
blood cells. Hemoglobin combines with oxygen as the
blood passes through the lungs. Then, as the blood
passes through the tissue capillaries, hemoglobin,
because of its own strong chemical affinity for oxygen,
does not release oxygen into the tissue fluid if too
much oxygen is already there. But if the oxygen concentration in the tissue fluid is too low, sufficient
oxygen is released to re-establish an adequate concentration. Thus, regulation of oxygen concentration
in the tissues is vested principally in the chemical
characteristics of hemoglobin itself. This regulation is
called the oxygen-buffering function of hemoglobin.
Carbon dioxide concentration in the extracellular
fluid is regulated in a much different way. Carbon
dioxide is a major end product of the oxidative reactions in cells. If all the carbon dioxide formed in the
cells continued to accumulate in the tissue fluids, the
mass action of the carbon dioxide itself would soon
halt all energy-giving reactions of the cells. Fortunately, a higher than normal carbon dioxide concentration in the blood excites the respiratory center,
causing a person to breathe rapidly and deeply. This
increases expiration of carbon dioxide and, therefore,
removes excess carbon dioxide from the blood and
tissue fluids. This process continues until the concentration returns to normal.
Regulation of Arterial Blood Pressure. Several systems con-
tribute to the regulation of arterial blood pressure.
One of these, the baroreceptor system, is a simple and
excellent example of a rapidly acting control mechanism. In the walls of the bifurcation region of the
carotid arteries in the neck, and also in the arch of the
aorta in the thorax, are many nerve receptors called
baroreceptors, which are stimulated by stretch of the
arterial wall. When the arterial pressure rises too high,
the baroreceptors send barrages of nerve impulses to
the medulla of the brain. Here these impulses inhibit
the vasomotor center, which in turn decreases the
number of impulses transmitted from the vasomotor
center through the sympathetic nervous system to the
heart and blood vessels. Lack of these impulses causes
diminished pumping activity by the heart and also
Chapter 1
7
Functional Organization of the Human Body and Control of the “Internal Environment”
dilation of the peripheral blood vessels, allowing
increased blood flow through the vessels. Both of these
effects decrease the arterial pressure back toward
normal.
Conversely, a decrease in arterial pressure below
normal relaxes the stretch receptors, allowing the
vasomotor center to become more active than usual,
thereby causing vasoconstriction and increased heart
pumping, and raising arterial pressure back toward
normal.
Normal Ranges and Physical Characteristics
of Important Extracellular Fluid Constituents
Table 1–1 lists the more important constituents and
physical characteristics of extracellular fluid, along
with their normal values, normal ranges, and maximum
limits without causing death. Note the narrowness of
the normal range for each one. Values outside these
ranges are usually caused by illness.
Most important are the limits beyond which abnormalities can cause death. For example, an increase
in the body temperature of only 11°F (7°C) above
normal can lead to a vicious cycle of increasing cellular metabolism that destroys the cells. Note also the
narrow range for acid-base balance in the body, with
a normal pH value of 7.4 and lethal values only
about 0.5 on either side of normal. Another important
factor is the potassium ion concentration, because
whenever it decreases to less than one third normal,
a person is likely to be paralyzed as a result of the
nerves’ inability to carry signals. Alternatively, if
the potassium ion concentration increases to two or
more times normal, the heart muscle is likely to be
severely depressed. Also, when the calcium ion concentration falls below about one half of normal, a
person is likely to experience tetanic contraction of
muscles throughout the body because of the spontaneous generation of excess nerve impulses in the
peripheral nerves. When the glucose concentration
falls below one half of normal, a person frequently
develops extreme mental irritability and sometimes
even convulsions.
These examples should give one an appreciation for
the extreme value and even the necessity of the
vast numbers of control systems that keep the body
operating in health; in the absence of any one of these
controls, serious body malfunction or death can result.
Characteristics of Control Systems
The aforementioned examples of homeostatic control
mechanisms are only a few of the many thousands in
the body, all of which have certain characteristics in
common. These characteristics are explained in this
section.
Negative Feedback Nature of Most
Control Systems
Most control systems of the body act by negative feedback, which can best be explained by reviewing some
of the homeostatic control systems mentioned previously. In the regulation of carbon dioxide concentration, a high concentration of carbon dioxide in the
extracellular fluid increases pulmonary ventilation.
This, in turn, decreases the extracellular fluid carbon
dioxide concentration because the lungs expire greater
amounts of carbon dioxide from the body. In other
words, the high concentration of carbon dioxide initiates events that decrease the concentration toward
normal, which is negative to the initiating stimulus.
Conversely, if the carbon dioxide concentration falls
too low, this causes feedback to increase the concentration. This response also is negative to the initiating
stimulus.
In the arterial pressure–regulating mechanisms, a
high pressure causes a series of reactions that promote
a lowered pressure, or a low pressure causes a series
of reactions that promote an elevated pressure. In both
instances, these effects are negative with respect to the
initiating stimulus.
Therefore, in general, if some factor becomes excessive or deficient, a control system initiates negative
feedback, which consists of a series of changes that
return the factor toward a certain mean value, thus
maintaining homeostasis.
“Gain” of a Control System. The degree of effectiveness
with which a control system maintains constant
Table 1–1
Important Constituents and Physical Characteristics of Extracellular Fluid
Oxygen
Carbon dioxide
Sodium ion
Potassium ion
Calcium ion
Chloride ion
Bicarbonate ion
Glucose
Body temperature
Acid-base
Normal Value
Normal Range
Approximate Short-Term
Nonlethal Limit
Unit
40
40
142
4.2
1.2
108
28
85
98.4 (37.0)
7.4
35–45
35–45
138–146
3.8–5.0
1.0–1.4
103–112
24–32
75–95
98–98.8 (37.0)
7.3–7.5
10–1000
5–80
115–175
1.5–9.0
0.5–2.0
70–130
8–45
20–1500
65–110 (18.3–43.3)
6.9–8.0
mm Hg
mm Hg
mmol/L
mmol/L
mmol/L
mmol/L
mmol/L
mg/dl
∞F (∞C)
pH
Unit I
Introduction to Physiology: The Cell and General Physiology
conditions is determined by the gain of the negative
feedback. For instance, let us assume that a large
volume of blood is transfused into a person whose
baroreceptor pressure control system is not functioning, and the arterial pressure rises from the normal
level of 100 mm Hg up to 175 mm Hg. Then, let us
assume that the same volume of blood is injected into
the same person when the baroreceptor system is functioning, and this time the pressure increases only
25 mm Hg. Thus, the feedback control system has
caused a “correction” of –50 mm Hg—that is, from
175 mm Hg to 125 mm Hg. There remains an increase
in pressure of +25 mm Hg, called the “error,” which
means that the control system is not 100 per cent effective in preventing change. The gain of the system is
then calculated by the following formula:
5
Pumping effectiveness of heart
(Liters pumped per minute)
8
Bled 1 liter
3
2
Positive Feedback Can Sometimes Cause
Vicious Cycles and Death
One might ask the question, Why do essentially all
control systems of the body operate by negative feedback rather than positive feedback? If one considers
the nature of positive feedback, one immediately sees
that positive feedback does not lead to stability but to
instability and often death.
Figure 1–3 shows an example in which death can
ensue from positive feedback. This figure depicts the
pumping effectiveness of the heart, showing that the
heart of a healthy human being pumps about 5 liters
of blood per minute. If the person is suddenly bled 2
liters, the amount of blood in the body is decreased to
such a low level that not enough blood is available for
the heart to pump effectively. As a result, the arterial
pressure falls, and the flow of blood to the heart muscle
through the coronary vessels diminishes. This results in
weakening of the heart, further diminished pumping,
a further decrease in coronary blood flow, and still
more weakness of the heart; the cycle repeats itself
again and again until death occurs. Note that each
cycle in the feedback results in further weakening of
the heart. In other words, the initiating stimulus causes
more of the same, which is positive feedback.
Bled 2 liters
1
Death
0
2
1
3
Hours
Correction
Gain =
Error
Thus, in the baroreceptor system example, the correction is –50 mm Hg and the error persisting is +25 mm
Hg. Therefore, the gain of the person’s baroreceptor
system for control of arterial pressure is –50 divided
by +25, or –2. That is, a disturbance that increases or
decreases the arterial pressure does so only one third
as much as would occur if this control system were not
present.
The gains of some other physiologic control systems
are much greater than that of the baroreceptor
system. For instance, the gain of the system controlling
internal body temperature when a person is exposed
to moderately cold weather is about –33. Therefore,
one can see that the temperature control system is
much more effective than the baroreceptor pressure
control system.
Return to
normal
4
Figure 1–3
Recovery of heart pumping caused by negative feedback after 1
liter of blood is removed from the circulation. Death is caused by
positive feedback when 2 liters of blood are removed.
Positive feedback is better known as a “vicious
cycle,” but a mild degree of positive feedback can be
overcome by the negative feedback control mechanisms of the body, and the vicious cycle fails to
develop. For instance, if the person in the aforementioned example were bled only 1 liter instead of 2
liters, the normal negative feedback mechanisms for
controlling cardiac output and arterial pressure would
overbalance the positive feedback and the person
would recover, as shown by the dashed curve of
Figure 1–3.
Positive Feedback Can Sometimes Be Useful. In some
instances, the body uses positive feedback to its advantage. Blood clotting is an example of a valuable use of
positive feedback. When a blood vessel is ruptured
and a clot begins to form, multiple enzymes called clotting factors are activated within the clot itself. Some
of these enzymes act on other unactivated enzymes
of the immediately adjacent blood, thus causing
more blood clotting. This process continues until the
hole in the vessel is plugged and bleeding no longer
occurs. On occasion, this mechanism can get out of
hand and cause the formation of unwanted clots.
In fact, this is what initiates most acute heart
attacks, which are caused by a clot beginning on the
inside surface of an atherosclerotic plaque in a coronary artery and then growing until the artery is
blocked.
Childbirth is another instance in which positive
feedback plays a valuable role. When uterine contractions become strong enough for the baby’s head to
begin pushing through the cervix, stretch of the cervix
sends signals through the uterine muscle back to the
Chapter 1
Functional Organization of the Human Body and Control of the “Internal Environment”
body of the uterus, causing even more powerful
contractions. Thus, the uterine contractions stretch
the cervix, and the cervical stretch causes stronger
contractions. When this process becomes powerful
enough, the baby is born. If it is not powerful enough,
the contractions usually die out, and a few days pass
before they begin again.
Another important use of positive feedback is
for the generation of nerve signals. That is, when
the membrane of a nerve fiber is stimulated, this
causes slight leakage of sodium ions through sodium
channels in the nerve membrane to the fiber’s interior.
The sodium ions entering the fiber then change
the membrane potential, which in turn causes more
opening of channels, more change of potential,
still more opening of channels, and so forth. Thus, a
slight leak becomes an explosion of sodium entering
the interior of the nerve fiber, which creates the
nerve action potential. This action potential in turn
causes electrical current to flow along both the outside
and the inside of the fiber and initiates additional
action potentials. This process continues again and
again until the nerve signal goes all the way to the end
of the fiber.
In each case in which positive feedback is useful, the
positive feedback itself is part of an overall negative
feedback process. For example, in the case of blood
clotting, the positive feedback clotting process is a
negative feedback process for maintenance of normal
blood volume. Also, the positive feedback that causes
nerve signals allows the nerves to participate in
thousands of negative feedback nervous control
systems.
More Complex Types of Control Systems—
Adaptive Control
Later in this text, when we study the nervous system,
we shall see that this system contains great numbers
of interconnected control mechanisms. Some are
simple feedback systems similar to those already
discussed. Many are not. For instance, some movements of the body occur so rapidly that there is not
enough time for nerve signals to travel from the
peripheral parts of the body all the way to the brain
and then back to the periphery again to control the
movement. Therefore, the brain uses a principle called
feed-forward control to cause required muscle contractions. That is, sensory nerve signals from the
moving parts apprise the brain whether the movement
is performed correctly. If not, the brain corrects the
feed-forward signals that it sends to the muscles the
next time the movement is required. Then, if still
further correction is needed, this will be done again for
subsequent movements. This is called adaptive control.
Adaptive control, in a sense, is delayed negative
feedback.
Thus, one can see how complex the feedback
control systems of the body can be. A person’s life
depends on all of them. Therefore, a major share of
this text is devoted to discussing these life-giving
mechanisms.
9
Summary—Automaticity
of the Body
The purpose of this chapter has been to point out, first,
the overall organization of the body and, second, the
means by which the different parts of the body operate
in harmony. To summarize, the body is actually a social
order of about 100 trillion cells organized into different functional structures, some of which are called
organs. Each functional structure contributes its share
to the maintenance of homeostatic conditions in the
extracellular fluid, which is called the internal environment. As long as normal conditions are maintained
in this internal environment, the cells of the body continue to live and function properly. Each cell benefits
from homeostasis, and in turn, each cell contributes its
share toward the maintenance of homeostasis. This
reciprocal interplay provides continuous automaticity
of the body until one or more functional systems lose
their ability to contribute their share of function.When
this happens, all the cells of the body suffer. Extreme
dysfunction leads to death; moderate dysfunction
leads to sickness.
References
Adolph EF: Physiological adaptations: hypertrophies and
superfunctions. Am Sci 60:608, 1972.
Bernard C: Lectures on the Phenomena of Life Common to
Animals and Plants. Springfield, IL: Charles C Thomas,
1974.
Cabanac M: Regulation and the ponderostat. Int J Obes
Relat Metab Disord 25(Suppl 5):S7, 2001.
Cannon WB: The Wisdom of the Body. New York: WW
Norton, 1932.
Conn PM, Goodman HM: Handbook of Physiology: Cellular Endocrinology. Bethesda: American Physiological
Society, 1997.
Csete ME, Doyle JC: Reverse engineering of biological complexity. Science 295:1664, 2002.
Danzler WH (ed): Handbook of Physiology, Sec 13: Comparative Physiology. Bethesda: American Physiological
Society, 1997.
Dickinson MH, Farley CT, Full RJ, et al: How animals move:
an integrative view. Science 288:100, 2000.
Garland T Jr, Carter PA: Evolutionary physiology.Annu Rev
Physiol 56:579, 1994.
Gelehrter TD, Collins FS: Principles of Medical Genetics.
Baltimore: Williams & Wilkins, 1995.
Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders, 1980.
Guyton AC, Jones CE, Coleman TG: Cardiac Output and Its
Regulation. Philadelphia: WB Saunders, 1973.
Guyton AC, Taylor AE, Granger HJ: Dynamics and
Control of the Body Fluids. Philadelphia: WB Saunders,
1975.
Hoffman JF, Jamieson JD: Handbook of Physiology: Cell
Physiology. Bethesda: American Physiological Society,
1997.
Krahe R, Gabbiani F: Burst firing in sensory systems. Nat
Rev Neurosci 5:13, 2004.
Lewin B: Genes VII. New York: Oxford University Press,
2000.
10
Unit I
Introduction to Physiology: The Cell and General Physiology
Masoro EJ (ed): Handbook of Physiology, Sec 11: Aging.
Bethesda: American Physiological Society, 1995.
Milhorn HT: The Application of Control Theory to
Physiological Systems. Philadelphia: WB Saunders,
1966.
Orgel LE: The origin of life on the earth. Sci Am 271:76,
1994.
Smith HW: From Fish to Philosopher. New York: Doubleday,
1961.
Thomson RC: Biomaterials Regulating Cell Function and
Tissue Development. Warrendale, PA: Materials Research
Society, 1998.
Tjian R: Molecular machines that control genes. Sci Am
272:54, 1995.
C
H
A
P
T
E
R
2
The Cell and Its Functions
Each of the 100 trillion cells in a human being is a
living structure that can survive for months or many
years, provided its surrounding fluids contain appropriate nutrients. To understand the function of
organs and other structures of the body, it is essential that we first understand the basic organization
of the cell and the functions of its component
parts.
Organization of the Cell
A typical cell, as seen by the light microscope, is shown in Figure 2–1. Its two
major parts are the nucleus and the cytoplasm. The nucleus is separated from
the cytoplasm by a nuclear membrane, and the cytoplasm is separated from the
surrounding fluids by a cell membrane, also called the plasma membrane.
The different substances that make up the cell are collectively called protoplasm. Protoplasm is composed mainly of five basic substances: water, electrolytes, proteins, lipids, and carbohydrates.
Water. The principal fluid medium of the cell is water, which is present in most
cells, except for fat cells, in a concentration of 70 to 85 per cent. Many cellular
chemicals are dissolved in the water. Others are suspended in the water as solid
particulates. Chemical reactions take place among the dissolved chemicals or at
the surfaces of the suspended particles or membranes.
Ions. The most important ions in the cell are potassium, magnesium, phosphate,
sulfate, bicarbonate, and smaller quantities of sodium, chloride, and calcium.
These are all discussed in more detail in Chapter 4, which considers the interrelations between the intracellular and extracellular fluids.
The ions provide inorganic chemicals for cellular reactions. Also, they are
necessary for operation of some of the cellular control mechanisms. For
instance, ions acting at the cell membrane are required for transmission of electrochemical impulses in nerve and muscle fibers.
Proteins. After water, the most abundant substances in most cells are proteins,
which normally constitute 10 to 20 per cent of the cell mass. These can be
divided into two types: structural proteins and functional proteins.
Structural proteins are present in the cell mainly in the form of long filaments
that themselves are polymers of many individual protein molecules. A prominent use of such intracellular filaments is to form microtubules that provide the
“cytoskeletons” of such cellular organelles as cilia, nerve axons, the mitotic
spindles of mitosing cells, and a tangled mass of thin filamentous tubules that
hold the parts of the cytoplasm and nucleoplasm together in their respective
compartments. Extracellularly, fibrillar proteins are found especially in the collagen and elastin fibers of connective tissue and in blood vessel walls, tendons,
ligaments, and so forth.
The functional proteins are an entirely different type of protein, usually composed of combinations of a few molecules in tubular-globular form. These
11
12
Unit I
Introduction to Physiology: The Cell and General Physiology
Cell
membrane
Cytoplasm
Nucleolus
Nuclear
membrane
Nucleoplasm
surrounding extracellular fluid so that it is readily
available to the cell. Also, a small amount of carbohydrate is virtually always stored in the cells in the form
of glycogen, which is an insoluble polymer of glucose
that can be depolymerized and used rapidly to supply
the cells’ energy needs.
Nucleus
Figure 2–1
Structure of the cell as seen with the light microscope.
proteins are mainly the enzymes of the cell and, in contrast to the fibrillar proteins, are often mobile in the
cell fluid. Also, many of them are adherent to membranous structures inside the cell. The enzymes come
into direct contact with other substances in the cell
fluid and thereby catalyze specific intracellular chemical reactions. For instance, the chemical reactions
that split glucose into its component parts and then
combine these with oxygen to form carbon dioxide
and water while simultaneously providing energy for
cellular function are all catalyzed by a series of protein
enzymes.
Lipids. Lipids are several types of substances that are
grouped together because of their common property
of being soluble in fat solvents. Especially important
lipids are phospholipids and cholesterol, which
together constitute only about 2 per cent of the total
cell mass. The significance of phospholipids and cholesterol is that they are mainly insoluble in water and,
therefore, are used to form the cell membrane and
intracellular membrane barriers that separate the different cell compartments.
In addition to phospholipids and cholesterol, some
cells contain large quantities of triglycerides, also
called neutral fat. In the fat cells, triglycerides often
account for as much as 95 per cent of the cell mass. The
fat stored in these cells represents the body’s main
storehouse of energy-giving nutrients that can later be
dissoluted and used to provide energy wherever in the
body it is needed.
Physical Structure of the Cell
The cell is not merely a bag of fluid, enzymes, and
chemicals; it also contains highly organized physical
structures, called intracellular organelles. The physical
nature of each organelle is as important as the cell’s
chemical constituents for cell function. For instance,
without one of the organelles, the mitochondria, more
than 95 per cent of the cell’s energy release from nutrients would cease immediately. The most important
organelles and other structures of the cell are shown
in Figure 2–2.
Membranous Structures of the Cell
Most organelles of the cell are covered by membranes
composed primarily of lipids and proteins.These membranes include the cell membrane, nuclear membrane,
membrane of the endoplasmic reticulum, and membranes of the mitochondria, lysosomes, and Golgi
apparatus.
The lipids of the membranes provide a barrier that
impedes the movement of water and water-soluble
substances from one cell compartment to another
because water is not soluble in lipids. However, protein
molecules in the membrane often do penetrate all the
way through the membrane, thus providing specialized
pathways, often organized into actual pores, for
passage of specific substances through the membrane.
Also, many other membrane proteins are enzymes that
catalyze a multitude of different chemical reactions,
discussed here and in subsequent chapters.
Cell Membrane
The cell membrane (also called the plasma membrane), which envelops the cell, is a thin, pliable,
elastic structure only 7.5 to 10 nanometers thick. It
is composed almost entirely of proteins and lipids.
The approximate composition is proteins, 55 per cent;
phospholipids, 25 per cent; cholesterol, 13 per cent;
other lipids, 4 per cent; and carbohydrates, 3 per cent.
Lipid Barrier of the Cell Membrane Impedes Water Penetration.
Carbohydrates. Carbohydrates have little structural
function in the cell except as parts of glycoprotein molecules, but they play a major role in nutrition of the
cell. Most human cells do not maintain large stores of
carbohydrates; the amount usually averages about 1
per cent of their total mass but increases to as much
as 3 per cent in muscle cells and, occasionally, 6 per
cent in liver cells. However, carbohydrate in the
form of dissolved glucose is always present in the
Figure 2–3 shows the structure of the cell membrane.
Its basic structure is a lipid bilayer, which is a thin,
double-layered film of lipids—each layer only one
molecule thick—that is continuous over the entire cell
surface. Interspersed in this lipid film are large globular protein molecules.
The basic lipid bilayer is composed of phospholipid
molecules. One end of each phospholipid molecule is
soluble in water; that is, it is hydrophilic. The other end
is soluble only in fats; that is, it is hydrophobic. The
Chapter 2
13
The Cell and Its Functions
Chromosomes and DNA
Centrioles
Secretory
granule
Golgi
apparatus
Microtubules
Nuclear
membrane
Cell
membrane
Nucleolus
Glycogen
Ribosomes
Lysosome
Figure 2–2
Reconstruction of a typical
cell, showing the internal
organelles in the cytoplasm
and in the nucleus.
Mitochondrion
Granular
endoplasmic
reticulum
phosphate end of the phospholipid is hydrophilic, and
the fatty acid portion is hydrophobic.
Because the hydrophobic portions of the phospholipid molecules are repelled by water but are mutually
attracted to one another, they have a natural tendency
to attach to one another in the middle of the membrane, as shown in Figure 2–3. The hydrophilic phosphate portions then constitute the two surfaces of the
complete cell membrane, in contact with intracellular
water on the inside of the membrane and extracellular
water on the outside surface.
The lipid layer in the middle of the membrane is
impermeable to the usual water-soluble substances,
such as ions, glucose, and urea. Conversely, fat-soluble
substances, such as oxygen, carbon dioxide, and
alcohol, can penetrate this portion of the membrane
with ease.
The cholesterol molecules in the membrane are also
lipid in nature because their steroid nucleus is highly
fat soluble. These molecules, in a sense, are dissolved
in the bilayer of the membrane. They mainly help
determine the degree of permeability (or impermeability) of the bilayer to water-soluble constituents of
Smooth
(agranular)
endoplasmic
reticulum
Microfilaments
body fluids. Cholesterol controls much of the fluidity
of the membrane as well.
Cell Membrane Proteins. Figure 2–3 also shows globular
masses floating in the lipid bilayer. These are membrane proteins, most of which are glycoproteins. Two
types of proteins occur: integral proteins that protrude
all the way through the membrane, and peripheral proteins that are attached only to one surface of the membrane and do not penetrate all the way through.
Many of the integral proteins provide structural
channels (or pores) through which water molecules
and water-soluble substances, especially ions, can
diffuse between the extracellular and intracellular
fluids. These protein channels also have selective properties that allow preferential diffusion of some substances over others.
Other integral proteins act as carrier proteins for
transporting substances that otherwise could not penetrate the lipid bilayer. Sometimes these even transport substances in the direction opposite to their
natural direction of diffusion, which is called “active
transport.” Still others act as enzymes.
14
Unit I
Introduction to Physiology: The Cell and General Physiology
Carbohydrate
Extracellular
fluid
Integral protein
Lipid
bilayer
Peripheral
protein
Intracellular
fluid
Cytoplasm
Integral protein
Integral membrane proteins can also serve as receptors for water-soluble chemicals, such as peptide hormones, that do not easily penetrate the cell membrane.
Interaction of cell membrane receptors with specific
ligands that bind to the receptor causes conformational changes in the receptor protein. This, in turn,
enzymatically activates the intracellular part of the
protein or induces interactions between the receptor
and proteins in the cytoplasm that act as second messengers, thereby relaying the signal from the extracellular part of the receptor to the interior of the cell. In
this way, integral proteins spanning the cell membrane
provide a means of conveying information about the
environment to the cell interior.
Peripheral protein molecules are often attached to
the integral proteins. These peripheral proteins function almost entirely as enzymes or as controllers of
transport of substances through the cell membrane
“pores.”
Figure 2–3
Structure of the cell membrane,
showing that it is composed
mainly of a lipid bilayer of phospholipid molecules, but with large
numbers of protein molecules
protruding through the layer.
Also, carbohydrate moieties are
attached to the protein molecules
on the outside of the membrane
and to additional protein molecules on the inside. (Redrawn
from Lodish HF, Rothman JE: The
assembly of cell membranes. Sci
Am 240:48, 1979. Copyright
George V. Kevin.)
cell surface. Many other carbohydrate compounds,
called proteoglycans—which are mainly carbohydrate
substances bound to small protein cores—are loosely
attached to the outer surface of the cell as well.
Thus, the entire outside surface of the cell often has a
loose carbohydrate coat called the glycocalyx.
The carbohydrate moieties attached to the outer
surface of the cell have several important functions:
(1) Many of them have a negative electrical charge,
which gives most cells an overall negative surface
charge that repels other negative objects. (2) The glycocalyx of some cells attaches to the glycocalyx of
other cells, thus attaching cells to one another. (3)
Many of the carbohydrates act as receptor substances
for binding hormones, such as insulin; when bound,
this combination activates attached internal proteins
that, in turn, activate a cascade of intracellular
enzymes. (4) Some carbohydrate moieties enter into
immune reactions, as discussed in Chapter 34.
Membrane Carbohydrates—The Cell “Glycocalyx.” Mem-
brane carbohydrates occur almost invariably in
combination with proteins or lipids in the form of glycoproteins or glycolipids. In fact, most of the integral
proteins are glycoproteins, and about one tenth of the
membrane lipid molecules are glycolipids. The “glyco”
portions of these molecules almost invariably protrude
to the outside of the cell, dangling outward from the
Cytoplasm and Its Organelles
The cytoplasm is filled with both minute and large dispersed particles and organelles. The clear fluid portion
of the cytoplasm in which the particles are dispersed
is called cytosol; this contains mainly dissolved proteins, electrolytes, and glucose.
Chapter 2
Dispersed in the cytoplasm are neutral fat globules,
glycogen granules, ribosomes, secretory vesicles, and
five especially important organelles: the endoplasmic
reticulum, the Golgi apparatus, mitochondria, lysosomes, and peroxisomes.
Endoplasmic Reticulum
Figure 2–2 shows a network of tubular and flat vesicular structures in the cytoplasm; this is the endoplasmic reticulum. The tubules and vesicles interconnect
with one another. Also, their walls are constructed
of lipid bilayer membranes that contain large amounts
of proteins, similar to the cell membrane. The total
surface area of this structure in some cells—the liver
cells, for instance—can be as much as 30 to 40 times
the cell membrane area.
The detailed structure of a small portion of endoplasmic reticulum is shown in Figure 2–4. The space
inside the tubules and vesicles is filled with endoplasmic matrix, a watery medium that is different from the
fluid in the cytosol outside the endoplasmic reticulum.
Electron micrographs show that the space inside the
endoplasmic reticulum is connected with the space
between the two membrane surfaces of the nuclear
membrane.
Substances formed in some parts of the cell enter
the space of the endoplasmic reticulum and are then
conducted to other parts of the cell. Also, the vast
surface area of this reticulum and the multiple enzyme
systems attached to its membranes provide machinery
for a major share of the metabolic functions of the cell.
Ribosomes
and
the
Granular
Endoplasmic
15
The Cell and Its Functions
Reticulum.
Attached to the outer surfaces of many parts of the
endoplasmic reticulum are large numbers of minute
granular particles called ribosomes. Where these are
present, the reticulum is called the granular endoplasmic reticulum. The ribosomes are composed of a
mixture of RNA and proteins, and they function to
synthesize new protein molecules in the cell, as discussed later in this chapter and in Chapter 3.
Agranular Endoplasmic Reticulum. Part of the endoplasmic
reticulum has no attached ribosomes. This part is
called the agranular, or smooth, endoplasmic reticulum. The agranular reticulum functions for the synthesis of lipid substances and for other processes of the
cells promoted by intrareticular enzymes.
Golgi Apparatus
The Golgi apparatus, shown in Figure 2–5, is closely
related to the endoplasmic reticulum. It has membranes similar to those of the agranular endoplasmic
reticulum. It usually is composed of four or more
stacked layers of thin, flat, enclosed vesicles lying near
one side of the nucleus. This apparatus is prominent
in secretory cells, where it is located on the side of
the cell from which the secretory substances are
extruded.
The Golgi apparatus functions in association with
the endoplasmic reticulum. As shown in Figure 2–5,
small “transport vesicles” (also called endoplasmic
reticulum vesicles, or ER vesicles) continually pinch off
from the endoplasmic reticulum and shortly thereafter
fuse with the Golgi apparatus. In this way, substances
entrapped in the ER vesicles are transported from the
endoplasmic reticulum to the Golgi apparatus. The
transported substances are then processed in the Golgi
apparatus to form lysosomes, secretory vesicles, and
other cytoplasmic components that are discussed later
in the chapter.
Golgi vesicles
Matrix
Golgi
apparatus
ER vesicles
Granular
endoplasmic
reticulum
Endoplasmic
reticulum
Agranular
endoplasmic
reticulum
Figure 2–4
Figure 2–5
Structure of the endoplasmic reticulum. (Modified from DeRobertis EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB Saunders, 1975.)
A typical Golgi apparatus and its relationship to the endoplasmic
reticulum (ER) and the nucleus.
16
Unit I
Introduction to Physiology: The Cell and General Physiology
Lysosomes
Lysosomes, shown in Figure 2–2, are vesicular
organelles that form by breaking off from the Golgi
apparatus and then dispersing throughout the cytoplasm. The lysosomes provide an intracellular digestive
system that allows the cell to digest (1) damaged cellular structures, (2) food particles that have been
ingested by the cell, and (3) unwanted matter such as
bacteria. The lysosome is quite different in different
types of cells, but it is usually 250 to 750 nanometers
in diameter. It is surrounded by a typical lipid bilayer
membrane and is filled with large numbers of small
granules 5 to 8 nanometers in diameter, which are
protein aggregates of as many as 40 different hydrolase (digestive) enzymes. A hydrolytic enzyme is
capable of splitting an organic compound into two or
more parts by combining hydrogen from a water molecule with one part of the compound and combining
the hydroxyl portion of the water molecule with the
other part of the compound. For instance, protein
is hydrolyzed to form amino acids, glycogen is
hydrolyzed to form glucose, and lipids are hydrolyzed
to form fatty acids and glycerol.
Ordinarily, the membrane surrounding the lysosome
prevents the enclosed hydrolytic enzymes from
coming in contact with other substances in the cell and,
therefore, prevents their digestive actions. However,
some conditions of the cell break the membranes of
some of the lysosomes, allowing release of the digestive enzymes. These enzymes then split the organic
substances with which they come in contact into small,
highly diffusible substances such as amino acids and
glucose. Some of the more specific functions of lysosomes are discussed later in the chapter.
Secretory
granules
Figure 2–6
Secretory granules (secretory vesicles) in acinar cells of the
pancreas.
Outer membrane
Inner membrane
Matrix
Crests
Outer chamber
Oxidative
phosphorylation
enzymes
Figure 2–7
Peroxisomes
Peroxisomes are similar physically to lysosomes, but
they are different in two important ways. First, they are
believed to be formed by self-replication (or perhaps
by budding off from the smooth endoplasmic reticulum) rather than from the Golgi apparatus. Second,
they contain oxidases rather than hydrolases. Several
of the oxidases are capable of combining oxygen with
hydrogen ions derived from different intracellular
chemicals to form hydrogen peroxide (H2O2). Hydrogen peroxide is a highly oxidizing substance and is
used in association with catalase, another oxidase
enzyme present in large quantities in peroxisomes, to
oxidize many substances that might otherwise be poisonous to the cell. For instance, about half the alcohol
a person drinks is detoxified by the peroxisomes of the
liver cells in this manner.
Secretory Vesicles
One of the important functions of many cells is secretion of special chemical substances. Almost all such
secretory substances are formed by the endoplasmic
reticulum–Golgi apparatus system and are then
released from the Golgi apparatus into the cytoplasm
in the form of storage vesicles called secretory vesicles
or secretory granules. Figure 2–6 shows typical secretory vesicles inside pancreatic acinar cells; these
Structure of a mitochondrion. (Modified from DeRobertis EDP,
Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB
Saunders, 1975.)
vesicles store protein proenzymes (enzymes that are
not yet activated). The proenzymes are secreted later
through the outer cell membrane into the pancreatic
duct and thence into the duodenum, where they
become activated and perform digestive functions on
the food in the intestinal tract.
Mitochondria
The mitochondria, shown in Figures 2–2 and 2–7, are
called the “powerhouses” of the cell. Without them,
cells would be unable to extract enough energy from
the nutrients, and essentially all cellular functions
would cease.
Mitochondria are present in all areas of each cell’s
cytoplasm, but the total number per cell varies from
less than a hundred up to several thousand, depending
on the amount of energy required by the cell. Further,
the mitochondria are concentrated in those portions
of the cell that are responsible for the major share of
its energy metabolism. They are also variable in size
and shape. Some are only a few hundred nanometers
Chapter 2
The Cell and Its Functions
in diameter and globular in shape, whereas others are
elongated—as large as 1 micrometer in diameter and
7 micrometers long; still others are branching and
filamentous.
The basic structure of the mitochondrion, shown
in Figure 2–7, is composed mainly of two lipid
bilayer–protein membranes: an outer membrane and
an inner membrane. Many infoldings of the inner
membrane form shelves onto which oxidative enzymes
are attached. In addition, the inner cavity of the mitochondrion is filled with a matrix that contains large
quantities of dissolved enzymes that are necessary for
extracting energy from nutrients. These enzymes
operate in association with the oxidative enzymes on
the shelves to cause oxidation of the nutrients, thereby
forming carbon dioxide and water and at the same
time releasing energy. The liberated energy is used to
synthesize a “high-energy” substance called adenosine
triphosphate (ATP). ATP is then transported out of the
mitochondrion, and it diffuses throughout the cell to
release its own energy wherever it is needed for performing cellular functions.The chemical details of ATP
formation by the mitochondrion are given in Chapter
67, but some of the basic functions of ATP in the cell
are introduced later in this chapter.
Mitochondria are self-replicative, which means that
one mitochondrion can form a second one, a third one,
and so on, whenever there is a need in the cell for
increased amounts of ATP. Indeed, the mitochondria
contain DNA similar to that found in the cell nucleus.
In Chapter 3 we will see that DNA is the basic chemical of the nucleus that controls replication of the cell.
The DNA of the mitochondrion plays a similar role,
controlling replication of the mitochondrion itself.
Filament and Tubular Structures of the Cell
The fibrillar proteins of the cell are usually organized
into filaments or tubules. These originate as precursor
protein molecules synthesized by ribosomes in the
cytoplasm. The precursor molecules then polymerize
to form filaments. As an example, large numbers of
actin filaments frequently occur in the outer zone of
the cytoplasm, called the ectoplasm, to form an elastic
support for the cell membrane. Also, in muscle cells,
actin and myosin filaments are organized into a special
contractile machine that is the basis for muscle contraction, as discussed in detail in Chapter 6.
A special type of stiff filament composed of polymerized tubulin molecules is used in all cells to construct very strong tubular structures, the microtubules.
Figure 2–8 shows typical microtubules that were
teased from the flagellum of a sperm.
Another example of microtubules is the tubular
skeletal structure in the center of each cilium that radiates upward from the cell cytoplasm to the tip of the
cilium. This structure is discussed later in the chapter
and is illustrated in Figure 2–17. Also, both the centrioles and the mitotic spindle of the mitosing cell are
composed of stiff microtubules.
Thus, a primary function of microtubules is to act as
a cytoskeleton, providing rigid physical structures for
certain parts of cells.
17
Figure 2–8
Microtubules teased from the flagellum of a sperm. (From
Wolstenholme GEW, O’Connor M, and The publisher, JA Churchill,
1967. Figure 4, page 314. Copyright the Novartis Foundation
formerly the Ciba Foundation.)
Nucleus
The nucleus is the control center of the cell. Briefly, the
nucleus contains large quantities of DNA, which are
the genes. The genes determine the characteristics of
the cell’s proteins, including the structural proteins, as
well as the intracellular enzymes that control cytoplasmic and nuclear activities.
The genes also control and promote reproduction of
the cell itself. The genes first reproduce to give two
identical sets of genes; then the cell splits by a special
process called mitosis to form two daughter cells, each
of which receives one of the two sets of DNA genes.
All these activities of the nucleus are considered in
detail in the next chapter.
Unfortunately, the appearance of the nucleus under
the microscope does not provide many clues to the
mechanisms by which the nucleus performs its control
activities. Figure 2–9 shows the light microscopic
appearance of the interphase nucleus (during the
period between mitoses), revealing darkly staining
chromatin material throughout the nucleoplasm.
During mitosis, the chromatin material organizes in
the form of highly structured chromosomes, which can
then be easily identified using the light microscope, as
illustrated in the next chapter.
Nuclear Membrane
The nuclear membrane, also called the nuclear envelope, is actually two separate bilayer membranes, one
inside the other. The outer membrane is continuous
with the endoplasmic reticulum of the cell cytoplasm,
and the space between the two nuclear membranes is
also continuous with the space inside the endoplasmic
reticulum, as shown in Figure 2–9.
18
Unit I
Pores
Introduction to Physiology: The Cell and General Physiology
Nucleoplasm
15 nm — Small virus
150 nm — Large virus
Endoplasmic
reticulum
350 nm — Rickettsia
Nucleolus
1 mm Bacterium
Nuclear envelope –
outer and inner
membranes
Cell
Chromatin material (DNA)
Cytoplasm
5 – 10 mm +
Figure 2–9
Figure 2–10
Structure of the nucleus.
Comparison of sizes of precellular organisms with that of the
average cell in the human body.
The nuclear membrane is penetrated by several
thousand nuclear pores. Large complexes of protein
molecules are attached at the edges of the pores so
that the central area of each pore is only about 9
nanometers in diameter. Even this size is large enough
to allow molecules up to 44,000 molecular weight to
pass through with reasonable ease.
Nucleoli and Formation of Ribosomes
The nuclei of most cells contain one or more highly
staining structures called nucleoli. The nucleolus,
unlike most other organelles discussed here, does not
have a limiting membrane. Instead, it is simply an accumulation of large amounts of RNA and proteins of the
types found in ribosomes. The nucleolus becomes considerably enlarged when the cell is actively synthesizing proteins.
Formation of the nucleoli (and of the ribosomes in
the cytoplasm outside the nucleus) begins in the
nucleus. First, specific DNA genes in the chromosomes
cause RNA to be synthesized. Some of this is stored
in the nucleoli, but most of it is transported outward
through the nuclear pores into cytoplasm. Here, it is
used in conjunction with specific proteins to assemble
“mature” ribosomes that play an essential role in
forming cytoplasmic proteins, as discussed more fully
in Chapter 3.
Comparison of the Animal Cell
with Precellular Forms of Life
Many of us think of the cell as the lowest level of life.
However, the cell is a very complicated organism that
required many hundreds of millions of years to
develop after the earliest form of life, an organism
similar to the present-day virus, first appeared on
earth. Figure 2–10 shows the relative sizes of (1) the
smallest known virus, (2) a large virus, (3) a rickettsia,
(4) a bacterium, and (5) a nucleated cell, demonstrating that the cell has a diameter about 1000 times that
of the smallest virus and, therefore, a volume about 1
billion times that of the smallest virus. Correspondingly, the functions and anatomical organization of the
cell are also far more complex than those of the virus.
The essential life-giving constituent of the small
virus is a nucleic acid embedded in a coat of protein.
This nucleic acid is composed of the same basic nucleic
acid constituents (DNA or RNA) found in mammalian
cells, and it is capable of reproducing itself under
appropriate conditions. Thus, the virus propagates its
lineage from generation to generation and is therefore
a living structure in the same way that the cell and the
human being are living structures.
As life evolved, other chemicals besides nucleic acid
and simple proteins became integral parts of the
organism, and specialized functions began to develop
in different parts of the virus. A membrane formed
around the virus, and inside the membrane, a fluid
matrix appeared. Specialized chemicals then developed inside the fluid to perform special functions;
many protein enzymes appeared that were capable of
catalyzing chemical reactions and, therefore, determining the organism’s activities.
In still later stages of life, particularly in the rickettsial and bacterial stages, organelles developed inside
the organism, representing physical structures of
chemical aggregates that perform functions in a more
efficient manner than can be achieved by dispersed
chemicals throughout the fluid matrix.
Finally, in the nucleated cell, still more complex
organelles developed, the most important of which is
the nucleus itself. The nucleus distinguishes this type
of cell from all lower forms of life; the nucleus provides a control center for all cellular activities, and it
provides for exact reproduction of new cells generation after generation, each new cell having almost
exactly the same structure as its progenitor.
Chapter 2
19
The Cell and Its Functions
Functional Systems of the Cell
Proteins
In the remainder of this chapter, we discuss several
representative functional systems of the cell that make
it a living organism.
Ingestion by the Cell—Endocytosis
If a cell is to live and grow and reproduce, it must
obtain nutrients and other substances from the surrounding fluids. Most substances pass through the cell
membrane by diffusion and active transport.
Diffusion involves simple movement through the
membrane caused by the random motion of the molecules of the substance; substances move either
through cell membrane pores or, in the case of lipidsoluble substances, through the lipid matrix of the
membrane.
Active transport involves the actual carrying of a
substance through the membrane by a physical protein structure that penetrates all the way through the
membrane. These active transport mechanisms are so
important to cell function that they are presented in
detail in Chapter 4.
Very large particles enter the cell by a specialized
function of the cell membrane called endocytosis. The
principal forms of endocytosis are pinocytosis and
phagocytosis. Pinocytosis means ingestion of minute
particles that form vesicles of extracellular fluid and
particulate constituents inside the cell cytoplasm.
Phagocytosis means ingestion of large particles, such
as bacteria, whole cells, or portions of degenerating
tissue.
Pinocytosis. Pinocytosis occurs continually in the cell
membranes of most cells, but it is especially rapid
in some cells. For instance, it occurs so rapidly in
macrophages that about 3 per cent of the total macrophage membrane is engulfed in the form of vesicles
each minute. Even so, the pinocytotic vesicles are so
small—usually only 100 to 200 nanometers in diameter—that most of them can be seen only with the electron microscope.
Pinocytosis is the only means by which most large
macromolecules, such as most protein molecules, can
enter cells. In fact, the rate at which pinocytotic vesicles form is usually enhanced when such macromolecules attach to the cell membrane.
Figure 2–11 demonstrates the successive steps of
pinocytosis, showing three molecules of protein
attaching to the membrane. These molecules usually
attach to specialized protein receptors on the surface
of the membrane that are specific for the type of
protein that is to be absorbed. The receptors generally
are concentrated in small pits on the outer surface of
the cell membrane, called coated pits. On the inside of
the cell membrane beneath these pits is a latticework
of fibrillar protein called clathrin, as well as other proteins, perhaps including contractile filaments of actin
and myosin. Once the protein molecules have bound
with the receptors, the surface properties of the local
Receptors
Coated pit
Clathrin
A
B
Actin and myosin
C
Dissolving clathrin
D
Figure 2–11
Mechanism of pinocytosis.
membrane change in such a way that the entire pit
invaginates inward, and the fibrillar proteins surrounding the invaginating pit cause its borders to close
over the attached proteins as well as over a small
amount of extracellular fluid. Immediately thereafter,
the invaginated portion of the membrane breaks away
from the surface of the cell, forming a pinocytotic
vesicle inside the cytoplasm of the cell.
What causes the cell membrane to go through the
necessary contortions to form pinocytotic vesicles
remains mainly a mystery.This process requires energy
from within the cell; this is supplied by ATP, a highenergy substance discussed later in the chapter. Also,
it requires the presence of calcium ions in the extracellular fluid, which probably react with contractile
protein filaments beneath the coated pits to provide
the force for pinching the vesicles away from the cell
membrane.
Phagocytosis. Phagocytosis occurs in much the same
way as pinocytosis, except that it involves large
particles rather than molecules. Only certain cells
have the capability of phagocytosis, most notably the
tissue macrophages and some of the white blood cells.
Phagocytosis is initiated when a particle such as a
bacterium, a dead cell, or tissue debris binds with
receptors on the surface of the phagocyte. In the case
of bacteria, each bacterium usually is already attached
to a specific antibody, and it is the antibody that
attaches to the phagocyte receptors, dragging the bacterium along with it. This intermediation of antibodies
is called opsonization, which is discussed in Chapters
33 and 34.
Phagocytosis occurs in the following steps:
1. The cell membrane receptors attach to the surface
ligands of the particle.
2. The edges of the membrane around the points of
attachment evaginate outward within a fraction of
a second to surround the entire particle; then,
progressively more and more membrane receptors
20
Unit I
Introduction to Physiology: The Cell and General Physiology
attach to the particle ligands. All this occurs
suddenly in a zipper-like manner to form a closed
phagocytic vesicle.
3. Actin and other contractile fibrils in the cytoplasm
surround the phagocytic vesicle and contract
around its outer edge, pushing the vesicle to the
interior.
4. The contractile proteins then pinch the stem of
the vesicle so completely that the vesicle
separates from the cell membrane, leaving the
vesicle in the cell interior in the same way that
pinocytotic vesicles are formed.
Digestion of Pinocytotic and
Phagocytic Foreign Substances Inside
the Cell—Function of the Lysosomes
Almost immediately after a pinocytotic or phagocytic
vesicle appears inside a cell, one or more lysosomes
become attached to the vesicle and empty their acid
hydrolases to the inside of the vesicle, as shown in
Figure 2–12. Thus, a digestive vesicle is formed inside
the cell cytoplasm in which the vesicular hydrolases
begin hydrolyzing the proteins, carbohydrates, lipids,
and other substances in the vesicle. The products of
digestion are small molecules of amino acids, glucose,
phosphates, and so forth that can diffuse through the
membrane of the vesicle into the cytoplasm. What is
left of the digestive vesicle, called the residual body,
represents indigestible substances. In most instances,
this is finally excreted through the cell membrane by
a process called exocytosis, which is essentially the
opposite of endocytosis.
Thus, the pinocytotic and phagocytic vesicles containing lysosomes can be called the digestive organs of
the cells.
occurs in the uterus after pregnancy, in muscles during
long periods of inactivity, and in mammary glands at
the end of lactation. Lysosomes are responsible for
much of this regression. The mechanism by which lack
of activity in a tissue causes the lysosomes to increase
their activity is unknown.
Another special role of the lysosomes is removal of
damaged cells or damaged portions of cells from
tissues. Damage to the cell—caused by heat, cold,
trauma, chemicals, or any other factor—induces lysosomes to rupture. The released hydrolases immediately begin to digest the surrounding organic
substances. If the damage is slight, only a portion of
the cell is removed, followed by repair of the cell. If
the damage is severe, the entire cell is digested, a
process called autolysis. In this way, the cell is completely removed, and a new cell of the same type ordinarily is formed by mitotic reproduction of an adjacent
cell to take the place of the old one.
The lysosomes also contain bactericidal agents that
can kill phagocytized bacteria before they can cause
cellular damage. These agents include (1) lysozyme,
which dissolves the bacterial cell membrane; (2) lysoferrin, which binds iron and other substances before
they can promote bacterial growth; and (3) acid at a
pH of about 5.0, which activates the hydrolases and
inactivates bacterial metabolic systems.
Synthesis and Formation of Cellular
Structures by Endoplasmic Reticulum
and Golgi Apparatus
Specific Functions of the
Endoplasmic Reticulum
Pinocytotic or
phagocytic
vesicle
The extensiveness of the endoplasmic reticulum and
the Golgi apparatus in secretory cells has already been
emphasized. These structures are formed primarily of
lipid bilayer membranes similar to the cell membrane,
and their walls are loaded with protein enzymes that
catalyze the synthesis of many substances required by
the cell.
Most synthesis begins in the endoplasmic reticulum.
The products formed there are then passed on to the
Golgi apparatus, where they are further processed
before being released into the cytoplasm. But first, let
us note the specific products that are synthesized in
specific portions of the endoplasmic reticulum and the
Golgi apparatus.
Digestive vesicle
Proteins Are Formed by the Granular Endoplasmic Reticulum.
Regression of Tissues and Autolysis of Cells. Tissues of the
body often regress to a smaller size. For instance, this
Lysosomes
Residual body
Excretion
Figure 2–12
Digestion of substances in pinocytotic or phagocytic vesicles by
enzymes derived from lysosomes.
The granular portion of the endoplasmic reticulum is
characterized by large numbers of ribosomes attached
to the outer surfaces of the endoplasmic reticulum
membrane. As we discuss in Chapter 3, protein molecules are synthesized within the structures of the ribosomes. The ribosomes extrude some of the synthesized
protein molecules directly into the cytosol, but they
also extrude many more through the wall of the endoplasmic reticulum to the interior of the endoplasmic
vesicles and tubules, that is, into the endoplasmic
matrix.
Chapter 2
21
The Cell and Its Functions
Synthesis of Lipids by the Smooth Endoplasmic Reticulum.
The endoplasmic reticulum also synthesizes lipids,
especially phospholipids and cholesterol. These are
rapidly incorporated into the lipid bilayer of the endoplasmic reticulum itself, thus causing the endoplasmic
reticulum to grow more extensive. This occurs mainly
in the smooth portion of the endoplasmic reticulum.
To keep the endoplasmic reticulum from growing
beyond the needs of the cell, small vesicles called ER
vesicles or transport vesicles continually break away
from the smooth reticulum; most of these vesicles then
migrate rapidly to the Golgi apparatus.
Other Functions of the Endoplasmic Reticulum. Other significant functions of the endoplasmic reticulum, especially the smooth reticulum, include the following:
1. It provides the enzymes that control glycogen
breakdown when glycogen is to be used for
energy.
2. It provides a vast number of enzymes that are
capable of detoxifying substances, such as drugs,
that might damage the cell. It achieves
detoxification by coagulation, oxidation,
hydrolysis, conjugation with glycuronic acid, and
in other ways.
Specific Functions of the Golgi Apparatus
Synthetic Functions of the Golgi Apparatus. Although the
major function of the Golgi apparatus is to provide
additional processing of substances already formed
in the endoplasmic reticulum, it also has the capability
of synthesizing certain carbohydrates that cannot be
formed in the endoplasmic reticulum. This is especially
true for the formation of large saccharide polymers
bound with small amounts of protein; the most important of these are hyaluronic acid and chondroitin sulfate.
A few of the many functions of hyaluronic acid and
chondroitin sulfate in the body are as follows: (1) they
are the major components of proteoglycans secreted
in mucus and other glandular secretions; (2) they are
the major components of the ground substance outside
the cells in the interstitial spaces, acting as filler
between collagen fibers and cells; and (3) they are
principal components of the organic matrix in both
cartilage and bone.
Processing of Endoplasmic Secretions by the Golgi Apparatus—
Formation of Vesicles. Figure 2–13 summarizes the
major functions of the endoplasmic reticulum and
Golgi apparatus. As substances are formed in the
endoplasmic reticulum, especially the proteins, they
are transported through the tubules toward portions
of the smooth endoplasmic reticulum that lie nearest
the Golgi apparatus. At this point, small transport vesicles composed of small envelopes of smooth endoplasmic reticulum continually break away and diffuse
to the deepest layer of the Golgi apparatus. Inside these
vesicles are the synthesized proteins and other products from the endoplasmic reticulum.
The transport vesicles instantly fuse with the Golgi
apparatus and empty their contained substances into
the vesicular spaces of the Golgi apparatus. Here,
Protein
Ribosomes formation
Glycosylation
Granular
endoplasmic
reticulum
Lipid
formation
Lysosomes
Secretory
vesicles
Transport
vesicles
Smooth
Golgi
endoplasmic apparatus
reticulum
Figure 2–13
Formation of proteins, lipids, and cellular vesicles by the endoplasmic reticulum and Golgi apparatus.
additional carbohydrate moieties are added to the
secretions. Also, an important function of the Golgi
apparatus is to compact the endoplasmic reticular
secretions into highly concentrated packets. As the
secretions pass toward the outermost layers of the
Golgi apparatus, the compaction and processing
proceed. Finally, both small and large vesicles continually break away from the Golgi apparatus, carrying
with them the compacted secretory substances, and in
turn, the vesicles diffuse throughout the cell.
To give an idea of the timing of these processes:
When a glandular cell is bathed in radioactive amino
acids, newly formed radioactive protein molecules can
be detected in the granular endoplasmic reticulum
within 3 to 5 minutes. Within 20 minutes, newly formed
proteins are already present in the Golgi apparatus,
and within 1 to 2 hours, radioactive proteins are
secreted from the surface of the cell.
Types of Vesicles Formed by the Golgi Apparatus—Secretory
Vesicles and Lysosomes. In a highly secretory cell, the
vesicles formed by the Golgi apparatus are mainly
secretory vesicles containing protein substances that
are to be secreted through the surface of the cell membrane. These secretory vesicles first diffuse to the cell
membrane, then fuse with it and empty their substances to the exterior by the mechanism called exocytosis. Exocytosis, in most cases, is stimulated by the
entry of calcium ions into the cell; calcium ions interact with the vesicular membrane in some way that is
not understood and cause its fusion with the cell membrane, followed by exocytosis—that is, opening of the
membrane’s outer surface and extrusion of its contents
outside the cell.
Some vesicles, however, are destined for intracellular use.
22
Unit I
Introduction to Physiology: The Cell and General Physiology
Use of Intracellular Vesicles to Replenish Cellular Membranes.
Some of the intracellular vesicles formed by the Golgi
apparatus fuse with the cell membrane or with the
membranes of intracellular structures such as the
mitochondria and even the endoplasmic reticulum.
This increases the expanse of these membranes and
thereby replenishes the membranes as they are used
up. For instance, the cell membrane loses much of its
substance every time it forms a phagocytic or pinocytotic vesicle, and the vesicular membranes of the Golgi
apparatus continually replenish the cell membrane.
In summary, the membranous system of the endoplasmic reticulum and Golgi apparatus represents a
highly metabolic organ capable of forming new intracellular structures as well as secretory substances to be
extruded from the cell.
all these digestive and metabolic functions are given
in Chapters 62 through 72.
Briefly, almost all these oxidative reactions occur
inside the mitochondria, and the energy that is
released is used to form the high-energy compound
ATP. Then, ATP, not the original foodstuffs, is used
throughout the cell to energize almost all the subsequent intracellular metabolic reactions.
Functional Characteristics of ATP
NH2
N
HC
N
Extraction of Energy from Nutrients—
Function of the Mitochondria
The principal substances from which cells extract
energy are foodstuffs that react chemically with
oxygen—carbohydrates, fats, and proteins. In the
human body, essentially all carbohydrates are converted into glucose by the digestive tract and liver
before they reach the other cells of the body. Similarly,
proteins are converted into amino acids and fats into
fatty acids. Figure 2–14 shows oxygen and the foodstuffs—glucose, fatty acids, and amino acids—all entering the cell. Inside the cell, the foodstuffs react
chemically with oxygen, under the influence of
enzymes that control the reactions and channel the
energy released in the proper direction. The details of
2ADP
Glucose
Gl
Fatty acids
FA
Amino acids
AA
2ATP
Pyruvic acid
Acetoacetic
acid
36 ADP
Acetyl-CoA
Acetyl-CoA
O2
O2
CO2
CO2
H2O
O2
ADP
CO2+H2O
ATP
H2O
Mitochondrion
36 ATP
Nucleus
Cell
membrane
Figure 2–14
Formation of adenosine triphosphate (ATP) in the cell, showing
that most of the ATP is formed in the mitochondria. ADP, adenosine diphosphate.
C
C
N
C
CH
N
O
H
Adenine
CH2 O
C H
H C
C
C H
O
O
O
P
O~P
O~P
O-
OPhosphate
O-
O-
OH OH
Ribose
Adenosine triphosphate
ATP is a nucleotide composed of (1) the nitrogenous
base adenine, (2) the pentose sugar ribose, and (3)
three phosphate radicals. The last two phosphate radicals are connected with the remainder of the molecule
by so-called high-energy phosphate bonds, which are
represented in the formula above by the symbol ~.
Under the physical and chemical conditions of the
body, each of these high-energy bonds contains about
12,000 calories of energy per mole of ATP, which is
many times greater than the energy stored in the
average chemical bond, thus giving rise to the term
high-energy bond. Further, the high-energy phosphate
bond is very labile, so that it can be split instantly on
demand whenever energy is required to promote
other intracellular reactions.
When ATP releases its energy, a phosphoric acid
radical is split away, and adenosine diphosphate (ADP)
is formed. This released energy is used to energize virtually all of the cell’s other functions, such as synthesis of substances and muscular contraction.
To reconstitute the cellular ATP as it is used up,
energy derived from the cellular nutrients causes ADP
and phosphoric acid to recombine to form new ATP,
and the entire process repeats over and over again. For
these reasons,ATP has been called the energy currency
of the cell because it can be spent and remade continually, having a turnover time of only a few minutes.
Chemical Processes in the Formation of ATP—Role of the
Mitochondria. On entry into the cells, glucose is sub-
jected to enzymes in the cytoplasm that convert it
into pyruvic acid (a process called glycolysis). A small
amount of ADP is changed into ATP by the energy
released during this conversion, but this amount
Chapter 2
23
The Cell and Its Functions
accounts for less than 5 per cent of the overall energy
metabolism of the cell.
By far, the major portion of the ATP formed in the
cell, about 95 per cent, is formed in the mitochondria.
The pyruvic acid derived from carbohydrates, fatty
acids from lipids, and amino acids from proteins are
eventually converted into the compound acetyl-CoA
in the matrix of the mitochondrion. This substance, in
turn, is further dissoluted (for the purpose of extracting its energy) by another series of enzymes in the
mitochondrion matrix, undergoing dissolution in a
sequence of chemical reactions called the citric acid
cycle, or Krebs cycle. These chemical reactions are
so important that they are explained in detail in
Chapter 67.
In this citric acid cycle, acetyl-CoA is split into its
component parts, hydrogen atoms and carbon dioxide.
The carbon dioxide diffuses out of the mitochondria
and eventually out of the cell; finally, it is excreted
from the body through the lungs.
The hydrogen atoms, conversely, are highly reactive,
and they combine instantly with oxygen that has also
diffused into the mitochondria. This releases a tremendous amount of energy, which is used by the mitochondria to convert very large amounts of ADP to
ATP. The processes of these reactions are complex,
requiring the participation of large numbers of protein
enzymes that are integral parts of mitochondrial membranous shelves that protrude into the mitochondrial
matrix. The initial event is removal of an electron from
the hydrogen atom, thus converting it to a hydrogen
ion. The terminal event is combination of hydrogen
ions with oxygen to form water plus the release of
tremendous amounts of energy to large globular proteins, called ATP synthetase, that protrude like knobs
from the membranes of the mitochondrial shelves.
Finally, the enzyme ATP synthetase uses the energy
from the hydrogen ions to cause the conversion of
ADP to ATP. The newly formed ATP is transported
out of the mitochondria into all parts of the cell cytoplasm and nucleoplasm, where its energy is used to
energize multiple cell functions.
This overall process for formation of ATP is called
the chemiosmotic mechanism of ATP formation. The
chemical and physical details of this mechanism are
presented in Chapter 67, and many of the detailed
metabolic functions of ATP in the body are presented
in Chapters 67 through 71.
Uses of ATP for Cellular Function. Energy from ATP is
used to promote three major categories of cellular
functions: (1) transport of substances through multiple
membranes in the cell, (2) synthesis of chemical
compounds throughout the cell, and (3) mechanical
work. These uses of ATP are illustrated by examples
in Figure 2–15: (1) to supply energy for the transport
of sodium through the cell membrane, (2) to promote
protein synthesis by the ribosomes, and (3) to supply
the energy needed during muscle contraction.
In addition to membrane transport of sodium,
energy from ATP is required for membrane transport
of potassium ions, calcium ions, magnesium ions, phos-
Ribosomes
Membrane
transport
Na+
Na+
Endoplasmic
reticulum
Protein synthesis
ATP
ADP
ADP
Mitochondrion
ATP
ATP
ADP
ATP
ADP
Muscle contraction
Figure 2–15
Use of adenosine triphosphate (ATP) (formed in the mitochondrion) to provide energy for three major cellular functions: membrane transport, protein synthesis, and muscle contraction. ADP,
adenosine diphosphate.
phate ions, chloride ions, urate ions, hydrogen ions,
and many other ions and various organic substances.
Membrane transport is so important to cell function
that some cells—the renal tubular cells, for instance—
use as much as 80 per cent of the ATP that they form
for this purpose alone.
In addition to synthesizing proteins, cells synthesize
phospholipids, cholesterol, purines, pyrimidines, and a
host of other substances. Synthesis of almost any
chemical compound requires energy. For instance, a
single protein molecule might be composed of as many
as several thousand amino acids attached to one
another by peptide linkages; the formation of each of
these linkages requires energy derived from the breakdown of four high-energy bonds; thus, many thousand
ATP molecules must release their energy as each
protein molecule is formed. Indeed, some cells use as
much as 75 per cent of all the ATP formed in the cell
simply to synthesize new chemical compounds, especially protein molecules; this is particularly true during
the growth phase of cells.
The final major use of ATP is to supply energy for
special cells to perform mechanical work. We see in
Chapter 6 that each contraction of a muscle fiber
requires expenditure of tremendous quantities of ATP
energy. Other cells perform mechanical work in other
ways, especially by ciliary and ameboid motion, which
are described later in this chapter. The source of
energy for all these types of mechanical work is ATP.
In summary, ATP is always available to release its
energy rapidly and almost explosively wherever in the
cell it is needed. To replace the ATP used by the cell,
24
Unit I
Introduction to Physiology: The Cell and General Physiology
much slower chemical reactions break down carbohydrates, fats, and proteins and use the energy derived
from these to form new ATP. More than 95 per cent
of this ATP is formed in the mitochondria, which
accounts for the mitochondria being called the “powerhouses” of the cell.
Locomotion of Cells
By far the most important type of movement that occurs
in the body is that of the muscle cells in skeletal, cardiac,
and smooth muscle, which constitute almost 50 per cent
of the entire body mass. The specialized functions of
these cells are discussed in Chapters 6 through 9. Two
other types of movement—ameboid locomotion and
ciliary movement—occur in other cells.
Ameboid Movement
Ameboid movement is movement of an entire cell in
relation to its surroundings, such as movement of white
blood cells through tissues. It receives its name from the
fact that amebae move in this manner and have provided an excellent tool for studying the phenomenon.
Typically, ameboid locomotion begins with protrusion
of a pseudopodium from one end of the cell. The
pseudopodium projects far out, away from the cell
body, and partially secures itself in a new tissue area.
Then the remainder of the cell is pulled toward the
pseudopodium. Figure 2–16 demonstrates this process,
showing an elongated cell, the right-hand end of which
is a protruding pseudopodium. The membrane of this
end of the cell is continually moving forward, and the
membrane at the left-hand end of the cell is continually
following along as the cell moves.
Mechanism of Ameboid Locomotion. Figure 2–16 shows the
general principle of ameboid motion. Basically, it results
from continual formation of new cell membrane at the
leading edge of the pseudopodium and continual
absorption of the membrane in mid and rear portions
of the cell. Also, two other effects are essential for
forward movement of the cell. The first effect is attachment of the pseudopodium to surrounding tissues so
that it becomes fixed in its leading position, while the
Movement of cell
Endocytosis
Pseudopodium
Exocytosis
Surrounding tissue
remainder of the cell body is pulled forward toward the
point of attachment. This attachment is effected by
receptor proteins that line the insides of exocytotic vesicles. When the vesicles become part of the pseudopodial
membrane, they open so that their insides evert to the
outside, and the receptors now protrude to the outside
and attach to ligands in the surrounding tissues.
At the opposite end of the cell, the receptors pull
away from their ligands and form new endocytotic vesicles. Then, inside the cell, these vesicles stream toward
the pseudopodial end of the cell, where they are used
to form still new membrane for the pseudopodium.
The second essential effect for locomotion is to
provide the energy required to pull the cell body in the
direction of the pseudopodium. Experiments suggest
the following as an explanation: In the cytoplasm of all
cells is a moderate to large amount of the protein actin.
Much of the actin is in the form of single molecules that
do not provide any motive power; however, these polymerize to form a filamentous network, and the network
contracts when it binds with an actin-binding protein
such as myosin. The whole process is energized by the
high-energy compound ATP. This is what happens in
the pseudopodium of a moving cell, where such
a network of actin filaments forms anew inside the
enlarging pseudopodium. Contraction also occurs in
the ectoplasm of the cell body, where a preexisting
actin network is already present beneath the cell
membrane.
Types of Cells That Exhibit Ameboid Locomotion. The most
common cells to exhibit ameboid locomotion in the
human body are the white blood cells when they move
out of the blood into the tissues in the form of tissue
macrophages. Other types of cells can also move by
ameboid locomotion under certain circumstances. For
instance, fibroblasts move into a damaged area to help
repair the damage, and even the germinal cells of the
skin, though ordinarily completely sessile cells, move
toward a cut area to repair the rent. Finally, cell locomotion is especially important in development of the
embryo and fetus after fertilization of an ovum. For
instance, embryonic cells often must migrate long distances from their sites of origin to new areas during
development of special structures.
Control of Ameboid Locomotion—Chemotaxis. The
most
important initiator of ameboid locomotion is the
process called chemotaxis. This results from the appearance of certain chemical substances in the tissues.
Any chemical substance that causes chemotaxis to
occur is called a chemotactic substance. Most cells
that exhibit ameboid locomotion move toward the
source of a chemotactic substance—that is, from an area
of lower concentration toward an area of higher concentration—which is called positive chemotaxis. Some
cells move away from the source, which is called negative chemotaxis.
But how does chemotaxis control the direction of
ameboid locomotion? Although the answer is not
certain, it is known that the side of the cell most exposed
to the chemotactic substance develops membrane
changes that cause pseudopodial protrusion.
Receptor binding
Cilia and Ciliary Movements
Figure 2–16
Ameboid motion by a cell.
A second type of cellular motion, ciliary movement, is a
whiplike movement of cilia on the surfaces of cells. This
Chapter 2
The Cell and Its Functions
Tip
Ciliary stalk
Membrane
Cross section
Filament
Forward stroke
Basal plate
Cell
membrane
Backward stroke
Basal body
Rootlet
Figure 2–17
Structure and function of the cilium. (Modified from Satir P: Cilia.
Sci Am 204:108, 1961. Copyright Donald Garber: Executor of the
estate of Bunji Tagawa.)
occurs in only two places in the human body: on the
sufaces of the respiratory airways and on the inside
surfaces of the uterine tubes (fallopian tubes) of the
reproductive tract. In the nasal cavity and lower respiratory airways, the whiplike motion of cilia causes a
layer of mucus to move at a rate of about 1 cm/min
toward the pharynx, in this way continually clearing
these passageways of mucus and particles that have
become trapped in the mucus. In the uterine tubes, the
cilia cause slow movement of fluid from the ostium of
the uterine tube toward the uterus cavity; this movement of fluid transports the ovum from the ovary to the
uterus.
As shown in Figure 2–17, a cilium has the appearance
of a sharp-pointed straight or curved hair that projects
2 to 4 micrometers from the surface of the cell. Many
cilia often project from a single cell—for instance, as
many as 200 cilia on the surface of each epithelial cell
inside the respiratory passageways. The cilium is
covered by an outcropping of the cell membrane, and it
is supported by 11 microtubules—9 double tubules
located around the periphery of the cilium, and 2 single
tubules down the center, as demonstrated in the cross
section shown in Figure 2–17. Each cilium is an outgrowth of a structure that lies immediately beneath the
cell membrane, called the basal body of the cilium.
25
The flagellum of a sperm is similar to a cilium; in fact,
it has much the same type of structure and same type
of contractile mechanism. The flagellum, however, is
much longer and moves in quasi-sinusoidal waves
instead of whiplike movements.
In the inset of Figure 2–17, movement of the cilium is
shown. The cilium moves forward with a sudden, rapid
whiplike stroke 10 to 20 times per second, bending
sharply where it projects from the surface of the cell.
Then it moves backward slowly to its initial position.
The rapid forward-thrusting, whiplike movement
pushes the fluid lying adjacent to the cell in the direction that the cilium moves; the slow, dragging movement
in the backward direction has almost no effect on fluid
movement. As a result, the fluid is continually propelled
in the direction of the fast-forward stroke. Because most
ciliated cells have large numbers of cilia on their surfaces and because all the cilia are oriented in the same
direction, this is an effective means for moving fluids
from one part of the surface to another.
Mechanism of Ciliary Movement. Although not all aspects of
ciliary movement are clear, we do know the following:
First, the nine double tubules and the two single tubules
are all linked to one another by a complex of protein
cross-linkages; this total complex of tubules and crosslinkages is called the axoneme. Second, even after
removal of the membrane and destruction of other elements of the cilium besides the axoneme, the cilium can
still beat under appropriate conditions. Third, there are
two necessary conditions for continued beating of the
axoneme after removal of the other structures of the
cilium: (1) the availability of ATP and (2) appropriate
ionic conditions, especially appropriate concentrations
of magnesium and calcium. Fourth, during forward
motion of the cilium, the double tubules on the front
edge of the cilium slide outward toward the tip of the
cilium, while those on the back edge remain in place.
Fifth, multiple protein arms composed of the protein
dynein, which has ATPase enzymatic activity, project
from each double tubule toward an adjacent double
tubule.
Given this basic information, it has been determined
that the release of energy from ATP in contact with the
ATPase dynein arms causes the heads of these arms to
“crawl” rapidly along the surface of the adjacent double
tubule. If the front tubules crawl outward while the back
tubules remain stationary, this will cause bending.
The way in which cilia contraction is controlled is not
understood. The cilia of some genetically abnormal cells
do not have the two central single tubules, and these
cilia fail to beat. Therefore, it is presumed that some
signal, perhaps an electrochemical signal, is transmitted
along these two central tubules to activate the dynein
arms.
References
Alberts B, Johnson A, Lewis J, et al: Molecular Biology of
the Cell. New York: Garland Science, 2002.
Bonifacino JS, Glick BS: The mechanisms of vesicle budding
and fusion. Cell 116:153, 2004.
Calakos N, Scheller RH: Synaptic vesicle biogenesis,
docking, and fusion: a molecular description. Physiol Rev
76:1, 1996.
Danial NN, Korsmeyer SJ: Cell death: critical control points.
Cell 116:205, 2004.
26
Unit I
Introduction to Physiology: The Cell and General Physiology
Deutsch C: The birth of a channel. Neuron 40:265, 2003.
Dröge W: Free radicals in the physiological control of cell
function. Physiol Rev 82:47, 2002.
Duchen MR: Roles of mitochondria in health and disease.
Diabetes 53(Suppl 1):S96, 2004.
Edidin M: Lipids on the frontier: a century of cell-membrane
bilayers. Nat Rev Mol Cell Biol 4:414, 2003.
Gerbi SA, Borovjagin AV, Lange TS: The nucleolus: a site of
ribonucleoprotein maturation. Curr Opin Cell Biol 15:318,
2003.
Hamill OP, Martinac B: Molecular basis of mechanotransduction in living cells. Physiol Rev 81:685, 2001.
Lange K: Role of microvillar cell surfaces in the regulation
of glucose uptake and organization of energy metabolism.
Am J Physiol Cell Physiol 282:C1, 2002.
Mattaj IW: Sorting out the nuclear envelope from the
endoplasmic reticulum. Nat Rev Mol Cell Biol 5:65,
2004.
Maxfield FR, McGraw TE: Endocytic recycling. Nat Rev
Mol Cell Biol 5:121, 2004.
Mazzanti M, Bustamante JO, Oberleithner H: Electrical
dimension of the nuclear envelope. Physiol Rev 81:1, 2001.
Perrios M: Nuclear Structure and Function. San Diego: Academic Press, 1998.
Ridley AJ, Schwartz MA, Burridge K, et al: Cell migration:
integrating signals from front to back. Science 302:1704,
2003.
Scholey JM: Intraflagellar transport. Annu Rev Cell Dev
Biol 19:423, 2003.
Schwab A: Function and spatial distribution of ion channels
and transporters in cell migration. Am J Physiol Renal
Physiol 280:F739, 2001.
Vereb G, Szollosi J, Matko J, et al: Dynamic, yet structured:
the cell membrane three decades after the SingerNicolson model. Proc Natl Acad Sci U S A 100:8053,
2003.
C
H
A
P
T
E
R
3
Genetic Control of Protein
Synthesis, Cell Function,
and Cell Reproduction
Virtually everyone knows that the genes, located in
the nuclei of all cells of the body, control heredity
from parents to children, but most people do not
realize that these same genes also control day-today function of all the body’s cells. The genes
control cell function by determining which substances are synthesized within the cell—which structures, which enzymes, which chemicals.
Figure 3–1 shows the general schema of genetic control. Each gene, which is
a nucleic acid called deoxyribonucleic acid (DNA), automatically controls the
formation of another nucleic acid, ribonucleic acid (RNA); this RNA then
spreads throughout the cell to control the formation of a specific protein.
Because there are more than 30,000 different genes in each cell, it is theoretically possible to form a very large number of different cellular proteins.
Some of the cellular proteins are structural proteins, which, in association with
various lipids and carbohydrates, form the structures of the various intracellular organelles discussed in Chapter 2. However, by far the majority of the proteins are enzymes that catalyze the different chemical reactions in the cells. For
instance, enzymes promote all the oxidative reactions that supply energy to the
cell, and they promote synthesis of all the cell chemicals, such as lipids, glycogen, and adenosine triphosphate (ATP).
Genes in the Cell Nucleus
In the cell nucleus, large numbers of genes are attached end on end in extremely
long double-stranded helical molecules of DNA having molecular weights
measured in the billions. A very short segment of such a molecule is shown in
Figure 3–2. This molecule is composed of several simple chemical compounds
bound together in a regular pattern, details of which are explained in the next
few paragraphs.
Basic Building Blocks of DNA. Figure 3–3 shows the basic chemical compounds
involved in the formation of DNA. These include (1) phosphoric acid, (2) a
sugar called deoxyribose, and (3) four nitrogenous bases (two purines, adenine
and guanine, and two pyrimidines, thymine and cytosine). The phosphoric acid
and deoxyribose form the two helical strands that are the backbone of the DNA
molecule, and the nitrogenous bases lie between the two strands and connect
them, as illustrated in Figure 3–6.
Nucleotides. The first stage in the formation of DNA is to combine one molecule of phosphoric acid, one molecule of deoxyribose, and one of the four bases
to form an acidic nucleotide. Four separate nucleotides are thus formed, one for
each of the four bases: deoxyadenylic, deoxythymidylic, deoxyguanylic, and
deoxycytidylic acids. Figure 3–4 shows the chemical structure of deoxyadenylic
acid, and Figure 3–5 shows simple symbols for the four nucleotides that form
DNA.
Organization of the Nucleotides to Form Two Strands of DNA Loosely Bound to Each Other.
Figure 3–6 shows the manner in which multiple numbers of nucleotides are
27
28
Introduction to Physiology: The Cell and General Physiology
Unit I
Gene (DNA)
RNA formation
Protein formation
Cell structure
Figure 3–2
Cell enzymes
The helical, double-stranded structure of the gene. The outside
strands are composed of phosphoric acid and the sugar deoxyribose. The internal molecules connecting the two strands of the
helix are purine and pyrimidine bases; these determine the “code”
of the gene.
Cell function
Figure 3–1
General schema by which the genes control cell function.
Phosphoric acid
O
H
O
P
O
H
O
H
H
Deoxyribose
H
O
H
H
C
C
O
C
H
H
C
O
C
H
H
H
O
H
Bases
H
N
H
N
H
H
C
C
N
C
C
O
C
N
N
O
N
C
H
C
N
C
H
H
H
C
C
H
H
H
Thymine
Adenine
H
O
N
H
C
N
C
N
H
C
H
N
C
N
C
O
N
H
H
H
Guanine
Purines
H
C
H
C
C
N
N
C
H
Cytosine
Pyrimidines
Figure 3–3
The basic building blocks of DNA.
H
H
H
Phosphate
H
O
H
O
C
P
O
H
O
C
H
29
Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
Chapter 3
Adenine
N
C
C
C
N
O
C
H O
N
H
C
N
N
T
A
H
C
P D
Deoxyadenylic acid
C
H Deoxyribose
C
H
P D
Deoxythymidylic acid
H
G
H
C
P D
Deoxyguanylic acid
P D
Deoxycytidylic acid
Figure 3–4
Deoxyadenylic acid, one of the nucleotides that make up DNA.
Figure 3–5
Symbols for the four nucleotides that combine to form DNA.
Each nucleotide contains phosphoric acid (P), deoxyribose (D),
and one of the four nucleotide bases: A, adenine; T, thymine;
G, guanine; or C, cytosine.
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
G
G
C
A
G
A
C
T
T
C
G
T
C
T
G
A
A
D
P
D
P
D
P
D
P
D
P
D
P
D
P
D
P
P
P
P
C
D
Figure 3–6
Arrangement of deoxyribose nucleotides
in a double strand of DNA.
bound together to form two strands of DNA. The two
strands are, in turn, loosely bonded with each other by
weak cross-linkages, illustrated in Figure 3–6 by the
central dashed lines. Note that the backbone of each
DNA strand is comprised of alternating phosphoric
acid and deoxyribose molecules. In turn, purine and
pyrimidine bases are attached to the sides of the
deoxyribose molecules. Then, by means of loose
hydrogen bonds (dashed lines) between the purine
and pyrimidine bases, the two respective DNA strands
are held together. But note the following:
1. Each purine base adenine of one strand always
bonds with a pyrimidine base thymine of the other
strand, and
2. Each purine base guanine always bonds with a
pyrimidine base cytosine.
Thus, in Figure 3–6, the sequence of complementary
pairs of bases is CG, CG, GC, TA, CG, TA, GC, AT, and
AT. Because of the looseness of the hydrogen bonds,
the two strands can pull apart with ease, and they do
so many times during the course of their function in
the cell.
To put the DNA of Figure 3–6 into its proper physical perspective, one could merely pick up the two ends
and twist them into a helix. Ten pairs of nucleotides
are present in each full turn of the helix in the DNA
molecule, as shown in Figure 3–2.
Genetic Code
The importance of DNA lies in its ability to control
the formation of proteins in the cell. It does this by
means of the so-called genetic code. That is, when the
two strands of a DNA molecule are split apart, this
exposes the purine and pyrimidine bases projecting to
the side of each DNA strand, as shown by the top
strand in Figure 3–7. It is these projecting bases that
form the genetic code.
30
Introduction to Physiology: The Cell and General Physiology
Unit I
DNA strand
D
P
D
P
D
P
D
C
A
G
A
C
P
D
P
D
G
R
P
R
P
R
P
R
G
RNA molecule
P
T
P
G
T
D
P
U
P
R
C
D
P
U
P
R
G
D
P
C
P
C
A
R
P
R
Figure 3–7
Triphosphate
P
P
P
P
RNA polymerase
C
P
R
C
P
R
Proline
G
P
R
U
P
R
C
P
R
U
P
Serine
R
G
P
R
A
P
R
A
P
Glutamic acid
The genetic code consists of successive “triplets” of
bases—that is, each three successive bases is a code
word. The successive triplets eventually control the
sequence of amino acids in a protein molecule that is
to be synthesized in the cell. Note in Figure 3–6 that
the top strand of DNA, reading from left to right, has
the genetic code GGC, AGA, CTT, the triplets being
separated from one another by the arrows. As we
follow this genetic code through Figures 3–7 and 3–8,
we see that these three respective triplets are responsible for successive placement of the three amino
acids, proline, serine, and glutamic acid, in a newly
formed molecule of protein.
The DNA Code in the Cell
Nucleus Is Transferred to
an RNA Code in the Cell
Cytoplasm—The Process
of Transcription
Because the DNA is located in the nucleus of the cell,
yet most of the functions of the cell are carried out in
the cytoplasm, there must be some means for the DNA
genes of the nucleus to control the chemical reactions
of the cytoplasm. This is achieved through the intermediary of another type of nucleic acid, RNA, the formation of which is controlled by the DNA of the
nucleus. Thus, as shown in Figure 3–7, the code is transferred to the RNA; this process is called transcription.
R
Combination of ribose nucleotides
with a strand of DNA to form a
molecule of RNA that carries the
genetic code from the gene to the
cytoplasm. The RNA polymerase
enzyme moves along the DNA
strand and builds the RNA molecule.
Figure 3–8
Portion of an RNA molecule, showing three RNA
“codons”—CCG, UCU, and GAA—which control
attachment of the three amino acids proline,
serine, and glutamic acid, respectively, to the
growing RNA chain.
The RNA, in turn, diffuses from the nucleus through
nuclear pores into the cytoplasmic compartment,
where it controls protein synthesis.
Synthesis of RNA
During synthesis of RNA, the two strands of the DNA
molecule separate temporarily; one of these strands is
used as a template for synthesis of an RNA molecule.
The code triplets in the DNA cause formation of complementary code triplets (called codons) in the RNA;
these codons, in turn, will control the sequence of
amino acids in a protein to be synthesized in the cell
cytoplasm.
Basic Building Blocks of RNA. The basic building blocks of
RNA are almost the same as those of DNA, except for
two differences. First, the sugar deoxyribose is not used
in the formation of RNA. In its place is another sugar
of slightly different composition, ribose, containing an
extra hydroxyl ion appended to the ribose ring structure. Second, thymine is replaced by another pyrimidine, uracil.
Formation of RNA Nucleotides. The basic building blocks
of RNA form RNA nucleotides, exactly as previously
described for DNA synthesis. Here again, four separate nucleotides are used in the formation of RNA.
These nucleotides contain the bases adenine, guanine,
cytosine, and uracil. Note that these are the same bases
Chapter 3
Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
as in DNA, except that uracil in RNA replaces
thymine in DNA.
“Activation” of the RNA Nucleotides. The next step in
the synthesis of RNA is “activation” of the RNA
nucleotides by an enzyme, RNA polymerase. This
occurs by adding to each nucleotide two extra phosphate radicals to form triphosphates (shown in Figure
3–7 by the two RNA nucleotides to the far right during
RNA chain formation). These last two phosphates are
combined with the nucleotide by high-energy phosphate bonds derived from ATP in the cell.
The result of this activation process is that
large quantities of ATP energy are made available to
each of the nucleotides, and this energy is used to
promote the chemical reactions that add each new
RNA nucleotide at the end of the developing RNA
chain.
Assembly of the RNA Chain from
Activated Nucleotides Using the DNA
Strand as a Template—The Process
of “Transcription”
Assembly of the RNA molecule is accomplished in
the manner shown in Figure 3–7 under the influence
of an enzyme, RNA polymerase. This is a large protein
enzyme that has many functional properties necessary
for formation of the RNA molecule. They are as
follows:
1. In the DNA strand immediately ahead of the
initial gene is a sequence of nucleotides called
the promoter. The RNA polymerase has an
appropriate complementary structure that
recognizes this promoter and becomes attached to
it. This is the essential step for initiating formation
of the RNA molecule.
2. After the RNA polymerase attaches to the
promoter, the polymerase causes unwinding of
about two turns of the DNA helix and separation
of the unwound portions of the two strands.
3. Then the polymerase moves along the DNA
strand, temporarily unwinding and separating the
two DNA strands at each stage of its movement.
As it moves along, it adds at each stage a new
activated RNA nucleotide to the end of the newly
forming RNA chain by the following steps:
a. First, it causes a hydrogen bond to form
between the end base of the DNA strand and
the base of an RNA nucleotide in the
nucleoplasm.
b. Then, one at a time, the RNA polymerase
breaks two of the three phosphate radicals
away from each of these RNA nucleotides,
liberating large amounts of energy from the
broken high-energy phosphate bonds; this
energy is used to cause covalent linkage of the
remaining phosphate on the nucleotide with
the ribose on the end of the growing RNA
chain.
31
c. When the RNA polymerase reaches the
end of the DNA gene, it encounters a new
sequence of DNA nucleotides called the
chain-terminating sequence; this causes the
polymerase and the newly formed RNA chain
to break away from the DNA strand. Then the
polymerase can be used again and again to
form still more new RNA chains.
d. As the new RNA strand is formed, its weak
hydrogen bonds with the DNA template break
away, because the DNA has a high affinity for
rebonding with its own complementary DNA
strand. Thus, the RNA chain is forced away
from the DNA and is released into the
nucleoplasm.
Thus, the code that is present in the DNA strand is
eventually transmitted in complementary form to the
RNA chain. The ribose nucleotide bases always
combine with the deoxyribose bases in the following
combinations:
DNA Base
guanine
cytosine
adenine
thymine
RNA Base
..................................
..................................
..................................
..................................
cytosine
guanine
uracil
adenine
Three Different Types of RNA. There are three different
types of RNA, each of which plays an independent and
entirely different role in protein formation:
1. Messenger RNA, which carries the genetic code to
the cytoplasm for controlling the type of protein
formed.
2. Transfer RNA, which transports activated amino
acids to the ribosomes to be used in assembling
the protein molecule.
3. Ribosomal RNA, which, along with about 75
different proteins, forms ribosomes, the physical
and chemical structures on which protein
molecules are actually assembled.
Messenger RNA—The Codons
Messenger RNA molecules are long, single RNA
strands that are suspended in the cytoplasm. These
molecules are composed of several hundred to several
thousand RNA nucleotides in unpaired strands, and
they contain codons that are exactly complementary
to the code triplets of the DNA genes. Figure 3–8
shows a small segment of a molecule of messenger
RNA. Its codons are CCG, UCU, and GAA. These are
the codons for the amino acids proline, serine, and glutamic acid. The transcription of these codons from the
DNA molecule to the RNA molecule is shown in
Figure 3–7.
RNA Codons for the Different Amino Acids. Table 3–1 gives
the RNA codons for the 20 common amino acids
32
Unit I
Introduction to Physiology: The Cell and General Physiology
found in protein molecules. Note that most of the
amino acids are represented by more than one codon;
also, one codon represents the signal “start manufacturing the protein molecule,” and three codons represent “stop manufacturing the protein molecule.” In
Table 3–1, these two types of codons are designated CI
for “chain-initiating” and CT for “chain-terminating.”
Transfer RNA—The Anticodons
Another type of RNA that plays an essential role in
protein synthesis is called transfer RNA, because it
transfers amino acid molecules to protein molecules as
the protein is being synthesized. Each type of transfer
RNA combines specifically with 1 of the 20 amino
acids that are to be incorporated into proteins. The
transfer RNA then acts as a carrier to transport its
specific type of amino acid to the ribosomes, where
protein molecules are forming. In the ribosomes, each
specific type of transfer RNA recognizes a particular
codon on the messenger RNA (described later) and
thereby delivers the appropriate amino acid to the
appropriate place in the chain of the newly forming
protein molecule.
Transfer RNA, which contains only about 80
nucleotides, is a relatively small molecule in comparison with messenger RNA. It is a folded chain of
nucleotides with a cloverleaf appearance similar to
that shown in Figure 3–9. At one end of the molecule
is always an adenylic acid; it is to this that the transported amino acid attaches at a hydroxyl group of the
ribose in the adenylic acid.
Because the function of transfer RNA is to cause
attachment of a specific amino acid to a forming
protein chain, it is essential that each type of transfer
RNA also have specificity for a particular codon in the
Forming protein
Table 3–1
RNA Codons for Amino Acids and for Start and Stop
Amino Acid
RNA
Codons
Alanine
Arginine
Asparagine
Aspartic acid
Cysteine
Glutamic acid
Glutamine
Glycine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Proline
Serine
Threonine
Tryptophan
Tyrosine
Valine
Start (CI)
Stop (CT)
GCU
CGU
AAU
GAU
UGU
GAA
CAA
GGU
CAU
AUU
CUU
AAA
AUG
UUU
CCU
UCU
ACU
UGG
UAU
GUU
AUG
UAA
GCC
CGC
AAC
GAC
UGC
GAG
CAG
GGC
CAC
AUC
CUC
AAG
GCA
CGA
GCG
CGG
GGA
GGG
AUA
CUA
UUC
CCC
UCC
ACC
AGA
AGG
CUG
UUA
UUG
CCA
UCA
ACA
CCG
UCG
ACG
AGC
AGU
UAC
GUC
GUA
GUG
UAG
UGA
CI, chain-initiating; CT, chain-terminating.
messenger RNA. The specific code in the transfer
RNA that allows it to recognize a specific codon is
again a triplet of nucleotide bases and is called an anticodon. This is located approximately in the middle
of the transfer RNA molecule (at the bottom of the
cloverleaf configuration shown in Figure 3–9). During
formation of the protein molecule, the anticodon bases
combine loosely by hydrogen bonding with the codon
Alanine
Cysteine
Histidine
Alanine
Phenylalanine
Serine
Proline
Transfer RNA
Start codon
GGG
AUG GCC UGU CAU GCC UUU UCC CCC AAA CAG GAC UAU
Ribosome
Messenger
RNA movement
Ribosome
Figure 3–9
A messenger RNA strand is moving through two
ribosomes. As each “codon” passes through,
an amino acid is added to the growing protein
chain, which is shown in the right-hand ribosome. The transfer RNA molecule transports
each specific amino acid to the newly forming
protein.
Chapter 3
Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
bases of the messenger RNA. In this way, the respective amino acids are lined up one after another along
the messenger RNA chain, thus establishing the
appropriate sequence of amino acids in the newly
forming protein molecule.
Ribosomal RNA
The third type of RNA in the cell is ribosomal RNA;
it constitutes about 60 per cent of the ribosome. The
remainder of the ribosome is protein, containing about
75 types of proteins that are both structural proteins
and enzymes needed in the manufacture of protein
molecules.
The ribosome is the physical structure in the cytoplasm on which protein molecules are actually synthesized. However, it always functions in association
with the other two types of RNA as well: transfer RNA
transports amino acids to the ribosome for incorporation into the developing protein molecule, whereas
messenger RNA provides the information necessary
for sequencing the amino acids in proper order for
each specific type of protein to be manufactured.
Thus, the ribosome acts as a manufacturing plant in
which the protein molecules are formed.
Formation of Ribosomes in the Nucleolus. The DNA genes
for formation of ribosomal RNA are located in five
pairs of chromosomes in the nucleus, and each of these
chromosomes contains many duplicates of these particular genes because of the large amounts of ribosomal RNA required for cellular function.
As the ribosomal RNA forms, it collects in the
nucleolus, a specialized structure lying adjacent to the
chromosomes.When large amounts of ribosomal RNA
are being synthesized, as occurs in cells that manufacture large amounts of protein, the nucleolus is a large
structure, whereas in cells that synthesize little protein,
the nucleolus may not even be seen. Ribosomal RNA
is specially processed in the nucleolus, where it binds
with “ribosomal proteins” to form granular condensation products that are primordial subunits of ribosomes. These subunits are then released from the
nucleolus and transported through the large pores of
the nuclear envelope to almost all parts of the cytoplasm. After the subunits enter the cytoplasm, they
are assembled to form mature, functional ribosomes.
Therefore, proteins are formed in the cytoplasm of the
cell, but not in the cell nucleus, because the nucleus
does not contain mature ribosomes.
Formation of Proteins on
the Ribosomes—The Process
of “Translation”
When a molecule of messenger RNA comes in contact
with a ribosome, it travels through the ribosome,
beginning at a predetermined end of the RNA molecule specified by an appropriate sequence of RNA
33
bases called the “chain-initiating” codon. Then, as
shown in Figure 3–9, while the messenger RNA travels
through the ribosome, a protein molecule is formed—
a process called translation. Thus, the ribosome reads
the codons of the messenger RNA in much the same
way that a tape is “read” as it passes through the playback head of a tape recorder. Then, when a “stop” (or
“chain-terminating”) codon slips past the ribosome,
the end of a protein molecule is signaled and the
protein molecule is freed into the cytoplasm.
Polyribosomes. A single messenger RNA molecule can
form protein molecules in several ribosomes at the
same time because the initial end of the RNA strand
can pass to a successive ribosome as it leaves the first,
as shown at the bottom left in Figure 3–9 and in Figure
3–10. The protein molecules are in different stages of
development in each ribosome. As a result, clusters of
ribosomes frequently occur, 3 to 10 ribosomes being
attached to a single messenger RNA at the same time.
These clusters are called polyribosomes.
It is especially important to note that a messenger
RNA can cause the formation of a protein molecule
in any ribosome; that is, there is no specificity of ribosomes for given types of protein. The ribosome is
simply the physical manufacturing plant in which the
chemical reactions take place.
Many Ribosomes Attach to the Endoplasmic Reticulum. In
Chapter 2, it was noted that many ribosomes become
attached to the endoplasmic reticulum. This occurs
because the initial ends of many forming protein molecules have amino acid sequences that immediately
attach to specific receptor sites on the endoplasmic
reticulum; this causes these molecules to penetrate the
reticulum wall and enter the endoplasmic reticulum
matrix. This gives a granular appearance to those portions of the reticulum where proteins are being formed
and entering the matrix of the reticulum.
Figure 3–10 shows the functional relation of messenger RNA to the ribosomes and the manner in
which the ribosomes attach to the membrane of the
endoplasmic reticulum. Note the process of translation
occurring in several ribosomes at the same time in
response to the same strand of messenger RNA. Note
also the newly forming polypeptide (protein) chains
passing through the endoplasmic reticulum membrane
into the endoplasmic matrix.
Yet it should be noted that except in glandular cells
in which large amounts of protein-containing secretory vesicles are formed, most proteins synthesized by
the ribosomes are released directly into the cytosol
instead of into the endoplasmic reticulum. These proteins are enzymes and internal structural proteins of
the cell.
Chemical Steps in Protein Synthesis. Some of the chemical
events that occur in synthesis of a protein molecule are
shown in Figure 3–11. This figure shows representative
reactions for three separate amino acids, AA1, AA2,
and AA20. The stages of the reactions are the following: (1) Each amino acid is activated by a chemical
34
Unit I
Transfer RNA
Introduction to Physiology: The Cell and General Physiology
Messenger
RNA
Small Ribosome
subunit
Figure 3–10
Amino acid
Amino acid
Activated amino acid
Large
subunit
Polypeptide
chain
AA1
+
ATP
AA2
+
ATP
AMP AA1
+
tRNA1
AA1
AA20
+
ATP
AMP AA2
+
tRNA2
AMP AA20
+
tRNA20
tRNA2
+
tRNA20
+
AA2
AA20
¸
Ô
Ô
˝
Ô
Ô
˛
RNA-amino acyl complex tRNA1
+
Physical structure of the ribosomes,
as well as their functional relation to
messenger RNA, transfer RNA, and
the endoplasmic reticulum during
the formation of protein molecules.
(Courtesy of Dr. Don W. Fawcett,
Montana.)
Messenger RNA
GCC UGU AAU
CAU CGU AUG GUU
GCC UGU AAU
CAU CGU AUG GUU
tRNA20
AA20
AA9
AA13
AA3
GTP
tRNA13
tRNA3
AA5
GTP GTP
AA2
tRNA5
AA1
AA1 AA5 AA3
tRNA2
tRNA1
Protein chain
tRNA9
Complex between tRNA,
messenger RNA, and
amino acid
GTP GTP GTP GTP
AA9
Figure 3–11
AA2 AA13 AA20
Chemical events in the formation of
a protein molecule.
process in which ATP combines with the amino acid
to form an adenosine monophosphate complex with the
amino acid, giving up two high-energy phosphate
bonds in the process. (2) The activated amino acid,
having an excess of energy, then combines with its
specific transfer RNA to form an amino acid–tRNA
complex and, at the same time, releases the adenosine
monophosphate. (3) The transfer RNA carrying the
amino acid complex then comes in contact with the
messenger RNA molecule in the ribosome, where
the anticodon of the transfer RNA attaches temporarily to its specific codon of the messenger RNA,
thus lining up the amino acid in appropriate sequence
to form a protein molecule. Then, under the influence
of the enzyme peptidyl transferase (one of the proteins
in the ribosome), peptide bonds are formed between
the successive amino acids, thus adding progressively
to the protein chain. These chemical events require
energy from two additional high-energy phosphate
bonds, making a total of four high-energy bonds used
for each amino acid added to the protein chain. Thus,
the synthesis of proteins is one of the most energy-consuming processes of the cell.
Peptide Linkage. The successive amino acids in the
protein chain combine with one another according to
the typical reaction:
NH2 O
R
C
C
OH + H
NH2 O
R
C
C
H
R
N
C
H
R
N
C
COOH
COOH + H2O
Chapter 3
Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
35
In this chemical reaction, a hydroxyl radical (OH–)
is removed from the COOH portion of the first amino
acid, and a hydrogen (H+) of the NH2 portion of the
other amino acid is removed. These combine to form
water, and the two reactive sites left on the two successive amino acids bond with each other, resulting
in a single molecule. This process is called peptide
linkage. As each additional amino acid is added, an
additional peptide linkage is formed.
There are basically two methods by which the biochemical activities in the cell are controlled. One of
these is genetic regulation, in which the degree of activation of the genes themselves is controlled, and the
other is enzyme regulation, in which the activity levels
of already formed enzymes in the cell are controlled.
Synthesis of Other
Substances in the Cell
The “Operon” of the Cell and Its Control of Biochemical Synthesis—Function of the Promoter. Synthesis of a cellular
Genetic Regulation
biochemical product usually requires a series of reactions, and each of these reactions is catalyzed by a
special protein enzyme. Formation of all the enzymes
needed for the synthetic process often is controlled by
a sequence of genes located one after the other on
the same chromosomal DNA strand. This area of the
DNA strand is called an operon, and the genes responsible for forming the respective enzymes are called
structural genes. In Figure 3–12, three respective
structural genes are shown in an operon, and it is
demonstrated that they control the formation of three
respective enzymes that in turn cause synthesis of a
specific intracellular product.
Note in the figure the segment on the DNA strand
called the promoter. This is a group of nucleotides that
has specific affinity for RNA polymerase, as already
discussed. The polymerase must bind with this promoter before it can begin traveling along the DNA
strand to synthesize RNA. Therefore, the promoter is
an essential element for activating the operon.
Many thousand protein enzymes formed in the
manner just described control essentially all the other
chemical reactions that take place in cells. These
enzymes promote synthesis of lipids, glycogen, purines,
pyrimidines, and hundreds of other substances. We
discuss many of these synthetic processes in relation
to carbohydrate, lipid, and protein metabolism in
Chapters 67 through 69. It is by means of all these
substances that the many functions of the cells are
performed.
Control of Gene Function and
Biochemical Activity in Cells
From our discussion thus far, it is clear that the genes
control both the physical and the chemical functions
of the cells. However, the degree of activation of
respective genes must be controlled as well; otherwise,
some parts of the cell might overgrow or some chemical reactions might overact until they kill the cell.
Each cell has powerful internal feedback control
mechanisms that keep the various functional operations of the cell in step with one another. For each gene
(more than 30,000 genes in all), there is at least one
such feedback mechanism.
Control of the Operon by a “Repressor Protein”—The
“Repressor Operator.” Also note in Figure 3–12 an addi-
tional band of nucleotides lying in the middle of the
promoter. This area is called a repressor operator
because a “regulatory” protein can bind here and
prevent attachment of RNA polymerase to the promoter, thereby blocking transcription of the genes of
Activator
operator
Repressor
operator
Operon
¸
Ô
Ô
Ô
˝
Ô
Ô
Ô
˛
Promoter
Figure 3–12
Function of an operon to control synthesis of a non
protein intracellular product, such as an intracellular metabolic chemical. Note that the synthesized product exerts negative feedback to inhibit
the function of the operon, in this way automatically
controlling the concentration of the product itself.
Structural
Gene A
Structural
Gene B
Enzyme A
Enzyme B
Structural
Gene C
Enzyme C
Inhibition of
the operator
Substrates
(Negative feedback)
Synthesized
product
36
Unit I
Introduction to Physiology: The Cell and General Physiology
this operon. Such a negative regulatory protein is
called a repressor protein.
Control of the Operon by an “Activator Protein”—The “Activator Operator.” Note in Figure 3–12 another operator,
called the activator operator, that lies adjacent to but
ahead of the promoter. When a regulatory protein
binds to this operator, it helps attract the RNA polymerase to the promoter, in this way activating the
operon. Therefore, a regulatory protein of this type is
called an activator protein.
Negative Feedback Control of the Operon. Finally, note in
Figure 3–12 that the presence of a critical amount
of a synthesized product in the cell can cause negative
feedback inhibition of the operon that is responsible
for its synthesis. It can do this either by causing a
regulatory repressor protein to bind at the repressor
operator or by causing a regulatory activator protein
to break its bond with the activator operator. In either
case, the operon becomes inhibited. Therefore, once
the required synthesized product has become abundant enough for proper cell function, the operon
becomes dormant. Conversely, when the synthesized
product becomes degraded in the cell and its concentration decreases, the operon once again becomes
active. In this way, the desired concentration of the
product is controlled automatically.
Other Mechanisms for Control of Transcription by the Operon.
Variations in the basic mechanism for control of the
operon have been discovered with rapidity in the past
2 decades. Without giving details, let us list some of
them:
1. An operon frequently is controlled by a
regulatory gene located elsewhere in the genetic
complex of the nucleus. That is, the regulatory
gene causes the formation of a regulatory protein
that in turn acts either as an activator or as a
repressor substance to control the operon.
2. Occasionally, many different operons are
controlled at the same time by the same
regulatory protein. In some instances, the same
regulatory protein functions as an activator
for one operon and as a repressor for another
operon. When multiple operons are controlled
simultaneously in this manner, all the operons
that function together are called a regulon.
3. Some operons are controlled not at the starting
point of transcription on the DNA strand but
farther along the strand. Sometimes the control is
not even at the DNA strand itself but during the
processing of the RNA molecules in the nucleus
before they are released into the cytoplasm;
rarely, control might occur at the level of protein
formation in the cytoplasm during RNA
translation by the ribosomes.
4. In nucleated cells, the nuclear DNA is packaged
in specific structural units, the chromosomes.
Within each chromosome, the DNA is wound
around small proteins called histones, which in
turn are held tightly together in a compacted state
by still other proteins. As long as the DNA is in
this compacted state, it cannot function to form
RNA. However, multiple control mechanisms are
beginning to be discovered that can cause selected
areas of chromosomes to become decompacted
one part at a time so that partial RNA
transcription can occur. Even then, some specific
“transcriptor factor” controls the actual rate of
transcription by the separate operon in the
chromosome. Thus, still higher orders of control
are used for establishing proper cell function. In
addition, signals from outside the cell, such as
some of the body’s hormones, can activate specific
chromosomal areas and specific transcription
factors, thus controlling the chemical machinery
for function of the cell.
Because there are more than 30,000 different genes
in each human cell, the large number of ways in which
genetic activity can be controlled is not surprising. The
gene control systems are especially important for controlling intracellular concentrations of amino acids,
amino acid derivatives, and intermediate substrates
and products of carbohydrate, lipid, and protein
metabolism.
Control of Intracellular Function
by Enzyme Regulation
In addition to control of cell function by genetic
regulation, some cell activities are controlled by intracellular inhibitors or activators that act directly on specific intracellular enzymes. Thus, enzyme regulation
represents a second category of mechanisms by which
cellular biochemical functions can be controlled.
Enzyme Inhibition. Some chemical substances formed in
the cell have direct feedback effects in inhibiting the
specific enzyme systems that synthesize them. Almost
always the synthesized product acts on the first
enzyme in a sequence, rather than on the subsequent
enzymes, usually binding directly with the enzyme and
causing an allosteric conformational change that inactivates it. One can readily recognize the importance of
inactivating the first enzyme: this prevents buildup of
intermediary products that are not used.
Enzyme inhibition is another example of negative
feedback control; it is responsible for controlling
intracellular concentrations of multiple amino acids,
purines, pyrimidines, vitamins, and other substances.
Enzyme Activation. Enzymes that are normally inactive
often can be activated when needed. An example of
this occurs when most of the ATP has been depleted
in a cell. In this case, a considerable amount of cyclic
adenosine monophosphate (cAMP) begins to be
formed as a breakdown product of the ATP; the presence of this cAMP, in turn, immediately activates the
glycogen-splitting enzyme phosphorylase, liberating
glucose molecules that are rapidly metabolized and
their energy used for replenishment of the ATP stores.
Thus, cAMP acts as an enzyme activator for the
Chapter 3
37
Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
enzyme phosphorylase and thereby helps control
intracellular ATP concentration.
Another interesting instance of both enzyme inhibition and enzyme activation occurs in the formation
of the purines and pyrimidines. These substances are
needed by the cell in approximately equal quantities
for formation of DNA and RNA. When purines are
formed, they inhibit the enzymes that are required for
formation of additional purines. However, they activate the enzymes for formation of pyrimidines. Conversely, the pyrimidines inhibit their own enzymes but
activate the purine enzymes. In this way, there is continual cross-feed between the synthesizing systems
for these two substances, resulting in almost exactly
equal amounts of the two substances in the cells at all
times.
Chromosome
Centromere
Nuclear
membrane
Nucleolus
Aster
Centriole
A
B
C
D
E
F
G
H
Summary. In summary, there are two principal methods
by which cells control proper proportions and proper
quantities of different cellular constituents: (1) the
mechanism of genetic regulation and (2) the mechanism of enzyme regulation. The genes can be either
activated or inhibited, and likewise, the enzyme
systems can be either activated or inhibited. These regulatory mechanisms most often function as feedback
control systems that continually monitor the cell’s
biochemical composition and make corrections as
needed. But on occasion, substances from without the
cell (especially some of the hormones discussed
throughout this text) also control the intracellular
biochemical reactions by activating or inhibiting one
or more of the intracellular control systems.
Figure 3–13
The DNA-Genetic System Also
Controls Cell Reproduction
Cell reproduction is another example of the ubiquitous role that the DNA-genetic system plays in all life
processes. The genes and their regulatory mechanisms
determine the growth characteristics of the cells and
also when or whether these cells will divide to form
new cells. In this way, the all-important genetic system
controls each stage in the development of the human
being, from the single-cell fertilized ovum to the whole
functioning body. Thus, if there is any central theme to
life, it is the DNA-genetic system.
Life Cycle of the Cell. The life cycle of a cell is the period
from cell reproduction to the next cell reproduction.
When mammalian cells are not inhibited and are reproducing as rapidly as they can, this life cycle may be as
little as 10 to 30 hours. It is terminated by a series of
distinct physical events called mitosis that cause division of the cell into two new daughter cells. The events
of mitosis are shown in Figure 3–13 and are described
later. The actual stage of mitosis, however, lasts for
only about 30 minutes, so that more than 95 per cent
of the life cycle of even rapidly reproducing cells is
represented by the interval between mitosis, called
interphase.
Stages of cell reproduction. A, B, and C, Prophase. D,
Prometaphase. E, Metaphase. F, Anaphase. G and H, Telophase.
(From Margaret C. Gladbach, Estate of Mary E. and Dan Todd,
Kansas.)
Except in special conditions of rapid cellular reproduction, inhibitory factors almost always slow or stop
the uninhibited life cycle of the cell. Therefore, different cells of the body actually have life cycle periods
that vary from as little as 10 hours for highly stimulated bone marrow cells to an entire lifetime of the
human body for most nerve cells.
Cell Reproduction Begins
with Replication of DNA
As is true of almost all other important events in the
cell, reproduction begins in the nucleus itself. The first
step is replication (duplication) of all DNA in the chromosomes. Only after this has occurred can mitosis take
place.
The DNA begins to be duplicated some 5 to 10
hours before mitosis, and this is completed in 4 to 8
hours. The net result is two exact replicas of all DNA.
These replicas become the DNA in the two new
38
Unit I
Introduction to Physiology: The Cell and General Physiology
daughter cells that will be formed at mitosis. After
replication of the DNA, there is another period of 1 to
2 hours before mitosis begins abruptly. Even during
this period, preliminary changes are beginning to take
place that will lead to the mitotic process.
Chemical and Physical Events of DNA Replication. DNA is
replicated in much the same way that RNA is transcribed in response to DNA, except for a few important differences:
1. Both strands of the DNA in each chromosome
are replicated, not simply one of them.
2. Both entire strands of the DNA helix are
replicated from end to end, rather than small
portions of them, as occurs in the transcription of
RNA.
3. The principal enzymes for replicating DNA
are a complex of multiple enzymes called DNA
polymerase, which is comparable to RNA
polymerase. It attaches to and moves along the
DNA template strand while another enzyme,
DNA ligase, causes bonding of successive DNA
nucleotides to one another, using high-energy
phosphate bonds to energize these attachments.
4. Formation of each new DNA strand occurs
simultaneously in hundreds of segments along
each of the two strands of the helix until the
entire strand is replicated. Then the ends of the
subunits are joined together by the DNA ligase
enzyme.
5. Each newly formed strand of DNA remains
attached by loose hydrogen bonding to the
original DNA strand that was used as its template.
Therefore, two DNA helixes are coiled together.
6. Because the DNA helixes in each chromosome
are approximately 6 centimeters in length and
have millions of helix turns, it would be
impossible for the two newly formed DNA helixes
to uncoil from each other were it not for some
special mechanism. This is achieved by enzymes
that periodically cut each helix along its entire
length, rotate each segment enough to cause
separation, and then resplice the helix. Thus, the
two new helixes become uncoiled.
DNA Repair, DNA “Proofreading,” and “Mutation.” During
the hour or so between DNA replication and the
beginning of mitosis, there is a period of very active
repair and “proofreading” of the DNA strands. That is,
wherever inappropriate DNA nucleotides have been
matched up with the nucleotides of the original template strand, special enzymes cut out the defective
areas and replace these with appropriate complementary nucleotides. This is achieved by the same DNA
polymerases and DNA ligases that are used in replication. This repair process is referred to as DNA
proofreading.
Because of repair and proofreading, the transcription process rarely makes a mistake. But when a
mistake is made, this is called a mutation. The mutation causes formation of some abnormal protein in the
cell rather than a needed protein, often leading to
abnormal cellular function and sometimes even cell
death. Yet, given that there are 30,000 or more genes
in the human genome and that the period from one
human generation to another is about 30 years, one
would expect as many as 10 or many more mutations
in the passage of the genome from parent to child. As
a further protection, however, each human genome is
represented by two separate sets of chromosomes with
almost identical genes. Therefore, one functional gene
of each pair is almost always available to the child
despite mutations.
Chromosomes and Their Replication
The DNA helixes of the nucleus are packaged in chromosomes. The human cell contains 46 chromosomes
arranged in 23 pairs. Most of the genes in the two chromosomes of each pair are identical or almost identical
to each other, so it is usually stated that the different
genes also exist in pairs, although occasionally this is
not the case.
In addition to DNA in the chromosome, there is a
large amount of protein in the chromosome, composed
mainly of many small molecules of electropositively
charged histones. The histones are organized into vast
numbers of small, bobbin-like cores. Small segments of
each DNA helix are coiled sequentially around one
core after another.
The histone cores play an important role in the regulation of DNA activity because as long as the DNA
is packaged tightly, it cannot function as a template for
either the formation of RNA or the replication of new
DNA. Further, some of the regulatory proteins have
been shown to decondense the histone packaging of
the DNA and to allow small segments at a time to form
RNA.
Several nonhistone proteins are also major components of chromosomes, functioning both as chromosomal structural proteins and, in connection with the
genetic regulatory machinery, as activators, inhibitors,
and enzymes.
Replication of the chromosomes in their entirety
occurs during the next few minutes after replication of
the DNA helixes has been completed; the new DNA
helixes collect new protein molecules as needed. The
two newly formed chromosomes remain attached to
each other (until time for mitosis) at a point called the
centromere located near their center. These duplicated
but still attached chromosomes are called chromatids.
Cell Mitosis
The actual process by which the cell splits into two new
cells is called mitosis. Once each chromosome has
been replicated to form the two chromatids, in many
cells, mitosis follows automatically within 1 or 2 hours.
Mitotic Apparatus: Function of the Centrioles. One of the
first events of mitosis takes place in the cytoplasm,
occurring during the latter part of interphase in or
Chapter 3
Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
around the small structures called centrioles. As shown
in Figure 3–13, two pairs of centrioles lie close to each
other near one pole of the nucleus. (These centrioles,
like the DNA and chromosomes, were also replicated
during interphase, usually shortly before replication of
the DNA.) Each centriole is a small cylindrical body
about 0.4 micrometer long and about 0.15 micrometer
in diameter, consisting mainly of nine parallel tubular
structures arranged in the form of a cylinder. The two
centrioles of each pair lie at right angles to each other.
Each pair of centrioles, along with attached pericentriolar material, is called a centrosome.
Shortly before mitosis is to take place, the two pairs
of centrioles begin to move apart from each other. This
is caused by polymerization of protein microtubules
growing between the respective centriole pairs and
actually pushing them apart. At the same time, other
microtubules grow radially away from each of the centriole pairs, forming a spiny star, called the aster, in
each end of the cell. Some of the spines of the aster
penetrate the nuclear membrane and help separate
the two sets of chromatids during mitosis. The complex
of microtubules extending between the two new centriole pairs is called the spindle, and the entire set of
microtubules plus the two pairs of centrioles is called
the mitotic apparatus.
Prophase. The first stage of mitosis, called prophase, is
shown in Figure 3–13A, B, and C. While the spindle is
forming, the chromosomes of the nucleus (which in
interphase consist of loosely coiled strands) become
condensed into well-defined chromosomes.
Prometaphase. During this stage (see Figure 3–13D),
the growing microtubular spines of the aster fragment
the nuclear envelope. At the same time, multiple
microtubules from the aster attach to the chromatids
at the centromeres, where the paired chromatids are
still bound to each other; the tubules then pull one
chromatid of each pair toward one cellular pole and
its partner toward the opposite pole.
Metaphase. During metaphase (see Figure 3–13E), the
two asters of the mitotic apparatus are pushed farther
apart. This is believed to occur because the microtubular spines from the two asters, where they interdigitate with each other to form the mitotic spindle,
actually push each other away. There is reason to
believe that minute contractile protein molecules
called “motor molecules,” perhaps composed of the
muscle protein actin, extend between the respective
spines and, using a stepping action as in muscle,
actively slide the spines in a reverse direction along
each other. Simultaneously, the chromatids are pulled
tightly by their attached microtubules to the very
center of the cell, lining up to form the equatorial plate
of the mitotic spindle.
Anaphase. During this phase (see Figure 3–13F), the
two chromatids of each chromosome are pulled apart
at the centromere. All 46 pairs of chromatids are
separated, forming two separate sets of 46 daughter
39
chromosomes. One of these sets is pulled toward one
mitotic aster and the other toward the other aster as
the two respective poles of the dividing cell are pushed
still farther apart.
Telophase. In telophase (see Figure 3–13G and H), the
two sets of daughter chromosomes are pushed completely apart. Then the mitotic apparatus dissolutes,
and a new nuclear membrane develops around each
set of chromosomes. This membrane is formed from
portions of the endoplasmic reticulum that are already
present in the cytoplasm. Shortly thereafter, the cell
pinches in two, midway between the two nuclei. This is
caused by formation of a contractile ring of microfilaments composed of actin and probably myosin (the
two contractile proteins of muscle) at the juncture of
the newly developing cells that pinches them off from
each other.
Control of Cell Growth and
Cell Reproduction
We know that certain cells grow and reproduce all the
time, such as the blood-forming cells of the bone
marrow, the germinal layers of the skin, and the epithelium of the gut. Many other cells, however, such as
smooth muscle cells, may not reproduce for many
years. A few cells, such as the neurons and most striated muscle cells, do not reproduce during the entire
life of a person, except during the original period of
fetal life.
In certain tissues, an insufficiency of some types of
cells causes these to grow and reproduce rapidly until
appropriate numbers of them are again available. For
instance, in some young animals, seven eighths of
the liver can be removed surgically, and the cells of
the remaining one eighth will grow and divide until the
liver mass returns almost to normal. The same occurs
for many glandular cells and most cells of the bone
marrow, subcutaneous tissue, intestinal epithelium,
and almost any other tissue except highly differentiated cells such as nerve and muscle cells.
We know little about the mechanisms that maintain
proper numbers of the different types of cells in
the body. However, experiments have shown at least
three ways in which growth can be controlled. First,
growth often is controlled by growth factors that come
from other parts of the body. Some of these circulate
in the blood, but others originate in adjacent tissues.
For instance, the epithelial cells of some glands, such
as the pancreas, fail to grow without a growth factor
from the sublying connective tissue of the gland.
Second, most normal cells stop growing when they
have run out of space for growth. This occurs when
cells are grown in tissue culture; the cells grow until
they contact a solid object, and then growth stops.
Third, cells grown in tissue culture often stop growing
when minute amounts of their own secretions are
allowed to collect in the culture medium. This, too,
could provide a means for negative feedback control
of growth.
40
Unit I
Introduction to Physiology: The Cell and General Physiology
Regulation of Cell Size. Cell size is determined almost
entirely by the amount of functioning DNA in the
nucleus. If replication of the DNA does not occur, the
cell grows to a certain size and thereafter remains at
that size. Conversely, it is possible, by use of the chemical colchicine, to prevent formation of the mitotic
spindle and therefore to prevent mitosis, even though
replication of the DNA continues. In this event, the
nucleus contains far greater quantities of DNA than
it normally does, and the cell grows proportionately
larger. It is assumed that this results simply from
increased production of RNA and cell proteins, which
in turn cause the cell to grow larger.
Cell Differentiation
A special characteristic of cell growth and cell division
is cell differentiation, which refers to changes in
physical and functional properties of cells as they proliferate in the embryo to form the different bodily
structures and organs. The description of an especially
interesting experiment that helps explain these processes follows.
When the nucleus from an intestinal mucosal cell of
a frog is surgically implanted into a frog ovum from
which the original ovum nucleus was removed, the
result is often the formation of a normal frog. This
demonstrates that even the intestinal mucosal cell,
which is a well-differentiated cell, carries all the
necessary genetic information for development of all
structures required in the frog’s body.
Therefore, it has become clear that differentiation
results not from loss of genes but from selective
repression of different genetic operons. In fact, electron micrographs suggest that some segments of DNA
helixes wound around histone cores become so condensed that they no longer uncoil to form RNA molecules. One explanation for this is as follows: It has
been supposed that the cellular genome begins at a
certain stage of cell differentiation to produce a regulatory protein that forever after represses a select
group of genes. Therefore, the repressed genes never
function again. Regardless of the mechanism, mature
human cells produce a maximum of about 8000 to
10,000 proteins rather than the potential 30,000 or
more if all genes were active.
Embryological experiments show that certain cells
in an embryo control differentiation of adjacent cells.
For instance, the primordial chorda-mesoderm is called
the primary organizer of the embryo because it forms
a focus around which the rest of the embryo develops.
It differentiates into a mesodermal axis that contains
segmentally arranged somites and, as a result of inductions in the surrounding tissues, causes formation of
essentially all the organs of the body.
Another instance of induction occurs when the
developing eye vesicles come in contact with the ectoderm of the head and cause the ectoderm to thicken
into a lens plate that folds inward to form the lens of
the eye. Therefore, a large share of the embryo develops as a result of such inductions, one part of the body
affecting another part, and this part affecting still other
parts.
Thus, although our understanding of cell differentiation is still hazy, we know many control mechanisms
by which differentiation could occur.
Apoptosis—Programmed
Cell Death
The 100 trillion cells of the body are members of a
highly organized community in which the total number
of cells is regulated not only by controlling the rate of
cell division but also by controlling the rate of cell
death. When cells are no longer needed or become a
threat to the organism, they undergo a suicidal programmed cell death, or apoptosis. This process involves
a specific proteolytic cascade that causes the cell to
shrink and condense, to disassemble its cytoskeleton,
and to alter its cell surface so that a neighboring
phagocytic cell, such as a macrophage, can attach to
the cell membrane and digest the cell.
In contrast to programmed death, cells that die as a
result of an acute injury usually swell and burst due to
loss of cell membrane integrity, a process called cell
necrosis. Necrotic cells may spill their contents, causing
inflammation and injury to neighboring cells. Apoptosis, however, is an orderly cell death that results in
disassembly and phagocytosis of the cell before any
leakage of its contents occurs, and neighboring cells
usually remain healthy.
Apoptosis is initiated by activation of a family of
proteases called caspases. These are enzymes that are
synthesized and stored in the cell as inactive procaspases. The mechanisms of activation of caspases are
complex, but once activated, the enzymes cleave and
activate other procaspases, triggering a cascade that
rapidly breaks down proteins within the cell. The cell
thus dismantles itself, and its remains are rapidly
digested by neighboring phagocytic cells.
A tremendous amount of apoptosis occurs in tissues
that are being remodeled during development. Even
in adult humans, billions of cells die each hour in
tissues such as the intestine and bone marrow and
are replaced by new cells. Programmed cell death,
however, is precisely balanced with the formation
of new cells in healthy adults. Otherwise, the body’s
tissues would shrink or grow excessively. Recent
studies suggest that abnormalities of apoptosis may
play a key role in neurodegenerative diseases such as
Alzheimer’s disease, as well as in cancer and autoimmune disorders. Some drugs that have been used
successfully for chemotherapy appear to induce apoptosis in cancer cells.
Cancer
Cancer is caused in all or almost all instances by mutation or by some other abnormal activation of cellular
genes that control cell growth and cell mitosis. The
Chapter 3
Genetic Control of Protein Synthesis, Cell Function, and Cell Reproduction
abnormal genes are called oncogenes. As many as 100
different oncogenes have been discovered.
Also present in all cells are antioncogenes, which
suppress the activation of specific oncogenes. Therefore, loss of or inactivation of antioncogenes can allow
activation of oncogenes that lead to cancer.
Only a minute fraction of the cells that mutate in the
body ever lead to cancer. There are several reasons for
this. First, most mutated cells have less survival capability than normal cells and simply die. Second, only
a few of the mutated cells that do survive become
cancerous, because even most mutated cells still
have normal feedback controls that prevent excessive
growth.
Third, those cells that are potentially cancerous are
often, if not usually, destroyed by the body’s immune
system before they grow into a cancer. This occurs in
the following way: Most mutated cells form abnormal
proteins within their cell bodies because of their
altered genes, and these proteins activate the body’s
immune system, causing it to form antibodies or sensitized lymphocytes that react against the cancerous
cells, destroying them. In support of this is the fact
that in people whose immune systems have been suppressed, such as in those taking immunosuppressant
drugs after kidney or heart transplantation, the probability of a cancer’s developing is multiplied as much
as fivefold.
Fourth, usually several different activated oncogenes are required simultaneously to cause a cancer.
For instance, one such gene might promote rapid
reproduction of a cell line, but no cancer occurs
because there is not a simultaneous mutant gene to
form the needed blood vessels.
But what is it that causes the altered genes? Considering that many trillions of new cells are formed
each year in humans, a better question might be, Why
is it that all of us do not develop millions or billions of
mutant cancerous cells? The answer is the incredible
precision with which DNA chromosomal strands are
replicated in each cell before mitosis can take place,
and also the proofreading process that cuts and repairs
any abnormal DNA strand before the mitotic process
is allowed to proceed. Yet, despite all these inherited
cellular precautions, probably one newly formed cell
in every few million still has significant mutant
characteristics.
Thus, chance alone is all that is required for mutations to take place, so we can suppose that a large
number of cancers are merely the result of an unlucky
occurrence.
However, the probability of mutations can be
increased manyfold when a person is exposed to
certain chemical, physical, or biological factors, including the following:
1. It is well known that ionizing radiation, such as
x-rays, gamma rays, and particle radiation from
radioactive substances, and even ultraviolet light
can predispose individuals to cancer. Ions formed
in tissue cells under the influence of such
radiation are highly reactive and can rupture
DNA strands, thus causing many mutations.
41
2. Chemical substances of certain types also have a
high propensity for causing mutations. It was
discovered long ago that various aniline dye
derivatives are likely to cause cancer, so that
workers in chemical plants producing such
substances, if unprotected, have a special
predisposition to cancer. Chemical substances that
can cause mutation are called carcinogens. The
carcinogens that currently cause the greatest
number of deaths are those in cigarette smoke.
They cause about one quarter of all cancer
deaths.
3. Physical irritants also can lead to cancer, such as
continued abrasion of the linings of the intestinal
tract by some types of food. The damage to
the tissues leads to rapid mitotic replacement of
the cells. The more rapid the mitosis, the greater
the chance for mutation.
4. In many families, there is a strong hereditary
tendency to cancer. This results from the fact that
most cancers require not one mutation but two or
more mutations before cancer occurs. In those
families that are particularly predisposed to
cancer, it is presumed that one or more cancerous
genes are already mutated in the inherited
genome. Therefore, far fewer additional mutations
must take place in such family members before a
cancer begins to grow.
5. In laboratory animals, certain types of viruses can
cause some kinds of cancer, including leukemia.
This usually results in one of two ways. In the case
of DNA viruses, the DNA strand of the virus can
insert itself directly into one of the chromosomes
and thereby cause a mutation that leads to
cancer. In the case of RNA viruses, some of these
carry with them an enzyme called reverse
transcriptase that causes DNA to be transcribed
from the RNA. The transcribed DNA then inserts
itself into the animal cell genome, leading to
cancer.
Invasive Characteristic of the Cancer Cell. The major differences between the cancer cell and the normal cell
are the following: (1) The cancer cell does not respect
usual cellular growth limits; the reason for this is that
these cells presumably do not require all the same
growth factors that are necessary to cause growth of
normal cells. (2) Cancer cells often are far less adhesive to one another than are normal cells. Therefore,
they have a tendency to wander through the tissues, to
enter the blood stream, and to be transported all
through the body, where they form nidi for numerous
new cancerous growths. (3) Some cancers also produce
angiogenic factors that cause many new blood vessels
to grow into the cancer, thus supplying the nutrients
required for cancer growth.
Why Do Cancer Cells Kill? The answer to this question
usually is simple. Cancer tissue competes with normal
tissues for nutrients. Because cancer cells continue to
proliferate indefinitely, their number multiplying day
by day, cancer cells soon demand essentially all the
42
Unit I
Introduction to Physiology: The Cell and General Physiology
nutrition available to the body or to an essential part
of the body. As a result, normal tissues gradually suffer
nutritive death.
References
Alberts B, Johnson A, Lewis J, et al: Molecular Biology of
the Cell. New York: Garland Science, 2002.
Aranda A, Pascal A: Nuclear hormone receptors and gene
expression. Physiol Rev 81:1269, 2001.
Balmain A, Gray J, Ponder B: The genetics and genomics of
cancer. Nat Genet 33(Suppl):238, 2003.
Bowen ID, Bowen SM, Jones AH: Mitosis and Apoptosis:
Matters of Life and Death. London: Chapman & Hall,
1998.
Burke W: Genomics as a probe for disease biology. N Engl
J Med 349:969, 2003.
Caplen NJ, Mousses S: Short interfering RNA (siRNA)mediated RNA interference (RNAi) in human cells. Ann
N Y Acad Sci 1002:56, 2003.
Cooke MS, Evans MD, Dizdaroglu M, Lunec J: Oxidative
DNA damage: mechanisms, mutation, and disease.
FASEB J 17:1195, 2003.
Cullen BR: Nuclear RNA export. J Cell Sci 116:587, 2003.
Fedier A, Fink D: Mutations in DNA mismatch repair genes:
implications for DNA damage signaling and drug sensitivity. Int J Oncol 24:1039, 2004.
Hahn S: Structure and mechanism of the RNA polymerase
II transcription machinery. Nat Struct Mol Biol 11:394,
2004.
Hall JG: Genomic imprinting: nature and clinical relevance.
Annu Rev Med 48:35, 1997.
Jockusch BM, Hüttelmaier S, Illenberger S: From the nucleus
toward the cell periphery: a guided tour for mRNAs. News
Physiol Sci 18:7, 2003.
Kazazian HH Jr: Mobile elements: drivers of genome
evolution. Science 303:1626, 2004.
Lewin B: Genes IV. Oxford: Oxford University Press, 2000.
Nabel GJ: Genetic, cellular and immune approaches to
disease therapy: past and future. Nat Med 10:135, 2004.
Pollard TD, Earnshaw WC: Cell Biology. Philadelphia:
Elsevier Science, 2002.
U
N
I
Membrane
Physiology, Nerve,
and Muscle
4. Transport of Substances Through the Cell Membrane
5. Membrane Potentials and Action Potentials
6. Contraction of Skeletal Muscle
7. Excitation of Skeletal Muscle: Neuromuscular
Transmission and Excitation-Contraction Coupling
8. Contraction and Excitation of Smooth Muscle
T
II
C
H
A
P
T
E
R
4
Transport of Substances Through
the Cell Membrane
Figure 4–1 gives the approximate concentrations of
important electrolytes and other substances in the
extracellular fluid and intracellular fluid. Note that
the extracellular fluid contains a large amount of
sodium but only a small amount of potassium.
Exactly the opposite is true of the intracellular fluid.
Also, the extracellular fluid contains a large amount
of chloride ions, whereas the intracellular fluid contains very little. But the concentrations of phosphates and proteins in the intracellular fluid are considerably greater than those in the extracellular fluid. These
differences are extremely important to the life of the cell. The purpose of this
chapter is to explain how the differences are brought about by the transport
mechanisms of the cell membranes.
The Lipid Barrier of the Cell Membrane,
and Cell Membrane Transport Proteins
The structure of the membrane covering the outside of every cell of the body
is discussed in Chapter 2 and illustrated in Figures 2–3 and 4–2. This membrane
consists almost entirely of a lipid bilayer, but it also contains large numbers of
protein molecules in the lipid, many of which penetrate all the way through the
membrane, as shown in Figure 4–2.
The lipid bilayer is not miscible with either the extracellular fluid or the intracellular fluid. Therefore, it constitutes a barrier against movement of water
molecules and water-soluble substances between the extracellular and intracellular fluid compartments. However, as demonstrated in Figure 4–2 by the leftmost arrow, a few substances can penetrate this lipid bilayer, diffusing directly
through the lipid substance itself; this is true mainly of lipid-soluble substances,
as described later.
The protein molecules in the membrane have entirely different properties for
transporting substances. Their molecular structures interrupt the continuity of
the lipid bilayer, constituting an alternative pathway through the cell membrane. Most of these penetrating proteins, therefore, can function as transport
proteins. Different proteins function differently. Some have watery spaces all
the way through the molecule and allow free movement of water as well as
selected ions or molecules; these are called channel proteins. Others, called
carrier proteins, bind with molecules or ions that are to be transported; conformational changes in the protein molecules then move the substances through
the interstices of the protein to the other side of the membrane. Both the
channel proteins and the carrier proteins are usually highly selective in the types
of molecules or ions that are allowed to cross the membrane.
“Diffusion” Versus “Active Transport.” Transport through the cell membrane, either
directly through the lipid bilayer or through the proteins, occurs by one of two
basic processes: diffusion or active transport.
Although there are many variations of these basic mechanisms, diffusion
means random molecular movement of substances molecule by molecule, either
through intermolecular spaces in the membrane or in combination with a carrier
45
46
Unit II
EXTRACELLULAR
FLUID
Membrane Physiology, Nerve, and Muscle
INTRACELLULAR
FLUID
Na+ --------------- 142 mEq/ L --------- 10 mEq/L
K+ ----------------- 4 mEq/ L ------------ 140 mEq/L
Ca++ -------------- 2.4 mEq/ L ---------- 0.0001 mEq/L
Mg++ -------------- 1.2 mEq/ L ---------- 58 mEq/L
Cl – ---------------- 103 mEq/ L --------- 4 mEq/L
HCO3– ----------- 28 mEq/ L ----------- 10 mEq/L
Phosphates----- 4 mEq/ L -------------75 mEq/L
SO4– ------------- 1 mEq/L ------------- 2 mEq/L
Glucose --------- 90 mg/dl ------------ 0 to 20 mg/dl
Amino acids ---- 30 mg/dl ------------ 200 mg/dl ?
Cholesterol
Phospholipids
Neutral fat
0.5 g/dl-------------- 2 to 95 g/dl
PO2 --------------- 35 mm Hg --------- 20 mm Hg ?
PCO2 ------------- 46 mm Hg --------- 50 mm Hg ?
pH ----------------- 7.4 ------------------- 7.0
Proteins ---------- 2 g/dl ---------------- 16 g/dl
(5 mEq/ L)
(40 mEq/ L)
Figure 4–1
Chemical compositions of extracellular and intracellular fluids.
Channel
protein
Carrier proteins
Energy
Simple
diffusion
Facilitated
diffusion
Diffusion
Active transport
Figure 4–2
Transport pathways through the cell membrane, and the basic
mechanisms of transport.
protein. The energy that causes diffusion is the energy
of the normal kinetic motion of matter.
By contrast, active transport means movement of
ions or other substances across the membrane in combination with a carrier protein in such a way that the
carrier protein causes the substance to move against
an energy gradient, such as from a low-concentration
state to a high-concentration state. This movement
requires an additional source of energy besides kinetic
energy. Following is a more detailed explanation of
Figure 4–3
Diffusion of a fluid molecule during a thousandth of a second.
the basic physics and physical chemistry of these two
processes.
Diffusion
All molecules and ions in the body fluids, including
water molecules and dissolved substances, are in constant motion, each particle moving its own separate
way. Motion of these particles is what physicists call
“heat”—the greater the motion, the higher the temperature—and the motion never ceases under any
condition except at absolute zero temperature. When
a moving molecule, A, approaches a stationary molecule, B, the electrostatic and other nuclear forces of
molecule A repel molecule B, transferring some of the
energy of motion of molecule A to molecule B. Consequently, molecule B gains kinetic energy of motion,
while molecule A slows down, losing some of its
kinetic energy. Thus, as shown in Figure 4–3, a single
molecule in a solution bounces among the other
molecules first in one direction, then another, then
another, and so forth, randomly bouncing thousands of
times each second. This continual movement of molecules among one another in liquids or in gases is called
diffusion.
Ions diffuse in the same manner as whole molecules,
and even suspended colloid particles diffuse in a
similar manner, except that the colloids diffuse far
less rapidly than molecular substances because of their
large size.
Diffusion Through the Cell Membrane
Diffusion through the cell membrane is divided into
two subtypes called simple diffusion and facilitated
diffusion. Simple diffusion means that kinetic movement of molecules or ions occurs through a membrane
opening or through intermolecular spaces without any
interaction with carrier proteins in the membrane.
The rate of diffusion is determined by the amount of
Chapter 4
47
Transport of Substances Through the Cell Membrane
substance available, the velocity of kinetic motion, and
the number and sizes of openings in the membrane
through which the molecules or ions can move.
Facilitated diffusion requires interaction of a carrier
protein. The carrier protein aids passage of the molecules or ions through the membrane by binding
chemically with them and shuttling them through the
membrane in this form.
Simple diffusion can occur through the cell membrane by two pathways: (1) through the interstices of
the lipid bilayer if the diffusing substance is lipid
soluble, and (2) through watery channels that penetrate all the way through some of the large transport
proteins, as shown to the left in Figure 4–2.
Diffusion of Lipid-Soluble Substances Through the Lipid Bilayer.
One of the most important factors that determines
how rapidly a substance diffuses through the lipid
bilayer is the lipid solubility of the substance. For
instance, the lipid solubilities of oxygen, nitrogen,
carbon dioxide, and alcohols are high, so that all these
can dissolve directly in the lipid bilayer and diffuse
through the cell membrane in the same manner that
diffusion of water solutes occurs in a watery solution.
For obvious reasons, the rate of diffusion of each of
these substances through the membrane is directly
proportional to its lipid solubility. Especially large
amounts of oxygen can be transported in this way;
therefore, oxygen can be delivered to the interior of
the cell almost as though the cell membrane did not
exist.
Diffusion of Water and Other Lipid-Insoluble Molecules Through
Protein Channels. Even though water is highly insoluble
in the membrane lipids, it readily passes through channels in protein molecules that penetrate all the way
through the membrane. The rapidity with which water
molecules can move through most cell membranes is
astounding. As an example, the total amount of water
that diffuses in each direction through the red cell
membrane during each second is about 100 times as
great as the volume of the red cell itself.
Other lipid-insoluble molecules can pass through
the protein pore channels in the same way as water
molecules if they are water soluble and small enough.
However, as they become larger, their penetration falls
off rapidly. For instance, the diameter of the urea
molecule is only 20 per cent greater than that of water,
yet its penetration through the cell membrane pores is
about 1000 times less than that of water. Even so, given
the astonishing rate of water penetration, this amount
of urea penetration still allows rapid transport of urea
through the membrane within minutes.
Diffusion Through Protein Channels,
and “Gating” of These Channels
Computerized three-dimensional reconstructions of
protein channels have demonstrated tubular pathways
all the way from the extracellular to the intracellular fluid. Therefore, substances can move by simple
diffusion directly along these channels from one side
of the membrane to the other. The protein channels
are distinguished by two important characteristics: (1)
they are often selectively permeable to certain substances, and (2) many of the channels can be opened
or closed by gates.
Selective Permeability of Protein Channels. Many of the
protein channels are highly selective for transport of
one or more specific ions or molecules. This results
from the characteristics of the channel itself, such as
its diameter, its shape, and the nature of the electrical
charges and chemical bonds along its inside surfaces.
To give an example, one of the most important of the
protein channels, the so-called sodium channel, is only
0.3 by 0.5 nanometer in diameter, but more important,
the inner surfaces of this channel are strongly negatively charged, as shown by the negative signs inside
the channel proteins in the top panel of Figure 4–4.
These strong negative charges can pull small dehydrated sodium ions into these channels, actually pulling
the sodium ions away from their hydrating water
molecules. Once in the channel, the sodium ions
diffuse in either direction according to the usual laws
of diffusion. Thus, the sodium channel is specifically
selective for passage of sodium ions.
Conversely, another set of protein channels is selective for potassium transport, shown in the lower panel
of Figure 4–4. These channels are slightly smaller than
the sodium channels, only 0.3 by 0.3 nanometer, but
they are not negatively charged, and their chemical
bonds are different. Therefore, no strong attractive force is pulling ions into the channels, and the
potassium ions are not pulled away from the water
Outside
Gate
closed
Na+
Na+
–
–
–
–
–
–
–
–
–
–
–
–
Gate open
–
–
–
–
Inside
Outside
Inside
Gate
closed
Gate open
K+
K+
Figure 4–4
Transport of sodium and potassium ions through protein channels.
Also shown are conformational changes in the protein molecules
to open or close “gates” guarding the channels.
Unit II
Membrane Physiology, Nerve, and Muscle
molecules that hydrate them. The hydrated form of
the potassium ion is considerably smaller than the
hydrated form of sodium because the sodium ion
attracts far more water molecules than does potassium. Therefore, the smaller hydrated potassium ions
can pass easily through this small channel, whereas the
larger hydrated sodium ions are rejected, thus providing selective permeability for a specific ion.
Gating of Protein Channels. Gating of protein channels
provides a means of controlling ion permeability of the
channels. This is shown in both panels of Figure 4–4
for selective gating of sodium and potassium ions. It is
believed that some of the gates are actual gatelike
extensions of the transport protein molecule, which
can close the opening of the channel or can be lifted
away from the opening by a conformational change in
the shape of the protein molecule itself.
The opening and closing of gates are controlled in
two principal ways:
1. Voltage gating. In this instance, the molecular
conformation of the gate or of its chemical bonds
responds to the electrical potential across the cell
membrane. For instance, in the top panel of Figure
4–4, when there is a strong negative charge
on the inside of the cell membrane, this presumably
could cause the outside sodium gates to remain
tightly closed; conversely, when the inside of the
membrane loses its negative charge, these gates
would open suddenly and allow tremendous
quantities of sodium to pass inward through the
sodium pores. This is the basic mechanism for
eliciting action potentials in nerves that are
responsible for nerve signals. In the bottom panel
of Figure 4–4, the potassium gates are on the
intracellular ends of the potassium channels, and
they open when the inside of the cell membrane
becomes positively charged. The opening of these
gates is partly responsible for terminating the
action potential, as is discussed more fully in
Chapter 5.
2. Chemical (ligand) gating. Some protein channel
gates are opened by the binding of a chemical
substance (a ligand) with the protein; this causes a
conformational or chemical bonding change in the
protein molecule that opens or closes the gate. This
is called chemical gating or ligand gating. One of
the most important instances of chemical gating
is the effect of acetylcholine on the so-called
acetylcholine channel. Acetylcholine opens the gate
of this channel, providing a negatively charged pore
about 0.65 nanometer in diameter that allows
uncharged molecules or positive ions smaller
than this diameter to pass through. This gate is
exceedingly important for the transmission of nerve
signals from one nerve cell to another (see Chapter
45) and from nerve cells to muscle cells to cause
muscle contraction (see Chapter 7).
Open sodium channel
3
Picoamperes
48
0
3
0
0
2
A
4
6
Milliseconds
8
10
Recorder
To recorder
Membrane
“patch”
B
Figure 4–5
A, Record of current flow through a single voltage-gated sodium
channel, demonstrating the “all or none” principle for opening and
closing of the channel. B, The “patch-clamp” method for recording current flow through a single protein channel. To the left,
recording is performed from a “patch” of a living cell membrane.
To the right, recording is from a membrane patch that has been
torn away from the cell.
Open-State Versus Closed-State of Gated Channels.
Figure 4–5A shows an especially interesting characteristic of most voltage-gated channels. This figure
shows two recordings of electrical current flowing
through a single sodium channel when there was an
approximate 25-millivolt potential gradient across the
membrane. Note that the channel conducts current
either “all or none.” That is, the gate of the channel
snaps open and then snaps closed, each open state
lasting for only a fraction of a millisecond up to several
milliseconds. This demonstrates the rapidity with
49
Transport of Substances Through the Cell Membrane
which changes can occur during the opening and
closing of the protein molecular gates. At one voltage
potential, the channel may remain closed all the time
or almost all the time, whereas at another voltage
level, it may remain open either all or most of the time.
At in-between voltages, as shown in the figure, the
gates tend to snap open and closed intermittently,
giving an average current flow somewhere between
the minimum and the maximum.
Patch-Clamp Method for Recording Ion Current Flow Through
Single Channels. One might wonder how it is technically
possible to record ion current flow through single
protein channels as shown in Figure 4–5A. This has been
achieved by using the “patch-clamp” method illustrated
in Figure 4–5B. Very simply, a micropipette, having a tip
diameter of only 1 or 2 micrometers, is abutted against
the outside of a cell membrane. Then suction is applied
inside the pipette to pull the membrane against the tip
of the pipette. This creates a seal where the edges of the
pipette touch the cell membrane. The result is a minute
membrane “patch” at the tip of the pipette through
which electrical current flow can be recorded.
Alternatively, as shown to the right in Figure 4–5B,
the small cell membrane patch at the end of the pipette
can be torn away from the cell. The pipette with
its sealed patch is then inserted into a free solution.
This allows the concentrations of ions both inside the
micropipette and in the outside solution to be altered
as desired. Also, the voltage between the two sides of
the membrane can be set at will—that is, “clamped” to
a given voltage.
It has been possible to make such patches small
enough so that only a single channel protein is found
in the membrane patch being studied. By varying the
concentrations of different ions, as well as the voltage
across the membrane, one can determine the transport
characteristics of the single channel and also its gating
properties.
Simple diffusion
Facilitated diffusion
Rate of diffusion
Chapter 4
Vmax
Concentration of substance
Figure 4–6
Effect of concentration of a substance on rate of diffusion through
a membrane by simple diffusion and facilitated diffusion. This
shows that facilitated diffusion approaches a maximum rate called
the Vmax.
Transported
molecule
Binding point
Carrier protein
and
conformational
change
Facilitated Diffusion
Facilitated diffusion is also called carrier-mediated diffusion because a substance transported in this manner
diffuses through the membrane using a specific carrier
protein to help. That is, the carrier facilitates diffusion
of the substance to the other side.
Facilitated diffusion differs from simple diffusion in
the following important way: Although the rate of
simple diffusion through an open channel increases
proportionately with the concentration of the diffusing substance, in facilitated diffusion the rate of
diffusion approaches a maximum, called Vmax, as the
concentration of the diffusing substance increases.This
difference between simple diffusion and facilitated diffusion is demonstrated in Figure 4–6. The figure shows
that as the concentration of the diffusing substance
increases, the rate of simple diffusion continues to
increase proportionately, but in the case of facilitated
diffusion, the rate of diffusion cannot rise greater than
the Vmax level.
What is it that limits the rate of facilitated diffusion?
A probable answer is the mechanism illustrated in
Figure 4–7. This figure shows a carrier protein with a
Release
of binding
Figure 4–7
Postulated mechanism for facilitated diffusion.
pore large enough to transport a specific molecule
partway through. It also shows a binding “receptor” on
the inside of the protein carrier. The molecule to be
transported enters the pore and becomes bound. Then,
in a fraction of a second, a conformational or chemical change occurs in the carrier protein, so that the
pore now opens to the opposite side of the membrane.
Because the binding force of the receptor is weak, the
thermal motion of the attached molecule causes it to
break away and to be released on the opposite side of
50
Unit II
Membrane Physiology, Nerve, and Muscle
the membrane. The rate at which molecules can be
transported by this mechanism can never be greater
than the rate at which the carrier protein molecule
can undergo change back and forth between its two
states. Note specifically, though, that this mechanism
allows the transported molecule to move—that is, to
“diffuse”—in either direction through the membrane.
Among the most important substances that cross
cell membranes by facilitated diffusion are glucose and
most of the amino acids. In the case of glucose, the
carrier molecule has been discovered, and it has a
molecular weight of about 45,000; it can also transport
several other monosaccharides that have structures
similar to that of glucose, including galactose. Also,
insulin can increase the rate of facilitated diffusion of
glucose as much as 10-fold to 20-fold. This is the principal mechanism by which insulin controls glucose use
in the body, as discussed in Chapter 78.
Factors That Affect Net Rate
of Diffusion
By now it is evident that many substances can diffuse
through the cell membrane. What is usually important
is the net rate of diffusion of a substance in the desired
direction. This net rate is determined by several
factors.
Effect of Concentration Difference on Net Diffusion Through a
Membrane. Figure 4–8A shows a cell membrane with a
substance in high concentration on the outside and low
concentration on the inside. The rate at which the substance diffuses inward is proportional to the concentration of molecules on the outside, because this
concentration determines how many molecules strike
the outside of the membrane each second. Conversely,
the rate at which molecules diffuse outward is proportional to their concentration inside the membrane.
Therefore, the rate of net diffusion into the cell is proportional to the concentration on the outside minus
the concentration on the inside, or:
Net diffusion µ (Co - Ci)
in which Co is concentration outside and Ci is concentration inside.
Effect of Membrane Electrical Potential on Diffusion of Ions—
The “Nernst Potential.” If an electrical potential is
applied across the membrane, as shown in Figure
4–8B, the electrical charges of the ions cause them to
move through the membrane even though no concentration difference exists to cause movement. Thus, in
the left panel of Figure 4–8B, the concentration of
negative ions is the same on both sides of the membrane, but a positive charge has been applied to the
right side of the membrane and a negative charge to
the left, creating an electrical gradient across the membrane. The positive charge attracts the negative ions,
whereas the negative charge repels them. Therefore,
net diffusion occurs from left to right.After much time,
large quantities of negative ions have moved to the
Outside
Inside
Co
Ci
Membrane
A
-
-
- - –
- -
- - - - - -
+
–
-
-
-
-
Piston
P1
-
+
-
-
-
-
-
-
-
-
-
- -
B
P2
C
Figure 4–8
Effect of concentration difference (A), electrical potential difference affecting negative ions (B), and pressure difference (C) to
cause diffusion of molecules and ions through a cell membrane.
right, creating the condition shown in the right panel
of Figure 4–8B, in which a concentration difference of
the ions has developed in the direction opposite to the
electrical potential difference. The concentration difference now tends to move the ions to the left, while
the electrical difference tends to move them to the
right. When the concentration difference rises high
enough, the two effects balance each other. At normal
body temperature (37°C), the electrical difference
that will balance a given concentration difference of
univalent ions—such as sodium (Na+) ions—can be
determined from the following formula, called the
Nernst equation:
EMF (in millivolts) = ± 61 log
C1
C2
in which EMF is the electromotive force (voltage)
between side 1 and side 2 of the membrane, C1 is the
concentration on side 1, and C2 is the concentration on
side 2. This equation is extremely important in understanding the transmission of nerve impulses and is discussed in much greater detail in Chapter 5.
Effect of a Pressure Difference Across the Membrane. At
times, considerable pressure difference develops
Chapter 4
51
Transport of Substances Through the Cell Membrane
between the two sides of a diffusible membrane.
This occurs, for instance, at the blood capillary membrane in all tissues of the body. The pressure is about
20 mm Hg greater inside the capillary than outside.
Pressure actually means the sum of all the forces of
the different molecules striking a unit surface area at
a given instant. Therefore, when the pressure is higher
on one side of a membrane than on the other, this
means that the sum of all the forces of the molecules
striking the channels on that side of the membrane is
greater than on the other side. In most instances, this
is caused by greater numbers of molecules striking the
membrane per second on one side than on the other
side. The result is that increased amounts of energy are
available to cause net movement of molecules from
the high-pressure side toward the low-pressure side.
This effect is demonstrated in Figure 4–8C, which
shows a piston developing high pressure on one side
of a “pore,” thereby causing more molecules to strike
the pore on this side and, therefore, more molecules to
“diffuse” to the other side.
Osmosis Across Selectively
Permeable Membranes—
“Net Diffusion” of Water
By far the most abundant substance that diffuses
through the cell membrane is water. Enough water
ordinarily diffuses in each direction through the red
cell membrane per second to equal about 100 times the
volume of the cell itself. Yet, normally, the amount that
diffuses in the two directions is balanced so precisely
that zero net movement of water occurs. Therefore, the
volume of the cell remains constant. However, under
certain conditions, a concentration difference for water
can develop across a membrane, just as concentration
differences for other substances can occur. When this
happens, net movement of water does occur across the
cell membrane, causing the cell either to swell or to
shrink, depending on the direction of the water movement. This process of net movement of water caused
by a concentration difference of water is called
osmosis.
To give an example of osmosis, let us assume the
conditions shown in Figure 4–9, with pure water on
one side of the cell membrane and a solution of
sodium chloride on the other side. Water molecules
pass through the cell membrane with ease, whereas
sodium and chloride ions pass through only with difficulty. Therefore, sodium chloride solution is actually a
mixture of permeant water molecules and nonpermeant sodium and chloride ions, and the membrane is
said to be selectively permeable to water but much less
so to sodium and chloride ions. Yet the presence of the
sodium and chloride has displaced some of the water
molecules on the side of the membrane where these
ions are present and, therefore, has reduced the concentration of water molecules to less than that of pure
water. As a result, in the example of Figure 4–9, more
water molecules strike the channels on the left side,
Water
NaCl solution
Osmosis
Figure 4–9
Osmosis at a cell membrane when a sodium chloride solution is
placed on one side of the membrane and water is placed on the
other side.
where there is pure water, than on the right side, where
the water concentration has been reduced. Thus, net
movement of water occurs from left to right—that is,
osmosis occurs from the pure water into the sodium
chloride solution.
Osmotic Pressure
If in Figure 4–9 pressure were applied to the sodium
chloride solution, osmosis of water into this solution
would be slowed, stopped, or even reversed. The exact
amount of pressure required to stop osmosis is called
the osmotic pressure of the sodium chloride solution.
The principle of a pressure difference opposing
osmosis is demonstrated in Figure 4–10, which shows
a selectively permeable membrane separating two
columns of fluid, one containing pure water and the
other containing a solution of water and any solute
that will not penetrate the membrane. Osmosis of
water from chamber B into chamber A causes the
levels of the fluid columns to become farther and
farther apart, until eventually a pressure difference
develops between the two sides of the membrane great
enough to oppose the osmotic effect. The pressure difference across the membrane at this point is equal to
the osmotic pressure of the solution that contains the
nondiffusible solute.
Importance of Number of Osmotic Particles (Molar Concentration) in Determining Osmotic Pressure. The osmotic pres-
sure exerted by particles in a solution, whether they
are molecules or ions, is determined by the number of
particles per unit volume of fluid, not by the mass of
the particles. The reason for this is that each particle
in a solution, regardless of its mass, exerts, on average,
the same amount of pressure against the membrane.
That is, large particles, which have greater mass (m)
than small particles, move at slower velocities (v). The
small particles move at higher velocities in such a way
52
Unit II
Membrane Physiology, Nerve, and Muscle
osmolality of the extracellular and intracellular fluids
is about 300 milliosmoles per kilogram of water.
A
B
Nondiffusible
solute
Semipermeable
membrane
Water
Relation of Osmolality to Osmotic Pressure. At normal body
temperature, 37°C, a concentration of 1 osmole per
liter will cause 19,300 mm Hg osmotic pressure in the
solution. Likewise, 1 milliosmole per liter concentration is equivalent to 19.3 mm Hg osmotic pressure.
Multiplying this value by the 300 milliosmolar concentration of the body fluids gives a total calculated
osmotic pressure of the body fluids of 5790 mm Hg.
The measured value for this, however, averages only
about 5500 mm Hg. The reason for this difference is
that many of the ions in the body fluids, such as sodium
and chloride ions, are highly attracted to one another;
consequently, they cannot move entirely unrestrained
in the fluids and create their full osmotic pressure
potential. Therefore, on average, the actual osmotic
pressure of the body fluids is about 0.93 times the calculated value.
The Term “Osmolarity.” Because of the difficulty of meas-
Figure 4–10
Demonstration of osmotic pressure caused by osmosis at a semipermeable membrane.
that their average kinetic energies (k), determined by
the equation
k=
mv 2
2
are the same for each small particle as for each large
particle. Consequently, the factor that determines the
osmotic pressure of a solution is the concentration of
the solution in terms of number of particles (which is
the same as its molar concentration if it is a nondissociated molecule), not in terms of mass of the solute.
“Osmolality”—The Osmole. To express the concentration
of a solution in terms of numbers of particles, the unit
called the osmole is used in place of grams.
One osmole is 1 gram molecular weight of osmotically active solute. Thus, 180 grams of glucose, which
is 1 gram molecular weight of glucose, is equal to 1
osmole of glucose because glucose does not dissociate
into ions. Conversely, if a solute dissociates into two
ions, 1 gram molecular weight of the solute will
become 2 osmoles because the number of osmotically
active particles is now twice as great as is the case for
the nondissociated solute. Therefore, when fully dissociated, 1 gram molecular weight of sodium chloride,
58.5 grams, is equal to 2 osmoles.
Thus, a solution that has 1 osmole of solute dissolved
in each kilogram of water is said to have an osmolality
of 1 osmole per kilogram, and a solution that has
1/1000 osmole dissolved per kilogram has an osmolality of 1 milliosmole per kilogram. The normal
uring kilograms of water in a solution, which is required
to determine osmolality, osmolarity, which is the
osmolar concentration expressed as osmoles per liter of
solution rather than osmoles per kilogram of water, is
used instead. Although, strictly speaking, it is osmoles
per kilogram of water (osmolality) that determines
osmotic pressure, for dilute solutions such as those in
the body, the quantitative differences between osmolarity and osmolality are less than 1 per cent. Because it
is far more practical to measure osmolarity than osmolality, this is the usual practice in almost all physiologic
studies.
“Active Transport” of
Substances Through
Membranes
At times, a large concentration of a substance is
required in the intracellular fluid even though the
extracellular fluid contains only a small concentration.
This is true, for instance, for potassium ions. Conversely, it is important to keep the concentrations of
other ions very low inside the cell even though their
concentrations in the extracellular fluid are great. This
is especially true for sodium ions. Neither of these two
effects could occur by simple diffusion, because simple
diffusion eventually equilibrates concentrations on the
two sides of the membrane. Instead, some energy
source must cause excess movement of potassium ions
to the inside of cells and excess movement of sodium
ions to the outside of cells. When a cell membrane
moves molecules or ions “uphill” against a concentration gradient (or “uphill” against an electrical or pressure gradient), the process is called active transport.
Different substances that are actively transported
through at least some cell membranes include sodium
ions, potassium ions, calcium ions, iron ions, hydrogen
ions, chloride ions, iodide ions, urate ions, several different sugars, and most of the amino acids.
Chapter 4
53
Transport of Substances Through the Cell Membrane
Primary Active Transport and Secondary Active Transport.
Active transport is divided into two types according to
the source of the energy used to cause the transport:
primary active transport and secondary active transport. In primary active transport, the energy is derived
directly from breakdown of adenosine triphosphate
(ATP) or of some other high-energy phosphate compound. In secondary active transport, the energy is
derived secondarily from energy that has been stored
in the form of ionic concentration differences of
secondary molecular or ionic substances between the
two sides of a cell membrane, created originally by
primary active transport. In both instances, transport
depends on carrier proteins that penetrate through the
cell membrane, as is true for facilitated diffusion.
However, in active transport, the carrier protein functions differently from the carrier in facilitated diffusion because it is capable of imparting energy to the
transported substance to move it against the electrochemical gradient. Following are some examples
of primary active transport and secondary active
transport, with more detailed explanations of their
principles of function.
Primary Active Transport
Sodium-Potassium Pump
Among the substances that are transported by primary
active transport are sodium, potassium, calcium,
hydrogen, chloride, and a few other ions.
The active transport mechanism that has been
studied in greatest detail is the sodium-potassium
(Na+-K+) pump, a transport process that pumps
sodium ions outward through the cell membrane of all
cells and at the same time pumps potassium ions from
the outside to the inside. This pump is responsible for
maintaining the sodium and potassium concentration
differences across the cell membrane, as well as for
establishing a negative electrical voltage inside the
cells. Indeed, Chapter 5 shows that this pump is also
the basis of nerve function, transmitting nerve signals
throughout the nervous system.
Figure 4–11 shows the basic physical components of
the Na+-K+ pump. The carrier protein is a complex of
two separate globular proteins: a larger one called the
a subunit, with a molecular weight of about 100,000,
and a smaller one called the b subunit, with a molecular weight of about 55,000. Although the function of
the smaller protein is not known (except that it might
anchor the protein complex in the lipid membrane),
the larger protein has three specific features that are
important for the functioning of the pump:
1. It has three receptor sites for binding sodium ions
on the portion of the protein that protrudes to the
inside of the cell.
2. It has two receptor sites for potassium ions on the
outside.
3. The inside portion of this protein near the sodium
binding sites has ATPase activity.
To put the pump into perspective: When two potassium ions bind on the outside of the carrier protein
3Na+
Outside
2K+
ATPase
ATP
Inside
3Na+
2K+
ADP
+
Pi
Figure 4–11
Postulated mechanism of the sodium-potassium pump. ADP,
adenosine diphosphate; ATP, adenosine triphosphate; Pi,
phosphate ion.
and three sodium ions bind on the inside, the ATPase
function of the protein becomes activated. This then
cleaves one molecule of ATP, splitting it to adenosine
diphosphate (ADP) and liberating a high-energy
phosphate bond of energy. This liberated energy is
then believed to cause a chemical and conformational
change in the protein carrier molecule, extruding the
three sodium ions to the outside and the two potassium ions to the inside.
As with other enzymes, the Na+-K+ ATPase pump
can run in reverse. If the electrochemical gradients for
Na+ and K+ are experimentally increased enough so
that the energy stored in their gradients is greater than
the chemical energy of ATP hydrolysis, these ions will
move down their concentration gradients and the Na+K+ pump will synthesize ATP from ADP and phosphate. The phosphorylated form of the Na+-K+ pump,
therefore, can either donate its phosphate to ADP to
produce ATP or use the energy to change its conformation and pump Na+ out of the cell and K+ into the
cell. The relative concentrations of ATP, ADP, and
phosphate, as well as the electrochemical gradients for
Na+ and K+, determine the direction of the enzyme
reaction. For some cells, such as electrically active
nerve cells, 60 to 70 per cent of the cells’ energy
requirement may be devoted to pumping Na+ out of
the cell and K+ into the cell.
Importance of the Na+-K+ Pump for Controlling Cell Volume.
One of the most important functions of the Na+-K+
pump is to control the volume of each cell. Without
function of this pump, most cells of the body would
swell until they burst. The mechanism for controlling
the volume is as follows: Inside the cell are large
numbers of proteins and other organic molecules that
cannot escape from the cell. Most of these are negatively charged and therefore attract large numbers of
potassium, sodium, and other positive ions as well. All
these molecules and ions then cause osmosis of water
to the interior of the cell. Unless this is checked, the
54
Unit II
Membrane Physiology, Nerve, and Muscle
cell will swell indefinitely until it bursts. The normal
mechanism for preventing this is the Na+-K+ pump.
Note again that this device pumps three Na+ ions to
the outside of the cell for every two K+ ions pumped
to the interior. Also, the membrane is far less permeable to sodium ions than to potassium ions, so that
once the sodium ions are on the outside, they have a
strong tendency to stay there. Thus, this represents a
net loss of ions out of the cell, which initiates osmosis
of water out of the cell as well.
If a cell begins to swell for any reason, this automatically activates the Na+-K+ pump, moving still more
ions to the exterior and carrying water with them.
Therefore, the Na+-K+ pump performs a continual surveillance role in maintaining normal cell volume.
Electrogenic Nature of the Na+-K+ Pump. The fact that the
Na+-K+ pump moves three Na+ ions to the exterior for
every two K+ ions to the interior means that a net of one
positive charge is moved from the interior of the cell to
the exterior for each cycle of the pump. This creates positivity outside the cell but leaves a deficit of positive ions
inside the cell; that is, it causes negativity on the inside.
Therefore, the Na+-K+ pump is said to be electrogenic
because it creates an electrical potential across the cell
membrane. As discussed in Chapter 5, this electrical
potential is a basic requirement in nerve and muscle
fibers for transmitting nerve and muscle signals.
Primary Active Transport of Calcium Ions
Another important primary active transport mechanism is the calcium pump. Calcium ions are normally
maintained at extremely low concentration in the
intracellular cytosol of virtually all cells in the body, at
a concentration about 10,000 times less than that in the
extracellular fluid. This is achieved mainly by two
primary active transport calcium pumps. One is in the
cell membrane and pumps calcium to the outside of
the cell. The other pumps calcium ions into one or
more of the intracellular vesicular organelles of the
cell, such as the sarcoplasmic reticulum of muscle cells
and the mitochondria in all cells. In each of these
instances, the carrier protein penetrates the membrane
and functions as an enzyme ATPase, having the same
capability to cleave ATP as the ATPase of the sodium
carrier protein. The difference is that this protein has
a highly specific binding site for calcium instead of for
sodium.
Primary Active Transport of Hydrogen Ions
At two places in the body, primary active transport of
hydrogen ions is very important: (1) in the gastric
glands of the stomach, and (2) in the late distal tubules
and cortical collecting ducts of the kidneys.
In the gastric glands, the deep-lying parietal cells
have the most potent primary active mechanism for
transporting hydrogen ions of any part of the body.
This is the basis for secreting hydrochloric acid in the
stomach digestive secretions. At the secretory ends of
the gastric gland parietal cells, the hydrogen ion concentration is increased as much as a millionfold and
then released into the stomach along with chloride
ions to form hydrochloric acid.
In the renal tubules are special intercalated cells in
the late distal tubules and cortical collecting ducts that
also transport hydrogen ions by primary active transport. In this case, large amounts of hydrogen ions are
secreted from the blood into the urine for the purpose
of eliminating excess hydrogen ions from the body
fluids. The hydrogen ions can be secreted into the
urine against a concentration gradient of about
900-fold.
Energetics of Primary Active Transport
The amount of energy required to transport a substance actively through a membrane is determined by
how much the substance is concentrated during transport. Compared with the energy required to concentrate a substance 10-fold, to concentrate it 100-fold
requires twice as much energy, and to concentrate
it 1000-fold requires three times as much energy.
In other words, the energy required is proportional
to the logarithm of the degree that the substance
is concentrated, as expressed by the following
formula:
Energy (in calories per osmole) = 1400 log
C1
C2
Thus, in terms of calories, the amount of energy
required to concentrate 1 osmole of substance 10-fold
is about 1400 calories; or to concentrate it 100-fold,
2800 calories. One can see that the energy expenditure
for concentrating substances in cells or for removing
substances from cells against a concentration gradient
can be tremendous. Some cells, such as those lining the
renal tubules and many glandular cells, expend as
much as 90 per cent of their energy for this purpose
alone.
Secondary Active Transport—
Co-Transport and Counter-Transport
When sodium ions are transported out of cells by
primary active transport, a large concentration gradient of sodium ions across the cell membrane usually
develops—high concentration outside the cell and
very low concentration inside. This gradient represents
a storehouse of energy because the excess sodium
outside the cell membrane is always attempting to
diffuse to the interior. Under appropriate conditions,
this diffusion energy of sodium can pull other substances along with the sodium through the cell membrane.This phenomenon is called co-transport; it is one
form of secondary active transport.
For sodium to pull another substance along with it,
a coupling mechanism is required. This is achieved by
means of still another carrier protein in the cell membrane. The carrier in this instance serves as an attachment point for both the sodium ion and the substance
to be co-transported. Once they both are attached, the
energy gradient of the sodium ion causes both the
sodium ion and the other substance to be transported
together to the interior of the cell.
Chapter 4
Transport of Substances Through the Cell Membrane
In counter-transport, sodium ions again attempt to
diffuse to the interior of the cell because of their large
concentration gradient. However, this time, the substance to be transported is on the inside of the cell and
must be transported to the outside. Therefore, the
sodium ion binds to the carrier protein where it projects to the exterior surface of the membrane, while
the substance to be counter-transported binds to the
interior projection of the carrier protein. Once both
have bound, a conformational change occurs, and
energy released by the sodium ion moving to the
interior causes the other substance to move to the
exterior.
Co-Transport of Glucose and Amino Acids
Along with Sodium Ions
Glucose and many amino acids are transported into
most cells against large concentration gradients; the
mechanism of this is entirely by co-transport, as shown
in Figure 4–12. Note that the transport carrier protein
has two binding sites on its exterior side, one for
sodium and one for glucose. Also, the concentration of
sodium ions is very high on the outside and very low
inside, which provides energy for the transport. A
special property of the transport protein is that a conformational change to allow sodium movement to the
interior will not occur until a glucose molecule also
attaches. When they both become attached, the conformational change takes place automatically, and the
sodium and glucose are transported to the inside of the
cell at the same time. Hence, this is a sodium-glucose
co-transport mechanism.
Sodium co-transport of the amino acids occurs in the
same manner as for glucose, except that it uses a different set of transport proteins. Five amino acid transport proteins have been identified, each of which is
responsible for transporting one subset of amino acids
with specific molecular characteristics.
Sodium co-transport of glucose and amino acids
occurs especially through the epithelial cells of the
intestinal tract and the renal tubules of the kidneys to
promote absorption of these substances into the
blood, as is discussed in later chapters.
Na+ Glucose
Na-binding
site
Glucose-binding
site
Na+
Glucose
Figure 4–12
Postulated mechanism for sodium co-transport of glucose.
55
Other important co-transport mechanisms in at
least some cells include co-transport of chloride ions,
iodine ions, iron ions, and urate ions.
Sodium Counter-Transport of Calcium and
Hydrogen Ions
Two especially important counter-transport mechanisms (transport in a direction opposite to the primary
ion) are sodium-calcium counter-transport and
sodium-hydrogen counter-transport.
Sodium-calcium counter-transport occurs through
all or almost all cell membranes, with sodium ions
moving to the interior and calcium ions to the exterior,
both bound to the same transport protein in a countertransport mode. This is in addition to primary active
transport of calcium that occurs in some cells.
Sodium-hydrogen counter-transport occurs in
several tissues. An especially important example is in
the proximal tubules of the kidneys, where sodium ions
move from the lumen of the tubule to the interior
of the tubular cell, while hydrogen ions are countertransported into the tubule lumen. As a mechanism for
concentrating hydrogen ions, counter-transport is not
nearly as powerful as the primary active transport of
hydrogen ions that occurs in the more distal renal
tubules, but it can transport extremely large numbers
of hydrogen ions, thus making it a key to hydrogen
ion control in the body fluids, as discussed in detail in
Chapter 30.
Active Transport Through
Cellular Sheets
At many places in the body, substances must be transported all the way through a cellular sheet instead of
simply through the cell membrane. Transport of this
type occurs through the (1) intestinal epithelium, (2)
epithelium of the renal tubules, (3) epithelium of all
exocrine glands, (4) epithelium of the gallbladder, and
(5) membrane of the choroid plexus of the brain and
other membranes.
The basic mechanism for transport of a substance
through a cellular sheet is (1) active transport through
the cell membrane on one side of the transporting cells
in the sheet, and then (2) either simple diffusion or
facilitated diffusion through the membrane on the
opposite side of the cell.
Figure 4–13 shows a mechanism for transport
of sodium ions through the epithelial sheet of the
intestines, gallbladder, and renal tubules. This figure
shows that the epithelial cells are connected together
tightly at the luminal pole by means of junctions called
“kisses.” The brush border on the luminal surfaces of
the cells is permeable to both sodium ions and water.
Therefore, sodium and water diffuse readily from the
lumen into the interior of the cell. Then, at the basal
and lateral membranes of the cells, sodium ions are
actively transported into the extracellular fluid of
the surrounding connective tissue and blood vessels.
This creates a high sodium ion concentration gradient
across these membranes, which in turn causes osmosis
56
Unit II
Na+
Active
transport
Osmosis
Active
transport
Na+
Osmosis
Active
transport
Lumen
Na+
Na+
References
Basement
membrane
Na+
and
H2O
Connective tissue
Brush
border
Membrane Physiology, Nerve, and Muscle
Osmosis
Diffusion
Figure 4–13
Basic mechanism of active transport across a layer of cells.
of water as well. Thus, active transport of sodium ions
at the basolateral sides of the epithelial cells results in
transport not only of sodium ions but also of water.
These are the mechanisms by which almost all the
nutrients, ions, and other substances are absorbed into
the blood from the intestine; they are also the way the
same substances are reabsorbed from the glomerular
filtrate by the renal tubules.
Throughout this text are numerous examples of the
different types of transport discussed in this chapter.
Agre P, Kozono D: Aquaporin water channels: molecular
mechanisms for human diseases. FEBS Lett 555:72, 2003.
Benos DJ, Stanton BA: Functional domains within the
degenerin/epithelial sodium channel (Deg/ENaC) superfamily of ion channels. J Physiol 520:631, 1999.
Caplan MJ: Ion pump sorting in polarized renal epithelial
cells. Kidney Int 60:427, 2001.
Decoursey TE: Voltage-gated proton channels and other
proton transfer pathways. Physiol Rev 83:475, 2003.
De Weer P: A century of thinking about cell membranes.
Annu Rev Physiol 62:919, 2000.
Dolphin AC: G protein modulation of voltage-gated calcium
channels. Pharmacol Rev 55:607, 2003.
Jentsch TJ, Stein V, Weinreich F, Zdebik AA: Molecular
structure and physiological function of chloride channels.
Physiol Rev 82:503, 2002.
Kaupp UB, Seifert R: Cyclic nucleotide-gated ion channels.
Physiol Rev 82:769, 2002.
Kellenberger S, Schild L: Epithelial sodium channel/
degenerin family of ion channels: a variety of functions for
a shared structure. Physiol Rev 82:735, 2002.
MacKinnon R: Potassium channels. FEBS Lett 555:62, 2003.
Peres A, Giovannardi S, Bossi E, Fesce R: Electrophysiological insights into the mechanism of ion-coupled cotransporters. News Physiol Sci 19:80, 2004.
Philipson KD, Nicoll DA, Ottolia M, et al: The Na+/Ca2+
exchange molecule: an overview. Ann N Y Acad Sci 976:1,
2002.
Rossier BC, Pradervand S, Schild L, Hummler E: Epithelial
sodium channel and the control of sodium balance: interaction between genetic and environmental factors. Annu
Rev Physiol 64:877, 2002.
Russell JM: Sodium-potassium-chloride cotransport. Physiol
Rev 80:211, 2000.
C
H
A
P
T
E
R
5
Membrane Potentials and
Action Potentials
Electrical potentials exist across the membranes of
virtually all cells of the body. In addition, some cells,
such as nerve and muscle cells, are capable of generating rapidly changing electrochemical impulses
at their membranes, and these impulses are used to
transmit signals along the nerve or muscle membranes. In still other types of cells, such as glandular
cells, macrophages, and ciliated cells, local changes
in membrane potentials also activate many of the cells’ functions. The present
discussion is concerned with membrane potentials generated both at rest and
during action by nerve and muscle cells.
Basic Physics of Membrane Potentials
Membrane Potentials Caused by Diffusion
“Diffusion Potential” Caused by an Ion Concentration Difference on the Two Sides of the
Membrane. In Figure 5–1A, the potassium concentration is great inside a nerve
fiber membrane but very low outside the membrane. Let us assume that the
membrane in this instance is permeable to the potassium ions but not to any
other ions. Because of the large potassium concentration gradient from inside
toward outside, there is a strong tendency for extra numbers of potassium ions
to diffuse outward through the membrane. As they do so, they carry positive
electrical charges to the outside, thus creating electropositivity outside the
membrane and electronegativity inside because of negative anions that remain
behind and do not diffuse outward with the potassium. Within a millisecond or
so, the potential difference between the inside and outside, called the diffusion
potential, becomes great enough to block further net potassium diffusion to the
exterior, despite the high potassium ion concentration gradient. In the normal
mammalian nerve fiber, the potential difference required is about 94 millivolts,
with negativity inside the fiber membrane.
Figure 5–1B shows the same phenomenon as in Figure 5–1A, but this time
with high concentration of sodium ions outside the membrane and low sodium
inside. These ions are also positively charged. This time, the membrane is highly
permeable to the sodium ions but impermeable to all other ions. Diffusion of
the positively charged sodium ions to the inside creates a membrane potential
of opposite polarity to that in Figure 5–1A, with negativity outside and positivity inside. Again, the membrane potential rises high enough within milliseconds
to block further net diffusion of sodium ions to the inside; however, this time,
in the mammalian nerve fiber, the potential is about 61 millivolts positive inside
the fiber.
Thus, in both parts of Figure 5–1, we see that a concentration difference of
ions across a selectively permeable membrane can, under appropriate conditions, create a membrane potential. In later sections of this chapter, we show
that many of the rapid changes in membrane potentials observed during nerve
and muscle impulse transmission result from the occurrence of such rapidly
changing diffusion potentials.
57
58
Unit II
Membrane Physiology, Nerve, and Muscle
DIFFUSION POTENTIALS
Nerve fiber
(Anions)– Nerve fiber
+ –
– +
+
– – +
(Anions)–
(Anions)
+ –
– +
+
– +
– +
+ –
+
– +
– +
+ –
+
– +
+
+
+
+
K
Na
K
Na
– +
+ –
+
– +
– +
+ –
+
– +
– +
+
+ –
– +
(– 94 mV)
(+ 61 mV)
+
– +
– +
+ –
+
– +
– +
+ –
– +
+
(Anions)–
A
–
–
–
–
–
–
–
–
–
–
B
Figure 5–1
A, Establishment of a “diffusion” potential across a nerve fiber
membrane, caused by diffusion of potassium ions from inside the
cell to outside through a membrane that is selectively permeable
only to potassium. B, Establishment of a “diffusion potential” when
the nerve fiber membrane is permeable only to sodium ions. Note
that the internal membrane potential is negative when potassium
ions diffuse and positive when sodium ions diffuse because of
opposite concentration gradients of these two ions.
Relation of the Diffusion Potential to the Concentration Difference—The Nernst Potential. The diffusion potential level
across a membrane that exactly opposes the net diffusion of a particular ion through the membrane is called
the Nernst potential for that ion, a term that was introduced in Chapter 4. The magnitude of this Nernst
potential is determined by the ratio of the concentrations of that specific ion on the two sides of the membrane. The greater this ratio, the greater the tendency
for the ion to diffuse in one direction, and therefore
the greater the Nernst potential required to prevent
additional net diffusion. The following equation, called
the Nernst equation, can be used to calculate the
Nernst potential for any univalent ion at normal body
temperature of 98.6°F (37°C):
EMF (millivolts) = ± 61 log
Concentration inside
Concentration outside
where EMF is electromotive force.
When using this formula, it is usually assumed that
the potential in the extracellular fluid outside the
membrane remains at zero potential, and the Nernst
potential is the potential inside the membrane. Also,
the sign of the potential is positive (+) if the ion diffusing from inside to outside is a negative ion, and it
is negative (–) if the ion is positive. Thus, when the concentration of positive potassium ions on the inside is
10 times that on the outside, the log of 10 is 1, so that
the Nernst potential calculates to be –61 millivolts
inside the membrane.
three factors: (1) the polarity of the electrical charge
of each ion, (2) the permeability of the membrane (P)
to each ion, and (3) the concentrations (C) of the
respective ions on the inside (i) and outside (o) of
the membrane. Thus, the following formula, called the
Goldman equation, or the Goldman-Hodgkin-Katz
equation, gives the calculated membrane potential on
the inside of the membrane when two univalent positive ions, sodium (Na+) and potassium (K+), and one
univalent negative ion, chloride (Cl–), are involved.
EMF (millivolts)
C Na +i PNa + + C K +i PK + + CCl - o PCl = -61 ◊ log
C Na + o PNa + + C K - o PK + + CCl -i PCl Let us study the importance and the meaning of this
equation. First, sodium, potassium, and chloride ions
are the most important ions involved in the development of membrane potentials in nerve and muscle
fibers, as well as in the neuronal cells in the nervous
system. The concentration gradient of each of these
ions across the membrane helps determine the voltage
of the membrane potential.
Second, the degree of importance of each of the ions
in determining the voltage is proportional to the membrane permeability for that particular ion.That is, if the
membrane has zero permeability to both potassium
and chloride ions, the membrane potential becomes
entirely dominated by the concentration gradient of
sodium ions alone, and the resulting potential will be
equal to the Nernst potential for sodium. The same
holds for each of the other two ions if the membrane
should become selectively permeable for either one of
them alone.
Third, a positive ion concentration gradient from
inside the membrane to the outside causes electronegativity inside the membrane. The reason for this is that
excess positive ions diffuse to the outside when their
concentration is higher inside than outside.This carries
positive charges to the outside but leaves the nondiffusible negative anions on the inside, thus creating
electronegativity on the inside. The opposite effect
occurs when there is a gradient for a negative ion. That
is, a chloride ion gradient from the outside to the inside
causes negativity inside the cell because excess negatively charged chloride ions diffuse to the inside, while
leaving the nondiffusible positive ions on the outside.
Fourth, as explained later, the permeability of the
sodium and potassium channels undergoes rapid
changes during transmission of a nerve impulse,
whereas the permeability of the chloride channels
does not change greatly during this process. Therefore,
rapid changes in sodium and potassium permeability
are primarily responsible for signal transmission in
nerves, which is the subject of most of the remainder
of this chapter.
Calculation of the Diffusion Potential
When the Membrane Is Permeable to
Several Different Ions
Measuring the Membrane
Potential
When a membrane is permeable to several different
ions, the diffusion potential that develops depends on
The method for measuring the membrane potential is
simple in theory but often difficult in practice because
Chapter 5
59
Membrane Potentials and Action Potentials
Nerve fiber
–+–+–+–+–+–+–+–
+–++––+–+––++–+
–+–+–+–+–+–+–+–
+–++––+–+––++–+
–+–+–+–+–+–+–+–
+–++––+–+––++–+
–+–+–+–+–+–+–+–
+–++––+–+––++–+
–+–+–+–+–+–+–+–
+–++––+–+––++–+
0
+
I
KC
+++++++++++
––––––––––
+++++
–––––
Silver–silver
chloride
electrode
– – – – – – – – – (– 90 – – – – – – –
+ + + + + + + + + mV) + + + + + + + +
Electrical potential
(millivolts)
—
0
–90
Figure 5–2
Measurement of the membrane potential of the nerve fiber using
a microelectrode.
of the small size of most of the fibers. Figure 5–2 shows
a small pipette filled with an electrolyte solution. The
pipette is impaled through the cell membrane to
the interior of the fiber. Then another electrode, called
the “indifferent electrode,” is placed in the extracellular
fluid, and the potential difference between the inside
and outside of the fiber is measured using an appropriate voltmeter. This voltmeter is a highly sophisticated
electronic apparatus that is capable of measuring very
small voltages despite extremely high resistance to electrical flow through the tip of the micropipette, which
has a lumen diameter usually less than 1 micrometer
and a resistance more than a million ohms. For recording rapid changes in the membrane potential during
transmission of nerve impulses, the microelectrode is
connected to an oscilloscope, as explained later in the
chapter.
The lower part of Figure 5–3 shows the electrical
potential that is measured at each point in or near the
nerve fiber membrane, beginning at the left side of the
figure and passing to the right. As long as the electrode
is outside the nerve membrane, the recorded potential
is zero, which is the potential of the extracellular fluid.
Then, as the recording electrode passes through the
voltage change area at the cell membrane (called the
electrical dipole layer), the potential decreases abruptly
to –90 millivolts. Moving across the center of the fiber,
the potential remains at a steady –90-millivolt level but
reverses back to zero the instant it passes through the
membrane on the opposite side of the fiber.
To create a negative potential inside the membrane,
only enough positive ions to develop the electrical
dipole layer at the membrane itself must be transported
outward. All the remaining ions inside the nerve fiber
can be both positive and negative, as shown in the upper
panel of Figure 5–3. Therefore, an incredibly small
number of ions needs to be transferred through the
membrane to establish the normal “resting potential” of
–90 millivolts inside the nerve fiber; this means that only
about 1/3,000,000 to 1/100,000,000 of the total positive
charges inside the fiber needs to be transferred. Also, an
Figure 5–3
Distribution of positively and negatively charged ions in the extracellular fluid surrounding a nerve fiber and in the fluid inside the
fiber; note the alignment of negative charges along the inside
surface of the membrane and positive charges along the outside
surface. The lower panel displays the abrupt changes in membrane potential that occur at the membranes on the two sides of
the fiber.
equally small number of positive ions moving from
outside to inside the fiber can reverse the potential from
–90 millivolts to as much as +35 millivolts within as little
as 1/10,000 of a second. Rapid shifting of ions in this
manner causes the nerve signals discussed in subsequent sections of this chapter.
Resting Membrane Potential
of Nerves
The resting membrane potential of large nerve fibers
when not transmitting nerve signals is about –90
millivolts. That is, the potential inside the fiber is 90
millivolts more negative than the potential in the extracellular fluid on the outside of the fiber. In the next few
paragraphs, we explain all the factors that determine
the level of this resting potential, but before doing so,
we must describe the transport properties of the resting
nerve membrane for sodium and potassium.
Active Transport of Sodium and Potassium Ions Through the
Membrane—The Sodium-Potassium (Na+-K+) Pump. First, let
us recall from Chapter 4 that all cell membranes of the
body have a powerful Na+-K+ that continually pumps
sodium ions to the outside of the cell and potassium
ions to the inside, as illustrated on the left-hand side in
Figure 5–4. Further, note that this is an electrogenic
pump because more positive charges are pumped to
the outside than to the inside (three Na+ ions to the
outside for each two K+ ions to the inside), leaving a
net deficit of positive ions on the inside; this causes a
negative potential inside the cell membrane.
60
Unit II
Membrane Physiology, Nerve, and Muscle
Outside
3Na+
2K+
Na+
K+
K+
4 mEq/L
K+
140 mEq/L
(–94 mV)
(-94 mV)
ATP Na+
K+
+
+
Na -K pump
Na+
ADP
K+
K+ -Na+
"leak" channels
A
Na+
K+
142 mEq/L
4 mEq/L
Na+
14 mEq/L
K+
140 mEq/L
(+61 mV)
(–94 mV)
Figure 5–4
Functional characteristics of the Na+-K+ pump and of the K+-Na+
“leak” channels. ADP, adenosine diphosphate; ATP, adenosine
triphosphate.
(–86 mV)
B
+ + Diffusion
The Na+-K+ also causes large concentration gradients for sodium and potassium across the resting nerve
membrane. These gradients are the following:
Na+
Na+ (outside): 142 mEq/L
Na+ (inside):
14 mEq/L
+
K (outside):
4 mEq/L
K+ (inside):
140 mEq/L
Na
K
+ pump
+ 4 mEq/L + + + + (Anions)- + K+
+
outside
/Na
= 0.1
+
/K outside = 35.0
Leakage of Potassium and Sodium Through the Nerve Membrane. The right side of Figure 5–4 shows a channel
protein in the nerve membrane through which potassium and sodium ions can leak, called a potassiumsodium (K+-Na+) “leak” channel. The emphasis is on
potassium leakage because, on average, the channels
are far more permeable to potassium than to sodium,
normally about 100 times as permeable. As discussed
later, this differential in permeability is exceedingly
important in determining the level of the normal
resting membrane potential.
Origin of the Normal Resting
Membrane Potential
Figure 5–5 shows the important factors in the establishment of the normal resting membrane potential of
–90 millivolts. They are as follows.
Contribution of the Potassium Diffusion Potential. In Figure
5–5A, we make the assumption that the only movement of ions through the membrane is diffusion of
potassium ions, as demonstrated by the open channels
between the potassium symbols (K+) inside and
outside the membrane. Because of the high ratio of
potassium ions inside to outside, 35:1, the Nernst
-
Na+
14 mEq/L
Diffusion
The ratios of these two respective ions from the inside
to the outside are
+
inside
+
inside
+
pump
+ 142 mEq/L + + + -
K+
140 mEq/L
(–90 mV)
(Anions)-
-
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
C
Figure 5–5
Establishment of resting membrane potentials in nerve fibers
under three conditions: A, when the membrane potential is caused
entirely by potassium diffusion alone; B, when the membrane
potential is caused by diffusion of both sodium and potassium
ions; and C, when the membrane potential is caused by diffusion
of both sodium and potassium ions plus pumping of both these
ions by the Na+-K+ pump.
potential corresponding to this ratio is –94 millivolts
because the logarithm of 35 is 1.54, and this times –61
millivolts is –94 millivolts. Therefore, if potassium ions
were the only factor causing the resting potential, the
resting potential inside the fiber would be equal to –94
millivolts, as shown in the figure.
Contribution of Sodium Diffusion Through the Nerve Membrane.
Figure 5–5B shows the addition of slight permeability
of the nerve membrane to sodium ions, caused by the
minute diffusion of sodium ions through the K+-Na+
61
Membrane Potentials and Action Potentials
leak channels. The ratio of sodium ions from inside to
outside the membrane is 0.1, and this gives a calculated
Nernst potential for the inside of the membrane of
+61 millivolts. But also shown in Figure 5–5B is the
Nernst potential for potassium diffusion of –94 millivolts. How do these interact with each other, and what
will be the summated potential? This can be answered
by using the Goldman equation described previously.
Intuitively, one can see that if the membrane is highly
permeable to potassium but only slightly permeable to
sodium, it is logical that the diffusion of potassium contributes far more to the membrane potential than does
the diffusion of sodium. In the normal nerve fiber, the
permeability of the membrane to potassium is about
100 times as great as its permeability to sodium. Using
this value in the Goldman equation gives a potential
inside the membrane of –86 millivolts, which is near
the potassium potential shown in the figure.
0
—
I
KC
+++++
–––––
––––
++++
++++
––––
––––––
++++++
+35
D ep
on
Millivolts
io n
0
rizati
Nerve signals are transmitted by action potentials,
which are rapid changes in the membrane potential
that spread rapidly along the nerve fiber membrane.
Each action potential begins with a sudden change
from the normal resting negative membrane potential
to a positive potential and then ends with an almost
equally rapid change back to the negative potential.
To conduct a nerve signal, the action potential moves
along the nerve fiber until it comes to the fiber’s
end.
The upper panel of Figure 5–6 shows the changes
that occur at the membrane during the action potential, with transfer of positive charges to the interior of
the fiber at its onset and return of positive charges to
the exterior at its end. The lower panel shows graphically the successive changes in membrane potential
over a few 10,000ths of a second, illustrating the
––––
++++
R e p ola
Nerve Action Potential
++++
––––
Silver–silver
chloride
electrode
Overshoot
Contribution of the Na+-K+ Pump. In Figure 5–5C, the
Na+-K+ pump is shown to provide an additional contribution to the resting potential. In this figure, there
is continuous pumping of three sodium ions to the
outside for each two potassium ions pumped to the
inside of the membrane. The fact that more sodium
ions are being pumped to the outside than potassium
to the inside causes continual loss of positive charges
from inside the membrane; this creates an additional
degree of negativity (about –4 millivolts additional) on
the inside beyond that which can be accounted for by
diffusion alone. Therefore, as shown in Figure 5–5C,
the net membrane potential with all these factors
operative at the same time is about –90 millivolts.
In summary, the diffusion potentials alone caused by
potassium and sodium diffusion would give a membrane potential of about –86 millivolts, almost all of
this being determined by potassium diffusion. Then, an
additional –4 millivolts is contributed to the membrane potential by the continuously acting electrogenic Na+-K+ pump, giving a net membrane potential
of –90 millivolts.
+
o l a ri z a t
Chapter 5
–90
Resting
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7
Milliseconds
Figure 5–6
Typical action potential recorded by the method shown in the
upper panel of the figure.
explosive onset of the action potential and the
almost equally rapid recovery.
The successive stages of the action potential are as
follows.
Resting Stage. This is the resting membrane potential
before the action potential begins. The membrane is
said to be “polarized” during this stage because of the
–90 millivolts negative membrane potential that is
present.
Depolarization Stage. At this time, the membrane sud-
denly becomes very permeable to sodium ions, allowing tremendous numbers of positively charged sodium
ions to diffuse to the interior of the axon. The normal
“polarized” state of –90 millivolts is immediately neutralized by the inflowing positively charged sodium
ions, with the potential rising rapidly in the positive
direction. This is called depolarization. In large nerve
fibers, the great excess of positive sodium ions moving
to the inside causes the membrane potential to actually “overshoot” beyond the zero level and to become
somewhat positive. In some smaller fibers, as well as in
62
Unit II
Membrane Physiology, Nerve, and Muscle
many central nervous system neurons, the potential
merely approaches the zero level and does not overshoot to the positive state.
Repolarization Stage. Within a few 10,000ths of a second
after the membrane becomes highly permeable to
sodium ions, the sodium channels begin to close and
the potassium channels open more than normal.
Then, rapid diffusion of potassium ions to the exterior
re-establishes the normal negative resting membrane potential. This is called repolarization of the
membrane.
To explain more fully the factors that cause both
depolarization and repolarization, we need to describe
the special characteristics of two other types of transport channels through the nerve membrane: the
voltage-gated sodium and potassium channels.
Voltage-Gated Sodium and
Potassium Channels
The necessary actor in causing both depolarization
and repolarization of the nerve membrane during the
action potential is the voltage-gated sodium channel. A
voltage-gated potassium channel also plays an important role in increasing the rapidity of repolarization of
the membrane. These two voltage-gated channels are
in addition to the Na+-K+ pump and the K+-Na+ leak
channels.
Voltage-Gated Sodium Channel—Activation
and Inactivation of the Channel
The upper panel of Figure 5–7 shows the voltage-gated
sodium channel in three separate states. This channel
has two gates—one near the outside of the channel
called the activation gate, and another near the inside
called the inactivation gate. The upper left of the figure
depicts the state of these two gates in the normal
resting membrane when the membrane potential is
–90 millivolts. In this state, the activation gate is closed,
which prevents any entry of sodium ions to the interior of the fiber through these sodium channels.
Activation of the Sodium Channel. When the membrane
potential becomes less negative than during the
resting state, rising from –90 millivolts toward zero, it
finally reaches a voltage—usually somewhere between
–70 and –50 millivolts—that causes a sudden conformational change in the activation gate, flipping it all
the way to the open position. This is called the activated state; during this state, sodium ions can pour
inward through the channel, increasing the sodium
permeability of the membrane as much as 500- to
5000-fold.
Inactivation of the Sodium Channel. The upper right panel
of Figure 5–7 shows a third state of the sodium
channel. The same increase in voltage that opens the
activation gate also closes the inactivation gate. The
inactivation gate, however, closes a few 10,000ths of
a second after the activation gate opens. That is, the
Activation
gate
Na+
Inactivation
gate
Resting
(-90 mV)
Inside
Resting
(-90 mV)
K+
Na+
Na+
Activated
(-90 to +35 mV)
Inactivated
(+35 to -90 mV,
delayed)
K+
Slow activation
(+35 to -90 mV)
Figure 5–7
Characteristics of the voltage-gated sodium (top) and potassium
(bottom) channels, showing successive activation and inactivation of the sodium channels and delayed activation of the potassium channels when the membrane potential is changed from the
normal resting negative value to a positive value.
conformational change that flips the inactivation gate
to the closed state is a slower process than the conformational change that opens the activation gate.
Therefore, after the sodium channel has remained
open for a few 10,000ths of a second, the inactivation
gate closes, and sodium ions no longer can pour to the
inside of the membrane. At this point, the membrane
potential begins to recover back toward the resting
membrane state, which is the repolarization process.
Another important characteristic of the sodium
channel inactivation process is that the inactivation
gate will not reopen until the membrane potential
returns to or near the original resting membrane
potential level. Therefore, it usually is not possible for
the sodium channels to open again without the nerve
fiber’s first repolarizing.
Voltage-Gated Potassium Channel and
Its Activation
The lower panel of Figure 5–7 shows the voltage-gated
potassium channel in two states: during the resting
state (left) and toward the end of the action potential
(right). During the resting state, the gate of the potassium channel is closed, and potassium ions are prevented from passing through this channel to the
exterior. When the membrane potential rises from
–90 millivolts toward zero, this voltage change causes
a conformational opening of the gate and allows
increased potassium diffusion outward through the
channel. However, because of the slight delay in
opening of the potassium channels, for the most part,
Membrane Potentials and Action Potentials
Chapter 5
they open just at the same time that the sodium channels are beginning to close because of inactivation.
Thus, the decrease in sodium entry to the cell and the
simultaneous increase in potassium exit from the cell
combine to speed the repolarization process, leading
to full recovery of the resting membrane potential
within another few 10,000ths of a second.
Research Method for Measuring the Effect of Voltage on Opening
and Closing of the Voltage-Gated Channels—The “Voltage
Clamp.” The original research that led to quantitative
understanding of the sodium and potassium channels
was so ingenious that it led to Nobel Prizes for the scientists responsible, Hodgkin and Huxley. The essence of
these studies is shown in Figures 5–8 and 5–9.
Amplifier
Electrode
in fluid
Voltage
electrode
Current
electrode
Figure 5–8
“Voltage clamp” method for studying flow of ions through specific
channels.
Activation
30
20
10
0
–90 mV
ac
In
Conductance
(mmho/cm2 )
Na+ channel
K+ channel
ti v
at
+10 mV
Membrane potential
0
1
2
Time (milliseconds)
–90 mV
3
Figure 5–9
Typical changes in conductance of sodium and potassium ion
channels when the membrane potential is suddenly increased
from the normal resting value of –90 millivolts to a positive value
of +10 millivolts for 2 milliseconds. This figure shows that the
sodium channels open (activate) and then close (inactivate)
before the end of the 2 milliseconds, whereas the potassium channels only open (activate), and the rate of opening is much slower
than that of the sodium channels.
63
Figure 5–8 shows an experimental apparatus called a
voltage clamp, which is used to measure flow of ions
through the different channels. In using this apparatus,
two electrodes are inserted into the nerve fiber. One of
these is to measure the voltage of the membrane potential, and the other is to conduct electrical current into
or out of the nerve fiber. This apparatus is used in the
following way: The investigator decides which voltage
he or she wants to establish inside the nerve fiber. The
electronic portion of the apparatus is then adjusted to
the desired voltage, and this automatically injects either
positive or negative electricity through the current electrode at whatever rate is required to hold the voltage,
as measured by the voltage electrode, at the level set by
the operator. When the membrane potential is suddenly
increased by this voltage clamp from –90 millivolts to
zero, the voltage-gated sodium and potassium channels
open, and sodium and potassium ions begin to pour
through the channels. To counterbalance the effect of
these ion movements on the desired setting of the intracellular voltage, electrical current is injected automatically through the current electrode of the voltage clamp
to maintain the intracellular voltage at the required
steady zero level. To achieve this, the current injected
must be equal to but of opposite polarity to the net
current flow through the membrane channels. To
measure how much current flow is occurring at each
instant, the current electrode is connected to an oscilloscope that records the current flow, as demonstrated on
the screen of the oscilloscope in Figure 5–8. Finally, the
investigator adjusts the concentrations of the ions to
other than normal levels both inside and outside the
nerve fiber and repeats the study. This can be done
easily when using large nerve fibers removed from some
crustaceans, especially the giant squid axon, which in
some cases is as large as 1 millimeter in diameter. When
sodium is the only permeant ion in the solutions inside
and outside the squid axon, the voltage clamp measures
current flow only through the sodium channels. When
potassium is the only permeant ion, current flow only
through the potassium channels is measured.
Another means for studying the flow of ions through
an individual type of channel is to block one type of
channel at a time. For instance, the sodium channels can
be blocked by a toxin called tetrodotoxin by applying it
to the outside of the cell membrane where the sodium
activation gates are located. Conversely, tetraethylammonium ion blocks the potassium channels when it is
applied to the interior of the nerve fiber.
Figure 5–9 shows typical changes in conductance of
the voltage-gated sodium and potassium channels when
the membrane potential is suddenly changed by use of
the voltage clamp from –90 millivolts to +10 millivolts
and then, 2 milliseconds later, back to –90 millivolts.
Note the sudden opening of the sodium channels (the
activation stage) within a small fraction of a millisecond
after the membrane potential is increased to the positive value. However, during the next millisecond or so,
the sodium channels automatically close (the inactivation stage).
Note the opening (activation) of the potassium channels. These open slowly and reach their full open state
only after the sodium channels have almost completely
closed. Further, once the potassium channels open, they
remain open for the entire duration of the positive
membrane potential and do not close again until after
the membrane potential is decreased back to a negative
value.
64
Unit II
Membrane Physiology, Nerve, and Muscle
Summary of the Events That Cause
the Action Potential
Figure 5–10 shows in summary form the sequential
events that occur during and shortly after the action
potential. The bottom of the figure shows the changes
in membrane conductance for sodium and potassium
ions. During the resting state, before the action
potential begins, the conductance for potassium ions is
50 to 100 times as great as the conductance for sodium
ions. This is caused by much greater leakage of potassium ions than sodium ions through the leak channels.
However, at the onset of the action potential, the
sodium channels instantaneously become activated
and allow up to a 5000-fold increase in sodium conductance. Then the inactivation process closes the
sodium channels within another fraction of a millisecond. The onset of the action potential also causes
voltage gating of the potassium channels, causing them
to begin opening more slowly a fraction of a millisecond after the sodium channels open. At the end of the
action potential, the return of the membrane potential
to the negative state causes the potassium channels to
close back to their original status, but again, only after
an additional millisecond or more delay.
The middle portion of Figure 5–10 shows the ratio
of sodium conductance to potassium conductance at
each instant during the action potential, and above this
is the action potential itself. During the early portion
of the action potential, the ratio of sodium to potassium conductance increases more than 1000-fold.
Therefore, far more sodium ions flow to the interior of
the fiber than do potassium ions to the exterior. This
is what causes the membrane potential to become
positive at the action potential onset. Then the sodium
channels begin to close and the potassium channels to
open, so that the ratio of conductance shifts far in
favor of high potassium conductance but low sodium
conductance. This allows very rapid loss of potassium
ions to the exterior but virtually zero flow of
sodium ions to the interior. Consequently, the action
potential quickly returns to its baseline level.
Roles of Other Ions During
the Action Potential
+ 60
+ 40
+ 20
Overshoot
Na+ conductance
K+ conductance
100
0
–20
–40
–60
–80
–100
10
1
0.1
Positive
afterpotential
0.01
0.001
100
Action potential
Ratio of conductances
Na+
K+
10
Conductance
(mmho/cm2 )
Membrane potential (mV)
Thus far, we have considered only the roles of sodium
and potassium ions in the generation of the action
potential. At least two other types of ions must be considered: negative anions and calcium ions.
1
0.1
0.01
0.005
0
0.5
1.0
Milliseconds
1.5
Figure 5–10
Changes in sodium and potassium conductance during the
course of the action potential. Sodium conductance increases
several thousand-fold during the early stages of the action potential, whereas potassium conductance increases only about 30-fold
during the latter stages of the action potential and for a short
period thereafter. (These curves were constructed from theory presented in papers by Hodgkin and Huxley but transposed from
squid axon to apply to the membrane potentials of large mammalian nerve fibers.)
Impermeant Negatively Charged Ions (Anions) Inside the Nerve
Axon. Inside the axon are many negatively charged ions
that cannot go through the membrane channels. They
include the anions of protein molecules and of many
organic phosphate compounds, sulfate compounds, and
so forth. Because these ions cannot leave the interior of
the axon, any deficit of positive ions inside the membrane leaves an excess of these impermeant negative
anions. Therefore, these impermeant negative ions are
responsible for the negative charge inside the fiber
when there is a net deficit of positively charged potassium ions and other positive ions.
Calcium Ions. The membranes of almost all cells of the
body have a calcium pump similar to the sodium pump,
and calcium serves along with (or instead of) sodium in
some cells to cause most of the action potential. Like
the sodium pump, the calcium pump pumps calcium
ions from the interior to the exterior of the cell membrane (or into the endoplasmic reticulum of the cell),
creating a calcium ion gradient of about 10,000-fold.
This leaves an internal cell concentration of calcium
ions of about 10–7 molar, in contrast to an external concentration of about 10–3 molar.
In addition, there are voltage-gated calcium channels.
These channels are slightly permeable to sodium ions as
well as to calcium ions; when they open, both calcium
and sodium ions flow to the interior of the fiber. Therefore, these channels are also called Ca++-Na+ channels.
The calcium channels are slow to become activated,
requiring 10 to 20 times as long for activation as the
sodium channels. Therefore, they are called slow channels, in contrast to the sodium channels, which are called
fast channels.
Calcium channels are numerous in both cardiac
muscle and smooth muscle. In fact, in some types of
smooth muscle, the fast sodium channels are hardly
Chapter 5
Membrane Potentials and Action Potentials
present, so that the action potentials are caused almost
entirely by activation of slow calcium channels.
Increased Permeability of the Sodium Channels When
There Is a Deficit of Calcium Ions. The concentration of
calcium ions in the extracellular fluid also has a profound effect on the voltage level at which the sodium
channels become activated. When there is a deficit of
calcium ions, the sodium channels become activated
(opened) by very little increase of the membrane potential from its normal, very negative level. Therefore, the
nerve fiber becomes highly excitable, sometimes discharging repetitively without provocation rather than
remaining in the resting state. In fact, the calcium ion
concentration needs to fall only 50 per cent below
normal before spontaneous discharge occurs in some
peripheral nerves, often causing muscle “tetany.” This is
sometimes lethal because of tetanic contraction of the
respiratory muscles.
The probable way in which calcium ions affect the
sodium channels is as follows: These ions appear to bind
to the exterior surfaces of the sodium channel protein
molecule. The positive charges of these calcium ions
in turn alter the electrical state of the channel protein
itself, in this way altering the voltage level required to
open the sodium gate.
Initiation of the Action Potential
fiber from –90 millivolts up to about –65 millivolts
usually causes the explosive development of an action
potential. This level of –65 millivolts is said to be the
threshold for stimulation.
Propagation of the
Action Potential
In the preceding paragraphs, we discussed the action
potential as it occurs at one spot on the membrane.
However, an action potential elicited at any one point
on an excitable membrane usually excites adjacent
portions of the membrane, resulting in propagation of
the action potential along the membrane. This mechanism is demonstrated in Figure 5–11. Figure 5–11A
shows a normal resting nerve fiber, and Figure 5–11B
shows a nerve fiber that has been excited in its midportion—that is, the midportion suddenly develops
increased permeability to sodium. The arrows show a
“local circuit” of current flow from the depolarized
areas of the membrane to the adjacent resting membrane areas. That is, positive electrical charges are
carried by the inward-diffusing sodium ions through
the depolarized membrane and then for several millimeters in both directions along the core of the axon.
These positive charges increase the voltage for a distance of 1 to 3 millimeters inside the large myelinated
Up to this point, we have explained the changing
sodium and potassium permeability of the membrane,
as well as the development of the action potential
itself, but we have not explained what initiates the
action potential. The answer is quite simple.
+++++++++++++++++++++++
–––––––––––––––––––––––
A Positive-Feedback Vicious Cycle Opens the Sodium Channels.
First, as long as the membrane of the nerve fiber
remains undisturbed, no action potential occurs in the
normal nerve. However, if any event causes enough
initial rise in the membrane potential from –90 millivolts toward the zero level, the rising voltage itself
causes many voltage-gated sodium channels to begin
opening. This allows rapid inflow of sodium ions, which
causes a further rise in the membrane potential, thus
opening still more voltage-gated sodium channels and
allowing more streaming of sodium ions to the interior
of the fiber. This process is a positive-feedback vicious
cycle that, once the feedback is strong enough, continues until all the voltage-gated sodium channels have
become activated (opened). Then, within another fraction of a millisecond, the rising membrane potential
causes closure of the sodium channels as well as
opening of potassium channels, and the action potential soon terminates.
–––––––––––––––––––––––
+++++++++++++++++++++++
A
++++++++++++––+++++++++
––––––––––––++–––––––––
––––––––––––++–––––––––
+ + + +++ + + +++ +– – + + + + + +++ +
B
++++++++++––––++++++++
––– –––– –– – + +++––––––––
––––––––––++++––––––––
++++++++++––––++++++++
C
++––––––––––––––––––++
– – + + +++ + + + + + + + + + + + + + – –
Threshold for Initiation of the Action Potential. An action
potential will not occur until the initial rise in membrane potential is great enough to create the vicious
cycle described in the preceding paragraph. This
occurs when the number of Na+ ions entering the fiber
becomes greater than the number of K+ ions leaving
the fiber. A sudden rise in membrane potential of 15
to 30 millivolts usually is required. Therefore, a sudden
increase in the membrane potential in a large nerve
65
– – + + +++ + + + + + + + + + + + + + – –
++––––––––––––––––––++
D
Figure 5–11
Propagation of action potentials in both directions along a conductive fiber.
Unit II
Membrane Physiology, Nerve, and Muscle
fiber to above the threshold voltage value for initiating an action potential. Therefore, the sodium channels
in these new areas immediately open, as shown in
Figure 5–11C and D, and the explosive action potential spreads. These newly depolarized areas produce
still more local circuits of current flow farther along
the membrane, causing progressively more and more
depolarization. Thus, the depolarization process
travels along the entire length of the fiber. This transmission of the depolarization process along a nerve or
muscle fiber is called a nerve or muscle impulse.
Direction of Propagation. As demonstrated in Figure
5–11, an excitable membrane has no single direction
of propagation, but the action potential travels in all
directions away from the stimulus—even along all
branches of a nerve fiber—until the entire membrane
has become depolarized.
All-or-Nothing Principle. Once an action potential has
been elicited at any point on the membrane of a
normal fiber, the depolarization process travels over
the entire membrane if conditions are right, or it does
not travel at all if conditions are not right. This is called
the all-or-nothing principle, and it applies to all normal
excitable tissues. Occasionally, the action potential
reaches a point on the membrane at which it does not
generate sufficient voltage to stimulate the next area
of the membrane. When this occurs, the spread of
depolarization stops. Therefore, for continued propagation of an impulse to occur, the ratio of action potential to threshold for excitation must at all times be
greater than 1. This “greater than 1” requirement is
called the safety factor for propagation.
Re-establishing Sodium and
Potassium Ionic Gradients
After Action Potentials Are
Completed—Importance
of Energy Metabolism
The transmission of each action potential along a
nerve fiber reduces very slightly the concentration
differences of sodium and potassium inside and
outside the membrane, because sodium ions diffuse to
the inside during depolarization and potassium ions
diffuse to the outside during repolarization. For a
single action potential, this effect is so minute that
it cannot be measured. Indeed, 100,000 to 50 million
impulses can be transmitted by large nerve fibers
before the concentration differences reach the point
that action potential conduction ceases. Even so, with
time, it becomes necessary to re-establish the sodium
and potassium membrane concentration differences.
This is achieved by action of the Na+-K+ pump in the
same way as described previously in the chapter for
the original establishment of the resting potential.That
is, sodium ions that have diffused to the interior of the
cell during the action potentials and potassium ions
that have diffused to the exterior must be returned to
Heat production
66
At rest
0
100
200
300
Impulses per second
Figure 5–12
Heat production in a nerve fiber at rest and at progressively
increasing rates of stimulation.
their original state by the Na+-K+ pump. Because this
pump requires energy for operation, this “recharging”
of the nerve fiber is an active metabolic process, using
energy derived from the adenosine triphosphate
(ATP) energy system of the cell. Figure 5–12 shows
that the nerve fiber produces excess heat during
recharging, which is a measure of energy expenditure
when the nerve impulse frequency increases.
A special feature of the Na+-K+ ATPase pump is that
its degree of activity is strongly stimulated when excess
sodium ions accumulate inside the cell membrane. In
fact, the pumping activity increases approximately in
proportion to the third power of this intracellular
sodium concentration. That is, as the internal sodium
concentration rises from 10 to 20 mEq/L, the activity
of the pump does not merely double but increases
about eightfold. Therefore, it is easy to understand
how the “recharging” process of the nerve fiber can be
set rapidly into motion whenever the concentration
differences of sodium and potassium ions across the
membrane begin to “run down.”
Plateau in Some Action
Potentials
In some instances, the excited membrane does not
repolarize immediately after depolarization; instead,
the potential remains on a plateau near the peak of the
spike potential for many milliseconds, and only then
does repolarization begin. Such a plateau is shown
in Figure 5–13; one can readily see that the plateau
greatly prolongs the period of depolarization. This
type of action potential occurs in heart muscle fibers,
where the plateau lasts for as long as 0.2 to 0.3 second
and causes contraction of heart muscle to last for this
same long period.
The cause of the plateau is a combination of several
factors. First, in heart muscle, two types of channels
Chapter 5
+60
Plateau
+60
+20
+40
0
+20
Millivolts
Millivolts
+40
67
Membrane Potentials and Action Potentials
–20
–40
Rhythmical
action
Potassium
conductance potentials Threshold
0
– 20
–60
– 40
–80
– 60
–100
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Seconds
Figure 5–13
Action potential (in millivolts) from a Purkinje fiber of the heart,
showing a “plateau.”
enter into the depolarization process: (1) the usual
voltage-activated sodium channels, called fast channels, and (2) voltage-activated calcium-sodium channels, which are slow to open and therefore are called
slow channels. Opening of fast channels causes the
spike portion of the action potential, whereas the slow,
prolonged opening of the slow calcium-sodium channels mainly allows calcium ions to enter the fiber,
which is largely responsible for the plateau portion of
the action potential as well.
A second factor that may be partly responsible for
the plateau is that the voltage-gated potassium channels are slower than usual to open, often not opening
very much until the end of the plateau. This delays the
return of the membrane potential toward its normal
negative value of –80 to –90 millivolts.
RHYTHMICITY OF SOME
EXCITABLE TISSUES—
REPETITIVE DISCHARGE
Repetitive self-induced discharges occur normally in
the heart, in most smooth muscle, and in many of the
neurons of the central nervous system. These rhythmical discharges cause (1) the rhythmical beat of the
heart, (2) rhythmical peristalsis of the intestines, and
(3) such neuronal events as the rhythmical control of
breathing.
Also, almost all other excitable tissues can discharge
repetitively if the threshold for stimulation of the
tissue cells is reduced low enough. For instance, even
large nerve fibers and skeletal muscle fibers, which
normally are highly stable, discharge repetitively when
they are placed in a solution that contains the drug
veratrine or when the calcium ion concentration falls
1
2
Seconds
3
Hyperpolarization
Figure 5–14
Rhythmical action potentials (in millivolts) similar to those recorded
in the rhythmical control center of the heart. Note their relationship
to potassium conductance and to the state of hyperpolarization.
below a critical value, both of which increase sodium
permeability of the membrane.
Re-excitation Process Necessary for Spontaneous Rhythmicity.
For spontaneous rhythmicity to occur, the membrane
even in its natural state must be permeable enough to
sodium ions (or to calcium and sodium ions through
the slow calcium-sodium channels) to allow automatic
membrane depolarization. Thus, Figure 5–14 shows
that the “resting” membrane potential in the rhythmical control center of the heart is only –60 to –70 millivolts. This is not enough negative voltage to keep the
sodium and calcium channels totally closed. Therefore,
the following sequence occurs: (1) some sodium and
calcium ions flow inward; (2) this increases the membrane voltage in the positive direction, which further
increases membrane permeability; (3) still more ions
flow inward; and (4) the permeability increases more,
and so on, until an action potential is generated. Then,
at the end of the action potential, the membrane repolarizes. After another delay of milliseconds or
seconds, spontaneous excitability causes depolarization again, and a new action potential occurs spontaneously. This cycle continues over and over and causes
self-induced rhythmical excitation of the excitable
tissue.
Why does the membrane of the heart control center
not depolarize immediately after it has become repolarized, rather than delaying for nearly a second
before the onset of the next action potential? The
answer can be found by observing the curve labeled
“potassium conductance” in Figure 5–14. This shows
that toward the end of each action potential, and continuing for a short period thereafter, the membrane
becomes excessively permeable to potassium ions. The
excessive outflow of potassium ions carries tremendous numbers of positive charges to the outside of the
68
Unit II
Membrane Physiology, Nerve, and Muscle
Axon
Myelin
sheath
Schwann cell
cytoplasm
Schwann cell
nucleus
Node of Ranvier
A
Figure 5–15
Unmyelinated axons
Cross section of a small nerve trunk containing both myelinated
and unmyelinated fibers.
Schwann cell nucleus
Schwann cell cytoplasm
membrane, leaving inside the fiber considerably more
negativity than would otherwise occur. This continues
for nearly a second after the preceding action potential is over, thus drawing the membrane potential
nearer to the potassium Nernst potential.This is a state
called hyperpolarization, also shown in Figure 5–14.As
long as this state exists, self–re-excitation will not
occur. But the excess potassium conductance (and the
state of hyperpolarization) gradually disappears, as
shown after each action potential is completed in the
figure, thereby allowing the membrane potential again
to increase up to the threshold for excitation. Then,
suddenly, a new action potential results, and the
process occurs again and again.
Special Characteristics
of Signal Transmission
in Nerve Trunks
Myelinated and Unmyelinated Nerve Fibers. Figure 5–15
shows a cross section of a typical small nerve, revealing
many large nerve fibers that constitute most of the
cross-sectional area. However, a more careful look
reveals many more very small fibers lying between
the large ones. The large fibers are myelinated, and the
small ones are unmyelinated. The average nerve trunk
contains about twice as many unmyelinated fibers as
myelinated fibers.
Figure 5–16 shows a typical myelinated fiber. The
central core of the fiber is the axon, and the membrane
of the axon is the membrane that actually conducts the
action potential. The axon is filled in its center with axoplasm, which is a viscid intracellular fluid. Surrounding
the axon is a myelin sheath that is often much thicker
than the axon itself. About once every 1 to 3 millimeters along the length of the myelin sheath is a node
of Ranvier.
B
Figure 5–16
Function of the Schwann cell to insulate nerve fibers. A, Wrapping
of a Schwann cell membrane around a large axon to form the
myelin sheath of the myelinated nerve fiber. B, Partial wrapping of
the membrane and cytoplasm of a Schwann cell around multiple
unmyelinated nerve fibers (shown in cross section). (A, Modified
from Leeson TS, Leeson R: Histology. Philadelphia: WB Saunders,
1979.)
The myelin sheath is deposited around the axon by
Schwann cells in the following manner: The membrane
of a Schwann cell first envelops the axon. Then the
Schwann cell rotates around the axon many times,
laying down multiple layers of Schwann cell membrane
containing the lipid substance sphingomyelin. This substance is an excellent electrical insulator that decreases
ion flow through the membrane about 5000-fold. At the
juncture between each two successive Schwann cells
along the axon, a small uninsulated area only 2 to 3
micrometers in length remains where ions still can flow
with ease through the axon membrane between the
extracellular fluid and the intracellular fluid inside the
axon. This area is called the node of Ranvier.
“Saltatory” Conduction in Myelinated Fibers from Node to Node.
Even though almost no ions can flow through the thick
myelin sheaths of myelinated nerves, they can flow with
ease through the nodes of Ranvier. Therefore, action
potentials occur only at the nodes. Yet the action potentials are conducted from node to node, as shown in
Figure 5–17; this is called saltatory conduction. That is,
electrical current flows through the surrounding extracellular fluid outside the myelin sheath as well as
through the axoplasm inside the axon from node to
node, exciting successive nodes one after another. Thus,
Chapter 5
Myelin sheath
Axoplasm
69
Membrane Potentials and Action Potentials
Node of Ranvier
+60
Action potentials
+40
Millivolts
+20
0
-20
-40
Acute
subthreshold
potentials
Threshold
-60
A
1
2
3
0
Figure 5–17
Saltatory conduction along a myelinated axon. Flow of electrical
current from node to node is illustrated by the arrows.
the nerve impulse jumps down the fiber, which is the
origin of the term “saltatory.”
Saltatory conduction is of value for two reasons. First,
by causing the depolarization process to jump long
intervals along the axis of the nerve fiber, this mechanism increases the velocity of nerve transmission in
myelinated fibers as much as 5- to 50-fold. Second,
saltatory conduction conserves energy for the axon
because only the nodes depolarize, allowing perhaps
100 times less loss of ions than would otherwise be necessary, and therefore requiring little metabolism for reestablishing the sodium and potassium concentration
differences across the membrane after a series of nerve
impulses.
Still another feature of saltatory conduction in
large myelinated fibers is the following: The excellent
insulation afforded by the myelin membrane and the 50fold decrease in membrane capacitance allow repolarization to occur with very little transfer of ions.
Velocity of Conduction in Nerve Fibers. The velocity of con-
duction in nerve fibers varies from as little as 0.25 m/sec
in very small unmyelinated fibers to as great as 100 m/
sec (the length of a football field in 1 second) in very
large myelinated fibers.
Excitation—The Process
of Eliciting the Action
Potential
Basically, any factor that causes sodium ions to begin to
diffuse inward through the membrane in sufficient
numbers can set off automatic regenerative opening of
the sodium channels. This can result from mechanical
disturbance of the membrane, chemical effects on the
membrane, or passage of electricity through the membrane. All these are used at different points in the body
to elicit nerve or muscle action potentials: mechanical
pressure to excite sensory nerve endings in the skin,
chemical neurotransmitters to transmit signals from one
neuron to the next in the brain, and electrical current to
transmit signals between successive muscle cells in the
heart and intestine. For the purpose of understanding
the excitation process, let us begin by discussing the
principles of electrical stimulation.
B
1
C
2
3
Milliseconds
D
4
Figure 5–18
Effect of stimuli of increasing voltages to elicit an action potential.
Note development of “acute subthreshold potentials” when the
stimuli are below the threshold value required for eliciting an
action potential.
Excitation of a Nerve Fiber by a Negatively Charged Metal
Electrode. The usual means for exciting a nerve or
muscle in the experimental laboratory is to apply electricity to the nerve or muscle surface through two small
electrodes, one of which is negatively charged and
the other positively charged. When this is done, the
excitable membrane becomes stimulated at the negative
electrode.
The cause of this effect is the following: Remember
that the action potential is initiated by the opening of
voltage-gated sodium channels. Further, these channels
are opened by a decrease in the normal resting electrical voltage across the membrane. That is, negative
current from the electrode decreases the voltage on the
outside of the membrane to a negative value nearer to
the voltage of the negative potential inside the fiber.
This decreases the electrical voltage across the membrane and allows the sodium channels to open, resulting in an action potential. Conversely, at the positive
electrode, the injection of positive charges on the
outside of the nerve membrane heightens the voltage
difference across the membrane rather than lessening
it. This causes a state of hyperpolarization, which actually decreases the excitability of the fiber rather than
causing an action potential.
Threshold for Excitation, and “Acute Local Potentials.” A weak
negative electrical stimulus may not be able to excite a
fiber. However, when the voltage of the stimulus is
increased, there comes a point at which excitation does
take place. Figure 5–18 shows the effects of successively
applied stimuli of progressing strength. A very weak
stimulus at point A causes the membrane potential to
change from –90 to –85 millivolts, but this is not a sufficient change for the automatic regenerative processes
of the action potential to develop. At point B, the stimulus is greater, but again, the intensity is still not enough.
The stimulus does, however, disturb the membrane
potential locally for as long as 1 millisecond or more
after both of these weak stimuli. These local potential
changes are called acute local potentials, and when they
fail to elicit an action potential, they are called acute
subthreshold potentials.
70
Membrane Physiology, Nerve, and Muscle
Unit II
decrease excitability. For instance, a high extracellular
fluid calcium ion concentration decreases membrane
permeability to sodium ions and simultaneously reduces
excitability. Therefore, calcium ions are said to be a
“stabilizer.”
At point C in Figure 5–18, the stimulus is even
stronger. Now the local potential has barely reached the
level required to elicit an action potential, called the
threshold level, but this occurs only after a short “latent
period.” At point D, the stimulus is still stronger, the
acute local potential is also stronger, and the action
potential occurs after less of a latent period.
Thus, this figure shows that even a very weak stimulus causes a local potential change at the membrane, but
the intensity of the local potential must rise to a threshold level before the action potential is set off.
Local Anesthetics. Among the most important stabilizers
are the many substances used clinically as local anesthetics, including procaine and tetracaine. Most of these
act directly on the activation gates of the sodium channels, making it much more difficult for these gates to
open, thereby reducing membrane excitability. When
excitability has been reduced so low that the ratio of
action potential strength to excitability threshold (called
the “safety factor”) is reduced below 1.0, nerve impulses
fail to pass along the anesthetized nerves.
“Refractory Period” After an Action
Potential, During Which a New
Stimulus Cannot Be Elicited
A new action potential cannot occur in an excitable fiber
as long as the membrane is still depolarized from the preceding action potential. The reason for this is that shortly
after the action potential is initiated, the sodium channels
(or calcium channels, or both) become inactivated, and
no amount of excitatory signal applied to these channels
at this point will open the inactivation gates. The only
condition that will allow them to reopen is for the membrane potential to return to or near the original resting
membrane potential level. Then, within another small
fraction of a second, the inactivation gates of the channels open, and a new action potential can be initiated.
The period during which a second action potential
cannot be elicited, even with a strong stimulus, is
called the absolute refractory period. This period for
large myelinated nerve fibers is about 1/2500 second.
Therefore, one can readily calculate that such a fiber
can transmit a maximum of about 2500 impulses per
second.
Recording Membrane
Potentials and Action
Potentials
Cathode Ray Oscilloscope. Earlier in this chapter, we noted
that the membrane potential changes extremely rapidly
during the course of an action potential. Indeed, most
of the action potential complex of large nerve fibers
takes place in less than 1/1000 second. In some figures
of this chapter, an electrical meter has been shown
recording these potential changes. However, it must be
understood that any meter capable of recording most
action potentials must be capable of responding
extremely rapidly. For practical purposes, the only
common type of meter that is capable of responding
accurately to the rapid membrane potential changes is
the cathode ray oscilloscope.
Figure 5–19 shows the basic components of a cathode
ray oscilloscope.The cathode ray tube itself is composed
basically of an electron gun and a fluorescent screen
against which electrons are fired. Where the electrons
hit the screen surface, the fluorescent material glows. If
the electron beam is moved across the screen, the spot
Inhibition of Excitability—
“Stabilizers” and Local Anesthetics
In contrast to the factors that increase nerve excitability, still others, called membrane-stabilizing factors, can
Recorded
action potential
Horizontal
plates
Electron gun
Electron
beam
Plugs
Stimulus
artifact
Vertical
plates
Electronic
sweep circuit
Electronic
amplifier
Electrical
stimulator
Nerve
Figure 5–19
Cathode ray oscilloscope for recording transient
action potentials.
Chapter 5
Membrane Potentials and Action Potentials
of glowing light also moves and draws a fluorescent line
on the screen.
In addition to the electron gun and fluorescent
surface, the cathode ray tube is provided with two sets
of electrically charged plates—one set positioned on the
two sides of the electron beam, and the other set positioned above and below. Appropriate electronic control
circuits change the voltage on these plates so that the
electron beam can be bent up or down in response to
electrical signals coming from recording electrodes on
nerves. The beam of electrons also is swept horizontally
across the screen at a constant time rate by an internal
electronic circuit of the oscilloscope. This gives the
record shown on the face of the cathode ray tube in
the figure, giving a time base horizontally and voltage
changes from the nerve electrodes shown vertically.
Note at the left end of the record a small stimulus artifact caused by the electrical stimulus used to elicit the
nerve action potential. Then further to the right is
the recorded action potential itself.
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Grillner S: The motor infrastructure: from ion channels to
neuronal networks. Nat Rev Neurosci 4:573, 2003.
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Hodgkin AL: The Conduction of the Nervous Impulse.
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Hodgkin AL, Huxley AF: Quantitative description of
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Kleber AG, Rudy Y: Basic mechanisms of cardiac impulse
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84:431, 2004.
Lu Z: Mechanism of rectification in inward-rectifier K+
channels. Annu Rev Physiol 66:103, 2004.
Matthews GG: Cellular Physiology of Nerve and Muscle.
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Perez-Reyes E: Molecular physiology of low-voltageactivated T-type calcium channels. Physiol Rev 83:117,
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Poliak S, Peles E: The local differentiation of myelinated
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Pollard TD, Earnshaw WC: Cell Biology. Philadelphia:
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C
H
A
P
T
E
R
Contraction of
Skeletal Muscle
About 40 per cent of the body is skeletal muscle,
and perhaps another 10 per cent is smooth and
cardiac muscle. Some of the same basic principles of
contraction apply to all these different types of
muscle. In this chapter, function of skeletal muscle
is considered mainly; the specialized functions of
smooth muscle are discussed in Chapter 8, and
cardiac muscle is discussed in Chapter 9.
Physiologic Anatomy of Skeletal Muscle
Skeletal Muscle Fiber
Figure 6–1 shows the organization of skeletal muscle, demonstrating that all
skeletal muscles are composed of numerous fibers ranging from 10 to 80
micrometers in diameter. Each of these fibers is made up of successively smaller
subunits, also shown in Figure 6–1 and described in subsequent paragraphs.
In most skeletal muscles, each fiber extends the entire length of the muscle.
Except for about 2 per cent of the fibers, each fiber is usually innervated by only
one nerve ending, located near the middle of the fiber.
Sarcolemma. The sarcolemma is the cell membrane of the muscle fiber. The sarcolemma consists of a true cell membrane, called the plasma membrane, and an
outer coat made up of a thin layer of polysaccharide material that contains
numerous thin collagen fibrils. At each end of the muscle fiber, this surface layer
of the sarcolemma fuses with a tendon fiber, and the tendon fibers in turn collect
into bundles to form the muscle tendons that then insert into the bones.
Myofibrils; Actin and Myosin Filaments. Each muscle fiber contains several hundred
to several thousand myofibrils, which are demonstrated by the many small open
dots in the cross-sectional view of Figure 6–1C. Each myofibril (Figure 6–1D
and E) is composed of about 1500 adjacent myosin filaments and 3000 actin filaments, which are large polymerized protein molecules that are responsible for
the actual muscle contraction.These can be seen in longitudinal view in the electron micrograph of Figure 6–2 and are represented diagrammatically in Figure
6–1, parts E through L. The thick filaments in the diagrams are myosin, and the
thin filaments are actin.
Note in Figure 6–1E that the myosin and actin filaments partially interdigitate and thus cause the myofibrils to have alternate light and dark bands, as
illustrated in Figure 6–2. The light bands contain only actin filaments and are
called I bands because they are isotropic to polarized light. The dark bands
contain myosin filaments, as well as the ends of the actin filaments where they
overlap the myosin, and are called A bands because they are anisotropic to
polarized light. Note also the small projections from the sides of the myosin
filaments in Figure 6–1E and L. These are cross-bridges. It is the interaction
between these cross-bridges and the actin filaments that causes contraction.
Figure 6–1E also shows that the ends of the actin filaments are attached to a
so-called Z disc. From this disc, these filaments extend in both directions to
72
6
73
Contraction of Skeletal Muscle
Chapter 6
SKELETAL MUSCLE
Muscle
A
Muscle fasciculus
B
C
H
band
Z
disc
A
band
Muscle fiber
I
band
Myofibril
Z Sarcomere Z
D
G-Actin molecules
H
J
Myofilaments
F-Actin filament
K
Figure 6–1
Organization of skeletal muscle,
from the gross to the molecular
level. F, G, H, and I are cross
sections at the levels indicated.
(Drawing by Sylvia Colard
Keene. Modified from Fawcett
DW: Bloom and Fawcett: A Textbook of Histology. Philadelphia:
WB Saunders, 1986.)
L
E
Myosin filament
Myosin molecule
M
N
F
G
interdigitate with the myosin filaments. The Z disc,
which itself is composed of filamentous proteins different from the actin and myosin filaments, passes
crosswise across the myofibril and also crosswise from
myofibril to myofibril, attaching the myofibrils to one
another all the way across the muscle fiber. Therefore,
the entire muscle fiber has light and dark bands, as do
the individual myofibrils.These bands give skeletal and
cardiac muscle their striated appearance.
The portion of the myofibril (or of the whole muscle
fiber) that lies between two successive Z discs is called
a sarcomere. When the muscle fiber is contracted, as
shown at the bottom of Figure 6–4, the length of
the sarcomere is about 2 micrometers. At this length,
the actin filaments completely overlap the myosin
filaments, and the tips of the actin filaments are just
H
I
Light
meromyosin
Heavy
meromyosin
beginning to overlap one another. We will see later
that, at this length, the muscle is capable of generating
its greatest force of contraction.
What Keeps the Myosin and Actin Filaments in Place?
Titin Filamentous Molecules. The side-by-side rela-
tionship between the myosin and actin filaments is difficult to maintain. This is achieved by a large number
of filamentous molecules of a protein called titin. Each
titin molecule has a molecular weight of about 3
million, which makes it one of the largest protein molecules in the body. Also, because it is filamentous, it is
very springy. These springy titin molecules act as a
framework that holds the myosin and actin filaments
in place so that the contractile machinery of the sarcomere will work. There is reason to believe that the
74
Unit II
Membrane Physiology, Nerve, and Muscle
Figure 6–2
Electron micrograph of muscle myofibrils showing the detailed
organization of actin and myosin filaments. Note the mitochondria
lying between the myofibrils. (From Fawcett DW: The Cell.
Philadelphia: WB Saunders, 1981.)
titin molecule itself acts as template for initial formation of portions of the contractile filaments of the
sarcomere, especially the myosin filaments.
Sarcoplasm. The many myofibrils of each muscle fiber
are suspended side by side in the muscle fiber. The
spaces between the myofibrils are filled with intracellular fluid called sarcoplasm, containing large quantities of potassium, magnesium, and phosphate, plus
multiple protein enzymes. Also present are tremendous numbers of mitochondria that lie parallel to the
myofibrils. These supply the contracting myofibrils
with large amounts of energy in the form of adenosine
triphosphate (ATP) formed by the mitochondria.
Sarcoplasmic Reticulum. Also in the sarcoplasm sur-
rounding the myofibrils of each muscle fiber is an
extensive reticulum (Figure 6–3), called the sarcoplasmic reticulum. This reticulum has a special organization that is extremely important in controlling muscle
contraction, as discussed in Chapter 7.The very rapidly
contracting types of muscle fibers have especially
extensive sarcoplasmic reticula.
General Mechanism of Muscle
Contraction
The initiation and execution of muscle contraction
occur in the following sequential steps.
1. An action potential travels along a motor nerve to
its endings on muscle fibers.
2. At each ending, the nerve secretes a small amount
of the neurotransmitter substance acetylcholine.
3. The acetylcholine acts on a local area of the muscle
fiber membrane to open multiple “acetylcholinegated” channels through protein molecules floating
in the membrane.
Figure 6–3
Sarcoplasmic reticulum in the extracellular spaces between the
myofibrils, showing a longitudinal system paralleling the myofibrils. Also shown in cross section are T tubules (arrows) that lead
to the exterior of the fiber membrane and are important for conducting the electrical signal into the center of the muscle fiber.
(From Fawcett DW: The Cell. Philadelphia: WB Saunders, 1981.)
4. Opening of the acetylcholine-gated channels allows
large quantities of sodium ions to diffuse to the
interior of the muscle fiber membrane. This initiates
an action potential at the membrane.
5. The action potential travels along the muscle fiber
membrane in the same way that action potentials
travel along nerve fiber membranes.
6. The action potential depolarizes the muscle
membrane, and much of the action potential
electricity flows through the center of the muscle
fiber. Here it causes the sarcoplasmic reticulum to
release large quantities of calcium ions that have
been stored within this reticulum.
7. The calcium ions initiate attractive forces between
the actin and myosin filaments, causing them to
slide alongside each other, which is the contractile
process.
8. After a fraction of a second, the calcium ions are
pumped back into the sarcoplasmic reticulum by a
Ca++ membrane pump, and they remain stored in
the reticulum until a new muscle action potential
comes along; this removal of calcium ions from the
myofibrils causes the muscle contraction to cease.
We now describe the molecular machinery of the
muscle contractile process.
Molecular Mechanism
of Muscle Contraction
Sliding Filament Mechanism of Muscle Contraction. Figure
6–4 demonstrates the basic mechanism of muscle contraction. It shows the relaxed state of a sarcomere
(top) and the contracted state (bottom). In the relaxed
state, the ends of the actin filaments extending from
two successive Z discs barely begin to overlap one
another. Conversely, in the contracted state, these actin
filaments have been pulled inward among the myosin
Chapter 6
I
A
I
Z
Head
A
Z
Tail
Two heavy chains
Relaxed
I
A
Z
75
Contraction of Skeletal Muscle
I
Light chains
Z
B
Actin filaments
Contracted
Figure 6–4
Relaxed and contracted states of a myofibril showing (top) sliding
of the actin filaments (pink) into the spaces between the myosin
filaments (red), and (bottom) pulling of the Z membranes toward
each other.
filaments, so that their ends overlap one another to
their maximum extent. Also, the Z discs have been
pulled by the actin filaments up to the ends of the
myosin filaments. Thus, muscle contraction occurs by a
sliding filament mechanism.
But what causes the actin filaments to slide inward
among the myosin filaments? This is caused by forces
generated by interaction of the cross-bridges from
the myosin filaments with the actin filaments. Under
resting conditions, these forces are inactive, but when
an action potential travels along the muscle fiber,
this causes the sarcoplasmic reticulum to release large
quantities of calcium ions that rapidly surround the
myofibrils. The calcium ions in turn activate the forces
between the myosin and actin filaments, and contraction begins. But energy is needed for the contractile
process to proceed. This energy comes from highenergy bonds in the ATP molecule, which is degraded
to adenosine diphosphate (ADP) to liberate the
energy. In the next few sections, we describe what is
known about the details of these molecular processes
of contraction.
Molecular Characteristics of the
Contractile Filaments
Myosin Filament. The myosin filament is composed of
multiple myosin molecules, each having a molecular
weight of about 480,000. Figure 6–5A shows an individual molecule; Figure 6–5B shows the organization
of many molecules to form a myosin filament, as well
as interaction of this filament on one side with the ends
of two actin filaments.
The myosin molecule (see Figure 6–5A) is composed
of six polypeptide chains—two heavy chains, each with
a molecular weight of about 200,000, and four light
chains with molecular weights of about 20,000 each.
Cross-bridges
Hinges
Body
Myosin filament
Figure 6–5
A, Myosin molecule. B, Combination of many myosin molecules
to form a myosin filament. Also shown are thousands of myosin
cross-bridges and interaction between the heads of the crossbridges with adjacent actin filaments.
The two heavy chains wrap spirally around each other
to form a double helix, which is called the tail of the
myosin molecule. One end of each of these chains is
folded bilaterally into a globular polypeptide structure
called a myosin head. Thus, there are two free heads
at one end of the double-helix myosin molecule. The
four light chains are also part of the myosin head, two
to each head. These light chains help control the function of the head during muscle contraction.
The myosin filament is made up of 200 or more individual myosin molecules. The central portion of one of
these filaments is shown in Figure 6–5B, displaying the
tails of the myosin molecules bundled together to form
the body of the filament, while many heads of the
molecules hang outward to the sides of the body. Also,
part of the body of each myosin molecule hangs to the
side along with the head, thus providing an arm that
extends the head outward from the body, as shown in
the figure. The protruding arms and heads together are
called cross-bridges. Each cross-bridge is flexible at
two points called hinges—one where the arm leaves
the body of the myosin filament, and the other where
the head attaches to the arm. The hinged arms allow
the heads either to be extended far outward from the
body of the myosin filament or to be brought close to
the body. The hinged heads in turn participate in the
actual contraction process, as discussed in the following sections.
The total length of each myosin filament is uniform,
almost exactly 1.6 micrometers. Note, however, that
there are no cross-bridge heads in the very center of
the myosin filament for a distance of about 0.2
micrometer because the hinged arms extend away
from the center.
76
Unit II
Membrane Physiology, Nerve, and Muscle
Now, to complete the picture, the myosin filament
itself is twisted so that each successive pair of crossbridges is axially displaced from the previous pair by
120 degrees. This ensures that the cross-bridges extend
in all directions around the filament.
ATPase Activity of the Myosin Head. Another feature
of the myosin head that is essential for muscle contraction is that it functions as an ATPase enzyme. As
explained later, this property allows the head to cleave
ATP and to use the energy derived from the ATP’s
high-energy phosphate bond to energize the contraction process.
Actin Filament. The actin filament is also complex.
It is composed of three protein components: actin,
tropomyosin, and troponin.
The backbone of the actin filament is a doublestranded F-actin protein molecule, represented by the
two lighter-colored strands in Figure 6–6. The two
strands are wound in a helix in the same manner as the
myosin molecule.
Each strand of the double F-actin helix is composed
of polymerized G-actin molecules, each having a
molecular weight of about 42,000. Attached to each
one of the G-actin molecules is one molecule of
ADP. It is believed that these ADP molecules are the
active sites on the actin filaments with which the crossbridges of the myosin filaments interact to cause
muscle contraction. The active sites on the two F-actin
strands of the double helix are staggered, giving one
active site on the overall actin filament about every 2.7
nanometers.
Each actin filament is about 1 micrometer long. The
bases of the actin filaments are inserted strongly into
the Z discs; the ends of the filaments protrude in both
directions to lie in the spaces between the myosin
molecules, as shown in Figure 6–4.
Tropomyosin Molecules. The actin filament also con-
tains another protein, tropomyosin. Each molecule of
tropomyosin has a molecular weight of 70,000 and a
length of 40 nanometers. These molecules are wrapped
spirally around the sides of the F-actin helix. In the
resting state, the tropomyosin molecules lie on top of
the active sites of the actin strands, so that attraction
Active sites
F-actin
Troponin complex
Tropomyosin
Figure 6–6
Actin filament, composed of two helical strands of F-actin molecules and two strands of tropomyosin molecules that fit in the
grooves between the actin strands. Attached to one end of each
tropomyosin molecule is a troponin complex that initiates
contraction.
cannot occur between the actin and myosin filaments
to cause contraction.
Troponin and Its Role in Muscle Contraction. Attached
intermittently along the sides of the tropomyosin molecules are still other protein molecules called troponin.
These are actually complexes of three loosely bound
protein subunits, each of which plays a specific role in
controlling muscle contraction. One of the subunits
(troponin I) has a strong affinity for actin, another
(troponin T) for tropomyosin, and a third (troponin C)
for calcium ions. This complex is believed to attach
the tropomyosin to the actin. The strong affinity of the
troponin for calcium ions is believed to initiate the
contraction process, as explained in the next section.
Interaction of One Myosin Filament,
Two Actin Filaments, and Calcium Ions
to Cause Contraction
Inhibition of the Actin Filament by the Troponin-Tropomyosin
Complex; Activation by Calcium Ions. A pure actin filament
without the presence of the troponin-tropomyosin
complex (but in the presence of magnesium ions and
ATP) binds instantly and strongly with the heads
of the myosin molecules. Then, if the troponintropomyosin complex is added to the actin filament,
the binding between myosin and actin does not take
place. Therefore, it is believed that the active sites on
the normal actin filament of the relaxed muscle are
inhibited or physically covered by the troponintropomyosin complex. Consequently, the sites cannot
attach to the heads of the myosin filaments to cause
contraction. Before contraction can take place, the
inhibitory effect of the troponin-tropomyosin complex
must itself be inhibited.
This brings us to the role of the calcium ions. In
the presence of large amounts of calcium ions, the
inhibitory effect of the troponin-tropomyosin on
the actin filaments is itself inhibited. The mechanism
of this is not known, but one suggestion is the following: When calcium ions combine with troponin C, each
molecule of which can bind strongly with up to four
calcium ions, the troponin complex supposedly undergoes a conformational change that in some way tugs
on the tropomyosin molecule and moves it deeper into
the groove between the two actin strands. This “uncovers” the active sites of the actin, thus allowing these to
attract the myosin cross-bridge heads and cause contraction to proceed. Although this is a hypothetical
mechanism, it does emphasize that the normal relation
between the troponin-tropomyosin complex and actin
is altered by calcium ions, producing a new condition
that leads to contraction.
Interaction Between the “Activated” Actin Filament and the
Myosin Cross-Bridges—The “Walk-Along” Theory of Contraction. As soon as the actin filament becomes activated
by the calcium ions, the heads of the cross-bridges
from the myosin filaments become attracted to the
active sites of the actin filament, and this, in some way,
causes contraction to occur. Although the precise
Chapter 6
Movement
Active sites
Hinges
Contraction of Skeletal Muscle
Actin filament
Power
stroke
Myosin filament
Figure 6–7
“Walk-along” mechanism for contraction of the muscle.
manner by which this interaction between the crossbridges and the actin causes contraction is still partly
theoretical, one hypothesis for which considerable evidence exists is the “walk-along” theory (or “ratchet”
theory) of contraction.
Figure 6–7 demonstrates this postulated walk-along
mechanism for contraction.The figure shows the heads
of two cross-bridges attaching to and disengaging from
active sites of an actin filament. It is postulated that
when a head attaches to an active site, this attachment simultaneously causes profound changes in the
intramolecular forces between the head and arm of its
cross-bridge. The new alignment of forces causes the
head to tilt toward the arm and to drag the actin filament along with it. This tilt of the head is called the
power stroke. Then, immediately after tilting, the head
automatically breaks away from the active site. Next,
the head returns to its extended direction. In this
position, it combines with a new active site farther
down along the actin filament; then the head tilts again
to cause a new power stroke, and the actin filament
moves another step. Thus, the heads of the crossbridges bend back and forth and step by step walk
along the actin filament, pulling the ends of two successive actin filaments toward the center of the myosin
filament.
Each one of the cross-bridges is believed to operate
independently of all others, each attaching and pulling
in a continuous repeated cycle. Therefore, the greater
the number of cross-bridges in contact with the actin
filament at any given time, the greater, theoretically,
the force of contraction.
ATP as the Source of Energy for Contraction—Chemical Events
in the Motion of the Myosin Heads. When a muscle con-
tracts, work is performed and energy is required. Large
amounts of ATP are cleaved to form ADP during the
contraction process; the greater the amount of work
performed by the muscle, the greater the amount of
ATP that is cleaved, which is called the Fenn effect. The
following sequence of events is believed to be the
means by which this occurs:
1. Before contraction begins, the heads of the crossbridges bind with ATP. The ATPase activity of the
myosin head immediately cleaves the ATP but
leaves the cleavage products, ADP plus phosphate
77
ion, bound to the head. In this state, the
conformation of the head is such that it extends
perpendicularly toward the actin filament but is
not yet attached to the actin.
2. When the troponin-tropomyosin complex binds
with calcium ions, active sites on the actin filament
are uncovered, and the myosin heads then bind
with these, as shown in Figure 6–7.
3. The bond between the head of the cross-bridge
and the active site of the actin filament causes a
conformational change in the head, prompting the
head to tilt toward the arm of the cross-bridge.
This provides the power stroke for pulling the
actin filament. The energy that activates the
power stroke is the energy already stored, like a
“cocked” spring, by the conformational change
that occurred in the head when the ATP molecule
was cleaved earlier.
4. Once the head of the cross-bridge tilts, this allows
release of the ADP and phosphate ion that were
previously attached to the head. At the site of
release of the ADP, a new molecule of ATP binds.
This binding of new ATP causes detachment of
the head from the actin.
5. After the head has detached from the actin, the
new molecule of ATP is cleaved to begin the next
cycle, leading to a new power stroke. That is,
the energy again “cocks” the head back to its
perpendicular condition, ready to begin the new
power stroke cycle.
6. When the cocked head (with its stored energy
derived from the cleaved ATP) binds with a new
active site on the actin filament, it becomes
uncocked and once again provides a new power
stroke.
Thus, the process proceeds again and again until the
actin filaments pull the Z membrane up against
the ends of the myosin filaments or until the load on
the muscle becomes too great for further pulling to
occur.
Effect of Amount of Actin and Myosin
Filament Overlap on Tension
Developed by the Contracting Muscle
Figure 6–8 shows the effect of sarcomere length and
amount of myosin-actin filament overlap on the active
tension developed by a contracting muscle fiber.To the
right, shown in black, are different degrees of overlap
of the myosin and actin filaments at different sarcomere lengths. At point D on the diagram, the actin
filament has pulled all the way out to the end of the
myosin filament, with no actin-myosin overlap. At this
point, the tension developed by the activated muscle
is zero. Then, as the sarcomere shortens and the actin
filament begins to overlap the myosin filament, the
tension increases progressively until the sarcomere
length decreases to about 2.2 micrometers. At this
point, the actin filament has already overlapped all
the cross-bridges of the myosin filament but has not
yet reached the center of the myosin filament. With
78
Unit II
Membrane Physiology, Nerve, and Muscle
Normal range of contraction
D
B C
C
Tension developed
(per cent)
B
A
A
50
D
0
0
Tension during
contraction
Tension of muscle
100
Increase in tension
during contraction
Tension
before contraction
D
0
1/2
normal
1
2
3
4
Length of sarcomere (micrometers)
Figure 6–8
Length-tension diagram for a single fully contracted sarcomere,
showing maximum strength of contraction when the sarcomere is
2.0 to 2.2 micrometers in length. At the upper right are the relative positions of the actin and myosin filaments at different sarcomere lengths from point A to point D. (Modified from Gordon
AM, Huxley AF, Julian FJ: The length-tension diagram of single
vertebrate striated muscle fibers. J Physiol 171:28P, 1964.)
further shortening, the sarcomere maintains full
tension until point B is reached, at a sarcomere length
of about 2 micrometers. At this point, the ends of the
two actin filaments begin to overlap each other in
addition to overlapping the myosin filaments. As the
sarcomere length falls from 2 micrometers down to
about 1.65 micrometers, at point A, the strength of
contraction decreases rapidly. At this point, the two Z
discs of the sarcomere abut the ends of the myosin filaments. Then, as contraction proceeds to still shorter
sarcomere lengths, the ends of the myosin filaments
are crumpled and, as shown in the figure, the strength
of contraction approaches zero, but the entire muscle
has now contracted to its shortest length.
Effect of Muscle Length on Force of Contraction in the Whole
Intact Muscle. The top curve of Figure 6–9 is similar to
that in Figure 6–8, but the curve in Figure 6–9 depicts
tension of the intact, whole muscle rather than of a
single muscle fiber. The whole muscle has a large
amount of connective tissue in it; also, the sarcomeres
in different parts of the muscle do not always contract
the same amount. Therefore, the curve has somewhat
different dimensions from those shown for the
individual muscle fiber, but it exhibits the same general
form for the slope in the normal range of contraction,
as noted in Figure 6–9.
Note in Figure 6–9 that when the muscle is at its
normal resting length, which is at a sarcomere length
of about 2 micrometers, it contracts upon activation
with the approximate maximum force of contraction.
However, the increase in tension that occurs during
contraction, called active tension, decreases as the
Normal
Length
2x
normal
Figure 6–9
Relation of muscle length to tension in the muscle both before and
during muscle contraction.
muscle is stretched beyond its normal length—that is,
to a sarcomere length greater than about 2.2 micrometers. This is demonstrated by the decreased length of
the arrow in the figure at greater than normal muscle
length.
Relation of Velocity of Contraction
to Load
A skeletal muscle contracts extremely rapidly when it
contracts against no load—to a state of full contraction
in about 0.1 second for the average muscle. When loads
are applied, the velocity of contraction becomes progressively less as the load increases, as shown in Figure
6–10. That is, when the load has been increased to equal
the maximum force that the muscle can exert, the velocity of contraction becomes zero and no contraction
results, despite activation of the muscle fiber.
This decreasing velocity of contraction with load is
caused by the fact that a load on a contracting muscle
is a reverse force that opposes the contractile force
caused by muscle contraction. Therefore, the net force
that is available to cause velocity of shortening is
correspondingly reduced.
Energetics of Muscle
Contraction
Work Output During Muscle
Contraction
When a muscle contracts against a load, it performs
work. This means that energy is transferred from the
muscle to the external load to lift an object to a greater
height or to overcome resistance to movement.
In mathematical terms, work is defined by the
following equation:
Velocity of contraction (cm/sec)
Chapter 6
Contraction of Skeletal Muscle
30
20
10
0
0
1
2
3
Load-opposing contraction (kg)
4
Figure 6–10
Relation of load to velocity of contraction in a skeletal muscle
with a cross section of 1 square centimeter and a length of 8
centimeters.
W=L¥D
in which W is the work output, L is the load, and D is
the distance of movement against the load. The energy
required to perform the work is derived from the
chemical reactions in the muscle cells during contraction, as described in the following sections.
Sources of Energy for Muscle
Contraction
We have already seen that muscle contraction depends
on energy supplied by ATP. Most of this energy is
required to actuate the walk-along mechanism by
which the cross-bridges pull the actin filaments, but
small amounts are required for (1) pumping calcium
ions from the sarcoplasm into the sarcoplasmic reticulum after the contraction is over, and (2) pumping
sodium and potassium ions through the muscle fiber
membrane to maintain appropriate ionic environment
for propagation of muscle fiber action potentials.
The concentration of ATP in the muscle fiber, about
4 millimolar, is sufficient to maintain full contraction
for only 1 to 2 seconds at most.The ATP is split to form
ADP, which transfers energy from the ATP molecule
to the contracting machinery of the muscle fiber. Then,
as described in Chapter 2, the ADP is rephosphorylated to form new ATP within another fraction of a
second, which allows the muscle to continue its contraction. There are several sources of the energy for
this rephosphorylation.
The first source of energy that is used to reconstitute the ATP is the substance phosphocreatine, which
carries a high-energy phosphate bond similar to the
bonds of ATP. The high-energy phosphate bond of
phosphocreatine has a slightly higher amount of free
79
energy than that of each ATP bond, as is discussed
more fully in Chapters 67 and 72. Therefore, phosphocreatine is instantly cleaved, and its released energy
causes bonding of a new phosphate ion to ADP to
reconstitute the ATP. However, the total amount of
phosphocreatine in the muscle fiber is also very little—
only about five times as great as the ATP. Therefore,
the combined energy of both the stored ATP and
the phosphocreatine in the muscle is capable of
causing maximal muscle contraction for only 5 to 8
seconds.
The second important source of energy, which is
used to reconstitute both ATP and phosphocreatine,
is “glycolysis” of glycogen previously stored in the
muscle cells. Rapid enzymatic breakdown of the glycogen to pyruvic acid and lactic acid liberates energy
that is used to convert ADP to ATP; the ATP can
then be used directly to energize additional muscle
contraction and also to re-form the stores of
phosphocreatine.
The importance of this glycolysis mechanism is
twofold. First, the glycolytic reactions can occur even
in the absence of oxygen, so that muscle contraction
can be sustained for many seconds and sometimes up
to more than a minute, even when oxygen delivery
from the blood is not available. Second, the rate of formation of ATP by the glycolytic process is about 2.5
times as rapid as ATP formation in response to cellular foodstuffs reacting with oxygen. However, so
many end products of glycolysis accumulate in the
muscle cells that glycolysis also loses its capability to
sustain maximum muscle contraction after about 1
minute.
The third and final source of energy is oxidative
metabolism. This means combining oxygen with the
end products of glycolysis and with various other cellular foodstuffs to liberate ATP. More than 95 per cent
of all energy used by the muscles for sustained, longterm contraction is derived from this source. The
foodstuffs that are consumed are carbohydrates, fats,
and protein. For extremely long-term maximal muscle
activity—over a period of many hours—by far the
greatest proportion of energy comes from fats, but for
periods of 2 to 4 hours, as much as one half of the
energy can come from stored carbohydrates.
The detailed mechanisms of these energetic
processes are discussed in Chapters 67 through 72.
In addition, the importance of the different mechanisms of energy release during performance of
different sports is discussed in Chapter 84 on sports
physiology.
Efficiency of Muscle Contraction. The efficiency of an
engine or a motor is calculated as the percentage of
energy input that is converted into work instead of heat.
The percentage of the input energy to muscle (the
chemical energy in nutrients) that can be converted into
work, even under the best conditions, is less than 25 per
cent, with the remainder becoming heat. The reason for
this low efficiency is that about one half of the energy
in foodstuffs is lost during the formation of ATP, and
even then, only 40 to 45 per cent of the energy in the
ATP itself can later be converted into work.
80
Unit II
Membrane Physiology, Nerve, and Muscle
Many features of muscle contraction can be demonstrated by eliciting single muscle twitches. This can be
accomplished by instantaneous electrical excitation of
the nerve to a muscle or by passing a short electrical
stimulus through the muscle itself, giving rise to a single,
sudden contraction lasting for a fraction of a second.
quadriceps muscle, a half million times as large as the
stapedius. Further, the fibers may be as small as 10
micrometers in diameter or as large as 80 micrometers.
Finally, the energetics of muscle contraction vary considerably from one muscle to another. Therefore, it is no
wonder that the mechanical characteristics of muscle
contraction differ among muscles.
Figure 6–12 shows records of isometric contractions
of three types of skeletal muscle: an ocular muscle,
which has a duration of isometric contraction of less
than 1/40 second; the gastrocnemius muscle, which has
a duration of contraction of about 1/15 second; and the
soleus muscle, which has a duration of contraction of about 1/3 second. It is interesting that these durations of contraction are adapted to the functions of the
respective muscles. Ocular movements must be
extremely rapid to maintain fixation of the eyes on
specific objects to provide accuracy of vision. The gastrocnemius muscle must contract moderately rapidly
to provide sufficient velocity of limb movement for
running and jumping, and the soleus muscle is concerned principally with slow contraction for continual,
long-term support of the body against gravity.
Isometric Versus Isotonic Contraction. Muscle contraction is
Fast Versus Slow Muscle Fibers. As we discuss more fully in
said to be isometric when the muscle does not shorten
during contraction and isotonic when it does shorten but
the tension on the muscle remains constant throughout
the contraction. Systems for recording the two types of
muscle contraction are shown in Figure 6–11.
In the isometric system, the muscle contracts against
a force transducer without decreasing the muscle
length, as shown on the right in Figure 6–11. In the isotonic system, the muscle shortens against a fixed load;
this is illustrated on the left in the figure, showing a
muscle lifting a pan of weights. The characteristics of
isotonic contraction depend on the load against which
the muscle contracts, as well as the inertia of the load.
However, the isometric system records strictly changes
in force of muscle contraction itself. Therefore, the isometric system is most often used when comparing the
functional characteristics of different muscle types.
Chapter 84 on sports physiology, every muscle of the
body is composed of a mixture of so-called fast and slow
muscle fibers, with still other fibers gradated between
these two extremes. The muscles that react rapidly are
composed mainly of “fast” fibers with only small
numbers of the slow variety. Conversely, the muscles
that respond slowly but with prolonged contraction are
composed mainly of “slow” fibers. The differences
between these two types of fibers are as follows.
Maximum efficiency can be realized only when the
muscle contracts at a moderate velocity. If the muscle
contracts slowly or without any movement, small
amounts of maintenance heat are released during contraction, even though little or no work is performed,
thereby decreasing the conversion efficiency to as little
as zero. Conversely, if contraction is too rapid, large proportions of the energy are used to overcome viscous
friction within the muscle itself, and this, too, reduces the
efficiency of contraction. Ordinarily, maximum efficiency is developed when the velocity of contraction is
about 30 per cent of maximum.
Characteristics of Whole
Muscle Contraction
Fast Fibers. (1) Large fibers for great strength of con-
traction. (2) Extensive sarcoplasmic reticulum for rapid
release of calcium ions to initiate contraction. (3) Large
amounts of glycolytic enzymes for rapid release of
energy by the glycolytic process. (4) Less extensive
blood supply because oxidative metabolism is of
Characteristics of Isometric Twitches Recorded from Different
Muscles. The human body has many sizes of skeletal
muscles—from the very small stapedius muscle in the
middle ear, measuring only a few millimeters long and
a millimeter or so in diameter, up to the very large
Kymograph
Stimulating
electrodes
Muscle
Weights
Electronic force
transducer
Force of contraction
Stimulating
electrodes
Duration of
depolarization
Soleus
Gastrocnemius
Ocular
muscle
0
40
80
120
160
200
Milliseconds
To electronic
recorder
ISOTONIC SYSTEM
ISOMETRIC SYSTEM
Figure 6–11
Isotonic and isometric systems for recording muscle contractions.
Figure 6–12
Duration of isometric contractions for different types of mammalian
skeletal muscles, showing a latent period between the action
potential (depolarization) and muscle contraction.
Chapter 6
81
Contraction of Skeletal Muscle
Slow Fibers. (1) Smaller fibers. (2) Also innervated by
smaller nerve fibers. (3) More extensive blood vessel
system and capillaries to supply extra amounts of
oxygen. (4) Greatly increased numbers of mitochondria,
also to support high levels of oxidative metabolism. (5)
Fibers contain large amounts of myoglobin, an ironcontaining protein similar to hemoglobin in red blood
cells. Myoglobin combines with oxygen and stores it
until needed; this also greatly speeds oxygen transport
to the mitochondria. The myoglobin gives the slow
muscle a reddish appearance and the name red muscle,
whereas a deficit of red myoglobin in fast muscle gives
it the name white muscle.
Mechanics of Skeletal Muscle
Contraction
Motor Unit. Each motoneuron that leaves the spinal cord
innervates multiple muscle fibers, the number depending on the type of muscle. All the muscle fibers innervated by a single nerve fiber are called a motor unit. In
general, small muscles that react rapidly and whose
control must be exact have more nerve fibers for fewer
muscle fibers (for instance, as few as two or three muscle
fibers per motor unit in some of the laryngeal muscles).
Conversely, large muscles that do not require fine
control, such as the soleus muscle, may have several
hundred muscle fibers in a motor unit. An average
figure for all the muscles of the body is questionable, but
a good guess would be about 80 to 100 muscle fibers to
a motor unit.
The muscle fibers in each motor unit are not all
bunched together in the muscle but overlap other motor
units in microbundles of 3 to 15 fibers. This interdigitation allows the separate motor units to contract in
support of one another rather than entirely as individual segments.
Muscle Contractions of Different Force—Force Summation.
Summation means the adding together of individual
twitch contractions to increase the intensity of overall
muscle contraction. Summation occurs in two ways: (1)
by increasing the number of motor units contracting
simultaneously, which is called multiple fiber summation, and (2) by increasing the frequency of contraction,
which is called frequency summation and can lead to
tetanization.
Multiple Fiber Summation. When the central nervous
system sends a weak signal to contract a muscle, the
smaller motor units of the muscle may be stimulated in
preference to the larger motor units. Then, as the
strength of the signal increases, larger and larger motor
units begin to be excited as well, with the largest motor
units often having as much as 50 times the contractile
force of the smallest units. This is called the size principle. It is important, because it allows the gradations of
muscle force during weak contraction to occur in small
steps, whereas the steps become progressively greater
when large amounts of force are required. The cause of
this size principle is that the smaller motor units are
driven by small motor nerve fibers, and the small
motoneurons in the spinal cord are more excitable than
the larger ones, so they naturally are excited first.
Strength of muscle contraction
secondary importance. (5) Fewer mitochondria, also
because oxidative metabolism is secondary.
Tetanization
5
10 15 20 25 30 35 40 45 50 55
Rate of stimulation (times per second)
Figure 6–13
Frequency summation and tetanization.
Another important feature of multiple fiber summation is that the different motor units are driven asynchronously by the spinal cord, so that contraction
alternates among motor units one after the other, thus
providing smooth contraction even at low frequencies
of nerve signals.
Frequency Summation and Tetanization. Figure 6–13
shows the principles of frequency summation and
tetanization. To the left are displayed individual twitch
contractions occurring one after another at low frequency of stimulation. Then, as the frequency increases,
there comes a point where each new contraction occurs
before the preceding one is over. As a result, the second
contraction is added partially to the first, so that the
total strength of contraction rises progressively with
increasing frequency. When the frequency reaches a
critical level, the successive contractions eventually
become so rapid that they fuse together, and the whole
muscle contraction appears to be completely smooth
and continuous, as shown in the figure. This is called
tetanization. At a slightly higher frequency, the strength
of contraction reaches its maximum, so that any additional increase in frequency beyond that point has no
further effect in increasing contractile force. This occurs
because enough calcium ions are maintained in the
muscle sarcoplasm, even between action potentials, so
that full contractile state is sustained without allowing
any relaxation between the action potentials.
Maximum Strength of Contraction. The maximum strength
of tetanic contraction of a muscle operating at a normal
muscle length averages between 3 and 4 kilograms per
square centimeter of muscle, or 50 pounds per square
inch. Because a quadriceps muscle can have as much as
16 square inches of muscle belly, as much as 800 pounds
of tension may be applied to the patellar tendon. Thus,
one can readily understand how it is possible for
muscles to pull their tendons out of their insertions in
bone.
Changes in Muscle Strength at the Onset of Contraction—The
Staircase Effect (Treppe). When a muscle begins to con-
tract after a long period of rest, its initial strength of
82
Unit II
Membrane Physiology, Nerve, and Muscle
contraction may be as little as one half its strength 10
to 50 muscle twitches later. That is, the strength of contraction increases to a plateau, a phenomenon called the
staircase effect, or treppe.
Although all the possible causes of the staircase effect
are not known, it is believed to be caused primarily by
increasing calcium ions in the cytosol because of the
release of more and more ions from the sarcoplasmic
reticulum with each successive muscle action potential
and failure of the sarcoplasm to recapture the ions
immediately.
Skeletal Muscle Tone. Even when muscles are at rest, a
certain amount of tautness usually remains. This is
called muscle tone. Because normal skeletal muscle
fibers do not contract without an action potential to
stimulate the fibers, skeletal muscle tone results entirely
from a low rate of nerve impulses coming from the
spinal cord. These, in turn, are controlled partly by
signals transmitted from the brain to the appropriate
spinal cord anterior motoneurons and partly by signals
that originate in muscle spindles located in the muscle
itself. Both of these are discussed in relation to muscle
spindle and spinal cord function in Chapter 54.
Muscle Fatigue. Prolonged and strong contraction of a
muscle leads to the well-known state of muscle fatigue.
Studies in athletes have shown that muscle fatigue
increases in almost direct proportion to the rate of
depletion of muscle glycogen. Therefore, fatigue results
mainly from inability of the contractile and metabolic
processes of the muscle fibers to continue supplying the
same work output. However, experiments have also
shown that transmission of the nerve signal through the
neuromuscular junction, which is discussed in Chapter
7, can diminish at least a small amount after intense prolonged muscle activity, thus further diminishing muscle
contraction. Interruption of blood flow through a contracting muscle leads to almost complete muscle fatigue
within 1 or 2 minutes because of the loss of nutrient
supply, especially loss of oxygen.
Lever Systems of the Body. Muscles operate by applying
tension to their points of insertion into bones, and the
bones in turn form various types of lever systems. Figure
6–14 shows the lever system activated by the biceps
muscle to lift the forearm. If we assume that a large
biceps muscle has a cross-sectional area of 6 square
inches, the maximum force of contraction would be
about 300 pounds. When the forearm is at right angles
with the upper arm, the tendon attachment of the biceps
is about 2 inches anterior to the fulcrum at the elbow,
and the total length of the forearm lever is about 14
inches. Therefore, the amount of lifting power of the
biceps at the hand would be only one seventh of the 300
pounds of muscle force, or about 43 pounds. When the
arm is fully extended, the attachment of the biceps is
much less than 2 inches anterior to the fulcrum, and the
force with which the hand can be brought forward is
also much less than 43 pounds.
In short, an analysis of the lever systems of the body
depends on knowledge of (1) the point of muscle insertion, (2) its distance from the fulcrum of the lever, (3)
the length of the lever arm, and (4) the position of the
lever. Many types of movement are required in the
body, some of which need great strength and others
of which need large distances of movement. For this
reason, there are many different types of muscle; some
are long and contract a long distance, and some are
short but have large cross-sectional areas and can
provide extreme strength of contraction over short distances. The study of different types of muscles, lever
systems, and their movements is called kinesiology
and is an important scientific component of human
physioanatomy.
“Positioning” of a Body Part by Contraction of Agonist and Antagonist Muscles on Opposite Sides of a Joint—“Coactivation”
of Antagonist Muscles. Virtually all body movements are
caused by simultaneous contraction of agonist and
antagonist muscles on opposite sides of joints. This
is called coactivation of the agonist and antagonist
muscles, and it is controlled by the motor control centers
of the brain and spinal cord.
The position of each separate part of the body, such
as an arm or a leg, is determined by the relative degrees
of contraction of the agonist and antagonist sets of
muscles. For instance, let us assume that an arm or a leg
is to be placed in a midrange position. To achieve
this, agonist and antagonist muscles are excited about
equally. Remember that an elongated muscle contracts
with more force than a shortened muscle, which was
demonstrated in Figure 6–9, showing maximum strength
of contraction at full functional muscle length and
almost no strength of contraction at half normal length.
Therefore, the elongated muscle on one side of a joint
can contract with far greater force than the shorter
muscle on the opposite side. As an arm or leg moves
toward its midposition, the strength of the longer
muscle decreases, whereas the strength of the shorter
muscle increases until the two strengths equal each
other. At this point, movement of the arm or leg stops.
Thus, by varying the ratios of the degree of activation
of the agonist and antagonist muscles, the nervous
system directs the positioning of the arm or leg.
We learn in Chapter 54 that the motor nervous
system has additional important mechanisms to compensate for different muscle loads when directing this
positioning process.
Remodeling of Muscle
to Match Function
Figure 6–14
Lever system activated by the biceps muscle.
All the muscles of the body are continually being
remodeled to match the functions that are required of
Chapter 6
Contraction of Skeletal Muscle
them. Their diameters are altered, their lengths are
altered, their strengths are altered, their vascular supplies are altered, and even the types of muscle fibers are
altered at least slightly. This remodeling process is often
quite rapid, within a few weeks. Indeed, experiments in
animals have shown that muscle contractile proteins in
some smaller, more active muscles can be replaced in as
little as 2 weeks.
Muscle Hypertrophy and Muscle Atrophy. When the total
mass of a muscle increases, this is called muscle hypertrophy. When it decreases, the process is called muscle
atrophy.
Virtually all muscle hypertrophy results from an
increase in the number of actin and myosin filaments in
each muscle fiber, causing enlargement of the individual muscle fibers; this is called simply fiber hypertrophy.
Hypertrophy occurs to a much greater extent when the
muscle is loaded during the contractile process. Only a
few strong contractions each day are required to cause
significant hypertrophy within 6 to 10 weeks.
The manner in which forceful contraction leads to
hypertrophy is not known. It is known, however, that the
rate of synthesis of muscle contractile proteins is far
greater when hypertrophy is developing, leading also to
progressively greater numbers of both actin and myosin
filaments in the myofibrils, often increasing as much as
50 per cent. In turn, some of the myofibrils themselves
have been observed to split within hypertrophying
muscle to form new myofibrils, but how important this
is in usual muscle hypertrophy is still unknown.
Along with the increasing size of myofibrils, the
enzyme systems that provide energy also increase. This
is especially true of the enzymes for glycolysis, allowing
rapid supply of energy during short-term forceful
muscle contraction.
When a muscle remains unused for many weeks, the
rate of decay of the contractile proteins is more rapid
than the rate of replacement. Therefore, muscle atrophy
occurs.
Adjustment of Muscle Length. Another type of hyper-
trophy occurs when muscles are stretched to greater
than normal length. This causes new sarcomeres to be
added at the ends of the muscle fibers, where they attach
to the tendons. In fact, new sarcomeres can be added
as rapidly as several per minute in newly developing
muscle, illustrating the rapidity of this type of
hypertrophy.
Conversely, when a muscle continually remains shortened to less than its normal length, sarcomeres at the
ends of the muscle fibers can actually disappear. It is by
these processes that muscles are continually remodeled
to have the appropriate length for proper muscle
contraction.
Hyperplasia of Muscle Fibers. Under rare conditions of
extreme muscle force generation, the actual number of
muscle fibers has been observed to increase (but only
by a few percentage points), in addition to the fiber
hypertrophy process. This increase in fiber number is
called fiber hyperplasia. When it does occur, the mechanism is linear splitting of previously enlarged fibers.
Effects of Muscle Denervation. When a muscle loses its
nerve supply, it no longer receives the contractile signals
that are required to maintain normal muscle size.
Therefore, atrophy begins almost immediately. After
83
about 2 months, degenerative changes also begin to
appear in the muscle fibers themselves. If the nerve
supply to the muscle grows back rapidly, full return
of function can occur in as little as 3 months, but from
that time onward, the capability of functional return
becomes less and less, with no further return of function
after 1 to 2 years.
In the final stage of denervation atrophy, most of the
muscle fibers are destroyed and replaced by fibrous and
fatty tissue. The fibers that do remain are composed of
a long cell membrane with a lineup of muscle cell nuclei
but with few or no contractile properties and little or no
capability of regenerating myofibrils if a nerve does
regrow.
The fibrous tissue that replaces the muscle fibers
during denervation atrophy also has a tendency to continue shortening for many months, which is called contracture. Therefore, one of the most important problems
in the practice of physical therapy is to keep atrophying
muscles from developing debilitating and disfiguring
contractures. This is achieved by daily stretching of the
muscles or use of appliances that keep the muscles
stretched during the atrophying process.
Recovery of Muscle Contraction in Poliomyelitis: Development of Macromotor Units. When some but not all
nerve fibers to a muscle are destroyed, as commonly
occurs in poliomyelitis, the remaining nerve fibers
branch off to form new axons that then innervate many
of the paralyzed muscle fibers. This causes large motor
units called macromotor units, which can contain as
many as five times the normal number of muscle fibers
for each motoneuron coming from the spinal cord. This
decreases the fineness of control one has over the
muscles but does allow the muscles to regain varying
degrees of strength.
Rigor Mortis
Several hours after death, all the muscles of the body go
into a state of contracture called “rigor mortis”; that is,
the muscles contract and become rigid, even without
action potentials. This rigidity results from loss of all the
ATP, which is required to cause separation of the crossbridges from the actin filaments during the relaxation
process. The muscles remain in rigor until the muscle
proteins deteriorate about 15 to 25 hours later, which
presumably results from autolysis caused by enzymes
released from lysosomes. All these events occur more
rapidly at higher temperatures.
References
Berchtold MW, Brinkmeier H, Muntener M: Calcium ion
in skeletal muscle: its crucial role for muscle function,
plasticity, and disease. Physiol Rev 80:1215, 2000.
Brooks SV: Current topics for teaching skeletal muscle physiology. Adv Physiol Educ 27:171, 2003.
Clausen T: Na+-K+ pump regulation and skeletal muscle contractility. Physiol Rev 83:1269, 2003.
Glass DJ: Molecular mechanisms modulating muscle mass.
Trends Mol Med 8:344, 2003.
Glass DJ: Signalling pathways that mediate skeletal muscle
hypertrophy and atrophy. Nat Cell Biol 5:87, 2003.
Gordon AM, Homsher E, Regnier M: Regulation of contraction in striated muscle. Physiol Rev 80:853, 2000.
84
Unit II
Membrane Physiology, Nerve, and Muscle
Gordon AM, Regnier M, Homsher E: Skeletal and cardiac
muscle contractile activation: tropomyosin “rocks and
rolls.” News Physiol Sci 16:49, 2001.
Huxley AF, Gordon AM: Striation patterns in active and
passive shortening of muscle. Nature (Lond) 193:280,
1962.
Huxley HE: A personal view of muscle and motility mechanisms. Annu Rev Physiol 58:1, 1996.
Jurkat-Rott K, Lerche H, Lehmann-Horn F: Skeletal muscle
channelopathies. J Neurol 249:1493, 2002.
Kjær M: Role of extracellular matrix in adaptation of tendon
and skeletal muscle to mechanical loading. Physiol Rev
84:649, 2004.
MacIntosh BR: Role of calcium sensitivity modulation in
skeletal muscle performance. News Physiol Sci 18:222,
2003.
Matthews GG: Cellular Physiology of Nerve and Muscle.
Malden, MA: Blackwell Science, 1998.
Sieck GC, Regnier M: Plasticity and energetic demands of
contraction in skeletal and cardiac muscle. J Appl Physiol
90:1158, 2001.
Stamler JS, Meissner G: Physiology of nitric oxide in skeletal muscle. Physiol Rev 81:209, 2001.
Szent-Gyorgyi AG: Regulation of contraction by calcium
binding myosins. Biophys Chem 59:357, 1996.
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7
Excitation of Skeletal Muscle:
Neuromuscular Transmission and
Excitation-Contraction Coupling
Transmission of Impulses
from Nerve Endings to
Skeletal Muscle Fibers:
The Neuromuscular
Junction
The skeletal muscle fibers are innervated by large,
myelinated nerve fibers that originate from large motoneurons in the anterior
horns of the spinal cord. As pointed out in Chapter 6, each nerve fiber, after
entering the muscle belly, normally branches and stimulates from three to
several hundred skeletal muscle fibers. Each nerve ending makes a junction,
called the neuromuscular junction, with the muscle fiber near its midpoint. The
action potential initiated in the muscle fiber by the nerve signal travels in both
directions toward the muscle fiber ends. With the exception of about 2 per cent
of the muscle fibers, there is only one such junction per muscle fiber.
Physiologic Anatomy of the Neuromuscular Junction—The Motor End Plate. Figure 7–1A
and B shows the neuromuscular junction from a large, myelinated nerve fiber
to a skeletal muscle fiber. The nerve fiber forms a complex of branching nerve
terminals that invaginate into the surface of the muscle fiber but lie outside the
muscle fiber plasma membrane. The entire structure is called the motor end
plate. It is covered by one or more Schwann cells that insulate it from the
surrounding fluids.
Figure 7–1C shows an electron micrographic sketch of the junction between
a single axon terminal and the muscle fiber membrane. The invaginated membrane is called the synaptic gutter or synaptic trough, and the space between the
terminal and the fiber membrane is called the synaptic space or synaptic cleft.
This space is 20 to 30 nanometers wide. At the bottom of the gutter are numerous smaller folds of the muscle membrane called subneural clefts, which greatly
increase the surface area at which the synaptic transmitter can act.
In the axon terminal are many mitochondria that supply adenosine triphosphate (ATP), the energy source that is used for synthesis of an excitatory
transmitter acetylcholine. The acetylcholine in turn excites the muscle
fiber membrane. Acetylcholine is synthesized in the cytoplasm of the terminal,
but it is absorbed rapidly into many small synaptic vesicles, about 300,000
of which are normally in the terminals of a single end plate. In the synaptic
space are large quantities of the enzyme acetylcholinesterase, which destroys
acetylcholine a few milliseconds after it has been released from the synaptic
vesicles.
Secretion of Acetylcholine by the Nerve Terminals
When a nerve impulse reaches the neuromuscular junction, about 125 vesicles
of acetylcholine are released from the terminals into the synaptic space. Some
of the details of this mechanism can be seen in Figure 7–2, which shows an
expanded view of a synaptic space with the neural membrane above and the
muscle membrane and its subneural clefts below.
85
86
Unit II
Myelin
sheath
Membrane Physiology, Nerve, and Muscle
Axon
Terminal nerve
branches
Teloglial cell
Muscle
nuclei
Myofibrils
A
B
Axon terminal in
synaptic trough
Synaptic vesicles
Figure 7–1
C
Different views of the motor end
plate. A, Longitudinal section
through the end plate. B, Surface
view of the end plate. C, Electron
micrographic appearance of the
contact point between a single
axon terminal and the muscle
fiber membrane. (Redrawn from
Fawcett DW, as modified from
Couteaux R, in Bloom W, Fawcett
DW: A Textbook of Histology.
Philadelphia: WB Saunders,
1986.)
Subneural clefts
On the inside surface of the neural membrane are
linear dense bars, shown in cross section in Figure 7–2.
To each side of each dense bar are protein particles
that penetrate the neural membrane; these are voltagegated calcium channels. When an action potential
spreads over the terminal, these channels open and
allow calcium ions to diffuse from the synaptic space
to the interior of the nerve terminal. The calcium ions,
in turn, are believed to exert an attractive influence on
the acetylcholine vesicles, drawing them to the neural
membrane adjacent to the dense bars. The vesicles
then fuse with the neural membrane and empty their
acetylcholine into the synaptic space by the process of
exocytosis.
Although some of the aforementioned details are
speculative, it is known that the effective stimulus for
causing acetylcholine release from the vesicles is entry
of calcium ions and that acetylcholine from the vesicles is then emptied through the neural membrane
adjacent to the dense bars.
Effect of Acetylcholine on the Postsynaptic Muscle Fiber Membrane to Open Ion Channels. Figure 7–2 also shows many
very small acetylcholine receptors in the muscle fiber
membrane; these are acetylcholine-gated ion channels,
and they are located almost entirely near the mouths
of the subneural clefts lying immediately below the
dense bar areas, where the acetylcholine is emptied
into the synaptic space.
Each receptor is a protein complex that has a total
molecular weight of 275,000. The complex is composed
Neural
Release
sites membrane
Vesicles
Dense bar
Calcium
channels
Basal lamina
and
acetylcholinesterase
Acetylcholine
receptors
Subneural cleft
Muscle
membrane
Figure 7–2
Release of acetylcholine from synaptic vesicles at the neural
membrane of the neuromuscular junction. Note the proximity of
the release sites in the neural membrane to the acetylcholine
receptors in the muscle membrane, at the mouths of the subneural
clefts.
Chapter 7
Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling
of five subunit proteins, two alpha proteins and one
each of beta, delta, and gamma proteins. These protein
molecules penetrate all the way through the membrane, lying side by side in a circle to form a tubular
channel, illustrated in Figure 7–3. The channel remains
constricted, as shown in section A of the figure, until
two acetylcholine molecules attach respectively to the
two alpha subunit proteins. This causes a conformational change that opens the channel, as shown in
section B of the figure.
The opened acetylcholine channel has a diameter of
about 0.65 nanometer, which is large enough to allow
the important positive ions—sodium (Na+), potassium
(K+), and calcium (Ca++)—to move easily through the
opening. Conversely, negative ions, such as chloride
ions, do not pass through because of strong negative
charges in the mouth of the channel that repel these
negative ions.
87
In practice, far more sodium ions flow through the
acetylcholine channels than any other ions, for two
reasons. First, there are only two positive ions in large
concentration: sodium ions in the extracellular fluid,
and potassium ions in the intracellular fluid. Second,
the very negative potential on the inside of the muscle
membrane, –80 to –90 millivolts, pulls the positively
charged sodium ions to the inside of the fiber, while
simultaneously preventing efflux of the positively
charged potassium ions when they attempt to pass
outward.
As shown in Figure 7–3B, the principal effect of
opening the acetylcholine-gated channels is to allow
large numbers of sodium ions to pour to the inside of
the fiber, carrying with them large numbers of positive
charges. This creates a local positive potential change
inside the muscle fiber membrane, called the end plate
potential. In turn, this end plate potential initiates an
action potential that spreads along the muscle membrane and thus causes muscle contraction.
Destruction of the Released Acetylcholine by Acetylcholinesterase. The acetylcholine, once released into
–
–
–
–
–
–
A
Na+
Ach
–
–
–
–
–
–
B
Figure 7–3
Acetylcholine channel. A, Closed state. B, After acetylcholine
(Ach) has become attached and a conformational change has
opened the channel, allowing sodium ions to enter the muscle
fiber and excite contraction. Note the negative charges at the
channel mouth that prevent passage of negative ions such as
chloride ions.
the synaptic space, continues to activate the acetylcholine receptors as long as the acetylcholine persists
in the space. However, it is removed rapidly by two
means: (1) Most of the acetylcholine is destroyed
by the enzyme acetylcholinesterase, which is attached
mainly to the spongy layer of fine connective tissue
that fills the synaptic space between the presynaptic
nerve terminal and the postsynaptic muscle membrane. (2) A small amount of acetylcholine diffuses out
of the synaptic space and is then no longer available
to act on the muscle fiber membrane.
The short time that the acetylcholine remains in the
synaptic space—a few milliseconds at most—normally
is sufficient to excite the muscle fiber. Then the rapid
removal of the acetylcholine prevents continued
muscle re-excitation after the muscle fiber has recovered from its initial action potential.
End Plate Potential and Excitation of the Skeletal Muscle Fiber.
The sudden insurgence of sodium ions into the muscle
fiber when the acetylcholine channels open causes the
electrical potential inside the fiber at the local area of
the end plate to increase in the positive direction as
much as 50 to 75 millivolts, creating a local potential
called the end plate potential. Recall from Chapter 5
that a sudden increase in nerve membrane potential
of more than 20 to 30 millivolts is normally sufficient
to initiate more and more sodium channel opening,
thus initiating an action potential at the muscle fiber
membrane.
Figure 7–4 shows the principle of an end plate
potential initiating the action potential. This figure
shows three separate end plate potentials. End plate
potentials A and C are too weak to elicit an action
potential, but they do produce weak local end plate
voltage changes, as recorded in the figure. By contrast,
end plate potential B is much stronger and causes
enough sodium channels to open so that the selfregenerative effect of more and more sodium ions
88
Unit II
Membrane Physiology, Nerve, and Muscle
+60
+40
Millivolts
+20
0
– 20
– 40
– 60
– 80
A
–100
0
B
15
30
C
45
60
75
Milliseconds
Figure 7–4
End plate potentials (in millivolts). A, Weakened end plate potential recorded in a curarized muscle, too weak to elicit an action
potential. B, Normal end plate potential eliciting a muscle action
potential. C, Weakened end plate potential caused by botulinum
toxin that decreases end plate release of acetylcholine, again too
weak to elicit a muscle action potential.
flowing to the interior of the fiber initiates an action
potential. The weakness of the end plate potential at
point A was caused by poisoning of the muscle fiber
with curare, a drug that blocks the gating action of
acetylcholine on the acetylcholine channels by competing for the acetylcholine receptor sites. The weakness of the end plate potential at point C resulted from
the effect of botulinum toxin, a bacterial poison that
decreases the quantity of acetylcholine release by the
nerve terminals.
Safety Factor for Transmission at the Neuromuscular Junction;
Fatigue of the Junction. Ordinarily, each impulse that
arrives at the neuromuscular junction causes about
three times as much end plate potential as that
required to stimulate the muscle fiber. Therefore, the
normal neuromuscular junction is said to have a high
safety factor. However, stimulation of the nerve fiber
at rates greater than 100 times per second for several
minutes often diminishes the number of acetylcholine
vesicles so much that impulses fail to pass into the
muscle fiber. This is called fatigue of the neuromuscular junction, and it is the same effect that causes fatigue
of synapses in the central nervous system when the
synapses are overexcited. Under normal functioning
conditions, measurable fatigue of the neuromuscular
junction occurs rarely, and even then only at the most
exhausting levels of muscle activity.
Molecular Biology of
Acetylcholine Formation
and Release
Because the neuromuscular junction is large enough to
be studied easily, it is one of the few synapses of the
nervous system for which most of the details of chemical transmission have been worked out. The formation
and release of acetylcholine at this junction occur in the
following stages:
1. Small vesicles, about 40 nanometers in size, are
formed by the Golgi apparatus in the cell body of
the motoneuron in the spinal cord. These vesicles
are then transported by axoplasm that “streams”
through the core of the axon from the central
cell body in the spinal cord all the way to the
neuromuscular junction at the tips of the peripheral
nerve fibers. About 300,000 of these small vesicles
collect in the nerve terminals of a single skeletal
muscle end plate.
2. Acetylcholine is synthesized in the cytosol of the
nerve fiber terminal but is immediately transported
through the membranes of the vesicles to their
interior, where it is stored in highly concentrated
form, about 10,000 molecules of acetylcholine in
each vesicle.
3. When an action potential arrives at the nerve
terminal, it opens many calcium channels in the
membrane of the nerve terminal because this
terminal has an abundance of voltage-gated
calcium channels. As a result, the calcium ion
concentration inside the terminal membrane
increases about 100-fold, which in turn increases
the rate of fusion of the acetylcholine vesicles with
the terminal membrane about 10,000-fold. This
fusion makes many of the vesicles rupture, allowing
exocytosis of acetylcholine into the synaptic space.
About 125 vesicles usually rupture with each
action potential. Then, after a few milliseconds, the
acetylcholine is split by acetylcholinesterase
into acetate ion and choline, and the choline is
reabsorbed actively into the neural terminal to be
reused to form new acetylcholine. This sequence of
events occurs within a period of 5 to 10
milliseconds.
4. The number of vesicles available in the nerve
ending is sufficient to allow transmission of only a
few thousand nerve-to-muscle impulses. Therefore,
for continued function of the neuromuscular
junction, new vesicles need to be re-formed rapidly.
Within a few seconds after each action potential is
over, “coated pits” appear in the terminal nerve
membrane, caused by contractile proteins in the
nerve ending, especially the protein clathrin, which
is attached to the membrane in the areas of the
original vesicles. Within about 20 seconds, the
proteins contract and cause the pits to break away
to the interior of the membrane, thus forming new
vesicles. Within another few seconds, acetylcholine
is transported to the interior of these vesicles, and
they are then ready for a new cycle of acetylcholine
release.
Drugs That Enhance or
Block Transmission at the
Neuromuscular Junction
Drugs That Stimulate the Muscle Fiber by Acetylcholine-Like
Action. Many compounds, including methacholine, car-
bachol, and nicotine, have the same effect on the muscle
fiber as does acetylcholine. The difference between
Chapter 7
Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling
these drugs and acetylcholine is that the drugs are not
destroyed by cholinesterase or are destroyed so slowly
that their action often persists for many minutes to
several hours. The drugs work by causing localized areas
of depolarization of the muscle fiber membrane at the
motor end plate where the acetylcholine receptors
are located. Then, every time the muscle fiber recovers
from a previous contraction, these depolarized areas, by
virtue of leaking ions, initiate a new action potential,
thereby causing a state of muscle spasm.
Drugs That Stimulate the Neuromuscular Junction by
Inactivating Acetylcholinesterase. Three particularly well-
known drugs, neostigmine, physostigmine, and
diisopropyl fluorophosphate, inactivate the acetylcholinesterase in the synapses so that it no longer
hydrolyzes acetylcholine. Therefore, with each successive nerve impulse, additional acetylcholine accumulates and stimulates the muscle fiber repetitively. This
causes muscle spasm when even a few nerve impulses
reach the muscle. Unfortunately, it also can cause death
due to laryngeal spasm, which smothers the person.
Neostigmine and physostigmine combine with acetylcholinesterase to inactivate the acetylcholinesterase
for up to several hours, after which these drugs are
displaced from the acetylcholinesterase so that the
esterase once again becomes active. Conversely, diisopropyl fluorophosphate, which has military potential
as a powerful “nerve” gas poison, inactivates acetylcholinesterase for weeks, which makes this a particularly lethal poison.
Drugs That Block Transmission at the Neuromuscular Junction. A
group of drugs known as curariform drugs can prevent
passage of impulses from the nerve ending into the
muscle. For instance, D-tubocurarine blocks the action
of acetylcholine on the muscle fiber acetylcholine receptors, thus preventing sufficient increase in permeability
of the muscle membrane channels to initiate an action
potential.
Myasthenia Gravis
Myasthenia gravis, which occurs in about 1 in every
20,000 persons, causes muscle paralysis because of
inability of the neuromuscular junctions to transmit
enough signals from the nerve fibers to the muscle
fibers. Pathologically, antibodies that attack the acetylcholine-gated sodium ion transport proteins have been
demonstrated in the blood of most patients with
myasthenia gravis. Therefore, it is believed that myasthenia gravis is an autoimmune disease in which the
patients have developed immunity against their own
acetylcholine-activated ion channels.
Regardless of the cause, the end plate potentials that
occur in the muscle fibers are mostly too weak to stimulate the muscle fibers. If the disease is intense enough,
the patient dies of paralysis—in particular, paralysis of
the respiratory muscles. The disease usually can be ameliorated for several hours by administering neostigmine
or some other anticholinesterase drug, which allows
larger than normal amounts of acetylcholine to accumulate in the synaptic space. Within minutes, some of
these paralyzed people can begin to function almost
normally, until a new dose of neostigmine is required a
few hours later.
89
Muscle Action Potential
Almost everything discussed in Chapter 5 regarding
initiation and conduction of action potentials in nerve
fibers applies equally to skeletal muscle fibers, except
for quantitative differences. Some of the quantitative
aspects of muscle potentials are the following:
1. Resting membrane potential: about –80 to –90
millivolts in skeletal fibers—the same as in large
myelinated nerve fibers.
2. Duration of action potential: 1 to 5 milliseconds in
skeletal muscle—about five times as long as in
large myelinated nerves.
3. Velocity of conduction: 3 to 5 m/sec—about 1/13
the velocity of conduction in the large myelinated
nerve fibers that excite skeletal muscle.
Spread of the Action Potential to the
Interior of the Muscle Fiber by Way of
“Transverse Tubules”
The skeletal muscle fiber is so large that action potentials spreading along its surface membrane cause
almost no current flow deep within the fiber. Yet, to
cause maximum muscle contraction, current must penetrate deeply into the muscle fiber to the vicinity of the
separate myofibrils. This is achieved by transmission of
action potentials along transverse tubules (T tubules)
that penetrate all the way through the muscle fiber
from one side of the fiber to the other, as illustrated in
Figure 7–5. The T tubule action potentials cause
release of calcium ions inside the muscle fiber in the
immediate vicinity of the myofibrils, and these calcium
ions then cause contraction. This overall process is
called excitation-contraction coupling.
Excitation-Contraction
Coupling
Transverse Tubule–Sarcoplasmic
Reticulum System
Figure 7–5 shows myofibrils surrounded by the T
tubule–sarcoplasmic reticulum system. The T tubules
are very small and run transverse to the myofibrils.
They begin at the cell membrane and penetrate all the
way from one side of the muscle fiber to the opposite
side. Not shown in the figure is the fact that these
tubules branch among themselves so that they form
entire planes of T tubules interlacing among all the
separate myofibrils. Also, where the T tubules originate
from the cell membrane, they are open to the exterior
of the muscle fiber. Therefore, they communicate with
the extracellular fluid surrounding the muscle fiber,
and they themselves contain extracellular fluid in their
lumens. In other words, the T tubules are actually
internal extensions of the cell membrane. Therefore,
when an action potential spreads over a muscle fiber
90
Unit II
Membrane Physiology, Nerve, and Muscle
Myofibrils
Sarcolemma
Terminal
cisternae
Triad of the
reticulum
Z line
Transverse
tubule
Mitochondrion
A band
Sarcoplasmic
reticulum
Transverse
tubule
I band
Sarcotubules
membrane, a potential change also spreads along the
T tubules to the deep interior of the muscle fiber. The
electrical currents surrounding these T tubules then
elicit the muscle contraction.
Figure 7–5 also shows a sarcoplasmic reticulum, in
yellow. This is composed of two major parts: (1) large
chambers called terminal cisternae that abut the T
tubules, and (2) long longitudinal tubules that surround all surfaces of the actual contracting myofibrils.
Release of Calcium Ions by the
Sarcoplasmic Reticulum
One of the special features of the sarcoplasmic reticulum is that within its vesicular tubules is an excess of
calcium ions in high concentration, and many of these
ions are released from each vesicle when an action
potential occurs in the adjacent T tubule.
Figure 7–6 shows that the action potential of the T
tubule causes current flow into the sarcoplasmic reticular cisternae where they abut the T tubule. This in
turn causes rapid opening of large numbers of calcium
channels through the membranes of the cisternae as
well as their attached longitudinal tubules. These
channels remain open for a few milliseconds; during
this time, enough calcium ions are released into the
Figure 7–5
Transverse (T) tubule–sarcoplasmic reticulum system. Note that
the T tubules communicate with the
outside of the cell membrane, and
deep in the muscle fiber, each T
tubule lies adjacent to the ends of
longitudinal sarcoplasmic reticulum
tubules that surround all sides of
the actual myofibrils that contract.
This illustration was drawn from frog
muscle, which has one T tubule per
sarcomere, located at the Z line. A
similar arrangement is found in
mammalian heart muscle, but
mammalian skeletal muscle has
two T tubules per sarcomere,
located at the A-I band junctions.
(Redrawn from Bloom W, Fawcett
DW: A Textbook of Histology.
Philadelphia: WB Saunders, 1986.
Modified after Peachey LD: J Cell
Biol 25:209, 1965. Drawn by Sylvia
Colard Keene.)
sarcoplasm surrounding the myofibrils to cause contraction, as discussed in Chapter 6.
Calcium Pump for Removing Calcium Ions from the Myofibrillar
Fluid After Contraction Occurs. Once the calcium ions
have been released from the sarcoplasmic tubules and
have diffused among the myofibrils, muscle contraction continues as long as the calcium ions remain in
high concentration. However, a continually active
calcium pump located in the walls of the sarcoplasmic
reticulum pumps calcium ions away from the myofibrils back into the sarcoplasmic tubules. This pump can
concentrate the calcium ions about 10,000-fold inside
the tubules. In addition, inside the reticulum is a
protein called calsequestrin that can bind up to 40
times more calcium.
Excitatory “Pulse” of Calcium Ions. The normal resting
state concentration (less than 10-7 molar) of calcium
ions in the cytosol that bathes the myofibrils is too
little to elicit contraction. Therefore, the troponintropomyosin complex keeps the actin filaments inhibited and maintains a relaxed state of the muscle.
Conversely, full excitation of the T tubule and
sarcoplasmic reticulum system causes enough release of calcium ions to increase the concentration
in the myofibrillar fluid to as high as 2 ¥ 10-4 molar
Chapter 7
Excitation of Skeletal Muscle: Neuromuscular Transmission and Excitation-Contraction Coupling
91
Action potential
Sarcolemma
Calcium pump
Ca
Ca
ATP
required
Figure 7–6
Excitation-contraction coupling in
the muscle, showing (1) an action
potential that causes release of
calcium ions from the sarcoplasmic reticulum and then (2) reuptake of the calcium ions by a
calcium pump.
concentration, a 500-fold increase, which is about 10
times the level required to cause maximum muscle
contraction. Immediately thereafter, the calcium pump
depletes the calcium ions again. The total duration of
this calcium “pulse” in the usual skeletal muscle fiber
lasts about 1/20 of a second, although it may last
several times as long in some fibers and several times
less in others. (In heart muscle, the calcium pulse lasts
about 1/3 of a second because of the long duration of
the cardiac action potential.)
During this calcium pulse, muscle contraction
occurs. If the contraction is to continue without interruption for long intervals, a series of calcium pulses
must be initiated by a continuous series of repetitive
action potentials, as discussed in Chapter 6.
References
Also see references for Chapters 5 and 6.
Allman BL, Rice CL: Neuromuscular fatigue and aging:
central and peripheral factors. Muscle Nerve 25:785, 2002.
Amonof MJ: Electromyography in Clinical Practice. New
York: Churchill Livingstone, 1998.
Brown RH Jr: Dystrophin-associated proteins and the muscular dystrophies. Annu Rev Med 48:457, 1997.
Chaudhuri A, Behan PO: Fatigue in neurological disorders.
Lancet 363:978, 2004.
Engel AG, Ohno K, Shen XM, Sine SM: Congenital myasthenic syndromes: multiple molecular targets at the
neuromuscular junction. Ann N Y Acad Sci 998:138,
2003.
Haouzi P, Chenuel B, Huszczuk A: Sensing vascular distension in skeletal muscle by slow conducting afferent fibers:
Ca++
Ca++
Actin filaments
Myosin filaments
neurophysiological basis and implication for respiratory
control. J Appl Physiol 96:407, 2004.
Hoch W: Molecular dissection of neuromuscular junction
formation. Trends Neurosci 26:335, 2003.
Keesey JC: Clinical evaluation and management of myasthenia gravis. Muscle Nerve 29:484, 2004.
Lee C: Conformation, action, and mechanism of action of
neuromuscular blocking muscle relaxants. Pharmacol Ther
98:143, 2003.
Leite JF, Rodrigues-Pinguet N, Lester HA: Insights into
channel function via channel dysfunction. J Clin Invest
111:436, 2003.
Payne AM, Delbono O: Neurogenesis of excitationcontraction uncoupling in aging skeletal muscle. Exerc
Sport Sci Rev 32:36, 2004.
Pette D: Historical perspectives: plasticity of mammalian
skeletal muscle. J Appl Physiol 90:1119, 2001.
Rekling JC, Funk GD, Bayliss DA, et al: Synaptic control of
motoneuronal excitability. Physiol Rev 80:767, 2000.
Schiaffino S, Serrano A: Calcineurin signaling and neural
control of skeletal muscle fiber type and size. Trends
Pharmacol Sci 23:569, 2002.
Tang W, Sencer S, Hamilton SL: Calmodulin modulation of
proteins involved in excitation-contraction coupling. Front
Biosci 7:583, 2002.
Toyoshima C, Nomura H, Sugita Y: Structural basis of ion
pumping by Ca2+-ATPase of sarcoplasmic reticulum.
FEBS Lett 555:106, 2003.
Van der Kloot W, Molgo J: Quantal acetylcholine release at
the vertebrate neuromuscular junction. Physiol Rev
74:899, 1994.
Vincent A: Unraveling the pathogenesis of myasthenia
gravis. Nat Rev Immunol 10:797, 2002.
Vincent A, McConville J, Farrugia ME, et al: Antibodies in
myasthenia gravis and related disorders. Ann N Y Acad
Sci 998:324, 2003.
C
H
A
P
T
E
R
Contraction and Excitation
of Smooth Muscle
Contraction of Smooth
Muscle
In Chapters 6 and 7, the discussion was concerned
with skeletal muscle. We now turn to smooth
muscle, which is composed of far smaller fibers—
usually 1 to 5 micrometers in diameter and only 20
to 500 micrometers in length. In contrast, skeletal
muscle fibers are as much as 30 times greater in diameter and hundreds of times
as long. Many of the same principles of contraction apply to smooth muscle as
to skeletal muscle. Most important, essentially the same attractive forces
between myosin and actin filaments cause contraction in smooth muscle as in
skeletal muscle, but the internal physical arrangement of smooth muscle fibers
is very different.
Types of Smooth Muscle
The smooth muscle of each organ is distinctive from that of most other organs
in several ways: (1) physical dimensions, (2) organization into bundles or sheets,
(3) response to different types of stimuli, (4) characteristics of innervation, and
(5) function. Yet, for the sake of simplicity, smooth muscle can generally be
divided into two major types, which are shown in Figure 8–1: multi-unit smooth
muscle and unitary (or single-unit) smooth muscle.
Multi-Unit Smooth Muscle. This type of smooth muscle is composed of discrete,
separate smooth muscle fibers. Each fiber operates independently of the others
and often is innervated by a single nerve ending, as occurs for skeletal muscle
fibers. Further, the outer surfaces of these fibers, like those of skeletal muscle
fibers, are covered by a thin layer of basement membrane–like substance, a
mixture of fine collagen and glycoprotein that helps insulate the separate fibers
from one another.
The most important characteristic of multi-unit smooth muscle fibers is that
each fiber can contract independently of the others, and their control is exerted
mainly by nerve signals. In contrast, a major share of control of unitary smooth
muscle is exerted by non-nervous stimuli. Some examples of multi-unit smooth
muscle are the ciliary muscle of the eye, the iris muscle of the eye, and the piloerector muscles that cause erection of the hairs when stimulated by the sympathetic nervous system.
Unitary Smooth Muscle. The term “unitary” is confusing because it does not mean
single muscle fibers. Instead, it means a mass of hundreds to thousands of
smooth muscle fibers that contract together as a single unit. The fibers usually
are arranged in sheets or bundles, and their cell membranes are adherent to one
another at multiple points so that force generated in one muscle fiber can be
transmitted to the next. In addition, the cell membranes are joined by many gap
junctions through which ions can flow freely from one muscle cell to the next
so that action potentials or simple ion flow without action potentials can travel
92
8
Chapter 8
93
Contraction and Excitation of Smooth Muscle
Adventitia
Actin
filaments
Dense bodies
Medial
muscle fibers
Endothelium
Small artery
Multi-unit smooth muscle
Unitary smooth muscle
A
B
Myosin filaments
Figure 8–1
Multi-unit (A) and unitary (B) smooth muscle.
from one fiber to the next and cause the muscle fibers
to contract together. This type of smooth muscle is also
known as syncytial smooth muscle because of its syncytial interconnections among fibers. It is also called
visceral smooth muscle because it is found in the walls
of most viscera of the body, including the gut, bile
ducts, ureters, uterus, and many blood vessels.
Cell membrane
Contractile Mechanism in
Smooth Muscle
Chemical Basis for Smooth
Muscle Contraction
Smooth muscle contains both actin and myosin filaments, having chemical characteristics similar to those
of the actin and myosin filaments in skeletal muscle.
It does not contain the normal troponin complex
that is required in the control of skeletal muscle contraction, so the mechanism for control of contraction
is different. This is discussed in detail later in this
chapter.
Chemical studies have shown that actin and myosin
filaments derived from smooth muscle interact with
each other in much the same way that they do in skeletal muscle. Further, the contractile process is activated
by calcium ions, and adenosine triphosphate (ATP) is
degraded to adenosine diphosphate (ADP) to provide
the energy for contraction.
There are, however, major differences between the
physical organization of smooth muscle and that of
skeletal muscle, as well as differences in excitationcontraction coupling, control of the contractile process
by calcium ions, duration of contraction, and amount
of energy required for contraction.
Figure 8–2
Physical structure of smooth muscle. The upper left-hand fiber
shows actin filaments radiating from dense bodies. The lower lefthand fiber and the right-hand diagram demonstrate the relation of
myosin filaments to actin filaments.
Physical Basis for Smooth Muscle Contraction
Smooth muscle does not have the same striated
arrangement of actin and myosin filaments as is found
in skeletal muscle. Instead, electron micrographic techniques suggest the physical organization exhibited in
Figure 8–2. This figure shows large numbers of actin
filaments attached to so-called dense bodies. Some of
these bodies are attached to the cell membrane.
Others are dispersed inside the cell. Some of the membrane dense bodies of adjacent cells are bonded
together by intercellular protein bridges. It is mainly
through these bonds that the force of contraction is
transmitted from one cell to the next.
94
Unit II
Membrane Physiology, Nerve, and Muscle
Interspersed among the actin filaments in the
muscle fiber are myosin filaments. These have a diameter more than twice that of the actin filaments. In
electron micrographs, one usually finds 5 to 10 times
as many actin filaments as myosin filaments.
To the right in Figure 8–2 is a postulated structure
of an individual contractile unit within a smooth
muscle cell, showing large numbers of actin filaments
radiating from two dense bodies; the ends of these
filaments overlap a myosin filament located midway
between the dense bodies. This contractile unit is
similar to the contractile unit of skeletal muscle, but
without the regularity of the skeletal muscle structure;
in fact, the dense bodies of smooth muscle serve the
same role as the Z discs in skeletal muscle.
There is another difference: Most of the myosin filaments have what are called “sidepolar” cross-bridges
arranged so that the bridges on one side hinge in one
direction and those on the other side hinge in the
opposite direction. This allows the myosin to pull an
actin filament in one direction on one side while simultaneously pulling another actin filament in the opposite direction on the other side. The value of this
organization is that it allows smooth muscle cells to
contract as much as 80 per cent of their length instead
of being limited to less than 30 per cent, as occurs in
skeletal muscle.
Comparison of Smooth Muscle Contraction
and Skeletal Muscle Contraction
Although most skeletal muscles contract and relax
rapidly, most smooth muscle contraction is prolonged
tonic contraction, sometimes lasting hours or even
days.Therefore, it is to be expected that both the physical and the chemical characteristics of smooth muscle
versus skeletal muscle contraction would differ.
Following are some of the differences.
Slow Cycling of the Myosin Cross-Bridges. The rapidity
of cycling of the myosin cross-bridges in smooth
muscle—that is, their attachment to actin, then release
from the actin, and reattachment for the next cycle—
is much, much slower in smooth muscle than in skeletal muscle; in fact, the frequency is as little as 1/10 to
1/300 that in skeletal muscle. Yet the fraction of time
that the cross-bridges remain attached to the actin filaments, which is a major factor that determines the
force of contraction, is believed to be greatly increased
in smooth muscle. A possible reason for the slow
cycling is that the cross-bridge heads have far less
ATPase activity than in skeletal muscle, so that degradation of the ATP that energizes the movements
of the cross-bridge heads is greatly reduced, with
corresponding slowing of the rate of cycling.
Energy Required to Sustain Smooth Muscle Contraction. Only
1/10 to 1/300 as much energy is required to sustain the
same tension of contraction in smooth muscle as in
skeletal muscle. This, too, is believed to result from the
slow attachment and detachment cycling of the crossbridges and because only one molecule of ATP is
required for each cycle, regardless of its duration.
This sparsity of energy utilization by smooth muscle
is exceedingly important to the overall energy
economy of the body, because organs such as the intestines, urinary bladder, gallbladder, and other viscera
often maintain tonic muscle contraction almost
indefinitely.
Slowness of Onset of Contraction and Relaxation of the Total
Smooth Muscle Tissue. A typical smooth muscle tissue
begins to contract 50 to 100 milliseconds after it is
excited, reaches full contraction about 0.5 second later,
and then declines in contractile force in another 1 to
2 seconds, giving a total contraction time of 1 to 3
seconds. This is about 30 times as long as a single
contraction of an average skeletal muscle fiber. But
because there are so many types of smooth muscle,
contraction of some types can be as short as 0.2 second
or as long as 30 seconds.
The slow onset of contraction of smooth muscle,
as well as its prolonged contraction, is caused by
the slowness of attachment and detachment of the
cross-bridges with the actin filaments. In addition, the
initiation of contraction in response to calcium ions is
much slower than in skeletal muscle, as discussed later.
Force of Muscle Contraction. Despite the relatively few
myosin filaments in smooth muscle, and despite the
slow cycling time of the cross-bridges, the maximum
force of contraction of smooth muscle is often greater
than that of skeletal muscle—as great as 4 to 6 kg/cm2
cross-sectional area for smooth muscle, in comparison
with 3 to 4 kilograms for skeletal muscle. This great
force of smooth muscle contraction results from the
prolonged period of attachment of the myosin crossbridges to the actin filaments.
“Latch” Mechanism for Prolonged Holding of Contractions of
Smooth Muscle. Once smooth muscle has developed full
contraction, the amount of continuing excitation
usually can be reduced to far less than the initial
level, yet the muscle maintains its full force of contraction. Further, the energy consumed to maintain
contraction is often minuscule, sometimes as little as
1/300 the energy required for comparable sustained
skeletal muscle contraction. This is called the “latch”
mechanism.
The importance of the latch mechanism is that it
can maintain prolonged tonic contraction in smooth
muscle for hours with little use of energy. Little continued excitatory signal is required from nerve fibers
or hormonal sources.
Stress-Relaxation of Smooth Muscle. Another impor-
tant characteristic of smooth muscle, especially the visceral unitary type of smooth muscle of many hollow
organs, is its ability to return to nearly its original
force of contraction seconds or minutes after it has
been elongated or shortened. For example, a sudden
increase in fluid volume in the urinary bladder, thus
stretching the smooth muscle in the bladder wall,
causes an immediate large increase in pressure in the
bladder. However, during the next 15 seconds to a
Chapter 8
Contraction and Excitation of Smooth Muscle
minute or so, despite continued stretch of the bladder
wall, the pressure returns almost exactly back to the
original level. Then, when the volume is increased by
another step, the same effect occurs again.
Conversely, when the volume is suddenly decreased,
the pressure falls very low at first but then rises back
in another few seconds or minutes to or near to the
original level. These phenomena are called stressrelaxation and reverse stress-relaxation. Their importance is that, except for short periods of time, they
allow a hollow organ to maintain about the same
amount of pressure inside its lumen despite long-term,
large changes in volume.
Regulation of Contraction
by Calcium Ions
As is true for skeletal muscle, the initiating stimulus
for most smooth muscle contraction is an increase in
intracellular calcium ions. This increase can be caused
in different types of smooth muscle by nerve stimulation of the smooth muscle fiber, hormonal stimulation,
stretch of the fiber, or even change in the chemical
environment of the fiber.
Yet smooth muscle does not contain troponin, the
regulatory protein that is activated by calcium ions
to cause skeletal muscle contraction. Instead, smooth
muscle contraction is activated by an entirely different
mechanism, as follows.
Combination of Calcium Ions with Calmodulin—Activation of
Myosin Kinase and Phosphorylation of the Myosin Head. In
place of troponin, smooth muscle cells contain a large
amount of another regulatory protein called calmodulin. Although this protein is similar to troponin, it is
different in the manner in which it initiates contraction. Calmodulin does this by activating the myosin
cross-bridges. This activation and subsequent contraction occur in the following sequence:
1. The calcium ions bind with calmodulin.
2. The calmodulin-calcium combination joins with
and activates myosin kinase, a phosphorylating
enzyme.
3. One of the light chains of each myosin
head, called the regulatory chain, becomes
phosphorylated in response to this myosin kinase.
When this chain is not phosphorylated, the
attachment-detachment cycling of the myosin
head with the actin filament does not occur. But
when the regulatory chain is phosphorylated, the
head has the capability of binding repetitively
with the actin filament and proceeding through
the entire cycling process of intermittent “pulls,”
the same as occurs for skeletal muscle, thus
causing muscle contraction.
Cessation of Contraction—Role of Myosin Phosphatase.
When the calcium ion concentration falls below a critical level, the aforementioned processes automatically
reverse, except for the phosphorylation of the myosin
head. Reversal of this requires another enzyme,
95
myosin phosphatase, located in the fluids of the
smooth muscle cell, which splits the phosphate from
the regulatory light chain. Then the cycling stops and
contraction ceases. The time required for relaxation of
muscle contraction, therefore, is determined to a great
extent by the amount of active myosin phosphatase in
the cell.
Possible Mechanism for Regulation of the
Latch Phenomenon
Because of the importance of the latch phenomenon
in smooth muscle, and because this phenomenon
allows long-term maintenance of tone in many smooth
muscle organs without much expenditure of energy,
many attempts have been made to explain it. Among
the many mechanisms that have been postulated, one
of the simplest is the following.
When the myosin kinase and myosin phosphatase
enzymes are both strongly activated, the cycling frequency of the myosin heads and the velocity of
contraction are great. Then, as the activation of the
enzymes decreases, the cycling frequency decreases,
but at the same time, the deactivation of these enzymes
allows the myosin heads to remain attached to the
actin filament for a longer and longer proportion of
the cycling period. Therefore, the number of heads
attached to the actin filament at any given time
remains large. Because the number of heads attached
to the actin determines the static force of contraction,
tension is maintained, or “latched”; yet little energy
is used by the muscle, because ATP is not degraded
to ADP except on the rare occasion when a head
detaches.
Nervous and Hormonal Control
of Smooth Muscle Contraction
Although skeletal muscle fibers are stimulated exclusively by the nervous system, smooth muscle can be
stimulated to contract by multiple types of signals: by
nervous signals, by hormonal stimulation, by stretch of
the muscle, and in several other ways. The principal
reason for the difference is that the smooth muscle
membrane contains many types of receptor proteins
that can initiate the contractile process. Still other
receptor proteins inhibit smooth muscle contraction,
which is another difference from skeletal muscle.
Therefore, in this section, we discuss nervous control
of smooth muscle contraction, followed by hormonal
control and other means of control.
Neuromuscular Junctions
of Smooth Muscle
Physiologic Anatomy of Smooth Muscle Neuromuscular Junctions. Neuromuscular junctions of the highly struc-
tured type found on skeletal muscle fibers do not occur
in smooth muscle. Instead, the autonomic nerve fibers
that innervate smooth muscle generally branch
96
Unit II
Membrane Physiology, Nerve, and Muscle
Varicosities
Visceral
Multi-unit
Figure 8–3
Innervation of smooth muscle.
diffusely on top of a sheet of muscle fibers, as shown
in Figure 8–3. In most instances, these fibers do not
make direct contact with the smooth muscle fiber cell
membranes but instead form so-called diffuse junctions that secrete their transmitter substance into the
matrix coating of the smooth muscle often a few
nanometers to a few micrometers away from the
muscle cells; the transmitter substance then diffuses to
the cells. Furthermore, where there are many layers of
muscle cells, the nerve fibers often innervate only the
outer layer, and muscle excitation travels from this
outer layer to the inner layers by action potential conduction in the muscle mass or by additional diffusion
of the transmitter substance.
The axons that innervate smooth muscle fibers do
not have typical branching end feet of the type in the
motor end plate on skeletal muscle fibers. Instead,
most of the fine terminal axons have multiple varicosities distributed along their axes. At these points the
Schwann cells that envelop the axons are interrupted
so that transmitter substance can be secreted through
the walls of the varicosities. In the varicosities are vesicles similar to those in the skeletal muscle end plate
that contain transmitter substance. But, in contrast to
the vesicles of skeletal muscle junctions, which always
contain acetylcholine, the vesicles of the autonomic
nerve fiber endings contain acetylcholine in some
fibers and norepinephrine in others—and occasionally
other substances as well.
In a few instances, particularly in the multi-unit type
of smooth muscle, the varicosities are separated from
the muscle cell membrane by as little as 20 to 30
nanometers—the same width as the synaptic cleft that
occurs in the skeletal muscle junction. These are called
contact junctions, and they function in much the same
way as the skeletal muscle neuromuscular junction; the
rapidity of contraction of these smooth muscle fibers
is considerably faster than that of fibers stimulated by
the diffuse junctions.
Excitatory and Inhibitory Transmitter Substances Secreted at
the Smooth Muscle Neuromuscular Junction. The most
important transmitter substances secreted by the
autonomic nerves innervating smooth muscle are
acetylcholine and norepinephrine, but they are never
secreted by the same nerve fibers. Acetylcholine is an
excitatory transmitter substance for smooth muscle
fibers in some organs but an inhibitory transmitter for
smooth muscle in other organs. When acetylcholine
excites a muscle fiber, norepinephrine ordinarily
inhibits it. Conversely, when acetylcholine inhibits a
fiber, norepinephrine usually excites it.
But why these different responses? The answer is
that both acetylcholine and norepinephrine excite or
inhibit smooth muscle by first binding with a receptor
protein on the surface of the muscle cell membrane.
Some of the receptor proteins are excitatory receptors,
whereas others are inhibitory receptors. Thus, the type
of receptor determines whether the smooth muscle is
inhibited or excited and also determines which of the
two transmitters, acetylcholine or norepinephrine, is
effective in causing the excitation or inhibition. These
receptors are discussed in more detail in Chapter 60 in
relation to function of the autonomic nervous system.
Membrane Potentials and Action
Potentials in Smooth Muscle
Membrane Potentials in Smooth Muscle. The quantitative
voltage of the membrane potential of smooth muscle
depends on the momentary condition of the muscle. In
the normal resting state, the intracellular potential is
usually about -50 to -60 millivolts, which is about 30
millivolts less negative than in skeletal muscle.
Action Potentials in Unitary Smooth Muscle. Action poten-
tials occur in unitary smooth muscle (such as visceral
muscle) in the same way that they occur in skeletal
muscle. They do not normally occur in many, if not
most, multi-unit types of smooth muscle, as discussed
in a subsequent section.
The action potentials of visceral smooth muscle
occur in one of two forms: (1) spike potentials or
(2) action potentials with plateaus.
Spike Potentials. Typical spike action potentials, such
as those seen in skeletal muscle, occur in most types of
unitary smooth muscle. The duration of this type of
action potential is 10 to 50 milliseconds, as shown in
Figure 8–4A. Such action potentials can be elicited in
many ways, for example, by electrical stimulation, by
the action of hormones on the smooth muscle, by the
action of transmitter substances from nerve fibers, by
stretch, or as a result of spontaneous generation in the
muscle fiber itself, as discussed subsequently.
Action Potentials with Plateaus. Figure 8–4C shows a
smooth muscle action potential with a plateau. The
onset of this action potential is similar to that of the
typical spike potential. However, instead of rapid
repolarization of the muscle fiber membrane, the repolarization is delayed for several hundred to as much
as 1000 milliseconds (1 second). The importance of
the plateau is that it can account for the prolonged
Chapter 8
Contraction and Excitation of Smooth Muscle
97
ions act directly on the smooth muscle contractile
mechanism to cause contraction. Thus, the calcium
performs two tasks at once.
0
Slow Wave Potentials in Unitary Smooth Muscle, and Spontaneous Generation of Action Potentials. Some smooth
Millivolts
+20
–40
Slow waves
–60
0
A
100
0
Milliseconds
50
B
10
20
30
Seconds
Millivolts
0
– 25
– 50
0
C
0.1
0.2
0.3
0.4
Seconds
Figure 8–4
A, Typical smooth muscle action potential (spike potential) elicited
by an external stimulus. B, Repetitive spike potentials, elicited by
slow rhythmical electrical waves that occur spontaneously in the
smooth muscle of the intestinal wall. C, Action potential with a
plateau, recorded from a smooth muscle fiber of the uterus.
contraction that occurs in some types of smooth
muscle, such as the ureter, the uterus under some conditions, and certain types of vascular smooth muscle.
(Also, this is the type of action potential seen in
cardiac muscle fibers that have a prolonged period of
contraction, as discussed in Chapters 9 and 10.)
Importance of Calcium Channels in Generating the
Smooth Muscle Action Potential. The smooth muscle
cell membrane has far more voltage-gated calcium
channels than does skeletal muscle but few voltagegated sodium channels. Therefore, sodium participates
little in the generation of the action potential in most
smooth muscle. Instead, flow of calcium ions to the
interior of the fiber is mainly responsible for the action
potential. This occurs in the same self-regenerative
way as occurs for the sodium channels in nerve fibers
and in skeletal muscle fibers. However, the calcium
channels open many times more slowly than do
sodium channels, and they also remain open much
longer. This accounts in large measure for the prolonged plateau action potentials of some smooth
muscle fibers.
Another important feature of calcium ion entry into
the cells during the action potential is that the calcium
muscle is self-excitatory. That is, action potentials arise
within the smooth muscle cells themselves without
an extrinsic stimulus. This often is associated with a
basic slow wave rhythm of the membrane potential.
A typical slow wave in a visceral smooth muscle of
the gut is shown in Figure 8–4B. The slow wave itself
is not the action potential. That is, it is not a selfregenerative process that spreads progressively over
the membranes of the muscle fibers. Instead, it is a
local property of the smooth muscle fibers that make
up the muscle mass.
The cause of the slow wave rhythm is unknown. One
suggestion is that the slow waves are caused by waxing
and waning of the pumping of positive ions (presumably sodium ions) outward through the muscle fiber
membrane; that is, the membrane potential becomes
more negative when sodium is pumped rapidly and
less negative when the sodium pump becomes less
active. Another suggestion is that the conductances of
the ion channels increase and decrease rhythmically.
The importance of the slow waves is that, when they
are strong enough, they can initiate action potentials.
The slow waves themselves cannot cause muscle contraction, but when the peak of the negative slow wave
potential inside the cell membrane rises in the positive
direction from -60 to about -35 millivolts (the
approximate threshold for eliciting action potentials in
most visceral smooth muscle), an action potential
develops and spreads over the muscle mass. Then contraction does occur. Figure 8–4B demonstrates this
effect, showing that at each peak of the slow wave, one
or more action potentials occur. These repetitive
sequences of action potentials elicit rhythmical contraction of the smooth muscle mass. Therefore, the
slow waves are called pacemaker waves. In Chapter 62,
we see that this type of pacemaker activity controls the
rhythmical contractions of the gut.
Excitation of Visceral Smooth Muscle by Muscle Stretch.
When visceral (unitary) smooth muscle is stretched
sufficiently, spontaneous action potentials usually
are generated. They result from a combination of (1)
the normal slow wave potentials and (2) decrease in
overall negativity of the membrane potential caused
by the stretch itself. This response to stretch allows the
gut wall, when excessively stretched, to contract automatically and rhythmically. For instance, when the gut
is overfilled by intestinal contents, local automatic contractions often set up peristaltic waves that move the
contents away from the overfilled intestine, usually in
the direction of the anus.
Depolarization of Multi-Unit Smooth Muscle
Without Action Potentials
The smooth muscle fibers of multi-unit smooth muscle
(such as the muscle of the iris of the eye or the
98
Unit II
Membrane Physiology, Nerve, and Muscle
piloerector muscle of each hair) normally contract
mainly in response to nerve stimuli. The nerve endings
secrete acetylcholine in the case of some multi-unit
smooth muscles and norepinephrine in the case of
others. In both instances, the transmitter substances
cause depolarization of the smooth muscle membrane,
and this in turn elicits contraction. Action potentials
usually do not develop; the reason is that the fibers are
too small to generate an action potential. (When
action potentials are elicited in visceral unitary smooth
muscle, 30 to 40 smooth muscle fibers must depolarize
simultaneously before a self-propagating action potential ensues.) Yet, in small smooth muscle cells, even
without an action potential, the local depolarization
(called the junctional potential) caused by the nerve
transmitter substance itself spreads “electrotonically”
over the entire fiber and is all that is needed to cause
muscle contraction.
Effect of Local Tissue Factors and
Hormones to Cause Smooth Muscle
Contraction Without Action Potentials
Probably half of all smooth muscle contraction is
initiated by stimulatory factors acting directly on
the smooth muscle contractile machinery and without
action potentials. Two types of non-nervous and
non–action potential stimulating factors often
involved are (1) local tissue chemical factors and (2)
various hormones.
Smooth Muscle Contraction in Response to Local Tissue
Chemical Factors. In Chapter 17, we discuss control of
contraction of the arterioles, meta-arterioles, and precapillary sphincters. The smallest of these vessels have
little or no nervous supply. Yet the smooth muscle is
highly contractile, responding rapidly to changes in
local chemical conditions in the surrounding interstitial fluid.
In the normal resting state, many of these small
blood vessels remain contracted. But when extra blood
flow to the tissue is needed, multiple factors can relax
the vessel wall, thus allowing for increased flow. In this
way, a powerful local feedback control system controls
the blood flow to the local tissue area. Some of the
specific control factors are as follows:
1. Lack of oxygen in the local tissues causes smooth
muscle relaxation and, therefore, vasodilatation.
2. Excess carbon dioxide causes vasodilatation.
3. Increased hydrogen ion concentration causes
vasodilatation.
Adenosine, lactic acid, increased potassium ions,
diminished calcium ion concentration, and increased
body temperature can all cause local vasodilatation.
Effects of Hormones on Smooth Muscle Contraction. Most
circulating hormones in the blood affect smooth
muscle contraction to some degree, and some have
profound effects. Among the more important of
these are norepinephrine, epinephrine, acetylcholine,
angiotensin, endothelin, vasopressin, oxytocin, serotonin, and histamine.
A hormone causes contraction of a smooth muscle
when the muscle cell membrane contains hormonegated excitatory receptors for the respective hormone.
Conversely, the hormone causes inhibition if the membrane contains inhibitory receptors for the hormone
rather than excitatory receptors.
Mechanisms of Smooth Muscle Excitation or Inhibition by Hormones or Local Tissue Factors. Some hormone receptors
in the smooth muscle membrane open sodium or
calcium ion channels and depolarize the membrane,
the same as after nerve stimulation. Sometimes action
potentials result, or action potentials that are already
occurring may be enhanced. In other cases, depolarization occurs without action potentials, and this depolarization allows calcium ion entry into the cell, which
promotes the contraction.
Inhibition, in contrast, occurs when the hormone (or
other tissue factor) closes the sodium and calcium
channels to prevent entry of these positive ions; inhibition also occurs if the normally closed potassium
channels are opened, allowing positive potassium ions
to diffuse out of the cell. Both of these actions increase
the degree of negativity inside the muscle cell, a
state called hyperpolarization, which strongly inhibits
muscle contraction.
Sometimes smooth muscle contraction or inhibition
is initiated by hormones without directly causing any
change in the membrane potential. In these instances,
the hormone may activate a membrane receptor that
does not open any ion channels but instead causes an
internal change in the muscle fiber, such as release of
calcium ions from the intracellular sarcoplasmic reticulum; the calcium then induces contraction. To
inhibit contraction, other receptor mechanisms are
known to activate the enzyme adenylate cyclase or
guanylate cyclase in the cell membrane; the portions of
the receptors that protrude to the interior of the cells
are coupled to these enzymes, causing the formation
of cyclic adenosine monophosphate (cAMP) or cyclic
guanosine monophosphate (cGMP), so-called second
messengers. The cAMP or cGMP has many effects, one
of which is to change the degree of phosphorylation of
several enzymes that indirectly inhibit contraction.
The pump that moves calcium ions from the sarcoplasm into the sarcoplasmic reticulum is activated,
as well as the cell membrane pump that moves calcium
ions out of the cell itself; these effects reduce the
calcium ion concentration in the sarcoplasm, thereby
inhibiting contraction.
Smooth muscles have considerable diversity in how
they initiate contraction or relaxation in response to
different hormones, neurotransmitters, and other
substances. In some instances, the same substance
may cause either relaxation or contraction of smooth
muscles in different locations. For example, norepinephrine inhibits contraction of smooth muscle in the
intestine but stimulates contraction of smooth muscle
in blood vessels.
Chapter 8
99
Contraction and Excitation of Smooth Muscle
Source of Calcium Ions That Cause
Contraction (1) Through the Cell
Membrane and (2) from the
Sarcoplasmic Reticulum
Although the contractile process in smooth muscle,
as in skeletal muscle, is activated by calcium ions,
the source of the calcium ions differs; the difference
is that the sarcoplasmic reticulum, which provides
virtually all the calcium ions for skeletal muscle contraction, is only slightly developed in most smooth
muscle. Instead, almost all the calcium ions that
cause contraction enter the muscle cell from the extracellular fluid at the time of the action potential or
other stimulus. That is, the concentration of calcium
ions in the extracellular fluid is greater than 10-3 molar,
in comparison with less than 10-7 molar inside the
smooth muscle cell; this causes rapid diffusion of
the calcium ions into the cell from the extracellular
fluid when the calcium pores open. The time required
for this diffusion to occur averages 200 to 300 milliseconds and is called the latent period before contraction begins. This latent period is about 50 times
as great for smooth muscle as for skeletal muscle
contraction.
Role of the Smooth Muscle Sarcoplasmic Reticulum. Figure
8–5 shows a few slightly developed sarcoplasmic
tubules that lie near the cell membrane in some larger
smooth muscle cells. Small invaginations of the
cell membrane, called caveolae, abut the surfaces of
these tubules. The caveolae suggest a rudimentary
analog of the transverse tubule system of skeletal
muscle. When an action potential is transmitted into
the caveolae, this is believed to excite calcium ion
release from the abutting sarcoplasmic tubules in the
same way that action potentials in skeletal muscle
transverse tubules cause release of calcium ions from
the skeletal muscle longitudinal sarcoplasmic tubules.
In general, the more extensive the sarcoplasmic reticulum in the smooth muscle fiber, the more rapidly
it contracts.
Effect on Smooth Muscle Contraction Caused by Changing of
Extracellular Calcium Ion Concentration. Although chang-
ing the extracellular fluid calcium ion concentration
from normal has little effect on the force of contraction of skeletal muscle, this is not true for most smooth
muscle. When the extracellular fluid calcium ion concentration falls to about 1/3 to 1/10 normal, smooth
muscle contraction usually ceases. Therefore, the
force of contraction of smooth muscle usually is
highly dependent on extracellular fluid calcium ion
concentration.
A Calcium Pump Is Required to Cause Smooth Muscle
Relaxation. To cause relaxation of smooth muscle after
it has contracted, the calcium ions must be removed
from the intracellular fluids. This removal is achieved
by a calcium pump that pumps calcium ions out
Caveolae
Sarcoplasmic
reticulum
Figure 8–5
Sarcoplasmic tubules in a large smooth muscle fiber showing their
relation to invaginations in the cell membrane called caveolae.
of the smooth muscle fiber back into the extracellular fluid, or into a sarcoplasmic reticulum, if it
is present. This pump is slow-acting in comparison
with the fast-acting sarcoplasmic reticulum pump in
skeletal muscle. Therefore, a single smooth muscle
contraction often lasts for seconds rather than hundredths to tenths of a second, as occurs for skeletal
muscle.
References
Also see references for Chapters 5 and 6.
Blaustein MP, Lederer WJ: Sodium/calcium exchange:
its physiological implications. Physiol Rev 79:763, 1999.
Davis MJ, Hill MA: Signaling mechanisms underlying
the vascular myogenic response. Physiol Rev 79:387,
1999.
Harnett KM, Biancani P: Calcium-dependent and calciumindependent contractions in smooth muscles. Am J Med
115(Suppl 3A):24S, 2003.
Horowitz A, Menice CB, Laporte R, Morgan KG: Mechanisms of smooth muscle contraction. Physiol Rev 76:967,
1996.
Kamm KE, Stull JT: Regulation of smooth muscle contractile elements by second messengers. Annu Rev Physiol
51:299, 1989.
Kuriyama H, Kitamura K, Itoh T, Inoue R: Physiological features of visceral smooth muscle cells, with special reference to receptors and ion channels. Physiol Rev 78:811,
1998.
Lee CH, Poburko D, Kuo KH, et al: Ca2+ oscillations, gradients, and homeostasis in vascular smooth muscle. Am J
Physiol Heart Circ Physiol 282:H1571, 2002.
Rybalkin SD, Yan C, Bornfeldt KE, Beavo JA: Cyclic GMP
phosphodiesterases and regulation of smooth muscle
function. Circ Res 93:280, 2003.
100
Unit II
Membrane Physiology, Nerve, and Muscle
Somlyo AP, Somlyo AV: Ca2+ sensitivity of smooth muscle
and nonmuscle myosin II: modulated by G proteins,
kinases, and myosin phosphatase. Physiol Rev 83:1325,
2003.
Stephens NL: Airway smooth muscle. Lung 179:333, 2001.
Walker JS, Wingard CJ, Murphy RA: Energetics of crossbridge phosphorylation and contraction in vascular
smooth muscle. Hypertension 23:1106, 1994.
Webb RC: Smooth muscle contraction and relaxation. Adv
Physiol Educ 27:201, 2003.
U
N
I
The Heart
9. Heart Muscle; The Heart as a Pump and Function
of the Heart Valves
10. Rhythmical Excitation of the Heart
11. The Normal Electrocardiogram
12. Electrocardiographic Interpretation of Cardiac
Muscle and Coronary Blood Flow Abnormalities:
Vectorial Analysis
13. Cardiac Arrhythmias and Their
Electrocardiographic Interpretation
T
III
C
H
A
P
T
E
R
9
Heart Muscle; The Heart as
a Pump and Function of the
Heart Valves
With this chapter we begin discussion of the heart
and circulatory system. The heart, shown in Figure
9–1, is actually two separate pumps: a right heart
that pumps blood through the lungs, and a left heart
that pumps blood through the peripheral organs. In
turn, each of these hearts is a pulsatile two-chamber
pump composed of an atrium and a ventricle. Each
atrium is a weak primer pump for the ventricle,
helping to move blood into the ventricle. The ventricles then supply the main
pumping force that propels the blood either (1) through the pulmonary circulation by the right ventricle or (2) through the peripheral circulation by the left
ventricle.
Special mechanisms in the heart cause a continuing succession of heart contractions called cardiac rhythmicity, transmitting action potentials throughout
the heart muscle to cause the heart’s rhythmical beat. This rhythmical control
system is explained in Chapter 10. In this chapter, we explain how the heart
operates as a pump, beginning with the special features of heart muscle itself.
Physiology of Cardiac Muscle
The heart is composed of three major types of cardiac muscle: atrial muscle, ventricular muscle, and specialized excitatory and conductive muscle fibers. The
atrial and ventricular types of muscle contract in much the same way as skeletal muscle, except that the duration of contraction is much longer. Conversely,
the specialized excitatory and conductive fibers contract only feebly because
they contain few contractile fibrils; instead, they exhibit either automatic rhythmical electrical discharge in the form of action potentials or conduction of the
action potentials through the heart, providing an excitatory system that controls
the rhythmical beating of the heart.
Physiologic Anatomy of Cardiac Muscle
Figure 9–2 shows a typical histological picture of cardiac muscle, demonstrating cardiac muscle fibers arranged in a latticework, with the fibers dividing,
recombining, and then spreading again. One also notes immediately from this
figure that cardiac muscle is striated in the same manner as in typical skeletal
muscle. Further, cardiac muscle has typical myofibrils that contain actin and
myosin filaments almost identical to those found in skeletal muscle; these filaments lie side by side and slide along one another during contraction in the
same manner as occurs in skeletal muscle (see Chapter 6). But in other ways,
cardiac muscle is quite different from skeletal muscle, as we shall see.
Cardiac Muscle as a Syncytium. The dark areas crossing the cardiac muscle fibers
in Figure 9–2 are called intercalated discs; they are actually cell membranes that
separate individual cardiac muscle cells from one another. That is, cardiac
muscle fibers are made up of many individual cells connected in series and in
parallel with one another.
103
104
Unit III
The Heart
HEAD AND UPPER EXTREMITY
Plateau
Aorta
Pulmonary artery
Superior
vena cava
Right atrium
Pulmonary
vein
Pulmonary
valve
Left atrium
Tricuspid
valve
Mitral valve
Right ventricle
Aortic valve
Left
ventricle
Inferior
vena cava
Millivolts
Lungs
+20
0
– 20
– 40
– 60
– 80
–100 Purkinje fiber
Plateau
+20
0
– 20
– 40
– 60
– 80
–100 Ventricular muscle
0
1
2
3
4
Seconds
TRUNK AND LOWER EXTREMITY
Figure 9–1
Structure of the heart, and course of blood flow through the heart
chambers and heart valves.
Figure 9–2
“Syncytial,” interconnecting nature of cardiac muscle fibers.
At each intercalated disc the cell membranes fuse
with one another in such a way that they form permeable “communicating” junctions (gap junctions) that
allow almost totally free diffusion of ions. Therefore,
from a functional point of view, ions move with ease
in the intracellular fluid along the longitudinal axes of
the cardiac muscle fibers, so that action potentials
travel easily from one cardiac muscle cell to the next,
past the intercalated discs. Thus, cardiac muscle is a
syncytium of many heart muscle cells in which the
cardiac cells are so interconnected that when one
of these cells becomes excited, the action potential
spreads to all of them, spreading from cell to cell
throughout the latticework interconnections.
Figure 9–3
Rhythmical action potentials (in millivolts) from a Purkinje fiber
and from a ventricular muscle fiber, recorded by means of
microelectrodes.
The heart actually is composed of two syncytiums:
the atrial syncytium that constitutes the walls of the
two atria, and the ventricular syncytium that constitutes the walls of the two ventricles. The atria are separated from the ventricles by fibrous tissue that
surrounds the atrioventricular (A-V) valvular openings between the atria and ventricles. Normally, potentials are not conducted from the atrial syncytium into
the ventricular syncytium directly through this fibrous
tissue. Instead, they are conducted only by way of a
specialized conductive system called the A-V bundle,
a bundle of conductive fibers several millimeters in
diameter that is discussed in detail in Chapter 10.
This division of the muscle of the heart into two
functional syncytiums allows the atria to contract a
short time ahead of ventricular contraction, which is
important for effectiveness of heart pumping.
Action Potentials in Cardiac Muscle
The action potential recorded in a ventricular muscle
fiber, shown in Figure 9–3, averages about 105 millivolts, which means that the intracellular potential rises
from a very negative value, about -85 millivolts,
between beats to a slightly positive value, about +20
millivolts, during each beat. After the initial spike, the
membrane remains depolarized for about 0.2 second,
exhibiting a plateau as shown in the figure, followed at
the end of the plateau by abrupt repolarization. The
presence of this plateau in the action potential causes
ventricular contraction to last as much as 15 times as
long in cardiac muscle as in skeletal muscle.
Chapter 9
105
Heart Muscle; The Heart as a Pump and Function of the Heart Valves
What Causes the Long Action Potential and the Plateau? At
this point, we must ask the questions: Why is the action
potential of cardiac muscle so long, and why does it
have a plateau, whereas that of skeletal muscle does
not? The basic biophysical answers to these questions
were presented in Chapter 5, but they merit summarizing here as well.
At least two major differences between the membrane properties of cardiac and skeletal muscle
account for the prolonged action potential and the
plateau in cardiac muscle. First, the action potential of
skeletal muscle is caused almost entirely by sudden
opening of large numbers of so-called fast sodium
channels that allow tremendous numbers of sodium
ions to enter the skeletal muscle fiber from the extracellular fluid. These channels are called “fast” channels
because they remain open for only a few thousandths
of a second and then abruptly close. At the end of this
closure, repolarization occurs, and the action potential
is over within another thousandth of a second or so.
In cardiac muscle, the action potential is caused by
opening of two types of channels: (1) the same fast
sodium channels as those in skeletal muscle and (2)
another entirely different population of slow calcium
channels, which are also called calcium-sodium
channels. This second population of channels differs
from the fast sodium channels in that they are slower
to open and, even more important, remain open
for several tenths of a second. During this time, a large
quantity of both calcium and sodium ions flows
through these channels to the interior of the cardiac
muscle fiber, and this maintains a prolonged period of
depolarization, causing the plateau in the action potential. Further, the calcium ions that enter during this
plateau phase activate the muscle contractile process,
while the calcium ions that cause skeletal muscle contraction are derived from the intracellular sarcoplasmic reticulum.
The second major functional difference between
cardiac muscle and skeletal muscle that helps account
for both the prolonged action potential and its plateau
is this: Immediately after the onset of the action potential, the permeability of the cardiac muscle membrane
for potassium ions decreases about fivefold, an effect
that does not occur in skeletal muscle. This decreased
potassium permeability may result from the excess
calcium influx through the calcium channels just
noted. Regardless of the cause, the decreased potassium permeability greatly decreases the outflux of
positively charged potassium ions during the action
potential plateau and thereby prevents early return of
the action potential voltage to its resting level. When
the slow calcium-sodium channels do close at the end
of 0.2 to 0.3 second and the influx of calcium and
sodium ions ceases, the membrane permeability for
potassium ions also increases rapidly; this rapid loss of
potassium from the fiber immediately returns the
membrane potential to its resting level, thus ending
the action potential.
Velocity of Signal Conduction in Cardiac Muscle. The velocity of conduction of the excitatory action potential
signal along both atrial and ventricular muscle fibers is
about 0.3 to 0.5 m/sec, or about 1/250 the velocity in very
large nerve fibers and about 1/10 the velocity in skeletal muscle fibers. The velocity of conduction in the
specialized heart conductive system—in the Purkinje
fibers—is as great as 4 m/sec in most parts of the
system, which allows reasonably rapid conduction of
the excitatory signal to the different parts of the heart,
as explained in Chapter 10.
Refractory Period of Cardiac Muscle. Cardiac muscle, like
all excitable tissue, is refractory to restimulation
during the action potential. Therefore, the refractory
period of the heart is the interval of time, as shown to
the left in Figure 9–4, during which a normal cardiac
impulse cannot re-excite an already excited area of
cardiac muscle. The normal refractory period of the
ventricle is 0.25 to 0.30 second, which is about the
duration of the prolonged plateau action potential.
There is an additional relative refractory period of
Refractory period
Force of ventricular heart muscle
contraction, showing also duration of the refractory period and
relative refractory period, plus the
effect of premature contraction.
Note that premature contractions
do not cause wave summation, as
occurs in skeletal muscle.
Force of contraction
Figure 9–4
Relative refractory
period
Early premature
contraction
0
1
2
Seconds
Later premature
contraction
3
106
Unit III
about 0.05 second during which the muscle is more difficult than normal to excite but nevertheless can be
excited by a very strong excitatory signal, as demonstrated by the early “premature” contraction in the
second example of Figure 9–4. The refractory period
of atrial muscle is much shorter than that for the ventricles (about 0.15 second for the atria compared with
0.25 to 0.30 second for the ventricles).
Excitation-Contraction Coupling—Function
of Calcium Ions and the Transverse Tubules
The term “excitation-contraction coupling” refers to
the mechanism by which the action potential causes
the myofibrils of muscle to contract. This was discussed
for skeletal muscle in Chapter 7. Once again, there are
differences in this mechanism in cardiac muscle that
have important effects on the characteristics of cardiac
muscle contraction.
As is true for skeletal muscle, when an action potential passes over the cardiac muscle membrane, the
action potential spreads to the interior of the cardiac
muscle fiber along the membranes of the transverse
(T) tubules. The T tubule action potentials in turn act
on the membranes of the longitudinal sarcoplasmic
tubules to cause release of calcium ions into the
muscle sarcoplasm from the sarcoplasmic reticulum. In
another few thousandths of a second, these calcium
ions diffuse into the myofibrils and catalyze the chemical reactions that promote sliding of the actin and
myosin filaments along one another; this produces the
muscle contraction.
Thus far, this mechanism of excitation-contraction
coupling is the same as that for skeletal muscle, but
there is a second effect that is quite different. In addition to the calcium ions that are released into the
sarcoplasm from the cisternae of the sarcoplasmic
reticulum, a large quantity of extra calcium ions also
diffuses into the sarcoplasm from the T tubules themselves at the time of the action potential. Indeed,
without this extra calcium from the T tubules, the
strength of cardiac muscle contraction would be
reduced considerably because the sarcoplasmic reticulum of cardiac muscle is less well developed than
that of skeletal muscle and does not store enough
calcium to provide full contraction. Conversely, the T
tubules of cardiac muscle have a diameter 5 times as
great as that of the skeletal muscle tubules, which
means a volume 25 times as great. Also, inside the T
tubules is a large quantity of mucopolysaccharides that
are electronegatively charged and bind an abundant
store of calcium ions, keeping these always available
for diffusion to the interior of the cardiac muscle fiber
when a T tubule action potential appears.
The strength of contraction of cardiac muscle
depends to a great extent on the concentration of
calcium ions in the extracellular fluids. The reason for
this is that the openings of the T tubules pass directly
through the cardiac muscle cell membrane into the
extracellular spaces surrounding the cells, allowing the
same extracellular fluid that is in the cardiac muscle
interstitium to percolate through the T tubules as well.
Consequently, the quantity of calcium ions in the T
The Heart
tubule system—that is, the availability of calcium ions
to cause cardiac muscle contraction—depends to a
great extent on the extracellular fluid calcium ion
concentration.
(By way of contrast, the strength of skeletal muscle
contraction is hardly affected by moderate changes in
extracellular fluid calcium concentration because
skeletal muscle contraction is caused almost entirely
by calcium ions released from the sarcoplasmic reticulum inside the skeletal muscle fiber itself.)
At the end of the plateau of the cardiac action
potential, the influx of calcium ions to the interior of
the muscle fiber is suddenly cut off, and the calcium
ions in the sarcoplasm are rapidly pumped back out of
the muscle fibers into both the sarcoplasmic reticulum
and the T tubule–extracellular fluid space. As a result,
the contraction ceases until a new action potential
comes along.
Duration of Contraction. Cardiac muscle begins to contract
a few milliseconds after the action potential begins and
continues to contract until a few milliseconds after the
action potential ends. Therefore, the duration of contraction of cardiac muscle is mainly a function of the
duration of the action potential, including the plateau—
about 0.2 second in atrial muscle and 0.3 second in ventricular muscle.
The Cardiac Cycle
The cardiac events that occur from the beginning of
one heartbeat to the beginning of the next are called
the cardiac cycle. Each cycle is initiated by spontaneous generation of an action potential in the sinus
node, as explained in Chapter 10. This node is located
in the superior lateral wall of the right atrium near the
opening of the superior vena cava, and the action
potential travels from here rapidly through both atria
and then through the A-V bundle into the ventricles.
Because of this special arrangement of the conducting
system from the atria into the ventricles, there is a
delay of more than 0.1 second during passage of the
cardiac impulse from the atria into the ventricles. This
allows the atria to contract ahead of ventricular contraction, thereby pumping blood into the ventricles
before the strong ventricular contraction begins. Thus,
the atria act as primer pumps for the ventricles, and
the ventricles in turn provide the major source of
power for moving blood through the body’s vascular
system.
Diastole and Systole
The cardiac cycle consists of a period of relaxation
called diastole, during which the heart fills with blood,
followed by a period of contraction called systole.
Figure 9–5 shows the different events during the
cardiac cycle for the left side of the heart.The top three
curves show the pressure changes in the aorta, left ventricle, and left atrium, respectively. The fourth curve
depicts the changes in left ventricular volume, the fifth
Chapter 9
Heart Muscle; The Heart as a Pump and Function of the Heart Valves
107
Isovolumic
relaxation
Isovolumic
contraction
Volume (ml)
Pressure (mm Hg)
120
100
Ejection
Rapid inflow
Diastasis
Atrial systole
Aortic valve
closes
Aortic
valve
opens
Aortic pressure
80
60
40
A-V valve
opens
A-V valve
closes
20
a
c
v
0
130
Atrial pressure
Ventricular pressure
Ventricular volume
90
R
50
P
Q
1st
2nd
3rd
T
Electrocardiogram
S
Phonocardiogram
Systole
Diastole
Systole
Figure 9–5
Events of the cardiac cycle for left ventricular function, showing changes in left atrial pressure, left ventricular pressure, aortic pressure,
ventricular volume, the electrocardiogram, and the phonocardiogram.
the electrocardiogram, and the sixth a phonocardiogram, which is a recording of the sounds produced by
the heart—mainly by the heart valves—as it pumps. It
is especially important that the reader study in detail
this figure and understand the causes of all the events
shown.
Relationship of the Electrocardiogram
to the Cardiac Cycle
The electrocardiogram in Figure 9–5 shows the P, Q,
R, S, and T waves, which are discussed in Chapters 11,
12, and 13. They are electrical voltages generated by
the heart and recorded by the electrocardiograph from
the surface of the body.
The P wave is caused by spread of depolarization
through the atria, and this is followed by atrial contraction, which causes a slight rise in the atrial pressure curve immediately after the electrocardiographic
P wave.
About 0.16 second after the onset of the P wave, the
QRS waves appear as a result of electrical depolarization of the ventricles, which initiates contraction of the
ventricles and causes the ventricular pressure to begin
rising, as also shown in the figure. Therefore, the QRS
complex begins slightly before the onset of ventricular systole.
Finally, one observes the ventricular T wave in the
electrocardiogram. This represents the stage of repolarization of the ventricles when the ventricular muscle
fibers begin to relax. Therefore, the T wave occurs
slightly before the end of ventricular contraction.
Function of the Atria as Primer Pumps
Blood normally flows continually from the great veins
into the atria; about 80 per cent of the blood flows
directly through the atria into the ventricles even
before the atria contract. Then, atrial contraction
usually causes an additional 20 per cent filling of the
ventricles. Therefore, the atria simply function as
108
Unit III
primer pumps that increase the ventricular pumping
effectiveness as much as 20 per cent. However, the
heart can continue to operate under most conditions
even without this extra 20 per cent effectiveness
because it normally has the capability of pumping 300
to 400 per cent more blood than is required by the
resting body. Therefore, when the atria fail to function,
the difference is unlikely to be noticed unless a person
exercises; then acute signs of heart failure occasionally
develop, especially shortness of breath.
Pressure Changes in the Atria—The a, c, and v Waves. In the
atrial pressure curve of Figure 9–5, three minor pressure
elevations, called the a, c, and v atrial pressure waves, are
noted.
The a wave is caused by atrial contraction. Ordinarily,
the right atrial pressure increases 4 to 6 mm Hg during
atrial contraction, and the left atrial pressure increases
about 7 to 8 mm Hg.
The c wave occurs when the ventricles begin to
contract; it is caused partly by slight backflow of blood
into the atria at the onset of ventricular contraction
but mainly by bulging of the A-V valves backward
toward the atria because of increasing pressure in the
ventricles.
The v wave occurs toward the end of ventricular
contraction; it results from slow flow of blood into the
atria from the veins while the A-V valves are closed
during ventricular contraction. Then, when ventricular
contraction is over, the A-V valves open, allowing this
stored atrial blood to flow rapidly into the ventricles and
causing the v wave to disappear.
Function of the Ventricles as Pumps
Filling of the Ventricles. During ventricular systole, large
amounts of blood accumulate in the right and left atria
because of the closed A-V valves. Therefore, as soon
as systole is over and the ventricular pressures fall
again to their low diastolic values, the moderately
increased pressures that have developed in the atria
during ventricular systole immediately push the A-V
valves open and allow blood to flow rapidly into the
ventricles, as shown by the rise of the left ventricular
volume curve in Figure 9–5. This is called the period of
rapid filling of the ventricles.
The period of rapid filling lasts for about the first
third of diastole. During the middle third of diastole,
only a small amount of blood normally flows into the
ventricles; this is blood that continues to empty into
the atria from the veins and passes through the atria
directly into the ventricles.
During the last third of diastole, the atria contract
and give an additional thrust to the inflow of blood
into the ventricles; this accounts for about 20 per cent
of the filling of the ventricles during each heart cycle.
Emptying of the Ventricles During Systole
Period of Isovolumic (Isometric) Contraction. Immediately after ventricular contraction begins, the ventricular pressure rises abruptly, as shown in Figure 9–5,
The Heart
causing the A-V valves to close. Then an additional
0.02 to 0.03 second is required for the ventricle to build
up sufficient pressure to push the semilunar (aortic
and pulmonary) valves open against the pressures in
the aorta and pulmonary artery. Therefore, during this
period, contraction is occurring in the ventricles, but
there is no emptying. This is called the period of isovolumic or isometric contraction, meaning that tension
is increasing in the muscle but little or no shortening
of the muscle fibers is occurring.
Period of Ejection. When the left ventricular pressure
rises slightly above 80 mm Hg (and the right ventricular pressure slightly above 8 mm Hg), the ventricular
pressures push the semilunar valves open. Immediately, blood begins to pour out of the ventricles, with
about 70 per cent of the blood emptying occurring
during the first third of the period of ejection and the
remaining 30 per cent emptying during the next two
thirds. Therefore, the first third is called the period of
rapid ejection, and the last two thirds, the period of
slow ejection.
Period of Isovolumic (Isometric) Relaxation. At the end of
systole, ventricular relaxation begins suddenly, allowing both the right and left intraventricular pressures
to decrease rapidly. The elevated pressures in the distended large arteries that have just been filled with
blood from the contracted ventricles immediately
push blood back toward the ventricles, which snaps the
aortic and pulmonary valves closed. For another 0.03
to 0.06 second, the ventricular muscle continues to
relax, even though the ventricular volume does not
change, giving rise to the period of isovolumic or isometric relaxation. During this period, the intraventricular pressures decrease rapidly back to their low
diastolic levels. Then the A-V valves open to begin a
new cycle of ventricular pumping.
End-Diastolic Volume, End-Systolic Volume, and Stroke Volume
Output. During diastole, normal filling of the ventricles
increases the volume of each ventricle to about 110 to
120 milliliters. This volume is called the end-diastolic
volume. Then, as the ventricles empty during systole,
the volume decreases about 70 milliliters, which is
called the stroke volume output. The remaining volume
in each ventricle, about 40 to 50 milliliters, is called the
end-systolic volume. The fraction of the end-diastolic
volume that is ejected is called the ejection fraction—
usually equal to about 60 per cent.
When the heart contracts strongly, the end-systolic
volume can be decreased to as little as 10 to 20 milliliters. Conversely, when large amounts of blood flow
into the ventricles during diastole, the ventricular enddiastolic volumes can become as great as 150 to 180
milliliters in the healthy heart. By both increasing the
end-diastolic volume and decreasing the end-systolic
volume, the stroke volume output can be increased to
more than double normal.
Chapter 9
Heart Muscle; The Heart as a Pump and Function of the Heart Valves
Function of the Valves
109
Aortic and Pulmonary Artery Valves. The aortic and pul-
Atrioventricular Valves. The A-V valves (the tricuspid
and mitral valves) prevent backflow of blood from the
ventricles to the atria during systole, and the semilunar valves (the aortic and pulmonary artery valves)
prevent backflow from the aorta and pulmonary arteries into the ventricles during diastole. These valves,
shown in Figure 9–6 for the left ventricle, close and
open passively. That is, they close when a backward
pressure gradient pushes blood backward, and they
open when a forward pressure gradient forces blood
in the forward direction. For anatomical reasons, the
thin, filmy A-V valves require almost no backflow to
cause closure, whereas the much heavier semilunar
valves require rather rapid backflow for a few
milliseconds.
Function of the Papillary Muscles. Figure 9–6 also
shows papillary muscles that attach to the vanes of the
A-V valves by the chordae tendineae. The papillary
muscles contract when the ventricular walls contract,
but contrary to what might be expected, they do not
help the valves to close. Instead, they pull the vanes of
the valves inward toward the ventricles to prevent
their bulging too far backward toward the atria during
ventricular contraction. If a chorda tendinea becomes
ruptured or if one of the papillary muscles becomes
paralyzed, the valve bulges far backward during ventricular contraction, sometimes so far that it leaks
severely and results in severe or even lethal cardiac
incapacity.
MITRAL VALVE
Cusp
Chordae tendineae
Papillary muscles
Cusp
AORTIC VALVE
monary artery semilunar valves function quite differently from the A-V valves. First, the high pressures in
the arteries at the end of systole cause the semilunar
valves to snap to the closed position, in contrast to the
much softer closure of the A-V valves. Second, because
of smaller openings, the velocity of blood ejection
through the aortic and pulmonary valves is far greater
than that through the much larger A-V valves. Also,
because of the rapid closure and rapid ejection, the
edges of the aortic and pulmonary valves are subjected
to much greater mechanical abrasion than are the
A-V valves. Finally, the A-V valves are supported by
the chordae tendineae, which is not true for the semilunar valves. It is obvious from the anatomy of the
aortic and pulmonary valves (as shown for the aortic
valve at the bottom of Figure 9–6) that they must be
constructed with an especially strong yet very pliable
fibrous tissue base to withstand the extra physical
stresses.
Aortic Pressure Curve
When the left ventricle contracts, the ventricular pressure increases rapidly until the aortic valve opens.
Then, after the valve opens, the pressure in the ventricle rises much less rapidly, as shown in Figure 9–5,
because blood immediately flows out of the ventricle
into the aorta and then into the systemic distribution
arteries.
The entry of blood into the arteries causes the walls
of these arteries to stretch and the pressure to increase
to about 120 mm Hg.
Next, at the end of systole, after the left ventricle
stops ejecting blood and the aortic valve closes, the
elastic walls of the arteries maintain a high pressure in
the arteries, even during diastole.
A so-called incisura occurs in the aortic pressure
curve when the aortic valve closes. This is caused by a
short period of backward flow of blood immediately
before closure of the valve, followed by sudden cessation of the backflow.
After the aortic valve has closed, the pressure in the
aorta decreases slowly throughout diastole because
the blood stored in the distended elastic arteries flows
continually through the peripheral vessels back to the
veins. Before the ventricle contracts again, the aortic
pressure usually has fallen to about 80 mm Hg (diastolic pressure), which is two thirds the maximal pressure of 120 mm Hg (systolic pressure) that occurs in
the aorta during ventricular contraction.
The pressure curves in the right ventricle and pulmonary artery are similar to those in the aorta, except
that the pressures are only about one sixth as great, as
discussed in Chapter 14.
Relationship of the Heart
Sounds to Heart Pumping
Figure 9–6
Mitral and aortic valves (the left ventricular valves).
When listening to the heart with a stethoscope, one does
not hear the opening of the valves because this is a
Unit III
relatively slow process that normally makes no noise.
However, when the valves close, the vanes of the valves
and the surrounding fluids vibrate under the influence
of sudden pressure changes, giving off sound that travels
in all directions through the chest.
When the ventricles contract, one first hears a sound
caused by closure of the A-V valves. The vibration is low
in pitch and relatively long-lasting and is known as the
first heart sound. When the aortic and pulmonary valves
close at the end of systole, one hears a rapid snap
because these valves close rapidly, and the surroundings
vibrate for a short period. This sound is called the
second heart sound. The precise causes of the heart
sounds are discussed more fully in Chapter 23, in relation to listening to the sounds with the stethoscope.
The Heart
300
Intraventricular pressure (mm Hg)
110
Systolic pressure
250
200 Isovolumic
relaxation
Period of ejection
150
Isovolumic
contraction
III
100
EW
IV
50
I
II
Diastolic
pressure
0
0
Work Output of the Heart
The stroke work output of the heart is the amount of
energy that the heart converts to work during each
heartbeat while pumping blood into the arteries. Minute
work output is the total amount of energy converted to
work in 1 minute; this is equal to the stroke work output
times the heart rate per minute.
Work output of the heart is in two forms. First, by far
the major proportion is used to move the blood from
the low-pressure veins to the high-pressure arteries.
This is called volume-pressure work or external work.
Second, a minor proportion of the energy is used to
accelerate the blood to its velocity of ejection through
the aortic and pulmonary valves. This is the kinetic
energy of blood flow component of the work output.
Right ventricular external work output is normally
about one sixth the work output of the left ventricle
because of the sixfold difference in systolic pressures
that the two ventricles pump. The additional work
output of each ventricle required to create kinetic
energy of blood flow is proportional to the mass of
blood ejected times the square of velocity of ejection.
Ordinarily, the work output of the left ventricle
required to create kinetic energy of blood flow is only
about 1 per cent of the total work output of the ventricle and therefore is ignored in the calculation of the
total stroke work output. But in certain abnormal conditions, such as aortic stenosis, in which blood flows with
great velocity through the stenosed valve, more than
50 per cent of the total work output may be required to
create kinetic energy of blood flow.
Graphical Analysis of Ventricular
Pumping
Figure 9–7 shows a diagram that is especially useful in
explaining the pumping mechanics of the left ventricle.
The most important components of the diagram are
the two curves labeled “diastolic pressure” and
“systolic pressure.” These curves are volume-pressure
curves.
The diastolic pressure curve is determined by filling
the heart with progressively greater volumes of blood
and then measuring the diastolic pressure immediately
before ventricular contraction occurs, which is the enddiastolic pressure of the ventricle.
The systolic pressure curve is determined by recording the systolic pressure achieved during ventricular
contraction at each volume of filling.
50
Period of filling
100
150
200
250
Left ventricular volume (ml)
Figure 9–7
Relationship between left ventricular volume and intraventricular
pressure during diastole and systole. Also shown by the heavy
red lines is the “volume-pressure diagram,” demonstrating
changes in intraventricular volume and pressure during the
normal cardiac cycle. EW, net external work.
Until the volume of the noncontracting ventricle rises
above about 150 milliliters, the “diastolic” pressure
does not increase greatly. Therefore, up to this volume,
blood can flow easily into the ventricle from the atrium.
Above 150 milliliters, the ventricular diastolic pressure
increases rapidly, partly because of fibrous tissue in the
heart that will stretch no more and partly because the
pericardium that surrounds the heart becomes filled
nearly to its limit.
During ventricular contraction, the “systolic” pressure increases even at low ventricular volumes and
reaches a maximum at a ventricular volume of 150 to
170 milliliters. Then, as the volume increases still
further, the systolic pressure actually decreases under
some conditions, as demonstrated by the falling systolic
pressure curve in Figure 9–7, because at these great
volumes, the actin and myosin filaments of the cardiac
muscle fibers are pulled apart far enough that the
strength of each cardiac fiber contraction becomes less
than optimal.
Note especially in the figure that the maximum systolic pressure for the normal left ventricle is between
250 and 300 mm Hg, but this varies widely with each
person’s heart strength and degree of heart stimulation
by cardiac nerves. For the normal right ventricle,
the maximum systolic pressure is between 60 and
80 mm Hg.
“Volume-Pressure Diagram” During the Cardiac Cycle; Cardiac
Work Output. The red lines in Figure 9–7 form a loop
called the volume-pressure diagram of the cardiac cycle
for normal function of the left ventricle. It is divided into
four phases.
Phase I: Period of filling. This phase in the volumepressure diagram begins at a ventricular volume of
about 45 milliliters and a diastolic pressure near
0 mm Hg. Forty-five milliliters is the amount of
Chapter 9
Heart Muscle; The Heart as a Pump and Function of the Heart Valves
blood that remains in the ventricle after the
previous heartbeat and is called the end-systolic
volume. As venous blood flows into the ventricle
from the left atrium, the ventricular volume
normally increases to about 115 milliliters, called
the end-diastolic volume, an increase of 70
milliliters. Therefore, the volume-pressure diagram
during phase I extends along the line labeled “I,”
with the volume increasing to 115 milliliters and the
diastolic pressure rising to about 5 mm Hg.
Phase II: Period of isovolumic contraction. During
isovolumic contraction, the volume of the ventricle
does not change because all valves are closed.
However, the pressure inside the ventricle increases
to equal the pressure in the aorta, at a pressure
value of about 80 mm Hg, as depicted by the arrow
end of the line labeled “II.”
Phase III: Period of ejection. During ejection, the
systolic pressure rises even higher because of still
more contraction of the ventricle. At the same time,
the volume of the ventricle decreases because the
aortic valve has now opened and blood flows out of
the ventricle into the aorta. Therefore, the curve
labeled “III” traces the changes in volume and
systolic pressure during this period of ejection.
Phase IV: Period of isovolumic relaxation. At the end
of the period of ejection, the aortic valve closes, and
the ventricular pressure falls back to the diastolic
pressure level. The line labeled “IV” traces this
decrease in intraventricular pressure without any
change in volume. Thus, the ventricle returns to its
starting point, with about 45 milliliters of blood
left in the ventricle and at an atrial pressure near
0 mm Hg.
Readers well trained in the basic principles of physics
should recognize that the area subtended by this functional volume-pressure diagram (the tan shaded area,
labeled EW) represents the net external work output of
the ventricle during its contraction cycle. In experimental studies of cardiac contraction, this diagram is used
for calculating cardiac work output.
When the heart pumps large quantities of blood, the
area of the work diagram becomes much larger. That is,
it extends far to the right because the ventricle fills with
more blood during diastole, it rises much higher because
the ventricle contracts with greater pressure, and it
usually extends farther to the left because the ventricle
contracts to a smaller volume—especially if the ventricle is stimulated to increased activity by the sympathetic
nervous system.
Concepts of Preload and Afterload. In assessing the contrac-
tile properties of muscle, it is important to specify the
degree of tension on the muscle when it begins to contract, which is called the preload, and to specify the load
against which the muscle exerts its contractile force,
which is called the afterload.
For cardiac contraction, the preload is usually considered to be the end-diastolic pressure when the ventricle has become filled.
The afterload of the ventricle is the pressure in the
artery leading from the ventricle. In Figure 9–7, this corresponds to the systolic pressure described by the phase
III curve of the volume-pressure diagram. (Sometimes
the afterload is loosely considered to be the resistance
in the circulation rather than the pressure.)
111
The importance of the concepts of preload and afterload is that in many abnormal functional states of the
heart or circulation, the pressure during filling of the
ventricle (the preload), the arterial pressure against
which the ventricle must contract (the afterload), or
both are severely altered from normal.
Chemical Energy Required
for Cardiac Contraction:
Oxygen Utilization by
the Heart
Heart muscle, like skeletal muscle, uses chemical energy
to provide the work of contraction. This energy is
derived mainly from oxidative metabolism of fatty acids
and, to a lesser extent, of other nutrients, especially
lactate and glucose. Therefore, the rate of oxygen consumption by the heart is an excellent measure of the
chemical energy liberated while the heart performs its
work. The different chemical reactions that liberate this
energy are discussed in Chapters 67 and 68.
Efficiency of Cardiac Contraction. During heart muscle contraction, most of the expended chemical energy is converted into heat and a much smaller portion into work
output. The ratio of work output to total chemical
energy expenditure is called the efficiency of cardiac
contraction, or simply efficiency of the heart. Maximum
efficiency of the normal heart is between 20 and 25 per
cent. In heart failure, this can decrease to as low as 5 to
10 per cent.
Regulation of Heart Pumping
When a person is at rest, the heart pumps only 4 to 6
liters of blood each minute. During severe exercise,
the heart may be required to pump four to seven times
this amount. The basic means by which the volume
pumped by the heart is regulated are (1) intrinsic
cardiac regulation of pumping in response to changes
in volume of blood flowing into the heart and (2)
control of heart rate and strength of heart pumping by
the autonomic nervous system.
Intrinsic Regulation of Heart
Pumping—The Frank-Starling
Mechanism
In Chapter 20, we will learn that under most conditions, the amount of blood pumped by the heart each
minute is determined almost entirely by the rate of
blood flow into the heart from the veins, which is called
venous return. That is, each peripheral tissue of the
body controls its own local blood flow, and all the local
tissue flows combine and return by way of the veins to
the right atrium. The heart, in turn, automatically
pumps this incoming blood into the arteries, so that it
can flow around the circuit again.
This intrinsic ability of the heart to adapt to increasing volumes of inflowing blood is called the FrankStarling mechanism of the heart, in honor of Frank and
112
Unit III
Starling, two great physiologists of a century ago. Basically, the Frank-Starling mechanism means that the
greater the heart muscle is stretched during filling,
the greater is the force of contraction and the greater
the quantity of blood pumped into the aorta. Or, stated
another way: Within physiologic limits, the heart pumps
all the blood that returns to it by the way of the veins.
What Is the Explanation of the Frank-Starling Mechanism?
When an extra amount of blood flows into the ventricles, the cardiac muscle itself is stretched to greater
length. This in turn causes the muscle to contract with
increased force because the actin and myosin filaments
are brought to a more nearly optimal degree of
overlap for force generation. Therefore, the ventricle,
because of its increased pumping, automatically
pumps the extra blood into the arteries.
This ability of stretched muscle, up to an optimal
length, to contract with increased work output is characteristic of all striated muscle, as explained in Chapter
6, and is not simply a characteristic of cardiac muscle.
In addition to the important effect of lengthening
the heart muscle, still another factor increases heart
pumping when its volume is increased. Stretch of the
right atrial wall directly increases the heart rate by
10 to 20 per cent; this, too, helps increase the amount
of blood pumped each minute, although its contribution is much less than that of the Frank-Starling
mechanism.
The Heart
work output for that side increases until it reaches the
limit of the ventricle’s pumping ability.
Figure 9–9 shows another type of ventricular function curve called the ventricular volume output curve.
The two curves of this figure represent function of the
two ventricles of the human heart based on data
extrapolated from lower animals. As the right and left
atrial pressures increase, the respective ventricular
volume outputs per minute also increase.
Thus, ventricular function curves are another way of
expressing the Frank-Starling mechanism of the heart.
That is, as the ventricles fill in response to higher atrial
pressures, each ventricular volume and strength of
cardiac muscle contraction increase, causing the
heart to pump increased quantities of blood into the
arteries.
Control of the Heart by the Sympathetic and
Parasympathetic Nerves
The pumping effectiveness of the heart also is controlled by the sympathetic and parasympathetic
(vagus) nerves, which abundantly supply the heart, as
shown in Figure 9–10. For given levels of input atrial
pressure, the amount of blood pumped each minute
(cardiac output) often can be increased more than 100
per cent by sympathetic stimulation. By contrast, the
output can be decreased to as low as zero or almost
zero by vagal (parasympathetic) stimulation.
Mechanisms of Excitation of the Heart by the Sympathetic
Nerves. Strong sympathetic stimulation can increase
One of the best ways to express the functional ability
of the ventricles to pump blood is by ventricular function curves, as shown in Figures 9–8 and 9–9. Figure
9–8 shows a type of ventricular function curve called
the stroke work output curve. Note that as the atrial
pressure for each side of the heart increases, the stroke
the heart rate in young adult humans from the normal
rate of 70 beats per minute up to 180 to 200 and, rarely,
even 250 beats per minute. Also, sympathetic stimulation increases the force of heart contraction to as much
as double normal, thereby increasing the volume of
blood pumped and increasing the ejection pressure.
Left ventricular
stroke work
(gram meters)
Right ventricular
stroke work
(gram meters)
40
4
30
3
20
2
10
1
0
0
0
10
20
Left mean atrial
pressure
(mm Hg)
0
10
20
Right mean atrial
pressure
(mm Hg)
Ventricular output (L /min)
Ventricular Function Curves
15
Right ventricle
10
Left ventricle
5
0
–4
0
+4
+8
+12
Atrial pressure (mm Hg)
+16
Figure 9–8
Left and right ventricular function curves recorded from dogs,
depicting ventricular stroke work output as a function of left and
right mean atrial pressures. (Curves reconstructed from data in
Sarnoff SJ: Myocardial contractility as described by ventricular
function curves. Physiol Rev 35:107, 1955.)
Figure 9–9
Approximate normal right and left ventricular volume output
curves for the normal resting human heart as extrapolated from
data obtained in dogs and data from human beings.
Chapter 9
Heart Muscle; The Heart as a Pump and Function of the Heart Valves
113
Sympathetic chains
Sympathetic
nerves
A-V
node
Sympathetic
nerves
Figure 9–10
20
Cardiac output (L/min)
S-A
node
Maximum sympathetic
stimulation
25
Vagi
15
Normal sympathetic
stimulation
10
Zero sympathetic
stimulation
(Parasympathetic
stimulation)
5
0
–4
Cardiac sympathetic and parasympathetic nerves. (The vagus
nerves to the heart are parasympathetic nerves.)
Thus, sympathetic stimulation often can increase the
maximum cardiac output as much as twofold to threefold, in addition to the increased output caused by the
Frank-Starling mechanism already discussed.
Conversely, inhibition of the sympathetic nerves to
the heart can decrease cardiac pumping to a moderate
extent in the following way: Under normal conditions,
the sympathetic nerve fibers to the heart discharge
continuously at a slow rate that maintains pumping at
about 30 per cent above that with no sympathetic stimulation. Therefore, when the activity of the sympathetic
nervous system is depressed below normal, this
decreases both heart rate and strength of ventricular
muscle contraction, thereby decreasing the level of
cardiac pumping as much as 30 per cent below normal.
Parasympathetic (Vagal) Stimulation of the Heart. Strong
stimulation of the parasympathetic nerve fibers in the
vagus nerves to the heart can stop the heartbeat for a
few seconds, but then the heart usually “escapes” and
beats at a rate of 20 to 40 beats per minute as long
as the parasympathetic stimulation continues. In
addition, strong vagal stimulation can decrease the
strength of heart muscle contraction by 20 to 30
per cent.
The vagal fibers are distributed mainly to the atria
and not much to the ventricles, where the power contraction of the heart occurs. This explains the effect of
vagal stimulation mainly to decrease heart rate rather
than to decrease greatly the strength of heart contraction. Nevertheless, the great decrease in heart rate
combined with a slight decrease in heart contraction
strength can decrease ventricular pumping 50 per cent
or more.
Effect of Sympathetic or Parasympathetic Stimulation on the
Cardiac Function Curve. Figure 9–11 shows four cardiac
0
+4
+8
Right atrial pressure (mm Hg)
Figure 9–11
Effect on the cardiac output curve of different degrees of sympathetic or parasympathetic stimulation.
function curves. They are similar to the ventricular
function curves of Figure 9–9. However, they represent
function of the entire heart rather than of a single ventricle; they show the relation between right atrial pressure at the input of the right heart and cardiac output
from the left ventricle into the aorta.
The curves of Figure 9–11 demonstrate that at any
given right atrial pressure, the cardiac output increases
during increased sympathetic stimulation and
decreases during increased parasympathetic stimulation. These changes in output caused by nerve stimulation result both from changes in heart rate and from
changes in contractile strength of the heart because
both change in response to the nerve stimulation.
Effect of Potassium and Calcium Ions
on Heart Function
In the discussion of membrane potentials in Chapter
5, it was pointed out that potassium ions have a
marked effect on membrane potentials, and in Chapter
6 it was noted that calcium ions play an especially
important role in activating the muscle contractile
process. Therefore, it is to be expected that the concentration of each of these two ions in the extracellular fluids should also have important effects on cardiac
pumping.
Effect of Potassium Ions. Excess potassium in the extra-
cellular fluids causes the heart to become dilated and
Unit III
flaccid and also slows the heart rate. Large quantities
also can block conduction of the cardiac impulse from
the atria to the ventricles through the A-V bundle.
Elevation of potassium concentration to only 8 to
12 mEq/L—two to three times the normal value—can
cause such weakness of the heart and abnormal
rhythm that this can cause death.
These effects result partially from the fact that a
high potassium concentration in the extracellular
fluids decreases the resting membrane potential in
the cardiac muscle fibers, as explained in Chapter 5.
As the membrane potential decreases, the intensity of
the action potential also decreases, which makes contraction of the heart progressively weaker.
The Heart
Normal range
Cardiac output (L/min)
114
5
4
3
2
1
0
0
50
100
150
200
Arterial pressure (mm Hg)
250
Effect of Calcium Ions. An excess of calcium ions causes
effects almost exactly opposite to those of potassium
ions, causing the heart to go toward spastic contraction. This is caused by a direct effect of calcium ions to
initiate the cardiac contractile process, as explained
earlier in the chapter.
Conversely, deficiency of calcium ions causes cardiac
flaccidity, similar to the effect of high potassium.
Fortunately, however, calcium ion levels in the blood
normally are regulated within a very narrow range.
Therefore, cardiac effects of abnormal calcium concentrations are seldom of clinical concern.
Effect of Temperature on
Heart Function
Increased body temperature, as occurs when one has
fever, causes a greatly increased heart rate, sometimes
to as fast as double normal. Decreased temperature
causes a greatly decreased heart rate, falling to as low
as a few beats per minute when a person is near death
from hypothermia in the body temperature range of
60° to 70°F. These effects presumably result from
the fact that heat increases the permeability of the
cardiac muscle membrane to ions that control heart
rate, resulting in acceleration of the self-excitation
process.
Contractile strength of the heart often is enhanced
temporarily by a moderate increase in temperature, as
occurs during body exercise, but prolonged elevation
of temperature exhausts the metabolic systems of the
heart and eventually causes weakness. Therefore,
optimal function of the heart depends greatly on
proper control of body temperature by the temperature control mechanisms explained in Chapter 73.
Increasing the Arterial Pressure Load
(up to a Limit) Does Not Decrease the
Cardiac Output
Note in Figure 9–12 that increasing the arterial pressure in the aorta does not decrease the cardiac output
until the mean arterial pressure rises above about
160 mm Hg. In other words, during normal function of
Figure 9–12
Constancy of cardiac output up to a pressure level of 160 mm Hg.
Only when the arterial pressure rises above this normal limit does
the increasing pressure load cause the cardiac output to fall
significantly.
the heart at normal systolic arterial pressures (80 to
140 mm Hg), the cardiac output is determined almost
entirely by the ease of blood flow through the body’s
tissues, which in turn controls venous return of
blood to the heart. This is the principal subject of
Chapter 20.
References
Bers DM: Cardiac excitation-contraction coupling. Nature
415:198, 2002.
Brette F, Orchard C:T-tubule function in mammalian cardiac
myocytes. Circ Res 92:1182, 2003.
Brutsaert DL: Cardiac endothelial-myocardial signaling: its
role in cardiac growth, contractile performance, and rhythmicity. Physiol Rev 83:59, 2003.
Clancy CE, Kass RS: Defective cardiac ion channels: from
mutations to clinical syndromes. J Clin Invest 110:1075,
2002.
Fozzard HA: Cardiac sodium and calcium channels: a history
of excitatory currents. Cardiovasc Res 55:1, 2002.
Fuchs F, Smith SH: Calcium, cross-bridges, and the FrankStarling relationship. News Physiol Sci 16:5, 2001.
Guyton AC: Determination of cardiac output by equating
venous return curves with cardiac response curves. Physiol
Rev 35:123, 1955.
Guyton AC, Jones CE, Coleman TG: Circulatory Physiology:
Cardiac Output and Its Regulation, 2nd ed. Philadelphia:
WB Saunders, 1973.
Herring N, Danson EJ, Paterson DJ: Cholinergic control of
heart rate by nitric oxide is site specific. News Physiol Sci
17:202, 2002.
Korzick DH: Regulation of cardiac excitation-contraction
coupling: a cellular update. Adv Physiol Educ 27:192, 2003.
Olson EN: A decade of discoveries in cardiac biology. Nat
Med 10:467, 2004.
Page E, Fozzard HA, Solaro JR: Handbook of Physiology,
sec 2: The Cardiovascular System, vol 1: The Heart. New
York: Oxford University Press, 2002.
Chapter 9
Heart Muscle; The Heart as a Pump and Function of the Heart Valves
Rudy Y: From genome to physiome: integrative models of
cardiac excitation. Ann Biomed Eng 28:945, 2000.
Sarnoff SJ: Myocardial contractility as described by ventricular function curves. Physiol Rev 35:107, 1955.
Starling EH: The Linacre Lecture on the Law of the Heart.
London: Longmans Green, 1918.
115
Sussman MA, Anversa P: Myocardial aging and senescence:
where have the stem cells gone? Annu Rev Physiol 66:29,
2004.
Zucker IH, Schultz HD, Li YF, et al: The origin of sympathetic outflow in heart failure: the roles of angiotensin II
and nitric oxide. Prog Biophys Mol Biol 84:217, 2004.
C
H
A
P
T
E
R
1
Rhythmical Excitation
of the Heart
The heart is endowed with a special system for (1)
generating rhythmical electrical impulses to cause
rhythmical contraction of the heart muscle and (2)
conducting these impulses rapidly through the
heart.When this system functions normally, the atria
contract about one sixth of a second ahead of ventricular contraction, which allows filling of the ventricles before they pump the blood through the
lungs and peripheral circulation. Another special importance of the system is
that it allows all portions of the ventricles to contract almost simultaneously,
which is essential for most effective pressure generation in the ventricular
chambers.
This rhythmical and conductive system of the heart is susceptible to damage
by heart disease, especially by ischemia of the heart tissues resulting from poor
coronary blood flow. The result is often a bizarre heart rhythm or abnormal
sequence of contraction of the heart chambers, and the pumping effectiveness
of the heart often is affected severely, even to the extent of causing death.
Specialized Excitatory and Conductive System
of the Heart
Figure 10–1 shows the specialized excitatory and conductive system of the heart
that controls cardiac contractions. The figure shows the sinus node (also called
sinoatrial or S-A node), in which the normal rhythmical impulse is generated;
the internodal pathways that conduct the impulse from the sinus node to the
atrioventricular (A-V) node; the A-V node, in which the impulse from the atria
is delayed before passing into the ventricles; the A-V bundle, which conducts
the impulse from the atria into the ventricles; and the left and right bundle
branches of Purkinje fibers, which conduct the cardiac impulse to all parts of
the ventricles.
Sinus (Sinoatrial) Node
The sinus node (also called sinoatrial node) is a small, flattened, ellipsoid strip
of specialized cardiac muscle about 3 millimeters wide, 15 millimeters long, and
1 millimeter thick. It is located in the superior posterolateral wall of the right
atrium immediately below and slightly lateral to the opening of the superior
vena cava. The fibers of this node have almost no contractile muscle filaments
and are each only 3 to 5 micrometers in diameter, in contrast to a diameter of
10 to 15 micrometers for the surrounding atrial muscle fibers. However, the
sinus nodal fibers connect directly with the atrial muscle fibers, so that any
action potential that begins in the sinus node spreads immediately into the atrial
muscle wall.
Automatic Electrical Rhythmicity of the Sinus Fibers
Some cardiac fibers have the capability of self-excitation, a process that can
cause automatic rhythmical discharge and contraction. This is especially true of
116
0
Rhythmical Excitation of the Heart
Chapter 10
A-V node
Sinus
node
A-V bundle
Left
bundle
branch
Internodal
pathways
Right
bundle
branch
Figure 10–1
Sinus node, and the Purkinje system of the heart, showing also
the A-V node, atrial internodal pathways, and ventricular bundle
branches.
Sinus
nodal fiber
Ventricular
muscle fiber
Threshold for
discharge
+20
Millivolts
0
– 40
“Resting
potential”
– 80
0
1
2
Seconds
3
Figure 10–2
Rhythmical discharge of a sinus nodal fiber. Also, the sinus nodal
action potential is compared with that of a ventricular muscle fiber.
117
fiber for three heartbeats and, by comparison, a single
ventricular muscle fiber action potential. Note that the
“resting membrane potential” of the sinus nodal fiber
between discharges has a negativity of about -55 to
-60 millivolts, in comparison with -85 to -90 millivolts
for the ventricular muscle fiber. The cause of this lesser
negativity is that the cell membranes of the sinus fibers
are naturally leaky to sodium and calcium ions, and
positive charges of the entering sodium and calcium
ions neutralize much of the intracellular negativity.
Before attempting to explain the rhythmicity of the
sinus nodal fibers, first recall from the discussions of
Chapters 5 and 9 that cardiac muscle has three types
of membrane ion channels that play important roles in
causing the voltage changes of the action potential.
They are (1) fast sodium channels, (2) slow sodiumcalcium channels, and (3) potassium channels. Opening
of the fast sodium channels for a few 10,000ths of a
second is responsible for the rapid upstroke spike of
the action potential observed in ventricular muscle,
because of rapid influx of positive sodium ions to the
interior of the fiber. Then the “plateau” of the ventricular action potential is caused primarily by slower
opening of the slow sodium-calcium channels, which
lasts for about 0.3 second. Finally, opening of potassium channels allows diffusion of large amounts of
positive potassium ions in the outward direction
through the fiber membrane and returns the membrane potential to its resting level.
But there is a difference in the function of these
channels in the sinus nodal fiber because the “resting”
potential is much less negative—only -55 millivolts in
the nodal fiber instead of the -90 millivolts in the ventricular muscle fiber. At this level of -55 millivolts,
the fast sodium channels mainly have already become
“inactivated,” which means that they have become
blocked. The cause of this is that any time the membrane potential remains less negative than about -55
millivolts for more than a few milliseconds, the inactivation gates on the inside of the cell membrane that
close the fast sodium channels become closed and
remain so. Therefore, only the slow sodium-calcium
channels can open (i.e., can become “activated”) and
thereby cause the action potential. As a result, the
atrial nodal action potential is slower to develop than
the action potential of the ventricular muscle. Also,
after the action potential does occur, return of the
potential to its negative state occurs slowly as well,
rather than the abrupt return that occurs for the
ventricular fiber.
Self-Excitation of Sinus Nodal Fibers. Because of the
the fibers of the heart’s specialized conducting system,
including the fibers of the sinus node. For this reason,
the sinus node ordinarily controls the rate of beat
of the entire heart, as discussed in detail later in
this chapter. First, let us describe this automatic
rhythmicity.
Mechanism of Sinus Nodal Rhythmicity. Figure 10–2 shows
action potentials recorded from inside a sinus nodal
high sodium ion concentration in the extracellular
fluid outside the nodal fiber, as well as a moderate
number of already open sodium channels, positive
sodium ions from outside the fibers normally tend to
leak to the inside. Therefore, between heartbeats,
influx of positively charged sodium ions causes a slow
rise in the resting membrane potential in the positive
direction. Thus, as shown in Figure 10–2, the “resting”
potential gradually rises between each two heartbeats.
When the potential reaches a threshold voltage of
118
Unit III
about -40 millivolts, the sodium-calcium channels
become “activated,” thus causing the action potential.
Therefore, basically, the inherent leakiness of the sinus
nodal fibers to sodium and calcium ions causes their
self-excitation.
Why does this leakiness to sodium and calcium ions
not cause the sinus nodal fibers to remain depolarized
all the time? The answer is that two events occur
during the course of the action potential to prevent
this. First, the sodium-calcium channels become inactivated (i.e., they close) within about 100 to 150 milliseconds after opening, and second, at about the same
time, greatly increased numbers of potassium channels
open. Therefore, influx of positive calcium and sodium
ions through the sodium-calcium channels ceases,
while at the same time large quantities of positive
potassium ions diffuse out of the fiber. Both of these
effects reduce the intracellular potential back to its
negative resting level and therefore terminate the
action potential. Furthermore, the potassium channels
remain open for another few tenths of a second, temporarily continuing movement of positive charges out
of the cell, with resultant excess negativity inside the
fiber; this is called hyperpolarization. The hyperpolarization state initially carries the “resting” membrane
potential down to about -55 to -60 millivolts at the
termination of the action potential.
Last, we must explain why this new state of hyperpolarization is not maintained forever. The reason is
that during the next few tenths of a second after the
action potential is over, progressively more and more
potassium channels close. The inward-leaking sodium
and calcium ions once again overbalance the outward
flux of potassium ions, and this causes the “resting”
potential to drift upward once more, finally reaching
the threshold level for discharge at a potential of about
-40 millivolts. Then the entire process begins again:
self-excitation to cause the action potential, recovery
from the action potential, hyperpolarization after the
action potential is over, drift of the “resting” potential
to threshold, and finally re-excitation to elicit another
cycle. This process continues indefinitely throughout a
person’s life.
Internodal Pathways and
Transmission of the Cardiac Impulse
Through the Atria
The ends of the sinus nodal fibers connect directly with
surrounding atrial muscle fibers. Therefore, action
potentials originating in the sinus node travel outward
into these atrial muscle fibers. In this way, the action
potential spreads through the entire atrial muscle mass
and, eventually, to the A-V node. The velocity of conduction in most atrial muscle is about 0.3 m/sec, but
conduction is more rapid, about 1 m/sec, in several
small bands of atrial fibers. One of these, called the
anterior interatrial band, passes through the anterior
walls of the atria to the left atrium. In addition, three
The Heart
Internodal
pathways
Transitional fibers
A-V node
(0.03)
Atrioventricular
fibrous tissue
(0.12)
Penetrating portion
of A-V bundle
Distal portion of
A-V bundle
Left bundle branch
Right bundle branch
(0.16)
Ventricular
septum
Figure 10–3
Organization of the A-V node. The numbers represent the interval
of time from the origin of the impulse in the sinus node. The values
have been extrapolated to human beings.
other small bands curve through the anterior, lateral,
and posterior atrial walls and terminate in the A-V
node; shown in Figures 10–1 and 10–3, these are called,
respectively, the anterior, middle, and posterior internodal pathways. The cause of more rapid velocity of
conduction in these bands is the presence of specialized conduction fibers. These fibers are similar to even
more rapidly conducting “Purkinje fibers” of the ventricles, which will be discussed.
Atrioventricular Node, and Delay
of Impulse Conduction from the Atria
to the Ventricles
The atrial conductive system is organized so that the
cardiac impulse does not travel from the atria into
the ventricles too rapidly; this delay allows time for the
atria to empty their blood into the ventricles before
ventricular contraction begins. It is primarily the A-V
node and its adjacent conductive fibers that delay this
transmission into the ventricles.
The A-V node is located in the posterior wall of the
right atrium immediately behind the tricuspid valve,
as shown in Figure 10–1. And Figure 10–3 shows diagrammatically the different parts of this node, plus
its connections with the entering atrial internodal
pathway fibers and the exiting A-V bundle. The figure
also shows the approximate intervals of time in
Chapter 10
Rhythmical Excitation of the Heart
fractions of a second between initial onset of the
cardiac impulse in the sinus node and its subsequent
appearance in the A-V nodal system. Note that the
impulse, after traveling through the internodal pathways, reaches the A-V node about 0.03 second after its
origin in the sinus node. Then there is a delay of
another 0.09 second in the A-V node itself before the
impulse enters the penetrating portion of the A-V
bundle, where it passes into the ventricles.A final delay
of another 0.04 second occurs mainly in this penetrating A-V bundle, which is composed of multiple small
fascicles passing through the fibrous tissue separating
the atria from the ventricles.
Thus, the total delay in the A-V nodal and A-V
bundle system is about 0.13 second. This, in addition to
the initial conduction delay of 0.03 second from the
sinus node to the A-V node, makes a total delay of 0.16
second before the excitatory signal finally reaches the
contracting muscle of the ventricles.
Cause of the Slow Conduction. The slow conduction in the
transitional, nodal, and penetrating A-V bundle fibers
is caused mainly by diminished numbers of gap junctions between successive cells in the conducting pathways, so that there is great resistance to conduction of
excitatory ions from one conducting fiber to the next.
Therefore, it is easy to see why each succeeding cell is
slow to be excited.
Rapid Transmission in the Ventricular
Purkinje System
Special Purkinje fibers lead from the A-V node
through the A-V bundle into the ventricles. Except for
the initial portion of these fibers where they penetrate
the A-V fibrous barrier, they have functional characteristics that are quite the opposite of those of the
A-V nodal fibers. They are very large fibers, even
larger than the normal ventricular muscle fibers, and
they transmit action potentials at a velocity of 1.5 to
4.0 m/sec, a velocity about 6 times that in the usual
ventricular muscle and 150 times that in some of the
A-V nodal fibers. This allows almost instantaneous
transmission of the cardiac impulse throughout the
entire remainder of the ventricular muscle.
The rapid transmission of action potentials by Purkinje fibers is believed to be caused by a very high level
of permeability of the gap junctions at the intercalated
discs between the successive cells that make up the
Purkinje fibers. Therefore, ions are transmitted easily
from one cell to the next, thus enhancing the velocity
of transmission. The Purkinje fibers also have very few
myofibrils, which means that they contract little or not
at all during the course of impulse transmission.
One-Way Conduction Through the A-V Bundle. A special
characteristic of the A-V bundle is the inability, except
in abnormal states, of action potentials to travel backward from the ventricles to the atria. This prevents
119
re-entry of cardiac impulses by this route from the
ventricles to the atria, allowing only forward conduction from the atria to the ventricles.
Furthermore, it should be recalled that everywhere,
except at the A-V bundle, the atrial muscle is separated from the ventricular muscle by a continuous
fibrous barrier, a portion of which is shown in Figure
10–3. This barrier normally acts as an insulator to
prevent passage of the cardiac impulse between atrial
and ventricular muscle through any other route
besides forward conduction through the A-V bundle
itself. (In rare instances, an abnormal muscle bridge
does penetrate the fibrous barrier elsewhere besides
at the A-V bundle. Under such conditions, the cardiac
impulse can re-enter the atria from the ventricles and
cause a serious cardiac arrhythmia.)
Distribution of the Purkinje Fibers in the Ventricles—The Left
and Right Bundle Branches. After penetrating the fibrous
tissue between the atrial and ventricular muscle, the
distal portion of the A-V bundle passes downward in
the ventricular septum for 5 to 15 millimeters toward
the apex of the heart, as shown in Figures 10–1 and
10–3. Then the bundle divides into left and right bundle
branches that lie beneath the endocardium on the two
respective sides of the ventricular septum. Each
branch spreads downward toward the apex of the ventricle, progressively dividing into smaller branches.
These branches in turn course sidewise around each
ventricular chamber and back toward the base of the
heart. The ends of the Purkinje fibers penetrate about
one third the way into the muscle mass and finally
become continuous with the cardiac muscle fibers.
From the time the cardiac impulse enters the bundle
branches in the ventricular septum until it reaches the
terminations of the Purkinje fibers, the total elapsed
time averages only 0.03 second. Therefore, once the
cardiac impulse enters the ventricular Purkinje conductive system, it spreads almost immediately to the
entire ventricular muscle mass.
Transmission of the Cardiac Impulse
in the Ventricular Muscle
Once the impulse reaches the ends of the Purkinje
fibers, it is transmitted through the ventricular muscle
mass by the ventricular muscle fibers themselves. The
velocity of transmission is now only 0.3 to 0.5 m/sec,
one sixth that in the Purkinje fibers.
The cardiac muscle wraps around the heart in a
double spiral, with fibrous septa between the spiraling
layers; therefore, the cardiac impulse does not necessarily travel directly outward toward the surface of the
heart but instead angulates toward the surface along
the directions of the spirals. Because of this, transmission from the endocardial surface to the epicardial
surface of the ventricle requires as much as another
0.03 second, approximately equal to the time required
for transmission through the entire ventricular portion
120
Unit III
of the Purkinje system. Thus, the total time for transmission of the cardiac impulse from the initial bundle
branches to the last of the ventricular muscle fibers in
the normal heart is about 0.06 second.
Summary of the Spread of the Cardiac
Impulse Through the Heart
Figure 10–4 shows in summary form the transmission
of the cardiac impulse through the human heart. The
numbers on the figure represent the intervals of time,
in fractions of a second, that lapse between the origin
of the cardiac impulse in the sinus node and its appearance at each respective point in the heart. Note that
the impulse spreads at moderate velocity through the
atria but is delayed more than 0.1 second in the A-V
nodal region before appearing in the ventricular septal
A-V bundle. Once it has entered this bundle, it spreads
very rapidly through the Purkinje fibers to the entire
endocardial surfaces of the ventricles.Then the impulse
once again spreads slightly less rapidly through the
ventricular muscle to the epicardial surfaces.
It is extremely important that the student learn in
detail the course of the cardiac impulse through the
heart and the precise times of its appearance in each
separate part of the heart, because a thorough quantitative knowledge of this process is essential to the
understanding of electrocardiography, to be discussed
in Chapters 11 through 13.
.07
.04
.06
S-A
.03
.07
.09
.22
.19
A-V
.03
.05
.07
.18
.17
Control of Excitation and
Conduction in the Heart
The Sinus Node as the Pacemaker
of the Heart
In the discussion thus far of the genesis and transmission of the cardiac impulse through the heart, we have
noted that the impulse normally arises in the sinus
node. In some abnormal conditions, this is not the case.
A few other parts of the heart can exhibit intrinsic
rhythmical excitation in the same way that the sinus
nodal fibers do; this is particularly true of the A-V
nodal and Purkinje fibers.
The A-V nodal fibers, when not stimulated from
some outside source, discharge at an intrinsic rhythmical rate of 40 to 60 times per minute, and the Purkinje fibers discharge at a rate somewhere between
15 and 40 times per minute. These rates are in contrast
to the normal rate of the sinus node of 70 to 80 times
per minute.
The question we must ask is: Why does the sinus
node rather than the A-V node or the Purkinje fibers
control the heart’s rhythmicity? The answer derives
from the fact that the discharge rate of the sinus node
is considerably faster than the natural self-excitatory
discharge rate of either the A-V node or the Purkinje
fibers. Each time the sinus node discharges, its impulse
is conducted into both the A-V node and the Purkinje
fibers, also discharging their excitable membranes. But
the sinus node discharges again before either the A-V
node or the Purkinje fibers can reach their own thresholds for self-excitation. Therefore, the new impulse
from the sinus node discharges both the A-V node and
the Purkinje fibers before self-excitation can occur in
either of these.
Thus, the sinus node controls the beat of the heart
because its rate of rhythmical discharge is faster than
that of any other part of the heart. Therefore, the sinus
node is virtually always the pacemaker of the normal
heart.
Abnormal Pacemakers—“Ectopic” Pacemaker. Occasionally
.16
.00
The Heart
.21
.17
.19
.18
.21
.20
Figure 10–4
Transmission of the cardiac impulse through the heart, showing
the time of appearance (in fractions of a second after initial
appearance at the sinoatrial node) in different parts of the heart.
some other part of the heart develops a rhythmical discharge rate that is more rapid than that of the sinus
node. For instance, this sometimes occurs in the A-V
node or in the Purkinje fibers when one of these
becomes abnormal. In either case, the pacemaker of
the heart shifts from the sinus node to the A-V node
or to the excited Purkinje fibers. Under rarer conditions, a place in the atrial or ventricular muscle develops excessive excitability and becomes the pacemaker.
A pacemaker elsewhere than the sinus node is
called an “ectopic” pacemaker. An ectopic pacemaker
causes an abnormal sequence of contraction of the
different parts of the heart and can cause significant
debility of heart pumping.
Another cause of shift of the pacemaker is blockage
of transmission of the cardiac impulse from the sinus
node to the other parts of the heart. The new pacemaker then occurs most frequently at the A-V node or
Chapter 10
Rhythmical Excitation of the Heart
in the penetrating portion of the A-V bundle on the
way to the ventricles.
When A-V block occurs—that is, when the cardiac
impulse fails to pass from the atria into the ventricles
through the A-V nodal and bundle system—the atria
continue to beat at the normal rate of rhythm of the
sinus node, while a new pacemaker usually develops
in the Purkinje system of the ventricles and drives the
ventricular muscle at a new rate somewhere between
15 and 40 beats per minute. After sudden A-V bundle
block, the Purkinje system does not begin to emit its
intrinsic rhythmical impulses until 5 to 20 seconds later
because, before the blockage, the Purkinje fibers had
been “overdriven” by the rapid sinus impulses and,
consequently, are in a suppressed state. During these
5 to 20 seconds, the ventricles fail to pump blood, and
the person faints after the first 4 to 5 seconds because
of lack of blood flow to the brain. This delayed pickup
of the heartbeat is called Stokes-Adams syndrome. If
the delay period is too long, it can lead to death.
Role of the Purkinje System
in Causing Synchronous Contraction
of the Ventricular Muscle
It is clear from our description of the Purkinje system
that normally the cardiac impulse arrives at almost all
portions of the ventricles within a narrow span of time,
exciting the first ventricular muscle fiber only 0.03 to
0.06 second ahead of excitation of the last ventricular
muscle fiber. This causes all portions of the ventricular
muscle in both ventricles to begin contracting at
almost the same time and then to continue contracting for about another 0.3 second.
Effective pumping by the two ventricular chambers
requires this synchronous type of contraction. If the
cardiac impulse should travel through the ventricles
slowly, much of the ventricular mass would contract
before contraction of the remainder, in which case the
overall pumping effect would be greatly depressed.
Indeed, in some types of cardiac debilities, several of
which are discussed in Chapters 12 and 13, slow transmission does occur, and the pumping effectiveness of
the ventricles is decreased as much as 20 to 30 per cent.
Control of Heart Rhythmicity and
Impulse Conduction by the Cardiac
Nerves: The Sympathetic and
Parasympathetic Nerves
The heart is supplied with both sympathetic and
parasympathetic nerves, as shown in Figure 9-10 of
Chapter 9. The parasympathetic nerves (the vagi) are
distributed mainly to the S-A and A-V nodes, to a
lesser extent to the muscle of the two atria, and very
little directly to the ventricular muscle. The sympathetic nerves, conversely, are distributed to all parts of
the heart, with strong representation to the ventricular muscle as well as to all the other areas.
121
Parasympathetic (Vagal) Stimulation Can Slow or Even Block
Cardiac Rhythm and Conduction—“Ventricular Escape.”
Stimulation of the parasympathetic nerves to the heart
(the vagi) causes the hormone acetylcholine to be
released at the vagal endings. This hormone has two
major effects on the heart. First, it decreases the rate
of rhythm of the sinus node, and second, it decreases
the excitability of the A-V junctional fibers between
the atrial musculature and the A-V node, thereby
slowing transmission of the cardiac impulse into the
ventricles.
Weak to moderate vagal stimulation slows the rate
of heart pumping, often to as little as one half normal.
And strong stimulation of the vagi can stop completely
the rhythmical excitation by the sinus node or block
completely transmission of the cardiac impulse from
the atria into the ventricles through the A-V mode. In
either case, rhythmical excitatory signals are no longer
transmitted into the ventricles. The ventricles stop
beating for 5 to 20 seconds, but then some point in the
Purkinje fibers, usually in the ventricular septal
portion of the A-V bundle, develops a rhythm of its
own and causes ventricular contraction at a rate of
15 to 40 beats per minute. This phenomenon is called
ventricular escape.
Mechanism of the Vagal Effects. The acetylcholine
released at the vagal nerve endings greatly increases
the permeability of the fiber membranes to potassium
ions, which allows rapid leakage of potassium out of
the conductive fibers. This causes increased negativity
inside the fibers, an effect called hyperpolarization,
which makes this excitable tissue much less excitable,
as explained in Chapter 5.
In the sinus node, the state of hyperpolarization
decreases the “resting” membrane potential of the
sinus nodal fibers to a level considerably more negative than usual, to -65 to -75 millivolts rather than
the normal level of -55 to -60 millivolts. Therefore,
the initial rise of the sinus nodal membrane potential
caused by inward sodium and calcium leakage
requires much longer to reach the threshold potential
for excitation. This greatly slows the rate of rhythmicity of these nodal fibers. If the vagal stimulation is
strong enough, it is possible to stop entirely the rhythmical self-excitation of this node.
In the A-V node, a state of hyperpolarization caused
by vagal stimulation makes it difficult for the small
atrial fibers entering the node to generate enough electricity to excite the nodal fibers. Therefore, the safety
factor for transmission of the cardiac impulse through
the transitional fibers into the A-V nodal fibers
decreases. A moderate decrease simply delays conduction of the impulse, but a large decrease blocks
conduction entirely.
Effect of Sympathetic Stimulation on Cardiac Rhythm and Conduction. Sympathetic stimulation causes essentially the
opposite effects on the heart to those caused by vagal
stimulation, as follows: First, it increases the rate of
sinus nodal discharge. Second, it increases the rate
of conduction as well as the level of excitability in all
122
Unit III
portions of the heart. Third, it increases greatly the
force of contraction of all the cardiac musculature,
both atrial and ventricular, as discussed in Chapter 9.
In short, sympathetic stimulation increases the
overall activity of the heart. Maximal stimulation
can almost triple the frequency of heartbeat and can
increase the strength of heart contraction as much as
twofold.
Mechanism of the Sympathetic Effect. Stimulation of
the sympathetic nerves releases the hormone norepinephrine at the sympathetic nerve endings. The precise
mechanism by which this hormone acts on cardiac
muscle fibers is somewhat unclear, but the belief is
that it increases the permeability of the fiber membrane to sodium and calcium ions. In the sinus node,
an increase of sodium-calcium permeability causes
a more positive resting potential and also causes
increased rate of upward drift of the diastolic membrane potential toward the threshold level for selfexcitation, thus accelerating self-excitation and,
therefore, increasing the heart rate.
In the A-V node and A-V bundles, increased
sodium-calcium permeability makes it easier for the
action potential to excite each succeeding portion of
the conducting fiber bundles, thereby decreasing the
conduction time from the atria to the ventricles.
The increase in permeability to calcium ions is at
least partially responsible for the increase in contractile strength of the cardiac muscle under the influence
of sympathetic stimulation, because calcium ions play
a powerful role in exciting the contractile process of
the myofibrils.
References
Blatter LA, Kockskamper J, Sheehan KA, et al: Local
calcium gradients during excitation-contraction coupling
and alternans in atrial myocytes. J Physiol 546:19, 2003.
Ferrier GR, Howlett SE: Cardiac excitation-contraction coupling: role of membrane potential in regulation of contraction.Am J Physiol Heart Circ Physiol 280:H1928, 2001.
The Heart
Gentlesk PJ, Markwood TT, Atwood JE: Chronotropic
incompetence in a young adult: case report and literature
review. Chest 125:297, 2004.
Huikuri HV, Castellanos A, Myerburg RJ: Sudden death due
to cardiac arrhythmias. N Engl J Med 345:1473, 2001.
Hume JR, Duan D, Collier ML, et al: Anion transport in
heart. Physiol Rev 80:31, 2000.
James TN: Structure and function of the sinus node, AV node
and His bundle of the human heart: part I—structure. Prog
Cardiovasc Dis 45:235, 2002.
James TN: Structure and function of the sinus node, AV node
and His bundle of the human heart: part II—function.
Prog Cardiovasc Dis 45:327, 2003.
Kaupp UB, Seifert R: Molecular diversity of pacemaker ion
channels. Annu Rev Physiol 63:235, 2001.
Kléber AG, Rudy Y: Basic mechanisms of cardiac impulse
propagation and associated arrhythmias. Physiol Rev
84:431, 2004.
Leclercq C, Hare JM: Ventricular resynchronization: current
state of the art. Circulation 109:296, 2004.
Mazgalev TN, Ho SY, Anderson RH: Anatomicelectrophysiological correlations concerning the pathways
for atrioventricular conduction. Circulation 103:2660,
2001.
Page E, Fozzard HA, Solaro JR: Handbook of Physiology,
sec 2: The Cardiovascular System, vol 1: The Heart. New
York: Oxford University Press, 2002.
Petrashevskaya NN, Koch SE, Bodi I, Schwartz A: Calcium
cycling, historic overview and perspectives: role for autonomic nervous system regulation. J Mol Cell Cardiol
34:885, 2002.
Priori SG: Inherited arrhythmogenic diseases: the complexity beyond monogenic disorders. Circ Res 94:140,
2004.
Roden DM, Balser JR, George AL Jr, Anderson ME:
Cardiac ion channels. Annu Rev Physiol 64:431, 2002.
Schram G, Pourrier M, Melnyk P, Nattel S: Differential distribution of cardiac ion channel expression as a basis for
regional specialization in electrical function. Circ Res
90:939, 2002.
Surawicz B: Electrophysiologic Basis of ECG and Cardiac
Arrhythmias. Baltimore: Williams & Wilkins, 1995.
Waldo AL: Mechanisms of atrial fibrillation. J Cardiovasc
Electrophysiol 14(12 Suppl):S267, 2003.
Yasuma F, Hayano J: Respiratory sinus arrhythmia: why does
the heartbeat synchronize with respiratory rhythm? Chest
125:683, 2004.
C
H
A
P
T
E
R
1
1
The Normal Electrocardiogram
When the cardiac impulse passes through the heart,
electrical current also spreads from the heart into
the adjacent tissues surrounding the heart. A small
portion of the current spreads all the way to the
surface of the body. If electrodes are placed on the
skin on opposite sides of the heart, electrical
potentials generated by the current can be recorded;
the recording is known as an electrocardiogram.
A normal electrocardiogram for two beats of the heart is shown in Figure 11–1.
Characteristics of the Normal
Electrocardiogram
The normal electrocardiogram (see Figure 11–1) is composed of a P wave, a
QRS complex, and a T wave. The QRS complex is often, but not always, three
separate waves: the Q wave, the R wave, and the S wave.
The P wave is caused by electrical potentials generated when the atria depolarize before atrial contraction begins. The QRS complex is caused by potentials generated when the ventricles depolarize before contraction, that is, as the
depolarization wave spreads through the ventricles. Therefore, both the P wave
and the components of the QRS complex are depolarization waves.
The T wave is caused by potentials generated as the ventricles recover from
the state of depolarization. This process normally occurs in ventricular muscle
0.25 to 0.35 second after depolarization, and the T wave is known as a repolarization wave.
Thus, the electrocardiogram is composed of both depolarization and repolarization waves. The principles of depolarization and repolarization are
discussed in Chapter 5. The distinction between depolarization waves and repolarization waves is so important in electrocardiography that further clarification
is needed.
Depolarization Waves Versus Repolarization Waves
Figure 11–2 shows a single cardiac muscle fiber in four stages of depolarization
and repolarization, the color red designating depolarization. During depolarization, the normal negative potential inside the fiber reverses and becomes
slightly positive inside and negative outside.
In Figure 11–2A, depolarization, demonstrated by red positive charges inside
and red negative charges outside, is traveling from left to right. The first half of
the fiber has already depolarized, while the remaining half is still polarized.
Therefore, the left electrode on the outside of the fiber is in an area of negativity, and the right electrode is in an area of positivity; this causes the meter to
record positively. To the right of the muscle fiber is shown a record of changes
in potential between the two electrodes, as recorded by a high-speed recording
meter. Note that when depolarization has reached the halfway mark in Figure
11–2A, the record has risen to a maximum positive value.
123
124
Unit III
The Heart
In Figure 11–2B, depolarization has extended over
the entire muscle fiber, and the recording to the right
has returned to the zero baseline because both electrodes are now in areas of equal negativity. The completed wave is a depolarization wave because it results
from spread of depolarization along the muscle fiber
membrane.
Atria Ventricles
+2
RR interval
+1
Millivolts
S-T
segment
R
Relation of the Monophasic Action Potential of Ventricular
Muscle to the QRS and T Waves in the Standard Electrocardiogram. The monophasic action potential of ventricular
T
P
0
muscle, discussed in Chapter 10, normally lasts between
0.25 and 0.35 second. The top part of Figure 11–3 shows
a monophasic action potential recorded from a microelectrode inserted to the inside of a single ventricular
muscle fiber. The upsweep of this action potential is
caused by depolarization, and the return of the potential to the baseline is caused by repolarization.
Note in the lower half of the figure a simultaneous
recording of the electrocardiogram from this same
ventricle, which shows the QRS waves appearing at
QS
Q-T interval
P-R interval
= 0.16 sec
–1
0
Figure 11–2C shows halfway repolarization of the
same muscle fiber, with positivity returning to the
outside of the fiber. At this point, the left electrode is
in an area of positivity, and the right electrode is in an
area of negativity. This is opposite to the polarity in
Figure 11–2A. Consequently, the recording, as shown
to the right, becomes negative.
In Figure 11–2D, the muscle fiber has completely
repolarized, and both electrodes are now in areas of
positivity, so that no potential difference is recorded
between them. Thus, in the recording to the right, the
potential returns once more to zero. This completed
negative wave is a repolarization wave because it
results from spread of repolarization along the muscle
fiber membrane.
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
Time (sec)
Figure 11–1
Normal electrocardiogram.
0
–
+
–
– – – – – – –+ + + + + + + + +
+ + ++ + ++– – – – – – – – –
A
+ + ++ + ++– – – – – – – – –
– – – – – – –+ + + + + + + + +
––––––– – –– – –– – – –
+ + ++ + ++ + + + + + + + + +
B
+ + ++ + ++ + + + + + + + + +
––––––– – –– – –– – – –
+ + ++ + ++ + + – – – – – – –
–––––––––+++++++
C
–––––––––+++++++
+ + ++ + ++ + + – – – – – – –
+ + ++ + ++ + + + + + + + + +
––––––––––––––––
D
––––––––––––––––
+ + ++ + ++ + + + + + + + + +
+
+
–
Depolarization
wave
+
–
+
–
+
–
Repolarization
wave
Figure 11–2
0.30 second
Recording the depolarization wave (A and B) and
the repolarization wave (C and D) from a cardiac
muscle fiber.
The Normal Electrocardiogram
Repolarization
Depolarization
Chapter 11
125
(the QRS complex), but in many other fibers, it takes
as long as 0.35 second. Thus, the process of ventricular
repolarization extends over a long period, about 0.15
second. For this reason, the T wave in the normal electrocardiogram is a prolonged wave, but the voltage of
the T wave is considerably less than the voltage of the
QRS complex, partly because of its prolonged length.
Voltage and Time Calibration of the
Electrocardiogram
R
T
Q
S
Figure 11–3
Above, Monophasic action potential from a ventricular muscle
fiber during normal cardiac function, showing rapid depolarization
and then repolarization occurring slowly during the plateau stage
but rapidly toward the end. Below, Electrocardiogram recorded
simultaneously.
the beginning of the monophasic action potential and
the T wave appearing at the end. Note especially that
no potential is recorded in the electrocardiogram when
the ventricular muscle is either completely polarized
or completely depolarized. Only when the muscle is
partly polarized and partly depolarized does current
flow from one part of the ventricles to another part,
and therefore current also flows to the surface of the
body to produce the electrocardiogram.
Relationship of Atrial and Ventricular
Contraction to the Waves of the
Electrocardiogram
Before contraction of muscle can occur, depolarization
must spread through the muscle to initiate the chemical processes of contraction. Refer again to Figure
11–1; the P wave occurs at the beginning of contraction of the atria, and the QRS complex of waves occurs
at the beginning of contraction of the ventricles. The
ventricles remain contracted until after repolarization
has occurred, that is, until after the end of the T wave.
The atria repolarize about 0.15 to 0.20 second after
termination of the P wave. This is also approximately when the QRS complex is being recorded in
the electrocardiogram. Therefore, the atrial repolarization wave, known as the atrial T wave, is usually
obscured by the much larger QRS complex. For
this reason, an atrial T wave seldom is observed in the
electrocardiogram.
The ventricular repolarization wave is the T wave of
the normal electrocardiogram. Ordinarily, ventricular
muscle begins to repolarize in some fibers about 0.20
second after the beginning of the depolarization wave
All recordings of electrocardiograms are made with
appropriate calibration lines on the recording paper.
Either these calibration lines are already ruled on the
paper, as is the case when a pen recorder is used, or
they are recorded on the paper at the same time that
the electrocardiogram is recorded, which is the case
with the photographic types of electrocardiographs.
As shown in Figure 11–1, the horizontal calibration
lines are arranged so that 10 of the small line divisions
upward or downward in the standard electrocardiogram represent 1 millivolt, with positivity in the upward
direction and negativity in the downward direction.
The vertical lines on the electrocardiogram are time
calibration lines. Each inch in the horizontal direction
is 1 second, and each inch is usually broken into five
segments by dark vertical lines; the intervals between
these dark lines represent 0.20 second. The 0.20 second
intervals are then broken into five smaller intervals by
thin lines, each of which represents 0.04 second.
Normal Voltages in the Electrocardiogram. The recorded
voltages of the waves in the normal electrocardiogram
depend on the manner in which the electrodes are
applied to the surface of the body and how close the
electrodes are to the heart. When one electrode is
placed directly over the ventricles and a second electrode is placed elsewhere on the body remote from the
heart, the voltage of the QRS complex may be as great
as 3 to 4 millivolts. Even this voltage is small in comparison with the monophasic action potential of 110
millivolts recorded directly at the heart muscle membrane. When electrocardiograms are recorded from
electrodes on the two arms or on one arm and one leg,
the voltage of the QRS complex usually is 1.0 to 1.5
millivolt from the top of the R wave to the bottom of
the S wave; the voltage of the P wave is between 0.1
and 0.3 millivolt; and that of the T wave is between 0.2
and 0.3 millivolt.
P-Q or P-R Interval. The time between the beginning of
the P wave and the beginning of the QRS complex is
the interval between the beginning of electrical excitation of the atria and the beginning of excitation of
the ventricles. This period is called the P-Q interval.
The normal P-Q interval is about 0.16 second. (Often
this interval is called the P-R interval because the Q
wave is likely to be absent.)
Q-T Interval. Contraction of the ventricle lasts almost
from the beginning of the Q wave (or R wave, if
the Q wave is absent) to the end of the T wave. This
126
Unit III
The Heart
interval is called the Q-T interval and ordinarily is
about 0.35 second.
0
Rate of Heartbeat as Determined from the Electrocardiogram.
The rate of heartbeat can be determined easily from
an electrocardiogram because the heart rate is the
reciprocal of the time interval between two successive
heartbeats. If the interval between two beats as determined from the time calibration lines is 1 second, the
heart rate is 60 beats per minute. The normal interval
between two successive QRS complexes in the adult
person is about 0.83 second. This is a heart rate of
60/0.83 times per minute, or 72 beats per minute.
–
+
–
Pen Recorder
Many modern clinical electrocardiographs use computer-based systems and electronic display, while
others use a direct pen recorder that writes the electrocardiogram with a pen directly on a moving sheet
of paper. Sometimes the pen is a thin tube connected
at one end to an inkwell, and its recording end is connected to a powerful electromagnet system that is
capable of moving the pen back and forth at high
speed. As the paper moves forward, the pen records
the electrocardiogram.The movement of the pen is controlled by appropriate electronic amplifiers connected
to electrocardiographic electrodes on the patient.
Other pen recording systems use special paper that
does not require ink in the recording stylus. One such
paper turns black when it is exposed to heat; the stylus
itself is made very hot by electrical current flowing
through its tip. Another type turns black when electrical current flows from the tip of the stylus through the
paper to an electrode at its back. This leaves a black
line on the paper where the stylus touches.
Flow of Current Around
the Heart During the
Cardiac Cycle
Recording Electrical Potentials from a
Partially Depolarized Mass of
Syncytial Cardiac Muscle
Figure 11–4 shows a syncytial mass of cardiac muscle
that has been stimulated at its centralmost point.
–
+
–
0
+
+
–
–
+
+
+++++
+ + ++++++++ +
+
+++++ +–+–+–+–+–+ +++++
++++++–––––––––––––––+–+++++ +
+++++
+
++++++ –– –– –– –– –– –– –– –– ––
+++++ + – – – – – – – – – +++++ +
–
–
–
–
–
–
–
–
–
+++++ – – – – – – – – – +++++
+++++ +––––––––––––––––– +++++ +
++++++ – – – –– –– –– –– ++++++
+++++ + + + – –+ ++++++
+
+++++++++++ +++
+++++++++++
Methods for Recording
Electrocardiograms
Sometimes the electrical currents generated by the
cardiac muscle during each beat of the heart change
electrical potentials and polarities on the respective
sides of the heart in less than 0.01 second. Therefore,
it is essential that any apparatus for recording electrocardiograms be capable of responding rapidly to these
changes in potentials.
0
Figure 11–4
Instantaneous potentials develop on the surface of a cardiac
muscle mass that has been depolarized in its center.
Before stimulation, all the exteriors of the muscle cells
had been positive and the interiors negative. For
reasons presented in Chapter 5 in the discussion of
membrane potentials, as soon as an area of cardiac
syncytium becomes depolarized, negative charges leak
to the outsides of the depolarized muscle fibers,
making this part of the surface electronegative, as represented by the negative signs in Figure 11–4. The
remaining surface of the heart, which is still polarized,
is represented by the positive signs. Therefore, a meter
connected with its negative terminal on the area of
depolarization and its positive terminal on one of the
still-polarized areas, as shown to the right in the figure,
records positively.
Two other electrode placements and meter readings
are also demonstrated in Figure 11–4. These should be
studied carefully, and the reader should be able to
explain the causes of the respective meter readings.
Because the depolarization spreads in all directions
through the heart, the potential differences shown
in the figure persist for only a few thousandths of
a second, and the actual voltage measurements can
be accomplished only with a high-speed recording
apparatus.
Flow of Electrical Currents in the
Chest Around the Heart
Figure 11–5 shows the ventricular muscle lying within
the chest. Even the lungs, although mostly filled
with air, conduct electricity to a surprising extent, and
fluids in other tissues surrounding the heart conduct
electricity even more easily. Therefore, the heart
is actually suspended in a conductive medium.
When one portion of the ventricles depolarizes and
therefore becomes electronegative with respect to the
Chapter 11
127
The Normal Electrocardiogram
+0.5 mV
0
+
-
+
Lead I
0
-
+
-
-
+
+
+ +
B
A
- 0 .2 mV
+ ++
---- + ++
-- - + +
-+
++ ++ -- -+ ++
+ ++-+
+
++
+
++
+
++ ++
+0.3 mV
+ 1 .2 mV
+0.7 mV
0
Figure 11–5
0
+
-
+
-
remainder, electrical current flows from the depolarized area to the polarized area in large circuitous
routes, as noted in the figure.
It should be recalled from the discussion of the
Purkinje system in Chapter 10 that the cardiac impulse
first arrives in the ventricles in the septum and shortly
thereafter spreads to the inside surfaces of the remainder of the ventricles, as shown by the red areas and the
negative signs in Figure 11–5. This provides electronegativity on the insides of the ventricles and electropositivity on the outer walls of the ventricles, with
electrical current flowing through the fluids surrounding the ventricles along elliptical paths, as demonstrated by the curving arrows in the figure. If one
algebraically averages all the lines of current flow (the
elliptical lines), one finds that the average current flow
occurs with negativity toward the base of the heart and
with positivity toward the apex.
During most of the remainder of the depolarization
process, current also continues to flow in this same
direction, while depolarization spreads from the endocardial surface outward through the ventricular
muscle mass. Then, immediately before depolarization
has completed its course through the ventricles, the
average direction of current flow reverses for about
0.01 second, flowing from the ventricular apex toward
the base, because the last part of the heart to become
depolarized is the outer walls of the ventricles near the
base of the heart.
+
-
Lead II
Flow of current in the chest around partially depolarized
ventricles.
+
-
Lead III
+ 1 .0 mV
Figure 11–6
Conventional arrangement of electrodes for recording the standard electrocardiographic leads. Einthoven’s triangle is superimposed on the chest.
Thus, in normal heart ventricles, current flows from
negative to positive primarily in the direction from
the base of the heart toward the apex during almost
the entire cycle of depolarization, except at the very
end. And if a meter is connected to electrodes on
the surface of the body as shown in Figure 11–5, the
electrode nearer the base will be negative, whereas
the electrode nearer the apex will be positive, and
the recording meter will show positive recording in the
electrocardiogram.
Electrocardiographic Leads
Three Bipolar Limb Leads
Figure 11–6 shows electrical connections between the
patient’s limbs and the electrocardiograph for recording electrocardiograms from the so-called standard
bipolar limb leads. The term “bipolar” means that the
electrocardiogram is recorded from two electrodes
128
Unit III
located on different sides of the heart, in this case, on
the limbs. Thus, a “lead” is not a single wire connecting from the body but a combination of two wires and
their electrodes to make a complete circuit between
the body and the electrocardiograph. The electrocardiograph in each instance is represented by an electrical meter in the diagram, although the actual
electrocardiograph is a high-speed recording meter
with a moving paper.
Lead I. In recording limb lead I, the negative terminal
of the electrocardiograph is connected to the right arm
and the positive terminal to the left arm. Therefore,
when the point where the right arm connects to the
chest is electronegative with respect to the point where
the left arm connects, the electrocardiograph records
positively, that is, above the zero voltage line in the
electrocardiogram. When the opposite is true, the electrocardiograph records below the line.
Lead II. To record limb lead II, the negative terminal of
the electrocardiograph is connected to the right arm and
the positive terminal to the left leg. Therefore, when the
right arm is negative with respect to the left leg, the
electrocardiograph records positively.
Lead III. To record limb lead III, the negative terminal
of the electrocardiograph is connected to the left arm
and the positive terminal to the left leg. This means that
the electrocardiograph records positively when the left
arm is negative with respect to the left leg.
The Heart
Now, note that the sum of the voltages in leads I and
III equals the voltage in lead II; that is, 0.5 plus
0.7 equals 1.2. Mathematically, this principle, called
Einthoven’s law, holds true at any given instant while
the three “standard” bipolar electrocardiograms are
being recorded.
Normal Electrocardiograms Recorded from the Three Standard
Bipolar Limb Leads. Figure 11–7 shows recordings of the
electrocardiograms in leads I, II, and III. It is obvious
that the electrocardiograms in these three leads are
similar to one another because they all record positive
P waves and positive T waves, and the major portion
of the QRS complex is also positive in each
electrocardiogram.
On analysis of the three electrocardiograms, it can
be shown, with careful measurements and proper
observance of polarities, that at any given instant the
sum of the potentials in leads I and III equals the
potential in lead II, thus illustrating the validity of
Einthoven’s law.
Because the recordings from all the bipolar limb
leads are similar to one another, it does not matter
greatly which lead is recorded when one wants to
diagnose different cardiac arrhythmias, because diagnosis of arrhythmias depends mainly on the time
relations between the different waves of the cardiac
cycle. But when one wants to diagnose damage in
the ventricular or atrial muscle or in the Purkinje
conducting system, it does matter greatly which leads
are recorded, because abnormalities of cardiac
muscle contraction or cardiac impulse conduction do
Einthoven’s Triangle. In Figure 11–6, the triangle, called
Einthoven’s triangle, is drawn around the area of the
heart. This illustrates that the two arms and the left leg
form apices of a triangle surrounding the heart. The
two apices at the upper part of the triangle represent
the points at which the two arms connect electrically
with the fluids around the heart, and the lower apex is
the point at which the left leg connects with the fluids.
I
Einthoven’s Law. Einthoven’s law states that if the
electrical potentials of any two of the three bipolar
limb electrocardiographic leads are known at any
given instant, the third one can be determined
mathematically by simply summing the first two (but
note that the positive and negative signs of the
different leads must be observed when making this
summation).
For instance, let us assume that momentarily, as
noted in Figure 11–6, the right arm is -0.2 millivolt
(negative) with respect to the average potential in
the body, the left arm is + 0.3 millivolt (positive), and
the left leg is +1.0 millivolt (positive). Observing the
meters in the figure, it can be seen that lead I records
a positive potential of +0.5 millivolt, because this is the
difference between the -0.2 millivolt on the right arm
and the +0.3 millivolt on the left arm. Similarly, lead
III records a positive potential of +0.7 millivolt, and
lead II records a positive potential of +1.2 millivolts
because these are the instantaneous potential differences between the respective pairs of limbs.
II
III
Figure 11–7
Normal electrocardiograms recorded from the three standard
electrocardiographic leads.
129
The Normal Electrocardiogram
Chapter 11
1 2
3 456
RA
V1
LA
V2
V3
V4
V5
V6
5000
ohms
Figure 11–9
5000
ohms
Normal electrocardiograms recorded from the six standard chest
leads.
0
-
+
+
5000
ohms
aVR
aVL
aVF
Figure 11–8
Connections of the body with the electrocardiograph for recording chest leads. LA, left arm; RA, right arm.
change the patterns of the electrocardiograms
markedly in some leads yet may not affect other leads.
Electrocardiographic interpretation of these two
types of conditions—cardiac myopathies and cardiac
arrhythmias—is discussed separately in Chapters 12
and 13.
Chest Leads (Precordial Leads)
Often electrocardiograms are recorded with one electrode placed on the anterior surface of the chest
directly over the heart at one of the points shown in
Figure 11–8. This electrode is connected to the positive
terminal of the electrocardiograph, and the negative
electrode, called the indifferent electrode, is connected
through equal electrical resistances to the right arm,
left arm, and left leg all at the same time, as also shown
in the figure. Usually six standard chest leads are
recorded, one at a time, from the anterior chest wall,
the chest electrode being placed sequentially at the six
points shown in the diagram. The different recordings
are known as leads V1, V2, V3, V4, V5, and V6.
Figure 11–9 illustrates the electrocardiograms of the
healthy heart as recorded from these six standard
chest leads. Because the heart surfaces are close
to the chest wall, each chest lead records mainly the
Figure 11–10
Normal electrocardiograms recorded from the three augmented
unipolar limb leads.
electrical potential of the cardiac musculature immediately beneath the electrode. Therefore, relatively
minute abnormalities in the ventricles, particularly in
the anterior ventricular wall, can cause marked
changes in the electrocardiograms recorded from individual chest leads.
In leads V1 and V2, the QRS recordings of the
normal heart are mainly negative because, as shown in
Figure 11–8, the chest electrode in these leads is nearer
to the base of the heart than to the apex, and the base
of the heart is the direction of electronegativity during
most of the ventricular depolarization process. Conversely, the QRS complexes in leads V4, V5, and V6 are
mainly positive because the chest electrode in these
leads is nearer the heart apex, which is the direction
of electropositivity during most of depolarization.
Augmented Unipolar Limb Leads
Another system of leads in wide use is the augmented
unipolar limb lead. In this type of recording, two of
the limbs are connected through electrical resistances
to the negative terminal of the electrocardiograph,
130
Unit III
and the third limb is connected to the positive terminal. When the positive terminal is on the right arm,
the lead is known as the aVR lead; when on the left
arm, the aVL lead; and when on the left leg, the aVF
lead.
Normal recordings of the augmented unipolar limb
leads are shown in Figure 11–10. They are all similar
to the standard limb lead recordings, except that the
The Heart
recording from the aVR lead is inverted. (Why does
this inversion occur? Study the polarity connections to
the electrocardiograph to determine this.)
References
See references for Chapter 13.
C
H
A
P
T
E
R
1
2
Electrocardiographic
Interpretation of Cardiac Muscle
and Coronary Blood Flow
Abnormalities: Vectorial Analysis
From the discussion in Chapter 10 of impulse transmission through the heart, it is obvious that any
change in the pattern of this transmission can cause
abnormal electrical potentials around the heart and,
consequently, alter the shapes of the waves in the
electrocardiogram. For this reason, almost all
serious abnormalities of the heart muscle can be
diagnosed by analyzing the contours of the different
waves in the different electrocardiographic leads.
Principles of Vectorial Analysis
of Electrocardiograms
Use of Vectors to Represent Electrical Potentials
Before it is possible to understand how cardiac abnormalities affect the contours of the electrocardiogram, one must first become thoroughly familiar with
the concept of vectors and vectorial analysis as applied to electrical potentials
in and around the heart.
Several times in Chapter 11 it was pointed out that heart current flows in a
particular direction in the heart at a given instant during the cardiac cycle. A
vector is an arrow that points in the direction of the electrical potential generated by the current flow, with the arrowhead in the positive direction. Also, by
convention, the length of the arrow is drawn proportional to the voltage of the
potential.
“Resultant” Vector in the Heart at Any Given Instant. Figure 12–1 shows, by the shaded
area and the negative signs, depolarization of the ventricular septum and parts
of the apical endocardial walls of the two ventricles. At this instant of heart excitation, electrical current flows between the depolarized areas inside the heart
and the nondepolarized areas on the outside of the heart, as indicated by the
long elliptical arrows. Some current also flows inside the heart chambers directly
from the depolarized areas toward the still polarized areas. Overall, considerably more current flows downward from the base of the ventricles toward the
apex than in the upward direction. Therefore, the summated vector of the generated potential at this particular instant, called the instantaneous mean vector,
is represented by the long black arrow drawn through the center of the ventricles in a direction from base toward apex. Furthermore, because the summated
current is considerable in quantity, the potential is large, and the vector is long.
Direction of a Vector Is Denoted in Terms of Degrees
When a vector is exactly horizontal and directed toward the person’s left side,
the vector is said to extend in the direction of 0 degrees, as shown in Figure
131
132
Unit III
+ + ++
+
+
+
- +
+
+
+
+ +
+
+
+
+
+
+
+
+
+
+
+
- +
+
+
+
-+
+
+
+
++
+
+
+- +
+
+
+
+
+
+
+
++
+
+ + + + +
The Heart
-
aVF
III
-
aVL +
+ aVR
210∞
-
-30∞
I
I
0∞
aVL
aVR
III
Figure 12–1
+
120∞
90∞
+
60∞
II
+
+
Mean vector through the partially depolarized ventricles.
Figure 12–3
Axes of the three bipolar and three unipolar leads.
-90∞
+270∞
through the center of Figure 12–2 in the +59-degree
direction. This means that during most of the depolarization wave, the apex of the heart remains positive
with respect to the base of the heart, as discussed later
in the chapter.
-100∞
0∞
180∞
Axis for Each Standard Bipolar Lead
and Each Unipolar Limb Lead
A
120∞
59∞
-90∞
Figure 12–2
Vectors drawn to represent potentials for several different hearts,
and the “axis” of the potential (expressed in degrees) for each
heart.
12–2. From this zero reference point, the scale of
vectors rotates clockwise: when the vector extends
from above and straight downward, it has a direction
of +90 degrees; when it extends from the person’s left
to right, it has a direction of +180 degrees; and when
it extends straight upward, it has a direction of -90 (or
+270) degrees.
In a normal heart, the average direction of the
vector during spread of the depolarization wave
through the ventricles, called the mean QRS vector, is
about +59 degrees, which is shown by vector A drawn
In Chapter 11, the three standard bipolar and the three
unipolar limb leads are described. Each lead is actually a pair of electrodes connected to the body on
opposite sides of the heart, and the direction from negative electrode to positive electrode is called the “axis”
of the lead. Lead I is recorded from two electrodes
placed respectively on the two arms. Because the electrodes lie exactly in the horizontal direction, with
the positive electrode to the left, the axis of lead I is
0 degrees.
In recording lead II, electrodes are placed on the
right arm and left leg. The right arm connects to the
torso in the upper right-hand corner and the left leg
connects in the lower left-hand corner. Therefore, the
direction of this lead is about +60 degrees.
By similar analysis, it can be seen that lead III has
an axis of about +120 degrees; lead aVR, +210 degrees;
aVF, +90 degrees; and aVL -30 degrees. The directions
of the axes of all these leads are shown in Figure 12–3,
which is known as the hexagonal reference system. The
polarities of the electrodes are shown by the plus and
minus signs in the figure. The reader must learn these
axes and their polarities, particularly for the bipolar
limb leads I, II, and III, to understand the remainder of
this chapter.
Chapter 12
133
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
Vectorial Analysis of Potentials
Recorded in Different Leads
Now that we have discussed, first, the conventions for
representing potentials across the heart by means of
vectors and, second, the axes of the leads, it is possible
to use these together to determine the instantaneous
potential that will be recorded in the electrocardiogram of each lead for a given vector in the heart, as
follows.
Figure 12–4 shows a partially depolarized heart;
vector A represents the instantaneous mean direction
of current flow in the ventricles. In this instance, the
direction of the vector is +55 degrees, and the voltage
of the potential, represented by the length of vector A,
is 2 millivolts. In the diagram below the heart, vector
A is shown again, and a line is drawn to represent the
axis of lead I in the 0-degree direction. To determine
how much of the voltage in vector A will be recorded
in lead I, a line perpendicular to the axis of lead I is
drawn from the tip of vector A to the lead I axis, and
a so-called projected vector (B) is drawn along the lead
I axis. The arrow of this projected vector points toward
the positive end of the lead I axis, which means that
the record momentarily being recorded in the electrocardiogram of lead I is positive. And the instantaneous
recorded voltage will be equal to the length of B
divided by the length of A times 2 millivolts, or about
1 millivolt.
Figure 12–5 shows another example of vectorial
analysis. In this example, vector A represents the electrical potential and its axis at a given instant during
ventricular depolarization in a heart in which the left
side of the heart depolarizes more rapidly than the
right. In this instance, the instantaneous vector has a
direction of 100 degrees, and its voltage is again 2 millivolts. To determine the potential actually recorded in
lead I, we draw a perpendicular line from the tip of
vector A to the lead I axis and find projected vector B.
Vector B is very short and this time in the negative
direction, indicating that at this particular instant, the
recording in lead I will be negative (below the zero line
in the electrocardiogram), and the voltage recorded
will be slight, about -0.3 millivolts. This figure demonstrates that when the vector in the heart is in a direction almost perpendicular to the axis of the lead, the
voltage recorded in the electrocardiogram of this lead
is very low. Conversely, when the heart vector has
almost exactly the same axis as the lead axis, essentially
the entire voltage of the vector will be recorded.
Vectorial Analysis of Potentials in the Three Standard
Bipolar Limb Leads. In Figure 12–6, vector A depicts the
-
I
B
+
A
Figure 12–5
Determination of the projected vector B along the axis of lead I
when vector A represents the instantaneous potential in the
ventricles.
-
-
A
I
II
III
-
B
I
I
D
+
A
C
-
I
B
I
+
III
+
II
+
A
Figure 12–4
Determination of a projected vector B along the axis of lead I when
vector A represents the instantaneous potential in the ventricles.
Figure 12–6
Determination of projected vectors in leads I, II, and III when
vector A represents the instantaneous potential in the ventricles.
134
Unit III
instantaneous electrical potential of a partially depolarized heart. To determine the potential recorded at
this instant in the electrocardiogram for each one of
the three standard bipolar limb leads, perpendicular
lines (the dashed lines) are drawn from the tip of
vector A to the three lines representing the axes of the
three different standard leads, as shown in the figure.
The projected vector B depicts the potential recorded
at that instant in lead I, projected vector C depicts the
potential in lead II, and projected vector D depicts the
potential in lead III. In each of these, the record in
the electrocardiogram is positive—that is, above the
zero line—because the projected vectors point in the
positive directions along the axes of all the leads.
The potential in lead I (vector B) is about one half that
of the actual potential in the heart (vector A); in lead
II (vector C), it is almost equal to that in the heart; and
in lead III (vector D), it is about one third that in the
heart.
An identical analysis can be used to determine
potentials recorded in augmented limb leads, except
that the respective axes of the augmented leads (see
Figure 12–3) are used in place of the standard bipolar
limb lead axes used for Figure 12–6.
Vectorial Analysis of the
Normal Electrocardiogram
Vectors That Occur at Successive
Intervals During Depolarization of the
Ventricles—The QRS Complex
When the cardiac impulse enters the ventricles
through the atrioventricular bundle, the first part of
the ventricles to become depolarized is the left endocardial surface of the septum. Then depolarization
spreads rapidly to involve both endocardial surfaces of
the septum, as demonstrated by the shaded portion
of the ventricle in Figure 12–7A. Next, depolarization
spreads along the endocardial surfaces of the remainder of the two ventricles, as shown in Figure 12–7B and
C. Finally, it spreads through the ventricular muscle to
the outside of the heart, as shown progressively in
Figure 12–7C, D, and E.
At each stage in Figure 12–7, parts A to E, the
instantaneous mean electrical potential of the ventricles is represented by a red vector superimposed on
the ventricle in each figure. Each of these vectors is
then analyzed by the method described in the preceding section to determine the voltages that will be
recorded at each instant in each of the three standard
electrocardiographic leads. To the right in each figure
is shown progressive development of the electrocardiographic QRS complex. Keep in mind that a positive
vector in a lead will cause recording in the electrocardiogram above the zero line, whereas a negative vector
will cause recording below the zero line.
Before proceeding with further consideration of
vectorial analysis, it is essential that this analysis of the
successive normal vectors presented in Figure 12–7 be
The Heart
understood. Each of these analyses should be studied
in detail by the procedure given here. A short
summary of this sequence follows.
In Figure 12–7A, the ventricular muscle has just
begun to be depolarized, representing an instant about
0.01 second after the onset of depolarization. At this
time, the vector is short because only a small portion
of the ventricles—the septum—is depolarized. Therefore, all electrocardiographic voltages are low, as
recorded to the right of the ventricular muscle for each
of the leads. The voltage in lead II is greater than the
voltages in leads I and III because the heart vector
extends mainly in the same direction as the axis of lead
II.
In Figure 12–7B, which represents about 0.02 second
after onset of depolarization, the heart vector is long
because much of the ventricular muscle mass has
become depolarized. Therefore, the voltages in all
electrocardiographic leads have increased.
In Figure 12–7C, about 0.035 second after onset of
depolarization, the heart vector is becoming shorter
and the recorded electrocardiographic voltages are
lower because the outside of the heart apex is now
electronegative, neutralizing much of the positivity on
the other epicardial surfaces of the heart. Also, the axis
of the vector is beginning to shift toward the left side
of the chest because the left ventricle is slightly slower
to depolarize than the right. Therefore, the ratio of the
voltage in lead I to that in lead III is increasing.
In Figure 12–7D, about 0.05 second after onset of
depolarization, the heart vector points toward the base
of the left ventricle, and it is short because only a
minute portion of the ventricular muscle is still polarized positive. Because of the direction of the vector at
this time, the voltages recorded in leads II and III are
both negative—that is, below the line—whereas the
voltage of lead I is still positive.
In Figure 12–7E, about 0.06 second after onset of
depolarization, the entire ventricular muscle mass is
depolarized, so that no current flows around the heart
and no electrical potential is generated. The vector
becomes zero, and the voltages in all leads become
zero.
Thus, the QRS complexes are completed in the
three standard bipolar limb leads.
Sometimes the QRS complex has a slight negative
depression at its beginning in one or more of the leads,
which is not shown in Figure 12–7; this depression is
the Q wave. When it occurs, it is caused by initial depolarization of the left side of the septum before the right
side, which creates a weak vector from left to right for
a fraction of a second before the usual base-to-apex
vector occurs. The major positive deflection shown in
Figure 12–7 is the R wave, and the final negative
deflection is the S wave.
Electrocardiogram During
Repolarization—The T Wave
After the ventricular muscle has become depolarized,
about 0.15 second later, repolarization begins and
−
Chapter 12
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
−
−
II
III
−
I
I
I
II
III
−
I
+
−
I
I
+
II
II
II
+
III
+
II
+
III
+
III
III
A
B
−
II
III
−
−
II
III
−
I
I
−
135
I
I
+
−
I
I
II
II
III
+
+
II
+
II
+
III
+
III
III
C
D
−
−
II
III
−
I
I
I
+
II
III
+
II
+
III
E
Figure 12–7
Shaded areas of the ventricles are depolarized (–); nonshaded areas are still polarized (+). The ventricular vectors and QRS complexes
0.01 second after onset of ventricular depolarization (A); 0.02 second after onset of depolarization (B); 0.035 second after onset of
depolarization (C); 0.05 second after onset of depolarization (D); and after depolarization of the ventricles is complete, 0.06 second after
onset (E).
proceeds until complete at about 0.35 second.This repolarization causes the T wave in the electrocardiogram.
Because the septum and endocardial areas of the
ventricular muscle depolarize first, it seems logical that
these areas should repolarize first as well. However,
this is not the usual case because the septum and other
endocardial areas have a longer period of contraction
than most of the external surfaces of the heart. Therefore, the greatest portion of ventricular muscle mass to
repolarize first is the entire outer surface of the ventricles, especially near the apex of the heart. The endocardial areas, conversely, normally repolarize last. This
sequence of repolarization is postulated to be caused
by the high blood pressure inside the ventricles during
contraction, which greatly reduces coronary blood
flow to the endocardium, thereby slowing repolarization in the endocardial areas.
Because the outer apical surfaces of the ventricles
repolarize before the inner surfaces, the positive end
of the overall ventricular vector during repolarization
is toward the apex of the heart. As a result, the normal
T wave in all three bipolar limb leads is positive, which
is also the polarity of most of the normal QRS complex.
In Figure 12–8, five stages of repolarization of the
ventricles are denoted by progressive increase of the
white areas—the repolarized areas. At each stage,
the vector extends from the base of the heart toward
the apex until it disappears in the last stage. At
first, the vector is relatively small because the area of
repolarization is small. Later, the vector becomes
136
Unit III
The Heart
-
-
II
III
-
I
I
III
+
P
+ + ++
++
+
-+ +
- +
+
+
+ ++
- +
+
+
+
+
+ SA
+
+ +
II
III
-
-
T
I
II
+
II
+
-
I
I
+
III
I
II
III
+
II
+
Figure 12–9
III
Figure 12–8
Generation of the T wave during repolarization of the ventricles,
showing also vectorial analysis of the first stage of repolarization.
The total time from the beginning of the T wave to its end is
approximately 0.15 second.
stronger because of greater degrees of repolarization.
Finally, the vector becomes weaker again because the
areas of depolarization still persisting become so slight
that the total quantity of current flow decreases. These
changes also demonstrate that the vector is greatest
when about half the heart is in the polarized state and
about half is depolarized.
The changes in the electrocardiograms of the three
standard limb leads during repolarization are noted
under each of the ventricles, depicting the progressive
stages of repolarization. Thus, over about 0.15 second,
the period of time required for the whole process to
take place, the T wave of the electrocardiogram is
generated.
Depolarization of the Atria—
The P Wave
Depolarization of the atria begins in the sinus node
and spreads in all directions over the atria. Therefore,
the point of original electronegativity in the atria is
about at the point of entry of the superior vena cava
where the sinus node lies, and the direction of initial
depolarization is denoted by the black vector in Figure
12–9. Furthermore, the vector remains generally in this
direction throughout the process of normal atrial
depolarization. Because this direction is generally
in the positive directions of the axes of the three
standard bipolar limb leads I, II, and III, the electrocardiograms recorded from the atria during depolarization are also usually positive in all three of these
leads, as shown in Figure 12–9. This record of atrial
depolarization is known as the atrial P wave.
Depolarization of the atria and generation of the P wave, showing
the maximum vector through the atria and the resultant vectors in
the three standard leads. At the right are the atrial P and T waves.
SA, sinoatrial node.
Repolarization of the Atria—The Atrial T Wave. Spread of
depolarization through the atrial muscle is much
slower than in the ventricles because the atria have no
Purkinje system for fast conduction of the depolarization signal. Therefore, the musculature around the
sinus node becomes depolarized a long time before the
musculature in distal parts of the atria. Because of this,
the area in the atria that also becomes repolarized first
is the sinus nodal region, the area that had originally
become depolarized first. Thus, when repolarization
begins, the region around the sinus node becomes positive with respect to the rest of the atria. Therefore, the
atrial repolarization vector is backward to the vector of
depolarization. (Note that this is opposite to the effect
that occurs in the ventricles.) Therefore, as shown to
the right in Figure 12–9, the so-called atrial T wave
follows about 0.15 second after the atrial P wave, but
this T wave is on the opposite side of the zero reference line from the P wave; that is, it is normally negative rather than positive in the three standard bipolar
limb leads.
In the normal electrocardiogram, the atrial T wave
appears at about the same time that the QRS complex
of the ventricles appears. Therefore, it is almost always
totally obscured by the large ventricular QRS
complex, although in some very abnormal states, it
does appear in the recorded electrocardiogram.
Vectorcardiogram
It has been noted in the discussion up to this point that
the vector of current flow through the heart changes
rapidly as the impulse spreads through the
myocardium. It changes in two aspects: First, the
vector increases and decreases in length because of
increasing and decreasing voltage of the vector.
Second, the vector changes direction because of
Chapter 12
1
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
2
4
3
4
III
- -60∞
5
5
I
1
137
I
+
I II
0∞
180∞
3
Depolarization
QRS
2
120∞
Repolarization
T
59∞
III
+
III
Figure 12–10
QRS and T vectorcardiograms.
Figure 12–11
Plotting the mean electrical axis of the ventricles from two electrocardiographic leads (leads I and III).
changes in the average direction of the electrical
potential from the heart. The so-called vectorcardiogram depicts these changes at different times during
the cardiac cycle, as shown in Figure 12–10.
In the large vectorcardiogram of Figure 12–10, point
5 is the zero reference point, and this point is the negative end of all the successive vectors. While the heart
muscle is polarized between heartbeats, the positive
end of the vector remains at the zero point because
there is no vectorial electrical potential. However,
as soon as current begins to flow through the ventricles at the beginning of ventricular depolarization, the
positive end of the vector leaves the zero reference
point.
When the septum first becomes depolarized, the
vector extends downward toward the apex of the ventricles, but it is relatively weak, thus generating the first
portion of the ventricular vectorcardiogram, as shown
by the positive end of vector 1. As more of the
ventricular muscle becomes depolarized, the vector
becomes stronger and stronger, usually swinging
slightly to one side. Thus, vector 2 of Figure 12–10 represents the state of depolarization of the ventricles
about 0.02 second after vector 1. After another 0.02
second, vector 3 represents the potential, and vector 4
occurs in another 0.01 second. Finally, the ventricles
become totally depolarized, and the vector becomes
zero once again, as shown at point 5.
The elliptical figure generated by the positive ends
of the vectors is called the QRS vectorcardiogram.
Vectorcardiograms can be recorded on an oscilloscope
by connecting body surface electrodes from the neck
and lower abdomen to the vertical plates of the oscilloscope and connecting chest surface electrodes from
each side of the heart to the horizontal plates. When
the vector changes, the spot of light on the oscilloscope
follows the course of the positive end of the changing
vector, thus inscribing the vectorcardiogram on the
oscilloscopic screen.
Mean Electrical Axis
of the Ventricular QRS—
And Its Significance
The vectorcardiogram during ventricular depolarization (the QRS vectorcardiogram) shown in Figure
12–10 is that of a normal heart. Note from this vectorcardiogram that the preponderant direction of the
vectors of the ventricles during depolarization is
mainly toward the apex of the heart. That is, during
most of the cycle of ventricular depolarization, the
direction of the electrical potential (negative to positive) is from the base of the ventricles toward the apex.
This preponderant direction of the potential during
depolarization is called the mean electrical axis of the
ventricles. The mean electrical axis of the normal
ventricles is 59 degrees. In many pathological conditions of the heart, this direction changes markedly—
sometimes even to opposite poles of the heart.
Determining the Electrical Axis from
Standard Lead Electrocardiograms
Clinically, the electrical axis of the heart usually is estimated from the standard bipolar limb lead electrocardiograms rather than from the vectorcardiogram.
Figure 12–11 shows a method for doing this. After
recording the standard leads, one determines the net
potential and polarity of the recordings in leads I and
III. In lead I of Figure 12–11, the recording is positive,
and in lead III, the recording is mainly positive but
negative during part of the cycle. If any part of a
recording is negative, this negative potential is subtracted from the positive part of the potential to determine the net potential for that lead, as shown by the
arrow to the right of the QRS complex for lead III.
138
Unit III
Then each net potential for leads I and III is plotted
on the axes of the respective leads, with the base of the
potential at the point of intersection of the axes, as
shown in Figure 12–11.
If the net potential of lead I is positive, it is plotted
in a positive direction along the line depicting lead I.
Conversely, if this potential is negative, it is plotted in
a negative direction. Also, for lead III, the net potential is placed with its base at the point of intersection,
and, if positive, it is plotted in the positive direction
along the line depicting lead III. If it is negative, it is
plotted in the negative direction.
To determine the vector of the total QRS ventricular mean electrical potential, one draws perpendicular
lines (the dashed lines in the figure) from the apices of
leads I and III, respectively. The point of intersection
of these two perpendicular lines represents, by vectorial analysis, the apex of the mean QRS vector in the
ventricles, and the point of intersection of the lead I
and lead III axes represents the negative end of the
mean vector. Therefore, the mean QRS vector is drawn
between these two points. The approximate average
potential generated by the ventricles during depolarization is represented by the length of this mean QRS
vector, and the mean electrical axis is represented by
the direction of the mean vector. Thus, the orientation
of the mean electrical axis of the normal ventricles,
as determined in Figure 12–11, is 59 degrees positive
(+59 degrees).
Abnormal Ventricular Conditions That
Cause Axis Deviation
The Heart
side of the heart than on the other side, and this allows
excess generation of electrical potential on that side.
Second, more time is required for the depolarization
wave to travel through the hypertrophied ventricle
than through the normal ventricle. Consequently, the
normal ventricle becomes depolarized considerably in
advance of the hypertrophied ventricle, and this causes
a strong vector from the normal side of the heart
toward the hypertrophied side, which remains strongly
positively charged. Thus, the axis deviates toward the
hypertrophied ventricle.
Vectorial Analysis of Left Axis Deviation Resulting from
Hypertrophy of the Left Ventricle. Figure 12–12 shows
the three standard bipolar limb lead electrocardiograms. Vectorial analysis demonstrates left axis deviation with mean electrical axis pointing in the
-15-degree direction. This is a typical electrocardiogram caused by increased muscle mass of the left ventricle. In this instance, the axis deviation was caused by
hypertension (high arterial blood pressure), which
caused the left ventricle to hypertrophy so that it could
pump blood against elevated systemic arterial pressure. A similar picture of left axis deviation occurs
when the left ventricle hypertrophies as a result of
aortic valvular stenosis, aortic valvular regurgitation, or
any number of congenital heart conditions in which the
left ventricle enlarges while the right ventricle remains
relatively normal in size.
Vectorial Analysis of Right Axis Deviation Resulting
from Hypertrophy of the Right Ventricle. The electro-
cardiogram of Figure 12–13 shows intense right axis
deviation, to an electrical axis of 170 degrees, which is
Although the mean electrical axis of the ventricles
averages about 59 degrees, this axis can swing even
in the normal heart from about 20 degrees to about
100 degrees. The causes of the normal variations are
mainly anatomical differences in the Purkinje distribution system or in the musculature itself of different
hearts. However, a number of abnormal conditions of
the heart can cause axis deviation beyond the normal
limits, as follows.
Change in the Position of the Heart in the Chest. If the heart
itself is angulated to the left, the mean electrical axis
of the heart also shifts to the left. Such shift occurs (1)
at the end of deep expiration, (2) when a person lies
down, because the abdominal contents press upward
against the diaphragm, and (3) quite frequently in
stocky, fat people whose diaphragms normally press
upward against the heart all the time.
Likewise, angulation of the heart to the right causes
the mean electrical axis of the ventricles to shift to the
right. This occurs (1) at the end of deep inspiration,
(2) when a person stands up, and (3) normally in tall,
lanky people whose hearts hang downward.
Hypertrophy of One Ventricle. When one ventricle greatly
hypertrophies, the axis of the heart shifts toward the
hypertrophied ventricle for two reasons. First, a far
greater quantity of muscle exists on the hypertrophied
I
II
III
III
-
I-
+I
+
III
Figure 12–12
Left axis deviation in a hypertensive heart (hypertrophic left ventricle). Note the slightly prolonged QRS complex as well.
Chapter 12
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
I
I
II
II
139
III
III
-
III
III
-
I-
+I
I-
+I
+
III
+
III
Figure 12–14
Figure 12–13
High-voltage electrocardiogram in congenital pulmonary valve
stenosis with right ventricular hypertrophy. Intense right axis deviation and a slightly prolonged QRS complex also are seen.
111 degrees to the right of the normal 59-degree mean
ventricular QRS axis. The right axis deviation demonstrated in this figure was caused by hypertrophy of the
right ventricle as a result of congenital pulmonary
valve stenosis. Right axis deviation also can occur in
other congenital heart conditions that cause hypertrophy of the right ventricle, such as tetralogy of Fallot and
interventricular septal defect.
Bundle Branch Block Causes Axis Deviation. Ordinarily, the
lateral walls of the two ventricles depolarize at almost
the same instant because both the left and the right
bundle branches of the Purkinje system transmit the
cardiac impulse to the two ventricular walls at almost
the same instant. As a result, the potentials generated
by the two ventricles (on the two opposite sides of the
heart) almost neutralize each other. But if only one of
the major bundle branches is blocked, the cardiac
impulse spreads through the normal ventricle long
before it spreads through the other. Therefore, depolarization of the two ventricles does not occur even
nearly simultaneously, and the depolarization potentials do not neutralize each other.As a result, axis deviation occurs as follows.
Vectorial Analysis of Left Axis Deviation in Left Bundle
Branch Block. When the left bundle branch is
Left axis deviation caused by left bundle branch block. Note also
the greatly prolonged QRS complex.
blocked, cardiac depolarization spreads through the
right ventricle two to three times as rapidly as through
the left ventricle. Consequently, much of the left ventricle remains polarized for as long as 0.1 second after
the right ventricle has become totally depolarized.
Thus, the right ventricle becomes electronegative,
whereas the left ventricle remains electropositive
during most of the depolarization process, and a strong
vector projects from the right ventricle toward the left
ventricle. In other words, there is intense left axis deviation of about -50 degrees because the positive end of
the vector points toward the left ventricle. This is
demonstrated in Figure 12–14, which shows typical left
axis deviation resulting from left bundle branch block.
Because of slowness of impulse conduction when
the Purkinje system is blocked, in addition to axis deviation, the duration of the QRS complex is greatly prolonged because of extreme slowness of depolarization
in the affected side of the heart. One can see this by
observing the excessive widths of the QRS waves in
Figure 12–14. This is discussed in greater detail later in
the chapter. This extremely prolonged QRS complex
differentiates bundle branch block from axis deviation
caused by hypertrophy.
Vectorial Analysis of Right Axis Deviation in Right
Bundle Branch Block. When the right bundle branch
is blocked, the left ventricle depolarizes far more
rapidly than the right ventricle, so that the left side of
the ventricles becomes electronegative as long as
140
Unit III
0.1 second before the right. Therefore, a strong vector
develops, with its negative end toward the left ventricle and its positive end toward the right ventricle. In
other words, intense right axis deviation occurs. Right
axis deviation caused by right bundle branch block is
demonstrated, and its vector is analyzed, in Figure
12–15, which shows an axis of about 105 degrees
instead of the normal 59 degrees and a prolonged QRS
complex because of slow conduction.
Conditions That Cause
Abnormal Voltages of the QRS
Complex
Increased Voltage in the Standard
Bipolar Limb Leads
Normally, the voltages in the three standard bipolar
limb leads, as measured from the peak of the R wave
to the bottom of the S wave, vary between 0.5 and 2.0
millivolts, with lead III usually recording the lowest
voltage and lead II the highest. However, these relations are not invariable, even for the normal heart. In
general, when the sum of the voltages of all the QRS
complexes of the three standard leads is greater than
4 millivolts, the patient is considered to have a highvoltage electrocardiogram.
The cause of high-voltage QRS complexes most
often is increased muscle mass of the heart, which
ordinarily results from hypertrophy of the muscle in
response to excessive load on one part of the heart or
the other. For example, the right ventricle hypertrophies when it must pump blood through a stenotic pulmonary valve, and the left ventricle hypertrophies
when a person has high blood pressure. The increased
quantity of muscle causes generation of increased
quantities of electricity around the heart. As a result,
The Heart
the electrical potentials recorded in the electrocardiographic leads are considerably greater than normal, as
shown in Figures 12–12 and 12–13.
Decreased Voltage of the
Electrocardiogram
Decreased Voltage Caused by Cardiac Myopathies. One of
the most common causes of decreased voltage of the
QRS complex is a series of old myocardial artery
infarctions with resultant diminished muscle mass. This
also causes the depolarization wave to move through
the ventricles slowly and prevents major portions of
the heart from becoming massively depolarized all at
once. Consequently, this condition causes some prolongation of the QRS complex along with the
decreased voltage. Figure 12–16 shows a typical lowvoltage electrocardiogram with prolongation of the
QRS complex, which is common after multiple small
infarctions of the heart have caused local delays of
impulse conduction and reduced voltages due to loss
of muscle mass throughout the ventricles.
Decreased Voltage Caused by Conditions Surrounding the
Heart. One of the most important causes of decreased
voltage in electrocardiographic leads is fluid in the
pericardium. Because extracellular fluid conducts electrical currents with great ease, a large portion of the
electricity flowing out of the heart is conducted from
one part of the heart to another through the pericardial fluid. Thus, this effusion effectively “short-circuits”
the electrical potentials generated by the heart,
decreasing the electrocardiographic voltages that
reach the outside surfaces of the body. Pleural effusion, to a lesser extent, also can “short-circuit” the
electricity around the heart, so that the voltages at the
III
-
I
I-
I
+I
II
II
III
+
III
Figure 12–15
Right axis deviation caused by right bundle branch block. Note
also the greatly prolonged QRS complex.
III
Figure 12–16
Low-voltage electrocardiogram following local damage throughout the ventricles caused by previous myocardial infarction.
Chapter 12
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
surface of the body and in the electrocardiograms are
decreased.
Pulmonary emphysema can decrease the electrocardiographic potentials, but by a different method from
that of pericardial effusion. In pulmonary emphysema,
conduction of electrical current through the lungs is
depressed considerably because of excessive quantity
of air in the lungs. Also, the chest cavity enlarges, and
the lungs tend to envelop the heart to a greater extent
than normally. Therefore, the lungs act as an insulator
to prevent spread of electrical voltage from the heart
to the surface of the body, and this results in decreased
electrocardiographic potentials in the various leads.
141
ventricular system, with replacement of this muscle by
scar tissue, and (2) multiple small local blocks in the
conduction of impulses at many points in the Purkinje
system. As a result, cardiac impulse conduction
becomes irregular, causing rapid shifts in voltages and
axis deviations. This often causes double or even triple
peaks in some of the electrocardiographic leads, such
as those shown in Figure 12–14.
Current of Injury
The QRS complex lasts as long as depolarization continues to spread through the ventricles—that is, as long
as part of the ventricles is depolarized and part is
still polarized. Therefore, prolonged conduction of the
impulse through the ventricles always causes a prolonged QRS complex. Such prolongation often occurs
when one or both ventricles are hypertrophied or
dilated, owing to the longer pathway that the impulse
must then travel. The normal QRS complex lasts 0.06
to 0.08 second, whereas in hypertrophy or dilatation of
the left or right ventricle, the QRS complex may be
prolonged to 0.09 to 0.12 second.
Many different cardiac abnormalities, especially those
that damage the heart muscle itself, often cause part
of the heart to remain partially or totally depolarized
all the time. When this occurs, current flows between
the pathologically depolarized and the normally polarized areas even between heartbeats. This is called a
current of injury. Note especially that the injured part
of the heart is negative, because this is the part that is
depolarized and emits negative charges into the surrounding fluids, whereas the remainder of the heart is
neutral or positive polarity.
Some abnormalities that can cause current of injury
are (1) mechanical trauma, which sometimes makes
the membranes remain so permeable that full repolarization cannot take place; (2) infectious processes
that damage the muscle membranes; and (3) ischemia
of local areas of heart muscle caused by local coronary
occlusions, which is by far the most common cause of
current of injury in the heart. During ischemia, not
enough nutrients from the coronary blood supply are
available to the heart muscle to maintain normal membrane polarization.
Prolonged QRS Complex Resulting
from Purkinje System Blocks
Effect of Current of Injury on the
QRS Complex
When the Purkinje fibers are blocked, the cardiac
impulse must then be conducted by the ventricular
muscle instead of by way of the Purkinje system. This
decreases the velocity of impulse conduction to about
one third of normal. Therefore, if complete block of
one of the bundle branches occurs, the duration of the
QRS complex usually is increased to 0.14 second or
greater.
In general, a QRS complex is considered to be
abnormally long when it lasts more than 0.09 second;
when it lasts more than 0.12 second, the prolongation
is almost certainly caused by pathological block somewhere in the ventricular conduction system, as shown
by the electrocardiograms for bundle branch block in
Figures 12–14 and 12–15.
In Figure 12–17, a small area in the base of the left ventricle is newly infarcted (loss of coronary blood flow).
Therefore, during the T-P interval—that is, when the
normal ventricular muscle is totally polarized—abnormal negative current still flows from the infarcted area
at the base of the left ventricle and spreads toward the
rest of the ventricles. The vector of this “current of
injury,” as shown in the first heart in the figure, is in a
direction of about 125 degrees, with the base of the
vector, the negative end, toward the injured muscle. As
shown in the lower portions of the figure, even before
the QRS complex begins, this vector causes an initial
record in lead I below the zero potential line, because
the projected vector of the current of injury in lead I
points toward the negative end of the lead I axis. In
lead II, the record is above the line because the projected vector points more toward the positive terminal
of the lead. In lead III, the projected vector points in
the same direction as the positive terminal of lead III,
so that the record is positive. Furthermore, because the
vector lies almost exactly in the direction of the axis
of lead III, the voltage of the current of injury in lead
III is much greater than in either lead I or lead II.
Prolonged and Bizarre
Patterns of the QRS Complex
Prolonged QRS Complex as a Result
of Cardiac Hypertrophy or Dilatation
Conditions That Cause Bizarre QRS
Complexes
Bizarre patterns of the QRS complex most frequently
are caused by two conditions: (1) destruction of
cardiac muscle in various areas throughout the
142
Unit III
The Heart
Injured area
-
-
I
II
III
-
I
I
III
+
+
II
+
J
II
Current
of injury
J
Figure 12–17
III
J
As the heart then proceeds through its normal
process of depolarization, the septum first becomes
depolarized; then the depolarization spreads down to
the apex and back toward the bases of the ventricles.
The last portion of the ventricles to become totally
depolarized is the base of the right ventricle, because
the base of the left ventricle is already totally and permanently depolarized. By vectorial analysis, the successive stages of electrocardiogram generation by the
depolarization wave traveling through the ventricles
can be constructed graphically, as demonstrated in the
lower part of Figure 12–17.
When the heart becomes totally depolarized, at the
end of the depolarization process (as noted by the
next-to-last stage in Figure 12–17), all the ventricular
muscle is in a negative state. Therefore, at this instant
in the electrocardiogram, no current flows from the
ventricles to the electrocardiographic electrodes
because now both the injured heart muscle and the
contracting muscle are depolarized.
Next, as repolarization takes place, all of the heart
finally repolarizes, except the area of permanent depolarization in the injured base of the left ventricle. Thus,
repolarization causes a return of the current of injury
in each lead, as noted at the far right in Figure 12–17.
The J Point—The Zero Reference
Potential for Analyzing Current
of Injury
One would think that the electrocardiograph
machines for recording electrocardiograms could
determine when no current is flowing around the
Current
of injury
Effect of a current of injury on
the electrocardiogram.
heart. However, many stray currents exist in the body,
such as currents resulting from “skin potentials” and
from differences in ionic concentrations in different
fluids of the body. Therefore, when two electrodes are
connected between the arms or between an arm and a
leg, these stray currents make it impossible for one to
predetermine the exact zero reference level in the
electrocardiogram. For these reasons, the following
procedure must be used to determine the zero potential level: First, one notes the exact point at which the
wave of depolarization just completes its passage
through the heart, which occurs at the end of the QRS
complex. At exactly this point, all parts of the ventricles have become depolarized, including both the
damaged parts and the normal parts, so that no current
is flowing around the heart. Even the current of injury
disappears at this point. Therefore, the potential of the
electrocardiogram at this instant is at zero voltage.This
point is known as the “J point” in the electrocardiogram, as shown in Figure 12–18.
Then, for analysis of the electrical axis of the injury
potential caused by a current of injury, a horizontal
line is drawn in the electrocardiogram for each lead at
the level of the J point. This horizontal line is then the
zero potential level in the electrocardiogram from
which all potentials caused by currents of injury must
be measured.
Use of the J Point in Plotting Axis of Injury Potential. Figure
12–18 shows electrocardiograms (leads I and III) from
an injured heart. Both records show injury potentials.
In other words, the J point of each of these two electrocardiograms is not on the same line as the T-P
segment. In the figure, a horizontal line has been
drawn through the J point to represent the zero
Chapter 12
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
voltage level in each of the two recordings. The injury
potential in each lead is the difference between the
voltage of the electrocardiogram immediately before
onset of the P wave and the zero voltage level determined from the J point. In lead I, the recorded voltage
of the injury potential is above the zero potential level
and is, therefore, positive. Conversely, in lead III, the
injury potential is below the zero voltage level and,
therefore, is negative.
At the bottom in Figure 12–18, the respective injury
potentials in leads I and III are plotted on the coordinates of these leads, and the resultant vector of the
injury potential for the whole ventricular muscle mass
is determined by vectorial analysis as described. In this
instance, the resultant vector extends from the right
side of the ventricles toward the left and slightly
upward, with an axis of about -30 degrees. If one
places this vector for the injury potential directly over
the ventricles, the negative end of the vector points
toward the permanently depolarized, “injured” area of
the ventricles. In the example shown in Figure 12–18,
the injured area would be in the lateral wall of the
right ventricle.
This analysis is obviously complex. However, it is
essential that the student go over it again and
again until he or she understands it thoroughly. No
other aspect of electrocardiographic analysis is more
important.
143
Coronary Ischemia as a Cause
of Injury Potential
Insufficient blood flow to the cardiac muscle depresses
the metabolism of the muscle for three reasons: (1)
lack of oxygen, (2) excess accumulation of carbon
dioxide, and (3) lack of sufficient food nutrients.
Consequently, repolarization of the muscle membrane
cannot occur in areas of severe myocardial ischemia.
Often the heart muscle does not die because the blood
flow is sufficient to maintain life of the muscle even
though it is not sufficient to cause repolarization of the
membranes. As long as this state exists, an injury
potential continues to flow during the diastolic portion
(the T-P portion) of each heart cycle.
Extreme ischemia of the cardiac muscle occurs after
coronary occlusion, and a strong current of injury
flows from the infarcted area of the ventricles during
the T-P interval between heartbeats, as shown in
Figures 12–19 and 12–20. Therefore, one of the most
important diagnostic features of electrocardiograms
recorded after acute coronary thrombosis is the
current of injury.
Acute Anterior Wall Infarction. Figure 12–19 shows the
electrocardiogram in the three standard bipolar limb
I
+
-
0
0
“J” point
“J” point
III
0
+
-
0
I
II
III
-
III
-
II-
III
+I
+I
+
III
+
III
V2
Figure 12–18
Figure 12–19
J point as the zero reference potential of the electrocardiograms
for leads I and II. Also, the method for plotting the axis of the injury
potential is shown by the lowermost panel.
Current of injury in acute anterior wall infarction. Note the intense
injury potential in lead V2.
144
Unit III
leads and in one chest lead (lead V2 ) recorded from a
patient with acute anterior wall cardiac infarction. The
most important diagnostic feature of this electrocardiogram is the intense injury potential in chest lead V2.
If one draws a zero horizontal potential line through
the J point of this electrocardiogram, a strong negative
injury potential during the T-P interval is found, which
means that the chest electrode over the front of the
heart is in an area of strongly negative potential. In
other words, the negative end of the injury potential
vector in this heart is against the anterior chest wall.
This means that the current of injury is emanating
from the anterior wall of the ventricles, which diagnoses this condition as anterior wall infarction.
Analyzing the injury potentials in leads I and III,
one finds a negative potential in lead I and a positive
potential in lead III. This means that the resultant
vector of the injury potential in the heart is about +150
degrees, with the negative end pointing toward the left
ventricle and the positive end pointing toward the
right ventricle. Thus, in this particular electrocardiogram, the current of injury is coming mainly from the
left ventricle as well as from the anterior wall of the
heart. Therefore, one would conclude that this anterior
wall infarction almost certainly is caused by thrombosis of the anterior descending branch of the left coronary artery.
Posterior Wall Infarction. Figure 12–20 shows the three
standard bipolar limb leads and one chest lead (lead
V2) from a patient with posterior wall infarction. The
major diagnostic feature of this electrocardiogram is
The Heart
also in the chest lead. If a zero potential reference line
is drawn through the J point of this lead, it is readily
apparent that during the T-P interval, the potential of
the current of injury is positive. This means that the
positive end of the vector is in the direction of the
anterior chest wall, and the negative end (injured end
of the vector) points away from the chest wall. In other
words, the current of injury is coming from the back
of the heart opposite to the anterior chest wall, which
is the reason this type of electrocardiogram is the basis
for diagnosing posterior wall infarction.
If one analyzes the injury potentials from leads II
and III of Figure 12–20, it is readily apparent that the
injury potential is negative in both leads. By vectorial
analysis, as shown in the figure, one finds that the
resultant vector of the injury potential is about -95
degrees, with the negative end pointing downward and
the positive end pointing upward. Thus, because the
infarct, as indicated by the chest lead, is on the posterior wall of the heart and, as indicated by the injury
potentials in leads II and III, is in the apical portion of
the heart, one would suspect that this infarct is near
the apex on the posterior wall of the left ventricle.
Infarction in Other Parts of the Heart. By the same proce-
dures demonstrated in the preceding discussions of
anterior and posterior wall infarctions, it is possible to
determine the locus of any infarcted area emitting a
current of injury, regardless of which part of the heart
is involved. In making such vectorial analyses, it must
be remembered that the positive end of the injury
potential vector points toward the normal cardiac
muscle, and the negative end points toward the injured
portion of the heart that is emitting the current of injury.
Recovery from Acute Coronary Thrombosis. Figure 12–21
shows a V3 chest lead from a patient with acute posterior wall infarction, demonstrating changes in the
electrocardiogram from the day of the attack to 1
I
II
II
-
III
V2
III
-
Same day
+
III
1 week
2 weeks
1 year
+
II
Figure 12–21
Figure 12–20
Injury potential in acute posterior wall, apical infarction.
Recovery of the myocardium after moderate posterior wall infarction, demonstrating disappearance of the injury potential that is
present on the first day after the infarction and still slightly present
at 1 week.
Chapter 12
Electrocardiographic Interpretation of Cardiac Muscle and Coronary Blood Flow Abnormalities
week later, 3 weeks later, and finally 1 year later.
From this electrocardiogram, one can see that the
injury potential is strong immediately after the
acute attack (T-P segment displaced positively from
the S-T segment). However, after about 1 week, the
injury potential has diminished considerably, and after
3 weeks, it is gone. After that, the electrocardiogram
does not change greatly during the next year. This
is the usual recovery pattern after acute cardiac
infarction of moderate degree, showing that the new
collateral coronary blood flow develops enough
to re-establish appropriate nutrition to most of the
infarcted area.
Conversely, in some patients with coronary infarction, the infarcted area never redevelops adequate
coronary blood supply. Often, some of the heart
muscle dies, but if the muscle does not die, it will continue to show an injury potential as long as the
ischemia exists, particularly during bouts of exercise
when the heart is overloaded.
Old Recovered Myocardial Infarction. Figure 12–22 shows
leads I and III after anterior infarction and leads I and
III after posterior infarction about 1 year after the
acute heart attack. The records show what might be
called the “ideal” configurations of the QRS complex
in these types of recovered myocardial infarction.
Usually a Q wave has developed at the beginning of
the QRS complex in lead I in anterior infarction
because of loss of muscle mass in the anterior wall of
the left ventricle, but in posterior infarction, a Q wave
has developed at the beginning of the QRS complex
in lead III because of loss of muscle in the posterior
apical part of the ventricle.
These configurations are certainly not found in all
cases of old cardiac infarction. Local loss of muscle and
local points of cardiac signal conduction block can
cause very bizarre QRS patterns (especially prominent Q waves, for instance), decreased voltage, and
QRS prolongation.
Anterior
Posterior
Q
145
Current of Injury in Angina Pectoris. “Angina pectoris”
means pain from the heart felt in the pectoral regions
of the upper chest. This pain usually also radiates into
the left neck area and down the left arm. The pain typically is caused by moderate ischemia of the heart.
Usually, no pain is felt as long as the person is quiet,
but as soon as he or she overworks the heart, the pain
appears.
An injury potential sometimes appears in the electrocardiogram during an attack of severe angina pectoris, because the coronary insufficiency becomes great
enough to prevent adequate repolarization of some
areas of the heart during diastole.
Abnormalities in the T Wave
Earlier in the chapter, it was pointed out that the T
wave is normally positive in all the standard bipolar
limb leads and that this is caused by repolarization of
the apex and outer surfaces of the ventricles ahead
of the intraventricular surfaces. That is, the T wave
becomes abnormal when the normal sequence of
repolarization does not occur. Several factors can
change this sequence of repolarization.
Effect of Slow Conduction of the
Depolarization Wave on the
Characteristics of the T Wave
Referring back to Figure 12–14, note that the QRS
complex is considerably prolonged. The reason for this
prolongation is delayed conduction in the left ventricle
resulting from left bundle branch block. This causes
the left ventricle to become depolarized about 0.08
second after depolarization of the right ventricle,
which gives a strong mean QRS vector to the left.
However, the refractory periods of the right and left
ventricular muscle masses are not greatly different
from each other. Therefore, the right ventricle begins
to repolarize long before the left ventricle; this causes
strong positivity in the right ventricle and negativity in
the left ventricle at the time that the T wave is developing. In other words, the mean axis of the T wave is
now deviated to the right, which is opposite the mean
electrical axis of the QRS complex in the same electrocardiogram. Thus, when conduction of the depolarization impulse through the ventricles is greatly
delayed, the T wave is almost always of opposite polarity to that of the QRS complex.
Q
I
III
I
III
Shortened Depolarization in Portions
of the Ventricular Muscle as a Cause
of T Wave Abnormalities
Figure 12–22
Electrocardiograms of anterior and posterior wall infarctions that
occurred about 1 year previously, showing a Q wave in lead I in
anterior wall infarction and a Q wave in lead III in posterior wall
infarction.
If the base of the ventricles should exhibit an abnormally short period of depolarization, that is, a shortened action potential, repolarization of the ventricles
would not begin at the apex as it normally does.
146
Unit III
T
T
T
The Heart
T
T
T
Figure 12–23
Inverted T wave resulting from mild ischemia at the apex of the
ventricles.
Figure 12–24
Instead, the base of the ventricles would repolarize
ahead of the apex, and the vector of repolarization
would point from the apex toward the base of the
heart, opposite to the standard vector of repolarization. Consequently, the T wave in all three standard
leads would be negative rather than the usual positive.
Thus, the simple fact that the base of the ventricles has
a shortened period of depolarization is sufficient to
cause marked changes in the T wave, even to the
extent of changing the entire T wave polarity, as shown
in Figure 12–23.
Mild ischemia is by far the most common cause of
shortening of depolarization of cardiac muscle,
because this increases current flow through the potassium channels. When the ischemia occurs in only one
area of the heart, the depolarization period of this area
decreases out of proportion to that in other portions.
As a result, definite changes in the T wave can take
place. The ischemia might result from chronic, progressive coronary occlusion; acute coronary occlusion;
or relative coronary insufficiency that occurs during
exercise.
One means for detecting mild coronary insufficiency
is to have the patient exercise and to record the electrocardiogram, noting whether changes occur in the T
waves. The changes in the T waves need not be specific, because any change in the T wave in any lead—
Biphasic T wave caused by digitalis toxicity.
inversion, for instance, or a biphasic wave—is often
evidence enough that some portion of the ventricular
muscle has a period of depolarization out of proportion to the rest of the heart, caused by mild to moderate coronary insufficiency.
Effect of Digitalis on the T Wave. As discussed in Chapter
22, digitalis is a drug that can be used during coronary
insufficiency to increase the strength of cardiac muscle
contraction. But when overdosages of digitalis are
given, depolarization duration in one part of the ventricles may be increased out of proportion to that of
other parts. As a result, nonspecific changes, such as T
wave inversion or biphasic T waves, may occur in one
or more of the electrocardiographic leads. A biphasic
T wave caused by excessive administration of digitalis
is shown in Figure 12–24. Therefore, changes in the T
wave during digitalis administration are often the earliest signs of digitalis toxicity.
References
See references for Chapter 13.
C
H
A
P
T
E
R
1
Cardiac Arrhythmias and
Their Electrocardiographic
Interpretation
Some of the most distressing types of heart malfunction
occur not as a result of abnormal heart muscle but
because of abnormal rhythm of the heart. For instance,
sometimes the beat of the atria is not coordinated with
the beat of the ventricles, so that the atria no longer
function as primer pumps for the ventricles.
The purpose of this chapter is to discuss the
physiology of common cardiac arrhythmias and their
effects on heart pumping, as well as their diagnosis by
electrocardiography.The causes of the cardiac arrhythmias are usually one or a combination of the following abnormalities in the rhythmicity-conduction system of the
heart:
1. Abnormal rhythmicity of the pacemaker
2. Shift of the pacemaker from the sinus node to another place in the heart
3. Blocks at different points in the spread of the impulse through the heart
4. Abnormal pathways of impulse transmission through the heart
5. Spontaneous generation of spurious impulses in almost any part of the heart
Abnormal Sinus Rhythms
Tachycardia
The term “tachycardia” means fast heart rate, usually defined in an adult person as
faster than 100 beats per minute. An electrocardiogram recorded from a patient
with tachycardia is shown in Figure 13–1. This electrocardiogram is normal except
that the heart rate, as determined from the time intervals between QRS complexes,
is about 150 per minute instead of the normal 72 per minute.
The general causes of tachycardia include increased body temperature, stimulation
of the heart by the sympathetic nerves, or toxic conditions of the heart.
The heart rate increases about 10 beats per minute for each degree Fahrenheit
(18 beats per degree Celsius) increase in body temperature, up to a body temperature of about 105°F (40.5°C); beyond this, the heart rate may decrease because of
progressive debility of the heart muscle as a result of the fever. Fever causes tachycardia because increased temperature increases the rate of metabolism of the sinus
node, which in turn directly increases its excitability and rate of rhythm.
Many factors can cause the sympathetic nervous system to excite the heart, as we
discuss at multiple points in this text. For instance, when a patient loses blood and
passes into a state of shock or semishock, sympathetic reflex stimulation of the heart
often increases the heart rate to 150 to 180 beats per minute.
Simple weakening of the myocardium usually increases the heart rate because
the weakened heart does not pump blood into the arterial tree to a normal extent,
and this elicits sympathetic reflexes to increase the heart rate.
Bradycardia
The term “bradycardia” means a slow heart rate, usually defined as fewer than 60
beats per minute. Bradycardia is shown by the electrocardiogram in Figure 13–2.
Bradycardia in Athletes. The athlete’s heart is larger and considerably stronger than
that of a normal person, which allows the athlete’s heart to pump a large stroke
volume output per beat even during periods of rest. When the athlete is at rest,
excessive quantities of blood pumped into the arterial tree with each beat initiate
feedback circulatory reflexes or other effects to cause bradycardia.
147
3
148
Unit III
The Heart
SA block
Figure 13–1
Sinus tachycardia (lead I).
Figure 13–2
Heart rate
Sinus bradycardia (lead III).
60
70
80
100
120
Figure 13–3
Sinus arrhythmia as recorded by a cardiotachometer. To the left is
the record when the subject was breathing normally; to the right,
when breathing deeply.
Vagal Stimulation as a Cause of Bradycardia. Any circulatory
reflex that stimulates the vagus nerves causes release of
acetylcholine at the vagal endings in the heart, thus
giving a parasympathetic effect. Perhaps the most striking example of this occurs in patients with carotid sinus
syndrome. In these patients, the pressure receptors
(baroreceptors) in the carotid sinus region of the carotid
artery walls are excessively sensitive. Therefore, even
mild external pressure on the neck elicits a strong
baroreceptor reflex, causing intense vagal-acetylcholine
effects on the heart, including extreme bradycardia.
Indeed, sometimes this reflex is so powerful that it actually stops the heart for 5 to 10 seconds.
Sinus Arrhythmia
Figure 13–3 shows a cardiotachometer recording of the
heart rate, at first during normal and then (in the second
half of the record) during deep respiration. A cardiotachometer is an instrument that records by the height of
successive spikes the duration of the interval between
the successive QRS complexes in the electrocardiogram. Note from this record that the heart rate
increased and decreased no more than 5 per cent during
Figure 13–4
Sinoatrial nodal block, with A-V nodal rhythm during the block
period (lead III).
quiet respiration (left half of the record). Then, during
deep respiration, the heart rate increased and decreased
with each respiratory cycle by as much as 30 per cent.
Sinus arrhythmia can result from any one of many circulatory conditions that alter the strengths of the sympathetic and parasympathetic nerve signals to the heart
sinus node. In the “respiratory” type of sinus arrhythmia, as shown in Figure 13–3, this results mainly from
“spillover” of signals from the medullary respiratory
center into the adjacent vasomotor center during inspiratory and expiratory cycles of respiration. The spillover
signals cause alternate increase and decrease in the
number of impulses transmitted through the sympathetic and vagus nerves to the heart.
Abnormal Rhythms That
Result from Block of Heart
Signals Within the
Intracardiac Conduction
Pathways
Sinoatrial Block
In rare instances, the impulse from the sinus node is
blocked before it enters the atrial muscle. This phenomenon is demonstrated in Figure 13–4, which shows
sudden cessation of P waves, with resultant standstill of
the atria. However, the ventricles pick up a new rhythm,
the impulse usually originating spontaneously in the
atrioventricular (A-V) node, so that the rate of the ventricular QRS-T complex is slowed but not otherwise
altered.
Atrioventricular Block
The only means by which impulses ordinarily can pass
from the atria into the ventricles is through the A-V
bundle, also known as the bundle of His. Conditions that
can either decrease the rate of impulse conduction in
this bundle or block the impulse entirely are as follows:
1. Ischemia of the A-V node or A-V bundle fibers
often delays or blocks conduction from the atria to
the ventricles. Coronary insufficiency can cause
ischemia of the A-V node and bundle in the same
way that it can cause ischemia of the myocardium.
2. Compression of the A-V bundle by scar tissue or by
calcified portions of the heart can depress or block
conduction from the atria to the ventricles.
3. Inflammation of the A-V node or A-V bundle
can depress conductivity from the atria to the
ventricles. Inflammation results frequently from
Chapter 13
149
Cardiac Arrhythmias and Their Electrocardiographic Interpretation
Dropped beat
P
P
P
P
P
P
P
P
P
P
P
Figure 13–5
Prolonged P-R interval caused by first degree A-V heart block
(lead II).
different types of myocarditis, caused, for example,
by diphtheria or rheumatic fever.
4. Extreme stimulation of the heart by the vagus nerves
in rare instances blocks impulse conduction
through the A-V node. Such vagal excitation
occasionally results from strong stimulation of the
baroreceptors in people with carotid sinus
syndrome, discussed earlier in relation to
bradycardia.
Figure 13–6
Second degree A-V block, showing occasional failure of the ventricles to receive the excitatory signals (lead V3).
P
P
P
P
P
P
P
P
P
P
Incomplete Atrioventricular
Heart Block
Prolonged P-R (or P-Q) Interval—First Degree Block. The usual
lapse of time between beginning of the P wave and
beginning of the QRS complex is about 0.16 second
when the heart is beating at a normal rate. This socalled P-R interval usually decreases in length with
faster heartbeat and increases with slower heartbeat. In
general, when the P-R interval increases to greater than
0.20 second, the P-R interval is said to be prolonged, and
the patient is said to have first degree incomplete heart
block.
Figure 13–5 shows an electrocardiogram with prolonged P-R interval; the interval in this instance is about
0.30 second instead of the normal 0.20 or less. Thus, first
degree block is defined as a delay of conduction from
the atria to the ventricles but not actual blockage of conduction. The P-R interval seldom increases above 0.35
to 0.45 second because, by that time, conduction through
the A-V bundle is depressed so much that conduction
stops entirely. One means for determining the severity
of some heart diseases—acute rheumatic heart disease,
for instance—is to measure the P-R interval.
Second Degree Block. When conduction through the A-V
bundle is slowed enough to increase the P-R interval to
0.25 to 0.45 second, the action potential sometimes is
strong enough to pass through the bundle into the ventricles and sometimes is not strong enough. In this
instance, there will be an atrial P wave but no QRS-T
wave, and it is said that there are “dropped beats” of the
ventricles. This condition is called second degree heart
block.
Figure 13–6 shows P-R intervals of 0.30 second, as
well as one dropped ventricular beat as a result of
failure of conduction from the atria to the ventricles.
At times, every other beat of the ventricles is
dropped, so that a “2:1 rhythm” develops, with the atria
beating twice for every single beat of the ventricles. At
other times, rhythms of 3:2 or 3:1 also develop.
Complete A-V Block (Third Degree Block). When the condi-
tion causing poor conduction in the A-V node or A-V
bundle becomes severe, complete block of the impulse
Figure 13–7
Complete A-V block (lead II).
from the atria into the ventricles occurs. In this instance,
the ventricles spontaneously establish their own signal,
usually originating in the A-V node or A-V bundle.
Therefore, the P waves become dissociated from the
QRS-T complexes, as shown in Figure 13–7. Note that
the rate of rhythm of the atria in this electrocardiogram
is about 100 beats per minute, whereas the rate of ventricular beat is less than 40 per minute. Furthermore,
there is no relation between the rhythm of the P waves
and that of the QRS-T complexes because the ventricles have “escaped” from control by the atria, and they
are beating at their own natural rate, controlled most
often by rhythmical signals generated in the A-V node
or A-V bundle.
Stokes-Adams Syndrome—Ventricular Escape. In some
patients with A-V block, the total block comes and goes;
that is, impulses are conducted from the atria into the
ventricles for a period of time and then suddenly
impulses are not conducted. The duration of block may
be a few seconds, a few minutes, a few hours, or even
weeks or longer before conduction returns. This condition occurs in hearts with borderline ischemia of the
conductive system.
Each time A-V conduction ceases, the ventricles often
do not start their own beating until after a delay of 5 to
30 seconds. This results from the phenomenon called
overdrive suppression. This means that ventricular
excitability is at first in a suppressed state because the
ventricles have been driven by the atria at a rate greater
than their natural rate of rhythm. However, after a few
seconds, some part of the Purkinje system beyond the
block, usually in the distal part of the A-V node beyond
the blocked point in the node, or in the A-V bundle,
begins discharging rhythmically at a rate of 15 to 40
times per minute and acting as the pacemaker of the
ventricles. This is called ventricular escape.
150
Unit III
Because the brain cannot remain active for more than
4 to 7 seconds without blood supply, most patients faint
a few seconds after complete block occurs because the
heart does not pump any blood for 5 to 30 seconds,
until the ventricles “escape.” After escape, however, the
slowly beating ventricles usually pump enough blood to
allow rapid recovery from the faint and then to sustain
the person. These periodic fainting spells are known as
the Stokes-Adams syndrome.
Occasionally the interval of ventricular standstill at
the onset of complete block is so long that it becomes
detrimental to the patient’s health or even causes death.
Consequently, most of these patients are provided
with an artificial pacemaker, a small battery-operated
electrical stimulator planted beneath the skin, with
electrodes usually connected to the right ventricle. The
pacemaker provides continued rhythmical impulses
that take control of the ventricles.
Incomplete Intraventricular Block—
Electrical Alternans
Most of the same factors that can cause A-V block can
also block impulse conduction in the peripheral ventricular Purkinje system. Figure 13–8 shows the condition known as electrical alternans, which results from
partial intraventricular block every other heartbeat.
This electrocardiogram also shows tachycardia (rapid
heart rate), which is probably the reason the block has
occurred, because when the rate of the heart is rapid, it
may be impossible for some portions of the Purkinje
system to recover from the previous refractory period
quickly enough to respond during every succeeding
heartbeat. Also, many conditions that depress the heart,
such as ischemia, myocarditis, or digitalis toxicity, can
cause incomplete intraventricular block, resulting in
electrical alternans.
Premature Contractions
A premature contraction is a contraction of the heart
before the time that normal contraction would have
been expected. This condition is also called extrasystole,
premature beat, or ectopic beat.
Causes of Premature Contractions. Most premature contractions result from ectopic foci in the heart, which emit
abnormal impulses at odd times during the cardiac
rhythm. Possible causes of ectopic foci are (1) local
The Heart
areas of ischemia; (2) small calcified plaques at different points in the heart, which press against the adjacent
cardiac muscle so that some of the fibers are irritated;
and (3) toxic irritation of the A-V node, Purkinje
system, or myocardium caused by drugs, nicotine, or caffeine. Mechanical initiation of premature contractions
is also frequent during cardiac catheterization; large
numbers of premature contractions often occur when
the catheter enters the right ventricle and presses
against the endocardium.
Premature Atrial Contractions
Figure 13–9 shows a single premature atrial contraction.
The P wave of this beat occurred too soon in the heart
cycle; the P-R interval is shortened, indicating that the
ectopic origin of the beat is in the atria near the A-V
node. Also, the interval between the premature contraction and the next succeeding contraction is slightly
prolonged, which is called a compensatory pause. One
of the reasons for this is that the premature contraction
originated in the atrium some distance from the sinus
node, and the impulse had to travel through a considerable amount of atrial muscle before it discharged the
sinus node. Consequently, the sinus node discharged late
in the premature cycle, and this made the succeeding
sinus node discharge also late in appearing.
Premature atrial contractions occur frequently in
otherwise healthy people. Indeed, they often occur in
athletes whose hearts are in very healthy condition.
Mild toxic conditions resulting from such factors as
smoking, lack of sleep, ingestion of too much coffee,
alcoholism, and use of various drugs can also initiate
such contractions.
Pulse Deficit. When the heart contracts ahead of schedule, the ventricles will not have filled with blood normally, and the stroke volume output during that
contraction is depressed or almost absent. Therefore,
the pulse wave passing to the peripheral arteries after a
premature contraction may be so weak that it cannot be
felt in the radial artery. Thus, a deficit in the number of
radial pulses occurs when compared with the actual
number of contractions of the heart.
A-V Nodal or A-V Bundle Premature
Contractions
Figure 13–10 shows a premature contraction that
originated in the A-V node or in the A-V bundle. The
P wave is missing from the electrocardiographic record
Premature beat
Figure 13–8
Partial intraventricular block—“electrical alternans” (lead III).
Figure 13–9
Atrial premature beat (lead I).
Cardiac Arrhythmias and Their Electrocardiographic Interpretation
Chapter 13
Premature beat
P
T
P
T
P
T
PT
P
T
Figure 13–10
A-V nodal premature contraction (lead III).
II
III
II
–
+
III
III
–
+
II
Figure 13–11
Premature ventricular contractions (PVCs) demonstrated by the
large abnormal QRS-T complexes (leads II and III). Axis of the
premature contractions is plotted in accordance with the principles of vectorial analysis explained in Chapter 12; this shows the
origin of the PVC to be near the base of the ventricles.
of the premature contraction. Instead, the P wave is
superimposed onto the QRS-T complex because the
cardiac impulse traveled backward into the atria at
the same time that it traveled forward into the ventricles; this P wave slightly distorts the QRS-T complex,
but the P wave itself cannot be discerned as such. In
general, A-V nodal premature contractions have
the same significance and causes as atrial premature
contractions.
Premature Ventricular Contractions
The electrocardiogram of Figure 13–11 shows a series
of premature ventricular contractions (PVCs) alternat-
151
ing with normal contractions. PVCs cause specific
effects in the electrocardiogram, as follows:
1. The QRS complex is usually considerably
prolonged. The reason is that the impulse is
conducted mainly through slowly conducting
muscle of the ventricles rather than through the
Purkinje system.
2. The QRS complex has a high voltage for the
following reasons: when the normal impulse passes
through the heart, it passes through both ventricles
nearly simultaneously; consequently, in the normal
heart, the depolarization waves of the two sides
of the heart—mainly of opposite polarity to each
other—partially neutralize each other in the
electrocardiogram. When a PVC occurs, the
impulse almost always travels in only one direction,
so that there is no such neutralization effect,
and one entire side or end of the ventricles is
depolarized ahead of the other; this causes large
electrical potentials, as shown for the PVCs in
Figure 13–11.
3. After almost all PVCs, the T wave has an electrical
potential polarity exactly opposite to that of the
QRS complex, because the slow conduction of the
impulse through the cardiac muscle causes the
muscle fibers that depolarize first also to repolarize
first.
Some PVCs are relatively benign in their effects on
overall pumping by the heart; they can result from such
factors as cigarettes, coffee, lack of sleep, various mild
toxic states, and even emotional irritability. Conversely,
many other PVCs result from stray impulses or reentrant signals that originate around the borders of
infarcted or ischemic areas of the heart. The presence
of such PVCs is not to be taken lightly. Statistics show
that people with significant numbers of PVCs have a
much higher than normal chance of developing spontaneous lethal ventricular fibrillation, presumably initiated by one of the PVCs. This is especially true when
the PVCs occur during the vulnerable period for
causing fibrillation, just at the end of the T wave when
the ventricles are coming out of refractoriness, as
explained later in the chapter.
Vector Analysis of the Origin of an Ectopic Premature Ventricular
Contraction. In Chapter 12, the principles of vectorial
analysis are explained. Applying these principles, one
can determine from the electrocardiogram in Figure
13–11 the point of origin of the PVC as follows: Note
that the potentials of the premature contractions in
leads II and III are both strongly positive. Plotting these
potentials on the axes of leads II and III and solving by
vectorial analysis for the mean QRS vector in the heart,
one finds that the vector of this premature contraction
has its negative end (origin) at the base of the heart and
its positive end toward the apex. Thus, the first portion
of the heart to become depolarized during this premature contraction is near the base of the ventricles, which
therefore is the locus of the ectopic focus.
Paroxysmal Tachycardia
Some abnormalities in different portions of the heart,
including the atria, the Purkinje system, or the ventricles, can occasionally cause rapid rhythmical discharge
of impulses that spread in all directions throughout the
heart. This is believed to be caused most frequently by
152
Unit III
The Heart
Figure 13–12
Atrial paroxysmal tachycardia—onset in middle of record (lead I).
Figure 13–13
Ventricular paroxysmal tachycardia (lead III).
re-entrant circus movement feedback pathways that set
up local repeated self–re-excitation. Because of the
rapid rhythm in the irritable focus, this focus becomes
the pacemaker of the heart.
The term “paroxysmal” means that the heart rate
becomes rapid in paroxysms, with the paroxysm
beginning suddenly and lasting for a few seconds, a
few minutes, a few hours, or much longer. Then the
paroxysm usually ends as suddenly as it began, with
the pacemaker of the heart instantly shifting back to
the sinus node.
Paroxysmal tachycardia often can be stopped by eliciting a vagal reflex. A type of vagal reflex sometimes
elicited for this purpose is to press on the neck in the
regions of the carotid sinuses, which may cause enough
of a vagal reflex to stop the paroxysm. Various drugs
may also be used. Two drugs frequently used are quinidine and lidocaine, either of which depresses the normal
increase in sodium permeability of the cardiac muscle
membrane during generation of the action potential,
thereby often blocking the rhythmical discharge of the
focal point that is causing the paroxysmal attack.
lar paroxysmal tachycardia has the appearance of a
series of ventricular premature beats occurring one
after another without any normal beats interspersed.
Ventricular paroxysmal tachycardia is usually a
serious condition for two reasons. First, this type of
tachycardia usually does not occur unless considerable
ischemic damage is present in the ventricles. Second,
ventricular tachycardia frequently initiates the lethal
condition of ventricular fibrillation because of rapid
repeated stimulation of the ventricular muscle, as we
discuss in the next section.
Sometimes intoxication from the heart treatment
drug digitalis causes irritable foci that lead to ventricular tachycardia. Conversely, quinidine, which increases
the refractory period and threshold for excitation of
cardiac muscle, may be used to block irritable foci
causing ventricular tachycardia.
Ventricular Fibrillation
Atrial Paroxysmal Tachycardia
Figure 13–12 demonstrates in the middle of the record
a sudden increase in the heart rate from about 95 to
about 150 beats per minute. On close study of the electrocardiogram during the rapid heartbeat, an inverted P
wave is seen before each QRS-T complex, and this P
wave is partially superimposed onto the normal T wave
of the preceding beat. This indicates that the origin
of this paroxysmal tachycardia is in the atrium, but
because the P wave is abnormal in shape, the origin is
not near the sinus node.
A-V Nodal Paroxysmal Tachycardia. Paroxysmal tachycardia
often results from an aberrant rhythm that involves the
A-V node. This usually causes almost normal QRS-T
complexes but totally missing or obscured P waves.
Atrial or A-V nodal paroxysmal tachycardia, both of
which are called supraventricular tachycardias, usually
occurs in young, otherwise healthy people, and they generally grow out of the predisposition to tachycardia
after adolescence. In general, supraventricular tachycardia frightens a person tremendously and may cause
weakness during the paroxysm, but only seldom does
permanent harm come from the attack.
Ventricular Paroxysmal Tachycardia
Figure 13–13 shows a typical short paroxysm of ventricular tachycardia. The electrocardiogram of ventricu-
The most serious of all cardiac arrhythmias is ventricular fibrillation, which, if not stopped within 1 to 3
minutes, is almost invariably fatal. Ventricular fibrillation results from cardiac impulses that have gone
berserk within the ventricular muscle mass, stimulating
first one portion of the ventricular muscle, then another
portion, then another, and eventually feeding back onto
itself to re-excite the same ventricular muscle over and
over—never stopping. When this happens, many small
portions of the ventricular muscle will be contracting at
the same time, while equally as many other portions will
be relaxing. Thus, there is never a coordinate contraction of all the ventricular muscle at once, which is
required for a pumping cycle of the heart. Despite
massive movement of stimulatory signals throughout
the ventricles, the ventricular chambers neither enlarge
nor contract but remain in an indeterminate stage of
partial contraction, pumping either no blood or negligible amounts. Therefore, after fibrillation begins, unconsciousness occurs within 4 to 5 seconds for lack of blood
flow to the brain, and irretrievable death of tissues
begins to occur throughout the body within a few
minutes.
Multiple factors can spark the beginning of ventricular fibrillation—a person may have a normal heartbeat
one moment, but 1 second later, the ventricles are in fibrillation. Especially likely to initiate fibrillation are (1)
sudden electrical shock of the heart, or (2) ischemia of
the heart muscle, of its specialized conducting system,
or both.
Chapter 13
Cardiac Arrhythmias and Their Electrocardiographic Interpretation
Phenomenon of Re-entry—“Circus
Movements” as the Basis for
Ventricular Fibrillation
153
the refractory state, and the impulse can continue
around the circle again and again.
Third, the refractory period of the muscle might
become greatly shortened. In this case, the impulse could
also continue around and around the circle.
All these conditions occur in different pathological
states of the human heart, as follows: (1) A long pathway
typically occurs in dilated hearts. (2) Decreased rate of
conduction frequently results from (a) blockage of the
Purkinje system, (b) ischemia of the muscle, (c) high
blood potassium levels, or (d) many other factors. (3)
A shortened refractory period commonly occurs in
response to various drugs, such as epinephrine, or after
repetitive electrical stimulation. Thus, in many cardiac
disturbances, re-entry can cause abnormal patterns of
cardiac contraction or abnormal cardiac rhythms that
ignore the pace-setting effects of the sinus node.
When the normal cardiac impulse in the normal heart
has traveled through the extent of the ventricles, it has
no place to go because all the ventricular muscle is
refractory and cannot conduct the impulse farther.
Therefore, that impulse dies, and the heart awaits a new
action potential to begin in the atrial sinus node.
Under some circumstances, however, this normal
sequence of events does not occur. Therefore, let us
explain more fully the background conditions that can
initiate re-entry and lead to “circus movements,” which
in turn cause ventricular fibrillation.
Figure 13–14 shows several small cardiac muscle
strips cut in the form of circles. If such a strip is stimulated at the 12 o’clock position so that the impulse travels
in only one direction, the impulse spreads progressively
around the circle until it returns to the 12 o’clock position. If the originally stimulated muscle fibers are still in
a refractory state, the impulse then dies out because
refractory muscle cannot transmit a second impulse. But
there are three different conditions that can cause this
impulse to continue to travel around the circle, that is,
to cause “re-entry” of the impulse into muscle that
has already been excited. This is called a “circus
movement.”
First, if the pathway around the circle is too long, by
the time the impulse returns to the 12 o’clock position,
the originally stimulated muscle will no longer be
refractory and the impulse will continue around the
circle again and again.
Second, if the length of the pathway remains constant
but the velocity of conduction becomes decreased
enough, an increased interval of time will elapse before
the impulse returns to the 12 o’clock position. By this
time, the originally stimulated muscle might be out of
Chain Reaction Mechanism
of Fibrillation
In ventricular fibrillation, one sees many separate and
small contractile waves spreading at the same time in
different directions over the cardiac muscle. The reentrant impulses in fibrillation are not simply a single
impulse moving in a circle, as shown in Figure 13–14.
Instead, they have degenerated into a series of multiple
wave fronts that have the appearance of a “chain reaction.” One of the best ways to explain this process in
fibrillation is to describe the initiation of fibrillation by
electric shock caused by 60-cycle alternating electric
current.
Fibrillation Caused by 60-Cycle Alternating Current. At a
central point in the ventricles of heart A in Figure 13–15,
a 60-cycle electrical stimulus is applied through a
stimulating electrode. The first cycle of the electrical
stimulus causes a depolarization wave to spread in all
directions, leaving all the muscle beneath the electrode
in a refractory state. After about 0.25 second, part of this
muscle begins to come out of the refractory state. Some
portions come out of refractoriness before other
NORMAL PATHWAY
Stimulus
point
Dividing
impulses
Absolutely
refractory
Absolutely
refractory
Relatively
refractory
LONG PATHWAY
Blocked
impulse
A
Figure 13–14
Circus movement, showing annihilation of the impulse in the short
pathway and continued propagation of the impulse in the long
pathway.
B
Figure 13–15
A, Initiation of fibrillation in a heart when patches of refractory musculature are present. B, Continued propagation of fibrillatory
impulses in the fibrillating ventricle.
154
Unit III
portions. This state of events is depicted in heart A by
many lighter patches, which represent excitable cardiac
muscle, and dark patches, which represent still refractory muscle. Now, continuing 60-cycle stimuli from the
electrode can cause impulses to travel only in certain
directions through the heart but not in all directions.
Thus, in heart A, certain impulses travel for short distances, until they reach refractory areas of the heart, and
then are blocked. But other impulses pass between the
refractory areas and continue to travel in the excitable
areas. Then, several events transpire in rapid succession,
all occurring simultaneously and eventuating in a state
of fibrillation.
First, block of the impulses in some directions but successful transmission in other directions creates one of
the necessary conditions for a re-entrant signal to
develop—that is, transmission of some of the depolarization waves around the heart in only some directions
but not other directions.
Second, the rapid stimulation of the heart causes two
changes in the cardiac muscle itself, both of which predispose to circus movement: (1) The velocity of conduction through the heart muscle decreases, which allows a
longer time interval for the impulses to travel around
the heart. (2) The refractory period of the muscle is
shortened, allowing re-entry of the impulse into previously excited heart muscle within a much shorter time
than normally.
Third, one of the most important features of fibrillation is the division of impulses, as demonstrated in heart
A. When a depolarization wave reaches a refractory
area in the heart, it travels to both sides around the
refractory area. Thus, a single impulse becomes two
impulses. Then, when each of these reaches another
refractory area, it, too, divides to form two more
impulses. In this way, many new wave fronts are continually being formed in the heart by progressive chain
reactions until, finally, there are many small depolarization waves traveling in many directions at the same
time. Furthermore, this irregular pattern of impulse
travel causes many circuitous routes for the impulses to
travel, greatly lengthening the conductive pathway, which
is one of the conditions that sustains the fibrillation. It
also results in a continual irregular pattern of patchy
refractory areas in the heart.
One can readily see when a vicious circle has been
initiated: More and more impulses are formed; these
cause more and more patches of refractory muscle, and
the refractory patches cause more and more division of
the impulses.Therefore, any time a single area of cardiac
muscle comes out of refractoriness, an impulse is close
at hand to re-enter the area.
Heart B in Figure 13–15 demonstrates the final state
that develops in fibrillation. Here one can see many
impulses traveling in all directions, some dividing and
increasing the number of impulses, whereas others are
blocked by refractory areas. In fact, a single electric
shock during this vulnerable period frequently can lead
to an odd pattern of impulses spreading multidirectionally around refractory areas of muscle, which will lead
to fibrillation.
Electrocardiogram in Ventricular
Fibrillation
In ventricular fibrillation, the electrocardiogram is
bizarre (Figure 13–16) and ordinarily shows no ten-
The Heart
Figure 13–16
Ventricular fibrillation (lead II).
dency toward a regular rhythm of any type. During the
first few seconds of ventricular fibrillation, relatively
large masses of muscle contract simultaneously, and this
causes coarse, irregular waves in the electrocardiogram.
After another few seconds, the coarse contractions of
the ventricles disappear, and the electrocardiogram
changes into a new pattern of low-voltage, very irregular waves. Thus, no repetitive electrocardiographic pattern can be ascribed to ventricular fibrillation. Instead,
the ventricular muscle contracts at as many as 30 to 50
small patches of muscle at a time, and electrocardiographic potentials change constantly and spasmodically
because the electrical currents in the heart flow first in
one direction and then in another and seldom repeat
any specific cycle.
The voltages of the waves in the electrocardiogram
in ventricular fibrillation are usually about 0.5 millivolt
when ventricular fibrillation first begins, but they
decay rapidly so that after 20 to 30 seconds, they
are usually only 0.2 to 0.3 millivolt. Minute voltages of
0.1 millivolt or less may be recorded for 10 minutes or
longer after ventricular fibrillation begins. As already
pointed out, because no pumping of blood occurs
during ventricular fibrillation, this state is lethal unless
stopped by some heroic therapy, such as immediate
electroshock through the heart, as explained in the next
section.
Electroshock Defibrillation
of the Ventricles
Although a moderate alternating-current voltage
applied directly to the ventricles almost invariably
throws the ventricles into fibrillation, a strong highvoltage alternating electrical current passed through the
ventricles for a fraction of a second can stop fibrillation
by throwing all the ventricular muscle into refractoriness simultaneously. This is accomplished by passing
intense current through large electrodes placed on two
sides of the heart. The current penetrates most of the
fibers of the ventricles at the same time, thus stimulating essentially all parts of the ventricles simultaneously
and causing them all to become refractory. All action
potentials stop, and the heart remains quiescent for 3 to
5 seconds, after which it begins to beat again, usually
with the sinus node or some other part of the heart
becoming the pacemaker. However, the same re-entrant
focus that had originally thrown the ventricles into fibrillation often is still present, in which case fibrillation
may begin again immediately.
When electrodes are applied directly to the two sides
of the heart, fibrillation can usually be stopped using 110
volts of 60-cycle alternating current applied for 0.1
Chapter 13
Cardiac Arrhythmias and Their Electrocardiographic Interpretation
second or 1000 volts of direct current applied for a few
thousandths of a second. When applied through two
electrodes on the chest wall, as shown in Figure 13–17,
the usual procedure is to charge a large electrical
capacitor up to several thousand volts and then to cause
the capacitor to discharge for a few thousandths of a
second through the electrodes and through the heart.
In our laboratory, the heart of a single anesthetized
dog was defibrillated 130 times through the chest wall,
and the animal lived thereafter in perfectly normal
condition.
Hand Pumping of the Heart
(Cardiopulmonary Resuscitation) as
an Aid to Defibrillation
Unless defibrillated within 1 minute after fibrillation
begins, the heart is usually too weak to be revived by
defibrillation because of the lack of nutrition from coronary blood flow. However, it is still possible to revive the
heart by preliminarily pumping the heart by hand
(intermittent hand squeezing) and then defibrillating
the heart later. In this way, small quantities of blood are
delivered into the aorta and a renewed coronary blood
supply develops. Then, after a few minutes of hand
pumping, electrical defibrillation often becomes possible. Indeed, fibrillating hearts have been pumped by
hand for as long as 90 minutes followed by successful
defibrillation.
A technique for pumping the heart without opening
the chest consists of intermittent thrusts of pressure on
the chest wall along with artificial respiration. This, plus
defibrillation, is called cardiopulmonary resuscitation,
or CPR.
Lack of blood flow to the brain for more than 5 to
8 minutes usually causes permanent mental impairment or even destruction of brain tissue. Even if the
heart is revived, the person may die from the effects of
brain damage or may live with permanent mental
impairment.
Several thousand volts
for a few milliseconds
155
Atrial Fibrillation
Remember that except for the conducting pathway
through the A-V bundle, the atrial muscle mass is
separated from the ventricular muscle mass by fibrous
tissue. Therefore, ventricular fibrillation often occurs
without atrial fibrillation. Likewise, fibrillation often
occurs in the atria without ventricular fibrillation
(shown to the right in Figure 13–19).
The mechanism of atrial fibrillation is identical to that
of ventricular fibrillation, except that the process occurs
only in the atrial muscle mass instead of the ventricular
mass. A frequent cause of atrial fibrillation is atrial
enlargement resulting from heart valve lesions that
prevent the atria from emptying adequately into the
ventricles, or from ventricular failure with excess
damming of blood in the atria. The dilated atrial walls
provide ideal conditions of a long conductive pathway
as well as slow conduction, both of which predispose to
atrial fibrillation.
Pumping Characteristics of the Atria During Atrial Fibrillation.
For the same reasons that the ventricles will not pump
blood during ventricular fibrillation, neither do the atria
pump blood in atrial fibrillation. Therefore, the atria
become useless as primer pumps for the ventricles. Even
so, blood flows passively through the atria into the
ventricles, and the efficiency of ventricular pumping is
decreased only 20 to 30 per cent. Therefore, in contrast
to the lethality of ventricular fibrillation, a person can
live for months or even years with atrial fibrillation,
although at reduced efficiency of overall heart pumping.
Electrocardiogram in Atrial Fibrillation. Figure 13–18 shows
the electrocardiogram during atrial fibrillation. Numerous small depolarization waves spread in all directions
through the atria during atrial fibrillation. Because the
waves are weak and many of them are of opposite
polarity at any given time, they usually almost completely electrically neutralize one another. Therefore, in
the electrocardiogram, one can see either no P waves
from the atria or only a fine, high-frequency, very low
voltage wavy record. Conversely, the QRS-T complexes
are normal unless there is some pathology of the ventricles, but their timing is irregular, as explained next.
Irregularity of Ventricular Rhythm During Atrial Fibrillation.
When the atria are fibrillating, impulses arrive from the
atrial muscle at the A-V node rapidly but also irregularly. Because the A-V node will not pass a second
impulse for about 0.35 second after a previous one, at
least 0.35 second must elapse between one ventricular
contraction and the next. Then an additional but
Handle for
application
of pressure
Electrode
Figure 13–17
Application of electrical current to the chest to stop ventricular
fibrillation.
Figure 13–18
Atrial fibrillation (lead I). The waves that can be seen are ventricular QRS and T waves.
156
Unit III
The Heart
Figure 13–20
Atrial flutter—2:1 and 3:1 atrial to ventricle rhythm (lead I).
Atrial flutter
Atrial fibrillation
Figure 13–19
Pathways of impulses in atrial flutter and atrial fibrillation.
variable interval of 0 to 0.6 second occurs before one of
the irregular atrial fibrillatory impulses happens to
arrive at the A-V node. Thus, the interval between successive ventricular contractions varies from a minimum
of about 0.35 second to a maximum of about 0.95
second, causing a very irregular heartbeat. In fact, this
irregularity, demonstrated by the variable spacing of the
heartbeats in the electrocardiogram of Figure 13–18, is
one of the clinical findings used to diagnose the condition. Also, because of the rapid rate of the fibrillatory
impulses in the atria, the ventricle is driven at a fast
heart rate, usually between 125 and 150 beats per
minute.
Electroshock Treatment of Atrial Fibrillation. In the same
manner that ventricular fibrillation can be converted
back to a normal rhythm by electroshock, so too can
atrial fibrillation be converted by electroshock. The procedure is essentially the same as for ventricular fibrillation conversion—passage of a single strong electric
shock through the heart, which throws the entire heart
into refractoriness for a few seconds; a normal rhythm
often follows if the heart is capable of this.
Atrial Flutter
Atrial flutter is another condition caused by a circus
movement in the atria. It is different from atrial fibrillation, in that the electrical signal travels as a single
large wave always in one direction around and around
the atrial muscle mass, as shown to the left in Figure
13–19. Atrial flutter causes a rapid rate of contraction of
the atria, usually between 200 and 350 beats per minute.
However, because one side of the atria is contracting
while the other side is relaxing, the amount of blood
pumped by the atria is slight. Furthermore, the signals
reach the A-V node too rapidly for all of them to be
passed into the ventricles, because the refractory
periods of the A-V node and A-V bundle are too long
to pass more than a fraction of the atrial signals. Therefore, there are usually two to three beats of the atria for
every single beat of the ventricles.
Figure 13–20 shows a typical electrocardiogram in
atrial flutter. The P waves are strong because of contraction of semicoordinate masses of muscle. However,
note in the record that a QRS-T complex follows an
atrial P wave only once for every two to three beats of
the atria, giving a 2:1 or 3:1 rhythm.
Cardiac Arrest
A final serious abnormality of the cardiac rhythmicityconduction system is cardiac arrest. This results from
cessation of all electrical control signals in the heart.
That is, no spontaneous rhythm remains.
Cardiac arrest is especially likely to occur during deep
anesthesia, when many patients develop severe hypoxia
because of inadequate respiration. The hypoxia prevents the muscle fibers and conductive fibers from maintaining normal electrolyte concentration differentials
across their membranes, and their excitability may be so
affected that the automatic rhythmicity disappears.
In most instances of cardiac arrest from anesthesia,
prolonged cardiopulmonary resuscitation (many
minutes or even hours) is quite successful in reestablishing a normal heart rhythm. In some patients,
severe myocardial disease can cause permanent or
semipermanent cardiac arrest, which can cause death.
To treat the condition, rhythmical electrical impulses
from an implanted electronic cardiac pacemaker have
been used successfully to keep patients alive for months
to years.
References
Al-Khatib SM, LaPointe NM, Kramer JM, Califf RM: What
clinicians should know about the QT interval. JAMA
289:2120, 2003.
Armoundas AA, Tomaselli GF, Esperer HD: Pathophysiological basis and clinical application of T-wave alternans.
J Am Coll Cardiol 40:207, 2002.
Bigi R, Cortigiani L, Desideri A: Exercise electrocardiography after acute coronary syndromes: still the first testing
modality? Clin Cardiol 8:390, 2003.
Cheitlin MD, Armstrong WF, Aurigemma GP, et al:
ACC/AHA/ASE 2003 Guideline update for the clinical
application of echocardiography: summary article. A
report of the American College of Cardiology/American
Heart Association Task Force on Practice Guidelines
(ACC/AHA/ASE Committee to Update the 1997 Guidelines for the Clinical Application of Echocardiography).
J Am Soc Echocardiogr 16:1091, 2003.
Cohn PF, Fox KM, Daly C: Silent myocardial ischemia.
Circulation 108:1263, 2003.
Falk RH: Atrial fibrillation. N Engl J Med 344:1067, 2001.
Frenneaux MP: Assessing the risk of sudden cardiac death
in a patient with hypertrophic cardiomyopathy. Heart
90:570, 2004.
Greenland P, Gaziano JM: Clinical practice: selecting asymptomatic patients for coronary computed tomography or
electrocardiographic exercise testing. N Engl J Med
349:465, 2003.
Chapter 13
Cardiac Arrhythmias and Their Electrocardiographic Interpretation
Hurst JW: Current status of clinical electrocardiography with
suggestions for the improvement of the interpretive
process. Am J Cardiol 92:1072, 2003.
Jalife J: Ventricular fibrillation: mechanisms of initiation and
maintenance. Annu Rev Physiol 62:25, 2000.
Lee TH, Boucher CA: Clinical practice: noninvasive tests in
patients with stable coronary artery disease. N Engl J Med
344:1840, 2001.
Lehmann MH, Morady F: QT interval: metric for cardiac
prognosis? Am J Med 115:732, 2003.
Levy S: Pharmacologic management of atrial fibrillation: current therapeutic strategies. Am Heart J 141(2 Suppl):S15,
2001.
Marban E: The surprising role of vascular K(ATP)
channels in vasospastic angina. J Clin Invest 110:153,
2002.
Maron BJ: Sudden death in young athletes. N Engl J Med
349:1064, 2003.
Nattel S: New ideas about atrial fibrillation 50 years on.
Nature 415:219, 2002.
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Roden DM: Drug-induced prolongation of the QT interval.
N Engl J Med 350:1013, 2004.
Swynghedauw B, Baillard C, Milliez P: The long QT interval
is not only inherited but is also linked to cardiac hypertrophy. J Mol Med 81:336, 2003.
Topol EJ: A guide to therapeutic decision-making in patients
with non-ST-segment elevation acute coronary syndromes. J Am Coll Cardiol 41(4 Suppl S):S123, 2003.
Wang K, Asinger RW, Marriott HJ: ST-segment elevation in
conditions other than acute myocardial infarction. N Engl
J Med 349:2128, 2003.
Yan GX, Lankipalli RS, Burke JF, et al: Ventricular repolarization components on the electrocardiogram: cellular
basis and clinical significance. J Am Coll Cardiol 42:401,
2003.
Zimetbaum PJ, Josephson ME: Use of the electrocardiogram
in acute myocardial infarction. N Engl J Med 348:933,
2003.
Zipes DP, Jalife J: Cardiac Electrophysiology, 3rd ed.
Philadelphia: WB Saunders, 1999.
U
N
I
The Circulation
14. Overview of the Circulation; Medical Physics of
Pressure, Flow, and Resistance
15. Vascular Distensibility and Functions of the
Arterial and Venous Systems
16. The Microcirculation and the Lymphatic System:
Capillary Fluid Exchange, Interstitial Fluid,
and Lymph Flow
17. Local and Humoral Control of Blood Flow
by the Tissues
18. Nervous Regulation of the Circulation, and
Rapid Control of Arterial Pressure
19. Dominant Role of the Kidney in Long-Term
Regulation of Arterial Pressure and in Hypertension:
The Integrated System for Pressure Control
20. Cardiac Output, Venous Return, and
Their Regulation
21. Muscle Blood Flow and Cardiac Output During
Exercise; the Coronary Circulation and Ischemic
Heart Disease
22. Cardiac Failure
23. Heart Valves and Heart Sounds; Dynamics of
Valvular and Congenital Heart Defects
24. Circulatory Shock and Physiology of Its Treatment
T
IV
C
H
A
P
T
E
R
1
4
Overview of the Circulation;
Medical Physics of Pressure,
Flow, and Resistance
The function of the circulation is to service the
needs of the body tissues—to transport nutrients to
the body tissues, to transport waste products away,
to conduct hormones from one part of the body to
another, and, in general, to maintain an appropriate
environment in all the tissue fluids of the body for
optimal survival and function of the cells.
The rate of blood flow through most tissues is
controlled in response to tissue need for nutrients. The heart and circulation in
turn are controlled to provide the necessary cardiac output and arterial pressure to cause the needed tissue blood flow. What are the mechanisms for controlling blood volume and blood flow, and how does this relate to all the other
functions of the circulation? These are some of the topics and questions that we
discuss in this section on the circulation.
Physical Characteristics of the Circulation
The circulation, shown in Figure 14–1, is divided into the systemic circulation
and the pulmonary circulation. Because the systemic circulation supplies blood
flow to all the tissues of the body except the lungs, it is also called the greater
circulation or peripheral circulation.
Functional Parts of the Circulation. Before discussing the details of circulatory func-
tion, it is important to understand the role of each part of the circulation.
The function of the arteries is to transport blood under high pressure to the
tissues. For this reason, the arteries have strong vascular walls, and blood flows
at a high velocity in the arteries.
The arterioles are the last small branches of the arterial system; they act as
control conduits through which blood is released into the capillaries. The arteriole has a strong muscular wall that can close the arteriole completely or can,
by relaxing, dilate it severalfold, thus having the capability of vastly altering
blood flow in each tissue bed in response to the need of the tissue.
The function of the capillaries is to exchange fluid, nutrients, electrolytes,
hormones, and other substances between the blood and the interstitial fluid.
To serve this role, the capillary walls are very thin and have numerous minute
capillary pores permeable to water and other small molecular substances.
The venules collect blood from the capillaries, and they gradually coalesce
into progressively larger veins.
The veins function as conduits for transport of blood from the venules back
to the heart; equally important, they serve as a major reservoir of extra blood.
Because the pressure in the venous system is very low, the venous walls are thin.
Even so, they are muscular enough to contract or expand and thereby act as a
controllable reservoir for the extra blood, either a small or a large amount,
depending on the needs of the circulation.
Volumes of Blood in the Different Parts of the Circulation. Figure 14–1 gives an
overview of the circulation and lists the percentage of the total blood volume
161
162
Unit IV
The Circulation
Pulmonary circulation–9%
Vessel
Aorta
Aorta
Small arteries
Arterioles
Capillaries
Venules
Small veins
Venae cavae
Cross-Sectional Area (cm2)
2.5
20
40
2500
250
80
8
Superior
vena cava
Heart–7%
Inferior
vena cava
Systemic
vessels
Arteries–13%
Arterioles
and
capillaries–7%
Veins, venules,
and venous
sinuses–64%
Figure 14–1
Distribution of blood (in percentage of total blood) in the different
parts of the circulatory system.
in major segments of the circulation. For instance,
about 84 per cent of the entire blood volume of the
body is in the systemic circulation, and 16 per cent in
heart and lungs. Of the 84 per cent in the systemic circulation, 64 per cent is in the veins, 13 per cent in the
arteries, and 7 per cent in the systemic arterioles and
capillaries. The heart contains 7 per cent of the blood,
and the pulmonary vessels, 9 per cent.
Most surprising is the low blood volume in the capillaries. It is here, however, that the most important
function of the circulation occurs, diffusion of substances back and forth between the blood and the
tissues. This function is discussed in detail in Chapter
16.
Cross-Sectional Areas and Velocities of Blood Flow. If all the
systemic vessels of each type were put side by side,
their approximate total cross-sectional areas for the
average human being would be as follows:
Note particularly the much larger cross-sectional
areas of the veins than of the arteries, averaging about
four times those of the corresponding arteries. This
explains the large storage of blood in the venous
system in comparison with the arterial system.
Because the same volume of blood must flow
through each segment of the circulation each minute,
the velocity of blood flow is inversely proportional to
vascular cross-sectional area. Thus, under resting conditions, the velocity averages about 33 cm/sec in the
aorta but only 1/1000 as rapidly in the capillaries, about
0.3 mm/sec. However, because the capillaries have a
typical length of only 0.3 to 1 millimeter, the blood
remains in the capillaries for only 1 to 3 seconds. This
short time is surprising because all diffusion of
nutrient food substances and electrolytes that occurs
through the capillary walls must do so in this exceedingly short time.
Pressures in the Various Portions of the Circulation. Because
the heart pumps blood continually into the aorta,
the mean pressure in the aorta is high, averaging about
100 mm Hg. Also, because heart pumping is pulsatile,
the arterial pressure alternates between a systolic pressure level of 120 mm Hg and a diastolic pressure level
of 80 mm Hg, as shown on the left side of Figure 14–2.
As the blood flows through the systemic circulation,
its mean pressure falls progressively to about 0 mm Hg
by the time it reaches the termination of the venae
cavae where they empty into the right atrium of the
heart.
The pressure in the systemic capillaries varies from
as high as 35 mm Hg near the arteriolar ends to as low
as 10 mm Hg near the venous ends, but their average
“functional” pressure in most vascular beds is about
17 mm Hg, a pressure low enough that little of the
plasma leaks through the minute pores of the capillary
walls, even though nutrients can diffuse easily through
these same pores to the outlying tissue cells.
Note at the far right side of Figure 14–2 the respective pressures in the different parts of the pulmonary
circulation. In the pulmonary arteries, the pressure is
pulsatile, just as in the aorta, but the pressure level is
far less: pulmonary artery systolic pressure averages
about 25 mm Hg and diastolic pressure 8 mm Hg, with
a mean pulmonary arterial pressure of only 16 mm Hg.
The mean pulmonary capillary pressure averages only
7 mm Hg. Yet the total blood flow through the lungs
each minute is the same as through the systemic circulation. The low pressures of the pulmonary system
are in accord with the needs of the lungs, because
all that is required is to expose the blood in the
Chapter 14
163
Overview of the Circulation; Medical Physics of Pressure, Flow, and Resistance
Pulmonary veins
Venules
Capillaries
Arterioles
Venae cavae
Large veins
Small veins
Arterioles
20
Small arteries
40
Large arteries
60
Venules
Capillaries
80
Aorta
Pressure (mm Hg)
100
Pulmonary arteries
120
0
0
Systemic
Pulmonary
Figure 14–2
Normal blood pressures in the different portions of the circulatory system when a person is lying in the horizontal position.
pulmonary capillaries to oxygen and other gases in the
pulmonary alveoli.
Basic Theory of Circulatory
Function
Although the details of circulatory function are
complex, there are three basic principles that underlie
all functions of the system.
1. The rate of blood flow to each tissue of the body
is almost always precisely controlled in relation to
the tissue need. When tissues are active, they need
greatly increased supply of nutrients and
therefore much more blood flow than when at
rest—occasionally as much as 20 to 30 times
the resting level. Yet the heart normally cannot
increase its cardiac output more than four to
seven times greater than resting levels. Therefore,
it is not possible simply to increase blood flow
everywhere in the body when a particular tissue
demands increased flow. Instead, the microvessels
of each tissue continuously monitor tissue needs,
such as the availability of oxygen and other
nutrients and the accumulation of carbon dioxide
and other tissue waste products, and these in turn
act directly on the local blood vessels, dilating or
constricting them, to control local blood flow
precisely to that level required for the tissue
activity. Also, nervous control of the circulation
from the central nervous system provides
additional help in controlling tissue blood
flow.
2. The cardiac output is controlled mainly by the
sum of all the local tissue flows. When blood flows
through a tissue, it immediately returns by way of
the veins to the heart. The heart responds
automatically to this increased inflow of blood by
pumping it immediately into the arteries from
whence it had originally come. Thus, the heart acts
as an automaton, responding to the demands of
the tissues. The heart, however, often needs help
in the form of special nerve signals to make it
pump the required amounts of blood flow.
3. In general the arterial pressure is controlled
independently of either local blood flow control
or cardiac output control. The circulatory
system is provided with an extensive system for
controlling the arterial blood pressure.
For instance, if at any time the pressure falls
significantly below the normal level of about
100 mm Hg, within seconds a barrage of nervous
reflexes elicits a series of circulatory changes to
raise the pressure back toward normal. The
nervous signals especially (a) increase the force of
heart pumping, (b) cause contraction of the large
venous reservoirs to provide more blood to the
heart, and (c) cause generalized constriction of
most of the arterioles throughout the body so that
more blood accumulates in the large arteries to
increase the arterial pressure. Then, over more
prolonged periods, hours and days, the kidneys
play an additional major role in pressure control
both by secreting pressure-controlling hormones
and by regulating the blood volume.
Thus, in summary, the needs of the individual tissues
are served specifically by the circulation. In the
remainder of this chapter, we begin to discuss the basic
details of the management of tissue blood flow and
control of cardiac output and arterial pressure.
164
Unit IV
The Circulation
Interrelationships Among
Pressure, Flow, and
Resistance
Blood flow through a blood vessel is determined by
two factors: (1) pressure difference of the blood between the two ends of the vessel, also sometimes called
“pressure gradient” along the vessel, which is the force
that pushes the blood through the vessel, and (2) the
impediment to blood flow through the vessel, which is
called vascular resistance. Figure 14–3 demonstrates
these relationships, showing a blood vessel segment
located anywhere in the circulatory system.
P1 represents the pressure at the origin of the vessel;
at the other end, the pressure is P2. Resistance occurs
as a result of friction between the flowing blood and
the intravascular endothelium all along the inside of
the vessel. The flow through the vessel can be calculated by the following formula, which is called
Ohm’s law:
F=
DP
R
in which F is blood flow, DP is the pressure difference
(P1 - P2) between the two ends of the vessel, and R is
the resistance. This formula states, in effect, that the
blood flow is directly proportional to the pressure difference but inversely proportional to the resistance.
Note that it is the difference in pressure between the
two ends of the vessel, not the absolute pressure in the
vessel, that determines rate of flow. For example, if
the pressure at both ends of a vessel is 100 mm Hg and
yet no difference exists between the two ends, there
will be no flow despite the presence of 100 mm Hg
pressure.
Ohm’s law, illustrated in Equation 1, expresses the
most important of all the relations that the reader
needs to understand to comprehend the hemodynamics of the circulation. Because of the extreme importance of this formula, the reader should also become
familiar with its other algebraic forms:
DP = F ¥ R
R=
DP
F
Blood Flow
Blood flow means simply the quantity of blood that
passes a given point in the circulation in a given period
P1
Pressure gradient
P2
Blood flow
Resistance
Figure 14–3
Interrelationships among pressure, resistance, and blood flow.
of time. Ordinarily, blood flow is expressed in milliliters
per minute or liters per minute, but it can be expressed
in milliliters per second or in any other unit of flow.
The overall blood flow in the total circulation of an
adult person at rest is about 5000 ml/min. This is called
the cardiac output because it is the amount of blood
pumped into the aorta by the heart each minute.
Methods for Measuring Blood Flow. Many mechanical and
mechanoelectrical devices can be inserted in series with
a blood vessel or, in some instances, applied to the
outside of the vessel to measure flow. They are called
flowmeters.
Electromagnetic Flowmeter. One of the most important
devices for measuring blood flow without opening the
vessel is the electromagnetic flowmeter, the principles
of which are illustrated in Figure 14–4. Figure 14–4A
shows the generation of electromotive force (electrical
voltage) in a wire that is moved rapidly in a cross-wise
direction through a magnetic field. This is the wellknown principle for production of electricity by the
electric generator. Figure 14–4B shows that the same
principle applies for generation of electromotive force
in blood that is moving through a magnetic field. In
this case, a blood vessel is placed between the poles
of a strong magnet, and electrodes are placed on the
two sides of the vessel perpendicular to the magnetic
lines of force. When blood flows through the vessel,
an electrical voltage proportional to the rate of blood
flow is generated between the two electrodes, and this
is recorded using an appropriate voltmeter or electronic
recording apparatus. Figure 14–4C shows an actual
“probe” that is placed on a large blood vessel to record
its blood flow. The probe contains both the strong
magnet and the electrodes.
A special advantage of the electromagnetic flowmeter is that it can record changes in flow in less than 1/100
of a second, allowing accurate recording of pulsatile
changes in flow as well as steady flow.
Ultrasonic Doppler Flowmeter. Another type of flowmeter
that can be applied to the outside of the vessel and that
has many of the same advantages as the electromagnetic
flowmeter is the ultrasonic Doppler flowmeter, shown in
Figure 14–5. A minute piezoelectric crystal is mounted
at one end in the wall of the device. This crystal, when
energized with an appropriate electronic apparatus,
transmits ultrasound at a frequency of several hundred thousand cycles per second downstream along the
flowing blood. A portion of the sound is reflected by the
red blood cells in the flowing blood. The reflected ultrasound waves then travel backward from the blood cells
toward the crystal. These reflected waves have a lower
frequency than the transmitted wave because the red
cells are moving away from the transmitter crystal. This
is called the Doppler effect. (It is the same effect that
one experiences when a train approaches and passes by
while blowing its whistle. Once the whistle has passed
by the person, the pitch of the sound from the whistle
suddenly becomes much lower than when the train is
approaching.)
For the flowmeter shown in Figure 14–5, the highfrequency ultrasound wave is intermittently cut off, and
the reflected wave is received back onto the crystal and
amplified greatly by the electronic apparatus. Another
portion of the electronic apparatus determines the
165
Overview of the Circulation; Medical Physics of Pressure, Flow, and Resistance
Chapter 14
+
+
0
–
S
N
+
A
0
S
N
–
–
B
+
–
Figure 14–4
Flowmeter of the electromagnetic
type, showing generation of an
electrical voltage in a wire as it
passes through an electromagnetic field (A); generation of an
electrical voltage in electrodes on
a blood vessel when the vessel
is placed in a strong magnetic
field and blood flows through
the vessel (B); and a modern
electromagnetic flowmeter probe
for chronic implantation around
blood vessels (C).
C
Crystal
A
B
Transmitted
wave
Reflected
wave
Figure 14–5
Ultrasonic Doppler flowmeter.
frequency difference between the transmitted wave and
the reflected wave, thus determining the velocity of
blood flow.
Like the electromagnetic flowmeter, the ultrasonic
Doppler flowmeter is capable of recording rapid, pulsatile changes in flow as well as steady flow.
Laminar Flow of Blood in Vessels. When blood flows at a
steady rate through a long, smooth blood vessel, it flows
in streamlines, with each layer of blood remaining the
same distance from the vessel wall. Also, the central
most portion of the blood stays in the center of the
vessel. This type of flow is called laminar flow or streamline flow, and it is the opposite of turbulent flow, which
is blood flowing in all directions in the vessel and
continually mixing within the vessel, as discussed
subsequently.
C
Figure 14–6
A, Two fluids (one dyed red, and the other clear) before flow
begins; B, the same fluids 1 second after flow begins; C, turbulent flow, with elements of the fluid moving in a disorderly pattern.
Parabolic Velocity Profile During Laminar Flow. When laminar
flow occurs, the velocity of flow in the center of the
vessel is far greater than that toward the outer edges.
This is demonstrated in Figure 14–6. In Figure 14–6A, a
vessel contains two fluids, the one at the left colored by
a dye and the one at the right a clear fluid, but there is
no flow in the vessel. When the fluids are made to flow,
a parabolic interface develops between them, as shown
1 second later in Figure 14–6B; the portion of fluid adjacent to the vessel wall has hardly moved, the portion
slightly away from the wall has moved a small distance,
and the portion in the center of the vessel has moved a
long distance. This effect is called the “parabolic profile
for velocity of blood flow.”
The cause of the parabolic profile is the following:The
fluid molecules touching the wall barely move because
166
Unit IV
The Circulation
of adherence to the vessel wall. The next layer of molecules slips over these, the third layer over the second,
the fourth layer over the third, and so forth. Therefore,
the fluid in the middle of the vessel can move rapidly
because many layers of slipping molecules exist
between the middle of the vessel and the vessel wall;
thus, each layer toward the center flows progressively
more rapidly than the outer layers.
Turbulent Flow of Blood Under Some Conditions. When the rate
of blood flow becomes too great, when it passes by an
obstruction in a vessel, when it makes a sharp turn, or
when it passes over a rough surface, the flow may then
become turbulent, or disorderly, rather than streamline
(see Figure 14–6C).Turbulent flow means that the blood
flows crosswise in the vessel as well as along the vessel,
usually forming whorls in the blood called eddy currents. These are similar to the whirlpools that one
frequently sees in a rapidly flowing river at a point of
obstruction.
When eddy currents are present, the blood flows with
much greater resistance than when the flow is streamline because eddies add tremendously to the overall
friction of flow in the vessel.
The tendency for turbulent flow increases in direct
proportion to the velocity of blood flow, the diameter of
the blood vessel, and the density of the blood, and is
inversely proportional to the viscosity of the blood, in
accordance with the following equation:
Re =
n◊d ◊r
h
where Re is Reynolds’ number and is the measure of the
tendency for turbulence to occur, n is the mean velocity
of blood flow (in centimeters/second), d is the vessel
diameter (in centimeters), r is density, and h is the viscosity (in poise). The viscosity of blood is normally
about 1/30 poise, and the density is only slightly greater
than 1. When Reynolds’ number rises above 200 to 400,
turbulent flow will occur at some branches of vessels but
will die out along the smooth portions of the vessels.
However, when Reynolds’ number rises above approximately 2000, turbulence will usually occur even in a
straight, smooth vessel.
Reynolds’ number for flow in the vascular system
even normally rises to 200 to 400 in large arteries; as a
result there is almost always some turbulence of flow at
the branches of these vessels. In the proximal portions
of the aorta and pulmonary artery, Reynolds’ number
can rise to several thousand during the rapid phase of
ejection by the ventricles; this causes considerable turbulence in the proximal aorta and pulmonary artery
where many conditions are appropriate for turbulence:
(1) high velocity of blood flow, (2) pulsatile nature of
the flow, (3) sudden change in vessel diameter, and
(4) large vessel diameter. However, in small vessels,
Reynolds’ number is almost never high enough to cause
turbulence.
Blood Pressure
Standard Units of Pressure. Blood pressure almost always
is measured in millimeters of mercury (mm Hg)
because the mercury manometer (shown in Figure
14–7) has been used since antiquity as the standard
100 mm Hg pressure
0 pressure
Moving sooted
paper
Float
Anticoagulant
solution
Mercury
Mercury
manometer
Figure 14–7
Recording arterial pressure with a mercury manometer, a method
that has been used in the manner shown for recording pressure
throughout the history of physiology.
reference for measuring pressure.Actually, blood pressure means the force exerted by the blood against any
unit area of the vessel wall. When one says that the
pressure in a vessel is 50 mm Hg, one means that the
force exerted is sufficient to push a column of mercury
against gravity up to a level 50 mm high. If the pressure is 100 mm Hg, it will push the column of mercury
up to 100 millimeters.
Occasionally, pressure is measured in centimeters of
water (cm H2O). A pressure of 10 cm H2O means a
pressure sufficient to raise a column of water against
gravity to a height of 10 centimeters. One millimeter of
mercury pressure equals 1.36 cm water pressure
because the specific gravity of mercury is 13.6 times
that of water, and 1 centimeter is 10 times as great as
1 millimeter.
High-Fidelity Methods for Measuring Blood Pressure. The
mercury in the mercury manometer has so much inertia
that it cannot rise and fall rapidly. For this reason, the
mercury manometer, although excellent for recording
steady pressures, cannot respond to pressure changes
that occur more rapidly than about one cycle every 2 to
3 seconds. Whenever it is desired to record rapidly
changing pressures, some other type of pressure
recorder is needed. Figure 14–8 demonstrates the basic
principles of three electronic pressure transducers commonly used for converting blood pressure and/or rapid
changes in pressure into electrical signals and then
recording the electrical signals on a high-speed electrical recorder. Each of these transducers uses a very thin,
highly stretched metal membrane that forms one wall
of the fluid chamber. The fluid chamber in turn is connected through a needle or catheter to the blood vessel
in which the pressure is to be measured. When the pressure is high, the membrane bulges slightly, and when it
is low, it returns toward its resting position.
Chapter 14
Overview of the Circulation; Medical Physics of Pressure, Flow, and Resistance
167
any direct means. Instead, resistance must be calculated from measurements of blood flow and pressure
difference between two points in the vessel. If the
pressure difference between two points is 1 mm Hg
and the flow is 1 ml/sec, the resistance is said to be 1
peripheral resistance unit, usually abbreviated PRU.
A
B
Expression of Resistance in CGS Units. Occasionally, a
basic physical unit called the CGS (centimeters, grams,
seconds) unit is used to express resistance. This unit is
dyne seconds/centimeters5. Resistance in these units can
be calculated by the following formula:
R Ê in
Ë
dyne sec ˆ 1333 ¥ mm Hg
=
cm5 ¯
ml sec
Total Peripheral Vascular Resistance and Total Pulmonary Vascular Resistance. The rate of blood flow through the
C
Figure 14–8
Principles of three types of electronic transducers for recording
rapidly changing blood pressures (explained in the text).
In Figure 14–8A, a simple metal plate is placed a few
hundredths of a centimeter above the membrane. When
the membrane bulges, the membrane comes closer to
the plate, which increases the electrical capacitance
between these two, and this change in capacitance can
be recorded using an appropriate electronic system.
In Figure 14–8B, a small iron slug rests on the membrane, and this can be displaced upward into a center
space inside an electrical wire coil. Movement of the
iron into the coil increases the inductance of the coil,
and this, too, can be recorded electronically.
Finally, in Figure 14–8C, a very thin, stretched resistance wire is connected to the membrane. When this wire
is stretched greatly, its resistance increases; when it is
stretched less, its resistance decreases. These changes,
too, can be recorded by an electronic system.
With some of these high-fidelity types of recording
systems, pressure cycles up to 500 cycles per second
have been recorded accurately. In common use are
recorders capable of registering pressure changes that
occur as rapidly as 20 to 100 cycles per second, in the
manner shown on the recording paper in Figure 14–8C.
Resistance to Blood Flow
Units of Resistance. Resistance is the impediment to
blood flow in a vessel, but it cannot be measured by
entire circulatory system is equal to the rate of blood
pumping by the heart—that is, it is equal to the cardiac
output. In the adult human being, this is approximately
100 ml/sec. The pressure difference from the systemic
arteries to the systemic veins is about 100 mm Hg.
Therefore, the resistance of the entire systemic circulation, called the total peripheral resistance, is about
100/100, or 1 PRU.
In conditions in which all the blood vessels throughout the body become strongly constricted, the total
peripheral resistance occasionally rises to as high as 4
PRU. Conversely, when the vessels become greatly
dilated, the resistance can fall to as little as 0.2 PRU.
In the pulmonary system, the mean pulmonary arterial pressure averages 16 mm Hg and the mean left
atrial pressure averages 2 mm Hg, giving a net pressure difference of 14 mm. Therefore, when the cardiac
output is normal at about 100 ml/sec, the total
pulmonary vascular resistance calculates to be about
0.14 PRU (about one seventh that in the systemic
circulation).
“Conductance” of Blood in a Vessel and Its Relation to Resistance. Conductance is a measure of the blood flow
through a vessel for a given pressure difference. This
is generally expressed in terms of milliliters per second
per millimeter of mercury pressure, but it can also be
expressed in terms of liters per second per millimeter
of mercury or in any other units of blood flow and
pressure.
It is evident that conductance is the exact reciprocal
of resistance in accord with the following equation:
Conductance =
1
Resistance
Very Slight Changes in Diameter of a Vessel Can Change Its
Conductance Tremendously! Slight changes in the diame-
ter of a vessel cause tremendous changes in the vessel’s
ability to conduct blood when the blood flow is streamlined. This is demonstrated by the experiment illustrated in Figure 14–9A, which shows three vessels with
relative diameters of 1, 2, and 4 but with the same pressure difference of 100 mm Hg between the two ends
of the vessels. Although the diameters of these vessels
168
Unit IV
A
d=1
P=
100 mm
Hg
d=2
The Circulation
1 ml/min
16 ml/min
d=4
256 ml/min
Note particularly in this equation that the rate of
blood flow is directly proportional to the fourth power
of the radius of the vessel, which demonstrates once
again that the diameter of a blood vessel (which is equal
to twice the radius) plays by far the greatest role of all
factors in determining the rate of blood flow through a
vessel.
Importance of the Vessel Diameter “Fourth Power Law” in
Determining Arteriolar Resistance. In the systemic circula-
B
Small vessel
Large vessel
Figure 14–9
A, Demonstration of the effect of vessel diameter on blood flow.
B, Concentric rings of blood flowing at different velocities; the
farther away from the vessel wall, the faster the flow.
increase only fourfold, the respective flows are 1, 16,
and 256 ml/mm, which is a 256-fold increase in flow.
Thus, the conductance of the vessel increases in proportion to the fourth power of the diameter, in accordance with the following formula:
Conductance µ Diameter 4
Poiseuille’s Law. The cause of this great increase in conductance when the diameter increases can be explained
by referring to Figure 14–9B, which shows cross sections
of a large and a small vessel. The concentric rings inside
the vessels indicate that the velocity of flow in each ring
is different from that in the adjacent rings because of
laminar flow, which was discussed earlier in the chapter.
That is, the blood in the ring touching the wall of the
vessel is barely flowing because of its adherence to
the vascular endothelium.The next ring of blood toward
the center of the vessel slips past the first ring and,
therefore, flows more rapidly. The third, fourth, fifth, and
sixth rings likewise flow at progressively increasing
velocities. Thus, the blood that is near the wall of the
vessel flows extremely slowly, whereas that in the
middle of the vessel flows extremely rapidly.
In the small vessel, essentially all the blood is near the
wall, so that the extremely rapidly flowing central
stream of blood simply does not exist. By integrating the
velocities of all the concentric rings of flowing blood and
multiplying them by the areas of the rings, one can
derive the following formula, known as Poiseuille’s law:
F=
pD Pr 4
8 hl
in which F is the rate of blood flow, DP is the pressure
difference between the ends of the vessel, r is the radius
of the vessel, l is length of the vessel, and h is viscosity
of the blood.
tion, about two thirds of the total systemic resistance
to blood flow is arteriolar resistance in the small
arterioles. The internal diameters of the arterioles
range from as little as 4 micrometers to as great as
25 micrometers. However, their strong vascular walls
allow the internal diameters to change tremendously,
often as much as fourfold. From the fourth power law
discussed above that relates blood flow to diameter of
the vessel, one can see that a fourfold increase in vessel
diameter can increase the flow as much as 256-fold.
Thus, this fourth power law makes it possible for
the arterioles, responding with only small changes in
diameter to nervous signals or local tissue chemical
signals, either to turn off almost completely the blood
flow to the tissue or at the other extreme to cause a
vast increase in flow. Indeed, ranges of blood flow of
more than 100-fold in separate tissue areas have been
recorded between the limits of maximum arteriolar
constriction and maximum arteriolar dilatation.
Resistance to Blood Flow in Series and Parallel Vascular Circuits. Blood pumped by the heart flows from the high
pressure part of the systemic circulation (i.e., aorta) to
the low pressure side (i.e., vena cava) through many
miles of blood vessels arranged in series and in parallel. The arteries, arterioles, capillaries, venules, and
veins are collectively arranged in series. When blood
vessels are arranged in series, flow through each blood
vessel is the same and the total resistance to blood flow
(Rtotal) is equal to the sum of the resistances of each
vessel:
R total = R 1 + R 2 + R 3 + R 4 . . .
The total peripheral vascular resistance is therefore
equal to the sum of resistances of the arteries, arterioles, capillaries, venules, and veins. In the example
shown in Figure 14–10A, the total vascular resistance
is equal to the sum of R1 and R2.
Blood vessels branch extensively to form parallel
circuits that supply blood to the many organs and
tissues of the body. This parallel arrangement permits
each tissue to regulate its own blood flow, to a great
extent, independently of flow to other tissues.
For blood vessels arranged in parallel (Figure
14–10B), the total resistance to blood flow is expressed
as:
1
1
1
1
1
=
+
+
+
...
R total R 1 R 2 R 3 R 4
It is obvious that for a given pressure gradient,
far greater amounts of blood will flow through this
Chapter 14
Overview of the Circulation; Medical Physics of Pressure, Flow, and Resistance
R1
R2
A
R1 R
2
B
R3
R4
169
100
100
100
90
90
90
80
80
80
70
70
70
60
60
60
50
50
50
40
40
40
30
30
30
20
20
20
10
10
10
0
0
0
Figure 14–10
Vascular resistances: A, in series and B, in parallel.
parallel system than through any of the individual
blood vessels. Therefore, the total resistance is far less
than the resistance of any single blood vessel. Flow
through each of the parallel vessels in Figure 14–10B
is determined by the pressure gradient and its own
resistance, not the resistance of the other parallel
blood vessels. However, increasing the resistance of
any of the blood vessels increases the total vascular
resistance.
It may seem paradoxical that adding more blood
vessels to a circuit reduces the total vascular resistance. Many parallel blood vessels, however, make it
easier for blood to flow through the circuit because
each parallel vessel provides another pathway, or conductance, for blood flow. The total conductance (Ctotal)
for blood flow is the sum of the conductance of each
parallel pathway:
C total = C1 + C 2 + C 3 + C 4 . . .
For example, brain, kidney, muscle, gastrointestinal,
skin, and coronary circulations are arranged in parallel, and each tissue contributes to the overall conductance of the systemic circulation. Blood flow through
each tissue is a fraction of the total blood flow (cardiac
output) and is determined by the resistance (the reciprocal of conductance) for blood flow in the tissue, as
well as the pressure gradient. Therefore, amputation of
a limb or surgical removal of a kidney also removes a
parallel circuit and reduces the total vascular conductance and total blood flow (i.e., cardiac output) while
increasing total peripheral vascular resistance.
Effect of Blood Hematocrit and Blood
Viscosity on Vascular Resistance and
Blood Flow
Note especially that another of the important factors
in Poiseuille’s equation is the viscosity of the blood.
The greater the viscosity, the less the flow in a vessel
if all other factors are constant. Furthermore, the viscosity of normal blood is about three times as great as
the viscosity of water.
But what makes the blood so viscous? It is mainly
the large numbers of suspended red cells in the blood,
Normal
Anemia
Polycythemia
Figure 14–11
Hematocrits in a healthy (normal) person and in patients with
anemia and polycythemia.
each of which exerts frictional drag against adjacent
cells and against the wall of the blood vessel.
Hematocrit. The percentage of the blood that is cells is
called the hematocrit. Thus, if a person has a hematocrit of 40, this means that 40 per cent of the blood
volume is cells and the remainder is plasma.The hematocrit of men averages about 42, while that of women
averages about 38. These values vary tremendously,
depending on whether the person has anemia, on the
degree of bodily activity, and on the altitude at which
the person resides. These changes in hematocrit are
discussed in relation to the red blood cells and their
oxygen transport function in Chapter 32.
Hematocrit is determined by centrifuging blood in
a calibrated tube, as shown in Figure 14–11. The calibration allows direct reading of the percentage of cells.
Effect of Hematocrit on Blood Viscosity. The viscosity of
blood increases drastically as the hematocrit increases,
as shown in Figure 14–12. The viscosity of whole blood
at normal hematocrit is about 3; this means that three
times as much pressure is required to force whole
blood as to force water through the same blood vessel.
When the hematocrit rises to 60 or 70, which it often
does in polycythemia, the blood viscosity can become
as great as 10 times that of water, and its flow through
blood vessels is greatly retarded.
Other factors that affect blood viscosity are the
plasma protein concentration and types of proteins in
the plasma, but these effects are so much less than
the effect of hematocrit that they are not significant
170
Unit IV
Viscosity (water = 1)
9
Viscosity of whole blood
7
Viscosity of plasma
Viscosity of water
6
Blood flow (ml/min)
10
The Circulation
8
7
6
5
4
Normal blood
3
2
Sympathetic
inhibition
Normal
Sympathetic
stimulation
5
4
Critical
closing
pressure
3
2
1
0
0
1
20
0
0
10
20
30
40
50
Hematocrit
60
40
60 80 100 120 140 160 180 200
Arterial pressure (mm Hg)
70
Figure 14–13
Figure 14–12
Effect of arterial pressure on blood flow through a blood vessel at
different degrees of vascular tone caused by increased or
decreased sympathetic stimulation of the vessel.
Effect of hematocrit on blood viscosity. (Water viscosity = 1.)
considerations in most hemodynamic studies. The viscosity of blood plasma is about 1.5 times that of water.
Effects of Pressure on Vascular
Resistance and Tissue Blood Flow
From the discussion thus far, one might expect an
increase in arterial pressure to cause a proportionate
increase in blood flow through the various tissues of
the body. However, the effect of pressure on blood
flow is greater than one would expect, as shown by the
upward curving lines in Figure 14–13. The reason for
this is that an increase in arterial pressure not only
increases the force that pushes blood through the
vessels but also distends the vessels at the same time,
which decreases vascular resistance. Thus, greater
pressure increases the flow in both of these ways.
Therefore, for most tissues, blood flow at 100 mm Hg
arterial pressure is usually four to six times as great as
blood flow at 50 mm Hg instead of two times as would
be true if there were no effect of increasing pressure
to increase vascular diameter.
Note also in Figure 14–13 the large changes in
blood flow that can be caused by either increased
or decreased sympathetic nerve stimulation of the
peripheral blood vessels. Thus, as shown in the figure,
inhibition of sympathetic activity greatly dilates the
vessels and can increase the blood flow twofold or
more. Conversely, very strong sympathetic stimulation
can constrict the vessels so much that blood flow occasionally decreases to as low as zero for a few seconds
despite high arterial pressure.
References
See references for Chapter 15.
C
H
A
P
T
E
R
1
5
Vascular Distensibility and
Functions of the Arterial and
Venous Systems
Vascular Distensibility
A valuable characteristic of the vascular system is
that all blood vessels are distensible. We have seen
one example of this in Chapter 14: When the pressure in blood vessels is increased, this dilates the
blood vessels and therefore decreases their resistance. The result is increased blood flow not only
because of increased pressure but also because of decreased resistance, usually
giving at least twice as much flow increase for each increase in pressure as one
might expect.
Vascular distensibility also plays other important roles in circulatory function.
For example, the distensible nature of the arteries allows them to accommodate
the pulsatile output of the heart and to average out the pressure pulsations. This
provides smooth, continuous flow of blood through the very small blood vessels
of the tissues.
The most distensible by far of all the vessels are the veins. Even slight
increases in venous pressure cause the veins to store 0.5 to 1.0 liter of extra
blood. Therefore, the veins provide a reservoir function for storing large quantities of extra blood that can be called into use whenever required elsewhere in
the circulation.
Units of Vascular Distensibility. Vascular distensibility normally is expressed as the
fractional increase in volume for each millimeter of mercury rise in pressure, in
accordance with the following formula:
Vascular distensibility =
Increase in volume
Increase in pressure ¥ Original volume
That is, if 1 mm Hg causes a vessel that originally contained 10 millimeters of
blood to increase its volume by 1 milliliter, the distensibility would be 0.1 per
mm Hg, or 10 per cent per mm Hg.
Difference in Distensibility of the Arteries and the Veins. Anatomically, the walls
of the arteries are far stronger than those of the veins. Consequently, the arteries, on average, are about eight times less distensible than the veins. That is, a
given increase in pressure causes about eight times as much increase in blood
in a vein as in an artery of comparable size.
In the pulmonary circulation, the pulmonary vein distensibilities are similar
to those of the systemic circulation. But, the pulmonary arteries normally
operate under pressures about one sixth of those in the systemic arterial system,
and their distensibilities are correspondingly greater, about six times the distensibility of systemic arteries.
Vascular Compliance (or Vascular Capacitance)
In hemodynamic studies, it usually is much more important to know the total
quantity of blood that can be stored in a given portion of the circulation for
each millimeter of mercury pressure rise than to know the distensibilities of the
171
172
Unit IV
The Circulation
individual vessels. This value is called the compliance
or capacitance of the respective vascular bed; that is,
Vascular compliance =
Increase in volume
Increase in pressure
Compliance and distensibility are quite different. A
highly distensible vessel that has a slight volume may
have far less compliance than a much less distensible
vessel that has a large volume because compliance is
equal to distensibility times volume.
The compliance of a systemic vein is about 24 times
that of its corresponding artery because it is about 8
times as distensible and it has a volume about 3 times
as great (8 ¥ 3 = 24).
Volume-Pressure Curves of the
Arterial and Venous Circulations
A convenient method for expressing the relation of
pressure to volume in a vessel or in any portion of the
circulation is to use the so-called volume-pressure
curve. The red and blue solid curves in Figure 15–1
represent, respectively, the volume-pressure curves
of the normal systemic arterial system and venous
system, showing that when the arterial system of the
average adult person (including all the large arteries,
small arteries, and arterioles) is filled with about 700
milliliters of blood, the mean arterial pressure is
100 mm Hg, but when it is filled with only 400 milliliters of blood, the pressure falls to zero.
In the entire systemic venous system, the volume
normally ranges from 2000 to 3500 milliliters, and a
change of several hundred millimeters in this volume
is required to change the venous pressure only 3 to
5 mm Hg. This mainly explains why as much as one
half liter of blood can be transfused into a healthy
person in only a few minutes without greatly altering
function of the circulation.
Effect of Sympathetic Stimulation or Sympathetic Inhibition on
the Volume-Pressure Relations of the Arterial and Venous
Systems. Also shown in Figure 15–1 are the effects that
exciting or inhibiting the vascular sympathetic nerves
has on the volume-pressure curves. It is evident that
increase in vascular smooth muscle tone caused by
sympathetic stimulation increases the pressure at each
volume of the arteries or veins, whereas sympathetic
inhibition decreases the pressure at each volume.
Control of the vessels in this manner by the sympathetics is a valuable means for diminishing the
dimensions of one segment of the circulation, thus
transferring blood to other segments. For instance, an
increase in vascular tone throughout the systemic circulation often causes large volumes of blood to shift
into the heart, which is one of the principal methods
that the body uses to increase heart pumping.
Sympathetic control of vascular capacitance is also
highly important during hemorrhage. Enhancement of
sympathetic tone, especially to the veins, reduces the
vessel sizes enough that the circulation continues to
operate almost normally even when as much as 25 per
cent of the total blood volume has been lost.
Delayed Compliance
(Stress-Relaxation) of Vessels
140
Pressure (mm Hg)
120
Sympathetic stimulation
100
Sympathetic inhibition
80
60
Normal volume
40
Arterial system
Venous system
20
0
0
500
1000 1500 2000 2500 3000 3500
Volume (ml)
Figure 15–1
“Volume-pressure curves” of the systemic arterial and venous
systems, showing the effects of stimulation or inhibition of the sympathetic nerves to the circulatory system.
The term “delayed compliance” means that a vessel
exposed to increased volume at first exhibits a large
increase in pressure, but progressive delayed stretching
of smooth muscle in the vessel wall allows the pressure
to return back toward normal over a period of minutes
to hours. This effect is shown in Figure 15–2. In this
figure, the pressure is recorded in a small segment of a
vein that is occluded at both ends. An extra volume of
blood is suddenly injected until the pressure rises from
5 to 12 mm Hg. Even though none of the blood is
removed after it is injected, the pressure begins to
decrease immediately and approaches about 9 mm Hg
after several minutes. In other words, the volume of
blood injected causes immediate elastic distention of the
vein, but then the smooth muscle fibers of the vein begin
to “creep” to longer lengths, and their tensions correspondingly decrease. This effect is a characteristic of all
smooth muscle tissue and is called stress-relaxation,
which was explained in Chapter 8.
Delayed compliance is a valuable mechanism by
which the circulation can accommodate much extra
blood when necessary, such as after too large a transfusion. Delayed compliance in the reverse direction is one
of the ways in which the circulation automatically
adjusts itself over a period of minutes or hours to diminished blood volume after serious hemorrhage.
Chapter 15
14
D
co elay
mp ed
lia
nc
e
8
6
4
+20
d
aye
Del liance
p
com
2
0
0
20
40
60
Minutes
80
Pressure (mm Hg)
Increased
volume
10
Exponential diastolic decline
(may be distorted by
reflected wave)
Sharp
incisura
Slow rise
to peak
Decreased
volume
Pressure (mm Hg)
12
173
Vascular Distensibility and Functions of the Arterial and Venous Systems
– 80
Sharp
upstroke
– 80
– 80
0
Figure 15–2
Effect on the intravascular pressure of injecting a volume of blood
into a venous segment and later removing the excess blood,
demonstrating the principle of delayed compliance.
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
Seconds
Figure 15–3
Pressure pulse contour recorded from the ascending aorta.
(Redrawn from Opdyke DF: Fed Proc 11:734, 1952.)
Arterial Pressure Pulsations
With each beat of the heart a new surge of blood fills
the arteries. Were it not for distensibility of the arterial system, all of this new blood would have to flow
through the peripheral blood vessels almost instantaneously, only during cardiac systole, and no flow would
occur during diastole. However, normally the compliance of the arterial tree reduces the pressure pulsations to almost no pulsations by the time the blood
reaches the capillaries; therefore, tissue blood flow is
mainly continuous with very little pulsation.
A typical record of the pressure pulsations at the
root of the aorta is shown in Figure 15–3. In the
healthy young adult, the pressure at the top of each
pulse, called the systolic pressure, is about 120 mm Hg.
At the lowest point of each pulse, called the diastolic
pressure, it is about 80 mm Hg. The difference between
these two pressures, about 40 mm Hg, is called the
pulse pressure.
Two major factors affect the pulse pressure: (1) the
stroke volume output of the heart and (2) the compliance (total distensibility) of the arterial tree. A third,
less important factor is the character of ejection from
the heart during systole.
In general, the greater the stroke volume output, the
greater the amount of blood that must be accommodated in the arterial tree with each heartbeat, and,
therefore, the greater the pressure rise and fall during
systole and diastole, thus causing a greater pulse pressure. Conversely, the less the compliance of the arterial system, the greater the rise in pressure for a given
stroke volume of blood pumped into the arteries. For
instance, as demonstrated by the middle top curves in
Figure 15–4, the pulse pressure in old age sometimes
rises to as much as twice normal, because the arteries
160
120
80
Normal
Arteriosclerosis Aortic stenosis
160
120
80
Normal
40
0
Patent ductus
arteriosus
Aortic
regurgitation
Figure 15–4
Aortic pressure pulse contours in arteriosclerosis, aortic stenosis,
patent ductus arteriosus, and aortic regurgitation.
have become hardened with arteriosclerosis and therefore are relatively noncompliant.
In effect, then, pulse pressure is determined approximately by the ratio of stroke volume output to
compliance of the arterial tree.Any condition of the circulation that affects either of these two factors also
affects the pulse pressure.
174
Unit IV
The Circulation
Abnormal Pressure Pulse Contours
Some conditions of the circulation also cause abnormal contours of the pressure pulse wave in addition
to altering the pulse pressure. Especially distinctive
among these are aortic stenosis, patent ductus arteriosus, and aortic regurgitation, each of which is shown in
Figure 15–4.
In aortic stenosis, the diameter of the aortic valve
opening is reduced significantly, and the aortic pressure pulse is decreased significantly because of diminished blood flow outward through the stenotic valve.
In patent ductus arteriosus, one half or more of the
blood pumped into the aorta by the left ventricle flows
immediately backward through the wide-open ductus
into the pulmonary artery and lung blood vessels, thus
allowing the diastolic pressure to fall very low before
the next heartbeat.
In aortic regurgitation, the aortic valve is absent or
will not close completely. Therefore, after each heartbeat, the blood that has just been pumped into the
aorta flows immediately backward into the left ventricle. As a result, the aortic pressure can fall all the way
to zero between heartbeats. Also, there is no incisura
in the aortic pulse contour because there is no aortic
valve to close.
Wave fronts
Figure 15–5
Progressive stages in transmission of the pressure pulse along the
aorta.
Transmission of Pressure Pulses
to the Peripheral Arteries
When the heart ejects blood into the aorta during
systole, at first only the proximal portion of the aorta
becomes distended because the inertia of the blood
prevents sudden blood movement all the way to the
periphery. However, the rising pressure in the proximal aorta rapidly overcomes this inertia, and the wave
front of distention spreads farther and farther along
the aorta, as shown in Figure 15–5. This is called transmission of the pressure pulse in the arteries.
The velocity of pressure pulse transmission in the
normal aorta is 3 to 5 m/sec; in the large arterial
branches, 7 to 10 m/sec; and in the small arteries, 15 to
35 m/sec. In general, the greater the compliance of
each vascular segment, the slower the velocity, which
explains the slow transmission in the aorta and the
much faster transmission in the much less compliant
small distal arteries. In the aorta, the velocity of transmission of the pressure pulse is 15 or more times the
velocity of blood flow because the pressure pulse is
simply a moving wave of pressure that involves little
forward total movement of blood volume.
Proximal aorta
Femoral artery
Radial artery
Arteriole
Capillary
Damping of the Pressure Pulses in the Smaller Arteries, Arterioles, and Capillaries. Figure 15–6 shows typical changes
in the contours of the pressure pulse as the pulse
travels into the peripheral vessels. Note especially in
the three lower curves that the intensity of pulsation
becomes progressively less in the smaller arteries,
the arterioles, and, especially, the capillaries. In fact,
only when the aortic pulsations are extremely large or
the arterioles are greatly dilated can pulsations be
observed in the capillaries.
Incisura
Systole Diastole
0
1
2
Time (seconds)
Figure 15–6
Changes in the pulse pressure contour as the pulse wave travels
toward the smaller vessels.
Chapter 15
Vascular Distensibility and Functions of the Arterial and Venous Systems
This progressive diminution of the pulsations in the
periphery is called damping of the pressure pulses. The
cause of this is twofold: (1) resistance to blood movement in the vessels and (2) compliance of the vessels.
The resistance damps the pulsations because a small
amount of blood must flow forward at the pulse wave
front to distend the next segment of the vessel; the
greater the resistance, the more difficult it is for this to
occur. The compliance damps the pulsations because
the more compliant a vessel, the greater the quantity
of blood required at the pulse wave front to cause an
increase in pressure. Therefore, in effect, the degree of
damping is almost directly proportional to the product
of resistance times compliance.
Clinical Methods for Measuring
Systolic and Diastolic Pressures
It is not reasonable to use pressure recorders that
require needle insertion into an artery for making
routine pressure measurements in human patients,
although these are used on occasion when special
studies are necessary. Instead, the clinician determines
systolic and diastolic pressures by indirect means,
usually by the auscultatory method.
Auscultatory Method. Figure 15–7 shows the auscultatory
method for determining systolic and diastolic arterial
pressures. A stethoscope is placed over the antecubital
artery and a blood pressure cuff is inflated around the
upper arm. As long as the cuff continues to compress
the arm with too little pressure to close the brachial
artery, no sounds are heard from the antecubital artery
with the stethoscope. However, when the cuff pressure
is great enough to close the artery during part of the
Sounds
120
100
175
arterial pressure cycle, a sound then is heard with each
pulsation. These sounds are called Korotkoff sounds.
The exact cause of Korotkoff sounds is still
debated, but they are believed to be caused mainly
by blood jetting through the partly occluded vessel.
The jet causes turbulence in the vessel beyond the cuff,
and this sets up the vibrations heard through the
stethoscope.
In determining blood pressure by the auscultatory
method, the pressure in the cuff is first elevated well
above arterial systolic pressure. As long as this cuff
pressure is higher than systolic pressure, the brachial
artery remains collapsed so that no blood jets into the
lower artery during any part of the pressure cycle.
Therefore, no Korotkoff sounds are heard in the lower
artery. But then the cuff pressure gradually is reduced.
Just as soon as the pressure in the cuff falls below systolic pressure, blood begins to slip through the artery
beneath the cuff during the peak of systolic pressure,
and one begins to hear tapping sounds from the antecubital artery in synchrony with the heartbeat. As soon
as these sounds begin to be heard, the pressure level
indicated by the manometer connected to the cuff is
about equal to the systolic pressure.
As the pressure in the cuff is lowered still more, the
Korotkoff sounds change in quality, having less of the
tapping quality and more of a rhythmical and harsher
quality. Then, finally, when the pressure in the cuff falls
to equal diastolic pressure, the artery no longer closes
during diastole, which means that the basic factor
causing the sounds (the jetting of blood through a
squeezed artery) is no longer present. Therefore, the
sounds suddenly change to a muffled quality, then disappear entirely after another 5- to 10-millimeter drop
in cuff pressure. One notes the manometer pressure
when the Korotkoff sounds change to the muffled
quality; this pressure is about equal to the diastolic
pressure. The auscultatory method for determining
systolic and diastolic pressures is not entirely accurate,
but it usually gives values within 10 per cent of those
determined by direct catheter measurement from
inside the arteries.
Normal Arterial Pressures as Measured by the Auscultatory
Method. Figure 15–8 shows the approximate normal
80
100
mm Hg
150
50
0
Figure 15–7
Auscultatory method for measuring systolic and diastolic arterial
pressures.
systolic and diastolic arterial pressures at different
ages. The progressive increase in pressure with age
results from the effects of aging on the blood pressure
control mechanisms. We shall see in Chapter 19 that
the kidneys are primarily responsible for this longterm regulation of arterial pressure; and it is well
known that the kidneys exhibit definitive changes with
age, especially after the age of 50 years.
A slight extra increase in systolic pressure usually
occurs beyond the age of 60 years. This results from
hardening of the arteries, which itself is an end-stage
result of atherosclerosis. The final effect is a bounding
systolic pressure with considerable increase in pulse
pressure, as previously explained.
Mean Arterial Pressure. The mean arterial pressure is the
average of the arterial pressures measured millisecond
176
Unit IV
The Circulation
Pressure (mm Hg)
200
Systolic
150
Mean
100
Diastolic
50
0
0
20
40
Age (years)
60
80
Figure 15–8
Changes in systolic, diastolic, and mean arterial pressures with
age. The shaded areas show the approximate normal ranges.
by millisecond over a period of time. It is not equal to
the average of systolic and diastolic pressure because
the arterial pressure remains nearer to diastolic pressure than to systolic pressure during the greater part
of the cardiac cycle. Therefore, the mean arterial pressure is determined about 60 per cent by the diastolic
pressure and 40 per cent by the systolic pressure. Note
in Figure 15–8 that the mean pressure (solid green
line) at all ages is nearer to the diastolic pressure than
to the systolic pressure.
Veins and Their Functions
For years, the veins were considered to be nothing
more than passageways for flow of blood to the heart,
but it has become apparent that they perform other
special functions that are necessary for operation of
the circulation. Especially important, they are capable
of constricting and enlarging and thereby storing
either small or large quantities of blood and making
this blood available when it is required by the remainder of the circulation. The peripheral veins can also
propel blood forward by means of a so-called venous
pump, and they even help to regulate cardiac output,
an exceedingly important function that is described in
detail in Chapter 20.
Venous Pressures—Right Atrial
Pressure (Central Venous Pressure)
and Peripheral Venous Pressures
To understand the various functions of the veins, it is
first necessary to know something about pressure in
the veins and what determines the pressure.
Blood from all the systemic veins flows into the right
atrium of the heart; therefore, the pressure in the right
atrium is called the central venous pressure.
Right atrial pressure is regulated by a balance
between (1) the ability of the heart to pump blood out
of the right atrium and ventricle into the lungs and (2)
the tendency for blood to flow from the peripheral veins
into the right atrium. If the right heart is pumping
strongly, the right atrial pressure decreases. Conversely, weakness of the heart elevates the right atrial
pressure. Also, any effect that causes rapid inflow of
blood into the right atrium from the peripheral veins
elevates the right atrial pressure. Some of the factors
that can increase this venous return (and thereby
increase the right atrial pressure) are (1) increased
blood volume, (2) increased large vessel tone throughout the body with resultant increased peripheral
venous pressures, and (3) dilatation of the arterioles,
which decreases the peripheral resistance and allows
rapid flow of blood from the arteries into the veins.
The same factors that regulate right atrial pressure
also enter into the regulation of cardiac output
because the amount of blood pumped by the heart
depends on both the ability of the heart to pump and
the tendency for blood to flow into the heart from the
peripheral vessels. Therefore, we will discuss regulation of right atrial pressure in much more depth in
Chapter 20 in connection with regulation of cardiac
output.
The normal right atrial pressure is about 0 mm Hg,
which is equal to the atmospheric pressure around the
body. It can increase to 20 to 30 mm Hg under very
abnormal conditions, such as (1) serious heart failure
or (2) after massive transfusion of blood, which greatly
increases the total blood volume and causes excessive
quantities of blood to attempt to flow into the heart
from the peripheral vessels.
The lower limit to the right atrial pressure is usually
about -3 to -5 mm Hg below atmospheric pressure.
This is also the pressure in the chest cavity that surrounds the heart. The right atrial pressure approaches
these low values when the heart pumps with exceptional vigor or when blood flow into the heart from the
peripheral vessels is greatly depressed, such as after
severe hemorrhage.
Venous Resistance and Peripheral
Venous Pressure
Large veins have so little resistance to blood flow when
they are distended that the resistance then is almost
zero and is of almost no importance. However, as
shown in Figure 15–9, most of the large veins that enter
the thorax are compressed at many points by the surrounding tissues, so that blood flow is impeded at these
points. For instance, the veins from the arms are compressed by their sharp angulations over the first rib.
Also, the pressure in the neck veins often falls so low
that the atmospheric pressure on the outside of the
neck causes these veins to collapse. Finally, veins
coursing through the abdomen are often compressed
by different organs and by the intra-abdominal pressure, so that they usually are at least partially collapsed
to an ovoid or slitlike state. For these reasons, the
large veins do usually offer some resistance to blood
flow, and because of this, the pressure in the more
Chapter 15
177
Vascular Distensibility and Functions of the Arterial and Venous Systems
Sagittal sinus
-1 0 mm
0 mm
0 mm
Atmospheric
pressure
collapse in neck
+ 6 mm
+ 8 mm
Rib collapse
Axillary collapse
Intrathoracic
pressure = - 4 mm Hg
+ 2 2 mm
+ 3 5 mm
Abdominal
pressure
collapse
+ 4 0 mm
Figure 15–9
Compression points that tend to collapse the veins entering the
thorax.
peripheral small veins in a person lying down is usually
+4 to +6 mm Hg greater than the right atrial pressure.
Effect of High Right Atrial Pressure on Peripheral Venous Pressure. When the right atrial pressure rises above its
normal value of 0 mm Hg, blood begins to back up in
the large veins. This enlarges the veins, and even the
collapse points in the veins open up when the right
atrial pressure rises above +4 to +6 mm Hg. Then, as
the right atrial pressure rises still further, the additional increase causes a corresponding rise in peripheral venous pressure in the limbs and elsewhere.
Because the heart must be weakened greatly to cause
a rise in right atrial pressure as high as +4 to +6 mm
Hg, one often finds that the peripheral venous pressure is not noticeably elevated even in the early stages
of heart failure.
Effect of Intra-abdominal Pressure on Venous Pressures of the
Leg. The pressure in the abdominal cavity of a recum-
bent person normally averages about +6 mm Hg,
but it can rise to +15 to +30 mm Hg as a result of
pregnancy, large tumors, or excessive fluid (called
“ascites”) in the abdominal cavity. When the intraabdominal pressure does rise, the pressure in the veins
of the legs must rise above the abdominal pressure
before the abdominal veins will open and allow
the blood to flow from the legs to the heart. Thus,
if the intra-abdominal pressure is +20 mm Hg, the
lowest possible pressure in the femoral veins is also
+20 mm Hg.
Effect of Gravitational Pressure on Venous
Pressure
In any body of water that is exposed to air, the pressure at the surface of the water is equal to atmospheric
+ 9 0 mm
Figure 15–10
Effect of gravitational pressure on the venous pressures throughout the body in the standing person.
pressure, but the pressure rises 1 mm Hg for each 13.6
millimeters of distance below the surface. This pressure results from the weight of the water and therefore is called gravitational pressure or hydrostatic
pressure.
Gravitational pressure also occurs in the vascular
system of the human being because of weight of the
blood in the vessels, as shown in Figure 15–10. When
a person is standing, the pressure in the right atrium
remains about 0 mm Hg because the heart pumps into
the arteries any excess blood that attempts to accumulate at this point. However, in an adult who is standing absolutely still, the pressure in the veins of the feet
is about +90 mm Hg simply because of the gravitational weight of the blood in the veins between
the heart and the feet. The venous pressures at other
levels of the body are proportionately between 0 and
90 mm Hg.
In the arm veins, the pressure at the level of the top
rib is usually about +6 mm Hg because of compression
of the subclavian vein as it passes over this rib. The
gravitational pressure down the length of the arm then
is determined by the distance below the level of this
rib. Thus, if the gravitational difference between the
178
Unit IV
The Circulation
level of the rib and the hand is +29 mm Hg, this gravitational pressure is added to the +6 mm Hg pressure
caused by compression of the vein as it crosses the rib,
making a total of +35 mm Hg pressure in the veins of
the hand.
The neck veins of a person standing upright collapse
almost completely all the way to the skull because of
atmospheric pressure on the outside of the neck. This
collapse causes the pressure in these veins to remain
at zero along their entire extent. The reason for this is
that any tendency for the pressure to rise above this
level opens the veins and allows the pressure to fall
back to zero because of flow of the blood. Conversely,
any tendency for the neck vein pressure to fall below
zero collapses the veins still more, which further
increases their resistance and again returns the pressure back to zero.
The veins inside the skull, on the other hand, are in
a noncollapsible chamber (the skull cavity) so that
they cannot collapse. Consequently, negative pressure
can exist in the dural sinuses of the head; in the standing position, the venous pressure in the sagittal sinus
at the top of the brain is about -10 mm Hg because of
the hydrostatic “suction” between the top of the skull
and the base of the skull. Therefore, if the sagittal sinus
is opened during surgery, air can be sucked immediately into the venous system; the air may even pass
downward to cause air embolism in the heart, and
death can ensue.
Effect of the Gravitational Factor on Arterial and Other Pressures. The gravitational factor also affects pressures in
the peripheral arteries and capillaries, in addition to its
effects in the veins. For instance, a standing person who
has a mean arterial pressure of 100 mm Hg at the level
of the heart has an arterial pressure in the feet of about
190 mm Hg. Therefore, when one states that the arterial pressure is 100 mm Hg, this generally means that
this is the pressure at the gravitational level of the
heart but not necessarily elsewhere in the arterial
vessels.
Venous Valves and the “Venous Pump”:
Their Effects on Venous Pressure
Were it not for valves in the veins, the gravitational
pressure effect would cause the venous pressure in the
feet always to be about +90 mm Hg in a standing adult.
However, every time one moves the legs, one tightens
the muscles and compresses the veins in or adjacent to
the muscles, and this squeezes the blood out of the
veins. But the valves in the veins, shown in Figure
15–11, are arranged so that the direction of venous
blood flow can be only toward the heart. Consequently, every time a person moves the legs or even
tenses the leg muscles, a certain amount of venous
blood is propelled toward the heart. This pumping
system is known as the “venous pump” or “muscle
pump,” and it is efficient enough that under ordinary
circumstances, the venous pressure in the feet of a
walking adult remains less than +20 mm Hg.
If a person stands perfectly still, the venous pump
does not work, and the venous pressures in the lower
Deep vein
Perforating
vein
Superficial
vein
Valve
Figure 15–11
Venous valves of the leg.
legs increase to the full gravitational value of 90 mm
Hg in about 30 seconds. The pressures in the capillaries also increase greatly, causing fluid to leak from the
circulatory system into the tissue spaces. As a result,
the legs swell, and the blood volume diminishes.
Indeed, 10 to 20 per cent of the blood volume can be
lost from the circulatory system within the 15 to 30
minutes of standing absolutely still, as often occurs
when a soldier is made to stand at rigid attention.
Venous Valve Incompetence Causes “Varicose” Veins. The
valves of the venous system frequently become
“incompetent” or sometimes even are destroyed. This
is especially true when the veins have been overstretched by excess venous pressure lasting weeks or
months, as occurs in pregnancy or when one stands
most of the time. Stretching the veins increases their
cross-sectional areas, but the leaflets of the valves do
not increase in size. Therefore, the leaflets of the valves
no longer close completely. When this develops, the
pressure in the veins of the legs increases greatly
because of failure of the venous pump; this further
increases the sizes of the veins and finally destroys the
function of the valves entirely. Thus, the person develops “varicose veins,” which are characterized by large,
bulbous protrusions of the veins beneath the skin of
the entire leg, particularly the lower leg.
Whenever people with varicose veins stand for more
than a few minutes, the venous and capillary pressures
become very high, and leakage of fluid from the capillaries causes constant edema in the legs. The edema
in turn prevents adequate diffusion of nutritional
materials from the capillaries to the muscle and skin
cells, so that the muscles become painful and weak,
and the skin frequently becomes gangrenous and
ulcerates. The best treatment for such a condition is
Chapter 15
Vascular Distensibility and Functions of the Arterial and Venous Systems
continual elevation of the legs to a level at least as high
as the heart. Tight binders on the legs also can be of
considerable assistance in preventing the edema and
its sequelae.
Clinical Estimation of Venous Pressure. The venous pressure
often can be estimated by simply observing the degree
of distention of the peripheral veins—especially of the
neck veins. For instance, in the sitting position, the neck
veins are never distended in the normal quietly resting
person. However, when the right atrial pressure becomes increased to as much as +10 mm Hg, the lower
veins of the neck begin to protrude; and at +15 mm Hg
atrial pressure essentially all the veins in the neck
become distended.
Direct Measurement of Venous Pressure and
Right Atrial Pressure
Venous pressure can also be measured with ease by
inserting a needle directly into a vein and connecting it
to a pressure recorder. The only means by which right
atrial pressure can be measured accurately is by inserting a catheter through the peripheral veins and into the
right atrium. Pressures measured through such central
venous catheters are used almost routinely in some types
of hospitalized cardiac patients to provide constant
assessment of heart pumping ability.
Pressure Reference Level for Measuring Venous and Other
Circulatory Pressures
In discussions up to this point, we often have spoken of
right atrial pressure as being 0 mm Hg and arterial pressure as being 100 mm Hg, but we have not stated the
gravitational level in the circulatory system to which this
pressure is referred. There is one point in the circulatory system at which gravitational pressure factors
caused by changes in body position of a healthy person
usually do not affect the pressure measurement by more
than 1 to 2 mm Hg. This is at or near the level of the tricuspid valve, as shown by the crossed axes in Figure
15–12. Therefore, all circulatory pressure measurements
discussed in this text are referred to this level, which is
called the reference level for pressure measurement.
The reason for lack of gravitational effects at the
tricuspid valve is that the heart automatically prevents
Right ventricle
Right atrium
Natural reference
point
Figure 15–12
Reference point for circulatory pressure measurement (located
near the tricuspid valve).
179
significant gravitational changes in pressure at this point
in the following way:
If the pressure at the tricuspid valve rises slightly
above normal, the right ventricle fills to a greater extent
than usual, causing the heart to pump blood more
rapidly and therefore to decrease the pressure at the tricuspid valve back toward the normal mean value. Conversely, if the pressure falls, the right ventricle fails to fill
adequately, its pumping decreases, and blood dams up
in the venous system until the pressure at the tricuspid
level again rises to the normal value. In other words,
the heart acts as a feedback regulator of pressure at the
tricuspid valve.
When a person is lying on his or her back, the tricuspid valve is located at almost exactly 60 per cent of the
chest thickness in front of the back. This is the zero pressure reference level for a person lying down.
Blood Reservoir Function of the Veins
As pointed out in Chapter 14, more than 60 per cent
of all the blood in the circulatory system is usually in
the veins. For this reason and also because the veins
are so compliant, it is said that the venous system
serves as a blood reservoir for the circulation.
When blood is lost from the body and the arterial
pressure begins to fall, nervous signals are elicited
from the carotid sinuses and other pressure-sensitive
areas of the circulation, as discussed in Chapter 18.
These in turn elicit nerve signals from the brain and
spinal cord mainly through sympathetic nerves to the
veins, causing them to constrict. This takes up much of
the slack in the circulatory system caused by the lost
blood. Indeed, even after as much as 20 per cent of the
total blood volume has been lost, the circulatory
system often functions almost normally because of this
variable reservoir function of the veins.
Specific Blood Reservoirs. Certain portions of the circulatory system are so extensive and/or so compliant that
they are called “specific blood reservoirs.” These
include (1) the spleen, which sometimes can decrease
in size sufficiently to release as much as 100 milliliters
of blood into other areas of the circulation; (2) the
liver, the sinuses of which can release several hundred
milliliters of blood into the remainder of the circulation; (3) the large abdominal veins, which can contribute as much as 300 milliliters; and (4) the venous
plexus beneath the skin, which also can contribute
several hundred milliliters. The heart and the lungs,
although not parts of the systemic venous reservoir
system, must also be considered blood reservoirs. The
heart, for instance, shrinks during sympathetic stimulation and in this way can contribute some 50 to 100
milliliters of blood; the lungs can contribute another
100 to 200 milliliters when the pulmonary pressures
decrease to low values.
The Spleen as a Reservoir for Storing Red Blood Cells.
Figure 15–13 shows that the spleen has two separate
areas for storing blood: the venous sinuses and the
pulp. The sinuses can swell the same as any other part
of the venous system and store whole blood.
180
Unit IV
The Circulation
Pulp
Capillaries
lined with similar cells. These cells function as part of a
cleansing system for the blood, acting in concert with a
similar system of reticuloendothelial cells in the venous
sinuses of the liver. When the blood is invaded by infectious agents, the reticuloendothelial cells of the spleen
rapidly remove debris, bacteria, parasites, and so forth.
Also, in many chronic infectious processes, the spleen
enlarges in the same manner that lymph nodes enlarge
and then performs its cleansing function even more
avidly.
Venous sinuses
Vein
Artery
Figure 15–13
Functional structures of the spleen. (Courtesy of Dr. Don W.
Fawcett, Montana.)
In the splenic pulp, the capillaries are so permeable
that whole blood, including the red blood cells, oozes
through the capillary walls into a trabecular mesh,
forming the red pulp. The red cells are trapped by the
trabeculae, while the plasma flows on into the venous
sinuses and then into the general circulation. As a consequence, the red pulp of the spleen is a special reservoir that contains large quantities of concentrated red
blood cells. These can then be expelled into the general
circulation whenever the sympathetic nervous system
becomes excited and causes the spleen and its vessels
to contract. As much as 50 milliliters of concentrated
red blood cells can be released into the circulation,
raising the hematocrit 1 to 2 per cent.
In other areas of the splenic pulp are islands of white
blood cells, which collectively are called the white pulp.
Here lymphoid cells are manufactured similar to those
manufactured in the lymph nodes. They are part of the
body’s immune system, described in Chapter 34.
Blood-Cleansing Function of the Spleen—Removal of Old Cells
Blood cells passing through the splenic pulp before
entering the sinuses undergo thorough squeezing.
Therefore, it is to be expected that fragile red blood cells
would not withstand the trauma. For this reason, many
of the red blood cells destroyed in the body have their
final demise in the spleen. After the cells rupture, the
released hemoglobin and the cell stroma are digested
by the reticuloendothelial cells of the spleen, and the
products of digestion are mainly reused by the body as
nutrients, often for making new blood cells.
Reticuloendothelial Cells of the Spleen
The pulp of the spleen contains many large phagocytic
reticuloendothelial cells, and the venous sinuses are
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Verdecchia P, Angeli F, Gattobigio R: Clinical usefulness of
ambulatory blood pressure monitoring. J Am Soc Nephrol
15(Suppl 1):S30, 2004.
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A
P
T
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6
The Microcirculation and the
Lymphatic System: Capillary
Fluid Exchange, Interstitial Fluid,
and Lymph Flow
The most purposeful function of the circulation
occurs in the microcirculation: this is transport of
nutrients to the tissues and removal of cell excreta.
The small arterioles control blood flow to each
tissue area, and local conditions in the tissues in turn
control the diameters of the arterioles. Thus, each
tissue, in most instances, controls its own blood flow
in relation to its individual needs, a subject that is
discussed in detail in Chapter 17.
The walls of the capillaries are extremely thin, constructed of single-layer,
highly permeable endothelial cells. Therefore, water, cell nutrients, and cell
excreta can all interchange quickly and easily between the tissues and the circulating blood.
The peripheral circulation of the whole body has about 10 billion capillaries
with a total surface area estimated to be 500 to 700 square meters (about oneeighth the surface area of a football field). Indeed, it is rare that any single functional cell of the body is more than 20 to 30 micrometers away from a capillary.
Structure of the Microcirculation and
Capillary System
The microcirculation of each organ is organized specifically to serve that organ’s
needs. In general, each nutrient artery entering an organ branches six to eight times
before the arteries become small enough to be called arterioles, which generally
have internal diameters of only 10 to 15 micrometers. Then the arterioles themselves
branch two to five times, reaching diameters of 5 to 9 micrometers at their ends
where they supply blood to the capillaries.
The arterioles are highly muscular, and their diameters can change manyfold. The
metarterioles (the terminal arterioles) do not have a continuous muscular coat, but
smooth muscle fibers encircle the vessel at intermittent points, as shown in Figure
16–1 by the black dots on the sides of the metarteriole.
At the point where each true capillary originates from a metarteriole, a smooth
muscle fiber usually encircles the capillary. This is called the precapillary sphincter.
This sphincter can open and close the entrance to the capillary.
The venules are larger than the arterioles and have a much weaker muscular coat.
Yet it must be remembered that the pressure in the venules is much less than that
in the arterioles, so that the venules still can contract considerably despite the weak
muscle.
This typical arrangement of the capillary bed is not found in all parts of the body;
however, some similar arrangement serves the same purposes. Most important, the
metarterioles and the precapillary sphincters are in close contact with the tissues
they serve. Therefore, the local conditions of the tissues—the concentrations of
nutrients, end products of metabolism, hydrogen ions, and so forth—can cause direct
effects on the vessels in controlling local blood flow in each small tissue area.
Structure of the Capillary Wall. Figure 16–2 shows the ultramicroscopic structure of
typical endothelial cells in the capillary wall as found in most organs of the body,
especially in muscles and connective tissue. Note that the wall is composed of
181
182
Unit IV
Metarteriole
Arteriole
The Circulation
Preferential channel
Precapillary
sphincter
True capillaries
Venule
Figure 16–1
Structure of the mesenteric capillary bed. (Redrawn from Zweifach
BW: Factors Regulating Blood Pressure. New York: Josiah Macy,
Jr., Foundation, 1950.)
Endothelial
cell
Vesicular
channel??
Plasmalemmal
vesicles
Intercellular
cleft
Basement
membrane
Figure 16–2
Structure of the capillary wall. Note especially the intercellular cleft
at the junction between adjacent endothelial cells; it is believed
that most water-soluble substances diffuse through the capillary
membrane along the clefts.
a unicellular layer of endothelial cells and is surrounded by a very thin basement membrane on the
outside of the capillary. The total thickness of the capillary wall is only about 0.5 micrometer. The internal
diameter of the capillary is 4 to 9 micrometers, barely
large enough for red blood cells and other blood cells
to squeeze through.
“Pores” in the Capillary Membrane. Studying Figure 16–2,
one sees two very small passageways connecting the
interior of the capillary with the exterior. One of these
is an intercellular cleft, which is the thin-slit, curving
channel that lies at the bottom of the figure between
adjacent endothelial cells. Each cleft is interrupted
periodically by short ridges of protein attachments
that hold the endothelial cells together, but between
these ridges fluid can percolate freely through the
cleft. The cleft normally has a uniform spacing with a
width of about 6 to 7 nanometers (60 to 70 angstroms),
slightly smaller than the diameter of an albumin
protein molecule.
Because the intercellular clefts are located only at
the edges of the endothelial cells, they usually represent no more than 1/1000 of the total surface area of
the capillary wall. Nevertheless, the rate of thermal
motion of water molecules as well as most watersoluble ions and small solutes is so rapid that all of
these diffuse with ease between the interior and exterior of the capillaries through these “slit-pores,” the
intercellular clefts.
Also present in the endothelial cells are many
minute plasmalemmal vesicles. These form at one
surface of the cell by imbibing small packets of plasma
or extracellular fluid. They can then move slowly
through the endothelial cell. It also has been postulated that some of these vesicles coalesce to form
vesicular channels all the way through the endothelial
cell, which is demonstrated to the right in Figure 16–2.
However, careful measurements in laboratory animals
probably have proved that these vesicular forms of
transport are quantitatively of little importance.
Special Types of “Pores” Occur in the Capillaries of Certain
Organs. The “pores” in the capillaries of some organs
have special characteristics to meet the peculiar needs
of the organs. Some of these characteristics are as
follows:
1. In the brain, the junctions between the capillary
endothelial cells are mainly “tight” junctions that
allow only extremely small molecules such as
water, oxygen, and carbon dioxide to pass into or
out of the brain tissues.
2. In the liver, the opposite is true. The clefts between
the capillary endothelial cells are wide open, so
that almost all dissolved substances of the plasma,
including the plasma proteins, can pass from the
blood into the liver tissues.
3. The pores of the gastrointestinal capillary
membranes are midway between those of the
muscles and those of the liver.
4. In the glomerular tufts of the kidney, numerous
small oval windows called fenestrae penetrate all
the way through the middle of the endothelial
cells, so that tremendous amounts of very small
molecular and ionic substances (but not the large
molecules of the plasma proteins) can filter through
the glomeruli without having to pass through the
clefts between the endothelial cells.
Flow of Blood in the
Capillaries—Vasomotion
Blood usually does not flow continuously through the
capillaries. Instead, it flows intermittently, turning on
and off every few seconds or minutes. The cause of this
intermittency is the phenomenon called vasomotion,
which means intermittent contraction of the metarterioles and precapillary sphincters (and sometimes
even the very small arterioles as well).
Regulation of Vasomotion. The most important factor
found thus far to affect the degree of opening and
closing of the metarterioles and precapillary sphincters is the concentration of oxygen in the tissues. When
Chapter 16
183
The Microcirculation and the Lymphatic System
the rate of oxygen usage by the tissue is great so that
tissue oxygen concentration decreases below normal,
the intermittent periods of capillary blood flow occur
more often, and the duration of each period of flow
lasts longer, thereby allowing the capillary blood to
carry increased quantities of oxygen (as well as other
nutrients) to the tissues. This effect, along with multiple other factors that control local tissue blood flow, is
discussed in Chapter 17.
Arterial end
Blood capillary
Venous end
Average Function of the
Capillary System
Despite the fact that blood flow through each capillary
is intermittent, so many capillaries are present in the
tissues that their overall function becomes averaged.
That is, there is an average rate of blood flow through
each tissue capillary bed, an average capillary pressure
within the capillaries, and an average rate of transfer of
substances between the blood of the capillaries and the
surrounding interstitial fluid. In the remainder of this
chapter, we will be concerned with these averages,
although one must remember that the average functions are, in reality, the functions of literally billions of
individual capillaries, each operating intermittently in
response to local conditions in the tissues.
Exchange of Water, Nutrients,
and Other Substances
Between the Blood and
Interstitial Fluid
Diffusion Through the
Capillary Membrane
By far the most important means by which substances
are transferred between the plasma and the interstitial
fluid is diffusion. Figure 16–3 demonstrates this
process, showing that as the blood flows along the
lumen of the capillary, tremendous numbers of water
molecules and dissolved particles diffuse back and
forth through the capillary wall, providing continual
mixing between the interstitial fluid and the plasma.
Diffusion results from thermal motion of the water
molecules and dissolved substances in the fluid, the
different molecules and ions moving first in one direction and then another, bouncing randomly in every
direction.
Lipid-Soluble Substances Can Diffuse Directly Through the Cell
Membranes of the Capillary Endothelium. If a substance is
lipid soluble, it can diffuse directly through the cell
membranes of the capillary without having to go
through the pores. Such substances include oxygen and
carbon dioxide. Because these substances can permeate all areas of the capillary membrane, their rates
of transport through the capillary membrane are
many times faster than the rates for lipid-insoluble
Lymphatic
capillary
Figure 16–3
Diffusion of fluid molecules and dissolved substances between
the capillary and interstitial fluid spaces.
substances, such as sodium ions and glucose that can
go only through the pores.
Water-Soluble, Non-Lipid-Soluble Substances Diffuse Only
Through Intercellular “Pores” in the Capillary Membrane.
Many substances needed by the tissues are soluble in
water but cannot pass through the lipid membranes of
the endothelial cells; such substances include water
molecules themselves, sodium ions, chloride ions, and
glucose. Despite the fact that not more than 1/1000 of
the surface area of the capillaries is represented by the
intercellular clefts between the endothelial cells, the
velocity of thermal molecular motion in the clefts is so
great that even this small area is sufficient to allow
tremendous diffusion of water and water-soluble substances through these cleft-pores. To give one an idea
of the rapidity with which these substances diffuse, the
rate at which water molecules diffuse through the capillary membrane is about 80 times as great as the rate at
which plasma itself flows linearly along the capillary.
That is, the water of the plasma is exchanged with
the water of the interstitial fluid 80 times before the
plasma can flow the entire distance through the
capillary.
Effect of Molecular Size on Passage Through the
Pores. The width of the capillary intercellular cleft-
pores, 6 to 7 nanometers, is about 20 times the diameter of the water molecule, which is the smallest
molecule that normally passes through the capillary
pores. Conversely, the diameters of plasma protein
molecules are slightly greater than the width of the
pores. Other substances, such as sodium ions, chloride
ions, glucose, and urea, have intermediate diameters.
Therefore, the permeability of the capillary pores for
184
Unit IV
The Circulation
Table 16–1
Relative Permeability of Skeletal Muscle Capillary Pores to
Different-Sized Molecules
Substance
Water
NaCl
Urea
Glucose
Sucrose
Inulin
Myoglobin
Hemoglobin
Albumin
Molecular Weight
Permeability
18
58.5
60
180
342
5,000
17,600
68,000
69,000
1.00
0.96
0.8
0.6
0.4
0.2
0.03
0.01
0.001
Free fluid
vesicles
Rivulets
of free
fluid
Data from Pappenheimer JR: Passage of molecules through capillary walls.
Physiol Rev 33:387, 1953.
Capillary
different substances varies according to their molecular diameters.
Table 16–1 gives the relative permeabilities of the
capillary pores in skeletal muscle for substances commonly encountered, demonstrating, for instance, that
the permeability for glucose molecules is 0.6 times that
for water molecules, whereas the permeability for
albumin molecules is very, very slight, only 1/1000 that
for water molecules.
A word of caution must be issued at this point.
The capillaries in different tissues have extreme differences in their permeabilities. For instance, the membrane of the liver capillary sinusoids is so permeable
that even plasma proteins pass freely through these
walls, almost as easily as water and other substances.
Also, the permeability of the renal glomerular membrane for water and electrolytes is about 500 times the
permeability of the muscle capillaries, but this is not
true for the plasma proteins; for these, the capillary
permeabilities are very slight, as in other tissues and
organs. When we study these different organs later in
this text, it should become clear why some tissues—the
liver, for instance—require greater degrees of capillary
permeability than others to transfer tremendous
amounts of nutrients between the blood and liver
parenchymal cells, and the kidneys to allow filtration
of large quantities of fluid for formation of urine.
Effect of Concentration Difference on Net Rate of Diffusion
Through the Capillary Membrane. The “net” rate of diffu-
sion of a substance through any membrane is proportional to the concentration difference of the substance
between the two sides of the membrane. That is, the
greater the difference between the concentrations of
any given substance on the two sides of the capillary
membrane, the greater the net movement of the substance in one direction through the membrane. For
instance, the concentration of oxygen in capillary
blood is normally greater than in the interstitial fluid.
Therefore, large quantities of oxygen normally move
from the blood toward the tissues. Conversely, the concentration of carbon dioxide is greater in the tissues
than in the blood, which causes excess carbon dioxide
Collagen fiber
bundles
Proteoglycan
filaments
Figure 16–4
Structure of the interstitium. Proteoglycan filaments are everywhere in the spaces between the collagen fiber bundles. Free fluid
vesicles and small amounts of free fluid in the form of rivulets
occasionally also occur.
to move into the blood and to be carried away from
the tissues.
The rates of diffusion through the capillary membranes of most nutritionally important substances are
so great that only slight concentration differences
suffice to cause more than adequate transport between
the plasma and interstitial fluid. For instance, the concentration of oxygen in the interstitial fluid immediately outside the capillary is no more than a few per
cent less than its concentration in the plasma of the
blood, yet this slight difference causes enough oxygen
to move from the blood into the interstitial spaces to
provide all the oxygen required for tissue metabolism,
often as much as several liters of oxygen per minute
during very active states of the body.
The Interstitium and
Interstitial Fluid
About one sixth of the total volume of the body consists of spaces between cells, which collectively are
called the interstitium. The fluid in these spaces is the
interstitial fluid.
The structure of the interstitium is shown in Figure
16–4. It contains two major types of solid structures:
(1) collagen fiber bundles and (2) proteoglycan filaments. The collagen fiber bundles extend long distances in the interstitium. They are extremely strong
and therefore provide most of the tensional strength
of the tissues. The proteoglycan filaments, however, are
extremely thin coiled or twisted molecules composed
of about 98 per cent hyaluronic acid and 2 per cent
protein. These molecules are so thin that they can
never be seen with a light microscope and are difficult
Chapter 16
185
The Microcirculation and the Lymphatic System
to demonstrate even with the electron microscope.
Nevertheless, they form a mat of very fine reticular filaments aptly described as a “brush pile.”
Capillary
pressure
Plasma colloid
osmotic pressure
(Pc)
(Pp)
Interstitial
fluid pressure
Interstitial fluid
colloid osmotic pressure
(Pif)
(Pif)
“Gel” in the Interstitium. The fluid in the interstitium is
derived by filtration and diffusion from the capillaries.
It contains almost the same constituents as plasma
except for much lower concentrations of proteins
because proteins do not pass outward through the
pores of the capillaries with ease. The interstitial fluid
is entrapped mainly in the minute spaces among the
proteoglycan filaments. This combination of proteoglycan filaments and fluid entrapped within them has
the characteristics of a gel and therefore is called tissue
gel.
Because of the large number of proteoglycan filaments, it is difficult for fluid to flow easily through the
tissue gel. Instead, it mainly diffuses through the gel;
that is, it moves molecule by molecule from one place
to another by kinetic, thermal motion rather than by
large numbers of molecules moving together.
Diffusion through the gel occurs about 95 to 99 per
cent as rapidly as it does through free fluid. For the
short distances between the capillaries and the tissue
cells, this diffusion allows rapid transport through the
interstitium not only of water molecules but also of
electrolytes, small molecular weight nutrients, cellular
excreta, oxygen, carbon dioxide, and so forth.
“Free” Fluid in the Interstitium. Although almost all the
fluid in the interstitium normally is entrapped within
the tissue gel, occasionally small rivulets of “free” fluid
and small free fluid vesicles are also present, which
means fluid that is free of the proteoglycan molecules
and therefore can flow freely. When a dye is injected
into the circulating blood, it often can be seen to flow
through the interstitium in the small rivulets, usually
coursing along the surfaces of collagen fibers or surfaces of cells.
The amount of “free” fluid present in normal tissues
is slight, usually much less than 1 per cent. Conversely,
when the tissues develop edema, these small pockets
and rivulets of free fluid expand tremendously until one
half or more of the edema fluid becomes freely flowing
fluid independent of the proteoglycan filaments.
Fluid Filtration Across
Capillaries Is Determined
by Hydrostatic and Colloid
Osmotic Pressures, and
Capillary Filtration Coefficient
The hydrostatic pressure in the capillaries tends to
force fluid and its dissolved substances through the
capillary pores into the interstitial spaces. Conversely,
osmotic pressure caused by the plasma proteins
(called colloid osmotic pressure) tends to cause fluid
movement by osmosis from the interstitial spaces
into the blood. This osmotic pressure exerted by
the plasma proteins normally prevents significant loss
Figure 16–5
Fluid pressure and colloid osmotic pressure forces operate at the
capillary membrane, tending to move fluid either outward or
inward through the membrane pores.
of fluid volume from the blood into the interstitial
spaces.
Also important is the lymphatic system, which
returns to the circulation the small amounts of excess
protein and fluid that leak from the blood into the
interstitial spaces. In the remainder of this chapter, we
discuss the mechanisms that control capillary filtration
and lymph flow function together to regulate the
respective volumes of the plasma and the interstitial
fluid.
Four Primary Hydrostatic and Colloid Osmotic Forces Determine
Fluid Movement Through the Capillary Membrane. Figure
16–5 shows the four primary forces that determine
whether fluid will move out of the blood into the interstitial fluid or in the opposite direction. These forces,
called “Starling forces” in honor of the physiologist
who first demonstrated their importance, are:
1. The capillary pressure (Pc), which tends to force
fluid outward through the capillary membrane.
2. The interstitial fluid pressure (Pif), which tends
to force fluid inward through the capillary
membrane when Pif is positive but outward when
Pif is negative.
3. The capillary plasma colloid osmotic pressure
(Pp), which tends to cause osmosis of fluid inward
through the capillary membrane.
4. The interstitial fluid colloid osmotic pressure (Pif),
which tends to cause osmosis of fluid outward
through the capillary membrane.
If the sum of these forces, the net filtration pressure,
is positive, there will be a net fluid filtration across the
capillaries. If the sum of the Starling forces is negative,
there will be a net fluid absorption from the interstitial spaces into the capillaries. The net filtration pressure (NFP) is calculated as:
NFP = Pc - Pif - Pp + Pif
As discussed later, the NFP is slightly positive under
normal conditions, resulting in a net filtration of fluid
across the capillaries into the interstitial space in most
organs. The rate of fluid filtration in a tissue is also
determined by the number and size of the pores in
each capillary as well as the number of capillaries in
186
Unit IV
The Circulation
which blood is flowing. These factors are usually
expressed together as the capillary filtration coefficient
(Kf). The Kf is therefore a measure of the capacity of
the capillary membranes to filter water for a given
NFP and is usually expressed as ml/min per mm Hg
net filtration pressure.
The rate of capillary fluid filtration is therefore
determined as:
Filtration = K f ¥ NFP
In the following sections we discuss in detail each
of the forces that determine the rate of capillary fluid
filtration.
Gut
Arterial pressure
Capillary Hydrostatic Pressure
Micropipette Method for Measuring Capillary Pressure. To
measure pressure in a capillary by cannulation, a
microscopic glass pipette is thrust directly into the capillary, and the pressure is measured by an appropriate
micromanometer system. Using this method, capillary
pressures have been measured in capillaries of
exposed tissues of animals and in large capillary loops
of the eponychium at the base of the fingernail in
humans. These measurements have given pressures of
30 to 40 mm Hg in the arterial ends of the capillaries,
10 to 15 mm Hg in the venous ends, and about 25 mm
Hg in the middle.
Isogravimetric Method for Indirectly Measuring “Functional”
Capillary Pressure. Figure 16–6 demonstrates an iso-
gravimetric method for indirectly estimating capillary
pressure. This figure shows a section of gut held up by
one arm of a gravimetric balance. Blood is perfused
through the blood vessels of the gut wall. When the
arterial pressure is decreased, the resulting decrease in
capillary pressure allows the osmotic pressure of the
plasma proteins to cause absorption of fluid out of the
gut wall and makes the weight of the gut decrease. This
immediately causes displacement of the balance arm.
To prevent this weight decrease, the venous pressure
is increased an amount sufficient to overcome the
effect of decreasing the arterial pressure. In other
words, the capillary pressure is kept constant while
simultaneously (1) decreasing the arterial pressure
and (2) increasing the venous pressure.
In the graph in the lower part of the figure, the
changes in arterial and venous pressures that exactly
nullify all weight changes are shown. The arterial and
venous lines meet each other at a value of 17 mm Hg.
Therefore, the capillary pressure must have remained
at this same level of 17 mm Hg throughout these
maneuvers; otherwise, either filtration or absorption of
100
80
Pressure
Two experimental methods have been used to estimate the capillary hydrostatic pressure: (1) direct
micropipette cannulation of the capillaries, which has
given an average mean capillary pressure of about 25
mm Hg, and (2) indirect functional measurement of the
capillary pressure, which has given a capillary pressure
averaging about 17 mm Hg.
Venous pressure
Ar
60
ter
ial
40
Capillary pressure
= 17 mm Hg
20
s
Venou
0
100
Arterial pressure
50
– venous pressure
0
Figure 16–6
Isogravimetric method for measuring capillary pressure.
fluid through the capillary walls would have occurred.
Thus, in a roundabout way, the “functional” capillary
pressure is measured to be about 17 mm Hg.
Why Is the Functional Capillary Pressure Lower than Capillary
Pressure Measured by the Micropipette Method? It is clear
that the aforementioned two methods do not give the
same capillary pressure. However, the isogravimetric
method determines the capillary pressure that exactly
balances all the forces tending to move fluid into or
out of the capillaries. Because such a balance of forces
is the normal state, the average functional capillary
pressure must be close to the pressure measured by
the isogravimetric method. Therefore, one is justified
in believing that the true functional capillary pressure
averages about 17 mm Hg.
It is easy to explain why the cannulation methods
give higher pressure values. The most important
reason is that these measurements usually are made
in capillaries whose arterial ends are open and when
Chapter 16
The Microcirculation and the Lymphatic System
blood is actively flowing into the capillary. However, it
should be recalled from the earlier discussion of capillary vasomotion that the metarterioles and precapillary sphincters normally are closed during a large part
of the vasomotion cycle. When closed, the pressure in
the capillaries beyond the closures should be almost
equal to the pressure at the venous ends of the capillaries, about 10 mm Hg. Therefore, when averaged
over a period of time, one would expect the functional
mean capillary pressure to be much nearer to the pressure in the venous ends of the capillaries than to the
pressure in the arterial ends.
There are two other reasons why the functional capillary pressure is less than the values measured by cannulation. First, there are far more capillaries nearer to
the venules than to the arterioles. Second, the venous
capillaries are several times as permeable as the arterial capillaries. Both of these effects further decrease
the functional capillary pressure to a lower value.
Interstitial Fluid Hydrostatic Pressure
As is true for the measurement of capillary pressure,
there are several methods for measuring interstitial
fluid pressure, and each of these gives slightly different values but usually values that are a few millimeters of mercury less than atmospheric pressure, that is,
values called negative interstitial fluid pressure. The
methods most widely used have been (1) direct cannulation of the tissues with a micropipette, (2) measurement of the pressure from implanted perforated
capsules, and (3) measurement of the pressure from a
cotton wick inserted into the tissue.
Measurement of Interstitial Fluid Pressure Using the
Micropipette. The same type of micropipette used for
measuring capillary pressure can also be used in some
tissues for measuring interstitial fluid pressure. The tip
of the micropipette is about 1 micrometer in diameter,
but even this is 20 or more times larger than the sizes
of the spaces between the proteoglycan filaments of the
interstitium. Therefore, the pressure that is measured is
probably the pressure in a free fluid pocket.
The first pressures measured using the micropipette
method ranged from -1 to +2 mm Hg but were usually
slightly positive. With experience and improved equipment for making such measurements, more recent pressures have averaged about -2 mm Hg, giving average
pressure values in loose tissues, such as skin, that are
slightly less than atmospheric pressure.
Measurement of Interstitial Free Fluid Pressure in Implanted Perforated Hollow Capsules. Interstitial free fluid pressure
measured by this method when using 2-centimeter
diameter capsules in normal loose subcutaneous tissue
averages about -6 mm Hg, but with smaller capsules,
the values are not greatly different from the -2 mm Hg
measured by the micropipette in Figure 16-7.
Measurement of Interstitial Free Fluid Pressure by Means of a
Cotton Wick. Still another method is to insert into a
tissue a small Teflon tube with about eight cotton fibers
187
protruding from its end. The cotton fibers form a “wick”
that makes excellent contact with the tissue fluids and
transmits interstitial fluid pressure into the Teflon tube:
the pressure can then be measured from the tube by
usual manometric means. Pressures measured by this
technique in loose subcutaneous tissue also have been
negative, usually measuring -1 to -3 mm Hg.
Interstitial Fluid Pressures in Tightly
Encased Tissues
Some tissues of the body are surrounded by tight
encasements, such as the cranial vault around the
brain, the strong fibrous capsule around the kidney, the
fibrous sheaths around the muscles, and the sclera
around the eye. In most of these, regardless of the
method used for measurement, the interstitial fluid
pressures are usually positive. However, these interstitial fluid pressures almost invariably are still less than
the pressures exerted on the outsides of the tissues by
their encasements. For instance, the cerebrospinal fluid
pressure surrounding the brain of an animal lying on
its side averages about +10 mm Hg, whereas the brain
interstitial fluid pressure averages about +4 to +6 mm
Hg. In the kidneys, the capsular pressure surrounding
the kidney averages about +13 mm Hg, whereas the
reported renal interstitial fluid pressures have averaged
about +6 mm Hg. Thus, if one remembers that the
pressure exerted on the skin is atmospheric pressure,
which is considered to be zero pressure, one might formulate a general rule that the normal interstitial fluid
pressure is usually several millimeters of mercury negative with respect to the pressure that surrounds each
tissue.
Is the True Interstitial Fluid Pressure in Loose
Subcutaneous Tissue Subatmospheric?
The concept that the interstitial fluid pressure is subatmospheric in many if not most tissues of the body
began with clinical observations that could not be
explained by the previously held concept that interstitial fluid pressure was always positive. Some of the pertinent observations are the following:
1. When a skin graft is placed on a concave surface
of the body, such as in an eye socket after
removal of the eye, before the skin becomes
attached to the sublying socket, fluid tends to
collect underneath the graft. Also, the skin
attempts to shorten, with the result that it tends to
pull it away from the concavity. Nevertheless,
some negative force underneath the skin causes
absorption of the fluid and usually literally pulls
the skin back into the concavity.
2. Less than 1 mm Hg of positive pressure is
required to inject tremendous volumes of fluid
into loose subcutaneous tissues, such as beneath
the lower eyelid, in the axillary space, and in the
scrotum. Amounts of fluid calculated to be more
than 100 times the amount of fluid normally in the
interstitial space, when injected into these areas,
cause no more than about 2 mm Hg of positive
pressure. The importance of these observations is
that they show that such tissues do not have
188
Unit IV
The Circulation
strong fibers that can prevent the accumulation of
fluid. Therefore, some other mechanism, such as a
negative fluid pressure system, must be available
to prevent such fluid accumulation.
3. In most natural cavities of the body where there
is free fluid in dynamic equilibrium with the
surrounding interstitial fluids, the pressures that
have been measured have been negative. Some of
these are the following:
Intrapleural space: -8 mm Hg
Joint synovial spaces: -4 to -6 mm Hg
Epidural space: -4 to -6 mm Hg
4. The implanted capsule for measuring the
interstitial fluid pressure can be used to record
dynamic changes in this pressure. The changes are
approximately those that one would calculate to
occur (1) when the arterial pressure is increased
or decreased, (2) when fluid is injected into the
surrounding tissue space, or (3) when a highly
concentrated colloid osmotic agent is injected into
the blood to absorb fluid from the tissue spaces. It
is not likely that these dynamic changes could be
recorded this accurately unless the capsule
pressure closely approximated the true interstitial
pressure.
Summary—An Average Value for Negative Interstitial Fluid
Pressure in Loose Subcutaneous Tissue. Although the
aforementioned different methods give slightly different values for interstitial fluid pressure, there currently
is a general belief among most physiologists that the
true interstitial fluid pressure in loose subcutaneous
tissue is slightly less subatmospheric, averaging about
-3 mm Hg.
Pumping by the Lymphatic System Is the
Basic Cause of the Negative Interstitial
Fluid Pressure
The lymphatic system is discussed later in the chapter,
but we need to understand here the basic role that this
system plays in determining interstitial fluid pressure.
The lymphatic system is a “scavenger” system that
removes excess fluid, excess protein molecules, debris,
and other matter from the tissue spaces. Normally,
when fluid enters the terminal lymphatic capillaries,
the lymph vessel walls automatically contract for a few
seconds and pump the fluid into the blood circulation.
This overall process creates the slight negative pressure that has been measured for fluid in the interstitial
spaces.
through the capillary pores, it is the proteins of the
plasma and interstitial fluids that are responsible for
the osmotic pressures on the two sides of the capillary
membrane. To distinguish this osmotic pressure from
that which occurs at the cell membrane, it is called
either colloid osmotic pressure or oncotic pressure. The
term “colloid” osmotic pressure is derived from the
fact that a protein solution resembles a colloidal solution despite the fact that it is actually a true molecular solution.
Normal Values for Plasma Colloid Osmotic Pressure. The
colloid osmotic pressure of normal human plasma
averages about 28 mm Hg; 19 mm of this is caused by
molecular effects of the dissolved protein and 9 mm by
the Donnan effect—that is, extra osmotic pressure
caused by sodium, potassium, and the other cations
held in the plasma by the proteins.
Effect of the Different Plasma Proteins on Colloid Osmotic
Pressure. The plasma proteins are a mixture that con-
tains albumin, with an average molecular weight of
69,000; globulins, 140,000; and fibrinogen, 400,000. Thus,
1 gram of globulin contains only half as many molecules
as 1 gram of albumin, and 1 gram of fibrinogen contains
only one sixth as many molecules as 1 gram of albumin.
It should be recalled from the discussion of osmotic
pressure in Chapter 4 that osmotic pressure is determined by the number of molecules dissolved in a fluid
rather than by the mass of these molecules. Therefore,
when corrected for number of molecules rather than
mass, the following chart gives both the relative mass
concentrations (g/dl) of the different types of proteins
in normal plasma and their respective contributions to
the total plasma colloid osmotic pressure (Pp).
Albumin
Globulins
Fibrinogen
Total
g/dl
Pp (mm Hg)
4.5
2.5
0.3
7.3
21.8
6.0
0.2
28.0
Thus, about 80 per cent of the total colloid osmotic
pressure of the plasma results from the albumin fraction, 20 per cent from the globulins, and almost none
from the fibrinogen. Therefore, from the point of view
of capillary and tissue fluid dynamics, it is mainly
albumin that is important.
Interstitial Fluid Colloid
Osmotic Pressure
Plasma Colloid Osmotic Pressure
Proteins in the Plasma Cause Colloid Osmotic Pressure. In the
basic discussion of osmotic pressure in Chapter 4, it
was pointed out that only those molecules or ions that
fail to pass through the pores of a semipermeable membrane exert osmotic pressure. Because the
proteins are the only dissolved constituents in the
plasma and interstitial fluids that do not readily pass
Although the size of the usual capillary pore is smaller
than the molecular sizes of the plasma proteins, this is
not true of all the pores. Therefore, small amounts of
plasma proteins do leak through the pores into the
interstitial spaces.
The total quantity of protein in the entire 12 liters
of interstitial fluid of the body is slightly greater than
the total quantity of protein in the plasma itself, but
Chapter 16
The Microcirculation and the Lymphatic System
because this volume is four times the volume of
plasma, the average protein concentration of the interstitial fluid is usually only 40 per cent of that in plasma,
or about 3 g/dl. Quantitatively, one finds that the
average interstitial fluid colloid osmotic pressure for
this concentration of proteins is about 8 mm Hg.
Exchange of Fluid Volume Through
the Capillary Membrane
Now that the different factors affecting fluid movement through the capillary membrane have been
discussed, it is possible to put all these together to
see how the capillary system maintains normal fluid
volume distribution between the plasma and the interstitial fluid.
The average capillary pressure at the arterial ends
of the capillaries is 15 to 25 mm Hg greater than at the
venous ends. Because of this difference, fluid “filters”
out of the capillaries at their arterial ends, but at their
venous ends fluid is reabsorbed back into the capillaries. Thus, a small amount of fluid actually “flows”
through the tissues from the arterial ends of the capillaries to the venous ends. The dynamics of this flow
are as follows.
Analysis of the Forces Causing Filtration at the Arterial End of
the Capillary. The approximate average forces operative
at the arterial end of the capillary that cause movement
through the capillary membrane are shown as follows:
189
mm Hg
Forces tending to move fluid inward:
Plasma colloid osmotic pressure
total inward force
28
28
Forces tending to move fluid outward:
Capillary pressure (venous end of capillary)
Negative interstitial free fluid pressure
Interstitial fluid colloid osmotic pressure
total outward force
10
3
8
21
Summation of forces:
Inward
Outward
net inward force
28
21
7
Thus, the force that causes fluid to move into the
capillary, 28 mm Hg, is greater than that opposing
reabsorption, 21 mm Hg. The difference, 7 mm Hg, is
the net reabsorption pressure at the venous ends of the
capillaries. This reabsorption pressure is considerably
less than the filtration pressure at the capillary arterial
ends, but remember that the venous capillaries are
more numerous and more permeable than the arterial
capillaries, so that less reabsorption pressure is
required to cause inward movement of fluid.
The reabsorption pressure causes about nine tenths
of the fluid that has filtered out of the arterial ends of
the capillaries to be reabsorbed at the venous ends.
The remaining one tenth flows into the lymph vessels
and returns to the circulating blood.
Starling Equilibrium for
Capillary Exchange
mm Hg
Forces tending to move fluid outward:
Capillary pressure (arterial end of capillary)
Negative interstitial free fluid pressure
Interstitial fluid colloid osmotic pressure
total outward force
30
3
8
41
Forces tending to move fluid inward:
Plasma colloid osmotic pressure
total inward force
28
28
Summation of forces:
Outward
Inward
net outward force (at arterial end)
41
28
13
Thus, the summation of forces at the arterial end of
the capillary shows a net filtration pressure of 13 mm
Hg, tending to move fluid outward through the capillary pores.
This 13 mm Hg filtration pressure causes, on the
average, about 1/200 of the plasma in the flowing blood
to filter out of the arterial ends of the capillaries into
the interstitial spaces each time the blood passes
through the capillaries.
Analysis of Reabsorption at the Venous End of the Capillary.
The low blood pressure at the venous end of the capillary changes the balance of forces in favor of absorption as follows:
E. H. Starling pointed out over a century ago that
under normal conditions, a state of near-equilibrium
exists at the capillary membrane. That is, the amount
of fluid filtering outward from the arterial ends of capillaries equals almost exactly the fluid returned to the
circulation by absorption. The slight disequilibrium
that does occur accounts for the small amount of fluid
that is eventually returned by way of the lymphatics.
The following chart shows the principles of the Starling equilibrium. For this chart, the pressures in the arterial and venous capillaries are averaged to calculate
mean functional capillary pressure for the entire length
of the capillary. This calculates to be 17.3 mm Hg.
mm Hg
Mean forces tending to move fluid outward:
Mean capillary pressure
Negative interstitial free fluid pressure
Interstitial fluid colloid osmotic pressure
total outward force
17.3
3.0
8.0
28.3
Mean force tending to move fluid inward:
Plasma colloid osmotic pressure
total inward force
28.0
28.0
Summation of mean forces:
Outward
Inward
net outward force
28.3
28.0
0.3
190
Unit IV
The Circulation
Thus, for the total capillary circulation, we find a
near-equilibrium between the total outward forces,
28.3 mm Hg, and the total inward force, 28.0 mm Hg.
This slight imbalance of forces, 0.3 mm Hg, causes
slightly more filtration of fluid into the interstitial
spaces than reabsorption.This slight excess of filtration
is called net filtration, and it is the fluid that must be
returned to the circulation through the lymphatics.The
normal rate of net filtration in the entire body is only
about 2 milliliters per minute.
Filtration Coefficient. In the above example, an average
net imbalance of forces at the capillary membranes
of 0.3 mm Hg causes net fluid filtration in the entire
body of 2 ml/min. Expressing this for each millimeter
of mercury imbalance, one finds a net filtration rate
of 6.67 milliliters of fluid per minute per millimeter of
mercury for the entire body. This is called the whole
body capillary filtration coefficient.
The filtration coefficient can also be expressed for
separate parts of the body in terms of rate of filtration
per minute per millimeter of mercury per 100 grams
of tissue. On this basis, the filtration coefficient of the
average tissue is about 0.01 ml/min/mm Hg/100 g of
tissue. But, because of extreme differences in permeabilities of the capillary systems in different tissues,
this coefficient varies more than 100-fold among the
different tissues. It is very small in both brain and
muscle, moderately large in subcutaneous tissue, large
in the intestine, and extreme in the liver and glomerulus of the kidney where the pores are either numerous
or wide open. By the same token, the permeation
of proteins through the capillary membranes varies
greatly as well. The concentration of protein in the
interstitial fluid of muscles is about 1.5 g/dl; in subcutaneous tissue, 2 g/dl; in intestine, 4 g/dl; and in liver,
6 g/dl.
Skin
To
measure
pressure
Implanted
capsule
Blood
vessels
Fluid filled cavity
FIGURE 16–7
Perforated capsule method for measuring interstitial fluid
pressure.
Effect of Abnormal Imbalance of Forces
at the Capillary Membrane
If the mean capillary pressure rises above 17 mm Hg,
the net force tending to cause filtration of fluid into the
tissue spaces rises. Thus, a 20 mm Hg rise in mean capillary pressure causes an increase in net filtration pressure from 0.3 mm Hg to 20.3 mm Hg, which results in
68 times as much net filtration of fluid into the interstitial spaces as normally occurs. To prevent accumulation of excess fluid in these spaces would require 68
times the normal flow of fluid into the lymphatic
system, an amount that is 2 to 5 times too much for the
lymphatics to carry away. As a result, fluid will begin
to accumulate in the interstitial spaces, and edema will
result.
Conversely, if the capillary pressure falls very low,
net reabsorption of fluid into the capillaries will occur
instead of net filtration, and the blood volume will
increase at the expense of the interstitial fluid volume.
These effects of imbalance at the capillary membrane
in relation to the development of different kinds of
edema are discussed in Chapter 25.
Lymphatic System
The lymphatic system represents an accessory route
through which fluid can flow from the interstitial
spaces into the blood. Most important, the lymphatics
can carry proteins and large particulate matter away
from the tissue spaces, neither of which can be
removed by absorption directly into the blood capillaries. This return of proteins to the blood from the
interstitial spaces is an essential function without
which we would die within about 24 hours.
Lymph Channels of the Body
Almost all tissues of the body have special lymph channels that drain excess fluid directly from the interstitial
spaces. The exceptions include the superficial portions
of the skin, the central nervous system, the endomysium
of muscles, and the bones. But, even these tissues have
minute interstitial channels called prelymphatics
through which interstitial fluid can flow; this fluid eventually empties either into lymphatic vessels or, in the
case of the brain, into the cerebrospinal fluid and then
directly back into the blood.
Essentially all the lymph vessels from the lower part
of the body eventually empty into the thoracic duct,
which in turn empties into the blood venous system at
the juncture of the left internal jugular vein and left subclavian vein, as shown in Figure 16–8.
Lymph from the left side of the head, the left arm, and
parts of the chest region also enters the thoracic duct
before it empties into the veins.
Lymph from the right side of the neck and head, the
right arm, and parts of the right thorax enters the right
lymph duct (much smaller than the thoracic duct), which
empties into the blood venous system at the juncture of
the right subclavian vein and internal jugular vein.
Terminal Lymphatic Capillaries and Their Permeability. Most
of the fluid filtering from the arterial ends of blood
Chapter 16
The Microcirculation and the Lymphatic System
191
Cervical nodes
Sentinel node
Subclavian vein
R. lymphatic duct
Thoracic duct
Axillary nodes
Cisterna chyli
Abdominal nodes
Inguinal nodes
Peripheral lymphatics
FIGURE 16–8
Lymphatic system.
capillaries flows among the cells and finally is reabsorbed back into the venous ends of the blood capillaries; but on the average, about 1/10 of the fluid
instead enters the lymphatic capillaries and returns to
the blood through the lymphatic system rather than
through the venous capillaries. The total quantity of all
this lymph is normally only 2 to 3 liters each day.
The fluid that returns to the circulation by way of
the lymphatics is extremely important because substances of high molecular weight, such as proteins,
cannot be absorbed from the tissues in any other way,
although they can enter the lymphatic capillaries
almost unimpeded. The reason for this is a special
structure of the lymphatic capillaries, demonstrated in
Figure 16–9. This figure shows the endothelial cells of
the lymphatic capillary attached by anchoring filaments to the surrounding connective tissue. At the
junctions of adjacent endothelial cells, the edge of one
endothelial cell overlaps the edge of the adjacent cell
in such a way that the overlapping edge is free to flap
inward, thus forming a minute valve that opens to the
interior of the lymphatic capillary. Interstitial fluid,
along with its suspended particles, can push the valve
open and flow directly into the lymphatic capillary. But
this fluid has difficulty leaving the capillary once it has
entered because any backflow closes the flap valve.
Thus, the lymphatics have valves at the very tips of the
terminal lymphatic capillaries as well as valves along
their larger vessels up to the point where they empty
into the blood circulation.
Formation of Lymph
Lymph is derived from interstitial fluid that flows into
the lymphatics. Therefore, lymph as it first enters the
terminal lymphatics has almost the same composition
as the interstitial fluid.
192
Unit IV
The Circulation
Endothelial cells
Valves
Relative lymph flow
4
2
2 times/
mm Hg
Anchoring filaments
FIGURE 16–9
0
–6
Special structure of the lymphatic capillaries that permits passage
of substances of high molecular weight into the lymph.
The protein concentration in the interstitial fluid of
most tissues averages about 2 g/dl, and the protein concentration of lymph flowing from these tissues is near
this value. Conversely, lymph formed in the liver has a
protein concentration as high as 6 g/dl, and lymph
formed in the intestines has a protein concentration as
high as 3 to 4 g/dl. Because about two thirds of all
lymph normally is derived from the liver and intestines, the thoracic duct lymph, which is a mixture of
lymph from all areas of the body, usually has a protein
concentration of 3 to 5 g/dl.
The lymphatic system is also one of the major routes
for absorption of nutrients from the gastrointestinal
tract, especially for absorption of virtually all fats in
food, as discussed in Chapter 65. Indeed, after a fatty
meal, thoracic duct lymph sometimes contains as much
as 1 to 2 per cent fat.
Finally, even large particles, such as bacteria, can
push their way between the endothelial cells of the
lymphatic capillaries and in this way enter the lymph.
As the lymph passes through the lymph nodes, these
particles are almost entirely removed and destroyed,
as discussed in Chapter 33.
Rate of Lymph Flow
About 100 milliliters per hour of lymph flows through
the thoracic duct of a resting human, and approximately another 20 milliliters flows into the circulation
each hour through other channels, making a total estimated lymph flow of about 120 ml/hr or 2 to 3 liters
per day.
Effect of Interstitial Fluid Pressure on Lymph Flow. Figure
16–10 shows the effect of different levels of interstitial
fluid pressure on lymph flow as measured in dog legs.
Note that normal lymph flow is very little at interstitial fluid pressures more negative than the normal
7 times/
mm Hg
–4
–2
0
2
PT (mm Hg)
4
FIGURE 16–10
Relation between interstitial fluid pressure and lymph flow in the
leg of a dog. Note that lymph flow reaches a maximum when the
interstitial pressure, PT, rises slightly above atmospheric pressure
(0 mm Hg). (Courtesy Drs. Harry Gibson and Aubrey Taylor.)
value of -6 mm Hg.Then, as the pressure rises to 0 mm
Hg (atmospheric pressure), flow increases more than
20-fold. Therefore, any factor that increases interstitial
fluid pressure also increases lymph flow if the lymph
vessels are functioning normally. Such factors include
the following:
∑ Elevated capillary pressure
∑ Decreased plasma colloid osmotic pressure
∑ Increased interstitial fluid colloid osmotic pressure
∑ Increased permeability of the capillaries
All of these cause a balance of fluid exchange at the
blood capillary membrane to favor fluid movement
into the interstitium, thus increasing interstitial fluid
volume, interstitial fluid pressure, and lymph flow all
at the same time.
However, note in Figure 16–10 that when the interstitial fluid pressure becomes 1 or 2 millimeters greater
than atmospheric pressure (greater than 0 mm Hg),
lymph flow fails to rise any further at still higher pressures. This results from the fact that the increasing
tissue pressure not only increases entry of fluid into
the lymphatic capillaries but also compresses the
outside surfaces of the larger lymphatics, thus impeding lymph flow. At the higher pressures, these two
factors balance each other almost exactly, so that
lymph flow reaches what is called the “maximum
lymph flow rate.” This is illustrated by the upper level
plateau in Figure 16–10.
Lymphatic Pump Increases Lymph Flow. Valves exist in all
lymph channels; typical valves are shown in Figure
Chapter 16
193
The Microcirculation and the Lymphatic System
Pores
Valves
Lymphatic
capillaries
FIGURE 16–11
Collecting
lymphatic
Structure of lymphatic capillaries
and a collecting lymphatic,
showing also the lymphatic
valves.
16–11 in collecting lymphatics into which the lymphatic capillaries empty.
Motion pictures of exposed lymph vessels, both
in animals and in human beings, show that when a
collecting lymphatic or larger lymph vessel becomes
stretched with fluid, the smooth muscle in the wall of
the vessel automatically contracts. Furthermore, each
segment of the lymph vessel between successive valves
functions as a separate automatic pump. That is, even
slight filling of a segment causes it to contract, and the
fluid is pumped through the next valve into the next
lymphatic segment. This fills the subsequent segment,
and a few seconds later it, too, contracts, the process
continuing all along the lymph vessel until the fluid is
finally emptied into the blood circulation. In a very
large lymph vessel such as the thoracic duct, this lymphatic pump can generate pressures as great as 50 to
100 mm Hg.
through the junctions between the endothelial cells.
Then, when the tissue is compressed, the pressure
inside the capillary increases and causes the overlapping edges of the endothelial cells to close like valves.
Therefore, the pressure pushes the lymph forward into
the collecting lymphatic instead of backward through
the cell junctions.
The lymphatic capillary endothelial cells also
contain a few contractile actomyosin filaments. In
some animal tissues (e.g., the bat’s wing) these filaments have been observed to cause rhythmical contraction of the lymphatic capillaries in the same way
that many of the small blood and larger lymphatic
vessels also contract rhythmically. Therefore, it is
probable that at least part of lymph pumping results
from lymph capillary endothelial cell contraction
in addition to contraction of the larger muscular
lymphatics.
Pumping Caused by External Intermittent Compression
of the Lymphatics. In addition to the pumping caused
Summary of Factors That Determine Lymph Flow. From the
by intrinsic intermittent contraction of the lymph
vessel walls, any external factor that intermittently
compresses the lymph vessel also can cause pumping.
In order of their importance, such factors are:
∑ Contraction of surrounding skeletal muscles
∑ Movement of the parts of the body
∑ Pulsations of arteries adjacent to the lymphatics
∑ Compression of the tissues by objects outside the
body
The lymphatic pump becomes very active during exercise, often increasing lymph flow 10- to 30-fold. Conversely, during periods of rest, lymph flow is sluggish,
almost zero.
Lymphatic Capillary Pump. The terminal lymphatic capil-
lary is also capable of pumping lymph, in addition to
the lymph pumping by the larger lymph vessels.
As explained earlier in the chapter, the walls of the
lymphatic capillaries are tightly adherent to the surrounding tissue cells by means of their anchoring
filaments. Therefore, each time excess fluid enters
the tissue and causes the tissue to swell, the anchoring
filaments pull on the wall of the lymphatic capillary,
and fluid flows into the terminal lymphatic capillary
above discussion, one can see that the two primary
factors that determine lymph flow are (1) the interstitial fluid pressure and (2) the activity of the lymphatic
pump. Therefore, one can state that, roughly, the rate
of lymph flow is determined by the product of interstitial fluid pressure times the activity of the lymphatic
pump.
Role of the Lymphatic System in
Controlling Interstitial Fluid Protein
Concentration, Interstitial Fluid
Volume, and Interstitial Fluid Pressure
It is already clear that the lymphatic system functions
as an “overflow mechanism” to return to the circulation excess proteins and excess fluid volume from the
tissue spaces. Therefore, the lymphatic system also
plays a central role in controlling (1) the concentration
of proteins in the interstitial fluids, (2) the volume of
interstitial fluid, and (3) the interstitial fluid pressure.
Let us explain how these factors interact.
First, remember that small amounts of proteins
leak continuously out of the blood capillaries into the
194
Unit IV
The Circulation
interstitium. Only minute amounts, if any, of the leaked
proteins return to the circulation by way of the venous
ends of the blood capillaries. Therefore, these proteins
tend to accumulate in the interstitial fluid, and this in
turn increases the colloid osmotic pressure of the
interstitial fluids.
Second, the increasing colloid osmotic pressure in
the interstitial fluid shifts the balance of forces at the
blood capillary membranes in favor of fluid filtration
into the interstitium. Therefore, in effect, fluid is
translocated osmotically outward through the capillary wall by the proteins and into the interstitium, thus
increasing both interstitial fluid volume and interstitial
fluid pressure.
Third, the increasing interstitial fluid pressure
greatly increases the rate of lymph flow, as explained
previously. This in turn carries away the excess interstitial fluid volume and excess protein that has accumulated in the spaces.
Thus, once the interstitial fluid protein concentration reaches a certain level and causes a comparable
increase in interstitial fluid volume and interstitial
fluid pressure, the return of protein and fluid by way
of the lymphatic system becomes great enough to
balance exactly the rate of leakage of these into the
interstitium from the blood capillaries. Therefore, the
quantitative values of all these factors reach a steady
state; they will remain balanced at these steady state
levels until something changes the rate of leakage of
proteins and fluid from the blood capillaries.
Significance of Negative Interstitial Fluid
Pressure as a Means for Holding the
Body Tissues Together
Traditionally, it has been assumed that the different
tissues of the body are held together entirely by
connective tissue fibers. However, at many places in
the body, connective tissue fibers are very weak or
even absent. This occurs particularly at points where
tissues slide over one another, such as the skin sliding
over the back of the hand or over the face. Yet even
at these places, the tissues are held together by the
negative interstitial fluid pressure, which is actually a
partial vacuum. When the tissues lose their negative
pressure, fluid accumulates in the spaces and the
condition known as edema occurs, which is discussed
in Chapter 25.
References
Aukland K, Reed RK: Interstitial-lymphatic mechanisms in
the control of extracellular fluid volume. Physiol Rev 73:1,
1993.
D’Amico G, Bazzi C: Pathophysiology of proteinuria.
Kidney Int 63:809, 2003.
Dejana E: Endothelial cell-cell junctions: happy together.
Nat Rev Mol Cell Biol 5:261, 2004.
Frank PG, Woodman SE, Park DS, Lisanti MP: Caveolin,
caveolae, and endothelial cell function. Arterioscler
Thromb Vasc Biol 23:1161, 2003.
Gashev AA: Physiologic aspects of lymphatic contractile
function: current perspectives. Ann N Y Acad Sci 979:178,
2002.
Guyton AC: Concept of negative interstitial pressure based
on pressures in implanted perforated capsules. Circ Res
12:399, 1963.
Guyton AC: Interstitial fluid pressure: II. Pressure-volume
curves of interstitial space. Circ Res 16:452, 1965.
Guyton AC, Granger HJ, Taylor AE: Interstitial fluid pressure. Physiol Rev 51:527, 1971.
Guyton AC, Prather J, Scheel K, McGehee J: Interstitial fluid
pressure: IV. Its effect on fluid movement through the
capillary wall. Circ Res 19:1022, 1966.
Guyton AC, Scheel K, Murphree D: Interstitial fluid pressure: III. Its effect on resistance to tissue fluid mobility.
Circ Res 19:412, 1966.
Guyton AC, Taylor AE, Granger HJ: Circulatory Physiology
II. Dynamics and Control of the Body Fluids. Philadelphia: WB Saunders Co, 1975.
Michel CC, Curry FE: Microvascular permeability. Physiol
Rev 79:703, 1999.
Miyasaka M, Tanaka T: Lymphocyte trafficking across high
endothelial venules: dogmas and enigmas. Nat Rev
Immunol 4:360, 2004.
Oliver G: Lymphatic vasculature development. Nat Rev
Immunol 4:35, 2004.
Rippe B, Rosengren BI, Carlsson O, Venturoli D:
Transendothelial transport: the vesicle controversy. J Vasc
Res 39:375, 2002.
Taylor AE, Granger DN: Exchange of macromolecules
across the microcirculation. In: Renkin EM, Michel CC
(eds): Handbook of Physiology. Sec. 2, Vol. IV. Bethesda:
American Physiological Society, 1984, p 467.
C
H
A
P
T
E
R
1
7
Local and Humoral Control of
Blood Flow by the Tissues
Local Control of Blood
Flow in Response to
Tissue Needs
One of the most fundamental principles of circulatory function is the ability of each tissue to control
its own local blood flow in proportion to its metabolic needs.
What are some of the specific needs of the tissues for blood flow? The answer
to this is manyfold, including the following:
1.
2.
3.
4.
5.
6.
Delivery of oxygen to the tissues
Delivery of other nutrients, such as glucose, amino acids, and fatty acids
Removal of carbon dioxide from the tissues
Removal of hydrogen ions from the tissues
Maintenance of proper concentrations of other ions in the tissues
Transport of various hormones and other substances to the different tissues
Certain organs have special requirements. For instance, blood flow to the skin
determines heat loss from the body and in this way helps to control body temperature. Also, delivery of adequate quantities of blood plasma to the kidneys
allows the kidneys to excrete the waste products of the body.
We shall see that most of these factors exert extreme degrees of local blood
flow control.
Variations in Blood Flow in Different Tissues and Organs. Note in Table 17–1 the very
large blood flows in some organs—for example, several hundred milliliters per
minute per 100 grams of thyroid or adrenal gland tissue and a total blood flow
of 1350 ml/min in the liver, which is 95 ml/min/100 g of liver tissue.
Also note the extremely large blood flow through the kidneys—1100 ml/min.
This extreme amount of flow is required for the kidneys to perform their function of cleansing the blood of waste products.
Conversely, most surprising is the low blood flow to all the inactive muscles
of the body, only a total of 750 ml/min, even though the muscles constitute
between 30 and 40 per cent of the total body mass. In the resting state, the metabolic activity of the muscles is very low, and so also is the blood flow, only
4 ml/min/100 g. Yet, during heavy exercise, muscle metabolic activity can
increase more than 60-fold and the blood flow as much as 20-fold, increasing to
as high as 16,000 ml/min in the body’s total muscle vascular bed (or 80 ml/
min/100 g of muscle).
Importance of Blood Flow Control by the Local Tissues. One might ask the simple question: Why not simply allow a very large blood flow all the time through every
tissue of the body, always enough to supply the tissue’s needs whether the activity of the tissue is little or great? The answer is equally simple: To do this would
require many times more blood flow than the heart can pump.
Experiments have shown that the blood flow to each tissue usually is regulated at the minimal level that will supply the tissue’s requirements— no more,
no less. For instance, in tissues for which the most important requirement is
delivery of oxygen, the blood flow is always controlled at a level only slightly
more than required to maintain full tissue oxygenation but no more than this.
195
196
Unit IV
The Circulation
Table 17–1
4
Brain
Heart
Bronchi
Kidneys
Liver
Portal
Arterial
Muscle (inactive state)
Bone
Skin (cool weather)
Thyroid gland
Adrenal glands
Other tissues
Total
Per cent
ml/min
14
4
2
22
27
(21)
(6)
15
5
6
1
0.5
3.5
100.0
700
200
100
1100
1350
1050
300
750
250
300
50
25
175
5000
ml/min/100 g
50
70
25
360
95
4
3
3
160
300
1.3
Based mainly on data compiled by Dr. L. A. Sapirstein.
By controlling local blood flow in such an exact way,
the tissues almost never suffer from oxygen nutritional
deficiency, and yet the workload on the heart is kept
at a minimum.
Mechanisms of Blood
Flow Control
Local blood flow control can be divided into two
phases: (1) acute control and (2) long-term control.
Acute control is achieved by rapid changes in local
vasodilation or vasoconstriction of the arterioles,
metarterioles, and precapillary sphincters, occurring
within seconds to minutes to provide very rapid maintenance of appropriate local tissue blood flow.
Long-term control, however, means slow, controlled
changes in flow over a period of days, weeks, or even
months. In general, these long-term changes provide
even better control of the flow in proportion to the
needs of the tissues. These changes come about as a
result of an increase or decrease in the physical sizes
and numbers of actual blood vessels supplying the
tissues.
Blood flow (x normal)
Blood Flow to Different Organs and Tissues Under Basal
Conditions
3
2
1
Normal level
0
0
1
2
3
4
5
6
7
Rate of metabolism (x normal)
8
Figure 17–1
Effect of increasing rate of metabolism on tissue blood flow.
(3) in carbon monoxide poisoning (which poisons the
ability of hemoglobin to transport oxygen), or (4) in
cyanide poisoning (which poisons the ability of the
tissues to use oxygen), the blood flow through the
tissues increases markedly. Figure 17–2 shows that as
the arterial oxygen saturation decreases to about 25
per cent of normal, the blood flow through an isolated
leg increases about threefold; that is, the blood flow
increases almost enough, but not quite enough, to
make up for the decreased amount of oxygen in the
blood, thus almost maintaining an exact constant
supply of oxygen to the tissues.
Total cyanide poisoning of oxygen usage by a local
tissue area can cause local blood flow to increase as
much as sevenfold, thus demonstrating the extreme
effect of oxygen deficiency to increase blood flow.
There are two basic theories for the regulation of
local blood flow when either the rate of tissue metabolism changes or the availability of oxygen changes.
They are (1) the vasodilator theory and (2) the oxygen
lack theory.
Vasodilator Theory for Acute Local Blood Flow Regulation—Possible Special Role of Adenosine. Accord-
Acute Control of Local Blood Flow
Effect of Tissue Metabolism on Local Blood Flow. Figure
17–1 shows the approximate quantitative acute effect
on blood flow of increasing the rate of metabolism in
a local tissue, such as in a skeletal muscle. Note that an
increase in metabolism up to eight times normal
increases the blood flow acutely about fourfold.
Acute Local Blood Flow Regulation When Oxygen Availability
Changes. One of the most necessary of the metabolic
nutrients is oxygen. Whenever the availability of
oxygen to the tissues decreases, such as (1) at high altitude at the top of a high mountain, (2) in pneumonia,
ing to this theory, the greater the rate of metabolism
or the less the availability of oxygen or some other
nutrients to a tissue, the greater the rate of formation
of vasodilator substances in the tissue cells. The
vasodilator substances then are believed to diffuse
through the tissues to the precapillary sphincters,
metarterioles, and arterioles to cause dilation. Some of
the different vasodilator substances that have been
suggested are adenosine, carbon dioxide, adenosine
phosphate compounds, histamine, potassium ions, and
hydrogen tons.
Most of the vasodilator theories assume that the
vasodilator substance is released from the tissue
mainly in response to oxygen deficiency. For instance,
Chapter 17
Local and Humoral Control of Blood Flow by the Tissues
197
Precapillary sphincter
Metarteriole
Blood flow (x normal)
3
2
1
Sidearm capillary
0
100
75
50
25
Arterial oxygen saturation (per cent)
Figure 17–2
Effect of decreasing arterial oxygen saturation on blood flow
through an isolated dog leg.
experiments have shown that decreased availability of
oxygen can cause both adenosine and lactic acid (containing hydrogen ions) to be released into the spaces
between the tissue cells; these substances then cause
intense acute vasodilation and therefore are responsible, or partially responsible, for the local blood flow
regulation.
Many physiologists have suggested that the substance adenosine is the most important of the local
vasodilators for controlling local blood flow. For
example, minute quantities of adenosine are released
from heart muscle cells when coronary blood flow
becomes too little, and this causes enough local vasodilation in the heart to return coronary blood flow
back to normal. Also, whenever the heart becomes
more active than normal and the heart’s metabolism
increases an extra amount, this, too, causes increased
utilization of oxygen, followed by (1) decreased
oxygen concentration in the heart muscle cells with (2)
consequent degradation of adenosine triphosphate
(ATP), which (3) increases the release of adenosine. It
is believed that much of this adenosine leaks out of the
heart muscle cells to cause coronary vasodilation, providing increased coronary blood flow to supply the
increased nutrient demands of the active heart.
Although research evidence is less clear, many physiologists also have suggested that the same adenosine
mechanism is the most important controller of blood
flow in skeletal muscle and many other tissues as
well as in the heart. The problem with the different
vasodilator theories of local blood flow regulation
has been the following: It has been difficult to prove
that sufficient quantities of any single vasodilator substance (including adenosine) are indeed formed in the
tissues to cause all the measured increase in blood
flow. But a combination of several different vasodilators could increase the blood flow sufficiently.
Oxygen Lack Theory for Local Blood Flow Control.
Although the vasodilator theory is widely accepted,
Figure 17–3
Diagram of a tissue unit area for explanation of acute local feedback control of blood flow, showing a metarteriole passing through
the tissue and a sidearm capillary with its precapillary sphincter
for controlling capillary blood flow.
several critical facts have made other physiologists
favor still another theory, which can be called either
the oxygen lack theory or, more accurately, the
nutrient lack theory (because other nutrients besides
oxygen are involved). Oxygen (and other nutrients
as well) is required as one of the metabolic nutrients
to cause vascular muscle contraction. Therefore, in the
absence of adequate oxygen, it is reasonable to
believe that the blood vessels simply would relax
and therefore naturally dilate. Also, increased utilization of oxygen in the tissues as a result of increased
metabolism theoretically could decrease the availability of oxygen to the smooth muscle fibers in the
local blood vessels, and this, too, would cause local
vasodilation.
A mechanism by which the oxygen lack theory
could operate is shown in Figure 17–3. This figure
shows a tissue unit, consisting of a metarteriole with a
single sidearm capillary and its surrounding tissue. At
the origin of the capillary is a precapillary sphincter,
and around the metarteriole are several other smooth
muscle fibers. Observing such a tissue under a microscope—for example, in a bat’s wing—one sees that the
precapillary sphincters are normally either completely
open or completely closed. The number of precapillary
sphincters that are open at any given time is roughly
proportional to the requirements of the tissue for
nutrition. The precapillary sphincters and metarterioles open and close cyclically several times per minute,
with the duration of the open phases being proportional to the metabolic needs of the tissues for oxygen.
The cyclical opening and closing is called vasomotion.
Let us explain how oxygen concentration in the
local tissue could regulate blood flow through the area.
Because smooth muscle requires oxygen to remain
198
Unit IV
The Circulation
contracted, one might assume that the strength of contraction of the sphincters would increase with an
increase in oxygen concentration. Consequently, when
the oxygen concentration in the tissue rises above
a certain level, the precapillary and metarteriole
sphincters presumably would close until the tissue
cells consume the excess oxygen. But when the excess
oxygen is gone and the oxygen concentration falls low
enough, the sphincters would open once more to begin
the cycle again.
Thus, on the basis of available data, either a
vasodilator substance theory or an oxygen lack theory
could explain acute local blood flow regulation in
response to the metabolic needs of the tissues.
Probably the truth lies in a combination of the two
mechanisms.
Possible Role of Other Nutrients Besides Oxygen in Control of
Local Blood Flow. Under special conditions, it has been
shown that lack of glucose in the perfusing blood can
cause local tissue vasodilation. Also, it is possible that
this same effect occurs when other nutrients, such as
amino acids or fatty acids, are deficient, although this
has not been studied adequately. In addition, vasodilation occurs in the vitamin deficiency disease beriberi,
in which the patient has deficiencies of the vitamin B
substances thiamine, niacin, and riboflavin. In this
disease, the peripheral vascular blood flow everywhere
in the body often increases twofold to threefold.
Because these vitamins all are needed for oxygeninduced phosphorylation that is required to produce
ATP in the tissue cells, one can well understand how
deficiency of these vitamins might lead to diminished
smooth muscle contractile ability and therefore also
local vasodilation.
emphasizes the close connection between local blood
flow regulation and delivery of oxygen and other nutrients to the tissues.
Active Hyperemia. When any tissue becomes highly
active, such as an exercising muscle, a gastrointestinal
gland during a hypersecretory period, or even the
brain during rapid mental activity, the rate of blood
flow through the tissue increases. Here again, by
simply applying the basic principles of local blood flow
control, one can easily understand this active hyperemia. The increase in local metabolism causes the cells
to devour tissue fluid nutrients extremely rapidly and
also to release large quantities of vasodilator substances. The result is to dilate the local blood vessels
and, therefore, to increase local blood flow. In this way,
the active tissue receives the additional nutrients
required to sustain its new level of function. As
pointed out earlier, active hyperemia in skeletal
muscle can increase local muscle blood flow as much
as 20-fold during intense exercise.
“Autoregulation” of Blood Flow When the
Arterial Pressure Changes from Normal—
“Metabolic” and “Myogenic” Mechanisms
In any tissue of the body, an acute increase in arterial
pressure causes immediate rise in blood flow. But,
within less than a minute, the blood flow in most
tissues returns almost to the normal level, even though
the arterial pressure is kept elevated. This return of
flow toward normal is called “autoregulation of blood
flow.” After autoregulation has occurred, the local
blood flow in most body tissues will be related to arterial pressure approximately in accord with the solid
“acute” curve in Figure 17–4. Note that between an
Special Examples of Acute “Metabolic”
Control of Local Blood Flow
Reactive Hyperemia. When the blood supply to a tissue
is blocked for a few seconds to as long an hour or more
and then is unblocked, blood flow through the tissue
usually increases immediately to four to seven times
normal; this increased flow will continue for a few
seconds if the block has lasted only a few seconds but
sometimes continues for as long as many hours if the
blood flow has been stopped for an hour or more. This
phenomenon is called reactive hyperemia.
Reactive hyperemia is another manifestation of the
local “metabolic” blood flow regulation mechanism;
that is, lack of flow sets into motion all of those factors
that cause vasodilation. After short periods of vascular occlusion, the extra blood flow during the reactive
hyperemia phase lasts long enough to repay almost
exactly the tissue oxygen deficit that has accrued
during the period of occlusion. This mechanism
2.5
Blood flow (x normal)
The mechanisms that we have described thus far for
local blood flow control are called “metabolic mechanisms” because all of them function in response to the
metabolic needs of the tissues. Two additional special
examples of metabolic control of local blood flow are
reactive hyperemia and active hyperemia.
Acute
2.0
1.5
1.0
Long-term
0.5
0
0
150
50
100
200
Arterial pressure (mm Hg)
250
Figure 17–4
Effect of different levels of arterial pressure on blood flow through
a muscle. The solid red curve shows the effect if the arterial pressure is raised over a period of a few minutes. The dashed green
curve shows the effect if the arterial pressure is raised extremely
slowly over a period of many weeks.
Chapter 17
Local and Humoral Control of Blood Flow by the Tissues
arterial pressure of about 70 mm Hg and 175 mm Hg,
the blood flow increases only 30 per cent even though
the arterial pressure increases 150 per cent.
For almost a century, two views have been proposed
to explain this acute autoregulation mechanism. They
have been called (1) the metabolic theory and (2) the
myogenic theory.
The metabolic theory can be understood easily by
applying the basic principles of local blood flow regulation discussed in previous sections. Thus, when the
arterial pressure becomes too great, the excess flow
provides too much oxygen and too many other nutrients to the tissues. These nutrients (especially oxygen)
then cause the blood vessels to constrict and the flow
to return nearly to normal despite the increased
pressure.
The myogenic theory, however, suggests that still
another mechanism not related to tissue metabolism
explains the phenomenon of autoregulation. This
theory is based on the observation that sudden stretch
of small blood vessels causes the smooth muscle of the
vessel wall to contract for a few seconds. Therefore, it
has been proposed that when high arterial pressure
stretches the vessel, this in turn causes reactive vascular constriction that reduces blood flow nearly back to
normal. Conversely, at low pressures, the degree of
stretch of the vessel is less, so that the smooth muscle
relaxes and allows increased flow.
The myogenic response is inherent to vascular
smooth muscle and can occur in the absence of neural
or hormonal influences. It is most pronounced in arterioles but can also be observed in arteries, venules,
veins, and even lymphatic vessels. Myogenic contraction is initiated by stretch-induced vascular depolarization, which then rapidly increases calcium ion entry
from the extracellular fluid into the cells, causing them
to contract. Changes in vascular pressure may also
open or close other ion channels that influence vascular contraction. The precise mechanisms by which
changes in pressure cause opening or closing of vascular ion channels are still uncertain, but likely involve
mechanical effects of pressure on extracellular proteins that are tethered to cytoskeleton elements of the
vascular wall or to the ion channels themselves.
The myogenic mechanism may be important in preventing excessive stretch of blood vessel when blood
pressure is increased. However, the importance of
the myogenic mechanism in blood flow regulation
is unclear because this pressure sensing mechanism
cannot directly detect changes in blood flow in the
tissue. Indeed metabolic factors appear to override
the myogenic mechanism in circumstances where the
metabolic demands of the tissues are significantly
increased, such as during vigorous muscle exercise,
which can cause dramatic increases in skeletal muscle
blood flow.
Special Mechanisms for Acute Blood Flow
Control in Specific Tissues
Although the general mechanisms for local blood flow
control discussed thus far are present in almost all
tissues of the body, distinctly different mechanisms
199
operate in a few special areas. They all are discussed
throughout this text in relation to specific organs, but
two notable ones are as follows:
1. In the kidneys, blood flow control is vested mainly
in a mechanism called tubuloglomerular feedback,
in which the composition of the fluid in the early
distal tubule is detected by an epithelial structure
of the distal tubule itself called the macula densa.
This is located where the distal tubule lies adjacent
to the afferent and efferent arterioles at the
nephron juxtaglomerular apparatus. When too
much fluid filters from the blood through the
glomerulus into the tubular system, appropriate
feedback signals from the macula densa cause
constriction of the afferent arterioles, in this way
reducing both renal blood flow and glomerular
filtration rate back to or near to normal. The
details of this mechanism are discussed in
Chapter 26.
2. In the brain, in addition to control of blood flow by
tissue oxygen concentration, the concentrations of
carbon dioxide and hydrogen ions play very
prominent roles. An increase of either or both of
these dilates the cerebral vessels and allows rapid
washout of the excess carbon dioxide or hydrogen
ions from the brain tissues. This is important
because the level of excitability of the brain itself is
highly dependent on exact control of both carbon
dioxide concentration and hydrogen ion
concentration. This special mechanism for cerebral
blood flow control is presented in Chapter 61.
Mechanism for Dilating Upstream Arteries
When Microvascular Blood Flow Increases—
The Endothelium-Derived Relaxing Factor
(Nitric Oxide)
The local mechanisms for controlling tissue blood flow
can dilate only the very small arteries and arterioles in
each tissue because tissue cell vasodilator substances
or tissue cell oxygen deficiency can reach only these
vessels, not the intermediate and larger arteries back
upstream. Yet, when blood flow through a microvascular portion of the circulation increases, this secondarily entrains another mechanism that does dilate
the larger arteries as well. This mechanism is the
following:
The endothelial cells lining the arterioles and small
arteries synthesize several substances that, when
released, can affect the degree of relaxation or contraction of the arterial wall. The most important of
these is a vasodilator substance called endotheliumderived relaxing factor (EDRF), which is composed
principally of nitric oxide, which has a half-life in the
blood of only 6 seconds. Rapid flow of blood through
the arteries and arterioles causes shear stress on the
endothelial cells because of viscous drag of the blood
against the vascular walls. This stress contorts the
endothelial cells in the direction of flow and causes significant increase in the release of nitric oxide. The
nitric oxide then relaxes the blood vessels. This is fortunate because it increases the diameters of the
upstream arterial blood vessels whenever microvascular blood flow increases downstream. Without such a
response, the effectiveness of local blood flow control
200
Unit IV
The Circulation
would be significantly decreased because a significant
part of the resistance to blood flow is in the upstream
small arteries.
Long-Term Blood Flow Regulation
Thus far, most of the mechanisms for local blood flow
regulation that we have discussed act within a few
seconds to a few minutes after the local tissue conditions have changed. Yet, even after full activation
of these acute mechanisms, the blood flow usually
is adjusted only about three quarters of the way to
the exact additional requirements of the tissues.
For instance, when the arterial pressure suddenly is
increased from 100 to 150 mm Hg, the blood flow
increases almost instantaneously about 100 per cent.
Then, within 30 seconds to 2 minutes, the flow
decreases back to about 15 per cent above the original control value. This illustrates the rapidity of the
acute mechanisms for local blood flow regulation, but
at the same time, it demonstrates that the regulation is
still incomplete because there remains an excess 15 per
cent increase in blood flow.
However, over a period of hours, days, and weeks, a
long-term type of local blood flow regulation develops
in addition to the acute regulation. This long-term
regulation gives far more complete regulation. For
instance, in the aforementioned example, if the arterial pressure remains at 150 mm Hg indefinitely, within
a few weeks the blood flow through the tissues gradually reapproaches almost exactly the normal flow level.
Figure 17–4 shows by the dashed green curve the
extreme effectiveness of this long-term local blood
flow regulation. Note that once the long-term regulation has had time to occur, long-term changes in arterial pressure between 50 and 250 mm Hg have little
effect on the rate of local blood flow.
Long-term regulation of blood flow is especially
important when the long-term metabolic demands
of a tissue change. Thus, if a tissue becomes chronically overactive and therefore requires chronically
increased quantities of oxygen and other nutrients, the
arterioles and capillary vessels usually increase both in
number and size within a few weeks to match the
needs of the tissue—unless the circulatory system has
become pathological or too old to respond.
Mechanism of Long-Term Regulation—
Change in “Tissue Vascularity”
The mechanism of long-term local blood flow regulation is principally to change the amount of vascularity
of the tissues. For instance, if the metabolism in a
given tissue is increased for a prolonged period,
vascularity increases; if the metabolism is decreased,
vascularity decreases.
Thus, there is actual physical reconstruction of
the tissue vasculature to meet the needs of the tissues.
This reconstruction occurs rapidly (within days) in
extremely young animals. It also occurs rapidly in new
growth tissue, such as in scar tissue and cancerous
tissue; however, it occurs much more slowly in old,
well-established tissues. Therefore, the time required
for long-term regulation to take place may be only
a few days in the neonate or as long as months in
the elderly person. Furthermore, the final degree of
response is much better in younger tissues than in
older, so that in the neonate, the vascularity will adjust
to match almost exactly the needs of the tissue for
blood flow, whereas in older tissues, vascularity frequently lags far behind the needs of the tissues.
Role of Oxygen in Long-Term Regulation. Oxygen is impor-
tant not only for acute control of local blood flow but
also for long-term control. One example of this is
increased vascularity in tissues of animals that live at
high altitudes, where the atmospheric oxygen is low. A
second example is that fetal chicks hatched in low
oxygen have up to twice as much tissue blood vessel
conductivity as is normally true.This same effect is also
dramatically demonstrated in premature human
babies put into oxygen tents for therapeutic purposes.
The excess oxygen causes almost immediate cessation
of new vascular growth in the retina of the premature
baby’s eyes and even causes degeneration of some of
the small vessels that already have formed. Then when
the infant is taken out of the oxygen tent, there is
explosive overgrowth of new vessels to make up for
the sudden decrease in available oxygen; indeed, there
is often so much overgrowth that the retinal vessels
grow out from the retina into the eye’s vitreous humor;
and this eventually causes blindness. (This condition is
called retrolental fibroplasia.)
Importance of Vascular Endothelial Growth
Factor in Formation of New Blood Vessels
A dozen or more factors that increase growth of new
blood vessels have been found, almost all of which are
small peptides. Three of those that have been best
characterized are vascular endothelial growth factor
(VEGF), fibroblast growth factor, and angiogenin, each
of which has been isolated from tissues that have inadequate blood supply. Presumably, it is deficiency of
tissue oxygen or other nutrients, or both, that leads to
formation of the vascular growth factors (also called
“angiogenic factors”).
Essentially all the angiogenic factors promote new
vessel growth in the same way. They cause new vessels
to sprout from other small vessels. The first step is dissolution of the basement membrane of the endothelial
cells at the point of sprouting. This is followed by rapid
reproduction of new endothelial cells that stream
outward through the vessel wall in extended cords
directed toward the source of the angiogenic factor.
The cells in each cord continue to divide and rapidly
fold over into a tube. Next, the tube connects with
another tube budding from another donor vessel
(another arteriole or venule) and forms a capillary
loop through which blood begins to flow. If the flow is
great enough, smooth muscle cells eventually invade
the wall, so that some of the new vessels eventually
grow to be new arterioles or venules or perhaps even
larger vessels. Thus, angiogenesis explains the manner
in which metabolic factors in local tissues can cause
growth of new vessels.
Chapter 17
Local and Humoral Control of Blood Flow by the Tissues
Certain other substances, such as some steroid hormones, have exactly the opposite effect on small blood
vessels, occasionally even causing dissolution of vascular cells and disappearance of vessels. Therefore,
blood vessels can also be made to disappear when not
needed.
Vascularity Is Determined by Maximum Blood Flow Need, Not
by Average Need. An especially valuable characteristic
of long-term vascular control is that vascularity is
determined mainly by the maximum level of blood
flow need rather than by average need. For instance,
during heavy exercise the need for whole body blood
flow often increases to six to eight times the resting
blood flow. This great excess of flow may not be
required for more than a few minutes each day. Nevertheless, even this short need can cause enough
VEGF to be formed by the muscles to increase their
vascularity as required. Were it not for this capability,
every time that a person attempted heavy exercise, the
muscles would fail to receive the required nutrients,
especially the required oxygen, so that the muscles
simply would fail to contract.
However, after extra vascularity does develop, the
extra blood vessels normally remain mainly vasoconstricted, opening to allow extra flow only when appropriate local stimuli such as oxygen lack, nerve
vasodilatory stimuli, or other stimuli call forth the
required extra flow.
Development of Collateral
Circulation—A Phenomenon of LongTerm Local Blood Flow Regulation
When an artery or a vein is blocked in virtually any
tissue of the body, a new vascular channel usually
develops around the blockage and allows at least
partial resupply of blood to the affected tissue. The
first stage in this process is dilation of small vascular
loops that already connect the vessel above the blockage to the vessel below. This dilation occurs within the
first minute or two, indicating that the dilation is
simply a neurogenic or metabolic relaxation of the
muscle fibers of the small vessels involved. After this
initial opening of collateral vessels, the blood flow
often is still less than one quarter that needed to
supply all the tissue needs. However, further opening
occurs within the ensuing hours, so that within 1 day
as much as half the tissue needs may be met, and
within a few days often all the tissue needs.
The collateral vessels continue to grow for many
months thereafter, almost always forming multiple
small collateral channels rather than one single large
vessel. Under resting conditions, the blood flow usually
returns very near to normal, but the new channels
seldom become large enough to supply the blood flow
needed during strenuous tissue activity. Thus, the
development of collateral vessels follows the usual
principles of both acute and long-term local blood flow
control, the acute control being rapid neurogenic and
201
metabolic dilation, followed chronically by manifold
growth and enlargement of new vessels over a period
of weeks and months.
The most important example of the development of
collateral blood vessels occurs after thrombosis of one
of the coronary arteries. Almost all people by the age
of 60 years have had at least one of the smaller branch
coronary vessels close. Yet most people do not know
that this has happened because collaterals have developed rapidly enough to prevent myocardial damage. It
is in those other instances in which coronary insufficiency occurs too rapidly or too severely for collaterals to develop that serious heart attacks occur.
Humoral Control of the
Circulation
Humoral control of the circulation means control by
substances secreted or absorbed into the body fluids—
such as hormones and ions. Some of these substances
are formed by special glands and transported in
the blood throughout the entire body. Others are
formed in local tissue areas and cause only local circulatory effects. Among the most important of the
humoral factors that affect circulatory function are the
following.
Vasoconstrictor Agents
Norepinephrine and Epinephrine. Norepinephrine is an
especially powerful vasoconstrictor hormone; epinephrine is less so and in some tissues even causes mild
vasodilation. (A special example of vasodilation
caused by epinephrine occurs to dilate the coronary
arteries during increased heart activity.)
When the sympathetic nervous system is stimulated
in most or all parts of the body during stress or exercise, the sympathetic nerve endings in the individual
tissues release norepinephrine, which excites the heart
and contracts the veins and arterioles. In addition,
the sympathetic nerves to the adrenal medullae cause
these glands to secrete both norepinephrine and epinephrine into the blood. These hormones then circulate to all areas of the body and cause almost the same
effects on the circulation as direct sympathetic stimulation, thus providing a dual system of control: (1)
direct nerve stimulation and (2) indirect effects of
norepinephrine and/or epinephrine in the circulating
blood.
Angiotensin II. Angiotensin II is another powerful vasoconstrictor substance. As little as one millionth of a
gram can increase the arterial pressure of a human
being 50 mm Hg or more.
The effect of angiotensin II is to constrict powerfully
the small arterioles. If this occurs in an isolated
tissue area, the blood flow to that area can be
severely depressed. However, the real importance of
angiotensin II is that it normally acts on many of the
arterioles of the body at the same time to increase the
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Unit IV
The Circulation
total peripheral resistance, thereby increasing the arterial pressure. Thus, this hormone plays an integral role
in the regulation of arterial pressure, as is discussed in
detail in Chapter 19.
Vasopressin. Vasopressin, also called antidiuretic
hormone, is even more powerful than angiotensin II as
a vasoconstrictor, thus making it one of the body’s
most potent vascular constrictor substances. It is
formed in nerve cells in the hypothalamus of the brain
(see Chapter 75) but is then transported downward by
nerve axons to the posterior pituitary gland, where it
is finally secreted into the blood.
It is clear that vasopressin could have enormous
effects on circulatory function. Yet, normally, only
minute amounts of vasopressin are secreted, so that
most physiologists have thought that vasopressin plays
little role in vascular control. However, experiments
have shown that the concentration of circulating blood
vasopressin after severe hemorrhage can rise high
enough to increase the arterial pressure as much as
60 mm Hg. In many instances, this can, by itself, bring
the arterial pressure almost back up to normal.
Vasopressin has a major function to increase greatly
water reabsorption from the renal tubules back into
the blood (discussed in Chapter 28), and therefore
to help control body fluid volume. That is why this
hormone is also called antidiuretic hormone.
Endothelin—A Powerful Vasoconstrictor in Damaged Blood
Vessels. Still another vasoconstrictor substance that
ranks along with angiotensin and vasopressin in its
vasoconstrictor capability is a large 21 amino acid
peptide called endothelin, which requires only
nanogram quantities to cause powerful vasoconstriction. This substance is present in the endothelial cells
of all or most blood vessels. The usual stimulus for
release is damage to the endothelium, such as that
caused by crushing the tissues or injecting a traumatizing chemical into the blood vessel. After severe
blood vessel damage, release of local endothelin and
subsequent vasoconstriction helps to prevent extensive bleeding from arteries as large as 5 millimeters in
diameter that might have been torn open by crushing
injury.
Vasodilator Agents
Bradykinin. Several substances called kinins cause pow-
erful vasodilation when formed in the blood and tissue
fluids of some organs.
The kinins are small polypeptides that are split away
by proteolytic enzymes from alpha2-globulins in the
plasma or tissue fluids. A proteolytic enzyme of particular importance for this purpose is kallikrein, which
is present in the blood and tissue fluids in an inactive
form. This inactive kallikrein is activated by maceration of the blood, by tissue inflammation, or by other
similar chemical or physical effects on the blood or
tissues. As kallikrein becomes activated, it acts immediately on alpha2-globulin to release a kinin called
kallidin that then is converted by tissue enzymes into
bradykinin. Once formed, bradykinin persists for only
a few minutes because it is inactivated by the enzyme
carboxypeptidase or by converting enzyme, the same
enzyme that also plays an essential role in activating
angiotensin, as discussed in Chapter 19. The activated
kallikrein enzyme is destroyed by a kallikrein inhibitor
also present in the body fluids.
Bradykinin causes both powerful arteriolar
dilation and increased capillary permeability. For
instance, injection of 1 microgram of bradykinin into
the brachial artery of a person increases blood flow
through the arm as much as sixfold, and even smaller
amounts injected locally into tissues can cause marked
local edema resulting from increase in capillary pore
size.
There is reason to believe that kinins play special
roles in regulating blood flow and capillary leakage of
fluids in inflamed tissues. It also is believed that
bradykinin plays a normal role to help regulate blood
flow in the skin as well as in the salivary and gastrointestinal glands.
Histamine. Histamine is released in essentially every
tissue of the body if the tissue becomes damaged or
inflamed or is the subject of an allergic reaction. Most
of the histamine is derived from mast cells in the
damaged tissues and from basophils in the blood.
Histamine has a powerful vasodilator effect on
the arterioles and, like bradykinin, has the ability to
increase greatly capillary porosity, allowing leakage
of both fluid and plasma protein into the tissues. In
many pathological conditions, the intense arteriolar
dilation and increased capillary porosity produced
by histamine cause tremendous quantities of fluid
to leak out of the circulation into the tissues, inducing
edema. The local vasodilatory and edema-producing
effects of histamine are especially prominent
during allergic reactions and are discussed in
Chapter 34.
Vascular Control by Ions and Other
Chemical Factors
Many different ions and other chemical factors can
either dilate or constrict local blood vessels. Most of
them have little function in overall regulation of the
circulation, but some specific effects are:
1. An increase in calcium ion concentration causes
vasoconstriction. This results from the general
effect of calcium to stimulate smooth muscle
contraction, as discussed in Chapter 8.
2. An increase in potassium ion concentration causes
vasodilation. This results from the ability of
potassium ions to inhibit smooth muscle
contraction.
3. An increase in magnesium ion concentration
causes powerful vasodilation because magnesium
ions inhibit smooth muscle contraction.
4. An increase in hydrogen ion concentration
(decrease in pH) causes dilation of the arterioles.
Chapter 17
Local and Humoral Control of Blood Flow by the Tissues
Conversely, slight decrease in hydrogen ion
concentration causes arteriolar constriction.
5. Anions that have significant effects on blood
vessels are acetate and citrate, both of which cause
mild degrees of vasodilation.
6. An increase in carbon dioxide concentration
causes moderate vasodilation in most tissues, but
marked vasodilation in the brain. Also, carbon
dioxide in the blood, acting on the brain
vasomotor center, has an extremely powerful
indirect effect, transmitted through the
sympathetic nervous vasoconstrictor system, to
cause widespread vasoconstriction throughout the
body.
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Chang L, Kaipainen A, Folkman J: Lymphangiogenesis new
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Cowley AW Jr, Mori T, Mattson D, Zou AP: Role of renal
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C
H
A
P
T
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R
1
8
Nervous Regulation of the
Circulation, and Rapid Control
of Arterial Pressure
Nervous Regulation of the
Circulation
As discussed in Chapter 17, adjustment of blood
flow tissue by tissue is mainly the function of local
tissue blood flow control mechanisms. We shall see
in this chapter that nervous control of the circulation has more global functions, such as redistributing blood flow to different areas of the body, increasing or decreasing pumping
activity by the heart, and, especially, providing very rapid control of systemic
arterial pressure.
The nervous system controls the circulation almost entirely through the autonomic nervous system. The total function of this system is presented in Chapter
60, and this subject was also introduced in Chapter 17. For our present discussion, we need to present still other specific anatomical and functional characteristics, as follows.
Autonomic Nervous System
By far the most important part of the autonomic nervous system for regulating
the circulation is the sympathetic nervous system. The parasympathetic nervous
system also contributes specifically to regulation of heart function, as we shall
see later in the chapter.
Sympathetic Nervous System. Figure 18–1 shows the anatomy of sympathetic
nervous control of the circulation. Sympathetic vasomotor nerve fibers leave
the spinal cord through all the thoracic spinal nerves and through the first one
or two lumbar spinal nerves. They then pass immediately into a sympathetic
chain, one of which lies on each side of the vertebral column. Next, they pass
by two routes to the circulation: (1) through specific sympathetic nerves that
innervate mainly the vasculature of the internal viscera and the heart, as shown
on the right side of Figure 18–1, and (2) almost immediately into peripheral portions of the spinal nerves distributed to the vasculature of the peripheral areas.
The precise pathways of these fibers in the spinal cord and in the sympathetic
chains are discussed more fully in Chapter 60.
Sympathetic Innervation of the Blood Vessels. Figure 18–2 shows distribution
of sympathetic nerve fibers to the blood vessels, demonstrating that in most
tissues all the vessels except the capillaries, precapillary sphincters, and metarterioles are innervated.
The innervation of the small arteries and arterioles allows sympathetic stimulation to increase resistance to blood flow and thereby to decrease rate of blood
flow through the tissues.
The innervation of the large vessels, particularly of the veins, makes it possible for sympathetic stimulation to decrease the volume of these vessels. This can
push blood into the heart and thereby play a major role in regulation of heart
pumping, as we shall see later in this and subsequent chapters.
204
Chapter 18
205
Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
Vasomotor center
Blood
vessels
Sympathetic chain
Vagus
Heart
Vasoconstrictor
Cardioinhibitor
Vasodilator
Blood
vessels
Figure 18–1
Anatomy of sympathetic nervous
control of the circulation. Also
shown by the red dashed line is a
vagus nerve that carries parasympathetic signals to the heart.
Sympathetic Nerve Fibers to the Heart. In addition to
sympathetic nerve fibers supplying the blood vessels,
sympathetic fibers also go directly to the heart, as
shown in Figure 18–1 and also discussed in Chapter 9.
It should be recalled that sympathetic stimulation
markedly increases the activity of the heart, both
increasing the heart rate and enhancing its strength
and volume of pumping.
Arteries
Arterioles
Sympathetic
vasoconstriction
Capillaries
Parasympathetic Control of Heart Function, Especially Heart
Rate. Although the parasympathetic nervous system is
exceedingly important for many other autonomic
functions of the body, such as control of multiple gastrointestinal actions, it plays only a minor role in
regulation of the circulation. Its most important circulatory effect is to control heart rate by way of parasympathetic nerve fibers to the heart in the vagus
nerves, shown in Figure 18–1 by the dashed red line
from the brain medulla directly to the heart.
The effects of parasympathetic stimulation on
heart function were discussed in detail in Chapter 9.
Veins
Venules
Figure 18–2
Sympathetic innervation of the systemic circulation.
206
Unit IV
The Circulation
Principally, parasympathetic stimulation causes a
marked decrease in heart rate and a slight decrease in
heart muscle contractility.
Sympathetic Vasoconstrictor System and
Its Control by the Central Nervous System
The sympathetic nerves carry tremendous numbers of
vasoconstrictor nerve fibers and only a few vasodilator
fibers. The vasoconstrictor fibers are distributed to
essentially all segments of the circulation, but more to
some tissues than others. This sympathetic vasoconstrictor effect is especially powerful in the kidneys,
intestines, spleen, and skin but much less potent in
skeletal muscle and the brain.
Vasomotor Center in the Brain and Its Control of the Vasoconstrictor System. Located bilaterally mainly in the retic-
ular substance of the medulla and of the lower third
of the pons, shown in Figures 18–1 and 18–3, is an area
called the vasomotor center. This center transmits
parasympathetic impulses through the vagus nerves to
the heart and transmits sympathetic impulses through
the spinal cord and peripheral sympathetic nerves
to virtually all arteries, arterioles, and veins of the
body.
Although the total organization of the vasomotor
center is still unclear, experiments have made it
Motor
Cingulate
Reticular
substance
Mesencephalon
Orbital
Temporal
Pons
Medulla
VASOMOTOR
CENTER
VASODILATOR
VASOCONSTRICTOR
Figure 18–3
Areas of the brain that play important roles in the nervous regulation of the circulation. The dashed lines represent inhibitory
pathways.
possible to identify certain important areas in this
center, as follows:
1. A vasoconstrictor area located bilaterally in the
anterolateral portions of the upper medulla. The
neurons originating in this area distribute their
fibers to all levels of the spinal cord, where they
excite preganglionic vasoconstrictor neurons of the
sympathetic nervous system.
2. A vasodilator area located bilaterally in the
anterolateral portions of the lower half of the
medulla. The fibers from these neurons project
upward to the vasoconstrictor area just described;
they inhibit the vasoconstrictor activity of this area,
thus causing vasodilation.
3. A sensory area located bilaterally in the tractus
solitarius in the posterolateral portions of the
medulla and lower pons. The neurons of this area
receive sensory nerve signals from the circulatory
system mainly through the vagus and
glossopharyngeal nerves, and output signals from
this sensory area then help to control activities of
both the vasoconstrictor and vasodilator areas of
the vasomotor center, thus providing “reflex”
control of many circulatory functions. An example
is the baroreceptor reflex for controlling arterial
pressure, which we describe later in this chapter.
Continuous Partial Constriction of the Blood Vessels Is Normally Caused by Sympathetic Vasoconstrictor Tone. Under
normal conditions, the vasoconstrictor area of the
vasomotor center transmits signals continuously to
the sympathetic vasoconstrictor nerve fibers over the
entire body, causing continuous slow firing of these
fibers at a rate of about one half to two impulses per
second. This continual firing is called sympathetic vasoconstrictor tone. These impulses normally maintain a
partial state of contraction in the blood vessels, called
vasomotor tone.
Figure 18–4 demonstrates the significance of vasoconstrictor tone. In the experiment of this figure,
total spinal anesthesia was administered to an animal.
This blocked all transmission of sympathetic nerve
impulses from the spinal cord to the periphery. As a
result, the arterial pressure fell from 100 to 50 mm Hg,
demonstrating the effect of losing vasoconstrictor tone
throughout the body. A few minutes later, a small
amount of the hormone norepinephrine was injected
into the blood (norepinephrine is the principal
vasoconstrictor hormonal substance secreted at the
endings of the sympathetic vasoconstrictor nerve
fibers throughout the body). As this injected hormone
was transported in the blood to all blood vessels, the
vessels once again became constricted, and the arterial
pressure rose to a level even greater than normal for
1 to 3 minutes, until the norepinephrine was destroyed.
Control of Heart Activity by the Vasomotor Center. At the
same time that the vasomotor center is controlling the
amount of vascular constriction, it also controls heart
activity. The lateral portions of the vasomotor center
transmit excitatory impulses through the sympathetic
nerve fibers to the heart when there is need to increase
heart rate and contractility. Conversely, when there is
need to decrease heart pumping, the medial portion of
Chapter 18
207
Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
Arterial pressure (mm Hg)
150
125
Total spinal
anesthesia
100
75
50
Injection of norepinephrine
25
Figure 18–4
0
Effect of total spinal anesthesia on
the arterial pressure, showing
marked decrease in pressure
resulting from loss of “vasomotor
tone.”
the vasomotor center sends signals to the adjacent
dorsal motor nuclei of the vagus nerves, which then
transmit parasympathetic impulses through the vagus
nerves to the heart to decrease heart rate and heart
contractility. Therefore, the vasomotor center can
either increase or decrease heart activity. Heart rate
and strength of heart contraction ordinarily increase
when vasoconstriction occurs and ordinarily decrease
when vasoconstriction is inhibited.
Control of the Vasomotor Center by Higher Nervous Centers.
Large numbers of small neurons located throughout
the reticular substance of the pons, mesencephalon,
and diencephalon can either excite or inhibit the
vasomotor center. This reticular substance is shown
in Figure 18–3 by the rose-colored area. In general, the
neurons in the more lateral and superior portions
of the reticular substance cause excitation, whereas
the more medial and inferior portions cause inhibition.
The hypothalamus plays a special role in controlling
the vasoconstrictor system because it can exert either
powerful excitatory or inhibitory effects on the vasomotor center. The posterolateral portions of the hypothalamus cause mainly excitation, whereas the anterior
portion can cause either mild excitation or inhibition,
depending on the precise part of the anterior hypothalamus stimulated.
Many parts of the cerebral cortex can also excite or
inhibit the vasomotor center. Stimulation of the motor
cortex, for instance, excites the vasomotor center
because of impulses transmitted downward into the
hypothalamus and thence to the vasomotor center.
Also, stimulation of the anterior temporal lobe, the
orbital areas of the frontal cortex, the anterior part of
0
5
10
Seconds
15
20
25
the cingulate gyrus, the amygdala, the septum, and the
hippocampus can all either excite or inhibit the vasomotor center, depending on the precise portions of
these areas that are stimulated and on the intensity of
stimulus. Thus, widespread basal areas of the brain can
have profound effects on cardiovascular function.
Norepinephrine—The Sympathetic Vasoconstrictor Transmitter
Substance. The substance secreted at the endings of the
vasoconstrictor nerves is almost entirely norepinephrine. Norepinephrine acts directly on the alpha adrenergic receptors of the vascular smooth muscle to cause
vasoconstriction, as discussed in Chapter 60.
Adrenal Medullae and Their Relation to the Sympathetic Vasoconstrictor System. Sympathetic impulses are transmit-
ted to the adrenal medullae at the same time that they
are transmitted to the blood vessels. They cause the
medullae to secrete both epinephrine and norepinephrine into the circulating blood. These two hormones are
carried in the blood stream to all parts of the body,
where they act directly on all blood vessels, usually
to cause vasoconstriction, but in an occasional tissue
epinephrine causes vasodilation because it also has
a “beta” adrenergic receptor stimulatory effect, which
dilates rather than constricts certain vessels, as discussed in Chapter 60.
Sympathetic Vasodilator System and its Control by the Central
Nervous System. The sympathetic nerves to skeletal
muscles carry sympathetic vasodilator fibers as well as
constrictor fibers. In lower animals such as the cat, these
dilator fibers release acetylcholine, not norepinephrine,
at their endings, although in primates, the vasodilator
208
Unit IV
The Circulation
effect is believed to be caused by epinephrine exciting
specific beta adrenergic receptors in the muscle
vasculature.
The pathway for central nervous system control of
the vasodilator system is shown by the dashed lines in
Figure 18–3. The principal area of the brain controlling
this system is the anterior hypothalamus.
Possible Unimportance of the Sympathetic Vasodilator System.
It is doubtful that the sympathetic vasodilator system
plays an important role in the control of the circulation
in the human being because complete block of the sympathetic nerves to the muscles hardly affects the ability
of these muscles to control their own blood flow in
response to their needs. Yet some experiments suggest
that at the onset of exercise, the sympathetic vasodilator system might cause initial vasodilation in skeletal
muscles to allow anticipatory increase in blood flow even
before the muscles require increased nutrients.
Emotional Fainting—Vasovagal Syncope. A particularly
interesting vasodilatory reaction occurs in people who
experience intense emotional disturbances that cause
fainting. In this case, the muscle vasodilator system
becomes activated, and at the same time, the vagal cardioinhibitory center transmits strong signals to the heart
to slow the heart rate markedly. The arterial pressure
falls rapidly, which reduces blood flow to the brain and
causes the person to lose consciousness. This overall
effect is called vasovagal syncope. Emotional fainting
begins with disturbing thoughts in the cerebral cortex.
The pathway probably then goes to the vasodilatory
center of the anterior hypothalamus next to the vagal
centers of the medulla, to the heart through the vagus
nerves, and also through the spinal cord to the sympathetic vasodilator nerves of the muscles.
Role of the Nervous System
in Rapid Control of
Arterial Pressure
One of the most important functions of nervous
control of the circulation is its capability to cause rapid
increases in arterial pressure. For this purpose, the
entire vasoconstrictor and cardioaccelerator functions
of the sympathetic nervous system are stimulated
together. At the same time, there is reciprocal inhibition of parasympathetic vagal inhibitory signals to the
heart. Thus, three major changes occur simultaneously,
each of which helps to increase arterial pressure. They
are as follows:
1. Almost all arterioles of the systemic circulation
are constricted. This greatly increases the total
peripheral resistance, thereby increasing the arterial
pressure.
2. The veins especially (but the other large vessels of
the circulation as well) are strongly constricted.
This displaces blood out of the large peripheral
blood vessels toward the heart, thus increasing the
volume of blood in the heart chambers. The stretch
of the heart then causes the heart to beat with far
greater force and therefore to pump increased
quantities of blood. This, too, increases the arterial
pressure.
3. Finally, the heart itself is directly stimulated by the
autonomic nervous system, further enhancing
cardiac pumping. Much of this is caused by an
increase in the heart rate, the rate sometimes
increasing to as great as three times normal. In
addition, sympathetic nervous signals have a
significant direct effect to increase contractile
force of the heart muscle, this, too, increasing the
capability of the heart to pump larger volumes of
blood. During strong sympathetic stimulation, the
heart can pump about two times as much blood as
under normal conditions. This contributes still more
to the acute rise in arterial pressure.
Rapidity of Nervous Control of Arterial Pressure. An especially important characteristic of nervous control of
arterial pressure is its rapidity of response, beginning
within seconds and often increasing the pressure to
two times normal within 5 to 10 seconds. Conversely,
sudden inhibition of nervous cardiovascular stimulation can decrease the arterial pressure to as little as
one half normal within 10 to 40 seconds. Therefore,
nervous control of arterial pressure is by far the most
rapid of all our mechanisms for pressure control.
Increase in Arterial Pressure
During Muscle Exercise and Other
Types of Stress
An important example of the ability of the nervous
system to increase the arterial pressure is the increase
in pressure that occurs during muscle exercise. During
heavy exercise, the muscles require greatly increased
blood flow. Part of this increase results from local
vasodilation of the muscle vasculature caused by
increased metabolism of the muscle cells, as explained
in Chapter 17. Additional increase results from simultaneous elevation of arterial pressure caused by sympathetic stimulation of the overall circulation during
exercise. In most heavy exercise, the arterial pressure
rises about 30 to 40 per cent, which increases blood
flow almost an additional twofold.
The increase in arterial pressure during exercise
results mainly from the following effect: At the same
time that the motor areas of the brain become activated to cause exercise, most of the reticular activating system of the brain stem is also activated, which
includes greatly increased stimulation of the vasoconstrictor and cardioacceleratory areas of the vasomotor
center. These increase the arterial pressure instantaneously to keep pace with the increase in muscle
activity.
In many other types of stress besides muscle exercise, a similar rise in pressure can also occur. For
instance, during extreme fright, the arterial pressure
sometimes rises to as high as double normal within a
few seconds. This is called the alarm reaction, and it
provides an excess of arterial pressure that can immediately supply blood to any or all muscles of the body
that might need to respond instantly to cause flight
from danger.
Chapter 18
Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
209
Reflex Mechanisms for Maintaining
Normal Arterial Pressure
Aside from the exercise and stress functions of the
autonomic nervous system to increase arterial pressure, there are multiple subconscious special nervous
control mechanisms that operate all the time to maintain the arterial pressure at or near normal. Almost
all of these are negative feedback reflex mechanisms,
which we explain in the following sections.
Glossopharyngeal nerve
Hering’s nerve
The Baroreceptor Arterial Pressure Control
System—Baroreceptor Reflexes
Carotid body
Carotid sinus
By far the best known of the nervous mechanisms for
arterial pressure control is the baroreceptor reflex.
Basically, this reflex is initiated by stretch receptors,
called either baroreceptors or pressoreceptors, located
at specific points in the walls of several large systemic
arteries. A rise in arterial pressure stretches the
baroreceptors and causes them to transmit signals into
the central nervous system. “Feedback” signals are
then sent back through the autonomic nervous system
to the circulation to reduce arterial pressure downward toward the normal level.
Vagus nerve
Aortic baroreceptors
Physiologic Anatomy of the Baroreceptors and Their Innervation. Baroreceptors are spray-type nerve endings that
Response of the Baroreceptors to Pressure. Figure 18–6
shows the effect of different arterial pressure levels on
the rate of impulse transmission in a Hering’s carotid
sinus nerve. Note that the carotid sinus baroreceptors
are not stimulated at all by pressures between 0 and
50 to 60 mm Hg, but above these levels, they respond
progressively more rapidly and reach a maximum at
about 180 mm Hg. The responses of the aortic baroreceptors are similar to those of the carotid receptors
except that they operate, in general, at pressure levels
about 30 mm Hg higher.
Note especially that in the normal operating range
of arterial pressure, around 100 mm Hg, even a slight
change in pressure causes a strong change in the
baroreflex signal to readjust arterial pressure back
toward normal. Thus, the baroreceptor feedback
mechanism functions most effectively in the pressure
range where it is most needed.
Figure 18–5
The baroreceptor system for controlling arterial pressure.
Number of impulses from carotid
sinus nerves per second
lie in the walls of the arteries; they are stimulated when
stretched. A few baroreceptors are located in the wall
of almost every large artery of the thoracic and neck
regions; but, as shown in Figure 18–5, baroreceptors
are extremely abundant in (1) the wall of each internal carotid artery slightly above the carotid bifurcation, an area known as the carotid sinus, and (2) the
wall of the aortic arch.
Figure 18–5 shows that signals from the “carotid
baroreceptors” are transmitted through very small
Hering’s nerves to the glossopharyngeal nerves in
the high neck, and then to the tractus solitarius in the
medullary area of the brain stem. Signals from the
“aortic baroreceptors” in the arch of the aorta are
transmitted through the vagus nerves also to the same
tractus solitarius of the medulla.
DI = maximum
DP
0
80
160
244
Arterial blood pressure (mm Hg)
Figure 18–6
Activation of the baroreceptors at different levels of arterial
pressure. DI, change in carotid sinus nerve impulses per second;
DP, change in arterial blood pressure in mm Hg.
210
Unit IV
The Circulation
The baroreceptors respond extremely rapidly to
changes in arterial pressure; in fact, the rate of impulse
firing increases in the fraction of a second during each
systole and decreases again during diastole. Furthermore, the baroreceptors respond much more to a
rapidly changing pressure than to a stationary pressure. That is, if the mean arterial pressure is 150 mm
Hg but at that moment is rising rapidly, the rate of
impulse transmission may be as much as twice that
when the pressure is stationary at 150 mm Hg.
Arterial pressure (mm Hg)
Circulatory Reflex Initiated by the Baroreceptors. After the
baroreceptor signals have entered the tractus solitarius of the medulla, secondary signals inhibit the vasoconstrictor center of the medulla and excite the vagal
parasympathetic center. The net effects are (1) vasodilation of the veins and arterioles throughout the
peripheral circulatory system and (2) decreased heart
rate and strength of heart contraction. Therefore, excitation of the baroreceptors by high pressure in the
arteries reflexly causes the arterial pressure to decrease
because of both a decrease in peripheral resistance
and a decrease in cardiac output. Conversely, low pressure has opposite effects, reflexly causing the pressure
to rise back toward normal.
Figure 18–7 shows a typical reflex change in arterial
pressure caused by occluding the two common carotid
arteries. This reduces the carotid sinus pressure; as a
result, the baroreceptors become inactive and lose
their inhibitory effect on the vasomotor center. The
vasomotor center then becomes much more active
than usual, causing the aortic arterial pressure to rise
and remain elevated during the 10 minutes that the
150
100
Both common
carotids clamped
Carotids released
50
0
0
2
4
6
8
10
Minutes
12
14
Figure 18–7
Typical carotid sinus reflex effect on aortic arterial pressure
caused by clamping both common carotids (after the two vagus
nerves have been cut).
carotids are occluded. Removal of the occlusion allows
the pressure in the carotid sinuses to rise, and the
carotid sinus reflex now causes the aortic pressure to
fall immediately to slightly below normal as a momentary overcompensation and then return to normal in
another minute.
Function of the Baroreceptors During Changes in Body
Posture. The ability of the baroreceptors to maintain
relatively constant arterial pressure in the upper body
is important when a person stands up after having
been lying down. Immediately on standing, the arterial
pressure in the head and upper part of the body tends
to fall, and marked reduction of this pressure could
cause loss of consciousness. However, the falling pressure at the baroreceptors elicits an immediate reflex,
resulting in strong sympathetic discharge throughout
the body. This minimizes the decrease in pressure in
the head and upper body.
Pressure “Buffer” Function of the Baroreceptor
Control System. Because the baroreceptor system
opposes either increases or decreases in arterial pressure, it is called a pressure buffer system, and the nerves
from the baroreceptors are called buffer nerves.
Figure 18–8 shows the importance of this buffer
function of the baroreceptors. The upper record in this
figure shows an arterial pressure recording for 2 hours
from a normal dog, and the lower record shows an
arterial pressure recording from a dog whose baroreceptor nerves from both the carotid sinuses and the
aorta had been removed. Note the extreme variability
of pressure in the denervated dog caused by simple
events of the day, such as lying down, standing, excitement, eating, defecation, and noises.
Figure 18–9 shows the frequency distributions of the
mean arterial pressures recorded for a 24-hour day in
both the normal dog and the denervated dog. Note
that when the baroreceptors were functioning normally the mean arterial pressure remained throughout
the day within a narrow range between 85 and
115 mm Hg—indeed, during most of the day at almost
exactly 100 mm Hg. Conversely, after denervation of
the baroreceptors, the frequency distribution curve
became the broad, low curve of the figure, showing
that the pressure range increased 2.5-fold, frequently
falling to as low as 50 mm Hg or rising to over 160 mm
Hg. Thus, one can see the extreme variability of pressure in the absence of the arterial baroreceptor
system.
In summary, a primary purpose of the arterial
baroreceptor system is to reduce the minute by minute
variation in arterial pressure to about one third that
which would occur were the baroreceptor system not
present.
Are the Baroreceptors Important in Long-Term Regulation of
Arterial Pressure? Although the arterial baroreceptors
provide powerful moment-to-moment control of
arterial pressure, their importance in long-term
blood pressure regulation has been controversial. One
reason that the baroreceptors have been considered
Chapter 18
211
Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
NORMAL
200
Percentage of occurrence
6
Arterial pressure (mm Hg)
100
0
24
DENERVATED
200
5
Normal
4
3
2
Denervated
1
0
0
50
100
150
200
250
Mean arterial pressure (mm Hg)
100
Figure 18–9
0
Time (min)
24
Frequency distribution curves of the arterial pressure for a 24-hour
period in a normal dog and in the same dog several weeks after
the baroreceptors had been denervated. (Redrawn from Cowley
AW Jr, Liard JP, Guyton AC: Role of baroreceptor reflex in daily
control of arterial blood pressure and other variables in dogs. Circ
Res 32:564, 1973. By permission of the American Heart Association, Inc.)
Figure 18–8
Two-hour records of arterial pressure in a normal dog (above) and
in the same dog (below) several weeks after the baroreceptors
had been denervated. (Redrawn from Cowley AW Jr, Liard JF,
Guyton AC: Role of baroreceptor reflex in daily control of arterial
blood pressure and other variables in dogs. Circ Res 32:564,
1973. By permission of the American Heart Association, Inc.)
by some physiologists to be relatively unimportant in
chronic regulation of arterial pressure chronically is
that they tend to reset in 1 to 2 days to the pressure
level to which they are exposed. That is, if the arterial
pressure rises from the normal value of 100 mm Hg to
160 mm Hg, a very high rate of baroreceptor impulses
are at first transmitted. During the next few minutes,
the rate of firing diminishes considerably; then it
diminishes much more slowly during the next 1 to 2
days, at the end of which time the rate of firing will
have returned to nearly normal despite the fact that
the mean arterial pressure still remains at 160 mm Hg.
Conversely, when the arterial pressure falls to a very
low level, the baroreceptors at first transmit no
impulses, but gradually, over 1 to 2 days, the rate of
baroreceptor firing returns toward the control level.
This “resetting” of the baroreceptors may attenuate
their potency as a control system for correcting disturbances that tend to change arterial pressure for
longer than a few days at a time. Experimental studies,
however, have suggested that the baroreceptors do
not completely reset and may therefore contribute to
long-term blood pressure regulation, especially by
influencing sympathetic nerve activity of the kidneys.
For example, with prolonged increases in arterial pressure, the baroreceptor reflexes may mediate decreases
in renal sympathetic nerve activity that promote
increased excretion of sodium and water by the
kidneys. This, in turn, causes a gradual decrease in
blood volume, which helps to restore arterial pressure
toward normal. Thus, long-term regulation of mean
arterial pressure by the baroreceptors requires
interaction with additional systems, principally the
renal–body fluid–pressure control system (along with
its associated nervous and hormonal mechanisms), discussed in Chapters 19 and 29.
Control of Arterial Pressure by the Carotid and Aortic
Chemoreceptors—Effect of Oxygen Lack on Arterial Pressure.
Closely associated with the baroreceptor pressure
control system is a chemoreceptor reflex that operates in
much the same way as the baroreceptor reflex except
that chemoreceptors, instead of stretch receptors, initiate the response.
The chemoreceptors are chemosensitive cells sensitive to oxygen lack, carbon dioxide excess, and hydrogen ion excess. They are located in several small
chemoreceptor organs about 2 millimeters in size (two
carotid bodies, one of which lies in the bifurcation of
each common carotid artery, and usually one to three
aortic bodies adjacent to the aorta). The chemoreceptors excite nerve fibers that, along with the
212
Unit IV
baroreceptor fibers, pass through Hering’s nerves and
the vagus nerves into the vasomotor center of the brain
stem.
Each carotid or aortic body is supplied with an
abundant blood flow through a small nutrient artery, so
that the chemoreceptors are always in close contact
with arterial blood. Whenever the arterial pressure falls
below a critical level, the chemoreceptors become stimulated because diminished blood flow causes decreased
oxygen as well as excess buildup of carbon dioxide and
hydrogen ions that are not removed by the slowly
flowing blood.
The signals transmitted from the chemoreceptors
excite the vasomotor center, and this elevates the arterial
pressure back toward normal. However, this chemoreceptor reflex is not a powerful arterial pressure controller until the arterial pressure falls below 80 mm Hg.
Therefore, it is at the lower pressures that this reflex
becomes important to help prevent still further fall in
pressure.
The chemoreceptors are discussed in much more
detail in Chapter 41 in relation to respiratory control,
in which they play a far more important role than in
pressure control.
Atrial and Pulmonary Artery Reflexes That Help Regulate Arterial
Pressure and Other Circulatory Factors. Both the atria and
the pulmonary arteries have in their walls stretch receptors called low-pressure receptors. They are similar to
the baroreceptor stretch receptors of the large systemic
arteries.These low-pressure receptors play an important
role, especially in minimizing arterial pressure changes
in response to changes in blood volume. To give an
example, if 300 milliliters of blood suddenly are infused
into a dog with allreceptors intact, the arterial pressure
rises only about 15 mm Hg. With the arterial baroreceptors denervated, the pressure rises about 40 mm Hg. If
the low-pressure receptors also are denervated, the pressure rises about 100 mm Hg.
Thus, one can see that even though the low-pressure
receptors in the pulmonary artery and in the atria
cannot detect the systemic arterial pressure, they do
detect simultaneous increases in pressure in the lowpressure areas of the circulation caused by increase in
volume, and they elicit reflexes parallel to the baroreceptor reflexes to make the total reflex system more
potent for control of arterial pressure.
Atrial Reflexes That Activate the Kidneys—The “Volume Reflex.”
Stretch of the atria also causes significant reflex dilation
of the afferent arterioles in the kidneys. And still
other signals are transmitted simultaneously from the
atria to the hypothalamus to decrease secretion of
antidiuretic hormone. The decreased afferent arteriolar
resistance in the kidneys causes the glomerular capillary
pressure to rise, with resultant increase in filtration of
fluid into the kidney tubules. The diminution of antidiuretic hormone diminishes the reabsorption of water
from the tubules. Combination of these two effects—
increase in glomerular filtration and decrease in reabsorption of the fluid—increases fluid loss by the kidneys
and reduces an increased blood volume back toward
normal. (We will also see in Chapter 19 that atrial
stretch caused by increased blood volume also elicits a
hormonal effect on the kidneys—release of atrial natriuretic peptide that adds still further to the excretion of
fluid in the urine and return of blood volume toward
normal.)
The Circulation
All these mechanisms that tend to return the blood
volume back toward normal after a volume overload act
indirectly as pressure controllers as well as blood
volume controllers because excess volume drives the
heart to greater cardiac output and leads, therefore, to
greater arterial pressure. This volume reflex mechanism
is discussed again in Chapter 29, along with other mechanisms of blood volume control.
Atrial Reflex Control of Heart Rate (the Bainbridge Reflex). An
increase in atrial pressure also causes an increase in
heart rate, sometimes increasing the heart rate as much
as 75 per cent. A small part of this increase is caused by
a direct effect of the increased atrial volume to stretch
the sinus node: it was pointed out in Chapter 10 that
such direct stretch can increase the heart rate as much
as 15 per cent. An additional 40 to 60 per cent increase
in rate is caused by a nervous reflex called the Bainbridge reflex. The stretch receptors of the atria that elicit
the Bainbridge reflex transmit their afferent signals
through the vagus nerves to the medulla of the brain.
Then efferent signals are transmitted back through
vagal and sympathetic nerves to increase heart rate and
strength of heart contraction. Thus, this reflex helps
prevent damming of blood in the veins, atria, and pulmonary circulation.
Central Nervous System Ischemic
Response—Control of Arterial
Pressure by the Brain’s Vasomotor
Center in Response to Diminished
Brain Blood Flow
Most nervous control of blood pressure is achieved
by reflexes that originate in the baroreceptors, the
chemoreceptors, and the low-pressure receptors, all of
which are located in the peripheral circulation outside
the brain. However, when blood flow to the vasomotor
center in the lower brain stem becomes decreased
severely enough to cause nutritional deficiency—that is,
to cause cerebral ischemia—the vasoconstrictor and
cardioaccelerator neurons in the vasomotor center
respond directly to the ischemia and become strongly
excited. When this occurs, the systemic arterial pressure
often rises to a level as high as the heart can possibly
pump. This effect is believed to be caused by failure of
the slowly flowing blood to carry carbon dioxide away
from the brain stem vasomotor center: at low levels of
blood flow to the vasomotor center, the local concentration of carbon dioxide increases greatly and
has an extremely potent effect in stimulating the sympathetic vasomotor nervous control areas in the brain’s
medulla.
It is possible that other factors, such as buildup of
lactic acid and other acidic substances in the vasomotor
center, also contribute to the marked stimulation and
elevation in arterial pressure. This arterial pressure elevation in response to cerebral ischemia is known as the
central nervous system ischemic response, or simply CNS
ischemic response.
The magnitude of the ischemic effect on vasomotor
activity is tremendous: it can elevate the mean arterial
pressure for as long as 10 minutes sometimes to as high
as 250 mm Hg. The degree of sympathetic vasoconstriction caused by intense cerebral ischemia is often so great
that some of the peripheral vessels become totally or
almost totally occluded. The kidneys, for instance, often
Chapter 18
Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
Pen
recorder
Arterial
pressure
Zero
pressure
213
CSF pressure
raised
CSF pressure
reduced
Moving paper
Figure 18–10
“Cushing reaction,” showing a
rapid rise in arterial pressure
resulting from increased cerebrospinal fluid (CSF) pressure.
Pressure
bottle
Connector to
subarachnoid
space
entirely cease their production of urine because of renal
arteriolar constriction in response to the sympathetic
discharge. Therefore, the CNS ischemic response is one
of the most powerful of all the activators of the sympathetic vasoconstrictor system.
Importance of the CNS Ischemic Response as a Regulator of Arterial Pressure. Despite the powerful nature of the CNS
ischemic response, it does not become significant until
the arterial pressure falls far below normal, down to
60 mm Hg and below, reaching its greatest degree of
stimulation at a pressure of 15 to 20 mm Hg. Therefore,
it is not one of the normal mechanisms for regulating
arterial pressure. Instead, it operates principally as an
emergency pressure control system that acts rapidly and
very powerfully to prevent further decrease in arterial
pressure whenever blood flow to the brain decreases dangerously close to the lethal level. It is sometimes called
the “last ditch stand” pressure control mechanism.
Cushing Reaction. The so-called Cushing reaction is a
special type of CNS ischemic response that results from
increased pressure of the cerebrospinal fluid around
the brain in the cranial vault. For instance, when the
cerebrospinal fluid pressure rises to equal the arterial
pressure, it compresses the whole brain as well as the
arteries in the brain and cuts off the blood supply to the
brain.This initiates a CNS ischemic response that causes
the arterial pressure to rise. When the arterial pressure
has risen to a level higher than the cerebrospinal fluid
pressure, blood will flow once again into the vessels of
the brain to relieve the brain ischemia. Ordinarily, the
blood pressure comes to a new equilibrium level slightly
higher than the cerebrospinal fluid pressure, thus allowing blood to begin again to flow through the brain. A
typical Cushing reaction is shown in Figure 18–10,
caused in this instance by pumping fluid under pressure
into the cranial vault around the brain. The Cushing
reaction helps protect the vital centers of the brain from
loss of nutrition if ever the cerebrospinal fluid pressure
rises high enough to compress the cerebral arteries.
Arterial
pressure
transducer
Special Features of Nervous
Control of Arterial Pressure
Role of the Skeletal Nerves and
Skeletal Muscles in Increasing
Cardiac Output and
Arterial Pressure
Although most rapidly acting nervous control of the circulation is effected through the autonomic nervous
system, at least two conditions in which the skeletal
nerves and muscles also play major roles in circulatory
responses are the following.
Abdominal Compression Reflex. When a baroreceptor or
chemoreceptor reflex is elicited, nerve signals are transmitted simultaneously through skeletal nerves to skeletal muscles of the body, particularly to the abdominal
muscles. This compresses all the venous reservoirs of
the abdomen, helping to translocate blood out of the
abdominal vascular reservoirs toward the heart. As a
result, increased quantities of blood are made available
for the heart to pump. This overall response is called the
abdominal compression reflex. The resulting effect on
the circulation is the same as that caused by sympathetic
vasoconstrictor impulses when they constrict the veins:
an increase in both cardiac output and arterial pressure.
The abdominal compression reflex is probably much
more important than has been realized in the past
because it is well known that people whose skeletal
muscles have been paralyzed are considerably more
prone to hypotensive episodes than are people with
normal skeletal muscles.
Increased Cardiac Output and Arterial Pressure Caused by
Skeletal Muscle Contraction During Exercise. When the
skeletal muscles contract during exercise, they compress
blood vessels throughout the body. Even anticipation of
exercise tightens the muscles, thereby compressing
the vessels in the muscles and in the abdomen. The
Unit IV
resulting effect is to translocate blood from the peripheral vessels into the heart and lungs and, therefore, to
increase the cardiac output. This is an essential effect in
helping to cause the fivefold to sevenfold increase in
cardiac output that sometimes occurs in heavy exercise.
The increase in cardiac output in turn is an essential
ingredient in increasing the arterial pressure during
exercise, an increase usually from a normal mean of
100 mm Hg up to 130 to 160 mm Hg.
Respiratory Waves in the
Arterial Pressure
With each cycle of respiration, the arterial pressure
usually rises and falls 4 to 6 mm Hg in a wavelike
manner, causing respiratory waves in the arterial pressure. The waves result from several different effects,
some of which are reflex in nature, as follows:
1. Many of the “breathing signals” that arise in the
respiratory center of the medulla “spill over” into
the vasomotor center with each respiratory cycle.
2. Every time a person inspires, the pressure in the
thoracic cavity becomes more negative than usual,
causing the blood vessels in the chest to expand.
This reduces the quantity of blood returning to the
left side of the heart and thereby momentarily
decreases the cardiac output and arterial pressure.
3. The pressure changes caused in the thoracic vessels
by respiration can excite vascular and atrial stretch
receptors.
Although it is difficult to analyze the exact relations
of all these factors in causing the respiratory pressure
waves, the net result during normal respiration is usually
an increase in arterial pressure during the early part
of expiration and a decrease in pressure during the
remainder of the respiratory cycle. During deep respiration, the blood pressure can rise and fall as much as
20 mm Hg with each respiratory cycle.
The Circulation
Pressure (mm Hg)
214
200
160
120
80
40
0
A
100
60
B
Figure 18–11
A, Vasomotor waves caused by oscillation of the CNS ischemic
response. B, Vasomotor waves caused by baroreceptor reflex
oscillation.
pressure in turn reduces the baroreceptor stimulation
and allows the vasomotor center to become active once
again, elevating the pressure to a high value. The
response is not instantaneous, and it is delayed until a
few seconds later. This high pressure then initiates
another cycle, and the oscillation continues on and on.
The chemoreceptor reflex can also oscillate to give the
same type of waves. This reflex usually oscillates simultaneously with the baroreceptor reflex. It probably plays
the major role in causing vasomotor waves when the
arterial pressure is in the range of 40 to 80 mm Hg
because in this low range, chemoreceptor control of the
circulation becomes powerful, whereas baroreceptor
control becomes weaker.
Oscillation of the CNS Ischemic Response. The record in
Arterial Pressure “Vasomotor”
Waves—Oscillation of Pressure
Reflex Control Systems
Often while recording arterial pressure from an animal,
in addition to the small pressure waves caused by respiration, some much larger waves are also noted—as
great as 10 to 40 mm Hg at times—that rise and fall
more slowly than the respiratory waves. The duration of
each cycle varies from 26 seconds in the anesthetized
dog to 7 to 10 seconds in the unanesthetized human.
These waves are called vasomotor waves or “Mayer
waves.” Such records are demonstrated in Figure 18–11,
showing the cyclical rise and fall in arterial pressure.
The cause of vasomotor waves is “reflex oscillation”
of one or more nervous pressure control mechanisms,
some of which are the following.
Oscillation of the Baroreceptor and Chemoreceptor Reflexes.
The vasomotor waves of Figure 18–11B are often seen
in experimental pressure recordings, although usually
much less intense than shown in the figure. They are
caused mainly by oscillation of the baroreceptor reflex.
That is, a high pressure excites the baroreceptors;
this then inhibits the sympathetic nervous system and
lowers the pressure a few seconds later. The decreased
Figure 18–11A resulted from oscillation of the CNS
ischemic pressure control mechanism. In this experiment, the cerebrospinal fluid pressure was raised to
160 mm Hg, which compressed the cerebral vessels and
initiated a CNS ischemic pressure response up to
200 mm Hg. When the arterial pressure rose to such a
high value, the brain ischemia was relieved and the sympathetic nervous system became inactive. As a result,
the arterial pressure fell rapidly back to a much lower
value, causing brain ischemia once again. The ischemia
then initiated another rise in pressure. Again the
ischemia was relieved and again the pressure fell. This
repeated itself cyclically as long as the cerebrospinal
fluid pressure remained elevated.
Thus, any reflex pressure control mechanism can
oscillate if the intensity of “feedback” is strong enough
and if there is a delay between excitation of the pressure receptor and the subsequent pressure response.
The vasomotor waves are of considerable theoretical
importance because they show that the nervous reflexes
that control arterial pressure obey the same principles
as those applicable to mechanical and electrical control
systems. For instance, if the feedback “gain” is too great
in the guiding mechanism of an automatic pilot for an
airplane and there is also delay in response time of the
guiding mechanism, the plane will oscillate from side to
side instead of following a straight course.
Chapter 18
Nervous Regulation of the Circulation, and Rapid Control of Arterial Pressure
References
Antunes-Rodrigues J, De Castro M, Elias LLK, et al: Neuroendocrine control of body fluid metabolism. Physiol Rev
84: 169, 2004.
Cao WH, Fan W, Morrison SF: Medullary pathways mediating specific sympathetic responses to activation of dorsomedial hypothalamus. Neuroscience 126:229, 2004.
Cowley AW Jr, Guyton AC: Baroreceptor reflex contribution in angiotensin II–induced hypertension. Circulation
50:61, 1974.
DiBona GF: Peripheral and central interactions between
the renin-angiotensin system and the renal sympathetic
nerves in control of renal function. Ann N Y Acad Sci
940:395, 2001.
DiCarlo SE, Bishop VS: Central baroreflex resetting as a
means of increasing and decreasing sympathetic outflow
and arterial pressure. Ann N Y Acad Sci 940:324, 2001.
Esler M, Lambert G, Brunner-La Rocca HP, et al: Sympathetic nerve activity and neurotransmitter release in
humans: translation from pathophysiology into clinical
practice. Acta Physiol Scand 177:275, 2003.
Floras JS: Arterial baroreceptor and cardiopulmonary reflex
control of sympathetic outflow in human heart failure.
Ann N Y Acad Sci 940:500, 2001.
Felder RB, Francis J, Zhang ZH, et al: Heart failure and the
brain: new perspectives. Am J Physiol Regul Integr Comp
Physiol 284:R259, 2003.
Goldstein DS, Robertson D, Esler M, et al: Dysautonomias:
clinical disorders of the autonomic nervous system. Ann
Intern Med 137:753, 2002.
Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders Co, 1980.
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Hall JE, Hildebrandt DA, Kuo J: Obesity hypertension: role
of leptin and sympathetic nervous system. Am J Hypertens 14:103S, 2001.
Ketch T, Biaggioni I, Robertson R, Robertson D: Four faces
of baroreflex failure: hypertensive crisis, volatile hypertension, orthostatic tachycardia, and malignant vagotonia.
Circulation 105:2518, 2002.
Krieger EM, Da Silva GJ, Negrao CE: Effects of exercise
training on baroreflex control of the cardiovascular
system. Ann N Y Acad Sci 940:338, 2001.
Lohmeier TE, Lohmeier JR, Warren S, et al: Sustained activation of the central baroreceptor pathway in angiotensin
hypertension. Hypertension 39:550, 2002.
Lohmeier TE: The sympathetic nervous system and longterm blood pressure regulation. Am J Hypertens 14:147S,
2001.
Malpas SC: What sets the long-term level of sympathetic
nerve activity: is there a role for arterial baroreceptors?
Am J Physiol Regul Integr Comp Physiol 286:R1, 2004.
Mifflin SW:What does the brain know about blood pressure?
News Physiol Sci 16:266, 2001.
Morrison SF: Differential control of sympathetic outflow.
Am J Physiol Regul Integr Comp Physiol 281:R683, 2001.
Sved AF, Ito S, Sved JC: Brainstem mechanisms of hypertension: role of the rostral ventrolateral medulla. Curr
Hypertens Rep 5:262, 2003 .
Thrasher TN: Unloading arterial baroreceptors causes neurogenic hypertension. Am J Physiol Regul Integr Comp
Physiol 282:R1044, 2002.
Zucker IH, Wang W, Pliquett RU, et al: The regulation
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N Y Acad Sci 940:431, 2001.
C
H
A
P
T
E
R
1
Dominant Role of the Kidney
in Long-Term Regulation of
Arterial Pressure and in
Hypertension: The Integrated
System for Pressure Control
Short-term control of arterial pressure by the sympathetic nervous system, as discussed in Chapter 18,
occurs primarily through the effects of the nervous
system on total peripheral vascular resistance and
capacitance, and on cardiac pumping ability.
The body, however, also has powerful mechanisms for regulating arterial pressure week
after week and month after month. This long-term
control of arterial pressure is closely intertwined with homeostasis of body fluid
volume, which is determined by the balance between the fluid intake and
output. For long-term survival, fluid intake and output must be precisely balanced, a task that is performed by multiple nervous and hormonal controls, and
by local control systems within the kidneys that regulate their excretion of salt
and water. In this chapter we discuss these renal–body fluid systems that play
a dominant role in long-term blood pressure regulation.
Renal–Body Fluid System for Arterial
Pressure Control
The renal–body fluid system for arterial pressure control is a simple one: When
the body contains too much extracellular fluid, the blood volume and arterial
pressure rise. The rising pressure in turn has a direct effect to cause the kidneys
to excrete the excess extracellular fluid, thus returning the pressure back toward
normal.
In the phylogenetic history of animal development, this renal–body fluid
system for pressure control is a primitive one. It is fully operative in one of the
lowest of vertebrates, the hagfish. This animal has a low arterial pressure, only
8 to 14 mm Hg, and this pressure increases almost directly in proportion to its
blood volume. The hagfish continually drinks sea water, which is absorbed into
its blood, increasing the blood volume as well as the pressure. However, when
the pressure rises too high, the kidney simply excretes the excess volume into
the urine and relieves the pressure. At low pressure, the kidney excretes far less
fluid than is ingested. Therefore, because the hagfish continues to drink, extracellular fluid volume, blood volume, and pressure all build up again to the higher
levels.
Throughout the ages, this primitive mechanism of pressure control has survived almost exactly as it functions in the hagfish; in the human being, kidney
output of water and salt is just as sensitive to pressure changes as in the hagfish,
if not more so. Indeed, an increase in arterial pressure in the human of only a
few millimeters of mercury can double renal output of water, which is called
pressure diuresis, as well as double the output of salt, which is called pressure
natriuresis.
216
9
217
The Integrated System for Pressure Control
4000
Cardiac output
(ml/min)
8
7
6
5
3000
2000
1000
3
2
1
0
0
Urinary output
(ml/min)
4
4
3
2
1
0
Arterial pressure
(mm Hg)
Urinary volume output (x normal)
Chapter 19
225
200
175
150
125
100
75
50
20 40 60 80 100 120 140 160 180 200
Arterial pressure (mm Hg)
Figure 19–1
Typical renal urinary output curve measured in a perfused isolated
kidney, showing pressure diuresis when the arterial pressure rises
above normal.
Infusion period
0 10 20 30 40 50 60
Time (minutes)
In the human being, the renal–body fluid system for
arterial pressure control, just as in the hagfish, is the
fundamental basis for long-term arterial pressure
control. However, through the stages of evolution,
multiple refinements have been added to make this
system much more exact in its control in the human
being. An especially important refinement, as we shall
see, has been addition of the renin-angiotensin
mechanism.
Quantitation of Pressure Diuresis as
a Basis for Arterial Pressure Control
Figure 19–1 shows the approximate average effect of
different arterial pressure levels on urinary volume
output by an isolated kidney, demonstrating markedly
increased output of volume as the pressure rises. This
increased urinary output is the phenomenon of pressure diuresis. The curve in this figure is called a renal
urinary output curve or a renal function curve. In the
human being, at an arterial pressure of 50 mm Hg, the
urine output is essentially zero. At 100 mm Hg it is
normal, and at 200 mm Hg it is about six to eight times
normal. Furthermore, not only does increasing the
arterial pressure increase urine volume output, but it
causes approximately equal increase in sodium output,
which is the phenomenon of pressure natriuresis.
An Experiment Demonstrating the Renal–Body Fluid System for
Arterial Pressure Control. Figure 19–2 shows the results
of a research experiment in dogs in which all the
nervous reflex mechanisms for blood pressure control
were first blocked. Then the arterial pressure was suddenly elevated by infusing about 400 milliliters of
blood intravenously. Note the instantaneous increase
120
Figure 19–2
Increases in cardiac output, urinary output, and arterial pressure
caused by increased blood volume in dogs whose nervous pressure control mechanisms had been blocked. This figure shows
return of arterial pressure to normal after about an hour of fluid
loss into the urine. (Courtesy Dr. William Dobbs.)
in cardiac output to about double normal and increase
in mean arterial pressure to 205 mm Hg, 115 mm Hg
above its resting level. Shown by the middle curve is
the effect of this increased arterial pressure on urine
output, which increased 12-fold. Along with this
tremendous loss of fluid in the urine, both the cardiac
output and the arterial pressure returned to normal
during the subsequent hour. Thus, one sees an extreme
capability of the kidneys to eliminate fluid volume
from the body in response to high arterial pressure and
in so doing to return the arterial pressure back to
normal.
Graphical Analysis of Pressure Control by the Renal–Body Fluid
Mechanism, Demonstrating an “Infinite Feedback Gain”
Feature. Figure 19–3 shows a graphical method that
can be used for analyzing arterial pressure control by
the renal–body fluid system. This analysis is based on
two separate curves that intersect each other: (1) the
renal output curve for water and salt in response to
rising arterial pressure, which is the same renal output
curve as that shown in Figure 19–1, and (2) the curve
(or line) that represents the net water and salt intake.
Over a long period, the water and salt output must
equal the intake. Furthermore, the only place on the
graph in Figure 19–3 at which output equals intake is
218
Unit IV
The Circulation
8
Renal output of water and salt
A
6
8
4
6
4
Equilibrium point
2
0
0
50
100
150
250
Arterial pressure (mm Hg)
Intake or output (x normal)
Intake or output (x normal)
Water and salt intake
Elevated
pressure
Normal
2
0
0
8
50
100
150
200
250
200
250
B
Elevated
pressure
6
4
Figure 19–3
Analysis of arterial pressure regulation by equating the “renal
output curve” with the “salt and water intake curve.” The equilibrium point describes the level to which the arterial pressure will
be regulated. (That small portion of the salt and water intake that
is lost from the body through nonrenal routes is ignored in this and
similar figures in this chapter.)
Normal
2
0
0
50
100
150
Arterial pressure (mm Hg)
Figure 19–4
where the two curves intersect, which is called the
equilibrium point. Now, let us see what happens if the
arterial pressure becomes some value that is different
from that at the equilibrium point.
First, assume that the arterial pressure rises to
150 mm Hg. At this level, the graph shows that renal
output of water and salt is about three times as great
as the intake. Therefore, the body loses fluid, the blood
volume decreases, and the arterial pressure decreases.
Furthermore, this “negative balance” of fluid will not
cease until the pressure falls all the way back exactly
to the equilibrium level. Indeed, even when the arterial pressure is only 1 mm Hg greater than the equilibrium level, there still is slightly more loss of water
and salt than intake, so that the pressure continues to
fall that last 1 mm Hg until the pressure eventually
returns exactly to the equilibrium point.
If the arterial pressure falls below the equilibrium
point, the intake of water and salt is greater than the
output. Therefore, body fluid volume increases, blood
volume increases, and the arterial pressure rises until
once again it returns exactly to the equilibrium point.
This return of the arterial pressure always exactly
back to the equilibrium point is the infinite feedback
gain principle for control of arterial pressure by the
renal–body fluid mechanism.
Two Determinants of the Long-Term Arterial Pressure Level. In
Figure 19–3, one can also see that two basic long-term
factors determine the long-term arterial pressure level.
This can be explained as follows.
Two ways in which the arterial pressure can be increased: A, by
shifting the renal output curve in the right-hand direction toward
a higher pressure level or B, by increasing the intake level of salt
and water.
As long as the two curves representing (1) renal
output of salt and water and (2) intake of salt and
water remain exactly as they are shown in Figure 19–3,
the long-term mean arterial pressure level will always
readjust exactly to 100 mm Hg, which is the pressure
level depicted by the equilibrium point of this figure.
Furthermore, there are only two ways in which the
pressure of this equilibrium point can be changed from
the 100 mm Hg level. One of these is by shifting the
pressure level of the renal output curve for salt and
water; and the other is by changing the level of the
water and salt intake line. Therefore, expressed simply,
the two primary determinants of the long-term arterial pressure level are as follows:
1. The degree of pressure shift of the renal output
curve for water and salt
2. The level of the water and salt intake line
Operation of these two determinants in the control
of arterial pressure is demonstrated in Figure 19–4. In
Figure 19–4A, some abnormality of the kidneys has
caused the renal output curve to shift 50 mm Hg in the
high-pressure direction (to the right). Note that the
equilibrium point has also shifted to 50 mm Hg higher
than normal. Therefore, one can state that if the renal
output curve shifts to a new pressure level, so will the
219
Arterial pressure
Hypothyroidism
c
Removal of four limbs
ia
100
outp
ut
50
0
40
Failure of Increased Total Peripheral
Resistance to Elevate the Long-Term Level
of Arterial Pressure if Fluid Intake and Renal
Function Do Not Change
Now is the chance for the reader to see whether he or
she really understands the renal–body fluid mechanism for arterial pressure control. Recalling the basic
equation for arterial pressure—arterial pressure equals
cardiac output times total peripheral resistance—it is
clear that an increase in total peripheral resistance
should elevate the arterial pressure. Indeed, when the
total peripheral resistance is acutely increased, the arterial pressure does rise immediately. Yet if the kidneys
continue to function normally, the acute rise in arterial
pressure usually is not maintained. Instead, the arterial pressure returns all the way to normal within a day
or so. Why?
The answer to this is the following: Increasing resistance in the blood vessels everywhere else in the body
besides in the kidneys does not change the equilibrium
point for blood pressure control as dictated by the
kidneys (see again Figures 19–3 and 19–4). Instead, the
kidneys immediately begin to respond to the high arterial pressure, causing pressure diuresis and pressure
natriuresis. Within hours, large amounts of salt and
water are lost from the body, and this continues until
the arterial pressure returns exactly to the pressure
level of the equilibrium point.
As proof of this principle that changes in total
peripheral resistance do not affect the long-term level
of arterial pressure if function of the kidneys is still
normal, carefully study Figure 19–5. This figure shows
the approximate cardiac outputs and the arterial pressures in different clinical conditions in which the longterm total peripheral resistance is either much less than
or much greater than normal, but kidney excretion of
salt and water is normal. Note in all these different
clinical conditions that the arterial pressure is also
exactly normal.
(A word of caution! Many times when the total
peripheral resistance increases, this increases the
intrarenal vascular resistance at the same time, which
alters the function of the kidney and can cause
rd
Normal
Ca
150
Pulmonary disease
Paget's disease
200
Anemia
arterial pressure follow to this new pressure level
within a few days.
Figure 19–4B shows how a change in the level of salt
and water intake also can change the arterial pressure.
In this case, the intake level has increased fourfold and
the equilibrium point has shifted to a pressure level of
160 mm Hg, 60 mm Hg above the normal level. Conversely, a decrease in the intake level would reduce the
arterial pressure.
Thus, it is impossible to change the long-term mean
arterial pressure level to a new value without changing
one or both of the two basic determinants of long-term
arterial pressure—either (1) the level of salt and water
intake or (2) the degree of shift of the renal function
curve along the pressure axis. However, if either of
these is changed, one finds the arterial pressure thereafter to be regulated at a new pressure level, at the
pressure level at which the two new curves intersect.
Beriberi
AV shunts
Hyperthyroidism
The Integrated System for Pressure Control
Arterial pressure and cardiac output
(per cent of normal)
Chapter 19
60
80
100 120 140
Total peripheral resistance
(per cent of normal)
160
Figure 19–5
Relations of total peripheral resistance to the long-term levels of
arterial pressure and cardiac output in different clinical abnormalities. In these conditions, the kidneys were functioning normally. Note that changing the whole-body total peripheral
resistance caused equal and opposite changes in cardiac output
but in all cases had no effect on arterial pressure. (Redrawn from
Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB
Saunders Co, 1980.)
hypertension by shifting the renal function curve to a
higher pressure level, in the manner shown in Figure
19–4A. We see an example of this later in this chapter
when we discuss hypertension caused by vasoconstrictor mechanisms. But it is the increase in renal resistance
that is the culprit, not the increased total peripheral
resistance—an important distinction!)
Increased Fluid Volume Can Elevate Arterial
Pressure by Increasing Cardiac Output or
Total Peripheral Resistance
The overall mechanism by which increased extracellular fluid volume elevates arterial pressure is given in
the schema of Figure 19–6. The sequential events are
(1) increased extracellular fluid volume (2) increases
the blood volume, which (3) increases the mean circulatory filling pressure, which (4) increases venous
return of blood to the heart, which (5) increases
cardiac output, which (6) increases arterial pressure.
Note especially in this schema the two ways in which
an increase in cardiac output can increase the arterial
pressure. One of these is the direct effect of increased
cardiac output to increase the pressure, and the other
is an indirect effect to raise total peripheral vascular
resistance through autoregulation of blood flow. The
second effect can be explained as follows.
Referring back to Chapter 17, let us recall that
whenever an excess amount of blood flows through a
220
Unit IV
The Circulation
Increased extracellular fluid volume
Increased blood volume
Increased mean circulatory filling pressure
Increased venous return of blood to the heart
Increased cardiac output
Autoregulation
Increased total
peripheral resistance
Increased arterial pressure
Figure 19–6
Sequential steps by which increased extracellular fluid volume
increases the arterial pressure. Note especially that increased
cardiac output has both a direct effect to increase arterial pressure and an indirect effect by first increasing the total peripheral
resistance.
tissue, the local tissue vasculature constricts and
decreases the blood flow back toward normal. This
phenomenon is called “autoregulation,” which means
simply regulation of blood flow by the tissue itself.
When increased blood volume increases the cardiac
output, the blood flow increases in all tissues of the
body, so that this autoregulation mechanism constricts
blood vessels all over the body. This in turn increases
the total peripheral resistance.
Finally, because arterial pressure is equal to cardiac
output times total peripheral resistance, the secondary
increase in total peripheral resistance that results from
the autoregulation mechanism helps greatly in increasing the arterial pressure. For instance, only a 5 to 10
per cent increase in cardiac output can increase the
arterial pressure from the normal mean arterial pressure of 100 mm Hg up to 150 mm Hg. In fact, the slight
increase in cardiac output is often unmeasurable.
Importance of Salt (NaCl) in the Renal–Body
Fluid Schema for Arterial Pressure Regulation
Although the discussions thus far have emphasized the
importance of volume in regulation of arterial pressure, experimental studies have shown that an increase
in salt intake is far more likely to elevate the arterial
pressure than is an increase in water intake.The reason
for this is that pure water is normally excreted by the
kidneys almost as rapidly as it is ingested, but salt
is not excreted so easily. As salt accumulates in the
body, it also indirectly increases the extracellular fluid
volume for two basic reasons:
1. When there is excess salt in the extracellular fluid,
the osmolality of the fluid increases, and this in
turn stimulates the thirst center in the brain,
making the person drink extra amounts of
water to return the extracellular salt
concentration to normal. This increases the
extracellular fluid volume.
2. The increase in osmolality caused by the excess
salt in the extracellular fluid also stimulates the
hypothalamic-posterior pituitary gland secretory
mechanism to secrete increased quantities of
antidiuretic hormone. (This is discussed in Chapter
28.) The antidiuretic hormone then causes the
kidneys to reabsorb greatly increased quantities
of water from the renal tubular fluid, thereby
diminishing the excreted volume of urine but
increasing the extracellular fluid volume.
Thus, for these important reasons, the amount of salt
that accumulates in the body is the main determinant
of the extracellular fluid volume. Because only small
increases in extracellular fluid and blood volume can
often increase the arterial pressure greatly, accumulation of even a small amount of extra salt in the body
can lead to considerable elevation of arterial pressure.
Chronic Hypertension (High Blood
Pressure) Is Caused by Impaired
Renal Fluid Excretion
When a person is said to have chronic hypertension (or
“high blood pressure”), it is meant that his or her mean
arterial pressure is greater than the upper range of the
accepted normal measure. A mean arterial pressure
greater than 110 mm Hg (normal is about 90 mm Hg)
is considered to be hypertensive. (This level of mean
pressure occurs when the diastolic blood pressure is
greater than about 90 mm Hg and the systolic pressure
is greater than about 135 mm Hg.) In severe hypertension, the mean arterial pressure can rise to 150
to 170 mm Hg, with diastolic pressure as high as
130 mm Hg and systolic pressure occasionally as high
as 250 mm Hg.
Even moderate elevation of arterial pressure leads
to shortened life expectancy. At severely high pressures—mean arterial pressures 50 per cent or more
above normal—a person can expect to live no more
than a few more years unless appropriately treated.
The lethal effects of hypertension are caused mainly
in three ways:
1. Excess workload on the heart leads to early heart
failure and coronary heart disease, often causing
death as a result of a heart attack.
2. The high pressure frequently damages a major
blood vessel in the brain, followed by death of
Chapter 19
221
The Integrated System for Pressure Control
0.9% NaCl Tap water 0.9% NaCl
150
Mean arterial pressure
(per cent of control)
140
130
120
35–45% of left
kidney removed
110
Entire right
kidney removed
100
0
0
20
40
60
80
100
Days
Figure 19–7
Average effect on arterial pressure of drinking 0.9 per cent saline solution instead of water in four dogs with 70 per cent of their
renal tissue removed. (Redrawn from Langston JB, Guyton AC, Douglas BH, Dorsett PE: Circ Res 12:508, 1963. By permission of the
American Heart Association, Inc.)
major portions of the brain; this is a cerebral
infarct. Clinically it is called a “stroke.” Depending
on which part of the brain is involved, a stroke
can cause paralysis, dementia, blindness, or
multiple other serious brain disorders.
3. High pressure almost always causes injury in
the kidneys, producing many areas of renal
destruction and, eventually, kidney failure, uremia,
and death.
Lessons learned from the type of hypertension
called “volume-loading hypertension” have been
crucial in understanding the role of the renal–body
fluid volume mechanism for arterial pressure regulation. Volume-loading hypertension means hypertension caused by excess accumulation of extracellular
fluid in the body, some examples of which follow.
Experimental Volume-Loading Hypertension Caused by Reduced
Renal Mass Along with Simultaneous Increase in Salt Intake.
Figure 19–7 shows a typical experiment demonstrating
volume-loading hypertension in a group of dogs with
70 per cent of their kidney mass removed. At the first
circled point on the curve, the two poles of one of the
kidneys were removed, and at the second circled point,
the entire opposite kidney was removed, leaving the
animals with only 30 per cent of normal renal mass.
Note that removal of this amount of kidney mass
increased the arterial pressure an average of only 6
mm Hg. Then, the dogs were given salt solution to
drink instead of water. Because salt solution fails to
quench the thirst, the dogs drank two to four times the
normal amounts of volume, and within a few days,
their average arterial pressure rose to about 40 mm Hg
above normal. After 2 weeks, the dogs were given tap
water again instead of salt solution; the pressure
returned to normal within 2 days. Finally, at the end of
the experiment, the dogs were given salt solution
again, and this time the pressure rose much more
rapidly to an even higher level because the dogs had
already learned to tolerate the salt solution and therefore drank much more. Thus, this experiment demonstrates volume-loading hypertension.
If the reader considers again the basic determinants
of long-term arterial pressure regulation, he or she can
immediately understand why hypertension occurred in
the volume-loading experiment of Figure 19–7. First,
reduction of the kidney mass to 30 per cent of normal
greatly reduced the ability of the kidneys to excrete
salt and water. Therefore, salt and water accumulated
in the body and in a few days raised the arterial pressure high enough to excrete the excess salt and water
intake.
Sequential Changes in Circulatory Function During the Development of Volume-Loading Hypertension. It is especially
instructive to study the sequential changes in circulatory function during progressive development of
volume-loading hypertension. Figure 19–8 shows these
sequential changes. A week or so before the point
labeled “0” days, the kidney mass had already been
decreased to only 30 per cent of normal. Then, at this
222
Extracellular
fluid volume
(liters)
Blood
volume
(liters)
6.0
5.5
5.0
7.0
6.5
6.0
5.5
5.0
Arterial
pressure
(mm Hg)
20
19
18
17
16
15
Total
peripheral
resistance Cardiac output
(L/min)
(mm Hg/L/min)
Unit IV
33%
4%
20%
5%
40%
5%
28
26
24
22
20
18
150
140
130
120
110
0
The Circulation
33%
–13%
Figure 19–8
40%
30%
0
2
4
6
8
10
12
Days
point, the intake of salt and water was increased to
about six times normal and kept at this high intake
thereafter. The acute effect was to increase extracellular fluid volume, blood volume, and cardiac output to
20 to 40 per cent above normal. Simultaneously, the
arterial pressure began to rise but not nearly so much
at first as did the fluid volumes and cardiac output. The
reason for this slower rise in pressure can be discerned
by studying the total peripheral resistance curve, which
shows an initial decrease in total peripheral resistance.
This decrease was caused by the baroreceptor mechanism discussed in Chapter 18, which tried to prevent
the rise in pressure. However, after 2 to 4 days, the
baroreceptors adapted (reset) and were no longer able
to prevent the rise in pressure. At this time, the arterial pressure had risen almost to its full height because
of the increase in cardiac output, even though the total
peripheral resistance was still almost at the normal
level.
After these early acute changes in the circulatory
variables had occurred, more prolonged secondary
changes occurred during the next few weeks. Especially important was a progressive increase in total
peripheral resistance, while at the same time the cardiac
output decreased almost all the way back to normal,
mainly as a result of the long-term blood flow autoregulation mechanism that is discussed in detail in
14
Progressive changes in important circulatory
system variables during the first few weeks of
volume-loading hypertension. Note especially the
initial increase in cardiac output as the basic
cause of the hypertension. Subsequently, the autoregulation mechanism returns the cardiac output
almost to normal while simultaneously causing a
secondary increase in total peripheral resistance.
(Modified from Guyton AC: Arterial Pressure and
Hypertension. Philadelphia: WB Saunders Co,
1980.)
Chapter 17 and earlier in this chapter. That is, after the
cardiac output had risen to a high level and had initiated the hypertension, the excess blood flow through
the tissues then caused progressive constriction of the
local arterioles, thus returning the local blood flows
in all the body tissues and also the cardiac output
almost all the way back to normal, while simultaneously causing a secondary increase in total peripheral
resistance.
Note, too, that the extracellular fluid volume and
blood volume returned almost all the way back to
normal along with the decrease in cardiac output. This
resulted from two factors: First, the increase in arteriolar resistance decreased the capillary pressure, which
allowed the fluid in the tissue spaces to be absorbed
back into the blood. Second, the elevated arterial
pressure now caused the kidneys to excrete the excess
volume of fluid that had initially accumulated in the
body.
Last, let us take stock of the final state of the circulation several weeks after the initial onset of volume
loading. We find the following effects:
1. Hypertension
2. Marked increase in total peripheral resistance
3. Almost complete return of the extracellular fluid
volume, blood volume, and cardiac output back to
normal
Chapter 19
223
The Integrated System for Pressure Control
Therefore, we can divide volume-loading hypertension into two separate sequential stages: The first stage
results from increased fluid volume causing increased
cardiac output. This increase in cardiac output causes
the hypertension. The second stage in volume-loading
hypertension is characterized by high blood pressure
and high total peripheral resistance but return of the
cardiac output so near to normal that the usual measuring techniques frequently cannot detect an abnormally elevated cardiac output.
Thus, the increased total peripheral resistance in
volume-loading hypertension occurs after the hypertension has developed and, therefore, is secondary to
the hypertension rather than being the cause of the
hypertension.
Volume-Loading Hypertension in Patients
Who Have No Kidneys but Are Being
Maintained on an Artificial Kidney
When a patient is maintained on an artificial kidney, it
is especially important to keep the patient’s body fluid
volume at a normal level—that is, it is important
to remove an appropriate amount of water and salt
each time the patient is dialyzed. If this is not done and
extracellular fluid volume is allowed to increase,
hypertension almost invariably develops in exactly the
same way as shown in Figure 19–8. That is, the cardiac
output increases at first and causes hypertension. Then
the autoregulation mechanism returns the cardiac
output back toward normal while causing a secondary
increase in total peripheral resistance. Therefore, in
the end, the hypertension is a high peripheral resistance type of hypertension.
Hypertension Caused by
Primary Aldosteronism
Another type of volume-loading hypertension is
caused by excess aldosterone in the body or, occasionally, by excesses of other types of steroids. A small
tumor in one of the adrenal glands occasionally
secretes large quantities of aldosterone, which is
the condition called “primary aldosteronism.” As discussed in Chapter 29, aldosterone increases the rate of
reabsorption of salt and water by the tubules of the
kidneys, thereby reducing the loss of these in the
urine while at the same time causing an increase in
blood volume and extracellular fluid volume. Consequently, hypertension occurs. And, if salt intake is
increased at the same time, the hypertension becomes
even greater. Furthermore, if the condition persists for
months or years, the excess arterial pressure often
causes pathological changes in the kidneys that make
the kidneys retain even more salt and water in addition to that caused directly by the aldosterone. Therefore, the hypertension often finally becomes lethally
severe.
Here again, in the early stages of this type of hypertension, the cardiac output is increased, but in later
stages, the cardiac output generally returns almost to
normal while the total peripheral resistance becomes
secondarily elevated, as explained earlier in the
chapter for primary volume-loading hypertension.
The Renin-Angiotensin
System: Its Role in Pressure
Control and in Hypertension
Aside from the capability of the kidneys to control
arterial pressure through changes in extracellular
fluid volume, the kidneys also have another powerful
mechanism for controlling pressure. It is the reninangiotensin system.
Renin is a protein enzyme released by the kidneys
when the arterial pressure falls too low. In turn, it
raises the arterial pressure in several ways, thus
helping to correct the initial fall in pressure.
Components of the Renin-Angiotensin
System
Figure 19–9 shows the functional steps by which the
renin-angiotensin system helps to regulate arterial
pressure.
Renin is synthesized and stored in an inactive form
called prorenin in the juxtaglomerular cells (JG cells)
of the kidneys. The JG cells are modified smooth
muscle cells located in the walls of the afferent
Decreased
arterial pressure
Renin (kidney)
Renin substrate
(angiotensinogen)
Angiotensin I
Converting
enzyme
(lung)
Angiotensin II
Angiotensinase
(Inactivated)
Renal retention Vasoconstriction
of salt and water
Increased arterial pressure
Figure 19–9
Renin-angiotensin vasoconstrictor mechanism for arterial pressure control.
Unit IV
The Circulation
arterioles immediately proximal to the glomeruli. When
the arterial pressure falls, intrinsic reactions in the
kidneys themselves cause many of the prorenin molecules in the JG cells to split and release renin. Most of
the renin enters the renal blood and then passes out
of the kidneys to circulate throughout the entire body.
However, small amounts of the renin do remain in the
local fluids of the kidney and initiate several intrarenal
functions.
Renin itself is an enzyme, not a vasoactive substance. As shown in the schema of Figure 19–9,
renin acts enzymatically on another plasma protein,
a globulin called renin substrate (or angiotensinogen),
to release a 10-amino acid peptide, angiotensin I.
Angiotensin I has mild vasoconstrictor properties but
not enough to cause significant changes in circulatory
function. The renin persists in the blood for 30 minutes
to 1 hour and continues to cause formation of still
more angiotensin I during this entire time.
Within a few seconds to minutes after formation of
angiotensin I, two additional amino acids are split
from the angiotensin I to form the 8-amino acid
peptide angiotensin II. This conversion occurs almost
entirely in the lungs while the blood flows through the
small vessels of the lungs, catalyzed by an enzyme
called converting enzyme that is present in the
endothelium of the lung vessels.
Angiotensin II is an extremely powerful vasoconstrictor, and it also affects circulatory function in other
ways as well. However, it persists in the blood only
for 1 or 2 minutes because it is rapidly inactivated by
multiple blood and tissue enzymes collectively called
angiotensinases.
During its persistence in the blood, angiotensin
II has two principal effects that can elevate arterial
pressure. The first of these, vasoconstriction in many
areas of the body, occurs rapidly. Vasoconstriction
occurs intensely in the arterioles and much less so in
the veins. Constriction of the arterioles increases the
total peripheral resistance, thereby raising the arterial
pressure, as demonstrated at the bottom of the schema
in Figure 19–9. Also, the mild constriction of the veins
promotes increased venous return of blood to the
heart, thereby helping the heart pump against the
increasing pressure.
The second principal means by which angiotensin
increases the arterial pressure is to decrease excretion
of both salt and water by the kidneys. This slowly
increases the extracellular fluid volume, which then
increases the arterial pressure during subsequent
hours and days. This long-term effect, acting through
the extracellular fluid volume mechanism, is even
more powerful than the acute vasoconstrictor mechanism in eventually raising the arterial pressure.
Rapidity and Intensity of the
Vasoconstrictor Pressure Response to
the Renin-Angiotensin System
Figure 19–10 shows a typical experiment demonstrating the effect of hemorrhage on the arterial
pressure under two separate conditions: (1) with the
renin-angiotensin system functioning and (2) without
Arterial pressure (mm Hg)
224
100
With
renin-angiotensin system
75
Without
renin-angiotensin system
50
25
Hemorrhage
0
0
10
20
30
40
Minutes
Figure 19–10
Pressure-compensating effect of the renin-angiotensin vasoconstrictor system after severe hemorrhage. (Drawn from experiments
by Dr. Royce Brough.)
the system functioning (the system was interrupted
by a renin-blocking antibody). Note that after hemorrhage—enough to cause acute decrease of the arterial
pressure to 50 mm Hg—the arterial pressure rose
back to 83 mm Hg when the renin-angiotensin
system was functional. Conversely, it rose to only
60 mm Hg when the renin-angiotensin system was
blocked. This shows that the renin-angiotensin system
is powerful enough to return the arterial pressure at
least halfway back to normal within a few minutes
after severe hemorrhage. Therefore, sometimes it
can be of lifesaving service to the body, especially in
circulatory shock.
Note also that the renin-angiotensin vasoconstrictor system requires about 20 minutes to become
fully active. Therefore, it is somewhat slower to act
for pressure control than are the nervous reflexes
and the sympathetic norepinephrine-epinephrine
system.
Effect of Angiotensin in the Kidneys to
Cause Renal Retention of Salt and Water—
An Especially Important Means for
Long-Term Control of Arterial Pressure
Angiotensin causes the kidneys to retain both salt and
water in two major ways:
1. Angiotensin acts directly on the kidneys to cause
salt and water retention.
2. Angiotensin causes the adrenal glands to secrete
aldosterone, and the aldosterone in turn increases
salt and water reabsorption by the kidney tubules.
Thus, whenever excess amounts of angiotensin circulate in the blood, the entire long-term renal–body
fluid mechanism for arterial pressure control automatically becomes set to a higher arterial pressure
level than normal.
Chapter 19
225
The Integrated System for Pressure Control
Mechanisms of the Direct Renal Effects of Angiotensin to Cause
Renal Retention of Salt and Water. Angiotensin has several
Stimulation of Aldosterone Secretion by Angiotensin, and the
Effect of Aldosterone in Increasing Salt and Water Retention
by the Kidneys. Angiotensin is also one of the most
powerful stimulators of aldosterone secretion by the
adrenal glands, as we shall discuss in relation to body
fluid regulation in Chapter 29 and in relation to
adrenal gland function in Chapter 77. Therefore, when
the renin-angiotensin system becomes activated, the
rate of aldosterone secretion usually also increases;
and an important subsequent function of aldosterone
is to cause marked increase in sodium reabsorption by
the kidney tubules, thus increasing the total body
extracellular fluid sodium. This increased sodium then
causes water retention, as already explained, increasing the extracellular fluid volume and leading secondarily to still more long-term elevation of the arterial
pressure.
Thus both the direct effect of angiotensin on the
kidney and its effect acting through aldosterone are
important in long-term arterial pressure control.
However, research in our own laboratory has suggested that the direct effect of angiotensin on the
kidneys is perhaps three or more times as potent as the
indirect effect acting through aldosterone—even
though the indirect effect is the one most widely
known.
Quantitative Analysis of Arterial Pressure Changes Caused by
Angiotensin. Figure 19–11 shows a quantitative analysis
of the effect of angiotensin in arterial pressure control.
This figure shows two renal output curves as well as a
line depicting normal level of sodium intake. The lefthand renal output curve is that measured in dogs whose
renin-angiotensin system had been blocked by the drug
captopril (which blocks the conversion of angiotensin I
to angiotensin II, the active form of angiotensin). The
right-hand curve was measured in dogs infused continuously with angiotensin II at a level about 2.5 times the
normal rate of angiotensin formation in the blood. Note
the shift of the renal output curve toward higher pressure levels under the influence of angiotensin II. This
shift is caused by both the direct effects of angiotensin
on the kidney and the indirect effect acting through
aldosterone secretion, as explained above.
Finally, note the two equilibrium points, one for zero
angiotensin showing an arterial pressure level of 75 mm
Hg, and one for elevated angiotensin showing a
pressure level of 115 mm Hg. Therefore, the effect of
Angiotensin levels in the blood
(times normal)
0
Sodium intake and output (times normal)
direct renal effects that make the kidneys retain salt
and water. One major effect is to constrict the renal
arterioles, thereby diminishing blood flow through
the kidneys. As a result, less fluid filters through the
glomeruli into the tubules. Also, the slow flow of blood
reduces the pressure in the peritubular capillaries,
which causes rapid reabsorption of fluid from the
tubules. And still a third effect is that angiotensin has
important direct actions on the tubular cells themselves to increase tubular reabsorption of sodium and
water. The total result of all these effects is significant,
sometimes decreasing urine output less than one fifth
of normal.
2.5
10
8
6
4
Equilibrium
points
2
Normal
Intake
0
0
60
80
100
120
140
160
Arterial pressure (mm Hg)
Figure 19–11
Effect of two angiotensin II levels in the blood on the renal output
curve, showing regulation of the arterial pressure at an equilibrium
point of 75 mm Hg when the angiotensin II level is low and at
115 mm Hg when the angiotensin II level is high.
angiotensin to cause renal retention of salt and water
can have a powerful effect in promoting chronic elevation of the arterial pressure.
Role of the Renin-Angiotensin System in
Maintaining a Normal Arterial Pressure
Despite Wide Variations in Salt Intake
One of the most important functions of the reninangiotensin system is to allow a person to eat either
very small or very large amounts of salt without
causing great changes in either extracellular fluid
volume or arterial pressure. This function is explained
by the schema in Figure 19–12, which shows that the
initial effect of increased salt intake is to elevate the
extracellular fluid volume and this in turn to elevate
the arterial pressure. Then, the increased arterial pressure causes increased blood flow through the kidneys,
which reduces the rate of secretion of renin to a much
lower level and leads sequentially to decreased renal
retention of salt and water, return of the extracellular
fluid volume almost to normal, and, finally, return of
the arterial pressure also almost to normal. Thus, the
renin-angiotensin system is an automatic feedback
mechanism that helps maintain the arterial pressure at
or near the normal level even when salt intake is
increased. Or, when salt intake is decreased below
normal, exactly opposite effects take place.
To emphasize the efficacy of the renin-angiotensin
system in controlling arterial pressure, when the
system functions normally, the pressure rises no more
226
Unit IV
The Circulation
Increased salt intake
Increased extracellular volume
Increased arterial pressure
Decreased renin and angiotensin
Renal artery constricted
Constriction released
Decreased renal retention of salt and water
Systemic arterial
pressure
200
Return of arterial pressure almost to normal
Pressure (mm Hg)
Return of extracellular volume almost to normal
Figure 19–12
than 4 to 6 mm Hg in response to as much as a 50-fold
increase in salt intake. Conversely, when the reninangiotensin system is blocked, the same increase in salt
intake sometimes causes the pressure to rise 10 times
the normal increase, often as much as 50 to 60 mm Hg.
Types of Hypertension in Which
Angiotensin Is Involved: Hypertension
Caused by a Renin-Secreting Tumor
or by Infusion of Angiotensin II
Occasionally a tumor of the renin-secreting juxtaglomerular cells (the JG cells) occurs and secretes
tremendous quantities of renin; in turn, equally large
quantities of angiotensin II are formed. In all patients
in whom this has occurred, severe hypertension has
developed. Also, when large amounts of angiotensin
are infused continuously for days or weeks into
animals, similar severe long-term hypertension
develops.
We have already noted that angiotensin can increase
the arterial pressure in two ways:
1. By constricting the arterioles throughout the entire
body, thereby increasing the total peripheral
resistance and arterial pressure; this effect occurs
within seconds after one begins to infuse
angiotensin.
Distal renal arterial
pressure
100
50
Times normal
Sequential events by which increased salt intake increases the
arterial pressure, but feedback decrease in activity of the renin
angiotensin system returns the arterial pressure almost to the
normal level.
150
7
Renin secretion
1
0
0
4
8
12
Days
Figure 19–13
Effect of placing a constricting clamp on the renal artery of one
kidney after the other kidney has been removed. Note the
changes in systemic arterial pressure, renal artery pressure distal
to the clamp, and rate of renin secretion. The resulting hypertension is called “one-kidney” Goldblatt hypertension.
2. By causing the kidneys to retain salt and water;
over a period of days, this, too, causes hypertension
and is the principal cause of the long-term
continuation of the elevated pressure.
“One-Kidney” Goldblatt Hypertension. When one kidney is
removed and a constrictor is placed on the renal artery
of the remaining kidney, as shown in Figure 19–13, the
immediate effect is greatly reduced pressure in the
renal artery beyond the constrictor, as demonstrated
by the dashed curve in the figure. Then, within seconds
or minutes, the systemic arterial pressure begins to rise
and continues to rise for several days. The pressure
usually rises rapidly for the first hour or so, and this is
Chapter 19
The Integrated System for Pressure Control
followed by a slower additional rise during the next
several days. When the systemic arterial pressure
reaches its new stable pressure level, the renal arterial
pressure (the dashed curve in the figure) will have
returned almost all the way back to normal.The hypertension produced in this way is called “one-kidney”
Goldblatt hypertension in honor of Dr. Goldblatt, who
first studied the important quantitative features of
hypertension caused by renal artery constriction.
The early rise in arterial pressure in Goldblatt
hypertension is caused by the renin-angiotensin vasoconstrictor mechanism. That is, because of poor blood
flow through the kidney after acute constriction of the
renal artery, large quantities of renin are secreted by
the kidney, as demonstrated by the lowermost curve in
Figure 19–13, and this causes increased angiotensin
II and aldosterone in the blood. The angiotensin in
turn raises the arterial pressure acutely. The secretion
of renin rises to a peak in an hour or so but returns
nearly to normal in 5 to 7 days because the renal arterial pressure by that time has also risen back to
normal, so that the kidney is no longer ischemic.
The second rise in arterial pressure is caused by
retention of salt and water by the constricted kidney
(that is also stimulated by angiotensin II and aldosterone). In 5 to 7 days, the body fluid volume will have
increased enough to raise the arterial pressure to its
new sustained level. The quantitative value of this sustained pressure level is determined by the degree of
constriction of the renal artery. That is, the aortic pressure must rise high enough so that renal arterial pressure distal to the constrictor is enough to cause normal
urine output.
“Two-Kidney” Goldblatt Hypertension. Hypertension also
can result when the artery to only one kidney is constricted while the artery to the other kidney is normal.
This hypertension results from the following mechanism: The constricted kidney secretes renin and also
retains salt and water because of decreased renal arterial pressure in this kidney. Then the “normal” opposite kidney retains salt and water because of the renin
produced by the ischemic kidney. This renin causes
formation of angiotension II and aldosterone both of
which circulate to the opposite kidney and cause it also
to retain salt and water. Thus, both kidneys, but for different reasons, become salt and water retainers. Consequently, hypertension develops.
Hypertension Caused by Diseased Kidneys That Secrete Renin
Chronically. Often, patchy areas of one or both kidneys
are diseased and become ischemic because of local
vascular constrictions, whereas other areas of the
kidneys are normal. When this occurs, almost identical
effects occur as in the two-kidney type of Goldblatt
hypertension. That is, the patchy ischemic kidney
tissue secretes renin, and this in turn, acting through
the formation of angiotensin II, causes the remaining
kidney mass also to retain salt and water. Indeed, one
of the most common causes of renal hypertension,
especially in older persons, is such patchy ischemic
kidney disease.
227
Other Types of Hypertension
Caused by Combinations of Volume
Loading and Vasoconstriction
Hypertension in the Upper Part of the Body Caused by Coarctation
of the Aorta. One out of every few thousand babies is
born with pathological constriction or blockage of the
aorta at a point beyond the aortic arterial branches to
the head and arms but proximal to the renal arteries,
a condition called coarctation of the aorta. When this
occurs, blood flow to the lower body is carried by multiple, small collateral arteries in the body wall, with
much vascular resistance between the upper aorta and
the lower aorta. As a consequence, the arterial pressure
in the upper part of the may be 40-50 per cent higher
than that in the lower body.
The mechanism of this upper-body hypertension is
almost identical to that of one-kidney Goldblatt hypertension.That is, when a constrictor is placed on the aorta
above the renal arteries, the blood pressure in both
kidneys at first falls, renin is secreted, angiotensin and
aldosterone are formed, and hypertension occurs in the
upper body. The arterial pressure in the lower body at
the level of the kidneys rises approximately to normal,
but high pressure persists in the upper body. The
kidneys are no longer ischemic, so that secretion of
renin and formation of angiotensin and aldosterone
return to normal. Likewise, in coarctation of the aorta,
the arterial pressure in the lower body is usually almost
normal, whereas the pressure in the upper body is far
higher than normal.
Role of Autoregulation in the Hypertension Caused by Aortic
Coarctation. A significant feature of hypertension caused
by aortic coarctation is that blood flow in the arms,
where the pressure may be 40 to 60 per cent above
normal, is almost exactly normal. Also, blood flow in the
legs, where the pressure is not elevated, is almost exactly
normal. How could this be, with the pressure in the
upper body 40 to 60 per cent greater than in the lower
body? The answer is not that there are differences in
vasoconstrictor substances in the blood of the upper and
lower body, because the same blood flows to both areas.
Likewise, the nervous system innervates both areas of
the circulation similarly, so that there is no reason to
believe that there is a difference in nervous control of
the blood vessels. The only reasonable answer is that
long-term autoregulation develops so nearly completely
that the local blood flow control mechanisms have compensated almost 100 per cent for the differences in pressure. The result is that, in both the high-pressure area
and the low-pressure area, the local blood flow is controlled almost exactly in accord with the needs of the
tissue and not in accord with the level of the pressure.
One of the reasons these observations are so important
is that they demonstrate how nearly complete the longterm autoregulation process can be.
Hypertension in Preeclampsia (Toxemia of Pregnancy). Approx-
imately 5 to 10 per cent of expectant mothers develop
a syndrome called preeclampsia (also called toxemia of
pregnancy). One of the manifestations of preeclampsia
is hypertension that usually subsides after delivery of
the baby. Although the precise causes of preeclampsia
are not completely understood, ischemia of the placenta
and subsequent release by the placenta of toxic factors
are believed to play a role in causing many of the man-
228
Unit IV
ifestations of this disorder, including hypertension in the
mother. Substances released by the ischemic placenta,
in turn, cause dysfunction of vascular endothelial cells
throughout the body, including the blood vessels of the
kidneys.This endothelial dysfunction decreases release of
nitric oxide and other vasodilator substances, causing
vasoconstriction, decreased rate of fluid filtration from
the glomeruli into the renal tubules, impaired renalpressure natriuresis, and development of hypertension.
Another pathological abnormality that may contribute to hypertension in preeclampsia is thickening of
the kidney glomerular membranes (perhaps caused by
an autoimmune process), which also reduces the rate of
glomerular fluid filtration. For obvious reasons, the arterial pressure level required to cause normal formation
of urine becomes elevated, and the long-term level of
arterial pressure becomes correspondingly elevated.
These patients are especially prone to extra degrees of
hypertension when they have excess salt intake.
Neurogenic Hypertension. Acute neurogenic hypertension
can be caused by strong stimulation of the sympathetic
nervous system. For instance, when a person becomes
excited for any reason or at times during states of
anxiety, the sympathetic system becomes excessively
stimulated, peripheral vasoconstriction occurs everywhere in the body, and acute hypertension ensues.
Acute Neurogenic Hypertension Caused by Sectioning the Baroreceptor Nerves. Another type of acute neurogenic hyper-
tension occurs when the nerves leading from the
baroreceptors are cut or when the tractus solitarius
is destroyed in each side of the medulla oblongata
(these are the areas where the nerves from the carotid
and aortic baroreceptors connect in the brain stem).
The sudden cessation of normal nerve signals from
the baroreceptors has the same effect on the nervous
pressure control mechanisms as a sudden reduction
of the arterial pressure in the aorta and carotid arteries.
That is, loss of the normal inhibitory effect on the
vasomotor center caused by normal baroreceptor
nervous signals allows the vasomotor center suddenly
to become extremely active and the mean arterial pressure to increase from 100 mm Hg to as high as 160 mm
Hg. The pressure returns to nearly normal within about
2 days because the response of the vasomotor center to
the absent baroreceptor signal fades away, which is
called central “resetting” of the baroreceptor pressure
control mechanism. Therefore, the neurogenic hypertension caused by sectioning the baroreceptor nerves is
mainly an acute type of hypertension, not a chronic
type.
Spontaneous Hereditary Hypertension in Lower Animals. Spon-
taneous hereditary hypertension has been observed in
a number of strains of lower animals, including several
different strains of rats, at least one strain of rabbits, and
at least one strain of dogs. In the strain of rats that has
been studied to the greatest extent, the Okamoto strain,
there is evidence that in early development of the
hypertension, the sympathetic nervous system is considerably more active than in normal rats. However, in
the late stages of this type of hypertension, two structural changes have been observed in the nephrons of the
kidneys: (1) increased preglomerular renal arterial
resistance and (2) decreased permeability of the
glomerular membranes. These structural changes could
easily be the basis for the long-term continuance of the
The Circulation
hypertension. In other strains of hypertensive rats,
impaired renal function also has been observed.
“Primary (Essential) Hypertension”
About 90 to 95 per cent of all people who have hypertension are said to have “primary hypertension,” also
widely known as “essential hypertension” by many clinicians. These terms mean simply that the hypertension
is of unknown origin, in contrast to those forms of
hypertension that are secondary to known causes, such
as renal artery stenosis. In some patients with primary
hypertension, there is a strong hereditary tendency, the
same as occurs in animal strains of genetic hypertension discussed above.
In most patients, excess weight gain and sedentary
lifestyle appear to play a major role in causing hypertension. The majority of patients with hypertension
are overweight, and studies of different populations
suggest that excess weight gain and obesity may
account for as much as 65 to 70 percent of the risk
for developing primary hypertension. Clinical studies
have clearly shown the value of weight loss for reducing blood pressure in most patients with hypertension.
In fact, new clinical guidelines for treating hypertension recommend increased physical activity and
weight loss as a first step in treating most patients with
hypertension.
Some of the characteristics of primary hypertension
caused by excess weight gain and obesity include:
1. Cardiac output is increased due, in part, to the
additional blood flow required for the extra
adipose tissue. However, blood flow in the heart,
kidneys, gastrointestinal tract, and skeletal muscle
also increases with weight gain due to increased
metabolic rate and growth of the organs and tissues
in response to their increased metabolic demands.
As the hypertension is sustained for many months
and years, total peripheral vascular resistance may
be increased.
2. Sympathetic nerve activity, especially in the kidneys,
is increased in overweight patients. The causes of
increased sympathetic activity in obesity are not
fully understood, but recent studies suggest that
hormones, such as leptin, released from fat cells
may directly stimulate multiple regions of the
hypothalamus, which, in turn, have an excitatory
influence on the vasomotor centers of the brain
medulla.
3. Angiotensin II and aldosterone levels are increased
two- to threefold in many obese patients. This may
be caused partly by increased sympathetic nerve
stimulation, which increases renin release by the
kidneys and therefore formation of angiotensin II,
which, in turn, stimulates the adrenal gland to
secrete aldosterone.
4. The renal-pressure natriuresis mechanism is
impaired, and the kidneys will not excrete adequate
amounts of salt and water unless the arterial
pressure is high or unless kidney function is
somehow improved. In other words, if the mean
arterial pressure in the essential hypertensive
person is 150 mm Hg, acute reduction of the mean
arterial pressure artificially to the normal value of
The Integrated System for Pressure Control
Chapter 19
100 mm Hg (but without otherwise altering renal
function except for the decreased pressure) will
cause almost total anuria, and the person will retain
salt and water until the pressure rises back to the
elevated value of 150 mm Hg. Chronic reductions
in arterial pressure with effective antihypertensive
therapies, however, usually do not cause marked
salt and water retention by the kidneys because
these therapies also improve renal-pressure
natriuresis, as discussed below.
Experimental studies in obese animals and obese
patients suggest that impaired renal-pressure natriuresis in obesity hypertension is caused mainly by
increased renal tubular reabsorption of salt and
water due to increased sympathetic nerve activity and
increased levels of angiotensin II and aldosterone.
However, if hypertension is not effectively treated,
there may also be vascular damage in the kidneys that
can reduce the glomerular filtration rate and increase
the severity of the hypertension. Eventually uncontrolled hypertension associated with obesity can lead
to severe vascular injury and complete loss of kidney
function.
Graphical Analysis of Arterial Pressure Control in Essential
Hypertension. Figure 19–14 is a graphical analysis of
essential hypertension. The curves of this figure are
called sodium-loading renal function curves because
the arterial pressure in each instance is increased
very slowly, over many days or weeks, by gradually
Normal
Nonsalt-sensitive
Salt-sensitive
Salt intake and output
(times normal)
6
5
4
High intake
E
B
B1
3
Normal
2
Normal intake
1
D
Essential
hypertension
A
C
0
0
50
100
150
Arterial pressure (mm Hg)
Figure 19–14
Analysis of arterial pressure regulation in (1) nonsalt-sensitive
essential hypertension and (2) salt-sensitive essential hypertension. (Redrawn from Guyton AC, Coleman TG, Young DB, et al:
Salt balance and long-term blood pressure control. Annu Rev Med
31:15, 1980. With permission, from the Annual Review of Medicine, ” 1980, by Annual Reviews http://www.AnnualReviews.org.)
229
increasing the level of sodium intake. The sodiumloading type of curve can be determined by increasing
the level of sodium intake to a new level every few
days, then waiting for the renal output of sodium to
come into balance with the intake, and at the same
time recording the changes in arterial pressure.
When this procedure is used in essential hypertensive patients, two types of curves, shown to the right in
Figure 19–14, can be recorded in essential hypertensive patients, one called (1) nonsalt-sensitive hypertension and the other (2) salt-sensitive hypertension. Note
in both instances that the curves are shifted to the
right, to a much higher pressure level than for normal
people. Now, let us plot on this same graph (1) a
normal level of salt intake and (2) a high level of salt
intake representing 3.5 times the normal intake. In
the case of the person with nonsalt-sensitive essential
hypertension, the arterial pressure does not increase
significantly when changing from normal salt intake
to high salt intake. Conversely, in those patients who
have salt-sensitive essential hypertension, the high salt
intake significantly exacerbates the hypertension.
Two additional points should be emphasized: (1)
Salt-sensitivity of blood pressure is not an all-or-none
characteristic—it is a quantitative characteristic, with
some individuals being more salt-sensitive than others.
(2) Salt-sensitivity of blood pressure is not a fixed
characteristic; instead, blood pressure usually becomes
more salt-sensitive as a person ages, especially after 50
or 60 years of age.
The reason for the difference between nonsaltsensitive essential hypertension and salt-sensitive
hypertension is presumably related to structural or
functional differences in the kidneys of these two types
of hypertensive patients. For example, salt-sensitive
hypertension may occur with different types of chronic
renal disease due to gradual loss of the functional units
of the kidneys (the nephrons) or to normal aging as
discussed in Chapter 31. Abnormal function of the
renin-angiotensin system can also cause blood pressure to become salt-sensitive, as discussed previously
in this chapter.
Treatment of Essential Hypertension. Current guidelines
for treating hypertension recommend, as a first step,
lifestyle modifications that are aimed at increasing
physical activity and weight loss in most patients.
Unfortunately, many patients are unable to lose
weight, and pharmacological treatment with antihypertensive drugs must be initiated.
Two general classes of drugs are used to treat hypertension: (1) vasodilator drugs that increase renal blood
flow and (2) natriuretic or diuretic drugs that decrease
tubular reabsorption of salt and water.
Vasodilator drugs usually cause vasodilation in
many other tissues of the body as well as in the
kidneys. Different ones act in one of the following
ways: (1) by inhibiting sympathetic nervous signals to
the kidneys or by blocking the action of the sympathetic transmitter substance on the renal vasculature,
(2) by directly relaxing the smooth muscle of the renal
vasculature, or (3) by blocking the action of the
The Circulation
Summary of the Integrated,
Multifaceted System for
Arterial Pressure Regulation
By now, it is clear that arterial pressure is regulated
not by a single pressure controlling system but instead
by several interrelated systems, each of which performs a specific function. For instance, when a person
bleeds severely so that the pressure falls suddenly, two
problems confront the pressure control system. The
first is survival, that is, to return the arterial pressure
immediately to a high enough level that the person can
live through the acute episode. The second is to return
the blood volume eventually to its normal level so that
the circulatory system can re-establish full normality,
including return of the arterial pressure all the way
back to its normal value, not merely back to a pressure
level required for survival.
In Chapter 18, we saw that the first line of defense against acute changes in arterial pressure is
the nervous control system. In this chapter, we have
emphasized a second line of defense achieved mainly
by kidney mechanisms for long-term control of arterial pressure. However, there are other pieces to the
puzzle. Figure 19–15 helps to put these together.
Figure 19–15 shows the approximate immediate
(seconds and minutes) and long-term (hours and days)
control responses, expressed as feedback gain, of eight
arterial pressure control mechanism. These mechanisms can be divided into three groups: (1) those that
react rapidly, within seconds or minutes; (2) those that
respond over an intermediate time period, minutes or
hours; and (3) those that provide long-term arterial
pressure regulation, days, months, and years. Let us see
how they fit together as a total, integrated system for
pressure control.
Rapidly Acting Pressure Control Mechanisms, Acting Within
Seconds or Minutes. The rapidly acting pressure control
mechanisms are almost entirely acute nervous reflexes
or other nervous responses. Note in Figure 19–15 the
three mechanisms that show responses within seconds.
They are (1) the baroreceptor feedback mechanism,
(2) the central nervous system ischemic mechanism,
and (3) the chemoreceptor mechanism. Not only do
these mechanisms begin to react within seconds, but
they are also powerful. After any acute fall in pressure,
as might be caused by severe hemorrhage, the nervous
mechanisms combine (1) to cause constriction of the
veins and provide transfer of blood into the heart,
Renin-angiotensin-vasoconstriction
•
CNS
isc
he
Baroreceptors
m
ic
re
sp
Chemorec
eptor
s
re
ess
Str
la
on
x a ti
on
se
Renal – b
11
10
9
8
7
6
5
4
3
2
1
0
• !!
volume pr lood
es
control sure
renin-angiotensin system on the renal vasculature or
renal tubules.
Those drugs that reduce reabsorption of salt and
water by the renal tubules include especially drugs that
block active transport of sodium through the tubular
wall; this blockage in turn also prevents the reabsorption of water, as explained earlier in the chapter. These
natriuretic or diuretic drugs are discussed in greater
detail in Chapter 31.
Acute change in pressure at this time
Unit IV
Maximum feedback gain at optimal pressure
230
sterone
A ldo
ry
id ift
Flu sh
illa
p
Ca
0 15 30 1 2 4 8 1632 1 2 4 816 1 2 4 8 16 •
Seconds Minutes
Hours
Days
Time after sudden change in pressure
Figure 19–15
Approximate potency of various arterial pressure control mechanisms at different time intervals after onset of a disturbance to the
arterial pressure. Note especially the infinite gain (∞) of the renal
body fluid pressure control mechanism that occurs after a few
weeks’ time. (Redrawn from Guyton AC: Arterial Pressure and
Hypertension. Philadelphia: WB Saunders Co, 1980.)
(2) to cause increased heart rate and contractility of
the heart to provide greater pumping capacity by the
heart, and (3) to cause constriction of most peripheral
arterioles to impede flow of blood out of the arteries;
all these effects occur almost instantly to raise the
arterial pressure back into a survival range.
When the pressure suddenly rises too high, as might
occur in response to rapid overadministration of a
blood transfusion, the same control mechanisms
operate in the reverse direction, again returning the
pressure back toward normal.
Pressure Control Mechanisms That Act After Many Minutes.
Several pressure control mechanisms exhibit significant responses only after a few minutes following
acute arterial pressure change. Three of these, shown
in Figure 19–15, are (1) the renin-angiotensin vasoconstrictor mechanism, (2) stress-relaxation of the
vasculature, and (3) shift of fluid through the tissue
capillary walls in and out of the circulation to readjust
the blood volume as needed.
We have already described at length the role of the
renin-angiotensin vasoconstrictor system to provide a
semi-acute means for increasing the arterial pressure
when this is needed. The stress-relaxation mechanism
is demonstrated by the following example: When the
pressure in the blood vessels becomes too high, they
become stretched and keep on stretching more and
more for minutes or hours; as a result, the pressure in
Chapter 19
The Integrated System for Pressure Control
the vessels falls toward normal. This continuing stretch
of the vessels, called stress-relaxation, can serve as an
intermediate-term pressure “buffer.”
The capillary fluid shift mechanism means simply
that any time capillary pressure falls too low, fluid is
absorbed through the capillary membranes from the
tissues into the circulation, thus building up the blood
volume and increasing the pressure in the circulation.
Conversely, when the capillary pressure rises too high,
fluid is lost out of the circulation into the tissues, thus
reducing the blood volume as well as virtually all the
pressures throughout the circulation.
These three intermediate mechanisms become
mostly activated within 30 minutes to several hours.
During this time, the nervous mechanisms usually
become less and less effective, which explains the
importance of these non-nervous, intermediate time
pressure control measures.
Long-Term Mechanisms for Arterial Pressure Regulation. The
goal of this chapter has been to explain the role of the
kidneys in long-term control of arterial pressure. To
the far right in Figure 19–15 is shown the renal-blood
volume pressure control mechanism (which is the
same as the renal–body fluid pressure control mechanism), demonstrating that it takes a few hours to begin
showing significant response. Yet it eventually develops a feedback gain for control of arterial pressure
equal to infinity. This means that this mechanism can
eventually return the arterial pressure all the way back,
not merely partway back, to that pressure level that
provides normal output of salt and water by the
kidneys. By now, the reader should be familiar with
this concept, which has been the major point of this
chapter.
It must also be remembered that many factors can
affect the pressure-regulating level of the renal–body
fluid mechanism. One of these, shown in Figure 19–15,
is aldosterone. A decrease in arterial pressure leads
within minutes to an increase in aldosterone secretion,
and over the next hour or days, this plays an important
role in modifying the pressure control characteristics
of the renal–body fluid mechanism. Especially important is interaction of the renin-angiotensin system with
the aldosterone and renal fluid mechanisms. For
instance, a person’s salt intake varies tremendously
from one day to another. We have seen in this chapter
that the salt intake can decrease to as little as 1/10
normal or can increase to 10 to 15 times normal and
yet the regulated level of the mean arterial pressure
will change only a few millimeters of mercury if the
renin-angiotensin-aldosterone system is fully operative. But, without a functional renin-angiotensin-aldosterone system, blood pressure becomes very sensitive
to changes in salt intake. Thus, arterial pressure control
begins with the lifesaving measures of the nervous
231
pressure controls, then continues with the sustaining
characteristics of the intermediate pressure controls,
and, finally, is stabilized at the long-term pressure level
by the renal–body fluid mechanism. This long-term
mechanism in turn has multiple interactions with the
renin-angiotensin-aldosterone system, the nervous
system, and several other factors that provide special
blood pressure control capabilities for special purposes.
References
Chobanian AV, Bakris GL, Black HR, et al: Joint National
Committee on Prevention, Detection, Evaluation, and
Treatment of High Blood Pressure. National High Blood
Pressure Education Program Coordinating Committee.
Seventh Report of the Joint National Committee on
prevention, detection, evaluation, and treatment of high
blood pressure. Hypertension 42:1206, 2003.
Cowley AW J: Long-term control of arterial blood pressure.
Physiol Rev 72:231, 1992.
Granger JP, Alexander BT, Bennett WA, Khalil RA: Pathophysiology of pregnancy-induced hypertension. Am J
Hypertens 14:178S, 2001.
Granger JP, Alexander BT: Abnormal pressure-natriuresis in
hypertension: role of nitric oxide. Acta Physiol Scand
168:161, 2000.
Guyton AC: Arterial Pressure and Hypertension. Philadelphia: WB Saunders Co, 1980.
Guyton AC: Blood pressure control—special role of the
kidneys and body fluids. Science 252:1813, 1991.
Guyton AC, Coleman TG, Cowley AW Jr, et al: Arterial pressure regulation: overriding dominance of the kidneys in
long-term regulation and in hypertension. Am J Med
52:584, 1972.
Hall JE:The kidney, hypertension, and obesity. Hypertension
41:625, 2003.
Hall JE, Brands MW, Henegar JR: Angiotensin II and longterm arterial pressure regulation: the overriding dominance of the kidney. J Am Soc Nephrol 10(Suppl 12):S258,
1999.
Hall JE, Guyton AC, Brands MW: Pressure-volume regulation in hypertension. Kidney Int Suppl 55:S35, 1996.
Hall JE, Jones DW, Kuo JJ, et al: Impact of the obesity epidemic on hypertension and renal disease. Curr Hypertens
Rep 5:386, 2003.
Lifton RP, Gharavi AG, Geller DS: Molecular mechanisms
of human hypertension. Cell 104:545, 2001.
Manning RD Jr, Hu L, Tan DY, Meng S: Role of abnormal
nitric oxide systems in salt-sensitive hypertension. Am J
Hypertens 14:68S, 2001.
Oparil S, Zaman MA, Calhoun DA: Pathogenesis of hypertension. Ann Intern Med 139:761, 2003.
O’Shaughnessy KM, Karet FE: Salt handling and hypertension. J Clin Invest 113:1075, 2004.
Reckelhoff JF: Gender differences in the regulation of blood
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Cardiac Output, Venous Return,
and Their Regulation
Cardiac output is the quantity of blood pumped into
the aorta each minute by the heart. This is
also the quantity of blood that flows through the
circulation. Cardiac output is perhaps the most
important factor that we have to consider in relation to the circulation.
Venous return is the quantity of blood flowing
from the veins into the right atrium each minute.
The venous return and the cardiac output must equal each other except for a
few heartbeats at a time when blood is temporarily stored in or removed from
the heart and lungs.
Normal Values for Cardiac Output at Rest and
During Activity
Cardiac output varies widely with the level of activity of the body. The following factors, among others, directly affect cardiac output: (1) the basic level of
body metabolism, (2) whether the person is exercising, (3) the person’s age, and
(4) size of the body.
For young, healthy men, resting cardiac output averages about 5.6 L/min.
For women, this value is about 4.9 L/min. When one considers the factor of age
as well—because with increasing age, body activity diminishes—the average
cardiac output for the resting adult, in round numbers, is often stated to be
almost exactly 5 L/min.
Cardiac Index
Experiments have shown that the cardiac output increases approximately in proportion to the surface area of the body.Therefore, cardiac output is frequently stated
in terms of the cardiac index, which is the cardiac output per square meter of body
surface area. The normal human being weighing 70 kilograms has a body surface
area of about 1.7 square meters, which means that the normal average cardiac index
for adults is about 3 L/min/m2 of body surface area.
Effect of Age on Cardiac Output. Figure 20–1 shows the cardiac output, expressed
as cardiac index, at different ages. Rising rapidly to a level greater than
4 L/min/m2 at age 10 years, the cardiac index declines to about 2.4 L/min/m2 at
age 80 years. We will see later in the chapter that the cardiac output is regulated throughout life almost directly in proportion to the overall bodily metabolic activity. Therefore, the declining cardiac index is indicative of declining
activity with age.
Control of Cardiac Output by Venous
Return—Role of the Frank-Starling
Mechanism of the Heart
When one states that cardiac output is controlled by venous return, this means
that it is not the heart itself that is the primary controller of cardiac output.
232
2
2
15
10
5
0
0
0
10
20
30
40
50
60
70
Cardiac output
and cardiac index
30
Oxygen
consumption
4
25
3
20
15
Dexter
Douglas
Christensen
Donald
10
5
0
0
1951
1922
1931
1055
2
1
0
200 400 600 800 1000 1200 14001600
Work output during exercise (kg-m/min)
1
1
35
Oxygen consumption (L/min)
Cardiac index (L/min/m2 )
3
3
Cardiac index (L/min/m2)
4
4
0
233
Cardiac Output, Venous Return, and Their Regulation
Cardiac output (L/min/m2)
Chapter 20
Figure 20–2
80
Age in years
Figure 20–1
Cardiac index for the human being (cardiac output per square
meter of surface area) at different ages. (Redrawn from Guyton
AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac
Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co,
1973.)
Instead, it is the various factors of the peripheral circulation that affect flow of blood into the heart from
the veins, called venous return, that are the primary
controllers.
The main reason peripheral factors are usually more
important than the heart itself in controlling cardiac
output is that the heart has a built-in mechanism that
normally allows it to pump automatically whatever
amount of blood that flows into the right atrium from
the veins. This mechanism, called the Frank-Starling
law of the heart, was discussed in Chapter 9. Basically,
this law states that when increased quantities of blood
flow into the heart, the increased blood stretches the
walls of the heart chambers. As a result of the stretch,
the cardiac muscle contracts with increased force, and
this empties the extra blood that has entered from the
systemic circulation. Therefore, the blood that flows
into the heart is automatically pumped without delay
into the aorta and flows again through the circulation.
Another important factor, discussed in Chapter 10,
is that stretching the heart causes the heart to pump
faster—at an increased heart rate.That is, stretch of the
sinus node in the wall of the right atrium has a direct
effect on the rhythmicity of the node itself to increase
heart rate as much as 10 to 15 per cent. In addition,
the stretched right atrium initiates a nervous reflex
called the Bainbridge reflex, passing first to the vasomotor center of the brain and then back to the heart
by way of the sympathetic nerves and vagi, also to
increase the heart rate.
Effect of increasing levels of exercise to increase cardiac output
(red solid line) and oxygen consumption (blue dashed line).
(Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory
Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
Under most normal unstressful conditions, the
cardiac output is controlled almost entirely by peripheral factors that determine venous return. However,
we shall see later in the chapter that if the returning
blood does become more than the heart can pump,
then the heart becomes the limiting factor that determines cardiac output.
Cardiac Output Regulation Is the Sum
of Blood Flow Regulation in All the
Local Tissues of the Body—Tissue
Metabolism Regulates Most Local
Blood Flow
The venous return to the heart is the sum of all the
local blood flows through all the individual tissue
segments of the peripheral circulation. Therefore, it
follows that cardiac output regulation is the sum of all
the local blood flow regulations.
The mechanisms of local blood flow regulation were
discussed in Chapter 17. In most tissues, blood flow
increases mainly in proportion to each tissue’s metabolism. For instance, local blood flow almost always
increases when tissue oxygen consumption increases;
this effect is demonstrated in Figure 20–2 for different
levels of exercise. Note that at each increasing level of
work output during exercise, the oxygen consumption
and the cardiac output increase in parallel to each
other.
To summarize, cardiac output is determined by the
sum of all the various factors throughout the body that
control local blood flow. All the local blood flows
summate to form the venous return, and the heart
automatically pumps this returning blood back into
the arteries to flow around the system again.
25
20
out
put
50
Cardiac output (L/min)
Anemia
100
ac
The Circulation
Hypothyroidism
di
r
150
Removal of both arms and legs
200
Pulmonary disease
Paget’s disease
Normal
Beriberi
AV shunts
Hyperthyroidism
Unit IV
Ca
Arterial pressure or cardiac output
(percentage of normal)
234
Hypereffective
15
Normal
10
Hypoeffective
5
0
40
60
80
100
120
140
160
0
–4
Total peripheral resistance
(percentage of normal)
0
+4
+8
Right atrial pressure (mm Hg)
Figure 20–3
Figure 20–4
Chronic effect of different levels of total peripheral resistance on
cardiac output, showing a reciprocal relationship between total
peripheral resistance and cardiac output. (Redrawn from Guyton
AC: Arterial Pressure and Hypertension. Philadelphia: WB
Saunders Co, 1980.)
Effect of Total Peripheral Resistance on the Long-Term Cardiac
Output Level. Figure 20–3 is the same as Figure 19–5.
It is repeated here to illustrate an extremely important
principle in cardiac output control: Under most
normal conditions, the long-term cardiac output level
varies reciprocally with changes in total peripheral
resistance. Note in Figure 20–3 that when the total
peripheral resistance is exactly normal (at the 100 per
cent mark in the figure), the cardiac output is also
normal. Then, when the total peripheral resistance
increases above normal, the cardiac output falls; conversely, when the total peripheral resistance decreases,
the cardiac output increases. One can easily understand this by reconsidering one of the forms of Ohm’s
law, as expressed in Chapter 14:
Cardiac Output =
Arterial Pressure
Total Peripheral Resistance
The meaning of this formula, and of Figure 20–3, is
simply the following: Any time the long-term level of
total peripheral resistance changes (but no other functions of the circulation change), the cardiac output
changes quantitatively in exactly the opposite direction.
The Heart Has Limits for the Cardiac
Output That It Can Achieve
There are definite limits to the amount of blood that
the heart can pump, which can be expressed quantitatively in the form of cardiac output curves.
Cardiac output curves for the normal heart and for hypoeffective
and hypereffective hearts. (Redrawn from Guyton AC, Jones CE,
Coleman TB: Circulatory Physiology: Cardiac Output and Its
Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
Figure 20–4 demonstrates the normal cardiac output
curve, showing the cardiac output per minute at each
level of right atrial pressure. This is one type of cardiac
function curve, which was discussed in Chapter 9. Note
that the plateau level of this normal cardiac output
curve is about 13 L/min, 2.5 times the normal cardiac
output of about 5 L/min. This means that the normal
human heart, functioning without any special stimulation, can pump an amount of venous return up to
about 2.5 times the normal venous return before the
heart becomes a limiting factor in the control of
cardiac output.
Shown in Figure 20–4 are several other cardiac
output curves for hearts that are not pumping normally. The uppermost curves are for hypereffective
hearts that are pumping better than normal. The lowermost curves are for hypoeffective hearts that are
pumping at levels below normal.
Factors That Can Cause Hypereffective Heart
Only two types of factors usually can make the heart
a better pump than normal. They are (1) nervous stimulation and (2) hypertrophy of the heart muscle.
Effect of Nervous Excitation to Increase Heart Pumping. In
Chapter 9, we saw that a combination of (1) sympathetic stimulation and (2) parasympathetic inhibition
does two things to increase the pumping effectiveness
of the heart: (1) it greatly increases the heart rate—
sometimes, in young people, from the normal level of
235
Cardiac Output, Venous Return, and Their Regulation
72 beats/min up to 180 to 200 beats/min—and (2) it
increases the strength of heart contraction (which is
called increased “contractility”) to twice its normal
strength. Combining these two effects, maximal
nervous excitation of the heart can raise the plateau
level of the cardiac output curve to almost twice the
plateau of the normal curve, as shown by the 25-liter
level of the uppermost curve in Figure 20–4.
Cardiac output
(L/min)
Chapter 20
6
5
4
3
2
0
Dinitrophenol
With pressure control
A long-term increased workload, but not so much
excess load that it damages the heart, causes the heart
muscle to increase in mass and contractile strength in
the same way that heavy exercise causes skeletal
muscles to hypertrophy. For instance, it is common for
the hearts of marathon runners to be increased in mass
by 50 to 75 per cent. This increases the plateau level of
the cardiac output curve, sometimes 60 to 100 per cent,
and therefore allows the heart to pump much greater
than usual amounts of cardiac output.
When one combines nervous excitation of the heart
and hypertrophy, as occurs in marathon runners, the
total effect can allow the heart to pump as much 30 to
40 L/min, about 21/2 times normal; this increased level
of pumping is one of the most important factors in
determining the runner’s running time.
Without pressure control
Factors That Cause a Hypoeffective Heart
Any factor that decreases the heart’s ability to pump
blood causes hypoeffectivity. Some of the factors that
can do this are the following:
Coronary artery blockage, causing a “heart
attack”
Inhibition of nervous excitation of the heart
Pathological factors that cause abnormal heart
rhythm or rate of heartbeat
Valvular heart disease
Increased arterial pressure against which the
heart must pump, such as in hypertension
Congenital heart disease
Myocarditis
Cardiac hypoxia
What Is the Role of the
Nervous System in Controlling
Cardiac Output?
Importance of the Nervous System in
Maintaining the Arterial Pressure When the
Venous Return and Cardiac Output Increase
Figure 20–5 shows an important difference in cardiac
output control with and without a functioning autonomic nervous system. The solid curves demonstrate
the effect in the normal dog of intense dilation of
the peripheral blood vessels caused by administering
the drug dinitrophenol, which increased the metabolism of virtually all tissues of the body about fourfold.
Note that with nervous control to keep the arterial
pressure from falling, dilating all the peripheral blood
vessels caused almost no change in arterial pressure
but increased the cardiac output almost fourfold.
Arterial pressure
(mm Hg)
Increased Pumping Effectiveness Caused by Heart Hypertrophy.
100
75
50
0
0
10
20
30
Minutes
Figure 20–5
Experiment in a dog to demonstrate the importance of nervous
maintenance of the arterial pressure as a prerequisite for cardiac
output control. Note that with pressure control, the metabolic stimulant dinitrophenol increases cardiac output greatly; without pressure control, the arterial pressure falls and the cardiac output rises
very little. (Drawn from experiments by Dr. M. Banet.)
However, after autonomic control of the nervous
system had been blocked, none of the normal circulatory reflexes for maintaining the arterial pressure
could function, and vasodilation of the vessels with
dinitrophenol (dashed curves) then caused a profound
fall in arterial pressure to about one half normal, and
the cardiac output rose only 1.6-fold instead of 4-fold.
Thus, maintenance of a normal arterial pressure by
the nervous reflexes, by mechanisms explained in
Chapter 18, is essential to achieve high cardiac outputs
when the peripheral tissues dilate their vessels to
increase the venous return.
Effect of the Nervous System to Increase the Arterial Pressure
During Exercise. During exercise, intense increase in
metabolism in active skeletal muscles acts directly on
the muscle arterioles to relax them and to allow adequate oxygen and other nutrients needed to sustain
muscle contraction. Obviously, this greatly decreases
the total peripheral resistance, which normally would
decrease the arterial pressure also. However, the
nervous system immediately compensates. The same
brain activity that sends motor signals to the muscles
sends simultaneous signals into the autonomic nervous
centers of the brain to excite circulatory activity,
causing large vein constriction, increased heart rate,
and increased contractility of the heart. All these
changes acting together increase the arterial pressure
above normal, which in turn forces still more blood
flow through the active muscles.
In summary, when local tissue vessels dilate and
thereby increase venous return and cardiac output
236
Unit IV
The Circulation
and at the same time increase the cardiac output to
above normal.
1. Beriberi. This disease is caused by insufficient
quantity of the vitamin thiamine (vitamin B1) in the
diet. Lack of this vitamin causes diminished ability
of the tissues to use some cellular nutrients, and the
local tissue blood flow mechanisms in turn cause
marked compensatory peripheral vasodilation.
Sometimes the total peripheral resistance decreases
to as little as one-half normal. Consequently, the
long-term levels of venous return and cardiac
output also often increase to twice normal.
2. Arteriovenous fistula (shunt). Earlier, we pointed
out that whenever a fistula (also called an AV
shunt) occurs between a major artery and a major
vein, tremendous amounts of blood flow directly
from the artery into the vein. This, too, greatly
decreases the total peripheral resistance and,
likewise, increases the venous return and cardiac
output.
3. Hyperthyroidism. In hyperthyroidism, the
metabolism of most tissues of the body becomes
greatly increased. Oxygen usage increases, and
vasodilator products are released from the tissues.
Therefore, the total peripheral resistance decreases
markedly because of the local tissue blood
flow control reactions throughout the body;
consequently, the venous return and cardiac output
often increase to 40 to 80 per cent above normal.
4. Anemia. In anemia, two peripheral effects greatly
decrease the total peripheral resistance. One of
these is reduced viscosity of the blood, resulting
from the decreased concentration of red blood
above normal, the nervous system plays an exceedingly important role in preventing the arterial pressure
from falling to disastrously low levels. In fact, during
exercise, the nervous system goes even further, providing additional signals to raise the arterial pressure
even above normal, which serves to increase the
cardiac output an extra 30 to 100 per cent.
Pathologically High and
Pathologically Low
Cardiac Outputs
In healthy human beings, the cardiac outputs are
surprisingly constant from one person to another.
However, multiple clinical abnormalities can cause
either high or low cardiac outputs. Some of the more
important of these are shown in Figure 20–6.
High Cardiac Output Caused
by Reduced Total Peripheral
Resistance
The left side of Figure 20–6 identifies conditions that
commonly cause cardiac outputs higher than normal.
One of the distinguishing features of these conditions is
that they all result from chronically reduced total peripheral resistance. None of them result from excessive
excitation of the heart itself, which we will explain subsequently. For the present, let us look at some of the
conditions that can decrease the peripheral resistance
200
7
175
6
125
4
Control (young adults)
100
2
Cardiac shock (7)
Traumatic shock (4)
Severe valve disease (29)
Mild shock (4)
Myocardial infarction (22)
Mild valve disease (31)
3
Hypertension (47)
Control (young adults) (308)
Paget’s disease (9)
Pregnancy (46)
Pulmonary disease (29)
0
Anxiety (21)
25
Beriberi (5)
50
Anemia (75)
75
Hyperthyroidism (29)
Average 45-year-old adult
Cardiac index
(L/min/m2)
5
AV shunts (33)
Cardiac output
(per cent of control)
150
1
0
Figure 20–6
Cardiac output in different pathological conditions. The numbers in parentheses indicate number of patients studied in each condition.
(Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia:
WB Saunders Co, 1973.)
Chapter 20
Cardiac Output, Venous Return, and Their Regulation
cells. The other is diminished delivery of oxygen to
the tissues, which causes local vasodilation. As a
consequence, the cardiac output increases greatly.
Any other factor that decreases the total peripheral
resistance chronically also increases the cardiac output.
Low Cardiac Output
Figure 20–6 shows at the far right several conditions that
cause abnormally low cardiac output. These conditions
fall into two categories: (1) those abnormalities that
cause the pumping effectiveness of the heart to fall too
low and (2) those that cause venous return to fall too
low.
Decreased Cardiac Output Caused by Cardiac Factors. When-
ever the heart becomes severely damaged, regardless
of the cause, its limited level of pumping may fall below
that needed for adequate blood flow to the tissues. Some
examples of this include (1) severe coronary blood vessel
blockage and consequent myocardial infarction, (2)
severe valvular heart disease, (3) myocarditis, (4) cardiac
tamponade, and (5) cardiac metabolic derangements.
The effects of several of these are shown on the right in
Figure 20–6, demonstrating the low cardiac outputs that
result.
When the cardiac output falls so low that the tissues
throughout the body begin to suffer nutritional deficiency, the condition is called cardiac shock. This is discussed fully in Chapter 22 in relation to cardiac failure.
Decrease in Cardiac Output Caused by Non-cardiac Peripheral
Factors—Decreased Venous Return. Anything that inter-
feres with venous return also can lead to decreased
cardiac output. Some of these factors are the following:
1. Decreased blood volume. By far, the most
common non-cardiac peripheral factor that leads
to decreased cardiac output is decreased blood
volume, resulting most often from hemorrhage.
It is clear why this condition decreases the cardiac
output: Loss of blood decreases the filling of the
vascular system to such a low level that there is not
enough blood in the peripheral vessels to create
peripheral vascular pressures high enough to push
the blood back to the heart.
2. Acute venous dilation. On some occasions, the
peripheral veins become acutely vasodilated. This
results most often when the sympathetic nervous
system suddenly becomes inactive. For instance,
fainting often results from sudden loss of
sympathetic nervous system activity, which causes
the peripheral capacitative vessels, especially the
veins, to dilate markedly. This decreases the filling
pressure of the vascular system because the blood
volume can no longer create adequate pressure
in the now flaccid peripheral blood vessels. As a
result, the blood “pools” in the vessels and does not
return to the heart.
3. Obstruction of the large veins. On rare occasions,
the large veins leading into the heart become
obstructed, so that the blood in the peripheral
vessels cannot flow back into the heart.
Consequently, the cardiac output falls markedly.
4. Decreased tissue mass, especially decreased skeletal
muscle mass. With normal aging or with prolonged
periods of physical inactivity, there is usually a
reduction in the size of the skeletal muscles. This, in
turn, decreases the total oxygen consumption and
237
blood flow needs of the muscles, resulting in
decreases in skeletal muscle blood flow and cardiac
output.
Regardless of the cause of low cardiac output,
whether it be a peripheral factor or a cardiac factor, if
ever the cardiac output falls below that level required
for adequate nutrition of the tissues, the person is said
to suffer circulatory shock. This condition can be lethal
within a few minutes to a few hours. Circulatory shock
is such an important clinical problem that it is discussed
in detail in Chapter 24.
A More Quantitative Analysis
of Cardiac Output Regulation
Our discussion of cardiac output regulation thus far is
adequate for understanding the factors that control
cardiac output in most simple conditions. However, to
understand cardiac output regulation in especially
stressful situations, such as the extremes of exercise,
cardiac failure, and circulatory shock, a more complex
quantitative analysis is presented in the following
sections.
To perform the more quantitative analysis, it is necessary to distinguish separately the two primary factors
concerned with cardiac output regulation: (1) the
pumping ability of the heart, as represented by cardiac
output curves, and (2) the peripheral factors that affect
flow of blood from the veins into the heart, as represented by venous return curves. Then one can put these
curves together in a quantitative way to show how they
interact with each other to determine cardiac output,
venous return, and right atrial pressure at the same time.
Cardiac Output Curves Used in the
Quantitative Analysis
Some of the cardiac output curves used to depict quantitative heart pumping effectiveness have already been
shown in Figure 20–4. However, an additional set of
curves is required to show the effect on cardiac output
caused by changing external pressures on the outside of
the heart, as explained in the next section.
Effect of External Pressure Outside the Heart on Cardiac Output
Curves. Figure 20–7 shows the effect of changes in exter-
nal cardiac pressure on the cardiac output curve. The
normal external pressure is equal to the normal
intrapleural pressure (the pressure in the chest cavity),
which is -4 mm Hg. Note in the figure that a rise in
intrapleural pressure, to -2 mm Hg, shifts the entire
cardiac output curve to the right by the same amount.
This shift occurs because to fill the cardiac chambers
with blood requires an extra 2 mm Hg right atrial pressure to overcome the increased pressure on the outside
of the heart. Likewise, an increase in intrapleural pressure to +2 mm Hg requires a 6 mm Hg increase in right
atrial pressure from the normal -4 mm Hg, which shifts
the entire cardiac output curve 6 mm Hg to the right.
Some of the factors that can alter the intrapleural
pressure and thereby shift the cardiac output curve are
the following:
1. Cyclical changes of intrapleural pressure during
respiration, which are about ±2 mm Hg during
normal breathing but can be as much as
±50 mm Hg during strenuous breathing.
238
Unit IV
The Circulation
Hypereffective–increased
intrapleural pressure
al pr
essu
re =
+
l eu r
e
onad
r ap
Int
I n tr
a mp
iac t
Card
N
0
–4
0
+4
+8
+12
Right atrial pressure (mm Hg)
15
Cardiac output (L/min)
Hg
2m
m
u r al
s s ure
press
ure =
–
–5
Hg
aple
ressu
re =
u r al p
ap le
Intr
ma
l (in
5
traple
ural
pre
10
)
Hg
2m
m
mm
.
=–
4m
m
5
or
Cardiac output (L/min)
15
Normal
10
Hypoeffective–reduced
intrapleural pressure
5
0
–4
0
+4
+8
+12
Right atrial pressure (mm Hg)
Figure 20–7
Cardiac output curves at different levels of intrapleural pressure
and at different degrees of cardiac tamponade. (Redrawn from
Guyton AC, Jones CE, Coleman TB: Circulatory Physiology:
Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB
Saunders Co, 1973.)
2. Breathing against a negative pressure, which shifts
the curve to a more negative right atrial pressure
(to the left).
3. Positive pressure breathing, which shifts the curve to
the right.
4. Opening the thoracic cage, which increases the
intrapleural pressure to 0 mm Hg and shifts the
cardiac output curve to the right 4 mm Hg.
5. Cardiac tamponade, which means accumulation of
a large quantity of fluid in the pericardial cavity
around the heart with resultant increase in external
cardiac pressure and shifting of the curve to the
right. Note in Figure 20–7 that cardiac tamponade
shifts the upper parts of the curves farther to the
right than the lower parts because the external
“tamponade” pressure rises to higher values as the
chambers of the heart fill to increased volumes
during high cardiac output.
Combinations of Different Patterns of Cardiac Output Curves.
Figure 20–8 shows that the final cardiac output curve can
change as a result of simultaneous changes in (a) external cardiac pressure and (b) effectiveness of the heart as
a pump. Thus, by knowing what is happening to the
external pressure as well as to the capability of the heart
as a pump, one can express the momentary ability of the
heart to pump blood by a single cardiac output curve.
Venous Return Curves
There remains the entire systemic circulation that must
be considered before total analysis of cardiac regulation
can be achieved. To analyze the function of the systemic
circulation, we first remove the heart and lungs from the
circulation of an animal and replace them with a pump
and artificial oxygenator system. Then, different factors,
such blood volume, vascular resistances, and central
venous pressure in the right atrium, are altered to
determine how the systemic circulation operates in
different circulatory states. In these studies, one finds
Figure 20–8
Combinations of two major patterns of cardiac output curves
showing the effect of alterations in both extracardiac pressure and
effectiveness of the heart as a pump. (Redrawn from Guyton AC,
Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output
and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
three principal factors that affect venous return to the
heart from the systemic circulation. They are as follows:
1. Right atrial pressure, which exerts a backward force
on the veins to impede flow of blood from the
veins into the right atrium.
2. Degree of filling of the systemic circulation
(measured by the mean systemic filling pressure),
which forces the systemic blood toward the heart
(this is the pressure measured everywhere in the
systemic circulation when all flow of blood is
stopped—we discuss this in detail later).
3. Resistance to blood flow between the peripheral
vessels and the right atrium.
These factors can all be expressed quantitatively
by the venous return curve, as we explain in the next
sections.
Normal Venous Return Curve
In the same way that the cardiac output curve relates
pumping of blood by the heart to right atrial pressure,
the venous return curve relates venous return also to
right atrial pressure—that is, the venous flow of blood
into the heart from the systemic circulation at different
levels of right atrial pressure.
The curve in Figure 20–9 is the normal venous return
curve. This curve shows that when heart pumping capability becomes diminished and causes the right atrial
pressure to rise, the backward force of the rising atrial
pressure on the veins of the systemic circulation
decreases venous return of blood to the heart. If all
nervous circulatory reflexes are prevented from acting,
venous return decreases to zero when the right atrial
pressure rises to about +7 mm Hg. Such a slight rise in
right atrial pressure causes a drastic decrease in venous
return because the systemic circulation is a distensible
bag, so that any increase in back pressure causes blood
to dam up in this bag instead of returning to the heart.
At the same time that the right atrial pressure is rising
and causing venous stasis, pumping by the heart also
Normal venous return curve. The
plateau is caused by collapse of
the large veins entering the chest
when the right atrial pressure falls
below atmospheric pressure.
Note also that venous return
becomes zero when the right
atrial pressure rises to equal the
mean systemic filling pressure.
Plateau
Transitional zone
5
Mean
systemic
filling
pressure
Do
wn
slo
pe
0
–8
approaches zero because of decreasing venous return.
Both the arterial and the venous pressures come to
equilibrium when all flow in the systemic circulation
ceases at a pressure of 7 mm Hg, which, by definition, is
the mean systemic filling pressure (Psf).
Plateau in the Venous Return Curve at Negative Atrial
Pressures—Caused by Collapse of the Large Veins. When the
right atrial pressure falls below zero—that is, below
atmospheric pressure—further increase in venous
return almost ceases. And by the time the right atrial
pressure has fallen to about -2 mm Hg, the venous
return will have reached a plateau. It remains at this
plateau level even though the right atrial pressure falls
to -20 mm Hg, -50 mm Hg, or even further.This plateau
is caused by collapse of the veins entering the chest. Negative pressure in the right atrium sucks the walls of the
veins together where they enter the chest, which prevents any additional flow of blood from the peripheral
veins. Consequently, even very negative pressures in the
right atrium cannot increase venous return significantly
above that which exists at a normal atrial pressure of
0 mm Hg.
Mean Circulatory Filling Pressure and Mean Systemic Filling
Pressure, and Their Effect on Venous Return
When heart pumping is stopped by shocking the heart
with electricity to cause ventricular fibrillation or is
stopped in any other way, flow of blood everywhere in
the circulation ceases a few seconds later.Without blood
flow, the pressures everywhere in the circulation
become equal. This equilibrated pressure level is called
the mean circulatory filling pressure.
–4
0
+4
+8
Right atrial pressure (mm Hg)
Mean circulatory filling pressure (mm Hg)
Figure 20–9
239
Cardiac Output, Venous Return, and Their Regulation
Venous return (L/min)
Chapter 20
Strong sympathetic
stimulation
Normal circulatory
system
Complete sympathetic
inhibition
Normal volume
14
12
10
8
6
4
2
0
–0
1000 2000 3000 4000 5000 6000 7000
Volume (ml)
Figure 20–10
Effect of changes in total blood volume on the mean circulatory
filling pressure (i.e., “volume-pressure curves” for the entire circulatory system). These curves also show the effects of strong
sympathetic stimulation and complete sympathetic inhibition.
Effect of Blood Volume on Mean Circulatory Filling Pressure.
Effect of Sympathetic Nervous Stimulation of the Circulation on
Mean Circulatory Filling Pressure. The green curve and blue
The greater the volume of blood in the circulation, the
greater is the mean circulatory filling pressure because
extra blood volume stretches the walls of the vasculature. The red curve in Figure 20–10 shows the approximate normal effect of different levels of blood volume
on the mean circulatory filling pressure. Note that at a
blood volume of about 4000 milliliters, the mean circulatory filling pressure is close to zero because this is the
“unstressed volume” of the circulation, but at a volume
of 5000 milliliters, the filling pressure is the normal value
of 7 mm Hg. Similarly, at still higher volumes, the mean
circulatory filling pressure increases almost linearly.
curve in Figure 20–10 show the effects, respectively,
of high and low levels of sympathetic nervous activity
on the mean circulatory filling pressure. Strong sympathetic stimulation constricts all the systemic blood
vessels as well as the larger pulmonary blood vessels
and even the chambers of the heart. Therefore, the
capacity of the system decreases, so that at each level of
blood volume, the mean circulatory filling pressure is
increased. At normal blood volume, maximal sympathetic stimulation increases the mean circulatory filling
pressure from 7 mm Hg to about 2.5 times that value, or
about 17 mm Hg.
240
Unit IV
The Circulation
Conversely, complete inhibition of the sympathetic
nervous system relaxes both the blood vessels and the
heart, decreasing the mean circulatory filling pressure
from the normal value of 7 mm Hg down to about 4 mm
Hg. Before leaving Figure 20–10, note specifically how
steep the curves are.This means that even slight changes
in blood volume or slight changes in the capacity of the
system caused by various levels of sympathetic activity
can have large effects on the mean circulatory filling
pressure.
Mean Systemic Filling Pressure and Its Relation to Mean Circulatory Filling Pressure. The mean systemic filling pressure,
Psf, is slightly different from the mean circulatory filling
pressure. It is the pressure measured everywhere in the
systemic circulation after blood flow has been stopped
by clamping the large blood vessels at the heart, so that
the pressures in the systemic circulation can be measured independently from those in the pulmonary circulation. The mean systemic pressure, although almost
impossible to measure in the living animal, is the important pressure for determining venous return. The mean
systemic filling pressure, however, is almost always nearly
equal to the mean circulatory filling pressure because the
pulmonary circulation has less than one eighth as much
capacitance as the systemic circulation and only about
one tenth as much blood volume.
Effect on the Venous Return Curve of Changes in Mean Systemic
Filling Pressure. Figure 20–11 shows the effects on the
Venous return (L/min)
venous return curve caused by increasing or decreasing
the mean systemic filling pressure (Psf). Note in Figure
20–11 that the normal mean systemic filling pressure is
7 mm Hg. Then, for the uppermost curve in the figure,
the mean systemic filling pressure has been increased to
14 mm Hg, and for the lowermost curve, has been
decreased to 3.5 mm Hg. These curves demonstrate that
the greater the mean systemic filling pressure (which
also means the greater the “tightness” with which the
circulatory system is filled with blood) the more the
venous return curve shifts upward and to the right.
10
Psf = 3.5
Psf = 7
No
5
Psf = 14
rm
al
0
–4
0
+4
+8
+12
Conversely, the lower the mean systemic filling pressure,
the more the curve shifts downward and to the left.
To express this another way, the greater the system is
filled, the easier it is for blood to flow into the heart.
The less the filling, the more difficult it is for blood to
flow into the heart.
“Pressure Gradient for Venous Return”—When This Is Zero, There
Is No Venous Return. When the right atrial pressure rises
to equal the mean systemic filling pressure, there is no
longer any pressure difference between the peripheral
vessels and the right atrium. Consequently, there can no
longer be any blood flow from any peripheral vessels
back to the right atrium. However, when the right atrial
pressure falls progressively lower than the mean systemic filling pressure, the flow to the heart increases
proportionately, as one can see by studying any of the
venous return curves in Figure 20–11. That is, the greater
the difference between the mean systemic filling pressure
and the right atrial pressure, the greater becomes the
venous return. Therefore, the difference between these
two pressures is called the pressure gradient for venous
return.
Resistance to Venous Return
In the same way that mean systemic filling pressure represents a pressure pushing venous blood from the
periphery toward the heart, there is also resistance to
this venous flow of blood. It is called the resistance to
venous return. Most of the resistance to venous return
occurs in the veins, although some occurs in the arterioles and small arteries as well.
Why is venous resistance so important in determining the resistance to venous return? The answer is that
when the resistance in the veins increases, blood begins
to be dammed up, mainly in the veins themselves. But
the venous pressure rises very little because the veins
are highly distensible. Therefore, this rise in venous
pressure is not very effective in overcoming the resistance, and blood flow into the right atrium decreases
drastically. Conversely, when arteriolar and small artery
resistances increase, blood accumulates in the arteries,
which have a capacitance only 1/30 as great as that of
the veins. Therefore, even slight accumulation of blood
in the arteries raises the pressure greatly—30 times as
much as in the veins—and this high pressure does overcome much of the increased resistance. Mathematically,
it turns out that about two thirds of the so-called “resistance to venous return” is determined by venous resistance, and about one third by the arteriolar and small
artery resistance.
Venous return can be calculated by the following
formula:
Right atrial pressure (mm Hg)
VR =
Figure 20–11
Venous return curves showing the normal curve when the mean
systemic filling pressure (Psf) is 7 mm Hg, and showing the effect
of altering the Psf to either 3.5 or 14 mm Hg. (Redrawn from
Guyton AC, Jones CE, Coleman TB: Circulatory Physiology:
Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB
Saunders Co, 1973.)
Psf - PRA
RVR
in which VR is venous return, Psf is mean systemic
filling pressure, PRA is right atrial pressure, and RVR
is resistance to venous return. In the healthy human
adult, the values for these are as follows: venous return
equals 5 L/min, mean systemic filling pressure equals
7 mm Hg, right atrial pressure equals 0 mm Hg, and
resistance to venous return equals 1.4 mm Hg per liter
of blood flow.
Chapter 20
241
Cardiac Output, Venous Return, and Their Regulation
20
Normal resistance
2 ¥ resistance
1/2 resistance
Venous return (L/min)
15
1/
10
2
re
sis
ta
nc
Norm
al r
es
ista
nc
e
2 ¥ resist
ance
e
Venous return (L/min)
15
5
1/3 resistance
10
5
Psf = 10.5
Psf = 10
Psf = 2.3
Psf = 7
0
–4
0
Psf = 7
+4
+8
+12
Right atrial pressure (mm Hg)
0
–4
0
+4
+8
Right atrial pressure (mm Hg)
Figure 20–13
Figure 20–12
Venous return curves depicting the effect of altering the “resistance to venous return.” Psf, mean systemic filling pressure.
(Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory
Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
Effect of Resistance to Venous Return on the Venous Return
Curve. Figure 20–12 demonstrates the effect of different
levels of resistance to venous return on the venous
return curve, showing that a decrease in this resistance
to one-half normal allows twice as much flow of blood
and, therefore, rotates the curve upward to twice as great
a slope. Conversely, an increase in resistance to twice
normal rotates the curve downward to one-half as great
a slope.
Note also that when the right atrial pressure rises to
equal the mean systemic filling pressure, venous return
becomes zero at all levels of resistance to venous return
because when there is no pressure gradient to cause
flow of blood, it makes no difference what the resistance
is in the circulation; the flow is still zero. Therefore, the
highest level to which the right atrial pressure can rise,
regardless of how much the heart might fail, is equal to
the mean systemic filling pressure.
Combinations of Venous Return Curve Patterns. Figure 20–13
shows effects on the venous return curve caused by
simultaneous changes in mean systemic pressure (Psf)
and resistance to venous return, demonstrating that
both these factors can operate simultaneously.
Analysis of Cardiac Output and
Right Atrial Pressure, Using
Simultaneous Cardiac Output and
Venous Return Curves
In the complete circulation, the heart and the systemic
circulation must operate together. This means that (1)
the venous return from the systemic circulation must
Combinations of the major patterns of venous return curves,
showing the effects of simultaneous changes in mean systemic
filling pressure (Psf) and in “resistance to venous return.”
(Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory
Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
equal the cardiac output from the heart and (2) the right
atrial pressure is the same for both the heart and the
systemic circulation.
Therefore, one can predict the cardiac output and
right atrial pressure in the following way: (1) Determine
the momentary pumping ability of the heart and depict
this in the form of a cardiac output curve; (2) determine
the momentary state of flow from the systemic circulation into the heart and depict this in the form of a
venous return curve; and (3) “equate” these curves
against each other, as shown in Figure 20–14.
Two curves in the figure depict the normal cardiac
output curve (red line) and the normal venous return
curve (blue line). There is only one point on the graph,
point A, at which the venous return equals the cardiac
output and at which the right atrial pressure is the same
for both the heart and the systemic circulation. Therefore, in the normal circulation, the right atrial pressure,
cardiac output, and venous return are all depicted by
point A, called the equilibrium point, giving a normal
value for cardiac output of 5 liters per minute and a
right atrial pressure of 0 mm Hg .
Effect of Increased Blood Volume on Cardiac Output. A sudden
increase in blood volume of about 20 per cent increases
the cardiac output to about 2.5 to 3 times normal. An
analysis of this effect is shown in Figure 20–14. Immediately on infusing the large quantity of extra blood, the
increased filling of the system causes the mean systemic
filling pressure (Psf) to increase to 16 mm Hg, which
shifts the venous return curve to the right. At the same
time, the increased blood volume distends the blood
vessels, thus reducing their resistance and thereby
reducing the resistance to venous return, which rotates
the curve upward. As a result of these two effects, the
venous return curve of Figure 20–14 is shifted to the
Unit IV
The Circulation
20
15
B
10
A
5
Psf = 7
Psf = 16
0
–4
0
+4
+8
+12
Right atrial pressure (mm Hg)
+16
Cardiac output and venous return (L/min)
Cardiac output and venous return (L/min)
242
25
Maximal sympathetic
stimulation
Moderate sympathetic
stimulation
20
Normal
Spinal anesthesia
15
D
10
C
A
5
B
0
–4
0
+4
+8
+12
Right atrial pressure (mm Hg)
+16
Figure 20–14
The two solid curves demonstrate an analysis of cardiac output
and right atrial pressure when the cardiac output (red line) and
venous return (blue line) curves are normal. Transfusion of blood
equal to 20 per cent of the blood volume causes the venous return
curve to become the dashed curve; as a result, the cardiac output
and right atrial pressure shift from point A to point B. Psf, mean
systemic filling pressure.
right. This new curve equates with the cardiac output
curve at point B, showing that the cardiac output and
venous return increase 2.5 to 3 times, and that the right
atrial pressure rises to about +8 mm Hg.
Further Compensatory Effects Initiated in Response to Increased
Blood Volume. The greatly increased cardiac output
caused by increased blood volume lasts for only a few
minutes because several compensatory effects immediately begin to occur: (1) The increased cardiac output
increases the capillary pressure so that fluid begins to
transude out of the capillaries into the tissues, thereby
returning the blood volume toward normal. (2) The
increased pressure in the veins causes the veins to
continue distending gradually by the mechanism called
stress-relaxation, especially causing the venous blood
reservoirs, such as the liver and spleen, to distend, thus
reducing the mean systemic pressure. (3) The excess
blood flow through the peripheral tissues causes
autoregulatory increase in the peripheral resistance,
thus increasing the resistance to venous return. These
factors cause the mean systemic filling pressure to
return back toward normal and the resistance vessels of
the systemic circulation to constrict. Therefore, gradually, over a period of 10 to 40 minutes, the cardiac output
returns almost to normal.
Effect of Sympathetic Stimulation on Cardiac Output. Sympathetic stimulation affects both the heart and the systemic circulation: (1) It makes the heart a stronger pump.
(2) In the systemic circulation, it increases the mean
systemic filling pressure because of contraction of
the peripheral vessels—especially the veins—and it
increases the resistance to venous return.
In Figure 20–15, the normal cardiac output and
venous return curves are depicted; these equate with
Figure 20–15
Analysis of the effect on cardiac output of (1) moderate sympathetic stimulation (from point A to point C), (2) maximal sympathetic stimulation (point D), and (3) sympathetic inhibition caused
by total spinal anesthesia (point B). (Redrawn from Guyton AC,
Jones CE, Coleman TB: Circulatory Physiology: Cardiac Output
and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
each other at point A, which represents a normal venous
return and cardiac output of 5 L/min and a right atrial
pressure of 0 min Hg. Note in the figure that maximal
sympathetic stimulation (green curves) increases the
mean systemic filling pressure to 17 mm Hg (depicted
by the point at which the venous return curve reaches
the zero venous return level).And the sympathetic stimulation also increases pumping effectiveness of the
heart by nearly 100 per cent. As a result, the cardiac
output rises from the normal value at equilibrium point
A to about double normal at equilibrium point D—and
yet the right atrial pressure hardly changes. Thus, different degrees of sympathetic stimulation can increase the
cardiac output progressively to about twice normal for
short periods of time, until other compensatory effects
occur within seconds or minutes.
Effect of Sympathetic Inhibition on Cardiac Output. The sym-
pathetic nervous system can be blocked by inducing
total spinal anesthesia or by using some drug, such as
hexamethonium, that blocks transmission of nerve
signals through the autonomic ganglia. The lowermost
curves in Figure 20–15 show the effect of sympathetic
inhibition caused by total spinal anesthesia, demonstrating that (1) the mean systemic filling pressure falls
to about 4 mm Hg and (2) the effectiveness of the heart
as a pump decreases to about 80 per cent of normal. The
cardiac output falls from point A to point B, which is a
decrease to about 60 per cent of normal.
Effect of Opening a Large Arteriovenous Fistula. Figure 20–16
shows various stages of circulatory changes that occur
after opening a large arteriovenous fistula, that is, after
making an opening directly between a large artery and
a large vein.
1. The two red curves crossing at point A show the
normal condition.
Chapter 20
Flow (L/min)
D
20
Cardiac output and venous return (L/min)
243
Cardiac Output, Venous Return, and Their Regulation
C
15
B
20
15
10
5
0
0
10
A
5
1
Seconds
2
Figure 20–17
Pulsatile blood flow in the root of the aorta recorded using an electromagnetic flowmeter.
0
–4
0
+4
+8
Right atrial pressure (mm Hg)
+12
Figure 20–16
Analysis of successive changes in cardiac output and right atrial
pressure in a human being after a large arteriovenous (AV) fistula
is suddenly opened. The stages of the analysis, as shown by the
equilibrium points, are A, normal conditions; B, immediately after
opening the AV fistula; C, 1 minute or so after the sympathetic
reflexes have become active; and D, several weeks after the blood
volume has increased and the heart has begun to hypertrophy.
(Redrawn from Guyton AC, Jones CE, Coleman TB: Circulatory
Physiology: Cardiac Output and Its Regulation. 2nd ed. Philadelphia: WB Saunders Co, 1973.)
2. The curves crossing at point B show the circulatory
condition immediately after opening the large
fistula. The principal effects are (1) a sudden and
precipitous rotation of the venous return curve
upward caused by the large decrease in resistance to
venous return when blood is allowed to flow with
almost no impediment directly from the large
arteries into the venous system, bypassing most
of the resistance elements of the peripheral
circulation, and (2) a slight increase in the level of
the cardiac output curve because opening the fistula
decreases the peripheral resistance and allows
an acute fall in arterial pressure against which
the heart can pump more easily. The net result,
depicted by point B, is an increase in cardiac output
from 5 L/min up to 13 L/min and an increase in
right atrial pressure to about +3 mm Hg.
3. Point C represents the effects about 1 minute later,
after the sympathetic nerve reflexes have restored
the arterial pressure almost to normal and caused
two other effects: (1) an increase in the mean
systemic filling pressure (because of constriction of
all veins and arteries) from 7 to 9 mm Hg, thus
shifting the venous return curve 2 mm Hg to the
right, and (2) further elevation of the cardiac
output curve because of sympathetic nervous
excitation of the heart. The cardiac output now
rises to almost 16 L/min, and the right atrial
pressure to about 4 mm Hg.
4. Point D shows the effect after several more weeks.
By this time, the blood volume has increased
because the slight reduction in arterial pressure and
the sympathetic stimulation have both reduced
kidney output of urine. The mean systemic filling
pressure has now risen to +12 mm Hg, shifting the
venous return curve another 3 mm Hg to the right.
Also, the prolonged increased workload on the
heart has caused the heart muscle to hypertrophy
slightly, raising the level of the cardiac output curve
still further. Therefore, point D shows a cardiac
output now of almost 20 L/min and a right atrial
pressure of about 6 mm Hg.
Other Analyses of Cardiac Output Regulation. In Chapter 21,
analysis of cardiac output regulation during exercise is
presented, and in Chapter 22, analyses of cardiac output
regulation at various stages of congestive heart failure
are shown.
Methods for Measuring
Cardiac Output
In animal experiments, one can cannulate the aorta, pulmonary artery, or great veins entering the heart and
measure the cardiac output using any type of flowmeter. An electromagnetic or ultrasonic flowmeter can also
be placed on the aorta or pulmonary artery to measure
cardiac output.
In the human, except in rare instances, cardiac output
is measured by indirect methods that do not require
surgery. Two of the methods commonly used are the
oxygen Fick method and the indicator dilution method.
Pulsatile Output of the Heart as
Measured by an Electromagnetic or
Ultrasonic Flowmeter
Figure 20–17 shows a recording in a dog of blood flow
in the root of the aorta made using an electromagnetic
flowmeter. It demonstrates that the blood flow rises
rapidly to a peak during systole, and then at the end of
systole reverses for a fraction of a second. This reverse
flow causes the aortic valve to close and the flow to
return to zero.
244
Unit IV
The Circulation
Measurement of Cardiac Output Using
the Oxygen Fick Principle
The Fick principle is explained by Figure 20–18. This
figure shows that 200 milliliters of oxygen are being
absorbed from the lungs into the pulmonary blood
each minute. It also shows that the blood entering the
right heart has an oxygen concentration of 160 milliliters per liter of blood, whereas that leaving the left
heart has an oxygen concentration of 200 milliliters
per liter of blood. From these data, one can calculate
that each liter of blood passing through the lungs
absorbs 40 milliliters of oxygen.
Because the total quantity of oxygen absorbed into
the blood from the lungs each minute is 200 milliliters,
dividing 200 by 40 calculates a total of five 1-liter portions of blood that must pass through the pulmonary
circulation each minute to absorb this amount of
oxygen. Therefore, the quantity of blood flowing
through the lungs each minute is 5 liters, which is also
a measure of the cardiac output. Thus, the cardiac
output can be calculated by the following formula:
Cardiac output (L min)
O2 absorbed per minute by the lungs (ml min)
=
Arteriovenous O2 difference (ml L of blood)
LUNGS
Oxygen used = 200 ml/min
O2 =
160 ml/L
right heart
Cardiac output =
5000 ml/min
To measure cardiac output by the so-called “indicator
dilution method,” a small amount of indicator, such as
a dye, is injected into a large systemic vein or, preferably, into the right atrium. This passes rapidly through
the right side of the heart, then through the blood
vessels of the lungs, through the left side of the heart
and, finally, into the systemic arterial system. The concentration of the dye is recorded as the dye passes
through one of the peripheral arteries, giving a curve as
shown in Figure 20–19. In each of these instances, 5 milligrams of Cardio-Green dye was injected at zero time.
In the top recording, none of the dye passed into the
arterial tree until about 3 seconds after the injection, but
then the arterial concentration of the dye rose rapidly
to a maximum in about 6 to 7 seconds. After that, the
concentration fell rapidly, but before the concentration
reached zero, some of the dye had already circulated all
the way through some of the peripheral systemic vessels
and returned through the heart for a second time. Consequently, the dye concentration in the artery began to
rise again. For the purpose of calculation, it is necessary
to extrapolate the early down-slope of the curve to the
zero point, as shown by the dashed portion of each
curve. In this way, the extrapolated time-concentration
curve of the dye in the systemic artery without recirculation can be measured in its first portion and estimated
reasonably accurately in its latter portion.
Once the extrapolated time-concentration curve has
been determined, one then calculates the mean concentration of dye in the arterial blood for the duration
of the curve. For instance, in the top example of Figure
20–19, this was done by measuring the area under the
5 mg
injected
Dye concentration in artery (mg/dl)
In applying this Fick procedure for measuring
cardiac output in the human being, mixed venous
blood is usually obtained through a catheter inserted
up the brachial vein of the forearm, through the subclavian vein, down to the right atrium, and, finally, into
the right ventricle or pulmonary artery. And systemic
arterial blood can then be obtained from any systemic
artery in the body. The rate of oxygen absorption by
the lungs is measured by the rate of disappearance of
oxygen from the respired air, using any type of oxygen
meter.
Indicator Dilution Method for
Measuring Cardiac Output
0.5
0.4
0.3
0.2
0.1
0
0
0.5
0.4
0.3
0.2
0.1
0
0
O2 =
200 ml/L
left heart
10
20
30
20
30
5 mg
injected
10
Seconds
Figure 20–19
Figure 20–18
Fick principle for determining cardiac output.
Extrapolated dye concentration curves used to calculate two separate cardiac outputs by the dilution method. (The rectangular
areas are the calculated average concentrations of dye in the arterial blood for the durations of the respective extrapolated curves.)
Chapter 20
Cardiac Output, Venous Return, and Their Regulation
entire initial and extrapolated curve and then averaging
the concentration of dye for the duration of the curve;
one can see from the shaded rectangle straddling the
curve in the upper figure that the average concentration
of dye was 0.25 mg/dl of blood and that the duration
of this average value was 12 seconds. A total of 5 milligrams of dye had been injected at the beginning of the
experiment. For blood carrying only 0.25 milligram of
dye in each deciliter to carry the entire 5 milligrams of
dye through the heart and lungs in 12 seconds, a total
of 20 1-deciliter portions of blood would have passed
through the heart during the 12 seconds, which would
be the same as a cardiac output of 2 L/12 sec, or
10 L/min. We leave it to the reader to calculate the
cardiac output from the bottom extrapolated curve of
Figure 20–19. To summarize, the cardiac output can be
determined using the following formula:
Cardiac output (ml min) =
Milligrams of dye injected ¥ 60
Ê Average concentration of dyeˆ
Ê Duration ofˆ
Á in each milliliter of blood ˜ ¥ Á the curve ˜
Ë for the duration of the curve ¯
Ë in seconds ¯
References
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atrial pressure determines venous return. J Appl Physiol
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Gaasch WH, Zile MR: Left ventricular diastolic dysfunction
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245
Guyton AC: Venous return. In: Hamilton WF (ed): Handbook of Physiology. Sec 2, Vol. 2. Baltimore, Williams &
Wilkins, 1963, p 1099.
Guyton AC: Determination of cardiac output by equating
venous return curves with cardiac response curves. Physiol
Rev 35:123, 1955.
Guyton AC, Coleman TG, Granger HJ: Circulation: overall
regulation. Annu Rev Physiol 34:13, 1972.
Guyton AC, Jones CE, Coleman TG: Circulatory Physiology:
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Saunders Co, 1973.
Guyton AC, Lindsey AW, Kaufmann BN: Effect of mean circulatory filling pressure and other peripheral circulatory
factors on cardiac output. Am J Physiol 180:463-468, 1955.
Koch WJ, Lefkowitz RJ, Rockman HA: Functional consequences of altering myocardial adrenergic receptor signaling. Annu Rev Physiol 62:237, 2000.
Rockman HA, Koch WJ, Lefkowitz RJ: Seven-transmembrane-spanning receptors and heart function. Nature
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Rothe CF: Mean circulatory filling pressure: its meaning and
measurement. J Appl Physiol 74:499, 1993.
Rothe CF: Reflex control of veins and vascular capacitance.
Physiol Rev 63:1281, 1983.
Sarnoff SJ, Berglund E: Ventricular function. 1. Starling’s law
of the heart, studied by means of simultaneous right and
left ventricular function curves in the dog. Circulation
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Uemura K, Sugimachi M, Kawada T, et al: A novel framework of circulatory equilibrium. Am J Physiol Heart Circ
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C
H
A
P
T
E
R
2
1
Muscle Blood Flow and Cardiac
Output During Exercise; the
Coronary Circulation and Ischemic
Heart Disease
In this chapter we consider (1) blood flow to the
skeletal muscles and (2) coronary blood flow
to the heart. Regulation of each of these is
achieved mainly by local control of vascular
resistance in response to muscle tissue metabolic
needs.
In addition, related subjects are discussed, such as
(1) cardiac output control during exercise, (2) characteristics of heart attacks, and (3) the pain of angina pectoris.
Blood Flow in Skeletal Muscle and Blood Flow
Regulation During Exercise
Very strenuous exercise is one of the most stressful conditions that the normal
circulatory system faces. This is true because there is such a large mass of skeletal muscle in the body, all of it requiring large amounts of blood flow. Also, the
cardiac output often must increase in the non-athlete to four to five times
normal, or in the well-trained athlete to six to seven times normal.
Rate of Blood Flow Through the Muscles
During rest, blood flow through skeletal muscle averages 3 to 4 ml/min/100 g of
muscle. During extreme exercise in the well-conditioned athlete, this can increase
15- to 25-fold, rising to 50 to 80 ml/min/100 g of muscle.
Blood Flow During Muscle Contractions. Figure 21–1 shows a record of blood flow
changes in a calf muscle of a human leg during strong rhythmical muscular exercise. Note that the flow increases and decreases with each muscle contraction.
At the end of the contractions, the blood flow remains very high for a few
seconds but then fades toward normal during the next few minutes.
The cause of the lower flow during the muscle contraction phase of exercise
is compression of the blood vessels by the contracted muscle. During strong
tetanic contraction, which causes sustained compression of the blood vessels, the
blood flow can be almost stopped, but this also causes rapid weakening of the
contraction.
Increased Blood Flow in Muscle Capillaries During Exercise. During rest, some muscle
capillaries have little or no flowing blood. But during strenuous exercise, all
the capillaries open. This opening of dormant capillaries diminishes the distance that oxygen and other nutrients must diffuse from the capillaries to the
contracting muscle fibers and sometimes contributes a twofold to threefold
increased capillary surface area through which oxygen and nutrients can diffuse
from the blood.
246
Chapter 21
Muscle Blood Flow and Cardiac Output During Exercise
247
Blood flow (100 ml/min)
(in some species of animals) sympathetic vasodilator
nerves as well.
Rhythmic exercise
40
20
Calf
flow
0
0
10
16
18
Minutes
Figure 21–1
Effects of muscle exercise on blood flow in the calf of a leg during
strong rhythmical contraction. The blood flow was much less
during contractions than between contractions. (Adapted from
Barcroft and Dornhorst: J Physiol 109:402, 1949.)
Control of Blood Flow Through the
Skeletal Muscles
Local Regulation—Decreased Oxygen in Muscle Greatly
Enhances Flow. The tremendous increase in muscle
blood flow that occurs during skeletal muscle activity
is caused primarily by chemical effects acting directly
on the muscle arterioles to cause dilation. One of the
most important chemical effects is reduction of oxygen
in the muscle tissues. That is, during muscle activity,
the muscle uses oxygen rapidly, thereby decreasing the
oxygen concentration in the tissue fluids. This in turn
causes local arteriolar vasodilation both because the
arteriolar walls cannot maintain contraction in the
absence of oxygen and because oxygen deficiency
causes release of vasodilator substances. The most
important vasodilator substance is probably adenosine, but experiments have shown that even large
amounts of adenosine infused directly into a muscle
artery cannot sustain vasodilation in skeletal muscle
for more than about 2 hours.
Fortunately, even after the muscle blood vessels
have become insensitive to the vasodilator effects of
adenosine, still other vasodilator factors continue to
maintain increased capillary blood flow as long as the
exercise continues. These factors include (1) potassium
ions, (2) adenosine triphosphate (ATP), (3) lactic acid,
and (4) carbon dioxide. We still do not know quantitatively how great a role each of these plays in increasing muscle blood flow during muscle activity;
this subject was discussed in additional detail in
Chapter 17.
Nervous Control of Muscle Blood Flow. In addition to local
tissue vasodilator mechanisms, skeletal muscles are
provided with sympathetic vasoconstrictor nerves and
Sympathetic Vasoconstrictor Nerves. The sympathetic vasoconstrictor nerve fibers secrete norepinephrine at their nerve endings. When maximally activated,
this can decrease blood flow through resting muscles
to as little as one half to one third normal. This vasoconstriction is of physiologic importance in circulatory
shock and during other periods of stress when it is
necessary to maintain a normal or even high arterial
pressure.
In addition to the norepinephrine secreted at the
sympathetic vasoconstrictor nerve endings, the medullae of the two adrenal glands also secrete large
amounts of norepinephrine plus even more epinephrine into the circulating blood during strenuous exercise. The circulating norepinephrine acts on the muscle
vessels to cause a vasoconstrictor effect similar to that
caused by direct sympathetic nerve stimulation. The
epinephrine, however, often has a slight vasodilator
effect because epinephrine excites more of the beta
adrenergic receptors of the vessels, which are vasodilator receptors, in contrast to the alpha vasoconstrictor
receptors excited especially by norepinephrine. These
receptors are discussed in Chapter 60.
Total Body Circulatory Readjustments
During Exercise
Three major effects occur during exercise that are
essential for the circulatory system to supply the
tremendous blood flow required by the muscles. They
are (1) mass discharge of the sympathetic nervous
system throughout the body with consequent stimulatory effects on the entire circulation, (2) increase in
arterial pressure, and (3) increase in cardiac output.
Effects of Mass Sympathetic Discharge
At the onset of exercise, signals are transmitted not
only from the brain to the muscles to cause muscle
contraction but also into the vasomotor center to initiate mass sympathetic discharge throughout the body.
Simultaneously, the parasympathetic signals to the
heart are attenuated. Therefore, three major circulatory effects result.
First, the heart is stimulated to greatly increased
heart rate and increased pumping strength as a result
of the sympathetic drive to the heart plus release of
the heart from normal parasympathetic inhibition.
Second, most of the arterioles of the peripheral
circulation are strongly contracted, except for the
arterioles in the active muscles, which are strongly
vasodilated by the local vasodilator effects in the
muscles as noted above. Thus, the heart is stimulated
to supply the increased blood flow required by the
muscles, while at the same time blood flow through
most nonmuscular areas of the body is temporarily
reduced, thereby temporarily “lending” their blood
supply to the muscles. This accounts for as much as
2 L/min of extra blood flow to the muscles, which is
Unit IV
The Circulation
exceedingly important when one thinks of a person
running for his life—even a fractional increase in
running speed may make the difference between life
and death. Two of the peripheral circulatory systems,
the coronary and cerebral systems, are spared this vasoconstrictor effect because both these circulatory areas
have poor vasoconstrictor innervation—fortunately so
because both the heart and the brain are as essential
to exercise as are the skeletal muscles.
Third, the muscle walls of the veins and other capacitative areas of the circulation are contracted powerfully, which greatly increases the mean systemic filling
pressure. As we learned in Chapter 20, this is one of
the most important factors in promoting increase in
venous return of blood to the heart and, therefore, in
increasing the cardiac output.
Increase in Arterial Pressure During Exercise—
An Important Result of Increased
Sympathetic Stimulation
One of the most important effects of increased sympathetic stimulation in exercise is to increase the arterial pressure. This results from multiple stimulatory
effects, including (1) vasoconstriction of the arterioles
and small arteries in most tissues of the body except
the active muscles, (2) increased pumping activity
by the heart, and (3) a great increase in mean systemic
filling pressure caused mainly by venous contraction.
These effects, working together, virtually always
increase the arterial pressure during exercise. This
increase can be as little as 20 mm Hg or as great as
80 mm Hg, depending on the conditions under which
the exercise is performed. When a person performs
exercise under tense conditions but uses only a few
muscles, the sympathetic nervous response still occurs
everywhere in the body. In the few active muscles,
vasodilation occurs, but everywhere else in the body
the effect is mainly vasoconstriction, often increasing
the mean arterial pressure to as high as 170 mm Hg.
Such a condition might occur in a person standing
on a ladder and nailing with a hammer on the ceiling
above. The tenseness of the situation is obvious.
Conversely, when a person performs massive
whole-body exercise, such as running or swimming,
the increase in arterial pressure is often only 20 to
40 mm Hg. This lack of a large increase in pressure
results from the extreme vasodilation that occurs
simultaneously in large masses of active muscle.
Why Is the Arterial Pressure Increase During Exercise Important? When muscles are stimulated maximally in a lab-
oratory experiment but without allowing the arterial
pressure to rise, muscle blood flow seldom rises more
than about eightfold. Yet, we know from studies of
marathon runners that muscle blood flow can increase
from as little as 1 L/min for the whole body during rest
to at least 20 L/min during maximal activity. Therefore,
it is clear that muscle blood flow can increase much
more than occurs in the aforementioned simple laboratory experiment. What is the difference? Mainly, the
arterial pressure rises during normal exercise. Let us
25
Cardiac output and
venous return (L/min)
248
B
20
15
10
A
5
0
–4
0
+4
+8
+12
+16
+20
+24
Right atrial pressure (mm Hg)
Figure 21–2
Graphical analysis of change in cardiac output and right atrial
pressure with onset of strenuous exercise. Black curves, normal
circulation. Red curves, heavy exercise.
assume, for instance, that the arterial pressure rises
30 per cent, a common increase during heavy exercise.
This 30 per cent increase causes 30 per cent more force
to push blood through the muscle tissue vessels. But
this is not the only important effect; the extra pressure
also stretches the walls of the vessels so much that
muscle total flow often rises to more than 20 times
normal.
Importance of the Increase in Cardiac Output
During Exercise
Many different physiologic effects occur at the same
time during exercise to increase cardiac output
approximately in proportion to the degree of exercise.
In fact, the ability of the circulatory system to provide
increased cardiac output for delivery of oxygen and
other nutrients to the muscles during exercise is
equally as important as the strength of the muscles
themselves in setting the limit for continued muscle
work. For instance, marathon runners who can
increase their cardiac outputs the most are generally
the same persons who have record-breaking running
times.
Graphical Analysis of the Changes in Cardiac Output During
Heavy Exercise. Figure 21–2 shows a graphical analysis
of the large increase in cardiac output that occurs
during heavy exercise. The cardiac output and venous
return curves crossing at point A give the analysis for
the normal circulation; and the curves crossing at point
B analyze heavy exercise. Note that the great increase
in cardiac output requires significant changes in both
the cardiac output curve and the venous return curve,
as follow.
The increased level of the cardiac output curve is
easy to understand. It results almost entirely from
sympathetic stimulation of the heart that causes (1)
Chapter 21
249
Muscle Blood Flow and Cardiac Output During Exercise
increased heart rate, often up to rates as high as 170 to
190 beats/min, and (2) increased strength of contraction of the heart, often to as much as twice normal.
Without this increased level of the output curve, the
increase in cardiac output would be limited to the
plateau level of the normal heart, which would be a
maximum increase of cardiac output of only about
2.5-fold rather than the 4-fold that can commonly be
achieved by the untrained runner and the 7-fold that
can be achieved in some marathon runners.
Now study the venous return curves. If no change
occurred from the normal venous return curve, the
cardiac output could hardly rise at all in exercise
because the upper plateau level of the normal venous
return curve is only 6 L/min. Yet two important
changes do occur:
1. The mean systemic filling pressure rises
tremendously at the onset of heavy exercise. This
results partly from the sympathetic stimulation
that contracts the veins and other capacitative parts
of the circulation. In addition, tensing of the
abdominal and other skeletal muscles of the body
compresses many of the internal vessels, thus
providing more compression of the entire
capacitative vascular system, causing a still greater
increase in mean systemic filling pressure.
During maximal exercise, these two effects
together can increase the mean systemic filling
pressure from a normal level of 7 mm Hg to as
high as 30 mm Hg.
2. The slope of the venous return curve rotates
upward. This is caused by decreased resistance in
virtually all the blood vessels in active muscle
tissue, which also causes resistance to venous
return to decrease, thus increasing the upward
slope of the venous return curve.
Therefore, the combination of increased mean systemic filling pressure and decreased resistance to
venous return raises the entire level of the venous
return curve.
In response to the changes in both the venous return
curve and the cardiac output curve, the new equilibrium point in Figure 21–2 for cardiac output and
right atrial pressure is now point B, in contrast to the
normal level at point A. Note especially that the right
atrial pressure has hardly changed, having risen only
1.5 mm Hg. In fact, in a person with a strong heart,
the right atrial pressure often falls below normal in
very heavy exercise because of the greatly increased
sympathetic stimulation of the heart during exercise.
Aortic valve
Left coronary
artery
Right coronary
artery
Figure 21–3
The coronary arteries.
Physiologic Anatomy of the Coronary
Blood Supply
Figure 21–3 shows the heart and its coronary blood
supply. Note that the main coronary arteries lie on the
surface of the heart and smaller arteries then penetrate from the surface into the cardiac muscle mass. It
is almost entirely through these arteries that the heart
receives its nutritive blood supply. Only the inner 1/10
millimeter of the endocardial surface can obtain significant nutrition directly from the blood inside the
cardiac chambers, so that this source of muscle nutrition is minuscule.
The left coronary artery supplies mainly the anterior
and left lateral portions of the left ventricle, whereas
the right coronary artery supplies most of the right
ventricle as well as the posterior part of the left ventricle in 80 to 90 per cent of people.
Most of the coronary venous blood flow from the
left ventricular muscle returns to the right atrium of
the heart by way of the coronary sinus—which is about
75 per cent of the total coronary blood flow. And most
of the coronary venous blood from the right ventricular muscle returns through small anterior cardiac veins
that flow directly into the right atrium, not by way of
the coronary sinus. A very small amount of coronary
venous blood also flows back into the heart through
very minute thebesian veins, which empty directly into
all chambers of the heart.
Coronary Circulation
Normal Coronary Blood Flow
About one third of all deaths in the affluent society of
the Western world result from coronary artery disease,
and almost all elderly people have at least some
impairment of the coronary artery circulation. For this
reason, understanding normal and pathological physiology of the coronary circulation is one of the most
important subjects in medicine.
The resting coronary blood flow in the human being
averages about 225 ml/min, which is about 4 to 5 per
cent of the total cardiac output.
During strenuous exercise, the heart in the young
adult increases its cardiac output fourfold to sevenfold, and it pumps this blood against a higher than
normal arterial pressure. Consequently, the work
250
Unit IV
The Circulation
Coronary blood flow (ml/min)
Epicardial coronary arteries
Cardiac
muscle
300
Subendocardial arterial plexus
200
Figure 21–5
Diagram of the epicardial, intramuscular, and subendocardial
coronary vasculature.
100
0
Systole
Diastole
Figure 21–4
Phasic flow of blood through the coronary capillaries of the human
left ventricle during cardiac systole and diastole (as extrapolated
from measured flows in dogs).
output of the heart under severe conditions may
increase sixfold to ninefold. At the same time, the
coronary blood flow increases threefold to fourfold to
supply the extra nutrients needed by the heart. This
increase is not as much as the increase in workload,
which means that the ratio of energy expenditure by
the heart to coronary blood flow increases. Thus, the
“efficiency” of cardiac utilization of energy increases
to make up for the relative deficiency of coronary
blood supply.
Phasic Changes in Coronary Blood Flow During Systole and
Diastole—Effect of Cardiac Muscle Compression. Figure
21–4 shows the changes in blood flow through the
nutrient capillaries of the left ventricular coronary
system in milliliters per minute in the human heart
during systole and diastole, as extrapolated from
experiments in lower animals. Note from this diagram
that the coronary capillary blood flow in the left ventricle muscle falls to a low value during systole, which
is opposite to flow in vascular beds elsewhere in the
body. The reason for this is strong compression of the
left ventricular muscle around the intramuscular
vessels during systolic contraction.
During diastole, the cardiac muscle relaxes and no
longer obstructs blood flow through the left ventricular muscle capillaries, so that blood flows rapidly
during all of diastole.
Blood flow through the coronary capillaries of the
right ventricle also undergoes phasic changes during
the cardiac cycle, but because the force of contraction
of the right ventricular muscle is far less than that of
the left ventricular muscle, the inverse phasic changes
are only partial in contrast to those in the left ventricular muscle.
Epicardial Versus Subendocardial Coronary Blood Flow—Effect
of Intramyocardial Pressure. Figure 21–5 demonstrates
the special arrangement of the coronary vessels at different depths in the heart muscle, showing on the outer
surface epicardial coronary arteries that supply most of
the muscle. Smaller, intramuscular arteries derived
from the epicardial arteries penetrate the muscle,
supplying the needed nutrients. Lying immediately
beneath the endocardium is a plexus of subendocardial arteries. During systole, blood flow through the
subendocardial plexus of the left ventricle, where the
intramuscular coronary vessels are compressed greatly
by ventricular muscle contraction, tends to be reduced.
But the extra vessels of the subendocardial plexus normally compensate for this. Later in the chapter, we will
see that this peculiar difference between blood flow in
the epicardial and subendocardial arteries plays an
important role in certain types of coronary ischemia.
Control of Coronary Blood Flow
Local Muscle Metabolism Is the Primary
Controller of Coronary Flow
Blood flow through the coronary system is regulated
mostly by local arteriolar vasodilation in response to
cardiac muscle need for nutrition. That is, whenever
the vigor of cardiac contraction is increased, regardless of cause, the rate of coronary blood flow also
increases. Conversely, decreased heart activity is
accompanied by decreased coronary flow. This local
regulation of coronary blood flow is almost identical
to that occurring in many other tissues of the body,
especially in the skeletal muscles all over the body.
Oxygen Demand as a Major Factor in Local Coronary Blood Flow
Regulation. Blood flow in the coronaries usually is reg-
ulated almost exactly in proportion to the need of the
cardiac musculature for oxygen. Normally, about 70
per cent of the oxygen in the coronary arterial blood
is removed as the blood flows through the heart
muscle. Because not much oxygen is left, very little
additional oxygen can be supplied to the heart musculature unless the coronary blood flow increases. Fortunately, the coronary blood flow does increase almost
in direct proportion to any additional metabolic consumption of oxygen by the heart.
However, the exact means by which increased
oxygen consumption causes coronary dilation has
not been determined. It is speculated by many
research workers that a decrease in the oxygen
Chapter 21
Muscle Blood Flow and Cardiac Output During Exercise
251
concentration in the heart causes vasodilator substances to be released from the muscle cells and that
these dilate the arterioles. A substance with great
vasodilator propensity is adenosine. In the presence of
very low concentrations of oxygen in the muscle cells,
a large proportion of the cell’s ATP degrades to adenosine monophosphate; then small portions of this are
further degraded and release adenosine into the tissue
fluids of the heart muscle, with resultant increase in
local coronary blood flow. After the adenosine causes
vasodilation, much of it is reabsorbed into the cardiac
cells to be reused.
Adenosine is not the only vasodilator product
that has been identified. Others include adenosine
phosphate compounds, potassium ions, hydrogen
ions, carbon dioxide, bradykinin, and, possibly,
prostaglandins and nitric oxide.
Yet, difficulties with the vasodilator hypothesis exist.
First, pharmacologic agents that block or partially
block the vasodilator effect of adenosine do not
prevent coronary vasodilation caused by increased
heart muscle activity. Second, studies in skeletal
muscle have shown that continued infusion of adenosine maintains vascular dilation for only 1 to 3 hours,
and yet muscle activity still dilates the local blood
vessels even when the adenosine can no longer dilate
them. Therefore, the other vasodilator mechanisms
listed above must be remembered.
There is much more extensive sympathetic innervation of the coronary vessels. In Chapter 60, we see that
the sympathetic transmitter substances norepinephrine and epinephrine can have either vascular constrictor or vascular dilator effects, depending on the
presence or absence of constrictor or dilator receptors
in the blood vessel walls. The constrictor receptors are
called alpha receptors and the dilator receptors are
called beta receptors. Both alpha and beta receptors
exist in the coronary vessels. In general, the epicardial
coronary vessels have a preponderance of alpha receptors, whereas the intramuscular arteries may have a
preponderance of beta receptors. Therefore, sympathetic stimulation can, at least theoretically, cause
slight overall coronary constriction or dilation, but
usually constriction. In some people, the alpha vasoconstrictor effects seem to be disproportionately
severe, and these people can have vasospastic myocardial ischemia during periods of excess sympathetic
drive, often with resultant anginal pain.
Metabolic factors—especially myocardial oxygen
consumption—are the major controllers of myocardial
blood flow. Whenever the direct effects of nervous
stimulation alter the coronary blood flow in the wrong
direction, the metabolic control of coronary flow
usually overrides the direct coronary nervous effects
within seconds.
Nervous Control of Coronary Blood Flow
Special Features of Cardiac
Muscle Metabolism
Stimulation of the autonomic nerves to the heart can
affect coronary blood flow both directly and indirectly.
The direct effects result from action of the nervous
transmitter substances acetylcholine from the vagus
nerves and norepinephrine and epinephrine from the
sympathetic nerves on the coronary vessels themselves. The indirect effects result from secondary
changes in coronary blood flow caused by increased or
decreased activity of the heart.
The indirect effects, which are mostly opposite to
the direct effects, play a far more important role in
normal control of coronary blood flow. Thus, sympathetic stimulation, which releases norepinephrine and
epinephrine, increases both heart rate and heart contractility as well as increases the rate of metabolism of
the heart. In turn, the increased metabolism of the
heart sets off local blood flow regulatory mechanisms
for dilating the coronary vessels, and the blood flow
increases approximately in proportion to the metabolic needs of the heart muscle. In contrast, vagal stimulation, with its release of acetylcholine, slows the
heart and has a slight depressive effect on heart contractility. These effects in turn decrease cardiac oxygen
consumption and, therefore, indirectly constrict the
coronary arteries.
Direct Effects of Nervous Stimuli on the Coronary Vasculature.
The distribution of parasympathetic (vagal) nerve
fibers to the ventricular coronary system is not very
great. However, the acetylcholine released by parasympathetic stimulation has a direct effect to dilate the coronary arteries.
The basic principles of cellular metabolism, discussed
in Chapters 67 through 72, apply to cardiac muscle the
same as for other tissues, but there are some quantitative differences. Most important, under resting conditions, cardiac muscle normally consumes fatty acids to
supply most of its energy instead of carbohydrates
(about 70 per cent of the energy is derived from fatty
acids). However, as is also true of other tissues, under
anaerobic or ischemic conditions, cardiac metabolism
must call on anaerobic glycolysis mechanisms for
energy. Unfortunately, glycolysis consumes tremendous quantities of the blood glucose and at the same
time forms large amounts of lactic acid in the cardiac
tissue, which is probably one of the causes of cardiac
pain in cardiac ischemic conditions, as discussed later
in this chapter.
As is true in other tissues, more than 95 per cent of
the metabolic energy liberated from foods is used to
form ATP in the mitochondria. This ATP in turn acts
as the conveyer of energy for cardiac muscular contraction and other cellular functions. In severe coronary ischemia, the ATP degrades first to adenosine
diphosphate, then to adenosine monophosphate and
adenosine. Because the cardiac muscle cell membrane
is slightly permeable to adenosine, much of this
can diffuse from the muscle cells into the circulating
blood.
The released adenosine is believed to be one of the
substances that causes dilation of the coronary arterioles during coronary hypoxia, as discussed earlier.
252
Unit IV
The Circulation
However, loss of adenosine also has a serious cellular
consequence. Within as little as 30 minutes of severe
coronary ischemia, as occurs after a myocardial infarct,
about one half of the adenine base can be lost from
the affected cardiac muscle cells. Furthermore, this loss
can be replaced by new synthesis of adenine at a rate
of only 2 per cent per hour. Therefore, once a serious
bout of coronary ischemia has persisted for 30 or more
minutes, relief of the ischemia may be too late to save
the lives of the cardiac cells. This almost certainly is
one of the major causes of cardiac cellular death
during myocardial ischemia.
Ischemic Heart Disease
The most common cause of death in Western culture
is ischemic heart disease, which results from insufficient coronary blood flow. About 35 per cent of people
in the United States die of this cause. Some deaths
occur suddenly as a result of acute coronary occlusion
or fibrillation of the heart, whereas other deaths occur
slowly over a period of weeks to years as a result of
progressive weakening of the heart pumping process.
In this chapter, we discuss acute coronary ischemia
caused by acute coronary occlusion and myocardial
infarction. In Chapter 22, we discuss congestive heart
failure, the most frequent cause of which is slowly
increasing coronary ischemia and weakening of the
cardiac muscle.
Atherosclerosis as a Cause of Ischemic Heart Disease. The
most frequent cause of diminished coronary blood
flow is atherosclerosis. The atherosclerotic process is
discussed in connection with lipid metabolism in
Chapter 68. Briefly, this process is the following.
In people who have genetic predisposition to atherosclerosis, or in people who eat excessive quantities
of cholesterol and have a sedentary lifestyle, large
quantities of cholesterol gradually become deposited
beneath the endothelium at many points in arteries
throughout the body. Gradually, these areas of deposit
are invaded by fibrous tissue and frequently become
calcified. The net result is the development of atherosclerotic plaques that actually protrude into the vessel
lumens and either block or partially block blood flow.
A common site for development of atherosclerotic
plaques is the first few centimeters of the major coronary arteries.
the flowing blood. Because the plaque presents an
unsmooth surface, blood platelets adhere to it,
fibrin is deposited, and red blood cells become
entrapped to form a blood clot that grows until
it occludes the vessel. Or, occasionally, the
clot breaks away from its attachment on the
atherosclerotic plaque and flows to a more
peripheral branch of the coronary arterial tree,
where it blocks the artery at that point. A
thrombus that flows along the artery in this way
and occludes the vessel more distally is called a
coronary embolus.
2. Many clinicians believe that local muscular spasm
of a coronary artery also can occur. The spasm
might result from direct irritation of the smooth
muscle of the arterial wall by the edges of an
arteriosclerotic plaque, or it might result from
local nervous reflexes that cause excess coronary
vascular wall contraction. The spasm may then
lead to secondary thrombosis of the vessel.
Lifesaving Value of Collateral Circulation in the Heart. The
degree of damage to the heart muscle caused either by
slowly developing atherosclerotic constriction of the
coronary arteries or by sudden coronary occlusion is
determined to a great extent by the degree of collateral circulation that has already developed or that can
open within minutes after the occlusion.
In a normal heart, almost no large communications
exist among the larger coronary arteries. But many
anastomoses do exist among the smaller arteries
sized 20 to 250 micrometers in diameter, as shown in
Figure 21–6.
When a sudden occlusion occurs in one of the larger
coronary arteries, the small anastomoses begin to
Artery
Vein
Acute Coronary Occlusion
Acute occlusion of a coronary artery most frequently
occurs in a person who already has underlying atherosclerotic coronary heart disease but almost never
in a person with a normal coronary circulation. Acute
occlusion can result from any one of several effects,
two of which are the following:
1. The atherosclerotic plaque can cause a local blood
clot called a thrombus, which in turn occludes
the artery. The thrombus usually occurs where the
arteriosclerotic plaque has broken through the
endothelium, thus coming in direct contact with
Artery
Vein
Figure 21–6
Minute anastomoses in the normal coronary arterial system.
Chapter 21
253
Muscle Blood Flow and Cardiac Output During Exercise
dilate within seconds. But the blood flow through these
minute collaterals is usually less than one half that
needed to keep alive most of the cardiac muscle that
they now supply; the diameters of the collateral vessels
do not enlarge much more for the next 8 to 24 hours.
But then collateral flow does begin to increase, doubling by the second or third day and often reaching
normal or almost normal coronary flow within about
1 month. Because of these developing collateral channels, many patients recover almost completely from
various degrees of coronary occlusion when the area
of muscle involved is not too great.
When atherosclerosis constricts the coronary arteries slowly over a period of many years rather than
suddenly, collateral vessels can develop at the same
time while the atherosclerosis becomes more and
more severe. Therefore, the person may never experience an acute episode of cardiac dysfunction. But,
eventually, the sclerotic process develops beyond the
limits of even the collateral blood supply to provide
the needed blood flow, and sometimes the collateral
blood vessels themselves develop atherosclerosis.
When this occurs, the heart muscle becomes severely
limited in its work output, often so much so that the
heart cannot pump even normally required amounts
of blood flow. This is one of the most common causes
of the cardiac failure that occurs in vast numbers of
older people.
Myocardial Infarction
Immediately after an acute coronary occlusion, blood
flow ceases in the coronary vessels beyond the occlusion except for small amounts of collateral flow from
surrounding vessels. The area of muscle that has either
zero flow or so little flow that it cannot sustain cardiac
muscle function is said to be infarcted. The overall
process is called a myocardial infarction.
Soon after the onset of the infarction, small amounts
of collateral blood begin to seep into the infarcted area,
and this, combined with progressive dilation of local
blood vessels, causes the area to become overfilled with
stagnant blood. Simultaneously the muscle fibers use
the last vestiges of the oxygen in the blood, causing
the hemoglobin to become totally de-oxygenated.
Therefore, the infarcted area takes on a bluish-brown
hue, and the blood vessels of the area appear to be
engorged despite lack of blood flow. In later stages, the
vessel walls become highly permeable and leak fluid;
the local muscle tissue becomes edematous, and the
cardiac muscle cells begin to swell because of diminished cellular metabolism.Within a few hours of almost
no blood supply, the cardiac muscle cells die.
Cardiac muscle requires about 1.3 milliliters of
oxygen per 100 grams of muscle tissue per minute just
to remain alive. This is in comparison with about 8 milliliters of oxygen per 100 grams delivered to the
normal resting left ventricle each minute. Therefore, if
there is even 15 to 30 per cent of normal resting
coronary blood flow, the muscle will not die. In the
central portion of a large infarct, however, where
there is almost no collateral blood flow, the muscle
does die.
Subendocardial Infarction. The subendocardial muscle
frequently becomes infarcted even when there is no
evidence of infarction in the outer surface portions of
the heart. The reason for this is that the subendocardial muscle has extra difficulty obtaining adequate
blood flow because the blood vessels in the subendocardium are intensely compressed by systolic contraction of the heart, as explained earlier. Therefore, any
condition that compromises blood flow to any area of
the heart usually causes damage first in the subendocardial regions, and the damage then spreads outward
toward the epicardium.
Causes of Death After Acute
Coronary Occlusion
The most common causes of death after acute
myocardial infarction are (1) decreased cardiac output;
(2) damming of blood in the pulmonary blood vessels
and then death resulting from pulmonary edema; (3)
fibrillation of the heart; and, occasionally, (4) rupture of
the heart.
Decreased Cardiac Output—Systolic Stretch and Cardiac
Shock. When some of the cardiac muscle fibers are not
functioning and others are too weak to contract with
great force, the overall pumping ability of the affected
ventricle is proportionately depressed. Indeed, the
overall pumping strength of the infarcted heart is often
decreased more than one might expect because of a
phenomenon called systolic stretch, shown in Figure
21–7. That is, when the normal portions of the ventricular muscle contract, the ischemic portion of the
muscle, whether this be dead or simply nonfunctional,
instead of contracting is forced outward by the pressure that develops inside the ventricle. Therefore,
Normal contraction
Non-functional
muscle
Systolic stretch
Figure 21–7
Systolic stretch in an area of ischemic cardiac muscle.
254
Unit IV
The Circulation
much of the pumping force of the ventricle is dissipated by bulging of the area of nonfunctional cardiac
muscle.
When the heart becomes incapable of contracting
with sufficient force to pump enough blood into the
peripheral arterial tree, cardiac failure and death of
peripheral tissues ensue as a result of peripheral
ischemia. This condition is called coronary shock, cardiogenic shock, cardiac shock, or low cardiac output
failure. It is discussed more fully in the next chapter.
Cardiac shock almost always occurs when more than
40 per cent of the left ventricle is infarcted. And death
occurs in about 85 per cent of patients once they
develop cardiac shock.
heart is not pumping blood forward, it must be
damming blood in the atria and in the blood vessels
of the lungs or in the systemic circulation. This leads
to increased capillary pressures, particularly in the
lungs.
This damming of blood in the veins often causes
little difficulty during the first few hours after myocardial infarction. Instead, symptoms develop a few days
later for the following reason: The acutely diminished
cardiac output leads to diminished blood flow to the
kidneys. Then, for reasons that are discussed in
Chapter 22. the kidneys fail to excrete enough urine.
This adds progressively to the total blood volume and,
therefore, leads to congestive symptoms. Consequently,
many patients who seemingly are getting along well
during the first few days after onset of heart failure will
suddenly develop acute pulmonary edema and often
will die within a few hours after appearance of the
initial pulmonary symptoms.
increases the irritability of the cardiac
musculature and, therefore, its likelihood of
fibrillating.
2. Ischemia of the muscle causes an “injury current,”
which is described in Chapter 12 in relation to
electrocardiograms in patients with acute
myocardial infarction. That is, the ischemic
musculature often cannot completely repolarize
its membranes after a heart beat, so that the
external surface of this muscle remains negative
with respect to normal cardiac muscle membrane
potential elsewhere in the heart. Therefore,
electric current flows from this ischemic area of
the heart to the normal area and can elicit
abnormal impulses that can cause fibrillation.
3. Powerful sympathetic reflexes often develop after
massive infarction, principally because the heart
does not pump an adequate volume of blood into
the arterial tree. The sympathetic stimulation also
increases irritability of the cardiac muscle and
thereby predisposes to fibrillation.
4. Cardiac muscle weakness caused by the
myocardial infarction often causes the ventricle to
dilate excessively. This increases the pathway length
for impulse conduction in the heart and frequently
causes abnormal conduction pathways all the way
around the infarcted area of the cardiac muscle.
Both of these effects predispose to development
of circus movements because, as discussed in
Chapter 13, excess prolongation of conduction
pathways in the ventricles allows impulses to reenter muscle that is already recovering from
refractoriness, thereby initiating a “circus
movement” cycle of new excitation and causing
the process to continue on and on.
Fibrillation of the Ventricles After Myocardial Infarction.
Rupture of the Infarcted Area. During the first day or so
Many people who die of coronary occlusion die
because of sudden ventricular fibrillation. The tendency to develop fibrillation is especially great after a
large infarction, but fibrillation can sometimes occur
after small occlusions as well. Indeed, some patients
with chronic coronary insufficiency die suddenly from
fibrillation without any acute infarction.
There are two especially dangerous periods after
coronary infarction during which fibrillation is most
likely to occur. The first is during the first 10 minutes
after the infarction occurs. Then there is a short period
of relative safety, followed by a secondary period of
cardiac irritability beginning 1 hour or so later and
lasting for another few hours. Fibrillation can also
occur many days after the infarct but less likely so.
At least four factors enter into the tendency for the
heart to fibrillate:
1. Acute loss of blood supply to the cardiac muscle
causes rapid depletion of potassium from the
ischemic musculature. This also increases the
potassium concentration in the extracellular
fluids surrounding the cardiac muscle fibers.
Experiments in which potassium has been injected
into the coronary system have demonstrated that
an elevated extracellular potassium concentration
after an acute infarct, there is little danger of rupture
of the ischemic portion of the heart, but a few days
later, the dead muscle fibers begin to degenerate, and
the heart wall becomes stretched very thin. When this
happens, the dead muscle bulges outward severely
with each heart contraction, and this systolic stretch
becomes greater and greater until finally the heart ruptures. In fact, one of the means used in assessing
progress of severe myocardial infarction is to record
by cardiac imaging (i.e., x-rays) whether the degree of
systolic stretch is worsening.
When a ventricle does rupture, loss of blood into the
pericardial space causes rapid development of cardiac
tamponade—that is, compression of the heart from the
outside by blood collecting in the pericardial cavity.
Because of this compression of the heart, blood cannot
flow into the right atrium, and the patient dies of suddenly decreased cardiac output.
Damming of Blood in the Body’s Venous System. When the
Stages of Recovery from Acute
Myocardial Infarction
The upper left part of Figure 21–8 shows the effects of
acute coronary occlusion in a patient with a small area
Chapter 21
Mild
ischemia
Nonfunctional
Muscle Blood Flow and Cardiac Output During Exercise
Mild
ischemia
Nonfunctional
Dead fibers
Nonfunctional
Dead fibers
Fibrous tissue
Figure 21–8
Top, Small and large areas of coronary ischemia. Bottom, Stages
of recovery from myocardial infarction.
of muscle ischemia; to the right is shown a heart with
a large area of ischemia. When the area of ischemia is
small, little or no death of the muscle cells may occur,
but part of the muscle often does become temporarily
nonfunctional because of inadequate nutrition to
support muscle contraction.
When the area of ischemia is large, some of the
muscle fibers in the center of the area die rapidly,
within 1 to 3 hours where there is total cessation of
coronary blood supply. Immediately around the dead
area is a nonfunctional area, with failure of contraction and usually failure of impulse conduction. Then,
extending circumferentially around the nonfunctional
area is an area that is still contracting but weakly so
because of mild ischemia.
Replacement of Dead Muscle by Scar Tissue. In the lower
part of Figure 21–8, the various stages of recovery after
a large myocardial infarction are shown. Shortly after
the occlusion, the muscle fibers in the center of the
ischemic area die. Then, during the ensuing days, this
area of dead fibers becomes bigger because many of
the marginal fibers finally succumb to the prolonged
ischemia. At the same time, because of enlargement of
collateral arterial channels supplying the outer rim of
the infarcted area, much of the nonfunctional muscle
recovers. After a few days to three weeks, most of
the nonfunctional muscle becomes functional again or
dies—one or the other. In the meantime, fibrous tissue
begins developing among the dead fibers because
ischemia can stimulate growth of fibroblasts and
promote development of greater than normal quantities of fibrous tissue. Therefore, the dead muscle tissue
is gradually replaced by fibrous tissue. Then, because
it is a general property of fibrous tissue to undergo
progressive contraction and dissolution, the fibrous
scar may grow smaller over a period of several months
to a year.
Finally, the normal areas of the heart gradually
hypertrophy to compensate at least partially for the
lost dead cardiac musculature. By these means, the
255
heart recovers either partially or almost completely
within a few months.
Value of Rest in Treating Myocardial Infarction. The degree
of cardiac cellular death is determined by the degree
of ischemia and the workload on the heart muscle.
When the workload is greatly increased, such as during
exercise, in severe emotional strain, or as a result of
fatigue, the heart needs increased oxygen and other
nutrients for sustaining its life. Furthermore, anastomotic blood vessels that supply blood to ischemic
areas of the heart must also still supply the areas of
the heart that they normally supply. When the heart
becomes excessively active, the vessels of the normal
musculature become greatly dilated. This allows most
of the blood flowing into the coronary vessels to flow
through the normal muscle tissue, thus leaving little
blood to flow through the small anastomotic channels
into the ischemic area, so that the ischemic condition
worsens. This condition is called the “coronary steal”
syndrome. Consequently, one of the most important
factors in the treatment of a patient with myocardial
infarction is observance of absolute body rest during
the recovery process.
Function of the Heart After Recovery
from Myocardial Infarction
Occasionally, a heart that has recovered from a large
myocardial infarction returns almost to full functional
capability, but more frequently its pumping capability
is permanently decreased below that of a healthy
heart. This does not mean that the person is necessarily a cardiac invalid or that the resting cardiac output
is depressed below normal, because the normal heart
is capable of pumping 300 to 400 per cent more blood
per minute than the body requires during rest—that is,
a normal person has a “cardiac reserve” of 300 to 400
per cent. Even when the cardiac reserve is reduced to
as little as 100 per cent, the person can still perform
normal activity of a quiet, restful type but not strenuous exercise that would overload the heart.
Pain in Coronary Heart Disease
Normally, a person cannot “feel” his or her heart, but
ischemic cardiac muscle often does cause pain sensation—sometimes severe pain. Exactly what causes this
pain is not known, but it is believed that ischemia
causes the muscle to release acidic substances, such as
lactic acid, or other pain-promoting products, such as
histamine, kinins, or cellular proteolytic enzymes, that
are not removed rapidly enough by the slowly moving
coronary blood flow. The high concentrations of these
abnormal products then stimulate pain nerve endings
in the cardiac muscle, sending pain impulses through
sensory afferent nerve fibers into the central nervous
system.
Angina Pectoris
In most people who develop progressive constriction
of their coronary arteries, cardiac pain, called angina
256
Unit IV
The Circulation
pectoris, begins to appear whenever the load on the
heart becomes too great in relation to the available
coronary blood flow. This pain is usually felt beneath
the upper sternum over the heart, and in addition it is
often referred to distant surface areas of the body,
most commonly to the left arm and left shoulder but
also frequently to the neck and even to the side of the
face. The reason for this distribution of pain is that the
heart originates during embryonic life in the neck, as
do the arms. Therefore, both the heart and these
surface areas of the body receive pain nerve fibers
from the same spinal cord segments.
Most people who have chronic angina pectoris feel
pain when they exercise or when they experience
emotions that increase metabolism of the heart or
temporarily constrict the coronary vessels because of
sympathetic vasoconstrictor nerve signals. The pain
usually lasts for only a few minutes. However, some
patients have such severe and lasting ischemia that the
pain is present all the time. The pain is frequently
described as hot, pressing, and constricting; it is of such
quality that it usually makes the patient stop all unnecessary body activity and come to a complete state of
rest.
Treatment with Drugs. Several vasodilator drugs, when
administered during an acute anginal attack, can
often give immediate relief from the pain. Commonly
used vasodilators are nitroglycerin and other nitrate
drugs.
A second class of drugs that are used for prolonged
treatment of angina pectoris is the beta blockers,
such as propranolol. These drugs block sympathetic
beta adrenergic receptors, which prevents sympathetic
enhancement of heart rate and cardiac metabolism
during exercise or emotional episodes. Therefore,
therapy with a beta blocker decreases the need
of the heart for extra metabolic oxygen during
stressful conditions. For obvious reasons, this can also
reduce the number of anginal attacks as well as their
severity.
Surgical Treatment of
Coronary Disease
Aortic-Coronary Bypass Surgery. In many patients with
coronary ischemia, the constricted areas of the coronary arteries are located at only a few discrete points
blocked by atherosclerotic disease, and the coronary
vessels elsewhere are normal or almost normal. A surgical procedure was developed in the 1960s, called
aortic-coronary bypass, for removing a section of a
subcutaneous vein from an arm or leg and then grafting this vein from the root of the aorta to the side of
a peripheral coronary artery beyond the atherosclerotic blockage point. One to five such grafts are usually
performed, each of which supplies a peripheral coronary artery beyond a block.
Anginal pain is relieved in most patients. Also, in
patients whose hearts have not become too severely
damaged before the operation, the coronary bypass
procedure may provide the patient with normal survival expectation. Conversely, if the heart has already
been severely damaged, the bypass procedure is likely
to be of little value.
Coronary Angioplasty. Since the 1980s, a procedure has
been used to open partially blocked coronary vessels
before they become totally occluded. This procedure,
called coronary artery angioplasty, is the following:
A small balloon-tipped catheter, about 1 millimeter in
diameter, is passed under radiographic guidance into
the coronary system and pushed through the partially
occluded artery until the balloon portion of the
catheter straddles the partially occluded point. Then
the balloon is inflated with high pressure, which
markedly stretches the diseased artery. After this procedure is performed, the blood flow through the vessel
often increases threefold to fourfold, and more than
three quarters of the patients who undergo the procedure are relieved of the coronary ischemic symptoms
for at least several years, although many of the patients
still eventually require coronary bypass surgery.
Still newer procedures for opening atherosclerotic
coronary arteries are constantly in experimental
development. One of these employs a laser beam from
the tip of a coronary artery catheter aimed at the
atherosclerotic lesion. The laser literally dissolves the
lesion without substantially damaging the rest of
the arterial wall.
Another development has been a minute metal
“sleeve” placed inside a coronary artery dilated by
angioplasty to hold the artery open, thus preventing its
restenosis.
References
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2003.
Cohn PF, Fox KM, Daly C: Silent myocardial ischemia.
Circulation 108:1263, 2003.
Dalal H, Evans PH, Campbell JL: Recent developments in
secondary prevention and cardiac rehabilitation after
acute myocardial infarction. BMJ 328:693, 2004.
Freedman SB, Isner JM: Therapeutic angiogenesis for coronary artery disease. Ann Intern Med 136:54, 2002.
Gehlbach BK, Geppert E: The pulmonary manifestations of
left heart failure. Chest 125:669, 2004.
Guyton AC, Jones CE, Coleman TG: Circulatory Pathology:
Cardiac Output and Its Regulation. Philadelphia: WB
Saunders Co, 1973.
Hao H, Gabbiani J, Bochaton-Piallat M: Arterial smooth
muscle cell heterogeneity: implications for atherosclerosis
and restenosis development. Arterioscler Thromb Vasc
Biol 23:1510, 2003.
Hester RL, Hammer LW: Venular-arteriolar communication
in the regulation of blood flow. Am J Physiol 282:R1280,
2002.
Hochman JS: Cardiogenic shock complicating acute myocardial infarction: expanding the paradigm. Circulation 107:
2998, 2003.
Joyner MJ, Dietz NM: Sympathetic vasodilation in human
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Chapter 21
Muscle Blood Flow and Cardiac Output During Exercise
Koerselman J, van der Graaf Y, de Jaegere PP, Grobbee DE:
Coronary collaterals: an important and underexposed
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2003.
Libby P: Inflammation in atherosclerosis. Nature 420:868,
2002.
Richardson RS: Oxygen transport and utilization: an integration of the muscle systems. Adv Physiol Educ 27:183,
2003.
257
Ridker PM: Clinical application of C-reactive protein for
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Tsai AG, Johnson PC, Intaglietta M: Oxygen gradients in the
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Yellon DM, Downey JM: Preconditioning the myocardium:
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83:1113, 2003.
C
H
A
P
T
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2
Cardiac Failure
One of the most important ailments that must be
treated by the physician is cardiac failure (“heart
failure”). This can result from any heart condition
that reduces the ability of the heart to pump blood.
The cause usually is decreased contractility of the
myocardium resulting from diminished coronary
blood flow. However, failure can also be caused by
damaged heart valves, external pressure around the
heart, vitamin B deficiency, primary cardiac muscle disease, or any other abnormality that makes the heart a hypoeffective pump. In this chapter, we discuss
mainly cardiac failure caused by ischemic heart disease resulting from partial
blockage of the coronary blood vessels. In Chapter 23, we discuss valvular and
congenital heart disease.
Definition of Cardiac Failure. The term “cardiac failure” means simply failure of
the heart to pump enough blood to satisfy the needs of the body.
Dynamics of the Circulation in Cardiac Failure
Acute Effects of Moderate Cardiac Failure
If a heart suddenly becomes severely damaged, such as by myocardial infarction, the pumping ability of the heart is immediately depressed. As a result, two
main effects occur: (1) reduced cardiac output and (2) damming of blood in the
veins, resulting in increased venous pressure.
The progressive changes in heart pumping effectiveness at different times
after an acute myocardial infarction are shown graphically in Figure 22–1. The
top curve of this figure shows a normal cardiac output curve. Point A on this
curve is the normal operating point, showing a normal cardiac output under
resting conditions of 5 L/min and a right atrial pressure of 0 mm Hg.
Immediately after the heart becomes damaged, the cardiac output curve
becomes greatly lowered, falling to the lowest curve at the bottom of the graph.
Within a few seconds, a new circulatory state is established at point B rather
than point A, illustrating that the cardiac output has fallen to 2 L/min, about
two-fifths normal, whereas the right atrial pressure has risen to +4 mm Hg
because venous blood returning to the heart from the body is dammed up in
the right atrium. This low cardiac output is still sufficient to sustain life for
perhaps a few hours, but it is likely to be associated with fainting. Fortunately,
this acute stage usually lasts for only a few seconds because sympathetic nerve
reflexes occur immediately and compensate, to a great extent, for the damaged
heart, as follows.
Compensation for Acute Cardiac Failure by Sympathetic Nervous Reflexes. When the
cardiac output falls precariously low, many of the circulatory reflexes discussed
in Chapter 18 are immediately activated. The best known of these is the baroreceptor reflex, which is activated by diminished arterial pressure. It is probable
that the chemoreceptor reflex, the central nervous system ischemic response,
and even reflexes that originate in the damaged heart also contribute to activating the sympathetic nervous system. But whatever the reflexes might be, the
sympathetics become strongly stimulated within a few seconds, and the
258
2
Chapter 22
Normal heart
Partially recovered heart
Damaged heart + sympathetic stimulation
Cardiac output (L/min)
15
Acutely damaged heart
10
A
5
D
B
0
–2
0
+2
+4
+6
259
The sympathetic reflexes become maximally developed in about 30 seconds. Therefore, a person who has
a sudden, moderate heart attack might experience
nothing more than cardiac pain and a few seconds of
fainting. Shortly thereafter, with the aid of the sympathetic reflex compensations, the cardiac output may
return to a level adequate to sustain the person if he
or she remains quiet, although the pain might persist.
Chronic Stage of Failure—Fluid
Retention Helps to Compensate
Cardiac Output
C
–4
Cardiac Failure
+8 +10 +12 +14
Right atrial pressure (mm Hg)
Figure 22–1
Progressive changes in the cardiac output curve after acute
myocardial infarction. Both the cardiac output and right atrial pressure change progressively from point A to point D (illustrated by
the black line) over a period of seconds, minutes, days, and
weeks.
parasympathetic nervous signals to the heart become
reciprocally inhibited at the same time.
Strong sympathetic stimulation has two major
effects on the circulation: first on the heart itself,
and second on the peripheral vasculature. If all the
ventricular musculature is diffusely damaged but is
still functional, sympathetic stimulation strengthens
this damaged musculature. If part of the muscle is nonfunctional and part of it is still normal, the normal
muscle is strongly stimulated by sympathetic stimulation, in this way partially compensating for the nonfunctional muscle. Thus, the heart, one way or another,
becomes a stronger pump. This effect is also demonstrated in Figure 22–1, showing after sympathetic compensation about twofold elevation of the very low
cardiac output curve.
Sympathetic stimulation also increases venous
return because it increases the tone of most of the
blood vessels of the circulation, especially the veins,
raising the mean systemic filling pressure to 12 to 14
mm Hg, almost 100 per cent above normal. As discussed in Chapter 20, this increased filling pressure
greatly increases the tendency for blood to flow from
the veins back into the heart. Therefore, the damaged
heart becomes primed with more inflowing blood than
usual, and the right atrial pressure rises still further,
which helps the heart to pump still larger quantities of
blood. Thus, in Figure 22–1, the new circulatory state
is depicted by point C, showing a cardiac output of
4.2 L/min and a right atrial pressure of 5 mm Hg.
After the first few minutes of an acute heart attack, a
prolonged semi-chronic state begins, characterized
mainly by two events: (1) retention of fluid by the
kidneys and (2) varying degrees of recovery of the
heart itself over a period of weeks to months, as illustrated by the light green curve in Figure 22–1; this was
also discussed in Chapter 21.
Renal Retention of Fluid and Increase in Blood
Volume Occur for Hours to Days
A low cardiac output has a profound effect on renal
function, sometimes causing anuria when the cardiac
output falls to one-half to two-thirds normal. In
general, the urine output remains reduced below
normal as long as the cardiac output and arterial pressure remain significantly less than normal, and urine
output usually does not return all the way to normal
after an acute heart attack until the cardiac output and
arterial pressure rise either all the way back to normal
or almost to normal.
Moderate Fluid Retention in Cardiac Failure Can Be Beneficial.
Many cardiologists formerly considered fluid retention
always to have a detrimental effect in cardiac failure.
But it is now known that a moderate increase in body
fluid and blood volume is an important factor in
helping to compensate for the diminished pumping
ability of the heart by increasing the venous return.
The increased blood volume increases venous return
in two ways: First, it increases the mean systemic filling
pressure, which increases the pressure gradient for
causing venous flow of blood toward the heart. Second,
it distends the veins, which reduces the venous resistance and allows even more ease of flow of blood to the
heart.
If the heart is not too greatly damaged, this
increased venous return can often fully compensate
for the heart’s diminished pumping ability—enough in
fact that even when the heart’s pumping ability is
reduced to as low as 40 to 50 per cent of normal, the
increased venous return can often cause an entirely
normal cardiac output as long as the person remains
in a quiet resting state.
When the heart’s pumping capability is reduced
still more, blood flow to the kidneys finally becomes
too low for the kidneys to excrete enough salt and
water to equal salt and water intake. Therefore, fluid
260
Unit IV
The Circulation
retention begins and continues indefinitely, unless
major therapeutic procedures are used to prevent this.
Furthermore, because the heart is already pumping at
its maximum pumping capacity, this excess fluid no
longer has a beneficial effect on the circulation. Instead,
severe edema develops throughout the body, which
can be very detrimental in itself and can lead to death.
thetic stimulation gradually abates toward normal for
the following reasons: The partial recovery of the
heart can elevate the cardiac output curve the same as
sympathetic stimulation can. Therefore, as the heart
recovers even slightly, the fast pulse rate, cold skin,
and pallor resulting from sympathetic stimulation in
the acute stage of cardiac failure gradually disappear.
Detrimental Effects of Excess Fluid Retention in Severe Cardiac
Failure. In contrast to the beneficial effects of moder-
Summary of the Changes That Occur
After Acute Cardiac Failure—
“Compensated Heart Failure”
ate fluid retention in cardiac failure, in severe failure
extreme excesses of fluid can have serious physiological consequences. They include (1) overstretching of
the heart, thus weakening the heart still more; (2)
filtration of fluid into the lungs, causing pulmonary
edema and consequent deoxygenation of the blood;
and (3) development of extensive edema in most parts
of the body.These detrimental effects of excessive fluid
are discussed in later sections of this chapter.
Recovery of the Myocardium After
Myocardial Infarction
After a heart becomes suddenly damaged as a result
of myocardial infarction, the natural reparative
processes of the body begin immediately to help
restore normal cardiac function. For instance, a new
collateral blood supply begins to penetrate the peripheral portions of the infarcted area of the heart, often
causing much of the heart muscle in the fringe areas
to become functional again. Also, the undamaged
portion of the heart musculature hypertrophies, in this
way offsetting much of the cardiac damage.
The degree of recovery depends on the type of
cardiac damage, and it varies from no recovery to
almost complete recovery. After acute myocardial
infarction, the heart ordinarily recovers rapidly during
the first few days and weeks and achieves most of its
final state of recovery within 5 to 7 weeks, although
mild degrees of additional recovery can continue for
months.
The Cardiac Output Curve After Partial Recovery. Figure 22–1
shows function of the partially recovered heart a week
or so after acute myocardial infarction. By this time,
considerable fluid has been retained in the body and
the tendency for venous return has increased
markedly as well; therefore, the right atrial pressure
has risen even more. As a result, the state of the circulation is now changed from point C to point D, which
shows a normal cardiac output of 5 L/min but right
atrial pressure increased to 6 mm Hg.
Because the cardiac output has returned to normal,
renal output of fluid also returns to normal, and no
further fluid retention occurs, except that the retention
of fluid that has already occurred continues to maintain
moderate excesses of fluid. Therefore, except for the
high right atrial pressure represented by point D in
this figure, the person now has essentially normal cardiovascular dynamics as long as he or she remains at
rest.
If the heart recovers to a significant extent and if
adequate fluid volume has been retained, the sympa-
To summarize the events discussed in the past few sections describing the dynamics of circulatory changes
after an acute, moderate heart attack, we can divide
the stages into (1) the instantaneous effect of the
cardiac damage; (2) compensation by the sympathetic
nervous system, which occurs mainly within the first 30
seconds to 1 minute; and (3) chronic compensations
resulting from partial heart recovery and renal retention of fluid. All these changes are shown graphically
by the black curve in Figure 22–1. The progression of
this curve shows the normal state of the circulation
(point A), the state a few seconds after the heart attack
but before sympathetic reflexes have occurred (point
B), the rise in cardiac output toward normal caused
by sympathetic stimulation (point C), and final return
of the cardiac output almost exactly to normal after
several days to several weeks of partial cardiac recovery and fluid retention (point D). This final state is
called compensated heart failure.
Compensated Heart Failure. Note especially in Figure
22–1 that the maximum pumping ability of the partly
recovered heart, as depicted by the plateau level of the
light green curve, is still depressed to less than one-half
normal. This demonstrates that an increase in right
atrial pressure can maintain the cardiac output at a
normal level despite continued weakness of the heart.
Thus many people, especially older people, have
normal resting cardiac outputs but mildly to moderately elevated right atrial pressures because of various
degrees of “compensated heart failure.” These persons
may not know that they have cardiac damage because
the damage often has occurred a little at a time, and
the compensation has occurred concurrently with the
progressive stages of damage.
When a person is in compensated heart failure, any
attempt to perform heavy exercise usually causes
immediate return of the symptoms of acute failure
because the heart is not able to increase its pumping
capacity to the levels required for the exercise. Therefore, it is said that the cardiac reserve is reduced in
compensated heart failure. This concept of cardiac
reserve is discussed more fully later in the chapter.
Dynamics of Severe Cardiac Failure—
Decompensated Heart Failure
If the heart becomes severely damaged, no amount of
compensation, either by sympathetic nervous reflexes
Chapter 22
or by fluid retention, can make the excessively weakened heart pump a normal cardiac output. As a consequence, the cardiac output cannot rise high enough
to make the kidneys excrete normal quantities of fluid.
Therefore, fluid continues to be retained, the person
develops more and more edema, and this state of
events eventually leads to death. This is called decompensated heart failure. Thus, the main cause of decompensated heart failure is failure of the heart to pump
sufficient blood to make the kidneys excrete daily the
necessary amounts of fluid.
Graphical Analysis of Decompensated Heart Failure. Figure
22–2 shows greatly depressed cardiac output at different times (points A to F) after the heart has become
severely weakened. Point A on this curve represents
the approximate state of the circulation before any
compensation has occurred, and point B, the state a
few minutes later after sympathetic stimulation has
compensated as much as it can but before fluid retention has begun. At this time, the cardiac output has
risen to 4 L/min and the right atrial pressure has risen
to 5 mm Hg. The person appears to be in reasonably
good condition, but this state will not remain stable
because the cardiac output has not risen high enough
to cause adequate kidney excretion of fluid; therefore,
fluid retention continues and can eventually be the
cause of death. These events can be explained quantitatively in the following way.
Note the straight line in Figure 22–2, at a cardiac
output level of 5 L/min. This is approximately the critical cardiac output level that is required in the normal
adult person to make the kidneys re-establish normal
fluid balance—that is, for the output of salt and water
to be as great as the intake of these. At any cardiac
output below this level, all the fluid-retaining mechanisms discussed in the earlier section remain in play
and the body fluid volume increases progressively.
And because of this progressive increase in fluid
volume, the mean systemic filling pressure of the
Cardiac output
(L/min)
Critical cardiac output level
for normal fluid balance
5.0
B
2.5
C
D
E
A
F
0
–4
0
+4
+8
+12
+16
Right atrial pressure (mm Hg)
Figure 22–2
Greatly depressed cardiac output that indicates decompensated
heart disease. Progressive fluid retention raises the right atrial
pressure over a period of days, and the cardiac output progresses
from point A to point F, until death occurs.
Cardiac Failure
261
circulation continues to rise; this forces progressively
increasing quantities of blood from the person’s
peripheral veins into the right atrium, thus increasing
the right atrial pressure. After 1 day or so, the state of
the circulation changes in Figure 22–2 from point B to
point C—the right atrial pressure rising to 7 mm Hg
and the cardiac output to 4.2 L/min. Note again that
the cardiac output is still not high enough to cause
normal renal output of fluid; therefore, fluid continues
to be retained. After another day or so, the right atrial
pressure rises to 9 mm Hg, and the circulatory state
becomes that depicted by point D. Still, the cardiac
output is not enough to establish normal fluid
balance.
After another few days of fluid retention, the right
atrial pressure has risen still further, but by now,
cardiac function is beginning to decline toward a lower
level.This decline is caused by overstretch of the heart,
edema of the heart muscle, and other factors that
diminish the heart’s pumping performance. It is
now clear that further retention of fluid will be more
detrimental than beneficial to the circulation. Yet the
cardiac output still is not high enough to bring about
normal renal function, so that fluid retention not
only continues but accelerates because of the falling
cardiac output (and falling arterial pressure that also
occurs). Consequently, within a few days, the state
of the circulation has reached point F on the curve,
with the cardiac output now less than 2.5 L/min and
the right atrial pressure 16 mm Hg. This state has
approached or reached incompatibility with life, and
the patient dies. This state of heart failure in which the
failure continues to worsen is called decompensated
heart failure.
Thus, one can see from this analysis that failure of
the cardiac output (and arterial pressure) to rise to the
critical level required for normal renal function results
in (1) progressive retention of more and more fluid,
which causes (2) progressive elevation of the mean
systemic filling pressure, and (3) progressive elevation
of the right atrial pressure until finally the heart is so
overstretched or so edematous that it cannot pump
even moderate quantities of blood and, therefore, fails
completely. Clinically, one detects this serious condition of decompensation principally by the progressing
edema, especially edema of the lungs, which leads
to bubbling rales in the lungs and to dyspnea (air
hunger). All clinicians know that lack of appropriate
therapy when this state of events occurs leads to rapid
death.
of Decompensation. The decompensation
process can often be stopped by (1) strengthening the
heart in any one of several ways, especially by administration of a cardiotonic drug, such as digitalis, so that
the heart becomes strong enough to pump adequate
quantities of blood required to make the kidneys function normally again, or (2) administering diuretic drugs
to increase kidney excretion while at the same time
reducing water and salt intake, which brings about a
balance between fluid intake and output despite low
cardiac output.
Treatment
262
Unit IV
The Circulation
Both methods stop the decompensation process by
re-establishing normal fluid balance, so that at least as
much fluid leaves the body as enters it.
it can cause death by suffocation in 20 to 30 minutes,
which we discuss more fully later in the chapter.
Mechanism of Action of the Cardiotonic Drugs Such as
Digitalis. Cardiotonic drugs, such as digitalis, when
Low-Output Cardiac Failure—
Cardiogenic Shock
administered to a person with a healthy heart, have
little effect on increasing the contractile strength of
the cardiac muscle. However, when administered to a
person with a chronically failing heart, the same drugs
can sometimes increase the strength of the failing
myocardium as much as 50 to 100 per cent. Therefore,
they are one of the mainstays of therapy in chronic
heart failure.
Digitalis and other cardiotonic glycosides are
believed to strengthen heart contraction by increasing
the quantity of calcium ions in muscle fibers. In the
failing heart muscle, the sarcoplasmic reticulum fails
to accumulate normal quantities of calcium and,
therefore, cannot release enough calcium ions into
the free-fluid compartment of the muscle fibers to
cause full contraction of the muscle. One effect of digitalis is to depress the calcium pump of the cell membrane of the cardiac muscle fibers. This pump normally
pumps calcium ions out of the muscle. However, in the
case of a failing heart, extra calcium is needed to
increase the muscle contractile force. Therefore, it is
usually beneficial to depress the calcium pumping
mechanism a moderate amount using digitalis, allowing the muscle fiber intracellular calcium level to rise
slightly.
Unilateral Left Heart Failure
In the discussions thus far in this chapter, we have considered failure of the heart as a whole. Yet, in a large
number of patients, especially those with early acute
failure, left-sided failure predominates over rightsided failure, and in rare instances, the right side fails
without significant failure of the left side. Therefore,
we need especially to discuss the special features of
unilateral heart failure.
When the left side of the heart fails without concomitant failure of the right side, blood continues to
be pumped into the lungs with usual right heart vigor,
whereas it is not pumped adequately out of the lungs
by the left heart into the systemic circulation. As
a result, the mean pulmonary filling pressure rises
because of shift of large volumes of blood from the systemic circulation into the pulmonary circulation.
As the volume of blood in the lungs increases, the
pulmonary capillary pressure increases, and if this rises
above a value approximately equal to the colloid
osmotic pressure of the plasma, about 28 mm Hg, fluid
begins to filter out of the capillaries into the lung interstitial spaces and alveoli, resulting in pulmonary
edema.
Thus, among the most important problems of left
heart failure are pulmonary vascular congestion and
pulmonary edema. In severe, acute left heart failure,
pulmonary edema occasionally occurs so rapidly that
In many instances after acute heart attacks and
often after prolonged periods of slow progressive
cardiac deterioration, the heart becomes incapable of
pumping even the minimal amount of blood flow
required to keep the body alive. Consequently, all the
body tissues begin to suffer and even to deteriorate,
often leading to death within a few hours to a few days.
The picture then is one of circulatory shock, as
explained in Chapter 24. Even the cardiovascular
system suffers from lack of nutrition, and it, too (along
with the remainder of the body), deteriorates, thus hastening death. This circulatory shock syndrome caused
by inadequate cardiac pumping is called cardiogenic
shock or simply cardiac shock. Once a person develops cardiogenic shock, the survival rate is often less
than 15 per cent.
Vicious Circle of Cardiac Deterioration in Cardiogenic Shock.
The discussion of circulatory shock in Chapter 24
emphasizes the tendency for the heart to become progressively more damaged when its coronary blood
supply is reduced during the course of the shock. That
is, the low arterial pressure that occurs during shock
reduces the coronary blood supply even more. This
makes the heart still weaker, which makes the arterial
pressure fall still more, which makes the shock still
worse, the process eventually becoming a vicious circle
of cardiac deterioration. In cardiogenic shock caused
by myocardial infarction, this problem is greatly compounded by already existing coronary vessel blockage.
For instance, in a healthy heart, the arterial pressure usually must be reduced below about 45 mm Hg
before cardiac deterioration sets in. However, in a
heart that already has a blocked major coronary
vessel, deterioration sets in when the coronary arterial
pressure falls below 80 to 90 mm Hg. In other words,
even a small decrease in arterial pressure can now
set off a vicious circle of cardiac deterioration. For
this reason, in treating myocardial infarction, it is
extremely important to prevent even short periods of
hypotension.
Physiology of Treatment. Often a patient dies of cardiogenic shock before the various compensatory
processes can return the cardiac output (and arterial
pressure) to a life-sustaining level. Therefore, treatment of this condition is one of the most important
problems in the management of acute heart attacks.
Immediate administration of digitalis is often used
for strengthening the heart if the ventricular muscle
shows signs of deterioration. Also, infusion of whole
blood, plasma, or a blood pressure–raising drug is used
to sustain the arterial pressure. If the arterial pressure
can be elevated high enough, the coronary blood flow
Chapter 22
often will increase enough to prevent the vicious circle
of deterioration. And this allows enough time for
appropriate compensatory mechanisms in circulatory
system to correct the shock.
Some success has also been achieved in saving the
lives of patients in cardiogenic shock by using one of
the following procedures: (1) surgically removing the
clot in the coronary artery, often in combination with
coronary bypass graft, or (2) catheterizing the blocked
coronary artery and infusing either streptokinase or
tissue-type plasminogen activator enzymes that cause
dissolution of the clot. The results occasionally are
astounding when one of these procedures is instituted
within the first hour of cardiogenic shock but of little,
if any, benefit after 3 hours.
Edema in Patients with
Cardiac Failure
Inability of Acute Cardiac Failure to Cause Peripheral Edema.
Acute left heart failure can cause terrific and rapid
congestion of the lungs, with development of pulmonary edema and even death within minutes to
hours.
However, either left or right heart failure is very
slow to cause peripheral edema. This can best be
explained by referring to Figure 22–3. When a previously healthy heart acutely fails as a pump, the aortic
pressure falls and the right atrial pressure rises. As the
cardiac output approaches zero, these two pressures
approach each other at an equilibrium value of about
13 mm Hg. Capillary pressure also falls from its normal
value of 17 mm Hg to the new equilibrium pressure
Mean aortic pressure
Capillary pressure
Pressure (mm Hg)
100
Right atrial pressure
80
60
40
13 mm Hg
20
0
Normal
1/2 Normal
Zero
Cardiac output
Figure 22–3
Progressive changes in mean aortic pressure, peripheral tissue
capillary pressure, and right atrial pressure as the cardiac output
falls from normal to zero.
Cardiac Failure
263
of 13 mm Hg. Thus, severe acute cardiac failure often
causes a fall in peripheral capillary pressure rather than
a rise. Therefore, animal experiments, as well as experience in humans, show that acute cardiac failure
almost never causes immediate development of peripheral edema.
Long-Term Fluid Retention by the Kidneys—
The Cause of Peripheral Edema in Persisting
Heart Failure
After the first day or so of overall heart failure or of
right-ventricular heart failure, peripheral edema does
begin to occur principally because of fluid retention by
the kidneys. The retention of fluid increases the mean
systemic filling pressure, resulting in increased tendency for blood to return to the heart. This elevates
the right atrial pressure to a still higher value and
returns the arterial pressure back toward normal.
Therefore, the capillary pressure now also rises
markedly, thus causing loss of fluid into the tissues and
development of severe edema.
There are three known causes of the reduced renal
output of urine during cardiac failure, all of which are
equally important but in different ways.
1. Decreased glomerular filtration. A decrease in
cardiac output has a tendency to reduce the
glomerular pressure in the kidneys because
of (1) reduced arterial pressure and (2) intense
sympathetic constriction of the afferent arterioles
of the kidney. As a consequence, except in the
mildest degrees of heart failure, the glomerular
filtration rate becomes less than normal. It is
clear from the discussion of kidney function in
Chapters 26 through 29 that even a slight decrease
in glomerular filtration often markedly decreases
urine output. When the cardiac output falls to
about one-half normal, this can result in almost
complete anuria.
2. Activation of the renin-angiotensin system and
increased reabsorption of water and salt by the
renal tubules. The reduced blood flow to the
kidneys causes marked increase in renin secretion
by the kidneys, and this in turn causes the
formation of angiotensin, as described in Chapter
19. The angiotensin in turn has a direct effect on
the arterioles of the kidneys to decrease further
the blood flow through the kidneys, which
especially reduces the pressure in the capillaries
surrounding the renal tubules, promoting greatly
increased reabsorption of both water and salt
from the tubules. Therefore, loss of water and salt
into the urine decreases greatly, and large
quantities of salt and water accumulate in the
blood and interstitial fluids everywhere in the
body.
3. Increased aldosterone secretion. In the chronic
stage of heart failure, large quantities of
aldosterone are secreted by the adrenal cortex.
This results mainly from the effect of angiotensin
to stimulate aldosterone secretion by the adrenal
cortex. But some of the increase in aldosterone
secretion often results from increased plasma
264
Unit IV
The Circulation
potassium. Excess potassium is one of the most
powerful stimuli known for aldosterone secretion,
and the potassium concentration rises in response
to reduced renal function in cardiac failure.
The elevated aldosterone level further increases
the reabsorption of sodium from the renal
tubules. This in turn leads to a secondary increase
in water reabsorption for two reasons: First, as the
sodium is reabsorbed, it reduces the osmotic
pressure in the tubules but increases the osmotic
pressure in the renal interstitial fluids; these
changes promote osmosis of water into the blood.
Second, the absorbed sodium and anions that go
with the sodium, mainly chloride ions, increase
the osmotic concentration of the extracellular
fluid everywhere in the body. This elicits
antidiuretic hormone secretion by the
hypothalamic–posterior pituitary gland system
(discussed in Chapter 29). The antidiuretic
hormone in turn promotes still greater increase in
tubular reabsorption of water.
Role of Atrial Natriuretic Factor to Delay Onset of Cardiac
Decompensation. Atrial natriuretic factor (ANF) is a
hormone released by the atrial walls of the heart when
they become stretched. Because heart failure almost
always causes excessive increase in both the right and
left atrial pressures that stretch the atrial walls, the circulating levels of ANF in the blood increase fivefold
to tenfold in severe heart failure. The ANF in turn has
a direct effect on the kidneys to increase greatly their
excretion of salt and water. Therefore, ANF plays a
natural role to help prevent extreme congestive symptoms during cardiac failure. The renal effects of ANF
are discussed in Chapter 29.
Acute Pulmonary Edema in Late-Stage Heart
Failure—Another Lethal Vicious Circle
A frequent cause of death in heart failure is acute pulmonary edema occurring in patients who have already
had chronic heart failure for a long time. When this
occurs in a person without new cardiac damage, it
usually is set off by some temporary overload of the
heart, such as might result from a bout of heavy exercise, some emotional experience, or even a severe cold.
The acute pulmonary edema is believed to result from
the following vicious circle:
1. A temporarily increased load on the already weak
left ventricle initiates the vicious circle. Because of
limited pumping capacity of the left heart, blood
begins to dam up in the lungs.
2. The increased blood in the lungs elevates the
pulmonary capillary pressure, and a small amount
of fluid begins to transude into the lung tissues
and alveoli.
3. The increased fluid in the lungs diminishes the
degree of oxygenation of the blood.
4. The decreased oxygen in the blood further
weakens the heart and also weakens the arterioles
everywhere in the body, thus causing peripheral
vasodilation.
5. The peripheral vasodilation increases venous
return of blood from the peripheral circulation
still more.
6. The increased venous return further increases the
damming of the blood in the lungs, leading to still
more transudation of fluid, more arterial oxygen
desaturation, more venous return, and so forth.
Thus, a vicious circle has been established.
Once this vicious circle has proceeded beyond a
certain critical point, it will continue until death of the
patient unless heroic therapeutic measures are used
within minutes. The types of heroic therapeutic measures that can reverse the process and save the patient’s
life include the following:
1. Putting tourniquets on both arms and legs to
sequester much of the blood in the veins and,
therefore, decrease the workload on the left side
of the heart
2. Bleeding the patient
3. Giving a rapidly acting diuretic, such as
furosemide, to cause rapid loss of fluid from the
body
4. Giving the patient pure oxygen to breathe to
reverse the blood oxygen desaturation, the heart
deterioration, and the peripheral vasodilation
5. Giving the patient a rapidly acting cardiotonic
drug, such as digitalis, to strengthen the heart
This vicious circle of acute pulmonary edema can
proceed so rapidly that death can occur in 20 minutes
to 1 hour. Therefore, any procedure that is to be successful must be instituted immediately.
Cardiac Reserve
The maximum percentage that the cardiac output can
increase above normal is called the cardiac reserve.
Thus, in the healthy young adult, the cardiac reserve is
300 to 400 per cent. In athletically trained persons, it
is occasionally 500 to 600 per cent. But in heart failure,
there is no cardiac reserve. As an example of normal
reserve, during severe exercise the cardiac output of a
healthy young adult can rise to about five times
normal; this is an increase above normal of 400 per
cent—that is, a cardiac reserve of 400 per cent.
Any factor that prevents the heart from pumping
blood satisfactorily will decrease the cardiac reserve.
This can result from ischemic heart disease, primary
myocardial disease, vitamin deficiency that affects
cardiac muscle, physical damage to the myocardium,
valvular heart disease, and many other factors, some of
which are shown in Figure 22–4.
Diagnosis of Low Cardiac Reserve—Exercise Test. As long as
persons with low cardiac reserve remain in a state of
rest, they usually will not know that they have heart
disease. However, a diagnosis of low cardiac reserve
usually can be easily made by requiring the person to
exercise either on a treadmill or by walking up and
down steps, either of which requires greatly increased
cardiac output. The increased load on the heart rapidly
uses up the small amount of reserve that is available,
Cardiac reserve (%)
600
Athlete
500
Normal
400
300
200
100
0
Normal
operation
Mild
valvular
disease
Moderate
coronary
disease
Diphtheria
Severe
coronary
thrombosis
Severe
valvular
disease
265
Cardiac Failure
Cardiac output and
venous return (L/min)
Chapter 22
15
Normal
10
A
5
C
D
B
0
–4
–2
0
2
4
6
8 10 12
Right atrial pressure (mm Hg)
14
Figure 22–5
Progressive changes in cardiac output and right atrial pressure
during different stages of cardiac failure.
Figure 22–4
Cardiac reserve in different conditions, showing less than zero
reserve for two of the conditions.
and the cardiac output soon fails to rise high enough
to sustain the body’s new level of activity. The acute
effects are as follows:
1. Immediate and sometimes extreme shortness of
breath (dyspnea) resulting from failure of the
heart to pump sufficient blood to the tissues,
thereby causing tissue ischemia and creating a
sensation of air hunger
2. Extreme muscle fatigue resulting from muscle
ischemia, thus limiting the person’s ability to
continue with the exercise
3. Excessive increase in heart rate because the
nervous reflexes to the heart overreact in an
attempt to overcome the inadequate cardiac
output
Exercise tests are part of the armamentarium of the
cardiologist. These tests take the place of cardiac
output measurements that cannot be made with ease
in most clinical settings.
Quantitative Graphical Method for
Analysis of Cardiac Failure
circulation. The two curves passing through Point A
are (1) the normal cardiac output curve and (2) the
normal venous return curve. As pointed out in Chapter
20, there is only one point on each of these two curves
at which the circulatory system can operate. This point
is where the two curves cross at point A. Therefore, the
normal state of the circulation is a cardiac output and
venous return of 5 L/min and a right atrial pressure of
0 mm Hg.
Effect of Acute Heart Attack. During the first few seconds
after a moderately severe heart attack, the cardiac
output curve falls to the lowermost curve. During these
few seconds, the venous return curve still has not
changed because the peripheral circulatory system is
still operating normally. Therefore, the new state of the
circulation is depicted by point B, where the new
cardiac output curve crosses the normal venous return
curve. Thus, the right atrial pressure rises immediately to 4 mm Hg, whereas the cardiac output falls to
2 L/min.
Effect of Sympathetic Reflexes. Within the next 30
Although it is possible to understand most general
principles of cardiac failure using mainly qualitative
logic, as we have done thus far in this chapter, one can
grasp the importance of the different factors in cardiac
failure with far greater depth by using more quantitative approaches. One such approach is the graphical
method for analysis of cardiac output regulation introduced in Chapter 20. In the remaining sections of this
chapter, we analyze several aspects of cardiac failure,
using this graphical technique.
Graphical Analysis of Acute Heart Failure and
Chronic Compensation
Figure 22–5 shows cardiac output and venous return
curves for different states of the heart and peripheral
seconds, the sympathetic reflexes become very active.
They affect both the cardiac output and the venous
return curves, raising both of them. Sympathetic stimulation can increase the plateau level of the cardiac
output curve as much as 30 to 100 per cent. It can also
increase the mean systemic filling pressure (depicted
by the point where the venous return curve crosses the
zero venous return axis) by several millimeters of
mercury—in this figure, from a normal value of 7 mm
Hg up to 10 mm Hg. This increase in mean systemic
filling pressure shifts the entire venous return curve to
the right and upward. The new cardiac output and
venous return curves now equilibrate at point C, that
is, at a right atrial pressure of +5 mm Hg and a cardiac
output of 4 L/min.
266
Unit IV
The Circulation
Compensation During the Next Few Days. During the
ensuing week, the cardiac output and venous return
curves rise further because of (1) some recovery of the
heart and (2) renal retention of salt and water, which
raises the mean systemic filling pressure still further—
this time up to +12 mm Hg. The two new curves now
equilibrate at point D. Thus, the cardiac output has
now returned to normal. The right atrial pressure,
however, has risen still further to +6 mm Hg. Because
the cardiac output is now normal, renal output is also
normal, so that a new state of equilibrated fluid
balance has been achieved. The circulatory system will
continue to function at point D and remain stable, with
a normal cardiac output and an elevated right atrial
pressure, until some additional extrinsic factor changes
either the cardiac output curve or the venous return
curve.
Using this technique for analysis, one can see especially the importance of moderate fluid retention and
how it eventually leads to a new stable state of the circulation in mild to moderate heart failure. And one
can also see the interrelation between mean systemic
filling pressure and cardiac pumping at various
degrees of heart failure.
Note that the events described in Figure 22–5 are
the same as those presented in Figure 22–1, but in
Figure 22–5, they are presented in a more quantitative
manner.
Graphical Analysis of “Decompensated”
Cardiac Failure
Cardiac output and venous return (L/min)
The black cardiac output curve in Figure 22–6 is the
same as the curve shown in Figure 22–2, a very low
curve that has already reached a degree of recovery as
great as this heart can achieve. In this figure, we have
added venous return curves that occur during successive days after the acute fall of the cardiac output
curve to this low level. At point A, the curve at time
zero equates with the normal venous return curve to
give a cardiac output of about 3 L/min. However, stimulation of the sympathetic nervous system, caused by
this low cardiac output, increases the mean systemic
filling pressure within 30 seconds from 7 to 10.5 mm
Hg. This shifts the venous return curve upward and to
the right to produce the curve labeled “autonomic
compensation.” Thus, the new venous return curve
equates with the cardiac output curve at point B. The
cardiac output has been improved to a level of 4 L/min
but at the expense of an additional rise in right atrial
pressure to 5 mm Hg.
The cardiac output of 4 L/min is still too low to cause
the kidneys to function normally. Therefore, fluid continues to be retained, and the mean systemic filling
pressure rises from 10.5 to almost 13 mm Hg. Now the
venous return curve becomes that labeled “2nd day”
and equilibrates with the cardiac output curve at point
C. The cardiac output rises to 4.2 L/min and the right
atrial pressure to 7 mm Hg.
During the succeeding days, the cardiac output
never rises quite high enough to re-establish normal
renal function. Fluid continues to be retained, the
mean systemic filling pressure continues to rise, the
venous return curve continues to shift to the right, and
the equilibrium point between the venous return curve
and the cardiac output curve also shifts progressively
to point D, to point E, and, finally, to point F. The equilibration process is now on the down slope of the
cardiac output curve, so that further retention of fluid
causes only more severe cardiac edema and a detrimental effect on cardiac output. The condition accelerates downhill until death occurs.
Thus, “decompensation” results from the fact that
the cardiac output curve never rises to the critical level
of 5 L/min needed to re-establish normal kidney excretion of fluid that would be required to cause balance
between fluid input and output.
Treatment of Decompensated Heart Disease with Digitalis. Let
15
8th
6th day
4th day
10
5
day
Critical cardiac
output level
for normal
fluid balance
2nd day
Autonomic
com
pen
sat
ion
ous
Normal v
en
retu
rn
A
B
C
D
E
F
0
–4 –2
0
2
4
6
8 10 12
Right atrial pressure (mm Hg)
14
16
Figure 22–6
Graphical analysis of decompensated heart disease showing
progressive shift of the venous return curve to the right as a result
of continued fluid retention.
us assume that the stage of decompensation has
already reached point E in Figure 22–6, and let us
proceed to the same point E in Figure 22–7. At this
time, digitalis is given to strengthen the heart. This
raises the cardiac output curve to the level shown in
Figure 22–7, but there is not an immediate change in
the venous return curve. Therefore, the new cardiac
output curve equates with the venous return curve at
point G. The cardiac output is now 5.7 L/min, a value
greater than the critical level of 5 liters required to
make the kidneys excrete normal amounts of urine.
Therefore, the kidneys eliminate much more fluid than
normally, causing diuresis, a well-known therapeutic
effect of digitalis.
The progressive loss of fluid over a period of several
days reduces the mean systemic filling pressure back
down to 11.5 mm Hg, and the new venous return curve
becomes the curve labeled “Several days later.” This
curve equates with the cardiac output curve of the
digitalized heart at point H, at an output of 5 L/min
and a right atrial pressure of 4.6 mm Hg. This cardiac
First d
ay
10 S everal d
G
H
t
e ar
dh
ize
f a ili n g
l
a
e ly
git
ver art
Di
Se he
5
0
–4 –2
0
2
4
6
E
8
10 12 14 16
Right atrial pressure (mm Hg)
25
20
15
Normal
venous
return
curve
10
5
A
la
ays
late
r
Critical cardiac output
level for normal
fluid balance
Cardiac output and
venous return (L/min)
15
267
Cardiac Failure
u
fist
AV
Cardiac output and
venous return (L/min)
Chapter 22
Normal cardiac
output curve
B
C
Beriberi
heart
disease
0
–4 –2 0 2 4 6 8 10 12 14 16
Right atrial pressure (mm Hg)
Figure 22–7
Treatment of decompensated heart disease showing the effect of
digitalis in elevating the cardiac output curve, this in turn causing
increased urine output and progressive shift of the venous return
curve to the left.
Figure 22–8
output is precisely that required for normal fluid
balance. Therefore, no additional fluid will be lost and
none will be gained. Consequently, the circulatory
system has now stabilized, or in other words, the
decompensation of the heart failure has been “compensated.” And to state this another way, the final
steady-state condition of the circulation is defined by
the crossing point of three curves: the cardiac output
curve, the venous return curve, and the critical level
for normal fluid balance. The compensatory mechanisms automatically stabilize the circulation when all
three curves cross at the same point.
Graphical Analysis of High-Output
Cardiac Failure
Figure 22–8 gives an analysis of two types of highoutput cardiac failure. One of these is caused by an
arteriovenous fistula that overloads the heart because
of excessive venous return, even though the pumping
capability of the heart is not depressed. The other is
caused by beriberi, in which the venous return is
greatly increased because of diminished systemic vascular resistance, but at the same time, the pumping
capability of the heart is depressed.
Arteriovenous Fistula. The “normal” curves of Figure
22–8 depict the normal cardiac output and normal
venous return curves. These equate with each other at
point A, which depicts a normal cardiac output of 5 L/
min and a normal right atrial pressure of 0 mm Hg.
Now let us assume that the systemic resistance (the
total peripheral resistance) becomes greatly decreased
because of opening a large arteriovenous fistula (a
direct opening between a large artery and a large
vein). The venous return curve rotates upward to give
the curve labeled “AV fistula.” This venous return
curve equates with the normal cardiac output curve at
point B, with a cardiac output of 12.5 L/min and a right
atrial pressure of 3 mm Hg. Thus, the cardiac output
has become greatly elevated, the right atrial pressure
Graphical analysis of two types of conditions that can cause highoutput cardiac failure: (1) arteriovenous (AV) fistula and (2)
beriberi heart disease.
is slightly elevated, and there are mild signs of peripheral congestion. If the person attempts to exercise, he
or she will have little cardiac reserve because the heart
is already being used almost to maximum capacity to
pump the extra blood through the arteriovenous
fistula. This condition resembles a failure condition
and is called “high-output failure,” but in reality, the
heart is overloaded by excess venous return.
Beriberi. Figure 22–8 shows the approximate changes
in the cardiac output and venous return curves caused
by beriberi. The decreased level of the cardiac output
curve is caused by weakening of the heart because of
the avitaminosis (mainly lack of thiamine) that causes
the beriberi syndrome. The weakening of the heart has
decreased the blood flow to the kidneys. Therefore, the
kidneys have retained a large amount of extra body
fluid, which in turn has increased the mean systemic
filling pressure (represented by the point where the
venous return curve now intersects the zero cardiac
output level) from the normal value of 7 mm Hg up to
11 mm Hg. This has shifted the venous return curve to
the right. Finally, the venous return curve has rotated
upward from the normal curve because the avitaminosis has dilated the peripheral blood vessels, as
explained in Chapter 17.
The two blue curves (cardiac output curve and
venous return curve) intersect with each other at
point C, which describes the circulatory condition in
beriberi, with a right atrial pressure in this instance of
9 mm Hg and a cardiac output about 65 per cent above
normal; this high cardiac output occurs despite the
weak heart, as demonstrated by the depressed plateau
level of the cardiac output curve.
268
Unit IV
The Circulation
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H
A
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2
3
Heart Valves and Heart Sounds;
Dynamics of Valvular and
Congenital Heart Defects
Function of the heart valves was discussed in
Chapter 9, where it was pointed out that closing of
the valves causes audible sounds. Ordinarily, no
audible sounds occur when the valves open. In this
chapter, we first discuss the factors that cause the
sounds in the heart under normal and abnormal
conditions. Then we discuss what happens in the
overall circulatory system when valvular or congenital heart defects are present.
Heart Sounds
Normal Heart Sounds
Listening with a stethoscope to a normal heart, one hears a sound usually
described as “lub, dub, lub, dub.”The “lub” is associated with closure of the atrioventricular (A-V) valves at the beginning of systole, and the “dub” is associated
with closure of the semilunar (aortic and pulmonary) valves at the end of
systole. The “lub” sound is called the first heart sound, and the “dub” is called
the second heart sound, because the normal pumping cycle of the heart is considered to start when the A-V valves close at the onset of ventricular systole.
Causes of the First and Second Heart Sounds. The earliest explanation for the cause
of the heart sounds was that the “slapping” together of the valve leaflets sets
up vibrations. However, this has been shown to cause little, if any, of the sound,
because the blood between the leaflets cushions the slapping effect and prevents significant sound. Instead, the cause is vibration of the taut valves immediately after closure, along with vibration of the adjacent walls of the heart and
major vessels around the heart. That is, in generating the first heart sound, contraction of the ventricles first causes sudden backflow of blood against the
A-V valves (the tricuspid and mitral valves), causing them to close and bulge
toward the atria until the chordae tendineae abruptly stop the back bulging.
The elastic tautness of the chordae tendineae and of the valves then causes the
back surging blood to bounce forward again into each respective ventricle. This
causes the blood and the ventricular walls, as well as the taut valves, to vibrate
and causes vibrating turbulence in the blood. The vibrations travel through the
adjacent tissues to the chest wall, where they can be heard as sound by using
the stethoscope.
The second heart sound results from sudden closure of the semilunar valves
at the end of systole. When the semilunar valves close, they bulge backward
toward the ventricles, and their elastic stretch recoils the blood back into the
arteries, which causes a short period of reverberation of blood back and forth
between the walls of the arteries and the semilunar valves, as well as between
these valves and the ventricular walls. The vibrations occurring in the arterial
walls are then transmitted mainly along the arteries. When the vibrations of the
vessels or ventricles come into contact with a “sounding board,” such as the
chest wall, they create sound that can be heard.
269
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Unit IV
The Circulation
Duration and Pitch of the First and Second Heart Sounds. The
duration of each of the heart sounds is slightly more
than 0.10 second—the first sound about 0.14 second,
and the second about 0.11 second. The reason for the
shorter second sound is that the semilunar valves are
more taut than the A-V valves, so that they vibrate for
a shorter time than do the A-V valves.
The audible range of frequency (pitch) in the first
and second heart sounds, as shown in Figure 23–1,
begins at the lowest frequency the ear can detect,
about 40 cycles/sec, and goes up above 500 cycles/sec.
When special electronic apparatus is used to record
these sounds, by far a larger proportion of the recorded sound is at frequencies and sound levels below
the audible range, going down to 3 to 4 cycles/sec and
peaking at about 20 cycles/sec, as illustrated by the
lower shaded area in Figure 23–1. For this reason,
major portions of the heart sounds can be recorded
electronically in phonocardiograms even though they
cannot be heard with a stethoscope.
The second heart sound normally has a higher frequency than the first heart sound for two reasons: (1)
the tautness of the semilunar valves in comparison
with the much less taut A-V valves, and (2) the greater
elastic coefficient of the taut arterial walls that provide
the principal vibrating chambers for the second sound,
in comparison with the much looser, less elastic ventricular chambers that provide the vibrating system for
the first heart sound. The clinician uses these differences to distinguish special characteristics of the two
respective sounds.
Third Heart Sound. Occasionally a weak, rumbling third
heart sound is heard at the beginning of the middle
Heart sounds
Inaudible and murmurs
100
Dynes/cm2
ho
0.1
Heart sounds
and murmurs
0.01
sound can sometimes be recorded in the phonocardiogram, but it can almost never be heard with a
stethoscope because of its weakness and very low
frequency—usually 20 cycles/sec or less. This sound
occurs when the atria contract, and presumably, it is
caused by the inrush of blood into the ventricles, which
initiates vibrations similar to those of the third heart
sound.
Chest Surface Areas for Auscultation
of Normal Heart Sounds
Listening to the sounds of the body, usually with the
aid of a stethoscope, is called auscultation. Figure 23–2
shows the areas of the chest wall from which the different heart valvular sounds can best be distinguished.
Although the sounds from all the valves can be heard
from all these areas, the cardiologist distinguishes
the sounds from the different valves by a process of
Pulmonic area
Speech
area
Th
res
Atrial Heart Sound (Fourth Heart Sound). An atrial heart
Aortic area
10
1
third of diastole. A logical but unproved explanation
of this sound is oscillation of blood back and forth
between the walls of the ventricles initiated by inrushing blood from the atria. This is analogous to running
water from a faucet into a paper sack, the inrushing
water reverberating back and forth between the walls
of the sack to cause vibrations in its walls. The reason
the third heart sound does not occur until the middle
third of diastole is believed to be that in the early part
of diastole, the ventricles are not filled sufficiently to
create even the small amount of elastic tension necessary for reverberation. The frequency of this sound is
usually so low that the ear cannot hear it, yet it can
often be recorded in the phonocardiogram.
ld
of
au
dib
ility
0.001
0.0001
0
8 32 64 128 256 512 1024 2048 4096
Frequency in cycles per second
Figure 23–1
Amplitude of different-frequency vibrations in the heart sounds
and heart murmurs in relation to the threshold of audibility,
showing that the range of sounds that can be heard is between
40 and 520 cycles/sec. (Modified from Butterworth JS, Chassin
JL, McGrath JJ: Cardiac Auscultation, 2nd ed. New York: Grune
& Stratton, 1960.)
Tricuspid area
Mitral area
Figure 23–2
Chest areas from which sound from each valve is best heard.
Chapter 23
Heart Valves and Heart Sounds; Dynamics of Valvular and Congenital Heart Defects
271
Valvular Lesions
1st
A
2nd
3rd
Rheumatic Valvular Lesions
Atrial
Normal
B
Aortic stenosis
C
Mitral regurgitation
D
Aortic regurgitation
E
Mitral stenosis
F
Patent ductus
arteriosus
Diastole
Systole
Diastole
Systole
Figure 23–3
Phonocardiograms from normal and abnormal hearts.
elimination. That is, he or she moves the stethoscope
from one area to another, noting the loudness of the
sounds in different areas and gradually picking out the
sound components from each valve.
The areas for listening to the different heart sounds
are not directly over the valves themselves. The aortic
area is upward along the aorta because of sound transmission up the aorta, and the pulmonic area is upward
along the pulmonary artery. The tricuspid area is over
the right ventricle, and the mitral area is over the apex
of the left ventricle, which is the portion of the heart
nearest the surface of the chest; the heart is rotated
so that the remainder of the left ventricle lies more
posteriorly.
Phonocardiogram
If a microphone specially designed to detect low-frequency sound is placed on the chest, the heart sounds
can be amplified and recorded by a high-speed recording apparatus. The recording is called a phonocardiogram, and the heart sounds appear as waves, as shown
schematically in Figure 23–3. Recording A is an
example of normal heart sounds, showing the vibrations of the first, second, and third heart sounds and
even the very weak atrial sound. Note specifically that
the third and atrial heart sounds are each a very low
rumble. The third heart sound can be recorded in only
one third to one half of all people, and the atrial heart
sound can be recorded in perhaps one fourth of all
people.
By far the greatest number of valvular lesions results
from rheumatic fever. Rheumatic fever is an autoimmune disease in which the heart valves are likely to be
damaged or destroyed. It is usually initiated by streptococcal toxin in the following manner.
The sequence of events almost always begins with a
preliminary streptococcal infection caused specifically
by group A hemolytic streptococci. These bacteria initially cause a sore throat, scarlet fever, or middle ear
infection. But the streptococci also release several different proteins against which the person’s reticuloendothelial system produces antibodies. The antibodies
react not only with the streptococcal protein but also
with other protein tissues of the body, often causing
severe immunologic damage. These reactions continue
to take place as long as the antibodies persist in the
blood—1 year or more.
Rheumatic fever causes damage especially in
certain susceptible areas, such as the heart valves. The
degree of heart valve damage is directly correlated
with the concentration and persistence of the antibodies. The principles of immunity that relate to this
type of reaction are discussed in Chapter 34, and it is
noted in Chapter 31 that acute glomerular nephritis of
the kidneys has a similar immunologic basis.
In rheumatic fever, large hemorrhagic, fibrinous,
bulbous lesions grow along the inflamed edges of
the heart valves. Because the mitral valve receives
more trauma during valvular action than any of the
other valves, it is the one most often seriously
damaged, and the aortic valve is second most frequently damaged. The right heart valves, the tricuspid
and pulmonary valves, are usually affected much less
severely, probably because the low-pressure stresses
that act on these valves are slight compared with
the high-pressure stresses that act on the left heart
valves.
Scarring of the Valves. The lesions of acute rheumatic
fever frequently occur on adjacent valve leaflets simultaneously, so that the edges of the leaflets become
stuck together. Then, weeks, months, or years later, the
lesions become scar tissue, permanently fusing portions of adjacent valve leaflets. Also, the free edges of
the leaflets, which are normally filmy and free-flapping,
often become solid, scarred masses.
A valve in which the leaflets adhere to one another
so extensively that blood cannot flow through it normally is said to be stenosed. Conversely, when the valve
edges are so destroyed by scar tissue that they cannot
close as the ventricles contract, regurgitation (backflow) of blood occurs when the valve should be closed.
Stenosis usually does not occur without the coexistence of at least some degree of regurgitation, and vice
versa.
Other Causes of Valvular Lesions. Stenosis or lack of one
or more leaflets of a valve also occurs occasionally as
a congenital defect. Complete lack of leaflets is rare;
272
Unit IV
The Circulation
congenital stenosis is more common, as is discussed
later in this chapter.
Heart Murmurs Caused by Valvular Lesions
As shown by the phonocardiograms in Figure 23–3,
many abnormal heart sounds, known as “heart
murmurs,” occur when there are abnormalities of the
valves, as follows.
Systolic Murmur of Aortic Stenosis. In aortic stenosis,
blood is ejected from the left ventricle through only a
small fibrous opening of the aortic valve. Because of
the resistance to ejection, sometimes the blood pressure in the left ventricle rises as high as 300 mm Hg,
while the pressure in the aorta is still normal. Thus,
a nozzle effect is created during systole, with blood
jetting at tremendous velocity through the small
opening of the valve. This causes severe turbulence of
the blood in the root of the aorta. The turbulent blood
impinging against the aortic walls causes intense vibration, and a loud murmur (see recording B, Figure 23–3)
occurs during systole and is transmitted throughout
the superior thoracic aorta and even into the large
arteries of the neck. This sound is harsh and in severe
stenosis may be so loud that it can be heard several
feet away from the patient. Also, the sound vibrations
can often be felt with the hand on the upper chest and
lower neck, a phenomenon known as a “thrill.”
Diastolic Murmur of Aortic Regurgitation. In aortic regurgitation, no abnormal sound is heard during systole,
but during diastole, blood flows backward from the
high-pressure aorta into the left ventricle, causing
a “blowing” murmur of relatively high pitch with a
swishing quality heard maximally over the left ventricle (see recording D, Figure 23–3). This murmur results
from turbulence of blood jetting backward into the
blood already in the low-pressure diastolic left
ventricle.
Systolic Murmur of Mitral Regurgitation. In mitral regurgi-
tation, blood flows backward through the mitral valve
into the left atrium during systole. This also causes a
high-frequency “blowing,” swishing sound (see recording C, Figure 23–3) similar to that of aortic regurgitation but occurring during systole rather than diastole.
It is transmitted most strongly into the left atrium.
However, the left atrium is so deep within the chest
that it is difficult to hear this sound directly over the
atrium. As a result, the sound of mitral regurgitation
is transmitted to the chest wall mainly through the left
ventricle to the apex of the heart.
Diastolic Murmur of Mitral Stenosis. In mitral stenosis,
blood passes with difficulty through the stenosed
mitral valve from the left atrium into the left ventricle, and because the pressure in the left atrium seldom
rises above 30 mm Hg, a large pressure differential
forcing blood from the left atrium into the left ventricle does not develop. Consequently, the abnormal
sounds heard in mitral stenosis (see recording E,
Figure 23–3) are usually weak and of very low
frequency, so that most of the sound spectrum is below
the low-frequency end of human hearing.
During the early part of diastole, a left ventricle with
a stenotic mitral valve has so little blood in it and its
walls are so flabby that blood does not reverberate
back and forth between the walls of the ventricle. For
this reason, even in severe mitral stenosis, no murmur
may be heard during the first third of diastole. Then,
after partial filling, the ventricle has stretched enough
for blood to reverberate, and a low rumbling murmur
begins.
of Valvular Murmurs. Phonocardiograms B, C, D, and E of Figure 23–3 show, respectively,
idealized records obtained from patients with aortic
stenosis, mitral regurgitation, aortic regurgitation, and
mitral stenosis. It is obvious from these phonocardiograms that the aortic stenotic lesion causes the loudest
murmur, and the mitral stenotic lesion causes the
weakest. The phonocardiograms show how the intensity of the murmurs varies during different portions of
systole and diastole, and the relative timing of each
murmur is also evident. Note especially that the
murmurs of aortic stenosis and mitral regurgitation
occur only during systole, whereas the murmurs of
aortic regurgitation and mitral stenosis occur only
during diastole. If the reader does not understand this
timing, extra review should be undertaken until it is
understood.
Phonocardiograms
Abnormal Circulatory
Dynamics in Valvular
Heart Disease
Dynamics of the Circulation in Aortic
Stenosis and Aortic Regurgitation
In aortic stenosis, the contracting left ventricle fails to
empty adequately, whereas in aortic regurgitation,
blood flows backward into the ventricle from the aorta
after the ventricle has just pumped the blood into the
aorta. Therefore, in either case, the net stroke volume
output of the heart is reduced.
Several important compensations take place that
can ameliorate the severity of the circulatory defects.
Some of these compensations are the following.
Hypertrophy of the Left Ventricle. In both aortic stenosis
and aortic regurgitation, the left ventricular musculature hypertrophies because of the increased ventricular workload.
In regurgitation, the left ventricular chamber also
enlarges to hold all the regurgitant blood from the
aorta. Sometimes the left ventricular muscle mass
increases fourfold to fivefold, creating a tremendously
large left side of the heart.
When the aortic valve is seriously stenosed, the
hypertrophied muscle allows the left ventricle to
develop as much as 400 mm Hg intraventricular pressure at systolic peak.
Chapter 23
Heart Valves and Heart Sounds; Dynamics of Valvular and Congenital Heart Defects
273
In severe aortic regurgitation, sometimes the hypertrophied muscle allows the left ventricle to pump a
stroke volume output as great as 250 milliliters,
although as much as three fourths of this blood returns
to the ventricle during diastole, and only one fourth
flows through the aorta to the body.
development of serious pulmonary edema. Ordinarily,
lethal edema does not occur until the mean left atrial
pressure rises above 25 mm Hg and sometimes as high
as 40 mm Hg, because the lung lymphatic vasculature
enlarges manyfold and can carry fluid away from the
lung tissues extremely rapidly.
Increase in Blood Volume. Another effect that helps
compensate for the diminished net pumping by the left
ventricle is increased blood volume. This results from
(1) an initial slight decrease in arterial pressure, plus
(2) peripheral circulatory reflexes that the decrease in
pressure induces. These together diminish renal output
of urine, causing the blood volume to increase and
the mean arterial pressure to return to normal. Also,
red cell mass eventually increases because of a slight
degree of tissue hypoxia.
The increase in blood volume tends to increase
venous return to the heart. This, in turn, causes the left
ventricle to pump with the extra power required to
overcome the abnormal pumping dynamics.
Enlarged Left Atrium and Atrial Fibrillation. The high left
atrial pressure in mitral valvular disease also causes
progressive enlargement of the left atrium, which
increases the distance that the cardiac electrical excitatory impulse must travel in the atrial wall. This
pathway may eventually become so long that it predisposes to development of excitatory signal circus
movements, as discussed in Chapter 13. Therefore,
in late stages of mitral valvular disease, especially in
mitral stenosis, atrial fibrillation usually occurs. This
further reduces the pumping effectiveness of the heart
and causes further cardiac debility.
Eventual Failure of the Left Ventricle, and
Development of Pulmonary Edema
In the early stages of aortic stenosis or aortic regurgitation, the intrinsic ability of the left ventricle to adapt
to increasing loads prevents significant abnormalities
in circulatory function in the person during rest, other
than increased work output required of the left ventricle. Therefore, considerable degrees of aortic stenosis or aortic regurgitation often occur before the
person knows that he or she has serious heart disease
(such as a resting left ventricular systolic pressure as
high as 200 mm Hg in aortic stenosis or a left ventricular stroke volume output as high as double normal in
aortic regurgitation).
Beyond a critical stage in these aortic valve lesions,
the left ventricle finally cannot keep up with the work
demand. As a consequence, the left ventricle dilates
and cardiac output begins to fall; blood simultaneously
dams up in the left atrium and in the lungs behind the
failing left ventricle. The left atrial pressure rises progressively, and at mean left atrial pressures above 25
to 40 mm Hg, serious edema appears in the lungs, as
discussed in detail in Chapter 38.
Dynamics of Mitral Stenosis and
Mitral Regurgitation
In mitral stenosis, blood flow from the left atrium into
the left ventricle is impeded, and in mitral regurgitation, much of the blood that has flowed into the left
ventricle during diastole leaks back into the left atrium
during systole rather than being pumped into the
aorta. Therefore, either of these conditions reduces net
movement of blood from the left atrium into the left
ventricle.
Pulmonary Edema in Mitral Valvular Disease. The buildup
of blood in the left atrium causes progressive increase
in left atrial pressure, and this eventually results in
Compensation in Early Mitral Valvular Disease. As also
occurs in aortic valvular disease and in many types of
congenital heart disease, the blood volume increases
in mitral valvular disease principally because of diminished excretion of water and salt by the kidneys. This
increased blood volume increases venous return to the
heart, thereby helping to overcome the effect of the
cardiac debility.Therefore, after compensation, cardiac
output may fall only minimally until the late stages of
mitral valvular disease, even though the left atrial pressure is rising.
As the left atrial pressure rises, blood begins to dam
up in the lungs, eventually all the way back to the pulmonary artery. In addition, incipient edema of the
lungs causes pulmonary arteriolar constriction. These
two effects together increase systolic pulmonary arterial pressure and also right ventricular pressure, sometimes to as high as 60 mm Hg, which is more than
double normal. This, in turn, causes hypertrophy of the
right side of the heart, which partially compensates for
its increased workload.
Circulatory Dynamics During Exercise
in Patients with Valvular Lesions
During exercise, large quantities of venous blood are
returned to the heart from the peripheral circulation.
Therefore, all the dynamic abnormalities that occur in
the different types of valvular heart disease become
tremendously exacerbated. Even in mild valvular
heart disease, in which the symptoms may be unrecognizable at rest, severe symptoms often develop
during heavy exercise. For instance, in patients with
aortic valvular lesions, exercise can cause acute left
ventricular failure followed by acute pulmonary
edema. Also, in patients with mitral disease, exercise
can cause so much damming of blood in the lungs that
serious or even lethal pulmonary edema may ensue in
as little as 10 minutes.
Even in mild to moderate cases of valvular disease,
the patient’s cardiac reserve diminishes in proportion
274
Unit IV
The Circulation
to the severity of the valvular dysfunction. That is, the
cardiac output does not increase as much as it should
during exercise. Therefore, the muscles of the body
fatigue rapidly because of too little increase in muscle
blood flow.
Head and upper
extremities
Right
lung
Ductus
arteriosus Aorta
Left lung
Abnormal Circulatory
Dynamics in Congenital
Heart Defects
Occasionally, the heart or its associated blood vessels
are malformed during fetal life; the defect is called a
congenital anomaly. There are three major types of
congenital anomalies of the heart and its associated
vessels: (1) stenosis of the channel of blood flow at
some point in the heart or in a closely allied major
blood vessel; (2) an anomaly that allows blood to flow
backward from the left side of the heart or aorta to the
right side of the heart or pulmonary artery, thus failing
to flow through the systemic circulation—called a leftto-right shunt; and (3) an anomaly that allows blood to
flow directly from the right side of the heart into the
left side of the heart, thus failing to flow through the
lungs—called a right-to-left shunt.
The effects of the different stenotic lesions are easily
understood. For instance, congenital aortic valve stenosis results in the same dynamic effects as aortic valve
stenosis caused by other valvular lesions, namely, a
tendency to develop serious pulmonary edema and a
reduced cardiac output.
Another type of congenital stenosis is coarctation
of the aorta, often occurring near the level of the
diaphragm. This causes the arterial pressure in the
upper part of the body (above the level of the coarctation) to be much greater than the pressure in the
lower body because of the great resistance to blood
flow through the coarctation to the lower body; part of
the blood must go around the coarctation through
small collateral arteries, as discussed in Chapter 19.
Patent Ductus Arteriosus—
A Left-to-Right Shunt
During fetal life, the lungs are collapsed, and the
elastic compression of the lungs that keeps the alveoli
collapsed keeps most of the lung blood vessels collapsed as well. Therefore, resistance to blood flow
through the lungs is so great that the pulmonary arterial pressure is high in the fetus. Also, because of low
resistance to blood flow from the aorta through the
large vessels of the placenta, the pressure in the aorta
of the fetus is lower than normal—in fact, lower than
in the pulmonary artery. This causes almost all the pulmonary arterial blood to flow through a special artery
present in the fetus that connects the pulmonary artery
with the aorta (Figure 23–4), called the ductus arteriosus, thus bypassing the lungs. This allows immediate
recirculation of the blood through the systemic arteries of the fetus without the blood going through the
Trunk and lower
extremities
Pulmonary
Left
artery
pulmonary
artery
Figure 23–4
Patent ductus arteriosus, showing by the intensity of the pink color
that dark venous blood changes into oxygenated blood at different points in the circulation. The right-hand diagram shows backflow of blood from the aorta into the pulmonary artery and then
through the lungs for a second time.
lungs. This lack of blood flow through the lungs is not
detrimental to the fetus because the blood is oxygenated by the placenta.
Closure of the Ductus Arteriosus After Birth. As soon as a
baby is born and begins to breathe, the lungs inflate;
not only do the alveoli fill with air, but also the resistance to blood flow through the pulmonary vascular
tree decreases tremendously, allowing the pulmonary
arterial pressure to fall. Simultaneously, the aortic
pressure rises because of sudden cessation of blood
flow from the aorta through the placenta. Thus, the
pressure in the pulmonary artery falls, while that in
the aorta rises. As a result, forward blood flow through
the ductus arteriosus ceases suddenly at birth, and in
fact, blood begins to flow backward through the ductus
from the aorta into the pulmonary artery. This new
state of backward blood flow causes the ductus arteriosus to become occluded within a few hours to a few
days in most babies, so that blood flow through the
ductus does not persist. The ductus is believed to close
because the oxygen concentration of the aortic blood
now flowing through it is about twice as high as that
of the blood flowing from the pulmonary artery into
the ductus during fetal life. The oxygen presumably
constricts the muscle in the ductus wall. This is discussed further in Chapter 83.
Unfortunately, in about 1 of every 5500 babies, the
ductus does not close, causing the condition known as
patent ductus arteriosus, which is shown in Figure 23–4.
Dynamics of the Circulation with a Persistent Patent Ductus.
During the early months of an infant’s life, a patent
ductus usually does not cause severely abnormal function. But as the child grows older, the differential
between the high pressure in the aorta and the lower
Chapter 23
Heart Valves and Heart Sounds; Dynamics of Valvular and Congenital Heart Defects
pressure in the pulmonary artery progressively increases, with corresponding increase in backward flow
of blood from the aorta into the pulmonary artery.
Also, the high aortic blood pressure usually causes the
diameter of the partially open ductus to increase with
time, making the condition even worse.
Head and upper
extremities
Right
lung
Recirculation Through the Lungs. In an older child
with a patent ductus, one half to two thirds of the aortic
blood flows backward through the ductus into the pulmonary artery, then through the lungs, and finally back
into the left ventricle and aorta, passing through the
lungs and left side of the heart two or more times for
every one time that it passes through the systemic circulation. These people do not show cyanosis until later
in life, when the heart fails or the lungs become congested. Indeed, early in life, the arterial blood is often
better oxygenated than normal because of the extra
times it passes through the lungs.
Diminished Cardiac and Respiratory Reserve. The
major effects of patent ductus arteriosus on the patient
are decreased cardiac and respiratory reserve. The left
ventricle is pumping about two or more times the
normal cardiac output, and the maximum that it can
pump after hypertrophy of the heart has occurred is
about four to seven times normal. Therefore, during
exercise, the net blood flow through the remainder of
the body can never increase to the levels required for
strenuous activity. With even moderately strenuous
exercise, the person is likely to become weak and may
even faint from momentary heart failure.
The high pressures in the pulmonary vessels caused
by excess flow through the lungs often lead to pulmonary congestion and pulmonary edema. As a result
of the excessive load on the heart, and especially
because the pulmonary congestion becomes progressively more severe with age, most patients with uncorrected patent ductus die from heart disease between
ages 20 and 40 years.
Heart Sounds: Machinery Murmur. In a newborn infant
with patent ductus arteriosus, occasionally no abnormal heart sounds are heard because the quantity of
reverse blood flow through the ductus may be insufficient to cause a heart murmur. But as the baby grows
older, reaching age 1 to 3 years, a harsh, blowing
murmur begins to be heard in the pulmonary artery
area of the chest, as shown in recording F, Figure 23–3.
This sound is much more intense during systole when
the aortic pressure is high and much less intense
during diastole when the aortic pressure falls low, so
that the murmur waxes and wanes with each beat of
the heart, creating the so-called machinery murmur.
Surgical Treatment. Surgical treatment of patent ductus
arteriosus is extremely simple; one need only ligate the
patent ductus or divide it and then close the two ends.
In fact, this was one of the first successful heart surgeries ever performed.
275
Left lung
Trunk and lower
extremities
Figure 23–5
Tetralogy of Fallot, showing by the intensity of the pink color that
most of the dark venous blood is shunted from the right ventricle
into the aorta without passing through the lungs.
Tetralogy of Fallot—
A Right-to-Left Shunt
Tetralogy of Fallot is shown in Figure 23–5; it is the
most common cause of “blue baby.” Most of the blood
bypasses the lungs, so the aortic blood is mainly unoxygenated venous blood. In this condition, four abnormalities of the heart occur simultaneously:
1. The aorta originates from the right ventricle
rather than the left, or it overrides a hole in the
septum, as shown in Figure 23–5, receiving blood
from both ventricles.
2. The pulmonary artery is stenosed, so that much
lower than normal amounts of blood pass from
the right ventricle into the lungs; instead, most of
the blood passes directly into the aorta, thus
bypassing the lungs.
3. Blood from the left ventricle flows either through
a ventricular septal hole into the right ventricle
and then into the aorta or directly into the aorta
that overrides this hole.
4. Because the right side of the heart must pump
large quantities of blood against the high pressure
in the aorta, its musculature is highly developed,
causing an enlarged right ventricle.
Abnormal Circulatory Dynamics. It is readily apparent that
the major physiological difficulty caused by tetralogy
of Fallot is the shunting of blood past the lungs without
its becoming oxygenated. As much as 75 per cent of
the venous blood returning to the heart passes directly
276
Unit IV
The Circulation
from the right ventricle into the aorta without becoming oxygenated.
A diagnosis of tetralogy of Fallot is usually based on
(1) the fact that the baby’s skin is cyanotic (blue); (2)
measurement of high systolic pressure in the right
ventricle, recorded through a catheter; (3) characteristic changes in the radiological silhouette of the
heart, showing an enlarged right ventricle; and (4)
angiograms (x-ray pictures) showing abnormal blood
flow through the interventricular septal hole and into
the overriding aorta, but much less flow through the
stenosed pulmonary artery.
Surgical Treatment. Tetralogy of Fallot can usually be
treated successfully by surgery. The usual operation is
to open the pulmonary stenosis, close the septal defect,
and reconstruct the flow pathway into the aorta. When
surgery is successful, the average life expectancy
increases from only 3 to 4 years to 50 or more years.
passing the blood between thin membranes or through
thin tubes that are permeable to oxygen and carbon
dioxide.
The different systems have all been fraught with
many difficulties, including hemolysis of the blood,
development of small clots in the blood, likelihood of
small bubbles of oxygen or small emboli of antifoam
agent passing into the arteries of the patient, necessity
for large quantities of blood to prime the entire
system, failure to exchange adequate quantities of
oxygen, and necessity to use heparin to prevent blood
coagulation in the extracorporeal system. Heparin also
interferes with adequate hemostasis during the surgical procedure. Yet despite these difficulties, in the
hands of experts, patients can be kept alive on artificial heart-lung machines for many hours while operations are performed on the inside of the heart.
Causes of Congenital Anomalies
Hypertrophy of the Heart
in Valvular and Congenital
Heart Disease
One of the most common causes of congenital heart
defects is a viral infection in the mother during the first
trimester of pregnancy when the fetal heart is being
formed. Defects are particularly prone to develop
when the expectant mother contracts German
measles; thus, obstetricians often advise termination of
pregnancy if German measles occurs in the first
trimester.
Some congenital defects of the heart are hereditary,
because the same defect has been known to occur in
identical twins as well as in succeeding generations.
Children of patients surgically treated for congenital
heart disease have about a 10 times greater chance of
having congenital heart disease than other children do.
Congenital defects of the heart are also frequently
associated with other congenital defects of the baby’s
body.
Hypertrophy of cardiac muscle is one of the most
important mechanisms by which the heart adapts to
increased workloads, whether these loads are caused
by increased pressure against which the heart muscle
must contract or by increased cardiac output that
must be pumped. Some physicians believe that the
increased strength of contraction of the heart muscle
causes the hypertrophy; others believe that the
increased metabolic rate of the muscle is the primary
stimulus. Regardless of which of these is correct, one
can calculate approximately how much hypertrophy
will occur in each chamber of the heart by multiplying
ventricular output by the pressure against which the
ventricle must work, with emphasis on pressure. Thus,
hypertrophy occurs in most types of valvular and congenital disease, sometimes causing heart weights as
great as 800 grams instead of the normal 300 grams.
Use of Extracorporeal
Circulation During
Cardiac Surgery
References
It is almost impossible to repair intracardiac defects
surgically while the heart is still pumping. Therefore,
many types of artificial heart-lung machines have been
developed to take the place of the heart and lungs
during the course of operation. Such a system is called
extracorporeal circulation. The system consists principally of a pump and an oxygenating device. Almost
any type of pump that does not cause hemolysis of the
blood seems to be suitable.
Methods used for oxygenating blood include (1)
bubbling oxygen through the blood and removing the
bubbles from the blood before passing it back into the
patient, (2) dripping the blood downward over the surfaces of plastic sheets in the presence of oxygen, (3)
passing the blood over surfaces of rotating discs, or (4)
Arad M, Seidman JG, Seidman CE: Phenotypic diversity in
hypertrophic cardiomyopathy. Hum Mol Genet 11:2499,
2002.
Braunwald E, Seidman CE, Sigwart U: Contemporary evaluation and management of hypertrophic cardiomyopathy.
Circulation 106:1312, 2002.
Brickner ME, Hillis LD, Lange RA: Congenital heart disease
in adults: second of two parts. N Engl J Med 342:334, 2000.
Gottdiener JS: Overview of stress echocardiography: uses,
advantages, and limitations. Curr Probl Cardiol 28:485,
2003.
Grech ED: Non-coronary percutaneous intervention. BMJ
327:97, 2003.
Guidelines for the management of patients with valvular
heart disease. A report of the American College of Cardiology/American Heart Association Task Force on Practice
Guidelines. Circulation 98:1949, 1998.
Hoffman JI, Kaplan S: The incidence of congenital heart
disease. J Am Coll Cardiol 39:1890, 2002.
Chapter 23
Heart Valves and Heart Sounds; Dynamics of Valvular and Congenital Heart Defects
Levi DS, Alejos JC, Moore JW: Future of interventional cardiology in pediatrics. Curr Opin Cardiol 18:79, 2003.
Maron BJ: Hypertrophic cardiomyopathy: a systematic
review. JAMA 287:1308, 2002.
McDonald M, Currie BJ, Carapetis JR: Acute rheumatic
fever: a chink in the chain that links the heart to the
throat? Lancet Infect Dis 4:240, 2004.
Nishimura RA, Holmes DR Jr: Clinical practice: hypertrophic obstructive cardiomyopathy. N Engl J Med 350:
1320, 2004.
Reimold SC, Rutherford JD: Clinical practice: valvular heart
disease in pregnancy. N Engl J Med 349:52, 2003.
277
Rosenhek R, Burder T, Pozenta G, et al: Predictors of
outcome in severe, asymptomatic aortic stenosis. N Engl J
Med 343:611, 2000.
Turgeman Y, Atar S, Rosenfeld T: The subvalvular apparatus
in rheumatic mitral stenosis: methods of assessment and
therapeutic implications. Chest 124:1929, 2003.
Yacoub MH, Cohn LH: Novel approaches to cardiac valve
repair: from structure to function. Part II. Circulation
109:942, 2004.
Yoerger DM, Weyman AE: Hypertrophic obstructive cardiomyopathy: mechanism of obstruction and response to
therapy. Rev Cardiovasc Med 4:199, 2003.
C
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P
T
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2
Circulatory Shock and
Physiology of Its Treatment
Circulatory shock means generalized inadequate
blood flow through the body, to the extent that the
body tissues are damaged because of too little flow,
especially because of too little oxygen and other
nutrients delivered to the tissue cells. Even the cardiovascular system itself—the heart musculature,
walls of the blood vessels, vasomotor system, and
other circulatory parts—begins to deteriorate, so
that the shock, once begun, is prone to become progressively worse.
Physiologic Causes of Shock
Circulatory Shock Caused by Decreased Cardiac Output
Shock usually results from inadequate cardiac output. Therefore, any condition
that reduces the cardiac output far below normal will likely lead to circulatory
shock. Two types of factors can severely reduce cardiac output:
1. Cardiac abnormalities that decrease the ability of the heart to pump blood.
These include especially myocardial infarction but also toxic states of
the heart, severe heart valve dysfunction, heart arrhythmias, and other
conditions. The circulatory shock that results from diminished cardiac
pumping ability is called cardiogenic shock. This is discussed in detail in
Chapter 22, where it is pointed out that as many as 85 per cent of people
who develop cardiogenic shock do not survive.
2. Factors that decrease venous return also decrease cardiac output because
the heart cannot pump blood that does not flow into it. The most common
cause of decreased venous return is diminished blood volume, but venous
return can also be reduced as a result of decreased vascular tone, especially
of the venous blood reservoirs, or obstruction to blood flow at some point
in the circulation, especially in the venous return pathway to the heart.
Circulatory Shock That Occurs Without Diminished
Cardiac Output
Occasionally, the cardiac output is normal or even greater than normal, yet the
person is in circulatory shock. This can result from (1) excessive metabolism of
the body, so that even a normal cardiac output is inadequate, or (2) abnormal
tissue perfusion patterns, so that most of the cardiac output is passing through
blood vessels besides those that supply the local tissues with nutrition.
The specific causes of shock are discussed later in the chapter. For the present,
it is important to note that all of them lead to inadequate delivery of nutrients
to critical tissues and critical organs and also cause inadequate removal of cellular waste products from the tissues.
278
4
Chapter 24
279
Circulatory Shock and Physiology of Its Treatment
In the minds of many physicians, the arterial pressure
level is the principal measure of adequacy of circulatory function. However, the arterial pressure can often
be seriously misleading. At times, a person may be in
severe shock and still have an almost normal arterial
pressure because of powerful nervous reflexes that
keep the pressure from falling. At other times, the
arterial pressure can fall to half of normal, but the
person still has normal tissue perfusion and is not in
shock.
In most types of shock, especially shock caused by
severe blood loss, the arterial blood pressure decreases
at the same time as the cardiac output decreases,
although usually not as much.
Cardiac output and
arterial pressure
(percentage of normal)
What Happens to the Arterial Pressure
in Circulatory Shock?
Arterial
pressure
100
50
Cardiac
output
0
0
10
20
30
40
50
Percentage of total blood removed
Figure 24–1
Effect of hemorrhage on cardiac output and arterial pressure.
Tissue Deterioration Is the End Result
of Circulatory Shock, Whatever
the Cause
Once circulatory shock reaches a critical state of severity, regardless of its initiating cause, the shock itself
breeds more shock. That is, the inadequate blood flow
causes the body tissues to begin deteriorating, including the heart and circulatory system itself. This causes
an even greater decrease in cardiac output, and a
vicious circle ensues, with progressively increasing circulatory shock, less adequate tissue perfusion, more
shock, and so forth until death. It is with this late stage
of circulatory shock that we are especially concerned,
because appropriate physiologic treatment can often
reverse the rapid slide to death.
Stages of Shock
Because the characteristics of circulatory shock
change with different degrees of severity, shock is
divided into the following three major stages:
1. A nonprogressive stage (sometimes called
the compensated stage), in which the normal
circulatory compensatory mechanisms eventually
cause full recovery without help from outside
therapy.
2. A progressive stage, in which, without therapy,
the shock becomes steadily worse until death.
3. An irreversible stage, in which the shock has
progressed to such an extent that all forms of
known therapy are inadequate to save the
person’s life, even though, for the moment, the
person is still alive.
Now, let us discuss the stages of circulatory shock
caused by decreased blood volume, which illustrate the
basic principles. Then we can consider special characteristics of shock initiated by other causes.
Shock Caused by
Hypovolemia—Hemorrhagic
Shock
Hypovolemia means diminished blood volume. Hemorrhage is the most common cause of hypovolemic
shock. Hemorrhage decreases the filling pressure of the
circulation and, as a consequence, decreases venous
return. As a result, the cardiac output falls below
normal, and shock may ensue.
Relationship of Bleeding Volume to
Cardiac Output and Arterial Pressure
Figure 24–1 shows the approximate effects on both
cardiac output and arterial pressure of removing blood
from the circulatory system over a period of about 30
minutes. About 10 per cent of the total blood volume
can be removed with almost no effect on either arterial pressure or cardiac output, but greater blood loss
usually diminishes the cardiac output first and later the
arterial pressure, both of which fall to zero when about
35 to 45 per cent of the total blood volume has been
removed.
Sympathetic Reflex Compensations in Shock—Their Special
Value to Maintain Arterial Pressure. The decrease in arte-
rial pressure after hemorrhage—as well as decreases
in pressures in the pulmonary arteries and veins in the
thorax—causes powerful sympathetic reflexes (initiated mainly by the arterial baroreceptors and other
vascular stretch receptors, as explained in Chapter 18).
These reflexes stimulate the sympathetic vasoconstrictor system throughout the body, resulting in three
important effects: (1) The arterioles constrict in most
parts of the systemic circulation, thereby increasing
the total peripheral resistance. (2) The veins and
venous reservoirs constrict, thereby helping to maintain adequate venous return despite diminished blood
280
Arterial pressure
(percentage of control value)
Unit IV
The Circulation
I
100
90
80
70
60
50
40
30
20
10
0
II
III
IV
V
VI
0
60
120
180
240
300
Time in minutes
volume. (3) Heart activity increases markedly, sometimes increasing the heart rate from the normal value
of 72 beats/min to as high as 160 to 180 beats/min.
Value of the Sympathetic Nervous Reflexes. In the
absence of the sympathetic reflexes, only 15 to 20 per
cent of the blood volume can be removed over a
period of 30 minutes before a person dies; this is in
contrast to a 30 to 40 per cent loss of blood volume
that a person can sustain when the reflexes are intact.
Therefore, the reflexes extend the amount of blood
loss that can occur without causing death to about
twice that which is possible in their absence.
360
Figure 24–2
Time course of arterial pressure in dogs after different degrees of acute hemorrhage. Each curve
represents average results from six dogs.
The sympathetic stimulation does not cause significant
constriction of either the cerebral or the cardiac
vessels. In addition, in both these vascular beds, local
blood flow autoregulation is excellent, which prevents
moderate decreases in arterial pressure from significantly decreasing their blood flows. Therefore, blood
flow through the heart and brain is maintained essentially at normal levels as long as the arterial pressure
does not fall below about 70 mm Hg, despite the fact
that blood flow in some other areas of the body might
be decreased to as little as one third to one quarter
normal by this time because of vasoconstriction.
Greater Effect of the Sympathetic Nervous Reflexes in
Maintaining Arterial Pressure than in Maintaining
Cardiac Output. Referring again to Figure 24–1, note
Progressive and Nonprogressive
Hemorrhagic Shock
that the arterial pressure is maintained at or near
normal levels in the hemorrhaging person longer than
is the cardiac output. The reason for this is that the
sympathetic reflexes are geared more for maintaining
arterial pressure than for maintaining output. They
increase the arterial pressure mainly by increasing the
total peripheral resistance, which has no beneficial
effect on cardiac output; however, the sympathetic constriction of the veins is important to keep venous return
and cardiac output from falling too much, in addition
to their role in maintaining arterial pressure.
Especially interesting is the second plateau occurring at about 50 mm Hg in the arterial pressure curve
of Figure 24–1. This results from activation of the
central nervous system ischemic response, which
causes extreme stimulation of the sympathetic nervous
system when the brain begins to suffer from lack of
oxygen or from excess buildup of carbon dioxide, as
discussed in Chapter 18. This effect of the central
nervous system ischemic response can be called the
“last-ditch stand” of the sympathetic reflexes in their
attempt to keep the arterial pressure from falling too
low.
Figure 24–2 shows an experiment that we performed
in dogs to demonstrate the effects of different degrees
of sudden acute hemorrhage on the subsequent course
of arterial pressure. The dogs were bled rapidly until
their arterial pressures fell to different levels. Those
dogs whose pressures fell immediately to no lower
than 45 mm Hg (groups I, II, and III) all eventually
recovered; the recovery occurred rapidly if the pressure fell only slightly (group I) but occurred slowly if
it fell almost to the 45 mm Hg level (group III). When
the arterial pressure fell below 45 mm Hg (groups IV,
V, and VI), all the dogs died, although many of them
hovered between life and death for hours before the
circulatory system deteriorated to the stage of death.
This experiment demonstrates that the circulatory
system can recover as long as the degree of hemorrhage is no greater than a certain critical amount.
Crossing this critical threshold by even a few milliliters
of blood loss makes the eventual difference between
life and death. Thus, hemorrhage beyond a certain critical level causes shock to become progressive. That is,
the shock itself causes still more shock, and the condition becomes a vicious circle that eventually leads to
deterioration of the circulation and to death.
Protection of Coronary and Cerebral Blood Flow by
the Reflexes. A special value of the maintenance
Nonprogressive Shock—Compensated Shock
of normal arterial pressure even in the presence of
decreasing cardiac output is protection of blood flow
through the coronary and cerebral circulatory systems.
If shock is not severe enough to cause its own progression, the person eventually recovers. Therefore,
shock of this lesser degree is called nonprogressive
Chapter 24
Circulatory Shock and Physiology of Its Treatment
shock. It is also called compensated shock, meaning
that the sympathetic reflexes and other factors compensate enough to prevent further deterioration of the
circulation.
The factors that cause a person to recover from
moderate degrees of shock are all the negative feedback control mechanisms of the circulation that
attempt to return cardiac output and arterial pressure
back to normal levels. They include the following:
1. Baroreceptor reflexes, which elicit powerful
sympathetic stimulation of the circulation.
2. Central nervous system ischemic response,
which elicits even more powerful sympathetic
stimulation throughout the body but is not
activated significantly until the arterial pressure
falls below 50 mm Hg.
3. Reverse stress-relaxation of the circulatory system,
which causes the blood vessels to contract around
the diminished blood volume, so that the blood
volume that is available more adequately fills the
circulation.
4. Formation of angiotensin by the kidneys, which
constricts the peripheral arteries and also causes
decreased output of water and salt by the
kidneys, both of which help prevent progression
of shock.
5. Formation of vasopressin (antidiuretic hormone)
by the posterior pituitary gland, which constricts
the peripheral arteries and veins and greatly
increases water retention by the kidneys.
6. Compensatory mechanisms that return the blood
volume back toward normal, including absorption
of large quantities of fluid from the intestinal
tract, absorption of fluid into the blood capillaries
from the interstitial spaces of the body,
conservation of water and salt by the kidneys, and
increased thirst and increased appetite for salt,
which make the person drink water and eat salty
foods if able.
The sympathetic reflexes provide immediate help
toward bringing about recovery because they become
maximally activated within 30 seconds to a minute
after hemorrhage.
The angiotensin and vasopressin mechanisms, as
well as the reverse stress-relaxation that causes
contraction of the blood vessels and venous reservoirs,
all require 10 minutes to 1 hour to respond completely,
but they aid greatly in increasing the arterial pressure
or increasing the circulatory filling pressure and
thereby increasing the return of blood to the heart.
Finally, readjustment of blood volume by absorption
of fluid from the interstitial spaces and intestinal tract,
as well as oral ingestion and absorption of additional
quantities of water and salt, may require from 1 to 48
hours, but recovery eventually takes place, provided
the shock does not become severe enough to enter the
progressive stage.
“Progressive Shock” Is Caused by a Vicious
Circle of Cardiovascular Deterioration
Figure 24–3 shows some of the positive feedbacks that
further depress cardiac output in shock, thus causing
281
the shock to become progressive. Some of the more
important feedbacks are the following.
Cardiac Depression. When the arterial pressure falls low
enough, coronary blood flow decreases below that
required for adequate nutrition of the myocardium.
This weakens the heart muscle and thereby decreases
the cardiac output more. Thus, a positive feedback
cycle has developed, whereby the shock becomes more
and more severe.
Figure 24–4 shows cardiac output curves extrapolated to the human heart from experiments in dogs,
demonstrating progressive deterioration of the heart
at different times after the onset of shock. A dog was
bled until the arterial pressure fell to 30 mm Hg, and
the pressure was held at this level by further bleeding
or retransfusion of blood as required. Note from the
second curve in the figure that there was little deterioration of the heart during the first 2 hours, but by 4
hours, the heart had deteriorated about 40 per cent;
then, rapidly, during the last hour of the experiment
(after 4 hours of low coronary blood pressure), the
heart deteriorated completely.
Thus, one of the important features of progressive
shock, whether it is hemorrhagic in origin or caused in
another way, is eventual progressive deterioration of
the heart. In the early stages of shock, this plays very
little role in the condition of the person, partly because
deterioration of the heart is not severe during the first
hour or so of shock, but mainly because the heart has
tremendous reserve capability that normally allows
it to pump 300 to 400 per cent more blood than is
required by the body for adequate bodywide tissue
nutrition. In the latest stages of shock, however, deterioration of the heart is probably the most important
factor in the final lethal progression of the shock.
Vasomotor Failure. In the early stages of shock, various
circulatory reflexes cause intense activity of the
sympathetic nervous system. This, as discussed earlier,
helps delay depression of the cardiac output and
especially helps prevent decreased arterial pressure.
However, there comes a point when diminished blood
flow to the brain’s vasomotor center depresses the
center so much that it, too, becomes progressively less
active and finally totally inactive. For instance, complete circulatory arrest to the brain causes, during the
first 4 to 8 minutes, the most intense of all sympathetic
discharges, but by the end of 10 to 15 minutes, the vasomotor center becomes so depressed that no further
evidence of sympathetic discharge can be demonstrated. Fortunately, the vasomotor center usually does
not fail in the early stages of shock if the arterial
pressure remains above 30 mm Hg.
Blockage of Very Small Vessels—“Sludged Blood.” In time,
blockage occurs in many of the very small blood
vessels in the circulatory system, and this also causes
the shock to progress. The initiating cause of this
blockage is sluggish blood flow in the microvessels.
Because tissue metabolism continues despite the low
flow, large amounts of acid, both carbonic acid and
282
Unit IV
The Circulation
Decreased cardiac output
Decreased arterial pressure
Decreased systemic blood flow
Decreased cardiac nutrition
Decreased nutrition of tissues
Decreased nutrition of brain
Decreased nutrition
of vascular system
Intravascular clotting
Tissue ischemia
Decreased vasomotor
activity
Increased
capillary
permeability
Release of
toxins
Vascular dilation
Venous pooling
of blood
Cardiac depression
Decreased
blood volume
Decreased venous return
Figure 24–3
Different types of “positive feedback” that can lead to progression of shock.
lactic acid, continue to empty into the local blood
vessels and greatly increase the local acidity of the
blood. This acid, plus other deterioration products
from the ischemic tissues, causes local blood agglutination, resulting in minute blood clots, leading to very
small plugs in the small vessels. Even if the vessels do
not become plugged, an increased tendency for the
blood cells to stick to one another makes it more difficult for blood to flow through the microvasculature,
giving rise to the term sludged blood.
Increased Capillary Permeability. After many hours of
capillary hypoxia and lack of other nutrients, the
permeability of the capillaries gradually increases,
and large quantities of fluid begin to transude into the
tissues. This decreases the blood volume even more,
with a resultant further decrease in cardiac output,
making the shock still more severe. Capillary hypoxia
does not cause increased capillary permeability until
the late stages of prolonged shock.
Release of Toxins by Ischemic Tissue. Throughout the
history of research in the field of shock, it has been
suggested that shock causes tissues to release toxic
substances, such as histamine, serotonin, and tissue
enzymes, that cause further deterioration of the circulatory system. Quantitative studies have proved the
significance of at least one toxin, endotoxin, in some
types of shock.
Cardiac Depression Caused by Endotoxin. Endotoxin
is released from the bodies of dead gram-negative
bacteria in the intestines. Diminished blood flow to
the intestines often causes enhanced formation and
absorption of this toxic substance. The circulating
toxin then causes increased cellular metabolism
despite inadequate nutrition of the cells; this has a specific effect on the heart muscle, causing cardiac depression. Endotoxin can play a major role in some types
of shock, especially “septic shock,” discussed later in
the chapter.
Chapter 24
Circulatory Shock and Physiology of Its Treatment
283
15
Cardiac output (L/min)
0 time
2 hours
10
4 hours
41/2 hours
43/4 hours
5
5 hours
0
–4
0
4
8
12
Right atrial pressure (mm Hg)
Figure 24–4
Cardiac output curves of the heart at different times after hemorrhagic shock begins. (These curves are extrapolated to the human
heart from data obtained in dog experiments by Dr. J. W. Crowell.)
Figure 24–5
Generalized Cellular Deterioration. As shock becomes
severe, many signs of generalized cellular deterioration occur throughout the body. One organ especially
affected is the liver, as illustrated in Figure 24–5. This
occurs mainly because of lack of enough nutrients to
support the normally high rate of metabolism in liver
cells, but also partly because of the extreme exposure
of the liver cells to any vascular toxin or other abnormal metabolic factor occurring in shock.
Among the damaging cellular effects that are
known to occur in most body tissues are the following:
1. Active transport of sodium and potassium
through the cell membrane is greatly diminished.
As a result, sodium and chloride accumulate in
the cells, and potassium is lost from the cells. In
addition, the cells begin to swell.
2. Mitochondrial activity in the liver cells, as well as
in many other tissues of the body, becomes
severely depressed.
3. Lysosomes in the cells in widespread tissue areas
begin to break open, with intracellular release of
hydrolases that cause further intracellular
deterioration.
4. Cellular metabolism of nutrients, such as glucose,
eventually becomes greatly depressed in the last
stages of shock. The actions of some hormones
are depressed as well, including almost 100 per
cent depression of the action of insulin.
All these effects contribute to further deterioration
of many organs of the body, including especially (1)
the liver, with depression of its many metabolic and
detoxification functions; (2) the lungs, with eventual
development of pulmonary edema and poor ability to
oxygenate the blood; and (3) the heart, thereby further
depressing its contractility.
Necrosis of the central portion of a liver lobule in severe circulatory shock. (Courtesy Dr. J. W. Crowell.)
Tissue Necrosis in Severe Shock—Patchy Areas of
Necrosis Occur Because of Patchy Blood Flows in
Different Organs. Not all cells of the body are equally
damaged by shock, because some tissues have better
blood supplies than others. For instance, the cells adjacent to the arterial ends of capillaries receive better
nutrition than cells adjacent to the venous ends of the
same capillaries. Therefore, one would expect more
nutritive deficiency around the venous ends of capillaries than elsewhere. This is precisely the effect that
Crowell discovered in studying tissue areas in many
parts of the body. For instance, Figure 24–5 shows
necrosis in the center of a liver lobule, the portion of
the lobule that is last to be exposed to the blood as it
passes through the liver sinusoids.
Similar punctate lesions occur in heart muscle,
although here a definite repetitive pattern, such as
occurs in the liver, cannot be demonstrated. Nevertheless, the cardiac lesions play an important role in
leading to the final irreversible stage of shock. Deteriorative lesions also occur in the kidneys, especially in
the epithelium of the kidney tubules, leading to kidney
failure and occasionally uremic death several days
later. Deterioration of the lungs also often leads to respiratory distress and death several days later—called
the shock lung syndrome.
Acidosis in Shock. Most metabolic derangements that
occur in shocked tissue can lead to blood acidosis all
through the body. This results from poor delivery of
oxygen to the tissues, which greatly diminishes oxidative metabolism of the foodstuffs. When this occurs,
the cells obtain most of their energy by the anaerobic
The Circulation
process of glycolysis, which leads to tremendous quantities of excess lactic acid in the blood. In addition, poor
blood flow through tissues prevents normal removal
of carbon dioxide. The carbon dioxide reacts locally
in the cells with water to form high concentrations of
intracellular carbonic acid; this, in turn, reacts with
various tissue chemicals to form still other intracellular acidic substances. Thus, another deteriorative effect
of shock is both generalized and local tissue acidosis,
leading to further progression of the shock itself.
Positive Feedback Deterioration of Tissues
in Shock and the Vicious Circle of
Progressive Shock
All the factors just discussed that can lead to further
progression of shock are types of positive feedback.
That is, each increase in the degree of shock causes a
further increase in the shock.
However, positive feedback does not necessarily
lead to a vicious circle. Whether a vicious circle develops depends on the intensity of the positive feedback.
In mild degrees of shock, the negative feedback mechanisms of the circulation—sympathetic reflexes,
reverse stress-relaxation mechanism of the blood
reservoirs, absorption of fluid into the blood from the
interstitial spaces, and others—can easily overcome
the positive feedback influences and, therefore, cause
recovery. But in severe degrees of shock, the deteriorative feedback mechanisms become more and more
powerful, leading to such rapid deterioration of the
circulation that all the normal negative feedback
systems of circulatory control acting together cannot
return the cardiac output to normal.
Considering once again the principles of positive
feedback and vicious circle discussed in Chapter 1, one
can readily understand why there is a critical cardiac
output level above which a person in shock recovers
and below which a person enters a vicious circle of circulatory deterioration that proceeds until death.
Irreversible Shock
After shock has progressed to a certain stage, transfusion or any other type of therapy becomes incapable
of saving the person’s life. The person is then said to
be in the irreversible stage of shock. Ironically, even in
this irreversible stage, therapy can, on rare occasions,
return the arterial pressure and even the cardiac
output to normal or near normal for short periods, but
the circulatory system nevertheless continues to deteriorate, and death ensues in another few minutes to
few hours.
Figure 24–6 demonstrates this effect, showing that
transfusion during the irreversible stage can sometimes cause the cardiac output (as well as the arterial
pressure) to return to normal. However, the cardiac
output soon begins to fall again, and subsequent transfusions have less and less effect. By this time, multiple
deteriorative changes have occurred in the muscle
cells of the heart that may not necessarily affect
100
Hemorrhage
Unit IV
Cardiac output
(percentage of normal)
284
75
50
Progressive
stage
Transfusion
25
Irreversible shock
0
0
30
60
90
Minutes
120
150
Figure 24–6
Failure of transfusion to prevent death in irreversible shock.
the heart’s immediate ability to pump blood but, over
a long period, depress heart pumping enough to cause
death. Beyond a certain point, so much tissue damage
has occurred, so many destructive enzymes have been
released into the body fluids, so much acidosis has
developed, and so many other destructive factors are
now in progress that even a normal cardiac output for
a few minutes cannot reverse the continuing deterioration. Therefore, in severe shock, a stage is eventually
reached at which the person will die even though vigorous therapy might still return the cardiac output to
normal for short periods.
Depletion of Cellular High-Energy Phosphate Reserves in
Irreversible Shock. The high-energy phosphate reserves
in the tissues of the body, especially in the liver and
the heart, are greatly diminished in severe degrees of
shock. Essentially all the creatine phosphate has been
degraded, and almost all the adenosine triphosphate
has downgraded to adenosine diphosphate, adenosine
monophosphate, and, eventually, adenosine. Then
much of this adenosine diffuses out of the cells into the
circulating blood and is converted into uric acid, a
substance that cannot re-enter the cells to reconstitute
the adenosine phosphate system. New adenosine can
be synthesized at a rate of only about 2 per cent of
the normal cellular amount an hour, meaning that
once the high-energy phosphate stores of the cells are
depleted, they are difficult to replenish.
Thus, one of the most devastating end results of
deterioration in shock, and the one that is perhaps
most significant for development of the final state of
irreversibility, is this cellular depletion of these highenergy compounds.
Hypovolemic Shock Caused
by Plasma Loss
Loss of plasma from the circulatory system, even
without loss of red blood cells, can sometimes be
severe enough to reduce the total blood volume
Chapter 24
Circulatory Shock and Physiology of Its Treatment
markedly, causing typical hypovolemic shock similar in
almost all details to that caused by hemorrhage. Severe
plasma loss occurs in the following conditions:
1. Intestinal obstruction is often a cause of severely
reduced plasma volume. Distention of the
intestine in intestinal obstruction partly blocks
venous blood flow in the intestinal walls, which
increases intestinal capillary pressure. This in turn
causes fluid to leak from the capillaries into the
intestinal walls and also into the intestinal lumen.
Because the lost fluid has a high protein content,
the result is reduced total blood plasma protein as
well as reduced plasma volume.
2. In almost all patients who have severe burns or
other denuding conditions of the skin, so much
plasma is lost through the denuded skin areas
that the plasma volume becomes markedly
reduced.
The hypovolemic shock that results from plasma
loss has almost the same characteristics as the shock
caused by hemorrhage, except for one additional complicating factor: the blood viscosity increases greatly as
a result of increased red blood cell concentration in
the remaining blood, and this exacerbates the sluggishness of blood flow.
Loss of fluid from all fluid compartments of the
body is called dehydration; this, too, can reduce the
blood volume and cause hypovolemic shock similar to
that resulting from hemorrhage. Some of the causes of
this type of shock are (1) excessive sweating, (2) fluid
loss in severe diarrhea or vomiting, (3) excess loss of
fluid by nephrotic kidneys, (4) inadequate intake of
fluid and electrolytes, or (5) destruction of the adrenal
cortices, with loss of aldosterone secretion and consequent failure of the kidneys to reabsorb sodium,
chloride, and water, which occurs in the absence of
the adrenocortical hormone aldosterone.
Hypovolemic Shock Caused
by Trauma
One of the most common causes of circulatory shock
is trauma to the body. Often the shock results simply
from hemorrhage caused by the trauma, but it can also
occur even without hemorrhage, because extensive
contusion of the body can damage the capillaries
sufficiently to allow excessive loss of plasma into the
tissues. This results in greatly reduced plasma volume,
with resultant hypovolemic shock.
Various attempts have been made to implicate toxic
factors released by the traumatized tissues as one of
the causes of shock after trauma. However, crosstransfusion experiments into normal animals have
failed to show significant toxic elements.
In summary, traumatic shock seems to result
mainly from hypovolemia, although there might also
be a moderate degree of concomitant neurogenic
shock caused by loss of vasomotor tone, as discussed
next.
285
Neurogenic Shock—Increased
Vascular Capacity
Shock occasionally results without any loss of blood
volume. Instead, the vascular capacity increases so
much that even the normal amount of blood becomes
incapable of filling the circulatory system adequately.
One of the major causes of this is sudden loss of vasomotor tone throughout the body, resulting especially in
massive dilation of the veins. The resulting condition
is known as neurogenic shock.
The role of vascular capacity in helping to regulate
circulatory function was discussed in Chapter 15,
where it was pointed out that either an increase in vascular capacity or a decrease in blood volume reduces
the mean systemic filling pressure, which reduces
venous return to the heart. Diminished venous return
caused by vascular dilation is called venous pooling of
blood.
Causes of Neurogenic Shock. Some neurogenic factors
that can cause loss of vasomotor tone include the
following:
1. Deep general anesthesia often depresses the
vasomotor center enough to cause vasomotor
paralysis, with resulting neurogenic shock.
2. Spinal anesthesia, especially when this extends all
the way up the spinal cord, blocks the sympathetic
nervous outflow from the nervous system and can
be a potent cause of neurogenic shock.
3. Brain damage is often a cause of vasomotor
paralysis. Many patients who have had brain
concussion or contusion of the basal regions of
the brain develop profound neurogenic shock.
Also, even though brain ischemia for a few
minutes almost always causes extreme vasomotor
stimulation, prolonged ischemia (lasting longer
than 5 to 10 minutes) can cause the opposite
effect—total inactivation of the vasomotor
neurons in the brain stem, with consequent
development of severe neurogenic shock.
Anaphylactic Shock and
Histamine Shock
Anaphylaxis is an allergic condition in which the
cardiac output and arterial pressure often decrease
drastically. This is discussed in Chapter 34. It results
primarily from an antigen-antibody reaction that takes
place immediately after an antigen to which the person
is sensitive enters the circulation. One of the principal
effects is to cause the basophils in the blood and mast
cells in the pericapillary tissues to release histamine or
a histamine-like substance. The histamine causes (1) an
increase in vascular capacity because of venous dilation, thus causing a marked decrease in venous return;
(2) dilation of the arterioles, resulting in greatly
reduced arterial pressure; and (3) greatly increased
capillary permeability, with rapid loss of fluid and
286
Unit IV
The Circulation
protein into the tissue spaces. The net effect is a great
reduction in venous return and sometimes such
serious shock that the person dies within minutes.
Intravenous injection of large amounts of histamine
causes “histamine shock,” which has characteristics
almost identical to those of anaphylactic shock.
Septic Shock
A condition that was formerly known by the popular
name “blood poisoning” is now called septic shock
by most clinicians. This refers to a bacterial infection
widely disseminated to many areas of the body, with
the infection being borne through the blood from one
tissue to another and causing extensive damage. There
are many varieties of septic shock because of the many
types of bacterial infections that can cause it and
because infection in different parts of the body
produces different effects.
Septic shock is extremely important to the clinician,
because other than cardiogenic shock, septic shock is
the most frequent cause of shock-related death in the
modern hospital.
Some of the typical causes of septic shock include
the following:
1. Peritonitis caused by spread of infection from the
uterus and fallopian tubes, sometimes resulting
from instrumental abortion performed under
unsterile conditions.
2. Peritonitis resulting from rupture of the
gastrointestinal system, sometimes caused by
intestinal disease and sometimes by wounds.
3. Generalized bodily infection resulting from
spread of a skin infection such as streptococcal
or staphylococcal infection.
4. Generalized gangrenous infection resulting
specifically from gas gangrene bacilli, spreading
first through peripheral tissues and finally by way
of the blood to the internal organs, especially the
liver.
5. Infection spreading into the blood from the
kidney or urinary tract, often caused by colon
bacilli.
Special Features of Septic Shock. Because of the multiple
types of septic shock, it is difficult to categorize this
condition. Some features often observed are:
1. High fever.
2. Often marked vasodilation throughout the body,
especially in the infected tissues.
3. High cardiac output in perhaps half of patients,
caused by arteriolar dilation in the infected tissues
and by high metabolic rate and vasodilation
elsewhere in the body, resulting from bacterial
toxin stimulation of cellular metabolism and from
high body temperature.
4. Sludging of the blood, caused by red cell
agglutination in response to degenerating tissues.
5. Development of micro–blood clots in widespread
areas of the body, a condition called disseminated
intravascular coagulation. Also, this causes the
blood clotting factors to be used up, so that
hemorrhaging occurs in many tissues, especially in
the gut wall of the intestinal tract.
In early stages of septic shock, the patient usually
does not have signs of circulatory collapse but only
signs of the bacterial infection. As the infection
becomes more severe, the circulatory system usually
becomes involved either because of direct extension
of the infection or secondarily as a result of toxins
from the bacteria, with resultant loss of plasma into the
infected tissues through deteriorating blood capillary
walls. There finally comes a point at which deterioration of the circulation becomes progressive in the same
way that progression occurs in all other types of shock.
The end stages of septic shock are not greatly different from the end stages of hemorrhagic shock, even
though the initiating factors are markedly different in
the two conditions.
Physiology of Treatment
in Shock
Replacement Therapy
Blood and Plasma Transfusion. If a person is in shock
caused by hemorrhage, the best possible therapy is
usually transfusion of whole blood. If the shock is
caused by plasma loss, the best therapy is administration of plasma; when dehydration is the cause, administration of an appropriate electrolyte solution can
correct the shock.
Whole blood is not always available, such as under
battlefield conditions. Plasma can usually substitute
adequately for whole blood because it increases the
blood volume and restores normal hemodynamics.
Plasma cannot restore a normal hematocrit, but the
human body can usually stand a decrease in hematocrit to about half of normal before serious consequences result, if cardiac output is adequate.
Therefore, in emergency conditions, it is reasonable to
use plasma in place of whole blood for treatment of
hemorrhagic or most other types of hypovolemic
shock.
Sometimes plasma is unavailable. In these instances,
various plasma substitutes have been developed that
perform almost exactly the same hemodynamic functions as plasma. One of these is dextran solution.
Dextran Solution as a Plasma Substitute. The principal
requirement of a truly effective plasma substitute is
that it remain in the circulatory system—that is, not
filter through the capillary pores into the tissue spaces.
In addition, the solution must be nontoxic and must
contain appropriate electrolytes to prevent derangement of the body’s extracellular fluid electrolytes on
administration.
To remain in the circulation, the plasma substitute
must contain some substance that has a large enough
molecular size to exert colloid osmotic pressure. One
of the most satisfactory substances developed for this
Chapter 24
Circulatory Shock and Physiology of Its Treatment
purpose is dextran, a large polysaccharide polymer of
glucose. Certain bacteria secrete dextran as a byproduct of their growth, and commercial dextran can
be manufactured using a bacterial culture procedure.
By varying the growth conditions of the bacteria, the
molecular weight of the dextran can be controlled to
the desired value. Dextrans of appropriate molecular
size do not pass through the capillary pores and, therefore, can replace plasma proteins as colloid osmotic
agents.
Few toxic reactions have been observed when using
purified dextran to provide colloid osmotic pressure;
therefore, solutions containing this substance have
proved to be a satisfactory substitute for plasma in
most fluid replacement therapy.
Treatment of Shock with
Sympathomimetic Drugs—Sometimes
Useful, Sometimes Not
287
the tissues, giving the patient oxygen to breathe can be
of benefit in many instances. However, this frequently
is far less beneficial than one might expect, because the
problem in most types of shock is not inadequate oxygenation of the blood by the lungs but inadequate
transport of the blood after it is oxygenated.
Treatment with Glucocorticoids (Adrenal Cortex Hormones That
Control Glucose Metabolism). Glucocorticoids are fre-
quently given to patients in severe shock for several
reasons: (1) experiments have shown empirically that
glucocorticoids frequently increase the strength of the
heart in the late stages of shock; (2) glucocorticoids
stabilize lysosomes in tissue cells and thereby prevent
release of lysosomal enzymes into the cytoplasm of the
cells, thus preventing deterioration from this source;
and (3) glucocorticoids might aid in the metabolism of
glucose by the severely damaged cells.
Circulatory Arrest
A sympathomimetic drug is a drug that mimics sympathetic stimulation. These drugs include norepinephrine, epinephrine, and a large number of long-acting
drugs that have the same effect as epinephrine and
norepinephrine.
In two types of shock, sympathomimetic drugs have
proved to be especially beneficial. The first of these is
neurogenic shock, in which the sympathetic nervous
system is severely depressed. Administering a sympathomimetic drug takes the place of the diminished
sympathetic actions and can often restore full circulatory function.
The second type of shock in which sympathomimetic drugs are valuable is anaphylactic shock, in
which excess histamine plays a prominent role. The
sympathomimetic drugs have a vasoconstrictor effect
that opposes the vasodilating effect of histamine.
Therefore, either norepinephrine or another sympathomimetic drug is often lifesaving.
Sympathomimetic drugs have not proved to be very
valuable in hemorrhagic shock. The reason is that in
this type of shock, the sympathetic nervous system is
almost always maximally activated by the circulatory
reflexes already; so much norepinephrine and epinephrine are already circulating in the blood that sympathomimetic drugs have essentially no additional
beneficial effect.
Other Therapy
Treatment by the Head-Down Position. When the pressure
falls too low in most types of shock, especially in
hemorrhagic and neurogenic shock, placing the patient
with the head at least 12 inches lower than the feet
helps tremendously in promoting venous return,
thereby also increasing cardiac output. This headdown position is the first essential step in the treatment of many types of shock.
Oxygen Therapy. Because the major deleterious effect of
most types of shock is too little delivery of oxygen to
A condition closely allied to circulatory shock is circulatory arrest, in which all blood flow stops. This
occurs frequently on the surgical operating table as a
result of cardiac arrest or ventricular fibrillation.
Ventricular fibrillation can usually be stopped by
strong electroshock of the heart, the basic principles
of which are described in Chapter 13.
Cardiac arrest often results from too little oxygen in
the anesthetic gaseous mixture or from a depressant
effect of the anesthesia itself. A normal cardiac rhythm
can usually be restored by removing the anesthetic and
immediately applying cardiopulmonary resuscitation
procedures, while at the same time supplying the
patient’s lungs with adequate quantities of ventilatory
oxygen.
Effect of Circulatory Arrest
on the Brain
A special problem in circulatory arrest is to prevent
detrimental effects in the brain as a result of the arrest.
In general, more than 5 to 8 minutes of total circulatory arrest can cause at least some degree of permanent brain damage in more than half of patients.
Circulatory arrest for as long as 10 to 15 minutes
almost always permanently destroys significant
amounts of mental power.
For many years, it was taught that this detrimental
effect on the brain was caused by the acute cerebral
hypoxia that occurs during circulatory arrest.
However, experiments have shown that if blood clots
are prevented from occurring in the blood vessels of
the brain, this will also prevent most of the early deterioration of the brain during circulatory arrest. For
instance, in animal experiments performed by Crowell,
all the blood was removed from the animal’s blood
vessels at the beginning of circulatory arrest and
then replaced at the end of circulatory arrest so that
no intravascular blood clotting could occur. In this
288
Unit IV
The Circulation
experiment, the brain was usually able to withstand up
to 30 minutes of circulatory arrest without permanent
brain damage.Also, administration of heparin or streptokinase (to prevent blood coagulation) before cardiac
arrest was shown to increase the survivability of the
brain up to two to four times longer than usual.
It is likely that the severe brain damage that occurs
from circulatory arrest is caused mainly by permanent
blockage of many small blood vessels by blood clots,
thus leading to prolonged ischemia and eventual death
of the neurons.
References
Annane D, Sebille V, Charpentier C, et al: Effect of treatment with low doses of hydrocortisone and fludrocortisone on mortality in patients with septic shock. JAMA
288:862, 2002.
Burry LD, Wax RS: Role of corticosteroids in septic shock.
Ann Pharmacother 38:464, 2004.
Creteur J, Vincent JL: Hemoglobin solutions. Crit Care Med
31(12 Suppl):S698, 2003.
Crowell JW, Guyton AC: Evidence favoring a cardiac mechanism in irreversible hemorrhagic shock. Am J Physiol
201:893, 1961.
Crowell JW, Smith EE: Oxygen deficit and irreversible
hemorrhagic shock. Am J Physiol 206:313, 1964.
de Jonge E, Levi M: Effects of different plasma substitutes
on blood coagulation: a comparative review. Crit Care
Med 29:1261, 2001.
Goodnough LT, Shander A: Evolution in alternatives to
blood transfusion. Hematol J 4:87, 2003.
Granger DN, Stokes KY, Shigematsu T, et al: Splanchnic
ischaemia-reperfusion injury: mechanistic insights provided by mutant mice. Acta Physiol Scand 173:83, 2001.
Guyton AC, Jones CE, Coleman TG: Circulatory Physiology:
Cardiac Output and Its Regulation. Philadelphia: WB
Saunders, 1973.
Ledgerwood AM, Lucas CE: A review of studies on the
effects of hemorrhagic shock and resuscitation on the
coagulation profile. J Trauma 54(5 Suppl):S68, 2003.
Martin GS, Mannino DM, Eaton S, Moss M: The epidemiology of sepsis in the United States from 1979 through 2000.
N Engl J Med 348:1546, 2003.
McLean-Tooke AP, Bethune CA, Fay AC, Spickett GP:
Adrenaline in the treatment of anaphylaxis: what is the
evidence? BMJ 327:1332, 2003.
Proctor KG: Blood substitutes and experimental models of
trauma. J Trauma 54(5 Suppl):S106, 2003.
Rivers E, Nguyen B, Havstad S, et al: The Early GoalDirected Therapy Collaborative Group. Early goaldirected therapy in the treatment of severe sepsis and
septic shock. N Engl J Med 345:1368, 2001.
Toh CH, Dennis M: Disseminated intravascular coagulation:
old disease, new hope. BMJ 327:974, 2003.
Wernly JA: Ischemia, reperfusion, and the role of surgery in
the treatment of cardiogenic shock secondary to acute
myocardial infarction: an interpretative review. J Surg Res
117:6, 2004.
Wilson M, Davis DP, Coimbra R: Diagnosis and monitoring
of hemorrhagic shock during the initial resuscitation of
multiple trauma patients: a review. J Emerg Med 24:413,
2003.
U
N
I
The Body Fluids
and Kidneys
25. The Body Fluid Compartments: Extracellular and
Intracellular Fluids; Interstitial Fluid and Edema
26. Urine Formation by the Kidneys: I. Glomerular
Filtration, Renal Blood Flow, and Their Control
27. Urine Formation by the Kidneys: II. Tubular
Processing of the Glomerular Filtrate
28. Regulation of Extracellular Fluid Osmolarity
and Sodium Concentration
29. Renal Regulation of Potassium, Calcium,
Phosphate, and Magnesium; Integration of Renal
Mechanisms for Control of Blood Volume and
Extracellular Fluid Volume
30. Regulation of Acid-Base Balance
31. Kidney Diseases and Diuretics
T
V
C
H
A
P
T
E
R
2
5
The Body Fluid Compartments:
Extracellular and
Intracellular Fluids;
Interstitial Fluid and Edema
The maintenance of a relatively constant volume
and a stable composition of the body fluids is essential for homeostasis, as discussed in Chapter 1. Some
of the most common and important problems in
clinical medicine arise because of abnormalities in
the control systems that maintain this constancy of
the body fluids. In this chapter and in the following
chapters on the kidneys, we discuss the overall regulation of body fluid volume, constituents of the extracellular fluid, acid-base
balance, and control of fluid exchange between extracellular and intracellular
compartments.
Fluid Intake and Output Are Balanced During
Steady-State Conditions
The relative constancy of the body fluids is remarkable because there is continuous exchange of fluid and solutes with the external environment as well as
within the different compartments of the body. For example, there is a highly
variable fluid intake that must be carefully matched by equal output from the
body to prevent body fluid volumes from increasing or decreasing.
Daily Intake of Water
Water is added to the body by two major sources: (1) it is ingested in the form
of liquids or water in the food, which together normally add about 2100 ml/day
to the body fluids, and (2) it is synthesized in the body as a result of oxidation
of carbohydrates, adding about 200 ml/day. This provides a total water intake
of about 2300 ml/day (Table 25–1). Intake of water, however, is highly variable
among different people and even within the same person on different days,
depending on climate, habits, and level of physical activity.
Daily Loss of Body Water
Insensible Water Loss. Some of the water losses cannot be precisely regulated. For
example, there is a continuous loss of water by evaporation from the respiratory tract and diffusion through the skin, which together account for about
700 ml/day of water loss under normal conditions. This is termed insensible
water loss because we are not consciously aware of it, even though it occurs continually in all living humans.
The insensible water loss through the skin occurs independently of sweating
and is present even in people who are born without sweat glands; the average
water loss by diffusion through the skin is about 300 to 400 ml/day. This loss is
minimized by the cholesterol-filled cornified layer of the skin, which provides a
barrier against excessive loss by diffusion. When the cornified layer becomes
291
The Body Fluids and Kidneys
Table 25–1
Daily Intake and Output of Water (ml/day)
Normal
Prolonged,
Heavy Exercise
Intake
Fluids ingested
From metabolism
Total intake
2100
200
2300
?
200
?
Output
Insensible—skin
Insensible—lungs
Sweat
Feces
Urine
Total output
350
350
100
100
1400
2300
350
650
5000
100
500
6600
denuded, as occurs with extensive burns, the rate of
evaporation can increase as much as 10-fold, to 3 to
5 L/day. For this reason, burn victims must be given
large amounts of fluid, usually intravenously, to
balance fluid loss.
Insensible water loss through the respiratory tract
averages about 300 to 400 ml/day. As air enters the
respiratory tract, it becomes saturated with moisture,
to a vapor pressure of about 47 mm Hg, before it is
expelled. Because the vapor pressure of the inspired
air is usually less than 47 mm Hg, water is continuously
lost through the lungs with respiration. In cold
weather, the atmospheric vapor pressure decreases to
nearly 0, causing an even greater loss of water from
the lungs as the temperature decreases. This explains
the dry feeling in the respiratory passages in cold
weather.
Fluid Loss in Sweat. The amount of water lost by sweating is highly variable, depending on physical activity
and environmental temperature. The volume of sweat
normally is about 100 ml/day, but in very hot weather
or during heavy exercise, water loss in sweat occasionally increases to 1 to 2 L/hour. This would rapidly
deplete the body fluids if intake were not also
increased by activating the thirst mechanism discussed
in Chapter 29.
OUTPUT
•Kidneys
•Lungs
•Feces
•Sweat
•Skin
INTAKE
Plasma
3.0 L
Capillary membrane
Interstitial
fluid
11.0 L
Lymphatics
Unit V
Extracellular
fluid (14.0 L)
292
Cell membrane
Intracellular
fluid
28.0 L
Figure 25–1
Summary of body fluid regulation, including the major body fluid
compartments and the membranes that separate these compartments. The values shown are for an average 70-kilogram person.
20 L/day in a person who has been drinking tremendous amounts of water.
This variability of intake is also true for most of the
electrolytes of the body, such as sodium, chloride, and
potassium. In some people, sodium intake may be as
low as 20 mEq/day, whereas in others, sodium intake
may be as high as 300 to 500 mEq/day. The kidneys are
faced with the task of adjusting the excretion rate of
water and electrolytes to match precisely the intake of
these substances, as well as compensating for excessive
losses of fluids and electrolytes that occur in certain
disease states. In Chapters 26 through 30, we discuss
the mechanisms that allow the kidneys to perform
these remarkable tasks.
Water Loss in Feces. Only a small amount of water
(100 ml/day) normally is lost in the feces. This can
increase to several liters a day in people with severe
diarrhea. For this reason, severe diarrhea can be life
threatening if not corrected within a few days.
Water Loss by the Kidneys. The remaining water loss from
the body occurs in the urine excreted by the kidneys.
There are multiple mechanisms that control the rate
of urine excretion. In fact, the most important means
by which the body maintains a balance between water
intake and output, as well as a balance between intake
and output of most electrolytes in the body, is by controlling the rates at which the kidneys excrete these
substances. For example, urine volume can be as low
as 0.5 L/day in a dehydrated person or as high as
Body Fluid Compartments
The total body fluid is distributed mainly between two
compartments: the extracellular fluid and the intracellular fluid (Figure 25–1). The extracellular fluid is
divided into the interstitial fluid and the blood plasma.
There is another small compartment of fluid that is
referred to as transcellular fluid. This compartment
includes fluid in the synovial, peritoneal, pericardial,
and intraocular spaces, as well as the cerebrospinal
fluid; it is usually considered to be a specialized type
of extracellular fluid, although in some cases, its composition may differ markedly from that of the plasma
or interstitial fluid. All the transcellular fluids together
constitute about 1 to 2 liters.
Chapter 25
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Interstitial Fluid and Edema
In the average 70-kilogram adult human, the total
body water is about 60 per cent of the body weight, or
about 42 liters. This percentage can change, depending
on age, gender, and degree of obesity. As a person
grows older, the percentage of total body weight that
is fluid gradually decreases. This is due in part to the
fact that aging is usually associated with an increased
percentage of the body weight being fat, which
decreases the percentage of water in the body. Because
women normally have more body fat than men, they
contain slightly less water than men in proportion to
their body weight. Therefore, when discussing the
“average” body fluid compartments, we should realize
that variations exist, depending on age, gender, and
percentage of body fat.
Intracellular Fluid Compartment
About 28 of the 42 liters of fluid in the body are inside
the 75 trillion cells and are collectively called the intracellular fluid. Thus, the intracellular fluid constitutes
about 40 per cent of the total body weight in an
“average” person.
The fluid of each cell contains its individual mixture
of different constituents, but the concentrations of
these substances are similar from one cell to another.
In fact, the composition of cell fluids is remarkably
similar even in different animals, ranging from the
most primitive microorganisms to humans. For this
reason, the intracellular fluid of all the different
cells together is considered to be one large fluid
compartment.
Extracellular Fluid
Compartment
All the fluids outside the cells are collectively called
the extracellular fluid. Together these fluids account for
about 20 per cent of the body weight, or about 14 liters
in a normal 70-kilogram adult. The two largest compartments of the extracellular fluid are the interstitial
fluid, which makes up more than three fourths of the
extracellular fluid, and the plasma, which makes up
almost one fourth of the extracellular fluid, or about 3
liters. The plasma is the noncellular part of the blood;
it exchanges substances continuously with the interstitial fluid through the pores of the capillary membranes. These pores are highly permeable to almost all
solutes in the extracellular fluid except the proteins.
Therefore, the extracellular fluids are constantly
mixing, so that the plasma and interstitial fluids have
about the same composition except for proteins, which
have a higher concentration in the plasma.
Blood Volume
Blood contains both extracellular fluid (the fluid in
plasma) and intracellular fluid (the fluid in the red
blood cells). However, blood is considered to be a separate fluid compartment because it is contained in a
293
chamber of its own, the circulatory system. The blood
volume is especially important in the control of cardiovascular dynamics.
The average blood volume of adults is about 7 per
cent of body weight, or about 5 liters. About 60 per
cent of the blood is plasma and 40 per cent is red blood
cells, but these percentages can vary considerably in
different people, depending on gender, weight, and
other factors.
Hematocrit (Packed Red Cell Volume). The hematocrit is
the fraction of the blood composed of red blood cells,
as determined by centrifuging blood in a “hematocrit
tube” until the cells become tightly packed in the
bottom of the tube. It is impossible to completely pack
the red cells together; therefore, about 3 to 4 per cent
of the plasma remains entrapped among the cells, and
the true hematocrit is only about 96 per cent of the
measured hematocrit.
In men, the measured hematocrit is normally about
0.40, and in women, it is about 0.36. In severe anemia,
the hematocrit may fall as low as 0.10, a value that is
barely sufficient to sustain life. Conversely, there are
some conditions in which there is excessive production
of red blood cells, resulting in polycythemia. In these
conditions, the hematocrit can rise to 0.65.
Constituents of Extracellular
and Intracellular Fluids
Comparisons of the composition of the extracellular
fluid, including the plasma and interstitial fluid, and the
intracellular fluid are shown in Figures 25–2 and 25–3
and in Table 25–2.
Ionic Composition of Plasma and
Interstitial Fluid Is Similar
Because the plasma and interstitial fluid are separated
only by highly permeable capillary membranes, their
ionic composition is similar. The most important difference between these two compartments is the higher
concentration of protein in the plasma; because the
capillaries have a low permeability to the plasma proteins, only small amounts of proteins are leaked into
the interstitial spaces in most tissues.
Because of the Donnan effect, the concentration of
positively charged ions (cations) is slightly greater
(about 2 per cent) in the plasma than in the interstitial fluid. The plasma proteins have a net negative
charge and, therefore, tend to bind cations, such as
sodium and potassium ions, thus holding extra
amounts of these cations in the plasma along with the
plasma proteins. Conversely, negatively charged ions
(anions) tend to have a slightly higher concentration
in the interstitial fluid compared with the plasma,
because the negative charges of the plasma proteins
repel the negatively charged anions. For practical
purposes, however, the concentration of ions in the
294
Unit V
Anions
100
50
100
Mg++
Cl-
Cholesterol – 150 mg/dl
Protein
Ca++
K+
HCO3-
Na+
PO –––4 and organic anions
mEq/L
50
0
Phospholipids – 280 mg/dl
EXTRACELLULAR
Cations
INTRACELLULAR
150
The Body Fluids and Kidneys
Neutral fat – 125 mg/dl
Glucose – 100 mg/dl
Urea – 15 mg/dl
Lactic acid – 10 mg/dl
Uric acid – 3 mg/dl
Creatinine – 1.5 mg/dl
Bilirubin – 0.5 mg/dl
Bile salts – trace
150
Figure 25–2
Major cations and anions of the intracellular and extracellular
fluids. The concentrations of Ca++ and Mg++ represent the sum of
these two ions. The concentrations shown represent the total of
free ions and complexed ions.
Figure 25–3
Nonelectrolytes of the plasma.
Table 25–2
Osmolar Substances in Extracellular and Intracellular Fluids
+
Na
K+
Ca++
Mg+
Cl–
HCO3–
HPO4–, H2PO4–
SO4–
Phosphocreatine
Carnosine
Amino acids
Creatine
Lactate
Adenosine triphosphate
Hexose monophosphate
Glucose
Protein
Urea
Others
Total mOsm/L
Corrected osmolar activity (mOsm/L)
Total osmotic pressure at 37∞C (mm Hg)
Plasma (mOsm/L H2O)
Interstitial (mOsm/L H2O)
Intracellular (mOsm/L H2O)
142
4.2
1.3
0.8
108
24
2
0.5
139
4.0
1.2
0.7
108
28.3
2
0.5
2
0.2
1.2
2
0.2
1.2
14
140
0
20
4
10
11
1
45
14
8
9
1.5
5
3.7
5.6
1.2
4
4.8
301.8
282.0
5443
5.6
0.2
4
3.9
300.8
281.0
5423
4
4
10
301.2
281.0
5423
Chapter 25
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Interstitial Fluid and Edema
interstitial fluid and in the plasma is considered to be
about equal.
Referring again to Figure 25–2, one can see that
the extracellular fluid, including the plasma and the
interstitial fluid, contains large amounts of sodium
and chloride ions, reasonably large amounts of bicarbonate ions, but only small quantities of potassium,
calcium, magnesium, phosphate, and organic acid ions.
The composition of extracellular fluid is carefully
regulated by various mechanisms, but especially by the
kidneys, as discussed later. This allows the cells to
remain continually bathed in a fluid that contains the
proper concentration of electrolytes and nutrients for
optimal cell function.
Important Constituents of the
Intracellular Fluid
295
Indicator Mass A = Volume A x Concentration A
Indicator Mass A = Indicator Mass B
Indicator Mass B = Volume B x Concentration B
Volume B = Indicator Mass B / Concentration B
The intracellular fluid is separated from the extracellular fluid by a cell membrane that is highly permeable
to water but not to most of the electrolytes in the body.
In contrast to the extracellular fluid, the intracellular fluid contains only small quantities of sodium and
chloride ions and almost no calcium ions. Instead, it
contains large amounts of potassium and phosphate
ions plus moderate quantities of magnesium and
sulfate ions, all of which have low concentrations in the
extracellular fluid. Also, cells contain large amounts of
protein, almost four times as much as in the plasma.
Measurement of Fluid
Volumes in the Different Body
Fluid Compartments—The
Indicator-Dilution Principle
The volume of a fluid compartment in the body can be
measured by placing an indicator substance in the
compartment, allowing it to disperse evenly throughout the compartment’s fluid, and then analyzing the
extent to which the substance becomes diluted. Figure
25–4 shows this “indicator-dilution” method of measuring the volume of a fluid compartment, which is
based on the principle of conservation of mass. This
means that the total mass of a substance after dispersion in the fluid compartment will be the same as the
total mass injected into the compartment.
In the example shown in Figure 25–4, a small
amount of dye or other substance contained in the
syringe is injected into a chamber, and the substance
is allowed to disperse throughout the chamber until it
becomes mixed in equal concentrations in all areas.
Then a sample of fluid containing the dispersed substance is removed and the concentration is analyzed
chemically, photoelectrically, or by other means. If
none of the substance leaks out of the compartment,
the total mass of substance in the compartment
(Volume B ¥ Concentration B) will equal the total
mass of the substance injected (Volume A ¥ Concentration A). By simple rearrangement of the equa-
Figure 25–4
Indicator-dilution method for measuring fluid volumes.
tion, one can calculate the unknown volume of
chamber B as
Volume B =
Volume A ¥ Concentration A
Concentration B
Note that all one needs to know for this calculation
is (1) the total amount of substance injected into
the chamber (the numerator of the equation) and (2)
the concentration of the fluid in the chamber after the
substance has been dispersed (the denominator).
For example, if 1 milliliter of a solution containing
10 mg/ml of dye is dispersed into chamber B and
the final concentration in the chamber is 0.01 milligram for each milliliter of fluid, the unknown volume
of the chamber can be calculated as follows:
Volume B =
1 ml ¥ 10 mg ml
= 1000 ml
0.01 mg ml
This method can be used to measure the volume of
virtually any compartment in the body as long as (1)
the indicator disperses evenly throughout the compartment, (2) the indicator disperses only in the compartment that is being measured, and (3) the indicator
is not metabolized or excreted. Several substances can
be used to measure the volume of each of the different body fluids.
Determination of Volumes
of Specific Body Fluid
Compartments
Measurement of Total Body Water. Radioactive
water
(tritium, 3H2O) or heavy water (deuterium, 2H2O) can
be used to measure total body water. These forms of
water mix with the total body water within a few hours
after being injected into the blood, and the dilution
296
Unit V
The Body Fluids and Kidneys
Table 25–3
Measurement of Body Fluid Volumes
Volume
Indicators
Total body water
3
Extracellular fluid
22
Intracellular fluid
(Calculated as Total body water –
Extracellular fluid volume)
Plasma volume
125
Blood volume
51
Interstitial fluid
(Calculated as Extracellular fluid
volume - Plasma volume)
H2O, 2H2O, antipyrine
Na, 125I-iothalamate, thiosulfate, inulin
I-albumin, Evans blue dye (T-1824)
Cr-labeled red blood cells, or calculated
as Blood volume = Plasma volume/
(1 - Hematocrit)
From Guyton AC, Hall JE: Human Physiology and Mechanisms of Disease,
6th ed. Philadelphia: WB Saunders, 1997.
principle can be used to calculate total body water
(Table 25–3). Another substance that has been used to
measure total body water is antipyrine, which is very
lipid soluble and can rapidly penetrate cell membranes
and distribute itself uniformly throughout the intracellular and extracellular compartments.
volume can also be calculated if one knows the
hematocrit (the fraction of the total blood volume composed of cells), using the following equation:
Total blood volume =
Plasma volume
1 - Hematocrit
For example, if plasma volume is 3 liters and hematocrit is 0.40, total blood volume would be calculated as
3 liters
= 5 liters
1 - 0.4
Another way to measure blood volume is to inject
into the circulation red blood cells that have been
labeled with radioactive material. After these mix in the
circulation, the radioactivity of a mixed blood sample
can be measured, and the total blood volume can be calculated using the dilution principle. A substance frequently used to label the red blood cells is radioactive
chromium (51Cr), which binds tightly with the red blood
cells.
Regulation of Fluid Exchange
and Osmotic Equilibrium
Between Intracellular and
Extracellular Fluid
Measurement of Extracellular Fluid Volume. The volume of
extracellular fluid can be estimated using any of several
substances that disperse in the plasma and interstitial
fluid but do not readily permeate the cell membrane.
They include radioactive sodium, radioactive chloride,
radioactive iothalamate, thiosulfate ion, and inulin.
When any one of these substances is injected into the
blood, it usually disperses almost completely throughout the extracellular fluid within 30 to 60 minutes. Some
of these substances, however, such as radioactive
sodium, may diffuse into the cells in small amounts.
Therefore, one frequently speaks of the sodium space or
the inulin space, instead of calling the measurement the
true extracellular fluid volume.
of Intracellular Volume. The intracellular
volume cannot be measured directly. However, it can be
calculated as
Calculation
Intracellular volume = Total body water
– Extracellular volume
Measurement of Plasma Volume. To measure plasma
volume, a substance must be used that does not readily
penetrate capillary membranes but remains in the vascular system after injection. One of the most commonly
used substances for measuring plasma volume is serum
albumin labeled with radioactive iodine (125I-albumin).
Also, dyes that avidly bind to the plasma proteins, such
as Evans blue dye (also called T-1824), can be used to
measure plasma volume.
Calculation of Interstitial Fluid Volume. Interstitial fluid
volume cannot be measured directly, but it can be calculated as
Interstitial fluid volume = Extracellular fluid volume
– Plasma volume
Measurement of Blood Volume. If one measures plasma
volume using the methods described earlier, blood
A frequent problem in treating seriously ill patients is
maintaining adequate fluids in one or both of the intracellular and extracellular compartments. As discussed
in Chapter 16 and later in this chapter, the relative
amounts of extracellular fluid distributed between the
plasma and interstitial spaces are determined mainly
by the balance of hydrostatic and colloid osmotic
forces across the capillary membranes.
The distribution of fluid between intracellular and
extracellular compartments, in contrast, is determined
mainly by the osmotic effect of the smaller solutes—
especially sodium, chloride, and other electrolytes—
acting across the cell membrane. The reason for this is
that the cell membranes are highly permeable to water
but relatively impermeable to even small ions such as
sodium and chloride. Therefore, water moves across
the cell membrane rapidly, so that the intracellular
fluid remains isotonic with the extracellular fluid.
In the next section, we discuss the interrelations
between intracellular and extracellular fluid volumes
and the osmotic factors that can cause shifts of fluid
between these two compartments.
Basic Principles of Osmosis
and Osmotic Pressure
The basic principles of osmosis and osmotic pressure
were presented in Chapter 4. Therefore, we review
here only the most important aspects of these principles as they apply to volume regulation.
Osmosis is the net diffusion of water across a selectively permeable membrane from a region of high water
concentration to one that has a lower water concentration. When a solute is added to pure water, this reduces
Chapter 25
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Interstitial Fluid and Edema
the concentration of water in the mixture. Thus, the
higher the solute concentration in a solution, the lower
the water concentration. Further, water diffuses from
a region of low solute concentration (high water concentration) to one with a high solute concentration
(low water concentration).
Because cell membranes are relatively impermeable
to most solutes but highly permeable to water (i.e.,
selectively permeable), whenever there is a higher
concentration of solute on one side of the cell membrane, water diffuses across the membrane toward the
region of higher solute concentration. Thus, if a solute
such as sodium chloride is added to the extracellular
fluid, water rapidly diffuses from the cells through the
cell membranes into the extracellular fluid until the
water concentration on both sides of the membrane
becomes equal. Conversely, if a solute such as sodium
chloride is removed from the extracellular fluid, water
diffuses from the extracellular fluid through the cell
membranes and into the cells. The rate of diffusion of
water is called the rate of osmosis.
Relation Between Moles and Osmoles. Because the water
concentration of a solution depends on the number of
solute particles in the solution, a concentration term is
needed to describe the total concentration of solute
particles, regardless of their exact composition. The
total number of particles in a solution is measured in
osmoles. One osmole (osm) is equal to 1 mole (mol)
(6.02 ¥ 1023) of solute particles. Therefore, a solution
containing 1 mole of glucose in each liter has a concentration of 1 osm/L. If a molecule dissociates into
two ions (giving two particles), such as sodium chloride ionizing to give chloride and sodium ions, then a
solution containing 1 mol/L will have an osmolar concentration of 2 osm/L. Likewise, a solution that contains 1 mole of a molecule that dissociates into three
ions, such as sodium sulfate (Na2SO4), will contain
3 osm/L. Thus, the term osmole refers to the number
of osmotically active particles in a solution rather than
to the molar concentration.
In general, the osmole is too large a unit for expressing osmotic activity of solutes in the body fluids.
The term milliosmole (mOsm), which equals 1/1000
osmole, is commonly used.
Osmolality and Osmolarity. The osmolal concentration of
a solution is called osmolality when the concentration
is expressed as osmoles per kilogram of water; it is
called osmolarity when it is expressed as osmoles per
liter of solution. In dilute solutions such as the body
fluids, these two terms can be used almost synonymously because the differences are small. In most
cases, it is easier to express body fluid quantities in
liters of fluid rather than in kilograms of water. Therefore, most of the calculations used clinically and the
calculations expressed in the next several chapters are
based on osmolarities rather than osmolalities.
Osmotic Pressure. Osmosis of water molecules across a
selectively permeable membrane can be opposed by
applying a pressure in the direction opposite that of
297
the osmosis. The precise amount of pressure required
to prevent the osmosis is called the osmotic pressure.
Osmotic pressure, therefore, is an indirect measurement of the water and solute concentrations of a solution. The higher the osmotic pressure of a solution,
the lower the water concentration and the higher the
solute concentration of the solution.
Relation Between Osmotic Pressure and Osmolarity. The
osmotic pressure of a solution is directly proportional
to the concentration of osmotically active particles in
that solution. This is true regardless of whether the
solute is a large molecule or a small molecule. For
example, one molecule of albumin with a molecular
weight of 70,000 has the same osmotic effect as one
molecule of glucose with a molecular weight of 180.
One molecule of sodium chloride, however, has two
osmotically active particles, Na+ and Cl–, and therefore
has twice the osmotic effect of either an albumin molecule or a glucose molecule. Thus, the osmotic pressure
of a solution is proportional to its osmolarity, a
measure of the concentration of solute particles.
Expressed mathematically, according to van’t Hoff’s
law, osmotic pressure (p) can be calculated as
p = CRT
where C is the concentration of solutes in osmoles per
liter, R is the ideal gas constant, and T is the absolute
temperature in degrees kelvin (273° + centigrade°). If
p is expressed in millimeters of mercury (mm Hg), the
unit of pressure commonly used for biological fluids,
and T is normal body temperature (273° + 37° =
310° kelvin), the value of p calculates to be about
19,300 mm Hg for a solution having a concentration
of 1 osm/L. This means that for a concentration of
1 mOsm/L, p is equal to 19.3 mm Hg. Thus, for each
milliosmole concentration gradient across the cell
membrane, 19.3 mm Hg osmotic pressure is exerted.
Calculation of the Osmolarity and Osmotic Pressure of a
Solution. Using van’t Hoff’s law, one can calculate the
potential osmotic pressure of a solution, assuming that
the cell membrane is impermeable to the solute.
For example, the osmotic pressure of a 0.9 per cent
sodium chloride solution is calculated as follows: A
0.9 per cent solution means that there is 0.9 gram of
sodium chloride per 100 milliliters of solution, or
9 g/L. Because the molecular weight of sodium chloride is 58.5 g/mol, the molarity of the solution is 9 g/L
divided by 58.5 g/mol, or about 0.154 mol/L. Because
each molecule of sodium chloride is equal to
2 osmoles, the osmolarity of the solution is 0.154 ¥ 2,
or 0.308 osm/L. Therefore, the osmolarity of this solution is 308 mOsm/L. The potential osmotic pressure
of this solution would therefore be 308 mOsm/L
¥ 19.3 mm Hg/mOsm/L, or 5944 mm Hg.
This calculation is only an approximation, because
sodium and chloride ions do not behave entirely independently in solution because of interionic attraction
between them. One can correct for these deviations
from the predictions of van’t Hoff’s law by using a
correction factor called the osmotic coefficient. For
298
Unit V
The Body Fluids and Kidneys
sodium chloride, the osmotic coefficient is about 0.93.
Therefore, the actual osmolarity of a 0.9 per cent
sodium chloride solution is 308 ¥ 0.93, or about
286 mOsm/L. For practical reasons, the osmotic coefficients of different solutes are sometimes neglected in
determining the osmolarity and osmotic pressures of
physiologic solutions.
A
Osmolarity of the Body Fluids. Turning back to Table 25–2,
B
note the approximate osmolarity of the various osmotically active substances in plasma, interstitial fluid, and
intracellular fluid. Note that about 80 per cent of the
total osmolarity of the interstitial fluid and plasma is
due to sodium and chloride ions, whereas for intracellular fluid, almost half the osmolarity is due to potassium ions, and the remainder is divided among many
other intracellular substances.
As shown in Table 25–2, the total osmolarity of each
of the three compartments is about 300 mOsm/L, with
the plasma being about 1 mOsm/L greater than that
of the interstitial and intracellular fluids. The slight difference between plasma and interstitial fluid is caused
by the osmotic effects of the plasma proteins, which
maintain about 20 mm Hg greater pressure in the capillaries than in the surrounding interstitial spaces, as
discussed in Chapter 16.
280 mOsm/L
C
ISOTONIC
No change
200 mOsm/L
360 mOsm/L
HYPOTONIC
Cell swells
HYPERTONIC
Cell shrinks
Figure 25–5
Effects of isotonic (A), hypertonic (B), and hypotonic (C) solutions
on cell volume.
Corrected Osmolar Activity of the Body Fluids. At the
bottom of Table 25–2 are shown corrected osmolar
activities of plasma, interstitial fluid, and intracellular
fluid.The reason for these corrections is that molecules
and ions in solution exert interionic and intermolecular attraction or repulsion from one solute molecule to
the next, and these two effects can cause, respectively,
a slight decrease or an increase in the osmotic “activity” of the dissolved substance.
potential osmotic pressure that can develop across the
cell membrane is more than 5400 mm Hg. This demonstrates the large force that can move water across the
cell membrane when the intracellular and extracellular fluids are not in osmotic equilibrium. As a result of
these forces, relatively small changes in the concentration of impermeant solutes in the extracellular fluid
can cause large changes in cell volume.
Total Osmotic Pressure Exerted by the Body Fluids. Table
Isotonic, Hypotonic, and Hypertonic Fluids. The effects of
25–2 also shows the total osmotic pressure in millimeters of mercury that would be exerted by each of the
different fluids if it were placed on one side of the cell
membrane with pure water on the other side. Note
that this total pressure averages about 5443 mm Hg for
plasma, which is 19.3 times the corrected osmolarity of
282 mOsm/L for plasma.
different concentrations of impermeant solutes in the
extracellular fluid on cell volume are shown in Figure
25–5. If a cell is placed in a solution of impermeant
solutes having an osmolarity of 282 mOsm/L, the cells
will not shrink or swell because the water concentration in the intracellular and extracellular fluids is equal
and the solutes cannot enter or leave the cell. Such a
solution is said to be isotonic because it neither shrinks
nor swells the cells. Examples of isotonic solutions
include a 0.9 per cent solution of sodium chloride or a
5 per cent glucose solution. These solutions are important in clinical medicine because they can be infused
into the blood without the danger of upsetting osmotic
equilibrium between the intracellular and extracellular fluids.
If a cell is placed into a hypotonic solution that has
a lower concentration of impermeant solutes (less than
282 mOsm/L), water will diffuse into the cell, causing
it to swell; water will continue to diffuse into the cell,
diluting the intracellular fluid while also concentrating
the extracellular fluid until both solutions have about
the same osmolarity. Solutions of sodium chloride with
a concentration of less than 0.9 per cent are hypotonic
and cause cells to swell.
Osmotic Equilibrium Is
Maintained Between
Intracellular and
Extracellular Fluids
Large osmotic pressures can develop across the cell
membrane with relatively small changes in the concentrations of solutes in the extracellular fluid. As discussed earlier, for each milliosmole concentration
gradient of an impermeant solute (one that will not
permeate the cell membrane), about 19.3 mm Hg
osmotic pressure is exerted across the cell membrane.
If the cell membrane is exposed to pure water and the
osmolarity of intracellular fluid is 282 mOsm/L, the
Chapter 25
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Interstitial Fluid and Edema
If a cell is placed in a hypertonic solution having a
higher concentration of impermeant solutes, water will
flow out of the cell into the extracellular fluid, concentrating the intracellular fluid and diluting the extracellular fluid. In this case, the cell will shrink until the
two concentrations become equal. Sodium chloride
solutions of greater than 0.9 per cent are hypertonic.
Isosmotic, Hyperosmotic, and Hypo-osmotic Fluids. The
terms isotonic, hypotonic, and hypertonic refer to
whether solutions will cause a change in cell volume.
The tonicity of solutions depends on the concentration
of impermeant solutes. Some solutes, however, can
permeate the cell membrane. Solutions with an osmolarity the same as the cell are called isosmotic, regardless of whether the solute can penetrate the cell
membrane.
The terms hyperosmotic and hypo-osmotic refer to
solutions that have a higher or lower osmolarity,
respectively, compared with the normal extracellular
fluid, without regard for whether the solute permeates
the cell membrane. Highly permeating substances,
such as urea, can cause transient shifts in fluid volume
between the intracellular and extracellular fluids, but
given enough time, the concentrations of these substances eventually become equal in the two compartments and have little effect on intracellular volume
under steady-state conditions.
Osmotic Equilibrium Between Intracellular and Extracellular
Fluids Is Rapidly Attained. The transfer of fluid across the
cell membrane occurs so rapidly that any differences
in osmolarities between these two compartments are
usually corrected within seconds or, at the most,
minutes. This rapid movement of water across the cell
membrane does not mean that complete equilibrium
occurs between the intracellular and extracellular
compartments throughout the whole body within the
same short period. The reason for this is that fluid
usually enters the body through the gut and must be
transported by the blood to all tissues before complete
osmotic equilibrium can occur. It usually takes about
30 minutes to achieve osmotic equilibrium everywhere
in the body after drinking water.
Volume and Osmolality of
Extracellular and Intracellular
Fluids in Abnormal States
Some of the different factors that can cause extracellular and intracellular volumes to change markedly are
ingestion of water, dehydration, intravenous infusion
of different types of solutions, loss of large amounts of
fluid from the gastrointestinal tract, and loss of abnormal amounts of fluid by sweating or through the
kidneys.
One can calculate both the changes in intracellular
and extracellular fluid volumes and the types of
therapy that should be instituted if the following basic
principles are kept in mind:
299
1. Water moves rapidly across cell membranes;
therefore, the osmolarities of intracellular and
extracellular fluids remain almost exactly equal
to each other except for a few minutes after a
change in one of the compartments.
2. Cell membranes are almost completely
impermeable to many solutes; therefore, the
number of osmoles in the extracellular or
intracellular fluid generally remains constant
unless solutes are added to or lost from the
extracellular compartment.
With these basic principles in mind, we can analyze
the effects of different abnormal fluid conditions on
extracellular and intracellular fluid volumes and
osmolarities.
Effect of Adding Saline Solution
to the Extracellular Fluid
If an isotonic saline solution is added to the extracellular fluid compartment, the osmolarity of the extracellular fluid does not change; therefore, no osmosis
occurs through the cell membranes. The only effect is
an increase in extracellular fluid volume (Figure
25–6A). The sodium and chloride largely remain in the
extracellular fluid because the cell membrane behaves
as though it were virtually impermeable to the sodium
chloride.
If a hypertonic solution is added to the extracellular
fluid, the extracellular osmolarity increases and causes
osmosis of water out of the cells into the extracellular
compartment (see Figure 25–6B). Again, almost all the
added sodium chloride remains in the extracellular
compartment, and fluid diffuses from the cells into the
extracellular space to achieve osmotic equilibrium.
The net effect is an increase in extracellular volume
(greater than the volume of fluid added), a decrease in
intracellular volume, and a rise in osmolarity in both
compartments.
If a hypotonic solution is added to the extracellular
fluid, the osmolarity of the extracellular fluid decreases
and some of the extracellular water diffuses into the
cells until the intracellular and extracellular compartments have the same osmolarity (see Figure 25–6C).
Both the intracellular and the extracellular volumes
are increased by the addition of hypotonic fluid,
although the intracellular volume increases to a
greater extent.
Calculation of Fluid Shifts and Osmolarities After Infusion
of Hypertonic Saline. We can calculate the sequential
effects of infusing different solutions on extracellular
and intracellular fluid volumes and osmolarities. For
example, if 2 liters of a hypertonic 3.0 per cent sodium
chloride solution are infused into the extracellular
fluid compartment of a 70-kilogram patient whose
initial plasma osmolarity is 280 mOsm/L, what would
be the intracellular and extracellular fluid volumes and
osmolarities after osmotic equilibrium?
The first step is to calculate the initial conditions,
including the volume, concentration, and total
300
Unit V
Intracellular fluid
The Body Fluids and Kidneys
Extracellular fluid
Normal State
A. Add Isotonic NaCl
Osmolarity
300
200
100
0
10
20
30
Volume (liters)
40
Figure 25–6
C. Add Hypotonic NaCl
B. Add Hypertonic NaCl
milliosmoles in each compartment. Assuming that
extracellular fluid volume is 20 per cent of body weight
and intracellular fluid volume is 40 per cent of body
weight, the following volumes and concentrations
can be calculated.
Step 1. Initial Conditions
Extracellular fluid
Intracellular fluid
Total body fluid
Volume
(Liters)
Concentration
(mOsm/L)
Total
(mOsm)
14
28
42
280
280
280
3,920
7,840
11,760
Next, we calculate the total milliosmoles added to
the extracellular fluid in 2 liters of 3.0 per cent sodium
chloride. A 3.0 per cent solution means that there are
3.0 g/100 ml, or 30 grams of sodium chloride per liter.
Because the molecular weight of sodium chloride is
about 58.5 g/mol, this means that there is about 0.513
mole of sodium chloride per liter of solution. For 2
liters of solution, this would be 1.026 mole of sodium
chloride. Because 1 mole of sodium chloride is about
equal to 2 osmoles (sodium chloride has two osmotically active particles per mole), the net effect of adding
2 liters of this solution is to add 2051 milliosmoles of
sodium chloride to the extracellular fluid.
In Step 2, we calculate the instantaneous effect of
adding 2051 milliosmoles of sodium chloride to the
extracellular fluid along with 2 liters of volume. There
would be no change in the intracellular fluid concentration or volume, and there would be no osmotic
equilibrium. In the extracellular fluid, however, there
Effect of adding isotonic, hypertonic, and hypotonic solutions
to the extracellular fluid after
osmotic equilibrium. The normal
state is indicated by the solid
lines, and the shifts from normal
are shown by the shaded areas.
The volumes of intracellular and
extracellular fluid compartments
are shown in the abscissa of each
diagram, and the osmolarities of
these compartments are shown
on the ordinates.
would be an additional 2051 milliosmoles of total
solute, yielding a total of 5791 milliosmoles. Because
the extracellular compartment now has 16 liters of
volume, the concentration can be calculated by dividing 5791 milliosmoles by 16 liters to yield a concentration of 373 mOsm/L. Thus, the following values
would occur instantly after adding the solution.
Step 2. Instantaneous Effect of Adding 2 Liters of
3.0 Per Cent Sodium Chloride
Volume
(Liters)
Extracellular fluid
Intracellular fluid
Total body fluid
16
28
44
Concentration
(mOsm/L)
Total
(mOsm)
373
280
No equilibrium
5,971
7,840
13,811
In the third step, we calculate the volumes and concentrations that would occur within a few minutes
after osmotic equilibrium develops. In this case, the
concentrations in the intracellular and extracellular
fluid compartments would be equal and can be calculated by dividing the total milliosmoles in the body,
13,811, by the total volume, which is now 44 liters. This
yields a concentration of 313.9 mOsm/L. Therefore, all
the body fluid compartments will have this same concentration after osmotic equilibrium. Assuming that
no solute or water has been lost from the body and
that there is no movement of sodium chloride into or
out of the cells, we then calculate the volumes of
the intracellular and extracellular compartments. The
intracellular fluid volume is calculated by dividing the
total milliosmoles in the intracellular fluid (7840) by
Chapter 25
301
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Interstitial Fluid and Edema
the concentration (313.9 mOsm/L), to yield a volume
of 24.98 liters. Extracellular fluid volume is calculated
by dividing the total milliosmoles in extracellular fluid
(5971) by the concentration (313.9 mOsm/L), to yield
a volume of 19.02 liters. Again, these calculations are
based on the assumption that the sodium chloride
added to the extracellular fluid remains there and does
not move into the cells.
Step 3. Effect of Adding 2 Liters of 3.0 Per Cent Sodium
Chloride After Osmotic Equilibrium
Extracellular fluid
Intracellular fluid
Total body fluid
Volume
(Liters)
Concentration
(mOsm/L)
Total
(mOsm)
19.02
24.98
44.0
313.9
313.9
313.9
5,971
7,840
13,811
Thus, one can see from this example that adding 2
liters of a hypertonic sodium chloride solution causes
more than a 5-liter increase in extracellular fluid
volume while decreasing intracellular fluid volume by
almost 3 liters.
This method of calculating changes in intracellular
and extracellular fluid volumes and osmolarities can
be applied to virtually any clinical problem of fluid
volume regulation. The reader should be familiar with
such calculations because an understanding of the
mathematical aspects of osmotic equilibrium between
intracellular and extracellular fluid compartments is
essential for understanding almost all fluid abnormalities of the body and their treatment.
Glucose and Other
Solutions Administered
for Nutritive Purposes
Many types of solutions are administered intravenously
to provide nutrition to people who cannot otherwise
take adequate amounts of nutrition. Glucose solutions
are widely used, and amino acid and homogenized fat
solutions are used to a lesser extent. When these solutions are administered, their concentrations of osmotically active substances are usually adjusted nearly to
isotonicity, or they are given slowly enough that they do
not upset the osmotic equilibrium of the body fluids.
After the glucose or other nutrients are metabolized, an
excess of water often remains, especially if additional
fluid is ingested. Ordinarily, the kidneys excrete this in
the form of a very dilute urine. The net result, therefore,
is the addition of only nutrients to the body.
Clinical Abnormalities of
Fluid Volume Regulation:
Hyponatremia and
Hypernatremia
The primary measurement that is readily available to
the clinician for evaluating a patient’s fluid status is the
plasma sodium concentration. Plasma osmolarity is not
routinely measured, but because sodium and its associated anions (mainly chloride) account for more than
90 per cent of the solute in the extracellular fluid,
plasma sodium concentration is a reasonable indicator
of plasma osmolarity under many conditions. When
plasma sodium concentration is reduced more than a
few milliequivalents below normal (about 142 mEq/L),
a person is said to have hyponatremia. When plasma
sodium concentration is elevated above normal, a
person is said to have hypernatremia.
Causes of Hyponatremia: Excess
Water or Loss of Sodium
Decreased plasma sodium concentration can result
from loss of sodium chloride from the extracellular fluid
or addition of excess water to the extracellular fluid
(Table 25–4). A primary loss of sodium chloride usually
results in hypo-osmotic dehydration and is associated
with decreased extracellular fluid volume. Conditions
that can cause hyponatremia owing to loss of sodium
chloride include diarrhea and vomiting. Overuse of
diuretics that inhibit the ability of the kidneys to conserve sodium and certain types of sodium-wasting
kidney diseases can also cause modest degrees of
hyponatremia. Finally, Addison’s disease, which results
from decreased secretion of the hormone aldosterone,
impairs the ability of the kidneys to reabsorb sodium
and can cause a modest degree of hyponatremia.
Hyponatremia can also be associated with excess
water retention, which dilutes the sodium in the extracellular fluid, a condition that is referred to as hypoosmotic overhydration. For example, excessive secretion
of antidiuretic hormone, which causes the kidney
Table 25–4
Abnormalities of Body Fluid Volume Regulation: Hyponatremia and Hypernatremia
Abnormality
Cause
Hypo-osmotic dehydration
Hypo-osmotic overhydration
Hyper-osmotic dehydration
Hyper-osmotic overhydration
Adrenal insufficiency; overuse of diuretics
Excess ADH; bronchogenic tumor
Diabetes insipidus; excessive sweating
Cushing’s disease; primary aldosteronism
ADH, antidiuretic hormone.
Plasma Na+
Concentration
Extracellular
Fluid Volume
Intracellular
Fluid Volume
Ø
Ø
≠
≠
Ø
≠
Ø
≠
≠
≠
Ø
Ø
302
Unit V
The Body Fluids and Kidneys
tubules to reabsorb more water, can lead to hyponatremia and overhydration.
Causes of Hypernatremia: Water
Loss or Excess Sodium
Increased plasma sodium concentration, which also
causes increased osmolarity, can be due to either loss of
water from the extracellular fluid, which concentrates
the sodium ions, or excess sodium in the extracellular
fluid. When there is primary loss of water from the
extracellular fluid, this results in hyperosmotic dehydration. This condition can occur from an inability to
secrete antidiuretic hormone, which is needed for the
kidneys to conserve water. As a result of lack of antidiuretic hormone, the kidneys excrete large amounts
of dilute urine (a disorder referred to as diabetes
insipidus), causing dehydration and increased concentration of sodium chloride in the extracellular fluid. In
certain types of renal diseases, the kidneys cannot
respond to antidiuretic hormone, also causing a type of
nephrogenic diabetes insipidus. A more common cause
of hypernatremia associated with decreased extracellular fluid volume is dehydration caused by water intake
that is less than water loss, as can occur with sweating
during prolonged, heavy exercise.
Hypernatremia can also occur as a result of excessive
sodium chloride added to the extracellular fluid. This
often results in hyperosmotic overhydration because
excess extracellular sodium chloride is usually associated with at least some degree of water retention by the
kidneys as well. For example, excessive secretion of the
sodium-retaining hormone aldosterone can cause a mild
degree of hypernatremia and overhydration. The reason
that the hypernatremia is not more severe is that
increased aldosterone secretion causes the kidneys to
reabsorb greater amounts of water as well as sodium.
Thus, in analyzing abnormalities of plasma sodium
concentration and deciding on proper therapy, one
should first determine whether the abnormality is
caused by a primary loss or gain of sodium or a primary
loss or gain of water.
Edema: Excess Fluid
in the Tissues
Edema refers to the presence of excess fluid in the
body tissues. In most instances, edema occurs mainly
in the extracellular fluid compartment, but it can
involve intracellular fluid as well.
Intracellular Edema
Two conditions are especially prone to cause intracellular swelling: (1) depression of the metabolic systems
of the tissues, and (2) lack of adequate nutrition to
the cells. For example, when blood flow to a tissue
is decreased, the delivery of oxygen and nutrients is
reduced. If the blood flow becomes too low to maintain normal tissue metabolism, the cell membrane
ionic pumps become depressed. When this occurs,
sodium ions that normally leak into the interior of the
cell can no longer be pumped out of the cells, and the
excess sodium ions inside the cells cause osmosis of
water into the cells. Sometimes this can increase intracellular volume of a tissue area—even of an entire
ischemic leg, for example—to two to three times
normal. When this occurs, it is usually a prelude to
death of the tissue.
Intracellular edema can also occur in inflamed
tissues. Inflammation usually has a direct effect on the
cell membranes to increase their permeability, allowing sodium and other ions to diffuse into the interior
of the cell, with subsequent osmosis of water into the
cells.
Extracellular Edema
Extracellular fluid edema occurs when there is excess
fluid accumulation in the extracellular spaces. There
are two general causes of extracellular edema: (1)
abnormal leakage of fluid from the plasma to the interstitial spaces across the capillaries, and (2) failure of
the lymphatics to return fluid from the interstitium
back into the blood. The most common clinical cause
of interstitial fluid accumulation is excessive capillary
fluid filtration.
Factors That Can Increase Capillary Filtration
To understand the causes of excessive capillary filtration, it is useful to review the determinants of capillary
filtration discussed in Chapter 16. Mathematically, capillary filtration rate can be expressed as
Filtration = Kf ¥ (Pc – Pif – pc + pif),
where Kf is the capillary filtration coefficient (the
product of the permeability and surface area of the
capillaries), Pc is the capillary hydrostatic pressure, Pif
is the interstitial fluid hydrostatic pressure, pc is the
capillary plasma colloid osmotic pressure, and pif is the
interstitial fluid colloid osmotic pressure. From this
equation, one can see that any one of the following
changes can increase the capillary filtration rate:
∑ Increased capillary filtration coefficient.
∑ Increased capillary hydrostatic pressure.
∑ Decreased plasma colloid osmotic pressure.
Lymphatic Blockage Causes Edema
When lymphatic blockage occurs, edema can become
especially severe because plasma proteins that leak
into the interstitium have no other way to be removed.
The rise in protein concentration raises the colloid
osmotic pressure of the interstitial fluid, which draws
even more fluid out of the capillaries.
Blockage of lymph flow can be especially severe
with infections of the lymph nodes, such as occurs with
infection by filaria nematodes. Blockage of the lymph
vessels can occur in certain types of cancer or after
surgery in which lymph vessels are removed or
obstructed. For example, large numbers of lymph
vessels are removed during radical mastectomy,
impairing removal of fluid from the breast and arm
areas and causing edema and swelling of the tissue
Chapter 25
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Interstitial Fluid and Edema
spaces. A few lymph vessels eventually regrow after
this type of surgery, so that the interstitial edema is
usually temporary.
Summary of Causes of
Extracellular Edema
A large number of conditions can cause fluid accumulation in the interstitial spaces by the abnormal leaking
of fluid from the capillaries or by preventing the lymphatics from returning fluid from the interstitium back
to the circulation. The following is a partial list of conditions that can cause extracellular edema by these two
types of abnormalities:
I. Increased capillary pressure
A. Excessive kidney retention of salt and water
1. Acute or chronic kidney failure
2. Mineralocorticoid excess
B. High venous pressure and venous constriction
1. Heart failure
2. Venous obstruction
3. Failure of venous pumps
(a) Paralysis of muscles
(b) Immobilization of parts of the body
(c) Failure of venous valves
C. Decreased arteriolar resistance
1. Excessive body heat
2. Insufficiency of sympathetic nervous system
3. Vasodilator drugs
II. Decreased plasma proteins
A. Loss of proteins in urine (nephrotic
syndrome)
B. Loss of protein from denuded skin areas
1. Burns
2. Wounds
C. Failure to produce proteins
1. Liver disease (e.g., cirrhosis)
2. Serious protein or caloric malnutrition
III. Increased capillary permeability
A. Immune reactions that cause release of
histamine and other immune products
B. Toxins
C. Bacterial infections
D. Vitamin deficiency, especially vitamin C
E. Prolonged ischemia
F. Burns
IV. Blockage of lymph return
A. Cancer
B. Infections (e.g., filaria nematodes)
C. Surgery
D. Congenital absence or abnormality of
lymphatic vessels
Edema Caused by Heart Failure. One of the most serious and
most common causes of edema is heart failure. In heart
failure, the heart fails to pump blood normally from the
veins into the arteries; this raises venous pressure and
capillary pressure, causing increased capillary filtration.
In addition, the arterial pressure tends to fall, causing
decreased excretion of salt and water by the kidneys,
which increases blood volume and further raises capillary hydrostatic pressure to cause still more edema.
Also, diminished blood flow to the kidneys stimulates
secretion of renin, causing increased formation of
angiotensin II and increased secretion of aldosterone,
both of which cause additional salt and water retention
by the kidneys. Thus, in untreated heart failure, all these
303
factors acting together cause serious generalized extracellular edema.
In patients with left-sided heart failure but without
significant failure of the right side of the heart, blood is
pumped into the lungs normally by the right side of the
heart but cannot escape easily from the pulmonary
veins to the left side of the heart because this part of the
heart has been greatly weakened. Consequently, all
the pulmonary vascular pressures, including pulmonary capillary pressure, rise far above normal, causing
serious and life-threatening pulmonary edema. When
untreated, fluid accumulation in the lungs can rapidly
progress, causing death within a few hours.
Edema Caused by Decreased Kidney Excretion of Salt and Water.
As discussed earlier, most sodium chloride added to the
blood remains in the extracellular compartment, and
only small amounts enter the cells. Therefore, in kidney
diseases that compromise urinary excretion of salt and
water, large amounts of sodium chloride and water are
added to the extracellular fluid. Most of this salt and
water leaks from the blood into the interstitial spaces,
but some remains in the blood. The main effects of this
are to cause (1) widespread increases in interstitial fluid
volume (extracellular edema) and (2) hypertension
because of the increase in blood volume, as explained
in Chapter 19. As an example, children who develop
acute glomerulonephritis, in which the renal glomeruli
are injured by inflammation and therefore fail to filter
adequate amounts of fluid, also develop serious
extracellular fluid edema in the entire body; along with
the edema, these children usually develop severe
hypertension.
Edema Caused by Decreased Plasma Proteins. A reduction in
plasma concentration of proteins because of either
failure to produce normal amounts of proteins or
leakage of proteins from the plasma causes the plasma
colloid osmotic pressure to fall. This leads to increased
capillary filtration throughout the body as well as extracellular edema.
One of the most important causes of decreased
plasma protein concentration is loss of proteins in the
urine in certain kidney diseases, a condition referred to
as nephrotic syndrome. Multiple types of renal diseases
can damage the membranes of the renal glomeruli,
causing the membranes to become leaky to the plasma
proteins and often allowing large quantities of these
proteins to pass into the urine. When this loss exceeds
the ability of the body to synthesize proteins, a reduction in plasma protein concentration occurs. Serious
generalized edema occurs when the plasma protein concentration falls below 2.5 g/100 ml.
Cirrhosis of the liver is another condition that causes
a reduction in plasma protein concentration. Cirrhosis
means development of large amounts of fibrous tissue
among the liver parenchymal cells. One result is failure
of these cells to produce sufficient plasma proteins,
leading to decreased plasma colloid osmotic pressure
and the generalized edema that goes with this condition.
Another way that liver cirrhosis causes edema is that
the liver fibrosis sometimes compresses the abdominal
portal venous drainage vessels as they pass through the
liver before emptying back into the general circulation.
Blockage of this portal venous outflow raises capillary
hydrostatic pressure throughout the gastrointestinal
area and further increases filtration of fluid out of
the plasma into the intra-abdominal areas. When this
The Body Fluids and Kidneys
Safety Factor Caused by Low Compliance
of the Interstitium in the Negative
Pressure Range
In Chapter 16, we noted that interstitial fluid hydrostatic pressure in most loose subcutaneous tissues of
the body is slightly less than atmospheric pressure,
averaging about –3 mm Hg. This slight suction in the
tissues helps hold the tissues together. Figure 25–7
shows the approximate relations between different
levels of interstitial fluid pressure and interstitial fluid
volume, as extrapolated to the human being from
animal studies. Note in Figure 25–7 that as long as the
interstitial fluid pressure is in the negative range, small
changes in interstitial fluid volume are associated with
relatively large changes in interstitial fluid hydrostatic
pressure. Therefore, in the negative pressure range, the
compliance of the tissues, defined as the change in
volume per millimeter of mercury pressure change, is
low.
How does the low compliance of the tissues in the
negative pressure range act as a safety factor against
edema? To answer this question, recall the determinants of capillary filtration discussed previously. When
interstitial fluid hydrostatic pressure increases, this
increased pressure tends to oppose further capillary
filtration. Therefore, as long as the interstitial fluid
hydrostatic pressure is in the negative pressure range,
small increases in interstitial fluid volume cause relatively large increases in interstitial fluid hydrostatic
pressure, opposing further filtration of fluid into the
tissues.
Because the normal interstitial fluid hydrostatic
pressure is –3 mm Hg, the interstitial fluid hydrostatic
pressure must increase by about 3 mm Hg before large
amounts of fluid will begin to accumulate in the tissues.
Therefore, the safety factor against edema is a change
of interstitial fluid pressure of about 3 mm Hg.
Once interstitial fluid pressure rises above 0 mm Hg,
the compliance of the tissues increases markedly,
allowing large amounts of fluid to accumulate in the
tissues with negative interstitial fluid pressure, virtually
all the fluid in the interstitium is in gel form. That is, the
fluid is bound in a proteoglycan meshwork so that there
are virtually no “free” fluid spaces larger than a few
hundredths of a micrometer in diameter. The importance of the gel is that it prevents fluid from flowing
easily through the tissues because of impediment from
the “brush pile” of trillions of proteoglycan filaments.
Also, when the interstitial fluid pressure falls to very
negative values, the gel does not contract greatly
because the meshwork of proteoglycan filaments offers
an elastic resistance to compression. In the negative
fluid pressure range, the interstitial fluid volume does
not change greatly, regardless of whether the degree of
suction is only a few millimeters of mercury negative
pressure or 10 to 20 mm Hg negative pressure. In other
words, the compliance of the tissues is very low in the
negative pressure range.
60
56
Free fluid
Gel fluid
52
48
44
id
Even though many disturbances can cause edema,
usually the abnormality must be severe before serious
edema develops. The reason for this is that three major
safety factors prevent excessive fluid accumulation in
the interstitial spaces: (1) low compliance of the interstitium when interstitial fluid pressure is in the negative pressure range, (2) the ability of lymph flow
to increase 10- to 50-fold, and (3) washdown of interstitial fluid protein concentration, which reduces
interstitial fluid colloid osmotic pressure as capillary
filtration increases.
Importance of Interstitial Gel in Preventing Fluid Accumulation in
the Interstitium. Note in Figure 25–7 that in normal
ial flu
Safety Factors That Normally
Prevent Edema
tissues with relatively small additional increases in
interstitial fluid hydrostatic pressure. Thus, in the positive tissue pressure range, this safety factor against
edema is lost because of the large increase in compliance of the tissues.
40
36
terstit
occurs, the combined effects of decreased plasma
protein concentration and high portal capillary pressures cause transudation of large amounts of fluid and
protein into the abdominal cavity, a condition referred
to as ascites.
32
Total
in
Unit V
Interstitial fluid volume (liters)
304
28
24
(High
compliance)
20
Normal
16
12
8
(Low compliance)
4
0
2
4
-10 -8 -6 -4 -2
0
Interstitial free fluid pressure
(mm Hg)
6
Figure 25–7
Relation between interstitial fluid hydrostatic pressure and interstitial fluid volumes, including total volume, free fluid volume, and
gel fluid volume, for loose tissues such as skin. Note that significant amounts of free fluid occur only when the interstitial fluid pressure becomes positive. (Modified from Guyton AC, Granger HJ,
Taylor AE: Interstitial fluid pressure. Physiol Rev 51:527, 1971.)
Chapter 25
The Body Fluid Compartments: Extracellular and Intracellular Fluids; Interstitial Fluid and Edema
By contrast, when interstitial fluid pressure rises to
the positive pressure range, there is a tremendous accumulation of free fluid in the tissues. In this pressure
range, the tissues are compliant, allowing large amounts
of fluid to accumulate with relatively small additional
increases in interstitial fluid hydrostatic pressure. Most
of the extra fluid that accumulates is “free fluid” because
it pushes the brush pile of proteoglycan filaments apart.
Therefore, the fluid can flow freely through the tissue
spaces because it is not in gel form. When this occurs,
the edema is said to be pitting edema because one can
press the thumb against the tissue area and push the
fluid out of the area. When the thumb is removed, a pit
is left in the skin for a few seconds until the fluid flows
back from the surrounding tissues. This type of edema
is distinguished from nonpitting edema, which occurs
when the tissue cells swell instead of the interstitium or
when the fluid in the interstitium becomes clotted with
fibrinogen so that it cannot move freely within the tissue
spaces.
Importance of the Proteoglycan Filaments as a “Spacer” for the
Cells and in Preventing Rapid Flow of Fluid in the Tissues. The
proteoglycan filaments, along with much larger collagen
fibrils in the interstitial spaces, act as a “spacer” between
the cells. Nutrients and ions do not diffuse readily
through cell membranes; therefore, without adequate
spacing between the cells, these nutrients, electrolytes,
and cell waste products could not be rapidly exchanged
between the blood capillaries and cells located at a distance from one another.
The proteoglycan filaments also prevent fluid from
flowing too easily through the tissue spaces. If it were
not for the proteoglycan filaments, the simple act of a
person standing up would cause large amounts of interstitial fluid to flow from the upper body to the lower
body. When too much fluid accumulates in the interstitium, as occurs in edema, this extra fluid creates large
channels that allow the fluid to flow readily through the
interstitium. Therefore, when severe edema occurs in
the legs, the edema fluid often can be decreased by
simply elevating the legs.
Even though fluid does not flow easily through the
tissues in the presence of the compacted proteoglycan
filaments, different substances within the fluid can
diffuse through the tissues at least 95 per cent as easily
as they normally diffuse. Therefore, the usual diffusion
of nutrients to the cells and the removal of waste products from the cells are not compromised by the proteoglycan filaments of the interstitium.
Increased Lymph Flow as a Safety Factor
Against Edema
A major function of the lymphatic system is to return
to the circulation the fluid and proteins filtered from
the capillaries into the interstitium. Without this continuous return of the filtered proteins and fluid to the
blood, the plasma volume would be rapidly depleted,
and interstitial edema would occur.
The lymphatics act as a safety factor against edema
because lymph flow can increase 10- to 50-fold when
fluid begins to accumulate in the tissues. This allows
the lymphatics to carry away large amounts of fluid
and proteins in response to increased capillary filtration, preventing the interstitial pressure from rising
into the positive pressure range. The safety factor
305
caused by increased lymph flow has been calculated to
be about 7 mm Hg.
“Washdown” of the Interstitial Fluid Protein as
a Safety Factor Against Edema
As increased amounts of fluid are filtered into the
interstitium, the interstitial fluid pressure increases,
causing increased lymph flow. In most tissues, the
protein concentration of the interstitium decreases as
lymph flow is increased, because larger amounts of
protein are carried away than can be filtered out of the
capillaries; the reason for this is that the capillaries
are relatively impermeable to proteins, compared
with the lymph vessels. Therefore, the proteins are
“washed out” of the interstitial fluid as lymph flow
increases.
Because the interstitial fluid colloid osmotic pressure caused by the proteins tends to draw fluid out of
the capillaries, decreasing the interstitial fluid proteins
lowers the net filtration force across the capillaries and
tends to prevent further accumulation of fluid. The
safety factor from this effect has been calculated to be
about 7 mm Hg.
Summary of Safety Factors That
Prevent Edema
Putting together all the safety factors against edema, we
find the following:
1. The safety factor caused by low tissue compliance
in the negative pressure range is about 3 mm Hg.
2. The safety factor caused by increased lymph flow is
about 7 mm Hg.
3. The safety factor caused by washdown of proteins
from the interstitial spaces is about 7 mm Hg.
Therefore, the total safety factor against edema is
about 17 mm Hg. This means that the capillary pressure in a peripheral tissue could theoretically rise by
17 mm Hg, or approximately double the normal value,
before marked edema would occur.
Fluids in the “Potential
Spaces” of the Body
Perhaps the best way to describe a “potential space”
is to list some examples: pleural cavity, pericardial
cavity, peritoneal cavity, and synovial cavities, including both the joint cavities and the bursae. Virtually all
these potential spaces have surfaces that almost touch
each other, with only a thin layer of fluid in between,
and the surfaces slide over each other. To facilitate the
sliding, a viscous proteinaceous fluid lubricates the
surfaces.
Fluid Is Exchanged Between the Capillaries and the Potential
Spaces. The surface membrane of a potential space
usually does not offer significant resistance to the
passage of fluids, electrolytes, or even proteins, which
all move back and forth between the space and the
interstitial fluid in the surrounding tissue with relative
ease. Therefore, each potential space is in reality a
306
Unit V
The Body Fluids and Kidneys
large tissue space. Consequently, fluid in the capillaries adjacent to the potential space diffuses not only
into the interstitial fluid but also into the potential
space.
Lymphatic Vessels Drain Protein from the Potential Spaces.
Proteins collect in the potential spaces because of
leakage out of the capillaries, similar to the collection
of protein in the interstitial spaces throughout the
body. The protein must be removed through lymphatics or other channels and returned to the circulation.
Each potential space is either directly or indirectly
connected with lymph vessels. In some cases, such as
the pleural cavity and peritoneal cavity, large lymph
vessels arise directly from the cavity itself.
Edema Fluid in the Potential Spaces Is Called “Effusion.”
When edema occurs in the subcutaneous tissues adjacent to the potential space, edema fluid usually collects
in the potential space as well, and this fluid is called
effusion. Thus, lymph blockage or any of the multiple
abnormalities that can cause excessive capillary filtration can cause effusion in the same way that interstitial edema is caused.The abdominal cavity is especially
prone to collect effusion fluid, and in this instance, the
effusion is called ascites. In serious cases, 20 liters or
more of ascitic fluid can accumulate.
The other potential spaces, such as the pleural
cavity, pericardial cavity, and joint spaces, can become
seriously swollen when there is generalized edema.
Also, injury or local infection in any one of the cavities often blocks the lymph drainage, causing isolated
swelling in the cavity.
The dynamics of fluid exchange in the pleural cavity
are discussed in detail in Chapter 38. These dynamics
are mainly representative of all the other potential
spaces as well. It is especially interesting that the
normal fluid pressure in most or all of the potential
spaces in the nonedematous state is negative in the
same way that this pressure is negative (subatmospheric) in loose subcutaneous tissue. For instance, the
interstitial fluid hydrostatic pressure is normally about
–7 to –8 mm Hg in the pleural cavity, –3 to –5 mm Hg
in the joint spaces, and –5 to –6 mm Hg in the pericardial cavity.
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6
Urine Formation by the Kidneys:
I. Glomerular Filtration, Renal
Blood Flow, and Their Control
Multiple Functions of the
Kidneys in Homeostasis
Most people are familiar with one important function of the kidneys—to rid the body of waste materials that are either ingested or produced by
metabolism. A second function that is especially
critical is to control the volume and composition of
the body fluids. For water and virtually all electrolytes in the body, the balance
between intake (due to ingestion or metabolic production) and output (due to
excretion or metabolic consumption) is maintained in large part by the kidneys.
This regulatory function of the kidneys maintains the stable environment of the
cells necessary for them to perform their various activities.
The kidneys perform their most important functions by filtering the plasma
and removing substances from the filtrate at variable rates, depending on the
needs of the body. Ultimately, the kidneys “clear” unwanted substances from
the filtrate (and therefore from the blood) by excreting them in the urine while
returning substances that are needed back to the blood.
Although this chapter and the next few chapters focus mainly on the control
of renal excretion, it is important to recognize that the kidneys serve multiple
functions, including the following:
∑ Excretion of metabolic waste products and foreign chemicals
∑ Regulation of water and electrolyte balances
∑ Regulation of body fluid osmolality and electrolyte concentrations
∑ Regulation of arterial pressure
∑ Regulation of acid-base balance
∑ Secretion, metabolism, and excretion of hormones
∑ Gluconeogenesis
Excretion of Metabolic Waste Products, Foreign Chemicals, Drugs, and Hormone Metabolites. The kidneys are the primary means for eliminating waste products of
metabolism that are no longer needed by the body. These products include urea
(from the metabolism of amino acids), creatinine (from muscle creatine), uric
acid (from nucleic acids), end products of hemoglobin breakdown (such as
bilirubin), and metabolites of various hormones. These waste products must be
eliminated from the body as rapidly as they are produced. The kidneys also
eliminate most toxins and other foreign substances that are either produced by
the body or ingested, such as pesticides, drugs, and food additives.
Regulation of Water and Electrolyte Balances. For maintenance of homeostasis, excretion of water and electrolytes must precisely match intake. If intake exceeds
excretion, the amount of that substance in the body will increase. If intake is
less than excretion, the amount of that substance in the body will decrease.
Intake of water and many electrolytes is governed mainly by a person’s eating
and drinking habits, requiring the kidneys to adjust their excretion rates to
match the intake of various substances. Figure 26–1 shows the response of the
kidneys to a sudden 10-fold increase in sodium intake from a low level of
30 mEq/day to a high level of 300 mEq/day. Within 2 to 3 days after raising the
307
308
Unit V
The Body Fluids and Kidneys
Regulation of Acid-Base Balance. The kidneys contribute
to acid-base regulation, along with the lungs and body
fluid buffers, by excreting acids and by regulating the
body fluid buffer stores. The kidneys are the only
means of eliminating from the body certain types of
acids, such as sulfuric acid and phosphoric acid, generated by the metabolism of proteins.
Extracellular
fluid volume
(Liters)
Sodium intake and
excretion
(mEq/day)
Sodium
retention
300
Intake
200
Excretion
100
Sodium
loss
0
15
10
5
-4 -2
0
2
4 6 8
Time (days)
10 12 14
Figure 26–1
Effect of increasing sodium intake 10-fold (from 30 to 300 mEq/
day) on urinary sodium excretion and extracellular fluid volume.
The shaded areas represent the net sodium retention or the net
sodium loss, determined from the difference between sodium
intake and sodium excretion.
Regulation of Erythrocyte Production. The kidneys secrete
erythropoietin, which stimulates the production of red
blood cells, as discussed in Chapter 32. One important
stimulus for erythropoietin secretion by the kidneys is
hypoxia. The kidneys normally account for almost all
the erythropoietin secreted into the circulation. In
people with severe kidney disease or who have had
their kidneys removed and have been placed on
hemodialysis, severe anemia develops as a result of
decreased erythropoietin production.
Regulation of 1,25–Dihydroxyvitamin D3 Production. The
kidneys produce the active form of vitamin D, 1,25dihydroxyvitamin D3 (calcitriol), by hydroxylating this
vitamin at the “number 1” position. Calcitriol is essential for normal calcium deposition in bone and calcium
reabsorption by the gastrointestinal tract. As discussed
in Chapter 79, calcitriol plays an important role in
calcium and phosphate regulation.
Glucose Synthesis. The kidneys synthesize glucose from
sodium intake, renal excretion also increases to about
300 mEq/day, so that a balance between intake and
output is re-established. However, during the 2 to 3
days of renal adaptation to the high sodium intake,
there is a modest accumulation of sodium that raises
extracellular fluid volume slightly and triggers hormonal changes and other compensatory responses
that signal the kidneys to increase their sodium
excretion.
The capacity of the kidneys to alter sodium excretion in response to changes in sodium intake is enormous. Experimental studies have shown that in many
people, sodium intake can be increased to 1500 mEq/
day (more than 10 times normal) or decreased to
10 mEq/day (less than one tenth normal) with relatively small changes in extracellular fluid volume or
plasma sodium concentration. This is also true for
water and for most other electrolytes, such as chloride,
potassium, calcium, hydrogen, magnesium, and phosphate ions. In the next few chapters, we discuss the specific mechanisms that permit the kidneys to perform
these amazing feats of homeostasis.
Regulation of Arterial Pressure. As discussed in Chapter
19, the kidneys play a dominant role in long-term regulation of arterial pressure by excreting variable
amounts of sodium and water. The kidneys also contribute to short-term arterial pressure regulation by
secreting vasoactive factors or substances, such as
renin, that lead to the formation of vasoactive products (e.g., angiotensin II).
amino acids and other precursors during prolonged
fasting, a process referred to as gluconeogenesis. The
kidneys’ capacity to add glucose to the blood during
prolonged periods of fasting rivals that of the liver.
With chronic kidney disease or acute failure of the
kidneys, these homeostatic functions are disrupted,
and severe abnormalities of body fluid volumes and
composition rapidly occur. With complete renal
failure, enough potassium, acids, fluid, and other substances accumulate in the body to cause death within
a few days, unless clinical interventions such as
hemodialysis are initiated to restore, at least partially,
the body fluid and electrolyte balances.
Physiologic Anatomy
of the Kidneys
General Organization of the Kidneys
and Urinary Tract
The two kidneys lie on the posterior wall of the
abdomen, outside the peritoneal cavity (Figure 26–2).
Each kidney of the adult human weighs about 150
grams and is about the size of a clenched fist. The
medial side of each kidney contains an indented region
called the hilum through which pass the renal artery
and vein, lymphatics, nerve supply, and ureter, which
carries the final urine from the kidney to the bladder,
where it is stored until emptied. The kidney is surrounded by a tough, fibrous capsule that protects its
delicate inner structures.
Chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
309
Minor calyx
Major calyx
Nephron (enlarged)
Papilla
Renal cortex
Renal medulla
Kidney
Renal pelvis
Renal pyramid
Renal
artery
Capsule
Ureter
Figure 26–2
General organization of the
kidneys and the urinary system.
Ureter
Kidney
Bladder
Urethra
If the kidney is bisected from top to bottom, the two
major regions that can be visualized are the outer
cortex and the inner region referred to as the medulla.
The medulla is divided into multiple cone-shaped
masses of tissue called renal pyramids. The base of
each pyramid originates at the border between the
cortex and medulla and terminates in the papilla,
which projects into the space of the renal pelvis, a
funnel-shaped continuation of the upper end of the
ureter. The outer border of the pelvis is divided into
open-ended pouches called major calyces that extend
downward and divide into minor calyces, which collect
urine from the tubules of each papilla. The walls of the
calyces, pelvis, and ureter contain contractile elements
that propel the urine toward the bladder, where urine
is stored until it is emptied by micturition, discussed in
later in this chapter.
much lower hydrostatic pressure in the peritubular
capillaries (about 13 mm Hg) permits rapid fluid reabsorption. By adjusting the resistance of the afferent
and efferent arterioles, the kidneys can regulate the
hydrostatic pressure in both the glomerular and the
peritubular capillaries, thereby changing the rate of
Interlobar
arteries
Renal artery
Arcuate arteries
Segmental
arteries
Interlobular
arterioles
Renal Blood Supply
Blood flow to the two kidneys is normally about 22 per
cent of the cardiac output, or 1100 ml/min. The renal
artery enters the kidney through the hilum and then
branches progressively to form the interlobar arteries,
arcuate arteries, interlobular arteries (also called radial
arteries) and afferent arterioles, which lead to the
glomerular capillaries, where large amounts of fluid
and solutes (except the plasma proteins) are filtered to
begin urine formation (Figure 26–3). The distal ends of
the capillaries of each glomerulus coalesce to form the
efferent arteriole, which leads to a second capillary
network, the peritubular capillaries, that surrounds the
renal tubules.
The renal circulation is unique in that it has two capillary beds, the glomerular and peritubular capillaries,
which are arranged in series and separated by the
efferent arterioles, which help regulate the hydrostatic
pressure in both sets of capillaries. High hydrostatic pressure in the glomerular capillaries (about
60 mm Hg) causes rapid fluid filtration, whereas a
Efferent Bowman's
arteriole capsule
Glomerulus
Juxtaglomerular
apparatus
Afferent
arteriole
Proximal tubule
Cortical
collecting tubule
Distal tubule
Arcuate
artery
Arcuate
vein
Loop of
Henle
Peritubular
capillaries
Collecting duct
Figure 26–3
Section of the human kidney showing the major vessels that
supply the blood flow to the kidney and schematic of the microcirculation of each nephron.
310
Unit V
The Body Fluids and Kidneys
glomerular filtration, tubular reabsorption, or both in
response to body homeostatic demands.
The peritubular capillaries empty into the vessels of
the venous system, which run parallel to the arteriolar
vessels and progressively form the interlobular vein,
arcuate vein, interlobar vein, and renal vein, which
leaves the kidney beside the renal artery and ureter.
The Nephron Is the Functional Unit
of the Kidney
Each kidney in the human contains about 1 million
nephrons, each capable of forming urine. The kidney
cannot regenerate new nephrons.Therefore, with renal
injury, disease, or normal aging, there is a gradual
decrease in nephron number. After age 40, the number
of functioning nephrons usually decreases about 10
per cent every 10 years; thus, at age 80, many people
have 40 per cent fewer functioning nephrons than they
did at age 40. This loss is not life threatening because
adaptive changes in the remaining nephrons allow
them to excrete the proper amounts of water,
electrolytes, and waste products, as discussed in
Chapter 31.
Each nephron contains (1) a tuft of glomerular capillaries called the glomerulus, through which large
amounts of fluid are filtered from the blood, and (2) a
long tubule in which the filtered fluid is converted into
urine on its way to the pelvis of the kidney (see Figure
26–3).
The glomerulus contains a network of branching
and anastomosing glomerular capillaries that, compared with other capillaries, have high hydrostatic
pressure (about 60 mm Hg). The glomerular capillaries are covered by epithelial cells, and the total
glomerulus is encased in Bowman’s capsule. Fluid
filtered from the glomerular capillaries flows into
Bowman’s capsule and then into the proximal tubule,
which lies in the cortex of the kidney (Figure 26–4).
From the proximal tubule, fluid flows into the loop
of Henle, which dips into the renal medulla. Each loop
consists of a descending and an ascending limb. The
walls of the descending limb and the lower end of the
ascending limb are very thin and therefore are called
the thin segment of the loop of Henle. After the ascending limb of the loop has returned partway back to the
cortex, its wall becomes much thicker, and it is referred
to as the thick segment of the ascending limb.
At the end of the thick ascending limb is a short
segment, which is actually a plaque in its wall, known
as the macula densa. As we discuss later, the macula
densa plays an important role in controlling nephron
function. Beyond the macula densa, fluid enters the
distal tubule, which, like the proximal tubule, lies in the
renal cortex. This is followed by the connecting tubule
and the cortical collecting tubule, which lead to the cortical collecting duct. The initial parts of 8 to 10 cortical
collecting ducts join to form a single larger collecting
duct that runs downward into the medulla and
becomes the medullary collecting duct. The collecting
ducts merge to form progressively larger ducts that
Proximal tubule
Distal tubule
Cortex
Connecting tubule
Bowman's capsule
Macula densa
Cortical
collecting tubule
Loop of Henle:
Thick segment of
ascending limb
Thin segment of
ascending limb
Medulla
Medullary
collecting tubule
Descending limb
Collecting duct
Figure 26–4
Basic tubular segments of the nephron. The relative lengths of the
different tubular segments are not drawn to scale.
eventually empty into the renal pelvis through the tips
of the renal papillae. In each kidney, there are about
250 of the very large collecting ducts, each of which
collects urine from about 4000 nephrons.
Regional Differences in Nephron Structure: Cortical and
Juxtamedullary Nephrons. Although each nephron has all
the components described earlier, there are some differences, depending on how deep the nephron lies
within the kidney mass. Those nephrons that have
glomeruli located in the outer cortex are called cortical nephrons; they have short loops of Henle that
penetrate only a short distance into the medulla
(Figure 26–5).
About 20 to 30 per cent of the nephrons have
glomeruli that lie deep in the renal cortex near the
medulla and are called juxtamedullary nephrons.
These nephrons have long loops of Henle that dip
deeply into the medulla, in some cases all the way to
the tips of the renal papillae.
The vascular structures supplying the juxtamedullary nephrons also differ from those supplying
the cortical nephrons. For the cortical nephrons, the
entire tubular system is surrounded by an extensive
network of peritubular capillaries. For the juxtamedullary nephrons, long efferent arterioles extend
from the glomeruli down into the outer medulla and
then divide into specialized peritubular capillaries
called vasa recta that extend downward into the
medulla, lying side by side with the loops of Henle.
Like the loops of Henle, the vasa recta return toward
the cortex and empty into the cortical veins. This
specialized network of capillaries in the medulla plays
an essential role in the formation of a concentrated
urine.
Chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
Micturition
Cortex
Micturition is the process by which the urinary bladder
empties when it becomes filled. This involves two main
steps: First, the bladder fills progressively until the
tension in its walls rises above a threshold level; this
elicits the second step, which is a nervous reflex called
the micturition reflex that empties the bladder or, if this
fails, at least causes a conscious desire to urinate.
Efferent
arteriole
Afferent
arteriole
Cortical
nephron
Juxtamedullary
nephron
Outer zone
Interlobar
artery
vein
Thick loop
of Henle
Inner zone
Medulla
Interlobar
artery
vein
Vasa
recta
Collecting
duct
Thin loop
of Henle
Duct of
Bellini
Figure 26–5
Schematic of relations between blood vessels and tubular structures and differences between cortical and juxtamedullary
nephrons.
311
Although the micturition reflex is an autonomic spinal
cord reflex, it can also be inhibited or facilitated by
centers in the cerebral cortex or brain stem.
Physiologic Anatomy and
Nervous Connections
of the Bladder
The urinary bladder, shown in Figure 26–6, is a smooth
muscle chamber composed of two main parts: (1) the
body, which is the major part of the bladder in which
urine collects, and (2) the neck, which is a funnel-shaped
extension of the body, passing inferiorly and anteriorly
into the urogenital triangle and connecting with the
urethra. The lower part of the bladder neck is also called
the posterior urethra because of its relation to the
urethra.
The smooth muscle of the bladder is called the detrusor muscle. Its muscle fibers extend in all directions and,
when contracted, can increase the pressure in the
bladder to 40 to 60 mm Hg. Thus, contraction of the
detrusor muscle is a major step in emptying the bladder.
Smooth muscle cells of the detrusor muscle fuse with
one another so that low-resistance electrical pathways
exist from one muscle cell to the other. Therefore, an
action potential can spread throughout the detrusor
muscle, from one muscle cell to the next, to cause contraction of the entire bladder at once.
On the posterior wall of the bladder, lying immediately above the bladder neck, is a small triangular area
called the trigone. At the lowermost apex of the trigone,
the bladder neck opens into the posterior urethra, and
the two ureters enter the bladder at the uppermost
angles of the trigone. The trigone can be identified by
the fact that its mucosa, the inner lining of the bladder,
is smooth, in contrast to the remaining bladder mucosa,
which is folded to form rugae. Each ureter, as it enters
the bladder, courses obliquely through the detrusor
muscle and then passes another 1 to 2 centimeters
beneath the bladder mucosa before emptying into the
bladder.
Ureter
L1
L2
L3
L4
L5
S1
S2
S3
S4
Sympathetics
Parasympathetics
Pudendal
Figure 26–6
Urinary bladder and its innervation.
Body
Trigone
Bladder neck
(posterior urethra)
External sphincter
312
Unit V
The Body Fluids and Kidneys
The bladder neck (posterior urethra) is 2 to 3 centimeters long, and its wall is composed of detrusor
muscle interlaced with a large amount of elastic tissue.
The muscle in this area is called the internal sphincter.
Its natural tone normally keeps the bladder neck and
posterior urethra empty of urine and, therefore, prevents emptying of the bladder until the pressure in the
main part of the bladder rises above a critical threshold.
Beyond the posterior urethra, the urethra passes
through the urogenital diaphragm, which contains a
layer of muscle called the external sphincter of the
bladder. This muscle is a voluntary skeletal muscle, in
contrast to the muscle of the bladder body and bladder
neck, which is entirely smooth muscle. The external
sphincter muscle is under voluntary control of the
nervous system and can be used to consciously prevent
urination even when involuntary controls are attempting to empty the bladder.
Innervation of the Bladder
The principal nerve supply of the bladder is by way of
the pelvic nerves, which connect with the spinal cord
through the sacral plexus, mainly connecting with cord
segments S-2 and S-3. Coursing through the pelvic
nerves are both sensory nerve fibers and motor nerve
fibers. The sensory fibers detect the degree of stretch in
the bladder wall. Stretch signals from the posterior
urethra are especially strong and are mainly responsible for initiating the reflexes that cause bladder
emptying.
The motor nerves transmitted in the pelvic nerves are
parasympathetic fibers. These terminate on ganglion
cells located in the wall of the bladder. Short postganglionic nerves then innervate the detrusor muscle.
In addition to the pelvic nerves, two other types of
innervation are important in bladder function. Most
important are the skeletal motor fibers transmitted
through the pudendal nerve to the external bladder
sphincter. These are somatic nerve fibers that innervate
and control the voluntary skeletal muscle of the sphincter. Also, the bladder receives sympathetic innervation
from the sympathetic chain through the hypogastric
nerves, connecting mainly with the L-2 segment of the
spinal cord. These sympathetic fibers stimulate mainly
the blood vessels and have little to do with bladder contraction. Some sensory nerve fibers also pass by way of
the sympathetic nerves and may be important in the
sensation of fullness and, in some instances, pain.
Transport of Urine from the
Kidney Through the Ureters
and into the Bladder
Urine that is expelled from the bladder has essentially
the same composition as fluid flowing out of the collecting ducts; there are no significant changes in the
composition of urine as it flows through the renal
calyces and ureters to the bladder.
Urine flowing from the collecting ducts into the
renal calyces stretches the calyces and increases their
inherent pacemaker activity, which in turn initiates
peristaltic contractions that spread to the renal pelvis
and then downward along the length of the ureter,
thereby forcing urine from the renal pelvis toward the
bladder. The walls of the ureters contain smooth
muscle and are innervated by both sympathetic and
parasympathetic nerves as well as by an intramural
plexus of neurons and nerve fibers that extends along
the entire length of the ureters. As with other visceral
smooth muscle, peristaltic contractions in the ureter are
enhanced by parasympathetic stimulation and inhibited
by sympathetic stimulation.
The ureters enter the bladder through the detrusor
muscle in the trigone region of the bladder, as shown
in Figure 26–6. Normally, the ureters course obliquely
for several centimeters through the bladder wall. The
normal tone of the detrusor muscle in the bladder wall
tends to compress the ureter, thereby preventing backflow of urine from the bladder when pressure builds
up in the bladder during micturition or bladder
compression. Each peristaltic wave along the ureter
increases the pressure within the ureter so that the
region passing through the bladder wall opens and
allows urine to flow into the bladder.
In some people, the distance that the ureter courses
through the bladder wall is less than normal, so that
contraction of the bladder during micturition does not
always lead to complete occlusion of the ureter. As a
result, some of the urine in the bladder is propelled
backward into the ureter, a condition called vesicoureteral reflux. Such reflux can lead to enlargement
of the ureters and, if severe, can increase the pressure
in the renal calyces and structures of the renal
medulla, causing damage to these regions.
Pain Sensation in the Ureters, and the Ureterorenal Reflex.
The ureters are well supplied with pain nerve fibers.
When a ureter becomes blocked (e.g., by a ureteral
stone), intense reflex constriction occurs, associated
with severe pain. Also, the pain impulses cause a sympathetic reflex back to the kidney to constrict the renal
arterioles, thereby decreasing urine output from the
kidney. This effect is called the ureterorenal reflex and
is important for preventing excessive flow of fluid into
the pelvis of a kidney with a blocked ureter.
Filling of the Bladder
and Bladder Wall Tone;
the Cystometrogram
Figure 26–7 shows the approximate changes in intravesicular pressure as the bladder fills with urine. When
there is no urine in the bladder, the intravesicular pressure is about 0, but by the time 30 to 50 milliliters of
urine has collected, the pressure rises to 5 to 10 centimeters of water. Additional urine—200 to 300 milliliters—can collect with only a small additional rise in
pressure; this constant level of pressure is caused by
intrinsic tone of the bladder wall itself. Beyond 300 to
400 milliliters, collection of more urine in the bladder
causes the pressure to rise rapidly.
Superimposed on the tonic pressure changes during
filling of the bladder are periodic acute increases in
pressure that last from a few seconds to more than a
minute. The pressure peaks may rise only a few centimeters of water or may rise to more than 100
Chapter 26
Intravesical pressure
(centimeters of water)
40
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
Micturition
contractions
30
20
10
gram
stometro
Basal cy
0
0
100
200
300
400
Volume (milliliters)
Figure 26–7
Normal cystometrogram, showing also acute pressure waves
(dashed spikes) caused by micturition reflexes.
centimeters of water. These pressure peaks are called
micturition waves in the cystometrogram and are caused
by the micturition reflex.
Micturition Reflex
Referring again to Figure 26–7, one can see that as the
bladder fills, many superimposed micturition contractions begin to appear, as shown by the dashed spikes.
They are the result of a stretch reflex initiated by
sensory stretch receptors in the bladder wall, especially
by the receptors in the posterior urethra when this
area begins to fill with urine at the higher bladder pressures. Sensory signals from the bladder stretch receptors are conducted to the sacral segments of the cord
through the pelvic nerves and then reflexively back
again to the bladder through the parasympathetic
nerve fibers by way of these same nerves.
When the bladder is only partially filled, these micturition contractions usually relax spontaneously after
a fraction of a minute, the detrusor muscles stop contracting, and pressure falls back to the baseline. As the
bladder continues to fill, the micturition reflexes
become more frequent and cause greater contractions
of the detrusor muscle.
Once a micturition reflex begins, it is “self-regenerative.” That is, initial contraction of the bladder activates the stretch receptors to cause a greater increase
in sensory impulses to the bladder and posterior
urethra, which causes a further increase in reflex contraction of the bladder; thus, the cycle is repeated again
and again until the bladder has reached a strong
degree of contraction. Then, after a few seconds to
more than a minute, the self-regenerative reflex begins
to fatigue and the regenerative cycle of the micturition
reflex ceases, permitting the bladder to relax.
313
Thus, the micturition reflex is a single complete cycle
of (1) progressive and rapid increase of pressure, (2) a
period of sustained pressure, and (3) return of the
pressure to the basal tone of the bladder. Once a micturition reflex has occurred but has not succeeded in
emptying the bladder, the nervous elements of this
reflex usually remain in an inhibited state for a few
minutes to 1 hour or more before another micturition
reflex occurs. As the bladder becomes more and more
filled, micturition reflexes occur more and more often
and more and more powerfully.
Once the micturition reflex becomes powerful
enough, it causes another reflex, which passes through
the pudendal nerves to the external sphincter to inhibit
it. If this inhibition is more potent in the brain than the
voluntary constrictor signals to the external sphincter,
urination will occur. If not, urination will not occur
until the bladder fills still further and the micturition
reflex becomes more powerful.
Facilitation or Inhibition of Micturition
by the Brain
The micturition reflex is a completely autonomic
spinal cord reflex, but it can be inhibited or facilitated
by centers in the brain. These centers include (1)
strong facilitative and inhibitory centers in the brain
stem, located mainly in the pons, and (2) several centers
located in the cerebral cortex that are mainly inhibitory
but can become excitatory.
The micturition reflex is the basic cause of micturition, but the higher centers normally exert final control
of micturition as follows:
1. The higher centers keep the micturition reflex
partially inhibited, except when micturition is
desired.
2. The higher centers can prevent micturition, even
if the micturition reflex occurs, by continual tonic
contraction of the external bladder sphincter until
a convenient time presents itself.
3. When it is time to urinate, the cortical centers can
facilitate the sacral micturition centers to help
initiate a micturition reflex and at the same time
inhibit the external urinary sphincter so that
urination can occur.
Voluntary urination is usually initiated in the following way: First, a person voluntarily contracts his or
her abdominal muscles, which increases the pressure
in the bladder and allows extra urine to enter the
bladder neck and posterior urethra under pressure,
thus stretching their walls. This stimulates the stretch
receptors, which excites the micturition reflex and
simultaneously inhibits the external urethral sphincter.
Ordinarily, all the urine will be emptied, with rarely
more than 5 to 10 milliliters left in the bladder.
Abnormalities of Micturition
Atonic Bladder Caused by Destruction of Sensory Nerve Fibers.
Micturition reflex contraction cannot occur if the
sensory nerve fibers from the bladder to the spinal cord
314
Unit V
The Body Fluids and Kidneys
are destroyed, thereby preventing transmission of
stretch signals from the bladder. When this happens, a
person loses bladder control, despite intact efferent
fibers from the cord to the bladder and despite intact
neurogenic connections within the brain. Instead of
emptying periodically, the bladder fills to capacity and
overflows a few drops at a time through the urethra.This
is called overflow incontinence.
A common cause of atonic bladder is crush injury to
the sacral region of the spinal cord. Certain diseases can
also cause damage to the dorsal root nerve fibers that
enter the spinal cord. For example, syphilis can cause
constrictive fibrosis around the dorsal root nerve fibers,
destroying them. This condition is called tabes dorsalis,
and the resulting bladder condition is called tabetic
bladder.
Afferent
arteriole
Uninhibited Neurogenic Bladder Caused by Lack of Inhibitory
Signals from the Brain. Another abnormality of micturi-
tion is the so-called uninhibited neurogenic bladder,
which results in frequent and relatively uncontrolled
micturition. This condition derives from partial damage
in the spinal cord or the brain stem that interrupts most
of the inhibitory signals. Therefore, facilitative impulses
passing continually down the cord keep the sacral
centers so excitable that even a small quantity of urine
elicits an uncontrollable micturition reflex, thereby promoting frequent urination.
Urine Formation Results
from Glomerular Filtration,
Tubular Reabsorption, and
Tubular Secretion
The rates at which different substances are excreted
in the urine represent the sum of three renal processes, shown in Figure 26–8: (1) glomerular filtration,
(2) reabsorption of substances from the renal tubules
into the blood, and (3) secretion of substances
from the blood into the renal tubules. Expressed
mathematically,
Urinary excretion rate = Filtration rate
- Reabsorption rate + Secretion rate
1. Filtration
2. Reabsorption
3. Secretion
4. Excretion
Glomerular
capillaries
Bowman's
capsule
1
2
Peritubular
capillaries
3
Automatic Bladder Caused by Spinal Cord Damage Above the
Sacral Region. If the spinal cord is damaged above the
sacral region but the sacral cord segments are still intact,
typical micturition reflexes can still occur. However,
they are no longer controlled by the brain. During the
first few days to several weeks after the damage to the
cord has occurred, the micturition reflexes are suppressed because of the state of “spinal shock” caused by
the sudden loss of facilitative impulses from the brain
stem and cerebrum. However, if the bladder is emptied
periodically by catheterization to prevent bladder injury
caused by overstretching of the bladder, the excitability
of the micturition reflex gradually increases until typical
micturition reflexes return; then, periodic (but unannounced) bladder emptying occurs.
Some patients can still control urination in this condition by stimulating the skin (scratching or tickling) in
the genital region, which sometimes elicits a micturition
reflex.
Efferent
arteriole
4
Renal
vein
Urinary excretion
Excretion = Filtration – Reabsorption + Secretion
Figure 26–8
Basic kidney processes that determine the composition of the
urine. Urinary excretion rate of a substance is equal to the rate at
which the substance is filtered minus its reabsorption rate plus the
rate at which it is secreted from the peritubular capillary blood into
the tubules.
Urine formation begins when a large amount of
fluid that is virtually free of protein is filtered from the
glomerular capillaries into Bowman’s capsule. Most
substances in the plasma, except for proteins, are freely
filtered, so that their concentration in the glomerular
filtrate in Bowman’s capsule is almost the same as in
the plasma. As filtered fluid leaves Bowman’s capsule
and passes through the tubules, it is modified by reabsorption of water and specific solutes back into the
blood or by secretion of other substances from the
peritubular capillaries into the tubules.
Figure 26–9 shows the renal handling of four hypothetical substances. The substance shown in panel A is
freely filtered by the glomerular capillaries but is
neither reabsorbed nor secreted. Therefore, its excretion rate is equal to the rate at which it was filtered.
Certain waste products in the body, such as creatinine,
are handled by the kidneys in this manner, allowing
excretion of essentially all that is filtered.
In panel B, the substance is freely filtered but is also
partly reabsorbed from the tubules back into the
blood. Therefore, the rate of urinary excretion is less
than the rate of filtration at the glomerular capillaries.
In this case, the excretion rate is calculated as the filtration rate minus the reabsorption rate. This is typical
for many of the electrolytes of the body.
In panel C, the substance is freely filtered at the
glomerular capillaries but is not excreted into the
Chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
A. Filtration only
B. Filtration, partial
reabsorption
Substance B
Substance A
Urine
Urine
C. Filtration, complete
D. Filtration, secretion
reabsorption
Substance D
Substance C
Urine
Urine
Figure 26–9
Renal handling of four hypothetical substances. A, The substance
is freely filtered but not reabsorbed. B, The substance is freely filtered, but part of the filtered load is reabsorbed back in the blood.
C, The substance is freely filtered but is not excreted in the urine
because all the filtered substance is reabsorbed from the tubules
into the blood. D, The substance is freely filtered and is not reabsorbed but is secreted from the peritubular capillary blood into the
renal tubules.
urine because all the filtered substance is reabsorbed
from the tubules back into the blood. This pattern
occurs for some of the nutritional substances in the
blood, such as amino acids and glucose, allowing them
to be conserved in the body fluids.
The substance in panel D is freely filtered at the
glomerular capillaries and is not reabsorbed, but additional quantities of this substance are secreted from
the peritubular capillary blood into the renal tubules.
This pattern often occurs for organic acids and bases,
permitting them to be rapidly cleared from the blood
and excreted in large amounts in the urine. The excretion rate in this case is calculated as filtration rate plus
tubular secretion rate.
For each substance in the plasma, a particular combination of filtration, reabsorption, and secretion
occurs. The rate at which the substance is excreted in
the urine depends on the relative rates of these three
basic renal processes.
315
Filtration, Reabsorption, and
Secretion of Different Substances
In general, tubular reabsorption is quantitatively more
important than tubular secretion in the formation of
urine, but secretion plays an important role in determining the amounts of potassium and hydrogen ions
and a few other substances that are excreted in the
urine. Most substances that must be cleared from the
blood, especially the end products of metabolism such
as urea, creatinine, uric acid, and urates, are poorly
reabsorbed and are therefore excreted in large
amounts in the urine. Certain foreign substances and
drugs are also poorly reabsorbed but, in addition, are
secreted from the blood into the tubules, so that their
excretion rates are high. Conversely, electrolytes, such
as sodium ions, chloride ions, and bicarbonate ions, are
highly reabsorbed, so that only small amounts appear
in the urine. Certain nutritional substances, such as
amino acids and glucose, are completely reabsorbed
from the tubules and do not appear in the urine even
though large amounts are filtered by the glomerular
capillaries.
Each of the processes—glomerular filtration,
tubular reabsorption, and tubular secretion—is regulated according to the needs of the body. For example,
when there is excess sodium in the body, the rate at
which sodium is filtered increases and a smaller fraction of the filtered sodium is reabsorbed, resulting in
increased urinary excretion of sodium.
For most substances, the rates of filtration and reabsorption are extremely large relative to the rates of
excretion.Therefore, subtle adjustments of filtration or
reabsorption can lead to relatively large changes in
renal excretion. For example, an increase in glomerular filtration rate (GFR) of only 10 per cent (from 180
to 198 L/day) would raise urine volume 13-fold (from
1.5 to 19.5 L/day) if tubular reabsorption remained
constant. In reality, changes in glomerular filtration
and tubular reabsorption usually act in a coordinated
manner to produce the necessary changes in renal
excretion.
Why Are Large Amounts of Solutes Filtered and Then Reabsorbed by the Kidneys? One might question the wisdom
of filtering such large amounts of water and solutes
and then reabsorbing most of these substances. One
advantage of a high GFR is that it allows the kidneys
to rapidly remove waste products from the body that
depend primarily on glomerular filtration for their
excretion. Most waste products are poorly reabsorbed
by the tubules and, therefore, depend on a high GFR
for effective removal from the body.
A second advantage of a high GFR is that it allows
all the body fluids to be filtered and processed by the
kidney many times each day. Because the entire
plasma volume is only about 3 liters, whereas the GFR
is about 180 L/day, the entire plasma can be filtered
and processed about 60 times each day. This high GFR
allows the kidneys to precisely and rapidly control the
volume and composition of the body fluids.
316
Unit V
The Body Fluids and Kidneys
Glomerular Filtration—The
First Step in Urine Formation
Proximal tubule
Podocytes
Composition of the Glomerular
Filtrate
Urine formation begins with filtration of large
amounts of fluid through the glomerular capillaries
into Bowman’s capsule. Like most capillaries, the
glomerular capillaries are relatively impermeable to
proteins, so that the filtered fluid (called the glomerular filtrate) is essentially protein-free and devoid of
cellular elements, including red blood cells.
The concentrations of other constituents of the
glomerular filtrate, including most salts and organic
molecules, are similar to the concentrations in the
plasma. Exceptions to this generalization include a few
low-molecular-weight substances, such as calcium and
fatty acids, that are not freely filtered because they are
partially bound to the plasma proteins. Almost one
half of the plasma calcium and most of the plasma fatty
acids are bound to proteins, and these bound portions
are not filtered through the glomerular capillaries.
Capillary loops
Bowman's space
A
As in other capillaries, the GFR is determined by (1)
the balance of hydrostatic and colloid osmotic forces
acting across the capillary membrane and (2) the capillary filtration coefficient (Kf), the product of the permeability and filtering surface area of the capillaries.
The glomerular capillaries have a much higher rate of
filtration than most other capillaries because of a high
glomerular hydrostatic pressure and a large Kf. In the
average adult human, the GFR is about 125 ml/min, or
180 L/day. The fraction of the renal plasma flow that is
filtered (the filtration fraction) averages about 0.2; this
means that about 20 per cent of the plasma flowing
through the kidney is filtered through the glomerular
capillaries. The filtration fraction is calculated as
follows:
Filtration fraction = GFR/Renal plasma flow
Glomerular Capillary Membrane
The glomerular capillary membrane is similar to that
of other capillaries, except that it has three (instead
of the usual two) major layers: (1) the endothelium of
the capillary, (2) a basement membrane, and (3) a layer
of epithelial cells (podocytes) surrounding the outer
surface of the capillary basement membrane (Figure
26–10). Together, these layers make up the filtration
barrier, which, despite the three layers, filters several
hundred times as much water and solutes as the usual
capillary membrane. Even with this high rate of filtration, the glomerular capillary membrane normally prevents filtration of plasma proteins.
Efferent arteriole
Slit pores
Epithelium
Basement
membrane
Endothelium
B
GFR Is About 20 Per Cent of the
Renal Plasma Flow
Afferent arteriole
Bowman's capsule
Fenestrations
Figure 26–10
A, Basic ultrastructure of the glomerular capillaries. B, Cross
section of the glomerular capillary membrane and its major components: capillary endothelium, basement membrane, and epithelium (podocytes).
The high filtration rate across the glomerular capillary membrane is due partly to its special characteristics. The capillary endothelium is perforated by
thousands of small holes called fenestrae, similar to the
fenestrated capillaries found in the liver. Although the
fenestrations are relatively large, endothelial cells are
richly endowed with fixed negative charges that hinder
the passage of plasma proteins.
Surrounding the endothelium is the basement membrane, which consists of a meshwork of collagen and
proteoglycan fibrillae that have large spaces through
which large amounts of water and small solutes can
filter. The basement membrane effectively prevents
filtration of plasma proteins, in part because of
strong negative electrical charges associated with the
proteoglycans.
The final part of the glomerular membrane is a layer
of epithelial cells that line the outer surface of the
glomerulus. These cells are not continuous but have
long footlike processes (podocytes) that encircle the
outer surface of the capillaries (see Figure 26–10).
The foot processes are separated by gaps called
slit pores through which the glomerular filtrate moves.
The epithelial cells, which also have negative charges,
provide additional restriction to filtration of plasma
proteins. Thus, all layers of the glomerular capillary
wall provide a barrier to filtration of plasma
proteins.
Chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
317
Table 26–1
Filterability of Substances by Glomerular Capillaries Based
on Molecular Weight
Molecular Weight
Filterability
Water
Sodium
Glucose
Inulin
Myoglobin
Albumin
18
23
180
5,500
17,000
69,000
1.0
1.0
1.0
1.0
0.75
0.005
Relative filterability
Substance
1.0
Polycationic dextran
Neutral dextran
Polyanionic dextran
0.8
0.6
0.4
0.2
Filterability of Solutes Is Inversely Related to Their
Size. The glomerular capillary membrane is thicker
than most other capillaries, but it is also much more
porous and therefore filters fluid at a high rate. Despite
the high filtration rate, the glomerular filtration barrier
is selective in determining which molecules will filter,
based on their size and electrical charge.
Table 26–1 lists the effect of molecular size on filterability of different molecules. A filterability of 1.0
means that the substance is filtered as freely as water;
a filterability of 0.75 means that the substance is filtered only 75 per cent as rapidly as water. Note that
electrolytes such as sodium and small organic compounds such as glucose are freely filtered. As the
molecular weight of the molecule approaches that of
albumin, the filterability rapidly decreases, approaching zero.
Negatively Charged Large Molecules Are Filtered Less Easily
Than Positively Charged Molecules of Equal Molecular Size.
The molecular diameter of the plasma protein albumin
is only about 6 nanometers, whereas the pores of
the glomerular membrane are thought to be about
8 nanometers (80 angstroms). Albumin is restricted
from filtration, however, because of its negative charge
and the electrostatic repulsion exerted by negative
charges of the glomerular capillary wall proteoglycans.
Figure 26–11 shows how electrical charge affects the
filtration of different molecular weight dextrans by the
glomerulus. Dextrans are polysaccharides that can be
manufactured as neutral molecules or with negative or
positive charges. Note that for any given molecular
radius, positively charged molecules are filtered much
more readily than negatively charged molecules.
Neutral dextrans are also filtered more readily than
negatively charged dextrans of equal molecular
weight. The reason for these differences in filterability
is that the negative charges of the basement membrane and the podocytes provide an important means
for restricting large negatively charged molecules,
including the plasma proteins.
In certain kidney diseases, the negative charges on
the basement membrane are lost even before there are
noticeable changes in kidney histology, a condition
referred to as minimal change nephropathy. As a result
of this loss of negative charges on the basement
membranes, some of the lower-molecular-weight
proteins, especially albumin, are filtered and appear
0
18
22
26
30
34
38
Effective molecular radius (A)
42
Figure 26–11
Effect of size and electrical charge of dextran on its filterability by
the glomerular capillaries. A value of 1.0 indicates that the substance is filtered as freely as water, whereas a value of 0 indicates
that it is not filtered. Dextrans are polysaccharides that can be
manufactured as neutral molecules or with negative or positive
charges and with varying molecular weights.
in the urine, a condition known as proteinuria or
albuminuria.
Determinants of the GFR
The GFR is determined by (1) the sum of the hydrostatic and colloid osmotic forces across the glomerular
membrane, which gives the net filtration pressure, and
(2) the glomerular capillary filtration coefficient,
Kf. Expressed mathematically, the GFR equals the
product of Kf and the net filtration pressure:
GFR = Kf ¥ Net filtration pressure
The net filtration pressure represents the sum of the
hydrostatic and colloid osmotic forces that either favor
or oppose filtration across the glomerular capillaries
(Figure 26–12). These forces include (1) hydrostatic
pressure inside the glomerular capillaries (glomerular
hydrostatic pressure, PG), which promotes filtration;
(2) the hydrostatic pressure in Bowman’s capsule (PB)
outside the capillaries, which opposes filtration; (3) the
colloid osmotic pressure of the glomerular capillary
plasma proteins (pG), which opposes filtration; and
(4) the colloid osmotic pressure of the proteins in
Bowman’s capsule (pB), which promotes filtration.
(Under normal conditions, the concentration of
protein in the glomerular filtrate is so low that the
colloid osmotic pressure of the Bowman’s capsule fluid
is considered to be zero.)
The GFR can therefore be expressed as
GFR = Kf ¥ (PG – PB – pG + pB)
318
Unit V
Afferent
arteriole
Glomerular
Glomerular
hydrostatic colloid osmotic
pressure
pressure
(60 mm Hg)
(32 mm Hg)
The Body Fluids and Kidneys
Efferent
arteriole
Bowman's
capsule
pressure
(18 mm Hg)
Net filtration
pressure
(10 mm Hg)
=
Glomerular
hydrostatic –
pressure
(60 mm Hg)
Bowman's
capsule
–
pressure
(18 mm Hg)
Glomerular
oncotic
pressure
(32 mm Hg)
4.2 ml/min/mm Hg, a value about 400 times as high as
the Kf of most other capillary systems of the body; the
average Kf of many other tissues in the body is only
about 0.01 ml/min/mm Hg per 100 grams. This high Kf
for the glomerular capillaries contributes tremendously to their rapid rate of fluid filtration.
Although increased Kf raises GFR and decreased Kf
reduces GFR, changes in Kf probably do not provide
a primary mechanism for the normal day-to-day regulation of GFR. Some diseases, however, lower Kf by
reducing the number of functional glomerular capillaries (thereby reducing the surface area for filtration)
or by increasing the thickness of the glomerular
capillary membrane and reducing its hydraulic conductivity. For example, chronic, uncontrolled hypertension and diabetes mellitus gradually reduce Kf by
increasing the thickness of the glomerular capillary
basement membrane and, eventually, by damaging the
capillaries so severely that there is loss of capillary
function.
Figure 26–12
Summary of forces causing filtration by the glomerular capillaries.
The values shown are estimates for healthy humans.
Although the normal values for the determinants of
GFR have not been measured directly in humans, they
have been estimated in animals such as dogs and rats.
Based on the results in animals, the approximate
normal forces favoring and opposing glomerular filtration in humans are believed to be as follows (see
Figure 26–12):
Forces Favoring Filtration (mm Hg)
Glomerular hydrostatic pressure
Bowman’s capsule colloid osmotic pressure
60
0
Forces Opposing Filtration (mm Hg)
Bowman’s capsule hydrostatic pressure
Glomerular capillary colloid osmotic pressure
18
32
Net filtration pressure = 60 – 18 – 32 = +10 mm Hg
Some of these values can change markedly under
different physiologic conditions, whereas others are
altered mainly in disease states, as discussed later.
Increased Glomerular Capillary
Filtration Coefficient Increases GFR
The Kf is a measure of the product of the hydraulic
conductivity and surface area of the glomerular capillaries. The Kf cannot be measured directly, but it is
estimated experimentally by dividing the rate of
glomerular filtration by net filtration pressure:
Kf = GFR/Net filtration pressure
Because total GFR for both kidneys is about 125 ml/
min and the net filtration pressure is 10 mm Hg, the
normal Kf is calculated to be about 12.5 ml/min/mm
Hg of filtration pressure. When Kf is expressed per
100 grams of kidney weight, it averages about
Increased Bowman’s Capsule
Hydrostatic Pressure Decreases GFR
Direct measurements, using micropipettes, of hydrostatic pressure in Bowman’s capsule and at different
points in the proximal tubule suggest that a reasonable
estimate for Bowman’s capsule pressure in humans is
about 18 mm Hg under normal conditions. Increasing
the hydrostatic pressure in Bowman’s capsule reduces
GFR, whereas decreasing this pressure raises GFR.
However, changes in Bowman’s capsule pressure normally do not serve as a primary means for regulating
GFR.
In certain pathological states associated with
obstruction of the urinary tract, Bowman’s capsule
pressure can increase markedly, causing serious reduction of GFR. For example, precipitation of calcium or
of uric acid may lead to “stones” that lodge in the
urinary tract, often in the ureter, thereby obstructing
outflow of the urinary tract and raising Bowman’s
capsule pressure. This reduces GFR and eventually
can damage or even destroy the kidney unless the
obstruction is relieved.
Increased Glomerular Capillary
Colloid Osmotic Pressure
Decreases GFR
As blood passes from the afferent arteriole through
the glomerular capillaries to the efferent arterioles,
the plasma protein concentration increases about
20 per cent (Figure 26–13). The reason for this is that
about one fifth of the fluid in the capillaries filters into
Bowman’s capsule, thereby concentrating the
glomerular plasma proteins that are not filtered.
Assuming that the normal colloid osmotic pressure
of plasma entering the glomerular capillaries is
28 mm Hg, this value usually rises to about 36 mm Hg
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
Normal
36
34
32
Filtration
fraction
30
28
100
Distance along
glomerular capillary
1400
Normal
60
800
Renal blood
flow
0
Afferent
end
0
Efferent
end
Glomerular filtration
rate (ml/min)
Increase in colloid osmotic pressure in plasma flowing through the
glomerular capillary. Normally, about one fifth of the fluid in the
glomerular capillaries filters into Bowman’s capsule, thereby concentrating the plasma proteins that are not filtered. Increases in
the filtration fraction (glomerular filtration rate/renal plasma flow)
increase the rate at which the plasma colloid osmotic pressure
rises along the glomerular capillary; decreases in the filtration
fraction have the opposite effect.
2000
100
1400
150
Normal
100
Glomerular
filtration
rate
50
Renal blood
flow
0
800
200
0
by the time the blood reaches the efferent end of the
capillaries. Therefore, the average colloid osmotic
pressure of the glomerular capillary plasma proteins
is midway between 28 and 36 mm Hg, or about
32 mm Hg.
Thus, two factors that influence the glomerular capillary colloid osmotic pressure are (1) the arterial
plasma colloid osmotic pressure and (2) the fraction
of plasma filtered by the glomerular capillaries (filtration fraction). Increasing the arterial plasma colloid
osmotic pressure raises the glomerular capillary
colloid osmotic pressure, which in turn decreases
GFR.
Increasing the filtration fraction also concentrates the
plasma proteins and raises the glomerular colloid
osmotic pressure (see Figure 26–13). Because the filtration fraction is defined as GFR/renal plasma flow,
the filtration fraction can be increased either by raising
GFR or by reducing renal plasma flow. For example, a
reduction in renal plasma flow with no initial change
in GFR would tend to increase the filtration fraction,
which would raise the glomerular capillary colloid
osmotic pressure and tend to reduce GFR. For this
reason, changes in renal blood flow can influence GFR
independently of changes in glomerular hydrostatic
pressure.
With increasing renal blood flow, a lower fraction of
the plasma is initially filtered out of the glomerular
capillaries, causing a slower rise in the glomerular
capillary colloid osmotic pressure and less inhibitory
effect on GFR. Consequently, even with a constant
glomerular hydrostatic pressure, a greater rate of blood
flow into the glomerulus tends to increase GFR, and a
lower rate of blood flow into the glomerulus tends to
decrease GFR.
200
1
2
3
4
Efferent arteriolar resistance
(X normal)
250
Figure 26–13
2000
Renal blood flow
(ml/min)
38
Glomerular
filtration
rate
150
Filtration
fraction
319
Renal blood flow
(ml/min)
Glomerular colloid
osmotic pressure
(mm Hg)
40
Glomerular filtration
rate (ml/min)
Chapter 26
1
2
3
4
Afferent arteriolar resistance
(X normal)
Figure 26–14
Effect of change in afferent arteriolar resistance or efferent arteriolar resistance on glomerular filtration rate and renal blood flow.
Increased Glomerular Capillary
Hydrostatic Pressure Increases GFR
The glomerular capillary hydrostatic pressure has
been estimated to be about 60 mm Hg under normal
conditions. Changes in glomerular hydrostatic pressure serve as the primary means for physiologic regulation of GFR. Increases in glomerular hydrostatic
pressure raise GFR, whereas decreases in glomerular
hydrostatic pressure reduce GFR.
Glomerular hydrostatic pressure is determined by
three variables, each of which is under physiologic
control: (1) arterial pressure, (2) afferent arteriolar
resistance, and (3) efferent arteriolar resistance.
Increased arterial pressure tends to raise glomerular hydrostatic pressure and, therefore, to increase
GFR. (However, as discussed later, this effect is
buffered by autoregulatory mechanisms that maintain
a relatively constant glomerular pressure as blood
pressure fluctuates.)
Increased resistance of afferent arterioles reduces
glomerular hydrostatic pressure and decreases GFR.
Conversely, dilation of the afferent arterioles increases
both glomerular hydrostatic pressure and GFR
(Figure 26–14).
320
Unit V
The Body Fluids and Kidneys
Constriction of the efferent arterioles increases the
resistance to outflow from the glomerular capillaries.
This raises the glomerular hydrostatic pressure, and as
long as the increase in efferent resistance does not
reduce renal blood flow too much, GFR increases
slightly (see Figure 26–14). However, because efferent
arteriolar constriction also reduces renal blood flow,
the filtration fraction and glomerular colloid osmotic
pressure increase as efferent arteriolar resistance
increases. Therefore, if the constriction of efferent
arterioles is severe (more than about a threefold
increase in efferent arteriolar resistance), the rise in
colloid osmotic pressure exceeds the increase in
glomerular capillary hydrostatic pressure caused by
efferent arteriolar constriction. When this occurs, the
net force for filtration actually decreases, causing a
reduction in GFR.
Thus, efferent arteriolar constriction has a biphasic
effect on GFR. At moderate levels of constriction,
there is a slight increase in GFR, but with severe constriction, there is a decrease in GFR. The primary
cause of the eventual decrease in GFR is as follows:
As efferent constriction becomes severe and as plasma
protein concentration increases, there is a rapid, nonlinear increase in colloid osmotic pressure caused by
the Donnan effect; the higher the protein concentration, the more rapidly the colloid osmotic pressure
rises because of the interaction of ions bound to the
plasma proteins, which also exert an osmotic effect, as
discussed in Chapter 16.
To summarize, constriction of afferent arterioles
reduces GFR. However, the effect of efferent arteriolar constriction depends on the severity of the constriction; modest efferent constriction raises GFR, but
severe efferent constriction (more than a threefold
increase in resistance) tends to reduce GFR.
Table 26–2 summarizes the factors that can decrease
GFR.
Table 26–2
Factors That Can Decrease the Glomerular Filtration Rate
(GFR)
Physical Determinants*
Physiologic/Pathophysiologic Causes
Ø Kf Æ Ø GFR
Renal disease, diabetes mellitus,
hypertension
Urinary tract obstruction (e.g., kidney
stones)
Ø Renal blood flow, increased plasma
proteins
≠ PB Æ Ø GFR
≠ pG Æ Ø GFR
Ø PG Æ Ø GFR
Ø AP Æ Ø PG
Ø RE Æ Ø PG
≠ RA Æ Ø PG
Ø Arterial pressure (has only small effect
due to autoregulation)
Ø Angiotensin II (drugs that block
angiotensin II formation)
≠ Sympathetic activity, vasoconstrictor
hormones (e.g., norepinephrine,
endothelin)
* Opposite changes in the determinants usually increase GFR.
Kf, glomerular filtration coefficient; PB, Bowman’s capsule hydrostatic pressure; pG, glomerular capillary colloid osmotic pressure; PG, glomerular capillary hydrostatic pressure; AP, systemic arterial pressure; RE, efferent arteriolar
resistance; RA, afferent arteriolar resistance.
Renal Blood Flow
In an average 70-kilogram man, the combined blood
flow through both kidneys is about 1100 ml/min, or
about 22 per cent of the cardiac output. Considering
the fact that the two kidneys constitute only about 0.4
per cent of the total body weight, one can readily see
that they receive an extremely high blood flow compared with other organs.
As with other tissues, blood flow supplies the
kidneys with nutrients and removes waste products.
However, the high flow to the kidneys greatly exceeds
this need. The purpose of this additional flow is to
supply enough plasma for the high rates of glomerular filtration that are necessary for precise regulation
of body fluid volumes and solute concentrations. As
might be expected, the mechanisms that regulate renal
blood flow are closely linked to the control of GFR
and the excretory functions of the kidneys.
Renal Blood Flow and Oxygen
Consumption
On a per gram weight basis, the kidneys normally
consume oxygen at twice the rate of the brain but have
almost seven times the blood flow of the brain. Thus,
the oxygen delivered to the kidneys far exceeds their
metabolic needs, and the arterial-venous extraction of
oxygen is relatively low compared with that of most
other tissues.
A large fraction of the oxygen consumed by the
kidneys is related to the high rate of active sodium
reabsorption by the renal tubules. If renal blood flow
and GFR are reduced and less sodium is filtered, less
sodium is reabsorbed and less oxygen is consumed.
Therefore, renal oxygen consumption varies in proportion to renal tubular sodium reabsorption, which in
turn is closely related to GFR and the rate of sodium
filtered (Figure 26–15). If glomerular filtration completely ceases, renal sodium reabsorption also ceases,
and oxygen consumption decreases to about one
fourth normal. This residual oxygen consumption
reflects the basic metabolic needs of the renal cells.
Determinants of Renal Blood Flow
Renal blood flow is determined by the pressure gradient across the renal vasculature (the difference
between renal artery and renal vein hydrostatic pressures), divided by the total renal vascular resistance:
(Renal artery pressure - Renal vein pressure)
Total renal vascular resistance
Renal artery pressure is about equal to systemic
arterial pressure, and renal vein pressure averages
about 3 to 4 mm Hg under most conditions. As in other
vascular beds, the total vascular resistance through the
kidneys is determined by the sum of the resistances in
the individual vasculature segments, including the
arteries, arterioles, capillaries, and veins (Table 26–3).
Chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
321
Table 26–3
Approximate Pressures and Vascular Resistances in the Circulation of a Normal Kidney
Pressure in Vessel (mm Hg)
Beginning
End
Vessel
Renal artery
Interlobar, arcuate, and interlobular arteries
Afferent arteriole
Glomerular capillaries
Efferent arteriole
Peritubular capillaries
Interlobar, interlobular, and arcuate veins
Renal vein
100
~100
85
60
59
18
8
4
~0
~16
~26
~1
~43
~10
~4
~0
range between 80 and 170 mm Hg, a process called
autoregulation. This capacity for autoregulation occurs
through mechanisms that are completely intrinsic to
the kidneys, as discussed later in this chapter.
3.0
Oxygen consumption
(ml/min/100 gm kidney weight)
100
85
60
59
18
8
4
~4
Per Cent of Total Renal Vascular Resistance
2.5
Blood Flow in the Vasa Recta of the
Renal Medulla Is Very Low Compared
with Flow in the Renal Cortex
2.0
1.5
1.0
0.5
Basal oxygen consumption
0
0
5
10
15
20
Sodium reabsorption
(mEq/min per 100 g kidney weight)
Figure 26–15
Relationship between oxygen consumption and sodium reabsorption in dog kidneys. (Kramer K, Deetjen P: Relation of renal
oxygen consumption to blood supply and glomerular filtration
during variations of blood pressure. Pflugers Arch Physiol
271:782, 1960.)
Most of the renal vascular resistance resides in three
major segments: interlobular arteries, afferent arterioles, and efferent arterioles. Resistance of these vessels
is controlled by the sympathetic nervous system,
various hormones, and local internal renal control
mechanisms, as discussed later. An increase in the
resistance of any of the vascular segments of the
kidneys tends to reduce the renal blood flow, whereas
a decrease in vascular resistance increases renal blood
flow if renal artery and renal vein pressures remain
constant.
Although changes in arterial pressure have some
influence on renal blood flow, the kidneys have effective mechanisms for maintaining renal blood flow and
GFR relatively constant over an arterial pressure
The outer part of the kidney, the renal cortex, receives
most of the kidney’s blood flow. Blood flow in the renal
medulla accounts for only 1 to 2 per cent of the total
renal blood flow. Flow to the renal medulla is supplied
by a specialized portion of the peritubular capillary
system called the vasa recta. These vessels descend into
the medulla in parallel with the loops of Henle and
then loop back along with the loops of Henle and
return to the cortex before emptying into the venous
system. As discussed in Chapter 28, the vasa recta play
an important role in allowing the kidneys to form a
concentrated urine.
Physiologic Control of
Glomerular Filtration and
Renal Blood Flow
The determinants of GFR that are most variable and
subject to physiologic control include the glomerular
hydrostatic pressure and the glomerular capillary
colloid osmotic pressure. These variables, in turn, are
influenced by the sympathetic nervous system, hormones and autacoids (vasoactive substances that are
released in the kidneys and act locally), and other
feedback controls that are intrinsic to the kidneys.
Sympathetic Nervous System
Activation Decreases GFR
Essentially all the blood vessels of the kidneys, including the afferent and the efferent arterioles, are richly
innervated by sympathetic nerve fibers. Strong activation of the renal sympathetic nerves can constrict the
renal arterioles and decrease renal blood flow and
322
Unit V
The Body Fluids and Kidneys
GFR. Moderate or mild sympathetic stimulation has
little influence on renal blood flow and GFR. For
example, reflex activation of the sympathetic nervous
system resulting from moderate decreases in pressure
at the carotid sinus baroreceptors or cardiopulmonary
receptors has little influence on renal blood flow or
GFR.
The renal sympathetic nerves seem to be most
important in reducing GFR during severe, acute disturbances lasting for a few minutes to a few hours,
such as those elicited by the defense reaction, brain
ischemia, or severe hemorrhage. In the healthy resting
person, sympathetic tone appears to have little influence on renal blood flow.
Hormonal and Autacoid Control
of Renal Circulation
There are several hormones and autacoids that can
influence GFR and renal blood flow, as summarized in
Table 26–4.
Norepinephrine, Epinephrine, and Endothelin Constrict Renal
Blood Vessels and Decrease GFR. Hormones that constrict
afferent and efferent arterioles, causing reductions in
GFR and renal blood flow, include norepinephrine and
epinephrine released from the adrenal medulla. In
general, blood levels of these hormones parallel the
activity of the sympathetic nervous system; thus, norepinephrine and epinephrine have little influence on
renal hemodynamics except under extreme conditions,
such as severe hemorrhage.
Another vasoconstrictor, endothelin, is a peptide
that can be released by damaged vascular endothelial
cells of the kidneys as well as by other tissues. The
physiologic role of this autacoid is not completely
understood. However, endothelin may contribute to
hemostasis (minimizing blood loss) when a blood
vessel is severed, which damages the endothelium
and releases this powerful vasoconstrictor. Plasma
endothelin levels also are increased in certain disease
states associated with vascular injury, such as toxemia
of pregnancy, acute renal failure, and chronic uremia,
and may contribute to renal vasoconstriction and
decreased GFR in some of these pathophysiologic
conditions.
Angiotensin II Constricts Efferent Arterioles. A powerful
renal vasoconstrictor, angiotensin II, can be considered a circulating hormone as well as a locally produced autacoid because it is formed in the kidneys
as well as in the systemic circulation. Because
angiotensin II preferentially constricts efferent arterioles, increased angiotensin II levels raise glomerular
hydrostatic pressure while reducing renal blood flow.
It should be kept in mind that increased angiotensin
II formation usually occurs in circumstances associated with decreased arterial pressure or volume
depletion, which tend to decrease GFR. In these circumstances, the increased level of angiotensin II, by
constricting efferent arterioles, helps prevent decreases
in glomerular hydrostatic pressure and GFR; at the
same time, though, the reduction in renal blood flow
caused by efferent arteriolar constriction contributes
to decreased flow through the peritubular capillaries,
which in turn increases reabsorption of sodium and
water, as discussed in Chapter 27.
Thus, increased angiotensin II levels that occur with
a low-sodium diet or volume depletion help preserve
GFR and maintain normal excretion of metabolic
waste products such as urea and creatinine that
depend on glomerular filtration for their excretion; at
the same time, the angiotensin II–induced constriction
of efferent arterioles increases tubular reabsorption of
sodium and water, which helps restore blood volume
and blood pressure. This effect of angiotensin II in
helping to “autoregulate” GFR is discussed in more
detail later in this chapter.
Endothelial-Derived Nitric Oxide Decreases Renal Vascular
Resistance and Increases GFR. An autacoid that de-
creases renal vascular resistance and is released by the
vascular endothelium throughout the body is endothelial-derived nitric oxide. A basal level of nitric oxide
production appears to be important for maintaining
vasodilation of the kidneys. This allows the kidneys to
excrete normal amounts of sodium and water. Therefore, administration of drugs that inhibit this normal
formation of nitric oxide increases renal vascular
resistance and decreases GFR and urinary sodium
excretion, eventually causing high blood pressure. In
some hypertensive patients, impaired nitric oxide production could be the cause of increased renal vasoconstriction and increased blood pressure.
Prostaglandins and Bradykinin Tend to Increase GFR. Hor-
Table 26–4
Hormones and Autacoids That Influence Glomerular
Filtration Rate (GFR)
Hormone or Autacoid
Effect on GFR
Norepinephrine
Epinephrine
Endothelin
Angiotensin II
Endothelial-derived nitric oxide
Prostaglandins
Ø
Ø
Ø
¨Æ (prevents Ø)
≠
≠
mones and autacoids that cause vasodilation and
increased renal blood flow and GFR include the
prostaglandins (PGE2 and PGI2) and bradykinin.
These substances are discussed in Chapter 17.
Although these vasodilators do not appear to be of
major importance in regulating renal blood flow or
GFR in normal conditions, they may dampen the renal
vasoconstrictor effects of the sympathetic nerves or
angiotensin II, especially their effects to constrict the
afferent arterioles.
By opposing vasoconstriction of afferent arterioles,
the prostaglandins may help prevent excessive reductions in GFR and renal blood flow. Under stressful
Chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
conditions, such as volume depletion or after surgery,
the administration of nonsteroidal anti-inflammatory
agents, such as aspirin, that inhibit prostaglandin synthesis may cause significant reductions in GFR.
Autoregulation of GFR and
Renal Blood Flow
1600
160
1200
120
800
80
400
0
Renal blood flow
Glomerular filtration
rate
40
0
Glomerular filtration
rate (ml/min)
Renal blood flow
(ml/min)
Feedback mechanisms intrinsic to the kidneys normally keep the renal blood flow and GFR relatively
constant, despite marked changes in arterial blood
pressure. These mechanisms still function in bloodperfused kidneys that have been removed from the
body, independent of systemic influences. This relative
constancy of GFR and renal blood flow is referred to
as autoregulation (Figure 26–16).
The primary function of blood flow autoregulation
in most tissues other than the kidneys is to maintain
the delivery of oxygen and nutrients at a normal level
and to remove the waste products of metabolism,
despite changes in the arterial pressure. In the kidneys,
the normal blood flow is much higher than that
required for these functions. The major function of
autoregulation in the kidneys is to maintain a relatively constant GFR and to allow precise control of
renal excretion of water and solutes.
The GFR normally remains autoregulated (that is,
remains relatively constant), despite considerable
arterial pressure fluctuations that occur during a
person’s usual activities. For instance, a decrease in
Urine output
(ml/min)
8
6
4
2
0
50
100
150
Arterial pressure
(mm Hg)
200
Figure 26–16
Autoregulation of renal blood flow and glomerular filtration rate but
lack of autoregulation of urine flow during changes in renal arterial pressure.
323
arterial pressure to as low as 75 mm Hg or an increase
to as high as 160 mm Hg changes GFR only a few
percentage points. In general, renal blood flow is autoregulated in parallel with GFR, but GFR is more efficiently autoregulated under certain conditions.
Importance of GFR Autoregulation
in Preventing Extreme Changes in
Renal Excretion
The autoregulatory mechanisms of the kidney are not
100 per cent perfect, but they do prevent potentially
large changes in GFR and renal excretion of water and
solutes that would otherwise occur with changes in
blood pressure. One can understand the quantitative
importance of autoregulation by considering the relative magnitudes of glomerular filtration, tubular reabsorption, and renal excretion and the changes in renal
excretion that would occur without autoregulatory
mechanisms.
Normally, GFR is about 180 L/day and tubular reabsorption is 178.5 L/day, leaving 1.5 L/day of fluid to be
excreted in the urine. In the absence of autoregulation,
a relatively small increase in blood pressure (from 100
to 125 mm Hg) would cause a similar 25 per cent
increase in GFR (from about 180 to 225 L/day). If
tubular reabsorption remained constant at 178.5 L/
day, this would increase the urine flow to 46.5 L/day
(the difference between GFR and tubular reabsorption)—a total increase in urine of more than 30-fold.
Because the total plasma volume is only about 3 liters,
such a change would quickly deplete the blood
volume.
But in reality, such a change in arterial pressure
exerts much less of an effect on urine volume for
two reasons: (1) renal autoregulation prevents large
changes in GFR that would otherwise occur, and (2)
there are additional adaptive mechanisms in the renal
tubules that allow them to increase their reabsorption
rate when GFR rises, a phenomenon referred to as
glomerulotubular balance (discussed in Chapter 27).
Even with these special control mechanisms, changes
in arterial pressure still have significant effects on
renal excretion of water and sodium; this is referred to
as pressure diuresis or pressure natriuresis, and it is
crucial in the regulation of body fluid volumes and
arterial pressure, as discussed in Chapters 19 and 29.
Role of Tubuloglomerular Feedback
in Autoregulation of GFR
To perform the function of autoregulation, the kidneys
have a feedback mechanism that links changes in
sodium chloride concentration at the macula densa
with the control of renal arteriolar resistance. This
feedback helps ensure a relatively constant delivery of
sodium chloride to the distal tubule and helps prevent
spurious fluctuations in renal excretion that would
otherwise occur. In many circumstances, this feedback
324
Unit V
The Body Fluids and Kidneys
autoregulates renal blood flow and GFR in parallel.
However, because this mechanism is specifically
directed toward stabilizing sodium chloride delivery to
the distal tubule, there are instances when GFR is
autoregulated at the expense of changes in renal blood
flow, as discussed later.
The tubuloglomerular feedback mechanism has
two components that act together to control GFR:
(1) an afferent arteriolar feedback mechanism and (2)
an efferent arteriolar feedback mechanism. These
feedback mechanisms depend on special anatomical
arrangements of the juxtaglomerular complex (Figure
26–17).
The juxtaglomerular complex consists of macula
densa cells in the initial portion of the distal tubule and
juxtaglomerular cells in the walls of the afferent and
efferent arterioles. The macula densa is a specialized
group of epithelial cells in the distal tubules that comes
in close contact with the afferent and efferent arterioles. The macula densa cells contain Golgi apparatus,
which are intracellular secretory organelles directed
toward the arterioles, suggesting that these cells may
be secreting a substance toward the arterioles.
Decreased Macula Densa Sodium Chloride Causes Dilation of
Afferent Arterioles and Increased Renin Release. The
macula densa cells sense changes in volume delivery
to the distal tubule by way of signals that are not completely understood. Experimental studies suggest that
decreased GFR slows the flow rate in the loop of
Henle, causing increased reabsorption of sodium and
chloride ions in the ascending loop of Henle, thereby
reducing the concentration of sodium chloride at the
macula densa cells. This decrease in sodium chloride
concentration initiates a signal from the macula densa
that has two effects (Figure 26–18): (1) it decreases
resistance to blood flow in the afferent arterioles,
which raises glomerular hydrostatic pressure and helps
return GFR toward normal, and (2) it increases renin
release from the juxtaglomerular cells of the afferent
and efferent arterioles, which are the major storage
sites for renin. Renin released from these cells then
functions as an enzyme to increase the formation of
angiotensin I, which is converted to angiotensin II.
Finally, the angiotensin II constricts the efferent arterioles, thereby increasing glomerular hydrostatic pressure and returning GFR toward normal.
These two components of the tubuloglomerular
feedback mechanism, operating together by way of the
special anatomical structure of the juxtaglomerular
apparatus, provide feedback signals to both the afferent and the efferent arterioles for efficient autoregulation of GFR during changes in arterial pressure.
When both of these mechanisms are functioning
together, the GFR changes only a few percentage
points, even with large fluctuations in arterial pressure
between the limits of 75 and 160 mm Hg.
Arterial pressure
-
Glomerular hydrostatic
pressure
Glomerular
epithelium
GFR
Proximal
NaCl
reabsorption
Juxtaglomerular
cells
Afferent
arteriole
Efferent
arteriole
-
Macula densa
NaCl
Renin
Angiotensin II
Internal
elastic
lamina
Macula densa
Smooth
muscle
fiber
Distal
tubule
Efferent
arteriolar
resistance
Basement
membrane
Afferent
arteriolar
resistance
Figure 26–18
Figure 26–17
Structure of the juxtaglomerular apparatus, demonstrating its possible feedback role in the control of nephron function.
Macula densa feedback mechanism for autoregulation of
glomerular hydrostatic pressure and glomerular filtration rate
(GFR) during decreased renal arterial pressure.
Chapter 26
Urine Formation by the Kidneys: I. Glomerular Filtration, Renal Blood Flow, and Their Control
Blockade of Angiotensin II Formation Further Reduces GFR During
Renal Hypoperfusion. As discussed earlier, a preferential
constrictor action of angiotensin II on efferent arterioles helps prevent serious reductions in glomerular
hydrostatic pressure and GFR when renal perfusion
pressure falls below normal. The administration of
drugs that block the formation of angiotensin II
(angiotensin-converting enzyme inhibitors) or that
block the action of angiotensin II (angiotensin II antagonists) causes greater reductions in GFR than usual
when the renal arterial pressure falls below normal.
Therefore, an important complication of using these
drugs to treat patients who have hypertension because
of renal artery stenosis (partial blockage of the renal
artery) is a severe decrease in GFR that can, in
some cases, cause acute renal failure. Nevertheless,
angiotensin II–blocking drugs can be useful therapeutic
agents in many patients with hypertension, congestive
heart failure, and other conditions, as long as they are
monitored to ensure that severe decreases in GFR do
not occur.
Myogenic Autoregulation of Renal
Blood Flow and GFR
Another mechanism that contributes to the maintenance of a relatively constant renal blood flow
and GFR is the ability of individual blood vessels to
resist stretching during increased arterial pressure, a
phenomenon referred to as the myogenic mechanism.
Studies of individual blood vessels (especially small
arterioles) throughout the body have shown that they
respond to increased wall tension or wall stretch by
contraction of the vascular smooth muscle. Stretch
of the vascular wall allows increased movement of
calcium ions from the extracellular fluid into the cells,
causing them to contract through the mechanisms discussed in Chapter 8. This contraction prevents overdistention of the vessel and at the same time, by raising
vascular resistance, helps prevent excessive increases
in renal blood flow and GFR when arterial pressure
increases.
Although the myogenic mechanism probably operates in most arterioles throughout the body, its importance in renal blood flow and GFR autoregulation has
been questioned by some physiologists because this
pressure-sensitive mechanism has no means of directly
detecting changes in renal blood flow or GFR per se.
Other Factors That Increase Renal
Blood Flow and GFR: High Protein
Intake and Increased Blood Glucose
Although renal blood flow and GFR are relatively
stable under most conditions, there are circumstances in
which these variables change significantly. For example,
a high protein intake is known to increase both renal
blood flow and GFR. With a chronic high-protein diet,
such as one that contains large amounts of meat, the
increases in GFR and renal blood flow are due partly to
growth of the kidneys. However, GFR and renal blood
flow increase 20 to 30 per cent within 1 or 2 hours after
a person eats a high-protein meal.
325
The exact mechanisms by which this occurs are still
not completely understood, but one possible explanation is the following: A high-protein meal increases the
release of amino acids into the blood, which are reabsorbed in the proximal tubule. Because amino acids
and sodium are reabsorbed together by the proximal
tubules, increased amino acid reabsorption also stimulates sodium reabsorption in the proximal tubules. This
decreases sodium delivery to the macula densa, which
elicits a tubuloglomerular feedback–mediated decrease
in resistance of the afferent arterioles, as discussed
earlier.The decreased afferent arteriolar resistance then
raises renal blood flow and GFR. This increased GFR
allows sodium excretion to be maintained at a nearly
normal level while increasing the excretion of the waste
products of protein metabolism, such as urea.
A similar mechanism may also explain the marked
increases in renal blood flow and GFR that occur with
large increases in blood glucose levels in uncontrolled
diabetes mellitus. Because glucose, like some of the
amino acids, is also reabsorbed along with sodium in
the proximal tubule, increased glucose delivery to the
tubules causes them to reabsorb excess sodium along
with glucose. This, in turn, decreases delivery of sodium
chloride to the macula densa, activating a tubuloglomerular feedback–mediated dilation of the afferent arterioles and subsequent increases in renal blood
flow and GFR.
These examples demonstrate that renal blood flow
and GFR per se are not the primary variables controlled
by the tubuloglomerular feedback mechanism. The
main purpose of this feedback is to ensure a constant
delivery of sodium chloride to the distal tubule, where
final processing of the urine takes place. Thus, disturbances that tend to increase reabsorption of sodium
chloride at tubular sites before the macula densa tend
to elicit increased renal blood flow and GFR, which
helps return distal sodium chloride delivery toward
normal so that normal rates of sodium and water excretion can be maintained (see Figure 26–18).
An opposite sequence of events occurs when proximal tubular reabsorption is reduced. For example, when
the proximal tubules are damaged (which can occur as
a result of poisoning by heavy metals, such as mercury,
or large doses of drugs, such as tetracyclines), their
ability to reabsorb sodium chloride is decreased. As a
consequence, large amounts of sodium chloride are
delivered to the distal tubule and, without appropriate
compensations, would quickly cause excessive volume
depletion. One of the important compensatory
responses appears to be a tubuloglomerular feedback–mediated renal vasoconstriction that occurs in
response to the increased sodium chloride delivery to
the macula densa in these circumstances. These examples again demonstrate the importance of this feedback
mechanism in ensuring that the distal tubule receives
the proper rate of delivery of sodium chloride, other
tubular fluid solutes, and tubular fluid volume so that
appropriate amounts of these substances are excreted
in the urine.
References
Beeuwkes R III: The vascular organization of the kidney.
Annu Rev Physiol 42:531, 1980.
Bell PD, Lapointe JY, Peti-Peterdi J: Macula densa cell signaling. Annu Rev Physiol 65:481, 2003.
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The Body Fluids and Kidneys
Blantz RC, Deng A, Lortie M, et al: The complex role of
nitric oxide in the regulation of glomerular ultrafiltration.
Kidney Int 61:782, 2002.
Cowley AW Jr, Mori T, Mattson D, Zou AP: Role of renal
NO production in the regulation of medullary blood flow.
Am J Physiol Regul Integr Comp Physiol 284:R1355,
2003.
Davis MJ, Hill MA: Signaling mechanisms underlying
the vascular myogenic response. Physiol Rev 79:387,
1999.
Deen WM, Lazzara MJ, Myers BD: Structural determinants
of glomerular permeability. Am J Physiol Renal Physiol
281:F579, 2001.
DiBona GF: Neural control of the kidney: past, present, and
future. Hypertension 41:621, 2003.
Hall JE: Angiotensin II and long-term arterial pressure regulation: the overriding dominance of the kidney. J Am Soc
Nephrol 10 (Suppl 12):s258, 1999.
Hall JE, Brands MW: The renin-angiotensin-aldosterone
system: renal mechanisms and circulatory homeostasis. In
Seldin DW, Giebisch G (eds): The Kidney—Physiology
and Pathophysiology, 3rd ed. New York: Raven Press,
2000, pp 1009-1046.
Haraldsson B, Sörensson J: Why do we not all have proteinuria? An update of our current understanding of the
glomerular barrier. News Physiol Sci 19:7, 2004.
Kriz W, Kaissling B: Structural organization of the mammalian kidney. In Seldin DW, Giebisch G (eds): The
Kidney—Physiology and Pathophysiology, 3rd ed. New
York: Raven Press, 2000, pp 587-654.
Navar LG, Kobori H, Prieto-Carrasquero M: Intrarenal
angiotensin II and hypertension. Curr Hypertens Rep
5:135, 2003.
Pallone TL, Zhang Z, Rhinehart K: Physiology of the renal
medullary microcirculation. Am J Physiol Renal Physiol
284:F253, 2003.
Roman RJ: P-450 metabolites of arachidonic acid in the
control of cardiovascular function. Physiol Rev 82:131,
2002.
Schnermann J, Levine DZ: Paracrine factors in tubuloglomerular feedback: adenosine, ATP, and nitric oxide.
Annu Rev Physiol 65:501, 2003.
Whelton A: Renal aspects of treatment with conventional
nonsteroidal anti-inflammatory drugs versus cyclooxygenase-2-specific inhibitors. Am J Med 110 (Suppl 3A):33S,
2001.
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7
Urine Formation by the Kidneys:
II. Tubular Processing of the
Glomerular Filtrate
Reabsorption and Secretion
by the Renal Tubules
As the glomerular filtrate enters the renal tubules,
it flows sequentially through the successive parts of
the tubule—the proximal tubule, the loop of Henle,
the distal tubule, the collecting tubule, and, finally,
the collecting duct—before it is excreted as urine.
Along this course, some substances are selectively reabsorbed from the tubules
back into the blood, whereas others are secreted from the blood into the tubular
lumen. Eventually, the urine that is formed and all the substances in the urine
represent the sum of three basic renal processes—glomerular filtration, tubular
reabsorption, and tubular secretion—as follows:
Urinary excretion = Glomerular filtration - Tubular reabsorption
+ Tubular secretion
For many substances, reabsorption plays a much more important role than
does secretion in determining the final urinary excretion rate. However, secretion accounts for significant amounts of potassium ions, hydrogen ions, and a
few other substances that appear in the urine.
Tubular Reabsorption Is Selective and Quantitatively Large
Table 27–1 shows the renal handling of several substances that are all freely filtered in the kidneys and reabsorbed at variable rates.
The rate at which each of these substances is filtered is calculated as
Filtration = Glomerular filtration rate ¥ Plasma concentration
This calculation assumes that the substance is freely filtered and not bound
to plasma proteins. For example, if plasma glucose concentration is 1 g/L, the
amount of glucose filtered each day is about 180 L/day ¥ 1 g/L, or 180 g/day.
Because virtually none of the filtered glucose is normally excreted, the rate of
glucose reabsorption is also 180 g/day.
From Table 27–1, two things are immediately apparent. First, the processes of
glomerular filtration and tubular reabsorption are quantitatively very large relative to urinary excretion for many substances. This means that a small change
in glomerular filtration or tubular reabsorption can potentially cause a relatively
large change in urinary excretion. For example, a 10 per cent decrease in tubular
reabsorption, from 178.5 to 160.7 L/day, would increase urine volume from 1.5
to 19.3 L/day (almost a 13-fold increase) if the glomerular filtration rate (GFR)
remained constant. In reality, however, changes in tubular reabsorption and
glomerular filtration are closely coordinated, so that large fluctuations in urinary
excretion are avoided.
Second, unlike glomerular filtration, which is relatively nonselective (that is,
essentially all solutes in the plasma are filtered except the plasma proteins or
substances bound to them), tubular reabsorption is highly selective. Some substances, such as glucose and amino acids, are almost completely reabsorbed
327
328
Unit V
The Body Fluids and Kidneys
Table 27–1
Filtration, Reabsorption, and Excretion Rates of Different Substances by the Kidneys
Glucose (g/day)
Bicarbonate (mEq/day)
Sodium (mEq/day)
Chloride (mEq/day)
Potassium (mEq/day)
Urea (g/day)
Creatinine (g/day)
Amount Filtered
Amount Reabsorbed
Amount Excreted
% of Filtered Load Reabsorbed
180
4,320
25,560
19,440
756
46.8
1.8
180
4,318
25,410
19,260
664
23.4
0
0
2
150
180
92
23.4
1.8
100
>99.9
99.4
99.1
87.8
50
0
from the tubules, so that the urinary excretion rate is
essentially zero. Many of the ions in the plasma, such
as sodium, chloride, and bicarbonate, are also highly
reabsorbed, but their rates of reabsorption and urinary
excretion are variable, depending on the needs of the
body. Certain waste products, such as urea and creatinine, conversely, are poorly reabsorbed from the
tubules and excreted in relatively large amounts.
Therefore, by controlling the rate at which they
reabsorb different substances, the kidneys regulate the
excretion of solutes independently of one another, a
capability that is essential for precise control of the
composition of body fluids. In this chapter, we discuss
the mechanisms that allow the kidneys to selectively
reabsorb or secrete different substances at variable
rates.
Tubular Reabsorption
Includes Passive and
Active Mechanisms
For a substance to be reabsorbed, it must first be transported (1) across the tubular epithelial membranes
into the renal interstitial fluid and then (2) through the
peritubular capillary membrane back into the blood
(Figure 27–1). Thus, reabsorption of water and solutes
includes a series of transport steps. Reabsorption
across the tubular epithelium into the interstitial fluid
includes active or passive transport by way of the same
basic mechanisms discussed in Chapter 4 for transport
across other membranes of the body. For instance,
water and solutes can be transported either through
the cell membranes themselves (transcellular route) or
through the junctional spaces between the cells (paracellular route). Then, after absorption across the
tubular epithelial cells into the interstitial fluid, water
and solutes are transported the rest of the way through
the peritubular capillary walls into the blood by ultrafiltration (bulk flow) that is mediated by hydrostatic
and colloid osmotic forces. The peritubular capillaries
behave very much like the venous ends of most other
capillaries because there is a net reabsorptive force
that moves the fluid and solutes from the interstitium
into the blood.
Peritubular
capillary
Tubular
cells
FILTRATION
Lumen
Paracellular
path
Bulk
flow
ATP
Blood
Active
Passive
(diffusion)
Osmosis
REABSORPTION
Transcellular
path
Solutes
H2O
EXCRETION
Figure 27–1
Reabsorption of filtered water and solutes from the tubular lumen
across the tubular epithelial cells, through the renal interstitium, and
back into the blood. Solutes are transported through the cells (transcellular route) by passive diffusion or active transport, or between
the cells (paracellular route) by diffusion. Water is transported
through the cells and between the tubular cells by osmosis. Transport of water and solutes from the interstitial fluid into the peritubular capillaries occurs by ultrafiltration (bulk flow).
Active Transport
Active transport can move a solute against an electrochemical gradient and requires energy derived
from metabolism. Transport that is coupled directly to
an energy source, such as the hydrolysis of adenosine
triphosphate (ATP), is termed primary active transport. A good example of this is the sodium-potassium
ATPase pump that functions throughout most parts of
the renal tubule. Transport that is coupled indirectly to
an energy source, such as that due to an ion gradient,
is referred to as secondary active transport. Reabsorption of glucose by the renal tubule is an example of
secondary active transport. Although solutes can be
reabsorbed by active and/or passive mechanisms by
the tubule, water is always reabsorbed by a passive
Chapter 27
329
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
(nonactive) physical mechanism called osmosis, which
means water diffusion from a region of low solute concentration (high water concentration) to one of high
solute concentration (low water concentration).
Peritubular
capillary
Tubular
epithelial cells
Na+
Solutes Can Be Transported Through Epithelial Cells or Between
Cells. Renal tubular cells, like other epithelial cells, are
held together by tight junctions. Lateral intercellular
spaces lie behind the tight junctions and separate the
epithelial cells of the tubule. Solutes can be reabsorbed
or secreted across the cells by way of the transcellular
pathway or between the cells by moving across the
tight junctions and intercellular spaces by way of
the paracellular pathway. Sodium is a substance that
moves through both routes, although most of the
sodium is transported through the transcellular
pathway. In some nephron segments, especially the
proximal tubule, water is also reabsorbed across the
paracellular pathway, and substances dissolved in
the water, especially potassium, magnesium, and chloride ions, are carried with the reabsorbed fluid
between the cells.
Primary Active Transport Through the Tubular Membrane Is
Linked to Hydrolysis of ATP. The special importance of
primary active transport is that it can move solutes
against an electrochemical gradient. The energy for this
active transport comes from the hydrolysis of ATP by
way of membrane-bound ATPase; the ATPase is also
a component of the carrier mechanism that binds and
moves solutes across the cell membranes. The primary
active transporters that are known include sodiumpotassium ATPase, hydrogen ATPase, hydrogenpotassium ATPase, and calcium ATPase.
A good example of a primary active transport
system is the reabsorption of sodium ions across the
proximal tubular membrane, as shown in Figure 27–2.
On the basolateral sides of the tubular epithelial cell,
the cell membrane has an extensive sodium-potassium
ATPase system that hydrolyzes ATP and uses the
released energy to transport sodium ions out of the
cell into the interstitium. At the same time, potassium
is transported from the interstitium to the inside of the
cell. The operation of this ion pump maintains low
intracellular sodium and high intracellular potassium
concentrations and creates a net negative charge of
about -70 millivolts within the cell. This pumping of
sodium out of the cell across the basolateral membrane
of the cell favors passive diffusion of sodium across the
luminal membrane of the cell, from the tubular lumen
into the cell, for two reasons: (1) There is a concentration gradient favoring sodium diffusion into the cell
because intracellular sodium concentration is low
(12 mEq/L) and tubular fluid sodium concentration is
high (140 mEq/L). (2) The negative, -70-millivolt,
intracellular potential attracts the positive sodium ions
from the tubular lumen into the cell.
Active reabsorption of sodium by sodiumpotassium ATPase occurs in most parts of the tubule.
In certain parts of the nephron, there are additional
provisions for moving large amounts of sodium into
the cell. In the proximal tubule, there is an extensive
Tubular
lumen
ATP
Na+
ATP
K+
K+
(-70 mV)
Basal
channels
Interstitial
fluid
Basement
membrane
Na+
(-3 mv)
Tight junction
Brush border
(luminal
membrane)
Intercellular space
Figure 27–2
Basic mechanism for active transport of sodium through the tubular
epithelial cell. The sodium-potassium pump transports sodium from
the interior of the cell across the basolateral membrane, creating a
low intracellular sodium concentration and a negative intracellular
electrical potential. The low intracellular sodium concentration and
the negative electrical potential cause sodium ions to diffuse from
the tubular lumen into the cell through the brush border.
brush border on the luminal side of the membrane
(the side that faces the tubular lumen) that multiplies
the surface area about 20-fold. There are also sodium
carrier proteins that bind sodium ions on the luminal
surface of the membrane and release them inside the
cell, providing facilitated diffusion of sodium through
the membrane into the cell. These sodium carrier proteins are also important for secondary active transport
of other substances, such as glucose and amino acids,
as discussed later.
Thus, the net reabsorption of sodium ions from the
tubular lumen back into the blood involves at least
three steps:
1. Sodium diffuses across the luminal membrane
(also called the apical membrane) into the cell
down an electrochemical gradient established
by the sodium-potassium ATPase pump on the
basolateral side of the membrane.
2. Sodium is transported across the basolateral
membrane against an electrochemical gradient
by the sodium-potassium ATPase pump.
3. Sodium, water, and other substances are
reabsorbed from the interstitial fluid into the
peritubular capillaries by ultrafiltration, a passive
process driven by the hydrostatic and colloid
osmotic pressure gradients.
Secondary Active Reabsorption Through the Tubular Membrane.
In secondary active transport, two or more substances
interact with a specific membrane protein (a carrier
molecule) and are transported together across the
membrane. As one of the substances (for instance,
sodium) diffuses down its electrochemical gradient,
the energy released is used to drive another substance
330
Unit V
Interstitial
fluid
Tubular
cells
The Body Fluids and Kidneys
Tubular
lumen
Co-transport
Glucose
Glucose
Na+
Na+
K+
K+
-70 mV
ATP
Na+
ATP
Amino acids
Amino acids
Na+
Na+
-70 mV
H+
Counter-transport
Figure 27–3
Mechanisms of secondary active transport. The upper cell shows the
co-transport of glucose and amino acids along with sodium ions
through the apical side of the tubular epithelial cells, followed by
facilitated diffusion through the basolateral membranes. The lower
cell shows the counter-transport of hydrogen ions from the interior
of the cell across the apical membrane and into the tubular lumen;
movement of sodium ions into the cell, down an electrochemical gradient established by the sodium-potassium pump on the basolateral
membrane, provides the energy for transport of the hydrogen ions
from inside the cell into the tubular lumen.
(for instance, glucose) against its electrochemical
gradient. Thus, secondary active transport does not
require energy directly from ATP or from other highenergy phosphate sources. Rather, the direct source of
the energy is that liberated by the simultaneous facilitated diffusion of another transported substance
down its own electrochemical gradient.
Figure 27–3 shows secondary active transport of
glucose and amino acids in the proximal tubule. In
both instances, a specific carrier protein in the brush
border combines with a sodium ion and an amino acid
or a glucose molecule at the same time. These transport mechanisms are so efficient that they remove virtually all the glucose and amino acids from the tubular
lumen. After entry into the cell, glucose and amino
acids exit across the basolateral membranes by facilitated diffusion, driven by the high glucose and amino
acid concentrations in the cell.
Although transport of glucose against a chemical
gradient does not directly use ATP, the reabsorption
of glucose depends on energy expended by the
primary active sodium-potassium ATPase pump in the
basolateral membrane. Because of the activity of
this pump, an electrochemical gradient for facilitated
diffusion of sodium across the luminal membrane is
maintained, and it is this downhill diffusion of sodium
to the interior of the cell that provides the energy for
the simultaneous uphill transport of glucose across the
luminal membrane. Thus, this reabsorption of glucose
is referred to as “secondary active transport” because
glucose itself is reabsorbed uphill against a chemical
gradient, but it is “secondary” to primary active transport of sodium.
Another important point is that a substance is said
to undergo “active” transport when at least one of the
steps in the reabsorption involves primary or secondary active transport, even though other steps in
the reabsorption process may be passive. For glucose
reabsorption, secondary active transport occurs at the
luminal membrane, but passive facilitated diffusion
occurs at the basolateral membrane, and passive
uptake by bulk flow occurs at the peritubular
capillaries.
Secondary Active Secretion into the Tubules. Some sub-
stances are secreted into the tubules by secondary
active transport. This often involves counter-transport
of the substance with sodium ions. In countertransport, the energy liberated from the downhill
movement of one of the substances (for example,
sodium ions) enables uphill movement of a second
substance in the opposite direction.
One example of counter-transport, shown in Figure
27–3, is the active secretion of hydrogen ions coupled
to sodium reabsorption in the luminal membrane of
the proximal tubule. In this case, sodium entry into the
cell is coupled with hydrogen extrusion from the cell
by sodium-hydrogen counter-transport. This transport
is mediated by a specific protein in the brush border
of the luminal membrane. As sodium is carried to the
interior of the cell, hydrogen ions are forced outward
in the opposite direction into the tubular lumen. The
basic principles of primary and secondary active transport are discussed in additional detail in Chapter 4.
Pinocytosis—An Active Transport Mechanism for Reabsorption
of Proteins. Some parts of the tubule, especially the
proximal tubule, reabsorb large molecules such as
proteins by pinocytosis. In this process, the protein
attaches to the brush border of the luminal membrane,
and this portion of the membrane then invaginates to
the interior of the cell until it is completely pinched off
and a vesicle is formed containing the protein. Once
inside the cell, the protein is digested into its constituent amino acids, which are reabsorbed through the
basolateral membrane into the interstitial fluid.
Because pinocytosis requires energy, it is considered a
form of active transport.
Transport Maximum for Substances That Are Actively Reabsorbed. For most substances that are actively reab-
sorbed or secreted, there is a limit to the rate at which
the solute can be transported, often referred to as the
transport maximum. This limit is due to saturation
of the specific transport systems involved when the
amount of solute delivered to the tubule (referred to
as tubular load) exceeds the capacity of the carrier
proteins and specific enzymes involved in the transport process.
Chapter 27
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
Glucose filtered load, reabsorption
or excretion (mg/min)
900
800
700
Filtered
load
600
Excretion
500
400
Transport
maximum
300
Reabsorption
Normal
200
100
331
nephrons have the same transport maximum for
glucose, and some of the nephrons excrete glucose
before others have reached their transport maximum.
The overall transport maximum for the kidneys, which
is normally about 375 mg/min, is reached when all
nephrons have reached their maximal capacity to reabsorb glucose.
The plasma glucose of a healthy person almost
never becomes high enough to cause excretion of
glucose in the urine, even after eating a meal.
However, in uncontrolled diabetes mellitus, plasma
glucose may rise to high levels, causing the filtered load
of glucose to exceed the transport maximum and
resulting in urinary glucose excretion. Some of the
important transport maximums for substances actively
reabsorbed by the tubules are as follows:
Threshold
Substance
0
0
100 200 300 400 500 600 700 800
Plasma glucose concentration
(mg/100 ml)
Figure 27–4
Relations among the filtered load of glucose, the rate of glucose
reabsorption by the renal tubules, and the rate of glucose excretion
in the urine. The transport maximum is the maximum rate at which
glucose can be reabsorbed from the tubules. The threshold for
glucose refers to the filtered load of glucose at which glucose first
begins to be excreted in the urine.
Glucose
Phosphate
Sulfate
Amino acids
Urate
Lactate
Plasma protein
375 mg/min
0.10 mM/min
0.06 mM/min
1.5 mM/min
15 mg/min
75 mg/min
30 mg/min
Transport Maximums for Substances That Are Actively
Secreted. Substances that are actively secreted also
exhibit transport maximums as follows:
Substance
The glucose transport system in the proximal tubule
is a good example. Normally, measurable glucose does
not appear in the urine because essentially all the filtered glucose is reabsorbed in the proximal tubule.
However, when the filtered load exceeds the capability of the tubules to reabsorb glucose, urinary excretion of glucose does occur.
In the adult human, the transport maximum for
glucose averages about 375 mg/min, whereas the filtered load of glucose is only about 125 mg/min (GFR
¥ plasma glucose = 125 ml/min ¥ 1 mg/ml). With large
increases in GFR and/or plasma glucose concentration
that increase the filtered load of glucose above 375 mg/
min, the excess glucose filtered is not reabsorbed and
passes into the urine.
Figure 27–4 shows the relation between plasma concentration of glucose, filtered load of glucose, tubular
transport maximum for glucose, and rate of glucose
loss in the urine. Note that when the plasma glucose
concentration is 100 mg/100 mL and the filtered load
is at its normal level, 125 mg/min, there is no loss of
glucose in the urine. However, when the plasma concentration of glucose rises above about 200 mg/100 ml,
increasing the filtered load to about 250 mg/min, a
small amount of glucose begins to appear in the urine.
This point is termed the threshold for glucose. Note
that this appearance of glucose in the urine (at the
threshold) occurs before the transport maximum is
reached. One reason for the difference between
threshold and transport maximum is that not all
Transport Maximum
Creatinine
Para-aminohippuric acid
Transport Maximum
16 mg/min
80 mg/min
Substances That Are Actively Transported but Do Not Exhibit a
Transport Maximum. The reason that actively trans-
ported solutes often exhibit a transport maximum is
that the transport carrier system becomes saturated as
the tubular load increases. Substances that are passively
reabsorbed do not demonstrate a transport maximum
because their rate of transport is determined by other
factors, such as (1) the electrochemical gradient for
diffusion of the substance across the membrane, (2)
the permeability of the membrane for the substance,
and (3) the time that the fluid containing the substance
remains within the tubule. Transport of this type is
referred to as gradient-time transport because the rate
of transport depends on the electrochemical gradient
and the time that the substance is in the tubule, which
in turn depends on the tubular flow rate.
Some actively transported substances also have characteristics of gradient-time transport. An example is
sodium reabsorption in the proximal tubule. The
main reason that sodium transport in the proximal
tubule does not exhibit a transport maximum is that
other factors limit the reabsorption rate besides the
maximum rate of active transport. For example, in the
proximal tubules, the maximum transport capacity of
the basolateral sodium-potassium ATPase pump is
usually far greater than the actual rate of net sodium
332
Unit V
The Body Fluids and Kidneys
reabsorption. One of the reasons for this is that a significant amount of sodium transported out of the
cell leaks back into the tubular lumen through the
epithelial tight junctions. The rate at which this backleak occurs depends on several factors, including (1)
the permeability of the tight junctions and (2) the
interstitial physical forces, which determine the rate of
bulk flow reabsorption from the interstitial fluid into
the peritubular capillaries. Therefore, sodium transport in the proximal tubules obeys mainly gradienttime transport principles rather than tubular
maximum transport characteristics. This means that
the greater the concentration of sodium in the proximal tubules, the greater its reabsorption rate. Also, the
slower the flow rate of tubular fluid, the greater the
percentage of sodium that can be reabsorbed from
the proximal tubules.
In the more distal parts of the nephron, the epithelial cells have much tighter junctions and transport
much smaller amounts of sodium. In these segments,
sodium reabsorption exhibits a transport maximum
similar to that for other actively transported substances.
Furthermore, this transport maximum can be increased
in response to certain hormones, such as aldosterone.
Passive Water Reabsorption
by Osmosis Is Coupled Mainly
to Sodium Reabsorption
When solutes are transported out of the tubule by
either primary or secondary active transport, their
concentrations tend to decrease inside the tubule
while increasing in the renal interstitium. This creates
a concentration difference that causes osmosis of
water in the same direction that the solutes are transported, from the tubular lumen to the renal interstitium. Some parts of the renal tubule, especially the
proximal tubule, are highly permeable to water, and
water reabsorption occurs so rapidly that there is only
a small concentration gradient for solutes across the
tubular membrane.
A large part of the osmotic flow of water occurs
through the so-called tight junctions between the
epithelial cells as well as through the cells themselves.
The reason for this, as already discussed, is that the
junctions between the cells are not as tight as their
name would imply, and they allow significant diffusion
of water and small ions. This is especially true in the
proximal tubules, which have a high permeability for
water and a smaller but significant permeability to
most ions, such as sodium, chloride, potassium,
calcium, and magnesium.
As water moves across the tight junctions by
osmosis, it can also carry with it some of the solutes, a
process referred to as solvent drag. And because the
reabsorption of water, organic solutes, and ions is
coupled to sodium reabsorption, changes in sodium
reabsorption significantly influence the reabsorption
of water and many other solutes.
In the more distal parts of the nephron, beginning
in the loop of Henle and extending through the col-
lecting tubule, the tight junctions become far less permeable to water and solutes, and the epithelial cells
also have a greatly decreased membrane surface area.
Therefore, water cannot move easily across the tubular
membrane by osmosis. However, antidiuretic hormone
(ADH) greatly increases the water permeability in the
distal and collecting tubules, as discussed later.
Thus, water movement across the tubular epithelium can occur only if the membrane is permeable to
water, no matter how large the osmotic gradient. In the
proximal tubule, the water permeability is always high,
and water is reabsorbed as rapidly as the solutes. In
the ascending loop of Henle, water permeability is
always low, so that almost no water is reabsorbed,
despite a large osmotic gradient. Water permeability in
the last parts of the tubules—the distal tubules, collecting tubules, and collecting ducts—can be high or
low, depending on the presence or absence of ADH.
Reabsorption of Chloride, Urea, and
Other Solutes by Passive Diffusion
When sodium is reabsorbed through the tubular
epithelial cell, negative ions such as chloride are transported along with sodium because of electrical potentials. That is, transport of positively charged sodium
ions out of the lumen leaves the inside of the lumen
negatively charged, compared with the interstitial
fluid. This causes chloride ions to diffuse passively
through the paracellular pathway. Additional reabsorption of chloride ions occurs because of a chloride
concentration gradient that develops when water is
reabsorbed from the tubule by osmosis, thereby concentrating the chloride ions in the tubular lumen
(Figure 27–5). Thus, the active reabsorption of sodium
is closely coupled to the passive reabsorption of chloride by way of an electrical potential and a chloride
concentration gradient.
Na+ reabsorption
H2O reabsorption
Lumen
negative
potential
Luminal Clconcentration
Passive Clreabsorption
Luminal
urea
concentration
Passive urea
reabsorption
Figure 27–5
Mechanisms by which water, chloride, and urea reabsorption are
coupled with sodium reabsorption.
Chapter 27
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
Chloride ions can also be reabsorbed by secondary
active transport. The most important of the secondary
active transport processes for chloride reabsorption
involves co-transport of chloride with sodium across
the luminal membrane.
Urea is also passively reabsorbed from the tubule,
but to a much lesser extent than chloride ions. As
water is reabsorbed from the tubules (by osmosis
coupled to sodium reabsorption), urea concentration
in the tubular lumen increases (see Figure 27–5). This
creates a concentration gradient favoring the reabsorption of urea. However, urea does not permeate
the tubule as readily as water. In some parts of the
nephron, especially the inner medullary collecting
duct, passive urea reabsorption is facilitated by specific
urea transporters. Yet only about one half of the urea
that is filtered by the glomerular capillaries is reabsorbed from the tubules. The remainder of the urea
passes into the urine, allowing the kidneys to excrete
large amounts of this waste product of metabolism.
Another waste product of metabolism, creatinine, is
an even larger molecule than urea and is essentially
impermeant to the tubular membrane. Therefore,
almost none of the creatinine that is filtered is reabsorbed, so that virtually all the creatinine filtered by
the glomerulus is excreted in the urine.
Reabsorption and Secretion
Along Different Parts of
the Nephron
In the previous sections, we discussed the basic principles by which water and solutes are transported across
the tubular membrane. With these generalizations in
mind, we can now discuss the different characteristics
of the individual tubular segments that enable them to
perform their specific excretory functions. Only the
tubular transport functions that are quantitatively
most important are discussed, especially as they relate
to the reabsorption of sodium, chloride, and water. In
subsequent chapters, we discuss the reabsorption and
secretion of other specific substances in different parts
of the tubular system.
Proximal Tubular Reabsorption
Normally, about 65 per cent of the filtered load of
sodium and water and a slightly lower percentage of
filtered chloride are reabsorbed by the proximal
tubule before the filtrate reaches the loops of Henle.
These percentages can be increased or decreased in
different physiologic conditions, as discussed later.
Proximal Tubules Have a High Capacity for Active and Passive
Reabsorption. The high capacity of the proximal tubule
for reabsorption results from its special cellular characteristics, as shown in Figure 27–6. The proximal
tubule epithelial cells are highly metabolic and have
large numbers of mitochondria to support potent
active transport processes. In addition, the proximal
333
65%
Proximal tubule
Na+, Cl-, HCO3-, K+,
H2O, glucose, amino acids
Isosmotic
H+, organic acids, bases
Figure 27–6
Cellular ultrastructure and primary transport characteristics of the
proximal tubule. The proximal tubules reabsorb about 65 per cent
of the filtered sodium, chloride, bicarbonate, and potassium and
essentially all the filtered glucose and amino acids. The proximal
tubules also secrete organic acids, bases, and hydrogen ions into the
tubular lumen.
tubular cells have an extensive brush border on the
luminal (apical) side of the membrane as well as an
extensive labyrinth of intercellular and basal channels,
all of which together provide an extensive membrane
surface area on the luminal and basolateral sides of the
epithelium for rapid transport of sodium ions and
other substances.
The extensive membrane surface of the epithelial
brush border is also loaded with protein carrier molecules that transport a large fraction of the sodium ions
across the luminal membrane linked by way of the cotransport mechanism with multiple organic nutrients
such as amino acids and glucose. The remainder of the
sodium is transported from the tubular lumen into the
cell by counter-transport mechanisms, which reabsorb
sodium while secreting other substances into the
tubular lumen, especially hydrogen ions. As discussed
in Chapter 30, the secretion of hydrogen ions into the
tubular lumen is an important step in the removal of
bicarbonate ions
from the tubule (by combining H+
_
with the HCO3 to form H2CO3, which then dissociates
into H2O and CO2).
Although the sodium-potassium ATPase pump provides the major force for reabsorption of sodium,
chloride, and water throughout the proximal tubule,
there are some differences in the mechanisms by
which sodium and chloride are transported through
the luminal side of the early and late portions of the
proximal tubular membrane.
In the first half of the proximal tubule, sodium is
reabsorbed by co-transport along with glucose, amino
acids, and other solutes. But in the second half of the
proximal tubule, little glucose and amino acids remain
to be reabsorbed. Instead, sodium is now reabsorbed
mainly with chloride ions. The second half of the proximal tubule has a relatively high concentration of
chloride (around 140 mEq/L) compared with the early
334
Unit V
The Body Fluids and Kidneys
Secretion of Organic Acids and Bases by the Proximal Tubule.
5.0
Tubular fluid / plasma concentration
Creatinine
2.0
Cl-
Urea
1.0
Na+
Osmolality
0.5
HCO3-
0.2
0.1
Glucose
0.05
Amino acids
0.01
0
20
40
60
80
% Total proximal tubule length
100
Figure 27–7
Changes in concentrations of different substances in tubular fluid
along the proximal convoluted tubule relative to the concentrations
of these substances in the plasma and in the glomerular filtrate. A
value of 1.0 indicates that the concentration of the substance in the
tubular fluid is the same as the concentration in the plasma. Values
below 1.0 indicate that the substance is reabsorbed more avidly than
water, whereas values above 1.0 indicate that the substance is reabsorbed to a lesser extent than water or is secreted into the tubules.
proximal tubule (about 105 mEq/L) because when
sodium is reabsorbed, it preferentially carries with it
glucose, bicarbonate, and organic ions in the early
proximal tubule, leaving behind a solution that has a
higher concentration of chloride. In the second half of
the proximal tubule, the higher chloride concentration
favors the diffusion of this ion from the tubule lumen
through the intercellular junctions into the renal interstitial fluid.
Concentrations of Solutes Along the Proximal Tubule. Figure
27–7 summarizes the changes in concentrations of
various solutes along the proximal tubule. Although
the amount of sodium in the tubular fluid decreases
markedly along the proximal tubule, the concentration
of sodium (and the total osmolarity) remains relatively
constant because water permeability of the proximal
tubules is so great that water reabsorption keeps pace
with sodium reabsorption. Certain organic solutes,
such as glucose, amino acids, and bicarbonate, are
much more avidly reabsorbed than water, so that their
concentrations decrease markedly along the length of
the proximal tubule. Other organic solutes that are less
permeant and not actively reabsorbed, such as creatinine, increase their concentration along the proximal
tubule. The total solute concentration, as reflected by
osmolarity, remains essentially the same all along the
proximal tubule because of the extremely high permeability of this part of the nephron to water.
The proximal tubule is also an important site for secretion of organic acids and bases such as bile salts,
oxalate, urate, and catecholamines. Many of these substances are the end products of metabolism and must
be rapidly removed from the body. The secretion of
these substances into the proximal tubule plus filtration into the proximal tubule by the glomerular capillaries and the almost total lack of reabsorption by the
tubules, all combined, contribute to rapid excretion in
the urine.
In addition to the waste products of metabolism, the
kidneys secrete many potentially harmful drugs or
toxins directly through the tubular cells into the
tubules and rapidly clear these substances from the
blood. In the case of certain drugs, such as penicillin
and salicylates, the rapid clearance by the kidneys
creates a problem in maintaining a therapeutically
effective drug concentration.
Another compound that is rapidly secreted by the
proximal tubule is para-aminohippuric acid (PAH).
PAH is secreted so rapidly that the average person
can clear about 90 per cent of the PAH from the
plasma flowing through the kidneys and excrete it in
the urine. For this reason, the rate of PAH clearance
can be used to estimate the renal plasma flow, as discussed later.
Solute and Water Transport in the
Loop of Henle
The loop of Henle consists of three functionally distinct segments: the thin descending segment, the thin
ascending segment, and the thick ascending segment.
The thin descending and thin ascending segments, as
their names imply, have thin epithelial membranes
with no brush borders, few mitochondria, and minimal
levels of metabolic activity (Figure 27–8).
The descending part of the thin segment is highly
permeable to water and moderately permeable to
most solutes, including urea and sodium. The function
of this nephron segment is mainly to allow simple diffusion of substances through its walls. About 20 per
cent of the filtered water is reabsorbed in the loop of
Henle, and almost all of this occurs in the thin descending limb. The ascending limb, including both the thin
and the thick portions, is virtually impermeable to
water, a characteristic that is important for concentrating the urine.
The thick segment of the loop of Henle, which
begins about halfway up the ascending limb, has thick
epithelial cells that have high metabolic activity and
are capable of active reabsorption of sodium, chloride,
and potassium (see Figure 27–8). About 25 per cent of
the filtered loads of sodium, chloride, and potassium
are reabsorbed in the loop of Henle, mostly in the
thick ascending limb. Considerable amounts of other
ions, such as calcium, bicarbonate, and magnesium, are
also reabsorbed in the thick ascending loop of Henle.
The thin segment of the ascending limb has a much
lower reabsorptive capacity than the thick segment,
Chapter 27
335
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
Renal
interstitial
fluid
Thin descending
loop of Henle
Tubular
lumen
(+8 mV)
Tubular
cells
Na+, K+
Mg++, Ca++
Paracellular
diffusion
H2O
Na+
Na+
K+
ATP
H+
Cl-
25%
K+
Na+
2Cl- K+
Thick ascending
loop of Henle
Loop diuretics
• Furosemide
• Ethacrynic acid
• Bumetanide
Na+, Cl-, K+,
HCO3-, Mg++
Ca++,
Hypoosmotic
Figure 27–9
H+
Figure 27–8
Cellular ultrastructure and transport characteristics of the thin
descending loop of Henle (top) and the thick ascending segment of
the loop of Henle (bottom). The descending part of the thin segment
of the loop of Henle is highly permeable to water and moderately
permeable to most solutes but has few mitochondria and little or no
active reabsorption. The thick ascending limb of the loop of Henle
reabsorbs about 25 per cent of the filtered loads of sodium, chloride,
and potassium, as well as large amounts of calcium, bicarbonate,
and magnesium. This segment also secretes hydrogen ions into the
tubular lumen.
and the thin descending limb does not reabsorb significant amounts of any of these solutes.
An important component of solute reabsorption in
the thick ascending limb is the sodium-potassium
ATPase pump in the epithelial cell basolateral membranes. As in the proximal tubule, the reabsorption of
other solutes in the thick segment of the ascending
loop of Henle is closely linked to the reabsorptive
capability of the sodium-potassium ATPase pump,
which maintains a low intracellular sodium concentration. The low intracellular sodium concentration in
turn provides a favorable gradient for movement
of sodium from the tubular fluid into the cell. In the
thick ascending loop, movement of sodium across
the luminal membrane is mediated primarily by a
1-sodium, 2-chloride, 1-potassium co-transporter
(Figure 27–9). This co-transport protein carrier in the
luminal membrane uses the potential energy released
by downhill diffusion of sodium into the cell to drive
the reabsorption of potassium into the cell against a
concentration gradient.
Mechanisms of sodium, chloride, and potassium transport in the
thick ascending loop of Henle. The sodium-potassium ATPase pump
in the basolateral cell membrane maintains a low intracellular
sodium concentration and a negative electrical potential in the cell.
The 1-sodium, 2-chloride, 1-potassium co-transporter in the luminal
membrane transports these three ions from the tubular lumen into
the cells, using the potential energy released by diffusion of sodium
down an electrochemical gradient into the cells. Sodium is also
transported into the tubular cell by sodium-hydrogen counter-transport. The positive charge (+8 mV) of the tubular lumen relative to
the interstitial fluid forces cations such as Mg++ and Ca++ to diffuse
from the lumen to the interstitial fluid via the paracellular pathway.
The thick ascending limb of the loop of Henle is
the site of action of the powerful “loop” diuretics
furosemide, ethacrynic acid, and bumetanide, all of
which inhibit the action of the sodium 2-chloride,
potassium co-transporter. These diuretics are discussed in Chapter 31.
There is also significant paracellular reabsorption of
cations, such as Mg++, Ca++, Na+, and K+, in the thick
ascending limb owing to the slight positive charge of
the tubular lumen relative to the interstitial fluid.
Although the 1-sodium, 2-chloride, 1-potassium cotransporter moves equal amounts of cations and
anions into the cell, there is a slight backleak of potassium ions into the lumen, creating a positive charge of
about +8 millivolts in the tubular lumen. This positive
charge forces cations such as Mg++ and Ca++ to diffuse
from the tubular lumen through the paracellular space
and into the interstitial fluid.
The thick ascending limb also has a sodiumhydrogen counter-transport mechanism in its luminal
cell membrane that mediates sodium reabsorption and
hydrogen secretion in this segment.
The thick segment of the ascending loop of Henle is
virtually impermeable to water. Therefore, most of the
water delivered to this segment remains in the tubule,
336
Unit V
The Body Fluids and Kidneys
despite reabsorption of large amounts of solute. The
tubular fluid in the ascending limb becomes very dilute
as it flows toward the distal tubule, a feature that is
important in allowing the kidneys to dilute or concentrate the urine under different conditions, as we
discuss much more fully in Chapter 28.
Distal Tubule
The thick segment of the ascending limb of the loop
of Henle empties into the distal tubule. The very first
portion of the distal tubule forms part of the juxtaglomerular complex that provides feedback control of
GFR and blood flow in this same nephron. The next
part of the distal tubule is highly convoluted and has
many of the same reabsorptive characteristics of the
thick segment of the ascending limb of the loop of
Henle. That is, it avidly reabsorbs most of the ions,
including sodium, potassium, and chloride, but is virtually impermeable to water and urea. For this reason,
it is referred to as the diluting segment because it also
dilutes the tubular fluid.
Approximately 5 percent of the filtered load of
sodium chloride is reabsorbed in the early distal
tubule. The sodium-chloride co-transporter moves
sodium chloride from the tubular lumen into the cell,
and the sodium-potassium ATPase pump transports
sodium out of the cell across the basolateral membrane (Figure 27–10). Chloride diffuses out of the cell
into the renal interstitial fluid through chloride channels in the basolateral membrane. The thiazide diuretics, which are widely used to treat disorders such as
hypertension and heart failure, inhibit the sodiumchloride co-transporter.
Renal
interstitial
fluid
Tubular
cells
Late Distal Tubule and Cortical
Collecting Tubule
The second half of the distal tubule and the subsequent cortical collecting tubule have similar functional
characteristics. Anatomically, they are composed of
two distinct cell types, the principal cells and the intercalated cells (Figure 27–11). The principal cells reabsorb sodium and water from the lumen and secrete
potassium ions into the lumen. The intercalated cells
reabsorb potassium ions and secrete hydrogen ions
into the tubular lumen.
Principal Cells Reabsorb Sodium and Secrete Potassium.
Sodium reabsorption and potassium secretion by the
principal cells depend on the activity of a sodiumpotassium ATPase pump in each cell’s basolateral
membrane (Figure 27–12). This pump maintains a low
sodium concentration inside the cell and, therefore,
favors sodium diffusion into the cell through special
Early distal tubule
Na+, Cl-, Ca++, Mg++
Late distal tubule
and collecting tubule
Principal
Na+, Clcells
Tubular
lumen
(-10mV)
K+
(+ADH) H2O
H+
Na+
K+
ATP
Na+
Intercalated
cells
-
K+
HCO3-
ClCl-
Figure 27–11
Thiazide diuretics:
Figure 27–10
Mechanism of sodium chloride transport in the early distal tubule.
Sodium and chloride are transported from the tubular lumen into
the cell by a co-transporter that is inhibited by thiazide diuretics.
Sodium is pumped out of the cell by sodium-potassium ATPase and
chloride diffuses into the interstitial fluid via chloride channels.
Cellular ultrastructure and transport characteristics of the early
distal tubule and the late distal tubule and collecting tubule. The
early distal tubule has many of the same characteristics as the thick
ascending loop of Henle and reabsorbs sodium, chloride, calcium,
and magnesium but is virtually impermeable to water and urea. The
late distal tubules and cortical collecting tubules are composed of
two distinct cell types, the principal cells and the intercalated cells.
The principal cells reabsorb sodium from the lumen and secrete
potassium ions into the lumen.The intercalated cells reabsorb potassium and bicarbonate ions from the lumen and secrete hydrogen
ions into the lumen. The reabsorption of water from this tubular
segment is controlled by the concentration of antidiuretic hormone.
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
Chapter 27
Renal
interstitial
fluid
Tubular
lumen
(-50 mV)
Tubular
cells
K+
Na+
K+
Na+
ATP
-
-
Cl-
Aldosterone antagonists
• Spironolactone
• Eplerenone
Na+ channel blockers
• Amiloride
• Triamterene
Figure 27–12
Mechanism of sodium chloride reabsorption and potassium secretion in the late distal tubules and cortical collecting tubules. Sodium
enters the cell through special channels and is transported out of the
cell by the sodium-potassium ATPase pump. Aldosterone antagonists compete with aldosterone for binding sites in the cell and
therefore inhibit the effects of aldosterone to stimulate sodium reabsorption and potassium secretion. Sodium channel blockers directly
inhibit the entry of sodium into the sodium channels.
channels. The secretion of potassium by these cells
from the blood into the tubular lumen involves two
steps: (1) Potassium enters the cell because of the
sodium-potassium ATPase pump, which maintains a
high intracellular potassium concentration, and then
(2) once in the cell, potassium diffuses down its concentration gradient across the luminal membrane into
the tubular fluid.
The principal cells are the primary sites of action
of the potassium-sparing diuretics, including spironolactone, eplerenone, amiloride, and triamterene.
Aldosterone antagonists compete with aldosterone for
receptor sites in the principal cells and therefore
inhibit the stimulatory effects of aldosterone on
sodium reabsorption and potassium secretion. Sodium
channel blockers directly inhibit the entry of sodium
into the sodium channels of the luminal membranes
and therefore reduce the amount of sodium that can
be transported across the basolateral membranes by
the sodium-potassium ATPase pump. This, in turn,
decreases transport of potassium into the cells and
ultimately reduces potassium secretion into the
tubular fluid. For this reason the sodium channel
blockers as well as the aldosterone antagonists
decrease urinary excretion of potassium and act as
potassium-sparing diuretics.
Intercalated Cells Avidly Secrete Hydrogen and Reabsorb Bicarbonate and Potassium Ions. Hydrogen ion secretion by
the intercalated cells is mediated by a hydrogenATPase transport mechanism. Hydrogen is generated
in this cell by the action of carbonic anhydrase on
water and carbon dioxide to form carbonic acid, which
337
then dissociates into hydrogen ions and bicarbonate
ions. The hydrogen ions are then secreted into the
tubular lumen, and for each hydrogen ion secreted, a
bicarbonate ion becomes available for reabsorption
across the basolateral membrane. A more detailed discussion of this mechanism is presented in Chapter 30.
The intercalated cells can also reabsorb potassium
ions.
The functional characteristics of the late distal tubule
and cortical collecting tubule can be summarized as
follows:
1. The tubular membranes of both segments are
almost completely impermeable to urea, similar
to the diluting segment of the early distal tubule;
thus, almost all the urea that enters these
segments passes on through and into the
collecting duct to be excreted in the urine,
although some reabsorption of urea occurs
in the medullary collecting ducts.
2. Both the late distal tubule and the cortical
collecting tubule segments reabsorb sodium ions,
and the rate of reabsorption is controlled by
hormones, especially aldosterone. At the same
time, these segments secrete potassium ions
from the peritubular capillary blood into the
tubular lumen, a process that is also controlled
by aldosterone and by other factors such as
the concentration of potassium ions in the body
fluids.
3. The intercalated cells of these nephron segments
avidly secrete hydrogen ions by an active
hydrogen-ATPase mechanism. This process is
different from the secondary active secretion of
hydrogen ions by the proximal tubule because it is
capable of secreting hydrogen ions against a large
concentration gradient, as much as 1000 to 1. This
is in contrast to the relatively small gradient (4- to
10-fold) for hydrogen ions that can be achieved
by secondary active secretion in the proximal
tubule. Thus, the intercalated cells play a key role
in acid-base regulation of the body fluids.
4. The permeability of the late distal tubule and
cortical collecting duct to water is controlled by
the concentration of ADH, which is also called
vasopressin. With high levels of ADH, these
tubular segments are permeable to water, but
in the absence of ADH, they are virtually
impermeable to water. This special characteristic
provides an important mechanism for controlling
the degree of dilution or concentration of the
urine.
Medullary Collecting Duct
Although the medullary collecting ducts reabsorb less
than 10 per cent of the filtered water and sodium, they
are the final site for processing the urine and, therefore, play an extremely important role in determining
the final urine output of water and solutes.
The epithelial cells of the collecting ducts are nearly
cuboidal in shape with smooth surfaces and relatively
Unit V
-
5
Cl
100.0
3
-
10.0
ine
tin
ea
r
C
lin Cl
Inu
Urea
K
and Na
5.0
2.0
1.0
0.20
0.10
ein
cids
Whether a solute will become concentrated in the
tubular fluid is determined by the relative degree of
reabsorption of that solute versus the reabsorption of
water. If a greater percentage of water is reabsorbed,
the substance becomes more concentrated. If a greater
percentage of the solute is reabsorbed, the substance
becomes more diluted.
Figure 27–14 shows the degree of concentration of
several substances in the different tubular segments.
All the values in this figure represent the tubular fluid
concentration divided by the plasma concentration of
HCO3
Prot
Summary of Concentrations of
Different Solutes in the Different
Tubular Segments
no a
0.05
Cl
Na
0.02
few mitochondria (Figure 27–13). Special characteristics of this tubular segment are as follows:
1. The permeability of the medullary collecting duct
to water is controlled by the level of ADH. With
high levels of ADH, water is avidly reabsorbed
into the medullary interstitium, thereby reducing
the urine volume and concentrating most of the
solutes in the urine.
2. Unlike the cortical collecting tubule, the
medullary collecting duct is permeable to urea.
Therefore, some of the tubular urea is reabsorbed
into the medullary interstitium, helping to raise
the osmolality in this region of the kidneys and
contributing to the kidneys’ overall ability to form
a concentrated urine.
3. The medullary collecting duct is capable of
secreting hydrogen ions against a large
concentration gradient, as also occurs in the
cortical collecting tubule. Thus, the medullary
collecting duct also plays a key role in regulating
acid-base balance.
K
0.50
Ami
Cellular ultrastructure and transport characteristics of the
medullary collecting duct. The medullary collecting ducts actively
reabsorb sodium and secrete hydrogen ions and are permeable to
urea, which is reabsorbed in these tubular segments. The reabsorption of water in medullary collecting ducts is controlled by the concentration of antidiuretic hormone.
20.0
ose
Gluc
Figure 27–13
Tubular fluid/plasma concentration
HCO
12
PAH
Urea
H+
5
50.0
to
H)
D
(+A
H 2O
40
58
+,
Na
to 1
Medullary
collecting duct
The Body Fluids and Kidneys
to
338
Proximal
tubule
Loop of
Henle
Distal
tubule
Collecting
tubule
Figure 27–14
Changes in average concentrations of different substances at different points in the tubular system relative to the concentration of that
substance in the plasma and in the glomerular filtrate. A value of 1.0
indicates that the concentration of the substance in the tubular fluid
is the same as the concentration of that substance in the plasma.
Values below 1.0 indicate that the substance is reabsorbed more
avidly than water, whereas values above 1.0 indicate that the substance is reabsorbed to a lesser extent than water or is secreted into
the tubules.
a substance. If plasma concentration of the substance
is assumed to be constant, any change in the ratio
of tubular fluid/plasma concentration rate reflects
changes in tubular fluid concentration.
As the filtrate moves along the tubular system, the
concentration rises to progressively greater than 1.0 if
more water is reabsorbed than solute, or if there has
been a net secretion of the solute into the tubular fluid.
If the concentration ratio becomes progressively less
than 1.0, this means that relatively more solute has
been reabsorbed than water.
The substances represented at the top of Figure
27–14, such as creatinine, become highly concentrated
in the urine. In general, these substances are not
needed by the body, and the kidneys have become
adapted to reabsorb them only slightly or not at all, or
even to secrete them into the tubules, thereby excreting especially great quantities into the urine. Conversely, the substances represented toward the bottom
of the figure, such as glucose and amino acids, are all
strongly reabsorbed; these are all substances that the
body needs to conserve, and almost none of them are
lost in the urine.
Chapter 27
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
Tubular Fluid /Plasma Inulin Concentration Ratio Can Be Used
to Measure Water Reabsorption by the Renal Tubules. Inulin,
339
a polysaccharide used to measure GFR, is not reabsorbed or secreted by the renal tubules. Changes in
inulin concentration at different points along the renal
tubule, therefore, reflect changes in the amount of
water present in the tubular fluid. For example, the
tubular fluid/plasma concentration ratio for inulin
rises to about 3.0 at the end of the proximal tubules,
indicating that inulin concentration in the tubular fluid
is 3 times greater than in the plasma and in the
glomerular filtrate. Since inulin is not secreted or reabsorbed from the tubules, a tubular fluid/plasma concentration ratio of 3.0 means that only one third of the
water that was filtered remains in the renal tubule and
that two thirds of the filtered water has been reabsorbed as the fluid passes through the proximal tubule.
At the end of the collecting ducts, the tubular
fluid/plasma inulin concentration ratio rises to about
125 (see Figure 27–14), indicating that only 1/125 of
the filtered water remains in the tubule and that more
than 99% has been reabsorbed.
are not fully understood but may be due partly to
changes in physical forces in the tubule and surrounding renal interstitium, as discussed later. It is clear that
the mechanisms for glomerulotubular balance can
occur independently of hormones and can be demonstrated in completely isolated kidneys or even in completely isolated proximal tubular segments.
The importance of glomerulotubular balance is that
it helps to prevent overloading of the distal tubular
segments when GFR increases. Glomerulotubular
balance acts as a second line of defense to buffer the
effects of spontaneous changes in GFR on urine
output. (The first line of defense, discussed earlier,
includes the renal autoregulatory mechanisms, especially tubuloglomerular feedback, which help prevent
changes in GFR.) Working together, the autoregulatory and glomerulotubular balance mechanisms
prevent large changes in fluid flow in the distal tubules
when the arterial pressure changes or when there
are other disturbances that would otherwise wreak
havoc with the maintenance of sodium and volume
homeostasis.
Regulation of Tubular
Reabsorption
Peritubular Capillary and Renal
Interstitial Fluid Physical Forces
Because it is essential to maintain a precise balance
between tubular reabsorption and glomerular filtration, there are multiple nervous, hormonal, and local
control mechanisms that regulate tubular reabsorption, just as there are for control of glomerular filtration. An important feature of tubular reabsorption is
that reabsorption of some solutes can be regulated
independently of others, especially through hormonal
control mechanisms.
Hydrostatic and colloid osmotic forces govern the rate
of reabsorption across the peritubular capillaries,
just as these physical forces control filtration in the
glomerular capillaries. Changes in peritubular capillary reabsorption can in turn influence the hydrostatic
and colloid osmotic pressures of the renal interstitium
and, ultimately, reabsorption of water and solutes from
the renal tubules.
Glomerulotubular Balance—The
Ability of the Tubules to Increase
Reabsorption Rate in Response
to Increased Tubular Load
One of the most basic mechanisms for controlling
tubular reabsorption is the intrinsic ability of the
tubules to increase their reabsorption rate in response
to increased tubular load (increased tubular inflow).
This phenomenon is referred to as glomerulotubular
balance. For example, if GFR is increased from 125 ml/
min to 150 ml/min, the absolute rate of proximal
tubular reabsorption also increases from about 81 ml/
min (65 per cent of GFR) to about 97.5 ml/min (65 per
cent of GFR). Thus, glomerulotubular balance refers
to the fact that the total rate of reabsorption increases
as the filtered load increases, even though the
percentage of GFR reabsorbed in the proximal
tubule remains relatively constant at about 65 per
cent.
Some degree of glomerulotubular balance also
occurs in other tubular segments, especially the loop
of Henle. The precise mechanisms responsible for this
Normal Values for Physical Forces and Reabsorption Rate. As
the glomerular filtrate passes through the renal
tubules, more than 99 per cent of the water and most
of the solutes are normally reabsorbed. Fluid and electrolytes are reabsorbed from the tubules into the renal
interstitium and from there into the peritubular capillaries. The normal rate of peritubular capillary reabsorption is about 124 ml/min.
Reabsorption across the peritubular capillaries can
be calculated as
Reabsorption = Kf ¥ Net reabsorptive force
The net reabsorptive force represents the sum of the
hydrostatic and colloid osmotic forces that either favor
or oppose reabsorption across the peritubular capillaries. These forces include (1) hydrostatic pressure
inside the peritubular capillaries (peritubular hydrostatic pressure [Pc]), which opposes reabsorption;
(2) hydrostatic pressure in the renal interstitium (Pif)
outside the capillaries, which favors reabsorption; (3)
colloid osmotic pressure of the peritubular capillary
plasma proteins (pc), which favors reabsorption; and
(4) colloid osmotic pressure of the proteins in the renal
interstitium (pif), which opposes reabsorption.
Figure 27–15 shows the approximate normal forces
that favor and oppose peritubular reabsorption.
340
Unit V
Peritubular
capillary
Pc
13 mm Hg
πc
32 mm Hg
Interstitial
fluid
Tubular
cells
The Body Fluids and Kidneys
Tubular
lumen
Pif
6 mm Hg
πif
15 mm Hg
Bulk
flow
H2O
H2O
+
10 mm Hg Na
Net reabsorption
pressure
ATP
Na+
Figure 27–15
Summary of the hydrostatic and colloid osmotic forces that determine fluid reabsorption by the peritubular capillaries. The numerical values shown are estimates of the normal values for humans. The
net reabsorptive pressure is normally about 10 mm Hg, causing fluid
and solutes to be reabsorbed into the peritubular capillaries as they
are transported across the renal tubular cells. ATP, adenosine
triphosphate; Pc, peritubular capillary hydrostatic pressure; Pif, interstitial fluid hydrostatic pressure; pc, peritubular capillary colloid
osmotic pressure; pif, interstitial fluid colloid osmotic pressure.
Because the normal peritubular capillary pressure
averages about 13 mm Hg and renal interstitial fluid
hydrostatic pressure averages 6 mm Hg, there is a positive hydrostatic pressure gradient from the peritubular capillary to the interstitial fluid of about 7 mm Hg,
which opposes fluid reabsorption. This is more than
counterbalanced by the colloid osmotic pressures that
favor reabsorption. The plasma colloid osmotic pressure, which favors reabsorption, is about 32 mm Hg,
and the colloid osmotic pressure of the interstitium,
which opposes reabsorption, is 15 mm Hg, causing a
net colloid osmotic force of about 17 mm Hg, favoring
reabsorption. Therefore, subtracting the net hydrostatic forces that oppose reabsorption (7 mm Hg) from
the net colloid osmotic forces that favor reabsorption
(17 mm Hg) gives a net reabsorptive force of about
10 mm Hg. This is a high value, similar to that found
in the glomerular capillaries, but in the opposite
direction.
The other factor that contributes to the high rate of
fluid reabsorption in the peritubular capillaries is a
large filtration coefficient (Kf) because of the high
hydraulic conductivity and large surface area of the
capillaries. Because the reabsorption rate is normally
about 124 ml/min and net reabsorption pressure is
10 mm Hg, Kf normally is about 12.4 ml/min/mm Hg.
Regulation of Peritubular Capillary Physical Forces. The two
determinants of peritubular capillary reabsorption
that are directly influenced by renal hemodynamic
changes are the hydrostatic and colloid osmotic pressures of the peritubular capillaries. The peritubular
capillary hydrostatic pressure is influenced by the arterial pressure and resistances of the afferent and efferent
arterioles. (1) Increases in arterial pressure tend to
raise peritubular capillary hydrostatic pressure and
decrease reabsorption rate. This effect is buffered to
some extent by autoregulatory mechanisms that maintain relatively constant renal blood flow as well as
relatively constant hydrostatic pressures in the renal
blood vessels. (2) Increase in resistance of either the
afferent or the efferent arterioles reduces peritubular
capillary hydrostatic pressure and tends to increase
reabsorption rate. Although constriction of the
efferent arterioles increases glomerular capillary
hydrostatic pressure, it lowers peritubular capillary
hydrostatic pressure.
The second major determinant of peritubular capillary reabsorption is the colloid osmotic pressure of the
plasma in these capillaries; raising the colloid osmotic
pressure increases peritubular capillary reabsorption.
The colloid osmotic pressure of peritubular capillaries
is determined by (1) the systemic plasma colloid
osmotic pressure; increasing the plasma protein concentration of systemic blood tends to raise peritubular
capillary colloid osmotic pressure, thereby increasing
reabsorption; and (2) the filtration fraction; the higher
the filtration fraction, the greater the fraction of
plasma filtered through the glomerulus and, consequently, the more concentrated the protein becomes in
the plasma that remains behind. Thus, increasing the
filtration fraction also tends to increase the peritubular capillary reabsorption rate. Because filtration fraction is defined as the ratio of GFR/renal plasma flow,
increased filtration fraction can occur as a result of
increased GFR or decreased renal plasma flow. Some
renal vasoconstrictors, such as angiotensin II, increase
peritubular capillary reabsorption by decreasing renal
plasma flow and increasing filtration fraction, as discussed later.
Changes in the peritubular capillary Kf can also
influence the reabsorption rate because Kf is a
measure of the permeability and surface area of the
capillaries. Increases in Kf raise reabsorption, whereas
decreases in Kf lower peritubular capillary reabsorption. Kf remains relatively constant in most physiologic
conditions. Table 27–2 summarizes the factors that
can influence the peritubular capillary reabsorption
rate.
Renal Interstitial Hydrostatic and Colloid Osmotic Pressures.
Ultimately, changes in peritubular capillary physical
forces influence tubular reabsorption by changing the
physical forces in the renal interstitium surrounding
the tubules. For example, a decrease in the reabsorptive force across the peritubular capillary membranes,
caused by either increased peritubular capillary hydrostatic pressure or decreased peritubular capillary
colloid osmotic pressure, reduces the uptake of fluid
and solutes from the interstitium into the peritubular
capillaries. This in turn raises renal interstitial fluid
hydrostatic pressure and decreases interstitial fluid
colloid osmotic pressure because of dilution of the
proteins in the renal interstitium. These changes then
decrease the net reabsorption of fluid from the renal
tubules into the interstitium, especially in the proximal
tubules.
Chapter 27
341
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
Normal
Table 27–2
Factors That Can Influence Peritubular Capillary
Reabsorption
≠ Pc Æ Ø Reabsorption
• Ø RA Æ ≠ Pc
• Ø RE Æ ≠ Pc
• ≠ Arterial Pressure Æ ≠ Pc
≠ pc Æ ≠ Reabsorption
• ≠ pA Æ ≠ pc
• ≠ FF Æ ≠ pc
≠ Kf Æ ≠ Reabsorption
Peritubular
capillary
Interstitial
fluid
Tubular
cells
Pc
πc
Net
reabsorption
ATP
Pc, peritubular capillary hydrostatic pressure; RA and RE, afferent and efferent arteriolar resistances, respectively; pc, peritubular capillary colloid osmotic
pressure; pA, arterial plasma colloid osmotic pressure; FF, filtration fraction;
Kf, peritubular capillary filtration coefficient.
The mechanisms by which changes in interstitial
fluid hydrostatic and colloid osmotic pressures influence tubular reabsorption can be understood by examining the pathways through which solute and water are
reabsorbed (Figure 27–16). Once the solutes enter the
intercellular channels or renal interstitium by active
transport or passive diffusion, water is drawn from the
tubular lumen into the interstitium by osmosis. And
once the water and solutes are in the interstitial spaces,
they can either be swept up into the peritubular capillaries or diffuse back through the epithelial junctions
into the tubular lumen. The so-called tight junctions
between the epithelial cells of the proximal tubule are
actually leaky, so that considerable amounts of sodium
can diffuse in both directions through these junctions.
With the normal high rate of peritubular capillary
reabsorption, the net movement of water and solutes
is into the peritubular capillaries with little backleak
into the lumen of the tubule. However, when peritubular capillary reabsorption is reduced, there is
increased interstitial fluid hydrostatic pressure and a
tendency for greater amounts of solute and water to
backleak into the tubular lumen, thereby reducing the
rate of net reabsorption (refer again to Figure 27–16).
The opposite is true when there is increased peritubular capillary reabsorption above the normal level.
An initial increase in reabsorption by the peritubular
capillaries tends to reduce interstitial fluid hydrostatic
pressure and raise interstitial fluid colloid osmotic
pressure. Both of these forces favor movement of fluid
and solutes out of the tubular lumen and into the interstitium; therefore, backleak of water and solutes into
the tubular lumen is reduced, and net tubular reabsorption increases.
Thus, through changes in the hydrostatic and colloid
osmotic pressures of the renal interstitium, the uptake
of water and solutes by the peritubular capillaries is
closely matched to the net reabsorption of water and
solutes from the tubular lumen into the interstitium.
Therefore, in general, forces that increase peritubular
capillary reabsorption also increase reabsorption from
the renal tubules. Conversely, hemodynamic changes
that inhibit peritubular capillary reabsorption also
inhibit tubular reabsorption of water and solutes.
Lumen
ATP
Backleak
Decreased reabsorption
Pc
πc
ATP
Decreased net
reabsorption
ATP
Increased
backleak
Figure 27–16
Proximal tubular and peritubular capillary reabsorption under
normal conditions (top) and during decreased peritubular capillary
reabsorption (bottom) caused by either increasing peritubular capillary hydrostatic pressure (Pc) or decreasing peritubular capillary
colloid osmotic pressure (pc). Reduced peritubular capillary reabsorption, in turn, decreases the net reabsorption of solutes and water
by increasing the amounts of solutes and water that leak back into
the tubular lumen through the tight junctions of the tubular epithelial cells, especially in the proximal tubule.
Effect of Arterial Pressure on Urine
Output—The Pressure-Natriuresis and
Pressure-Diuresis Mechanisms
Even small increases in arterial pressure often cause
marked increases in urinary excretion of sodium and
water, phenomena that are referred to as pressure
natriuresis and pressure diuresis. Because of the
autoregulatory mechanisms described in Chapter 26,
increasing the arterial pressure between the limits of
75 and 160 mm Hg usually has only a small effect on
renal blood flow and GFR. The slight increase in GFR
that does occur contributes in part to the effect of
increased arterial pressure on urine output. When
GFR autoregulation is impaired, as often occurs in
kidney disease, increases in arterial pressure cause
much larger increases in GFR.
342
Unit V
The Body Fluids and Kidneys
A second effect of increased renal arterial pressure
that raises urine output is that it decreases the percentage of the filtered load of sodium and water that
is reabsorbed by the tubules. The mechanisms responsible for this effect include a slight increase in peritubular capillary hydrostatic pressure, especially in the
vasa recta of the renal medulla, and a subsequent
increase in the renal interstitial fluid hydrostatic pressure. As discussed earlier, an increase in the renal
interstitial fluid hydrostatic pressure enhances backleak of sodium into the tubular lumen, thereby reducing the net reabsorption of sodium and water and
further increasing the rate of urine output when renal
arterial pressure rises.
A third factor that contributes to the pressurenatriuresis and pressure-diuresis mechanisms is
reduced angiotensin II formation. Angiotensin II itself
increases sodium reabsorption by the tubules; it also
stimulates aldosterone secretion, which further
increases sodium reabsorption. Therefore, decreased
angiotensin II formation contributes to the decreased
tubular sodium reabsorption that occurs when arterial
pressure is increased.
Hormonal Control of Tubular
Reabsorption
Precise regulation of body fluid volumes and solute
concentrations requires the kidneys to excrete different solutes and water at variable rates, sometimes
independently of one another. For example, when
potassium intake is increased, the kidneys must
excrete more potassium while maintaining normal
excretion of sodium and other electrolytes. Likewise,
when sodium intake is changed, the kidneys must
appropriately adjust urinary sodium excretion without
major changes in excretion of other electrolytes.
Several hormones in the body provide this specificity
of tubular reabsorption for different electrolytes and
water. Table 27–3 summarizes some of the most important hormones for regulating tubular reabsorption,
their principal sites of action on the renal tubule, and
their effects on solute and water excretion. Some
of these hormones are discussed in more detail in
Chapters 28 and 29, but we briefly review their renal
tubular actions in the next few paragraphs.
Aldosterone Increases Sodium Reabsorption and Increases
Potassium Secretion. Aldosterone, secreted by the zona
glomerulosa cells of the adrenal cortex, is an important regulator of sodium reabsorption and potassium
secretion by the renal tubules. The primary site
of aldosterone action is on the principal cells of the
cortical collecting tubule. The mechanism by which
aldosterone increases sodium reabsorption while at
the same time increasing potassium secretion is by
stimulating the sodium-potassium ATPase pump on
the basolateral side of the cortical collecting tubule
membrane. Aldosterone also increases the sodium
permeability of the luminal side of the membrane.
The cellular mechanisms of aldosterone action are
discussed in Chapter 77.
In the absence of aldosterone, as occurs with adrenal
destruction or malfunction (Addison’s disease), there
is marked loss of sodium from the body and accumulation of potassium. Conversely, excess aldosterone
secretion, as occurs in patients with adrenal tumors
(Conn’s syndrome) is associated with sodium retention
and potassium depletion. Although day-to-day regulation of sodium balance can be maintained as long as
minimal levels of aldosterone are present, the inability to appropriately adjust aldosterone secretion
greatly impairs the regulation of renal potassium
excretion and potassium concentration of the body
fluids. Thus, aldosterone is even more important as a
regulator of potassium concentration than it is for
sodium concentration.
Angiotensin II Increases Sodium and Water Reabsorption.
Angiotensin II is perhaps the body’s most powerful
sodium-retaining hormone. As discussed in Chapter
19, angiotensin II formation increases in circumstances
associated with low blood pressure and/or low extracellular fluid volume, such as during hemorrhage or
loss of salt and water from the body fluids. The
increased formation of angiotensin II helps to return
blood pressure and extracellular volume toward
normal by increasing sodium and water reabsorption
from the renal tubules through three main effects:
1. Angiotensin II stimulates aldosterone secretion,
which in turn increases sodium reabsorption.
2. Angiotensin II constricts the efferent arterioles,
which has two effects on peritubular capillary
dynamics that raise sodium and water
Table 27–3
Hormones That Regulate Tubular Reabsorption
Hormone
Site of Action
Effects
Aldosterone
Angiotensin II
Collecting tubule and duct
Proximal tubule, thick ascending loop of Henle/distal
tubule, collecting tubule
Distal tubule/collecting tubule and duct
Distal tubule/collecting tubule and duct
Proximal tubule, thick ascending loop of
Henle/distal tubule
≠ NaCl, H2O reabsorption, ≠ K+ secretion
≠ NaCl, H2O reabsorption, ≠ H+ secretion
Antidiuretic hormone
Atrial natriuretic peptide
Parathyroid hormone
≠ H2O reabsorption
Ø NaCl reabsorption
Ø PO4- - - reabsorption, ≠ Ca++ reabsorption
Chapter 27
343
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
reabsorption. First, efferent arteriolar constriction
reduces peritubular capillary hydrostatic pressure,
which increases net tubular reabsorption,
especially from the proximal tubules. Second,
efferent arteriolar constriction, by reducing renal
blood flow, raises filtration fraction in the
glomerulus and increases the concentration of
proteins and the colloid osmotic pressure in
the peritubular capillaries; this increases the
reabsorptive force at the peritubular capillaries
and raises tubular reabsorption of sodium and
water.
3. Angiotensin II directly stimulates sodium
reabsorption in the proximal tubules, the loops of
Henle, the distal tubules, and the collecting tubules.
One of the direct effects of angiotensin II is to
stimulate the sodium-potassium ATPase pump on
the tubular epithelial cell basolateral membrane.
A second effect is to stimulate sodium-hydrogen
exchange in the luminal membrane, especially
in the proximal tubule. Thus, angiotensin II
stimulates sodium transport across both the
luminal and the basolateral surfaces of the
epithelial cell membrane in the tubules.
These multiple actions of angiotensin II cause
marked sodium retention by the kidneys when
angiotensin II levels are increased.
ADH Increases Water Reabsorption. The most important
renal action of ADH is to increase the water permeability of the distal tubule, collecting tubule, and collecting duct epithelia. This effect helps the body to
conserve water in circumstances such as dehydration.
In the absence of ADH, the permeability of the distal
tubules and collecting ducts to water is low, causing the
kidneys to excrete large amounts of dilute urine. Thus,
the actions of ADH play a key role in controlling the
degree of dilution or concentration of the urine, as discussed further in Chapters 28 and 75.
ADH binds to specific V2 receptors in the late distal
tubules, collecting tubules, and collecting ducts,
increasing the formation of cyclic AMP and activating
protein kinases. This, in turn, stimulates the movement
of an intracellular protein, called aquaporin-2 (AQP2), to the luminal side of the cell membranes. The molecules of AQP-2 cluster together and fuse with the cell
membrane by exocytosis to form water channels that
permit rapid diffusion of water through the cells. There
are other aquaporins, AQP-3 and AQP-4, in the basolateral side of the cell membrane that provide a path
for water to rapidly exit the cells, although these
are not believed to be regulated by ADH. Chronic
increases in ADH levels also increase the formation of
AQP-2 protein in the renal tubular cells by stimulating AQP-2 gene transcription. When the concentration
of ADH decreases, the molecules of AQP-2 are shuttled back to the cell cytoplasm, thereby removing the
water channels from the luminal membrane and
reducing water permeability.
Atrial Natriuretic Peptide Decreases Sodium and Water
Reabsorption. Specific cells of the cardiac atria, when
distended because of plasma volume expansion,
secrete a peptide called atrial natriuretic peptide.
Increased levels of this peptide in turn inhibit the reabsorption of sodium and water by the renal tubules,
especially in the collecting ducts. This decreased
sodium and water reabsorption increases urinary
excretion, which helps to return blood volume back
toward normal.
Parathyroid
Hormone
Increases
Calcium
Reabsorption.
Parathyroid hormone is one of the most important
calcium-regulating hormones in the body. Its principal
action in the kidneys is to increase tubular reabsorption of calcium, especially in the distal tubules and
perhaps also in the loops of Henle. Parathyroid
hormone also has other actions, including inhibition of
phosphate reabsorption by the proximal tubule and
stimulation of magnesium reabsorption by the loop of
Henle, as discussed in Chapter 29.
Sympathetic Nervous System
Activation Increases Sodium
Reabsorption
Activation of the sympathetic nervous system can
decrease sodium and water excretion by constricting
the renal arterioles, thereby reducing GFR. Sympathetic activation also increases sodium reabsorption in
the proximal tubule, the thick ascending limb of the
loop of Henle, and perhaps in more distal parts of the
renal tubule. And finally, sympathetic nervous system
stimulation increases renin release and angiotensin II
formation, which adds to the overall effect to increase
tubular reabsorption and decrease renal excretion of
sodium.
Use of Clearance Methods
to Quantify Kidney Function
The rates at which different substances are “cleared”
from the plasma provide a useful way of quantitating
the effectiveness with which the kidneys excrete
various substances (Table 27–4). By definition, the
renal clearance of a substance is the volume of plasma
that is completely cleared of the substance by the
kidneys per unit time. This concept is somewhat
abstract because there is no single volume of plasma
that is completely cleared of a substance. However,
renal clearance provides a useful way of quantifying
the excretory function of the kidneys and, as discussed
later, can be used to quantify the rate at which blood
flows through the kidneys as well as the basic functions
of the kidneys: glomerular filtration, tubular reabsorption, and tubular secretion.
To illustrate the clearance principle, consider the following example: If the plasma passing through the
kidneys contains 1 milligram of a substance in each milliliter and if 1 milligram of this substance is also
excreted into the urine each minute, then 1 ml/min
of the plasma is “cleared” of the substance. Thus,
344
Unit V
The Body Fluids and Kidneys
Table 27–4
Use of Clearance to Quantify Kidney Function
Term
Equation
Units
Us ¥ V
Ps
Uinulin ¥ V
GFR =
Pinulin
Cs =
Clearance rate (Cs)
Glomerular filtration rate (GFR)
ml/min
Cs
C inulin
Clearance ratio
Clearance ratio =
Effective renal plasma flow (ERPF)
ERPF = C PAH =
Renal plasma flow (RPF)
RPF =
Renal blood flow (RBF)
RBF =
Excretion rate
Excretion rate = Us ¥ V
Reabsorption rate = Filtered load - Excretion rate
= (GFR ¥ Ps) - (Us ¥ V)
Secretion rate = Excretion rate - Filtered load
Reabsorption rate
Secretion rate
None
UPAH ¥ V
PPAH
ml/min
(UPAH ¥ V PPAH )
C PAH
=
EPAH (PPAH - VPAH ) PPAH
UPAH ¥ VPAH
=
PPAH - VPAH
RPF
1 - Hematocrit
ml/min
ml/min
mg/min, mmol/min, or mEq/min
mg/min, mmol/min, or mEq/min
mg/min, mmol/min, or mEq/min
S, a substance; U, urine concentration; V, urine flow rate; P, plasma concentration; PAH, para-aminohippuric acid; PPAH, renal arterial PAH concentration; EPAH, PAH
extraction ratio; VPAH, renal venous PAH concentration.
clearance refers to the volume of plasma that would be
necessary to supply the amount of substance excreted
in the urine per unit time. Stated mathematically,
Cs ¥ Ps = Us ¥ V,
where Cs is the clearance rate of a substance s, Ps is the
plasma concentration of the substance, Us is the urine
concentration of that substance, and V is the urine flow
rate. Rearranging this equation, clearance can be
expressed as
Cs =
Us ¥ V
Ps
Thus, renal clearance of a substance is calculated from
the urinary excretion rate (Us ¥ V) of that substance
divided by its plasma concentration.
Inulin Clearance Can Be Used
to Estimate GFR
If a substance is freely filtered (filtered as freely as
water) and is not reabsorbed or secreted by the renal
tubules, then the rate at which that substance is excreted
in the urine (Us ¥ V) is equal to the filtration rate of the
substance by the kidneys (GFR ¥ Ps). Thus,
GFR ¥ Ps = Us ¥ V
The GFR, therefore, can be calculated as the clearance
of the substance as follows:
GFR =
Us ¥ V
= Cs
Ps
A substance that fits these criteria is inulin, a polysaccharide molecule with a molecular weight of about
5200. Inulin, which is not produced in the body, is found
in the roots of certain plants and must be administered
intravenously to a patient to measure GFR.
Figure 27–17 shows the renal handling of inulin. In
this example, the plasma concentration is 1 mg/ml, urine
concentration is 125 mg/ml, and urine flow rate is 1 ml/
min. Therefore, 125 mg/min of inulin passes into the
urine. Then, inulin clearance is calculated as the urine
excretion rate of inulin divided by the plasma concentration, which yields a value of 125 ml/min. Thus, 125
milliliters of plasma flowing through the kidneys must
be filtered to deliver the inulin that appears in the urine.
Inulin is not the only substance that can be used for
determining GFR. Other substances that have been
used clinically to estimate GFR include radioactive
iothalamate and creatinine.
Creatinine Clearance and Plasma
Creatinine Concentration Can Be
Used to Estimate GFR
Creatinine is a by-product of muscle metabolism and is
cleared from the body fluids almost entirely by glomerular filtration. Therefore, the clearance of creatinine can
also be used to assess GFR. Because measurement of
creatinine clearance does not require intravenous infusion into the patient, this method is much more widely
used than inulin clearance for estimating GFR clinically.
However, creatinine clearance is not a perfect marker
of GFR because a small amount of it is secreted by the
tubules, so that the amount of creatinine excreted
slightly exceeds the amount filtered. There is normally
a slight error in measuring plasma creatinine that
leads to an overestimate of the plasma creatinine
Chapter 27
345
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
Pinulin = 1 mg/ml
GFR (ml/min)
100
Amount filtered = Amount excreted
GFR x Pinulin = Uinulin x V
Uinulin x V
0
Pinulin
GFR = 125 ml/min
Uinulin = 125 mg/ml
Serum creatinine
concentration (mg/dl)
GFR =
50
2
1
0
Figure 27–17
Measurement of glomerular filtration rate (GFR) from the renal
clearance of inulin. Inulin is freely filtered by the glomerular capillaries but is not reabsorbed by the renal tubules. Pinulin, plasma
inulin concentration; Uinulin, urine inulin concentration; V, urine flow
rate.
concentration, and fortuitously, these two errors tend to
cancel each other. Therefore, creatinine clearance provides a reasonable estimate of GFR.
In some cases, it may not be practical to collect urine
in a patient for measuring creatinine clearance (CCr). An
approximation of changes in GFR, however, can be
obtained by simply measuring plasma creatinine concentration (PCr), which is inversely proportional to GFR:
GFR ª CCr =
UCr ¥ V
PCr
If GFR suddenly decreases by 50%, the kidneys will
transiently filter and excrete only half as much creatinine, causing accumulation of creatinine in the body
fluids and raising plasma concentration. Plasma concentration of creatinine will continue to rise until the filtered load of creatinine (PCr ¥ GFR) and creatinine
excretion (UCr ¥ V) return to normal and a balance
between creatinine production and creatinine excretion
is reestablished. This will occur when plasma creatinine
increases to approximately twice normal, as shown in
Figure 27–18. If GFR falls to one-fourth normal, plasma
creatinine would increase to about 4 times normal, and
a decrease of GFR to one-eighth normal would raise
plasma creatinine to 8 times normal. Thus, under steadystate conditions, the creatinine excretion rate equals the
rate of creatinine production, despite reductions in
GFR. However, this normal rate of creatinine excretion
occurs at the expense of elevated plasma creatinine concentration, as shown in Figure 27–19.
PAH Clearance Can Be Used to
Estimate Renal Plasma Flow
Theoretically, if a substance is completely cleared from
the plasma, the clearance rate of that substance is equal
Creatinine production and
renal excretion (g/day)
V = 1 ml/min
Positive balance
Production
2
Excretion
GFR x PCreatinine
1
0
0
1
2
Days
3
4
Figure 27–18
Effect of reducing glomerular filtration rate (GRF) by 50 per cent
on serum creatinine concentration and on creatinine excretion
rate when the production rate of creatinine remains constant.
PCreatinine, plasma creatinine concentration.
to the total renal plasma flow. In other words, the
amount of the substance delivered to the kidneys in
the blood (renal plasma flow ¥ Ps) would be equal to
the amount excreted in the urine (Us ¥ V). Thus, renal
plasma flow (RPF) could be calculated as
RPF =
Us ¥ V
= Cs
Ps
Because the GFR is only about 20 per cent of the
total plasma flow, a substance that is completely cleared
from the plasma must be excreted by tubular secretion
as well as glomerular filtration (Figure 27–20). There is
no known substance that is completely cleared by the
kidneys. One substance, however, PAH, is about 90 per
cent cleared from the plasma. Therefore, the clearance
of PAH can be used as an approximation of renal
plasma flow. To be more accurate, one can correct for
the percentage of PAH that is still in the blood when it
leaves the kidneys. The percentage of PAH removed
from the blood is known as the extraction ratio of PAH
and averages about 90 per cent in normal kidneys. In
diseased kidneys, this extraction ratio may be reduced
346
Unit V
The Body Fluids and Kidneys
PPAH = 0.01 mg/ml
Plasma creatinine concentration
(mg/100 ml)
14
12
10
Renal plasma flow
UPAH x V
=
PPAH
8
6
4
2
Renal venous
PAH =
0.001 mg/ml
Normal
25
50
75
100
125
Glomerular filtration rate
(ml/min)
Figure 27–19
Approximate relationship between glomerular filtration rate (GFR)
and plasma creatinine concentration under steady-state conditions.
Decreasing GFR by 50 per cent will increase plasma creatinine to
twice normal if creatinine production by the body remains constant.
because of inability of damaged tubules to secrete PAH
into the tubular fluid.
The calculation of RPF can be demonstrated by the
following example: Assume that the plasma concentration of PAH is 0.01 mg/ml, urine concentration is 5.85
mg/ml, and urine flow rate is 1 ml/min. PAH clearance
can be calculated from the rate of urinary PAH excretion (5.85 mg/ml ¥ 1 ml/min) divided by the plasma PAH
concentration (0.01 mg/ml). Thus, clearance of PAH calculates to be 585 ml/min.
If the extraction ratio for PAH is 90 per cent, the
actual renal plasma flow can be calculated by dividing
585 ml/min by 0.9, yielding a value of 650 ml/min. Thus,
total renal plasma flow can be calculated as
Total renal plasma flow =
Clearance of PAH/Extraction ratio of PAH
The extraction ratio (EPAH) is calculated as the difference between the renal arterial PAH (PPAH) and renal
venous PAH (VPAH) concentrations, divided by the renal
arterial PAH concentration:
EPAH =
UPAH = 5.85 mg/ml
150
PPAH - VPAH
PPAH
One can calculate the total blood flow through the
kidneys from the total renal plasma flow and hematocrit
(the percentage of red blood cells in the blood). If the
hematocrit is 0.45 and the total renal plasma flow is
650 ml/min, the total blood flow through both kidneys
is 650/(1 - 0.45), or 1182 ml/min.
V = 1 ml/min
Figure 27–20
Measurement of renal plasma flow from the renal clearance of paraaminohippuric acid (PAH). PAH is freely filtered by the glomerular
capillaries and is also secreted from the peritubular capillary blood
into the tubular lumen. The amount of PAH in the plasma of the
renal artery is about equal to the amount of PAH excreted in
the urine. Therefore, the renal plasma flow can be calculated from
the clearance of PAH (CPAH). To be more accurate, one can correct
for the percentage of PAH that is still in the blood when it leaves
the kidneys. PPAH, arterial plasma PAH concentration; UPAH, urine
PAH concentration; V, urine flow rate.
Filtration Fraction Is Calculated
from GFR Divided by Renal
Plasma Flow
To calculate the filtration fraction, which is the fraction
of plasma that filters through the glomerular membrane, one must first know the renal plasma flow (PAH
clearance) and the GFR (inulin clearance). If renal
plasma flow is 650 ml/min and GFR is 125 ml/min, the
filtration fraction (FF) is calculated as
FF = GFR/RPF = 125/650 = 0.19
Calculation of Tubular Reabsorption
or Secretion from Renal Clearances
If the rates of glomerular filtration and renal excretion
of a substance are known, one can calculate whether
there is a net reabsorption or a net secretion of that substance by the renal tubules. For example, if the rate of
excretion of the substance (Us ¥ V) is less than the filtered load of the substance (GFR ¥ Ps), then some of
the substance must have been reabsorbed from the
renal tubules. Conversely, if the excretion rate of the
substance is greater than its filtered load, then the rate
at which it appears in the urine represents the sum of
the rate of glomerular filtration plus tubular secretion.
The following example demonstrates the calculation
of tubular reabsorption. Assume the following laboratory values for a patient were obtained:
Chapter 27
Urine Formation by the Kidneys: II. Tubular Processing of the Glomerular Filtrate
Urine flow rate = 1 ml/min
Urine concentration of sodium (UNa) = 70 mEq/L
= 70 mEq/ml
Plasma sodium concentration = 140 mEq/L
= 140 mEq/ml
GFR (inulin clearance) = 100 ml/min
In this example, the filtered sodium load is GFR ¥ PNa,
or 100 ml/min ¥ 140 mEq/ml = 14,000 mEq/min. Urinary
sodium excretion (UNa ¥ urine flow rate) is 70 mEq/min.
Therefore, tubular reabsorption of sodium is the difference between the filtered load and urinary excretion, or
14,000 mEq/min - 70 mEq/min = 13,930 mEq/min.
Comparisons of Inulin Clearance with Clearances of Different
Solutes. The following generalizations can be made by
comparing the clearance of a substance with the clearance of inulin, a measure of GFR: (1) if the clearance
rate of the substance equals that of inulin, the substance
is only filtered and not reabsorbed or secreted; (2) if the
clearance rate of a substance is less than inulin clearance, the substance must have been reabsorbed by the
nephron tubules; and (3) if the clearance rate of a substance is greater than that of inulin, the substance must
be secreted by the nephron tubules. Listed below are
the approximate clearance rates for some of the substances normally handled by the kidneys:
Substance
Clearance Rate (ml/min)
Glucose
Sodium
Chloride
Potassium
Phosphate
Inulin
Creatinine
0
0.9
1.3
12.0
25.0
125.0
140.0
References
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C
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8
Regulation of Extracellular Fluid
Osmolarity and Sodium
Concentration
For the cells of the body to function properly, they
must be bathed in extracellular fluid with a relatively constant concentration of electrolytes and
other solutes. The total concentration of solutes in
the extracellular fluid—and therefore the osmolarity—is determined by the amount of solute divided
by the volume of the extracellular fluid. Thus, to a
large extent, extracellular fluid sodium concentration and osmolarity are regulated by the amount of extracellular water. The
body water in turn is controlled by (1) fluid intake, which is regulated by factors
that determine thirst, and (2) renal excretion of water, which is controlled by
multiple factors that influence glomerular filtration and tubular reabsorption.
In this chapter, we discuss specifically (1) the mechanisms that cause the
kidneys to eliminate excess water by excreting a dilute urine; (2) the mechanisms that cause the kidneys to conserve water by excreting a concentrated
urine; (3) the renal feedback mechanisms that control the extracellular fluid
sodium concentration and osmolarity; and (4) the thirst and salt appetite mechanisms that determine the intakes of water and salt, which also help to control
extracellular fluid volume, osmolarity, and sodium concentration.
The Kidneys Excrete Excess Water by Forming
a Dilute Urine
The normal kidney has tremendous capability to vary the relative proportions
of solutes and water in the urine in response to various challenges. When there
is excess water in the body and body fluid osmolarity is reduced, the kidney can
excrete urine with an osmolarity as low as 50 mOsm/L, a concentration that is
only about one sixth the osmolarity of normal extracellular fluid. Conversely,
when there is a deficit of water and extracellular fluid osmolarity is high, the
kidney can excrete urine with a concentration of 1200 to 1400 mOsm/L. Equally
important, the kidney can excrete a large volume of dilute urine or a small
volume of concentrated urine without major changes in rates of excretion of
solutes such as sodium and potassium. This ability to regulate water excretion
independently of solute excretion is necessary for survival, especially when fluid
intake is limited.
Antidiuretic Hormone Controls Urine Concentration
There is a powerful feedback system for regulating plasma osmolarity and
sodium concentration that operates by altering renal excretion of water independently of the rate of solute excretion. A primary effector of this feedback is
antidiuretic hormone (ADH), also called vasopressin.
When osmolarity of the body fluids increases above normal (that is, the
solutes in the body fluids become too concentrated), the posterior pituitary
gland secretes more ADH, which increases the permeability of the distal tubules
and collecting ducts to water, as discussed in Chapter 27. This allows large
amounts of water to be reabsorbed and decreases urine volume but does not
348
Chapter 28
349
Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
markedly alter the rate of renal excretion of the
solutes.
When there is excess water in the body and extracellular fluid osmolarity is reduced, the secretion of
ADH by the posterior pituitary decreases, thereby
reducing the permeability of the distal tubule and collecting ducts to water, which causes large amounts of
dilute urine to be excreted. Thus, the rate of ADH
secretion determines, to a large extent, whether the
kidney excretes a dilute or a concentrated urine.
Renal Mechanisms for Excreting a
Dilute Urine
When there is a large excess of water in the body, the
kidney can excrete as much as 20 L/day of dilute urine,
with a concentration as low as 50 mOsm/L. The kidney
performs this impressive feat by continuing to reabsorb solutes while failing to reabsorb large amounts of
water in the distal parts of the nephron, including the
late distal tubule and the collecting ducts.
Figure 28–1 shows the approximate renal responses
in a human after ingestion of 1 liter of water. Note that
urine volume increases to about six times normal
within 45 minutes after the water has been drunk.
However, the total amount of solute excreted remains
relatively constant because the urine formed becomes
very dilute and urine osmolarity decreases from 600 to
about 100 mOsm/L. Thus, after ingestion of excess
water, the kidney rids the body of the excess water but
does not excrete excess amounts of solutes.
When the glomerular filtrate is initially formed,
its osmolarity is about the same as that of plasma
(300 mOsm/L). To excrete excess water, it is necessary
to dilute the filtrate as it passes along the tubule. This
is achieved by reabsorbing solutes to a greater extent
than water, as shown in Figure 28–2, but this occurs
only in certain segments of the tubular system as
follows.
Tubular Fluid Remains Isosmotic in the Proximal Tubule. As
fluid flows through the proximal tubule, solutes and
water are reabsorbed in equal proportions, so that
little change in osmolarity occurs; that is, the proximal
tubule fluid remains isosmotic to the plasma, with an
osmolarity of about 300 mOsm/L.As fluid passes down
the descending loop of Henle, water is reabsorbed by
osmosis and the tubular fluid reaches equilibrium with
the surrounding interstitial fluid of the renal medulla,
which is very hypertonic—about two to four times the
osmolarity of the original glomerular filtrate. Therefore, the tubular fluid becomes more concentrated as
it flows into the inner medulla.
Tubular Fluid Becomes Dilute in the Ascending Loop of Henle.
Drink 1.0 L H2O
Urine
osmolarity
Plasma
osmolarity
400
NaCl
0
300
300
6
Urine flow rate
(ml/min)
Urinary solute
excretion
(mOsm/min)
NaCl
H2O
100
300
4
100
Cortex
NaCl
2
0
1.2
400
0.6
400
NaCl
H 2O
400
70
Medulla
Osmolarity
(mOsm/L)
800
In the ascending limb of the loop of Henle, especially
in the thick segment, sodium, potassium, and chloride
are avidly reabsorbed. However, this portion of the
tubular segment is impermeable to water, even in
NaCl
0
0
60
120
Time (minutes)
180
600
600
600
50
Figure 28–2
Figure 28–1
Water diuresis in a human after ingestion of 1 liter of water. Note
that after water ingestion, urine volume increases and urine osmolarity decreases, causing the excretion of a large volume of dilute
urine; however, the total amount of solute excreted by the kidneys
remains relatively constant. These responses of the kidneys
prevent plasma osmolarity from decreasing markedly during
excess water ingestion.
Formation of a dilute urine when antidiuretic hormone (ADH) levels
are very low. Note that in the ascending loop of Henle, the tubular
fluid becomes very dilute. In the distal tubules and collecting
tubules, the tubular fluid is further diluted by the reabsorption of
sodium chloride and the failure to reabsorb water when ADH
levels are very low. The failure to reabsorb water and continued
reabsorption of solutes lead to a large volume of dilute urine.
(Numerical values are in milliosmoles per liter.)
350
Unit V
The Body Fluids and Kidneys
the presence of large amounts of ADH. Therefore, the
tubular fluid becomes more dilute as it flows up the
ascending loop of Henle into the early distal tubule,
with the osmolarity decreasing progressively to about
100 mOsm/L by the time the fluid enters the early
distal tubular segment. Thus, regardless of whether
ADH is present or absent, fluid leaving the early distal
tubular segment is hypo-osmotic, with an osmolarity of
only about one third the osmolarity of plasma.
Tubular Fluid in Distal and Collecting Tubules Is Further Diluted
in the Absence of ADH. As the dilute fluid in the early
distal tubule passes into the late distal convoluted
tubule, cortical collecting duct, and collecting duct,
there is additional reabsorption of sodium chloride. In
the absence of ADH, this portion of the tubule is also
impermeable to water, and the additional reabsorption
of solutes causes the tubular fluid to become even
more dilute, decreasing its osmolarity to as low as
50 mOsm/L.The failure to reabsorb water and the continued reabsorption of solutes lead to a large volume
of dilute urine.
To summarize, the mechanism for forming a dilute
urine is to continue reabsorbing solutes from the distal
segments of the tubular system while failing to reabsorb water. In healthy kidneys, fluid leaving the
ascending loop of Henle and early distal tubule is
always dilute, regardless of the level of ADH. In the
absence of ADH, the urine is further diluted in the late
distal tubule and collecting ducts, and a large volume
of dilute urine is excreted.
The Kidneys Conserve Water
by Excreting a Concentrated
Urine
The ability of the kidney to form a urine that is more
concentrated than plasma is essential for survival of
mammals that live on land, including humans. Water is
continuously lost from the body through various
routes, including the lungs by evaporation into the
expired air, the gastrointestinal tract by way of the
feces, the skin through evaporation and perspiration,
and the kidneys through the excretion of urine. Fluid
intake is required to match this loss, but the ability of
the kidney to form a small volume of concentrated
urine minimizes the intake of fluid required to maintain homeostasis, a function that is especially important when water is in short supply.
When there is a water deficit in the body, the kidney
forms a concentrated urine by continuing to excrete
solutes while increasing water reabsorption and
decreasing the volume of urine formed. The human
kidney can produce a maximal urine concentration of
1200 to 1400 mOsm/L, four to five times the osmolarity of plasma. Some desert animals, such as the Australian hopping mouse, can concentrate urine to as
high as 10,000 mOsm/L. This allows the mouse to
survive in the desert without drinking water; sufficient
water can be obtained through the food ingested and
water produced in the body by metabolism of the food.
Animals adapted to aquatic environments, such as the
beaver, have minimal urine concentrating ability; they
can concentrate the urine to only about 500 mOsm/L.
Obligatory Urine Volume
The maximal concentrating ability of the kidney dictates how much urine volume must be excreted each
day to rid the body of waste products of metabolism and
ions that are ingested. A normal 70-kilogram human
must excrete about 600 milliosmoles of solute each day.
If maximal urine concentrating ability is 1200 mOsm/L,
the minimal volume of urine that must be excreted,
called the obligatory urine volume, can be calculated as
600 mOsm day
= 0.5 L day
1200 mOsm L
This minimal loss of volume in the urine contributes to
dehydration, along with water loss from the skin, respiratory tract, and gastrointestinal tract, when water is not
available to drink.
The limited ability of the human kidney to concentrate the urine to a maximal concentration of
1200 mOsm/L explains why severe dehydration occurs
if one attempts to drink seawater. Sodium chloride
concentration in the oceans averages about 3.0 to 3.5
per cent, with an osmolarity between about 1000 and
1200 mOsm/L. Drinking 1 liter of seawater with a concentration of 1200 mOsm/L would provide a total
sodium chloride intake of 1200 milliosmoles. If maximal
urine concentrating ability is 1200 mOsm/L, the amount
of urine volume needed to excrete 1200 milliosmoles
would be 1200 milliosmoles divided by 1200 mOsm/L,
or 1.0 liter. Why then does drinking seawater cause
dehydration? The answer is that the kidney must also
excrete other solutes, especially urea, which contribute
about 600 mOsm/L when the urine is maximally concentrated. Therefore, the maximum concentration of
sodium chloride that can be excreted by the kidneys is
about 600 mOsm/L. Thus, for every liter of seawater
drunk, 2 liters of urine volume would be required to rid
the body of 1200 milliosmoles of sodium chloride
ingested in addition to other solutes such as urea. This
would result in a net fluid loss of 1 liter for every liter
of seawater drunk, explaining the rapid dehydration
that occurs in shipwreck victims who drink seawater.
However, a shipwreck victim’s pet Australian hopping
mouse could drink with impunity all the seawater it
wanted.
Requirements for Excreting a
Concentrated Urine—High ADH Levels
and Hyperosmotic Renal Medulla
The basic requirements for forming a concentrated
urine are (1) a high level of ADH, which increases the
permeability of the distal tubules and collecting ducts
to water, thereby allowing these tubular segments to
avidly reabsorb water, and (2) a high osmolarity of the
renal medullary interstitial fluid, which provides the
osmotic gradient necessary for water reabsorption to
occur in the presence of high levels of ADH.
Chapter 28
351
Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
The renal medullary interstitium surrounding the
collecting ducts normally is very hyperosmotic, so that
when ADH levels are high, water moves through the
tubular membrane by osmosis into the renal interstitium; from there it is carried away by the vasa recta
back into the blood. Thus, the urine concentrating
ability is limited by the level of ADH and by the
degree of hyperosmolarity of the renal medulla. We
discuss the factors that control ADH secretion later,
but for now, what is the process by which renal
medullary interstitial fluid becomes hyperosmotic?
This process involves the operation of the countercurrent mechanism.
The countercurrent mechanism depends on the
special anatomical arrangement of the loops of Henle
and the vasa recta, the specialized peritubular capillaries of the renal medulla. In the human, about 25 per
cent of the nephrons are juxtamedullary nephrons,
with loops of Henle and vasa recta that go deeply into
the medulla before returning to the cortex. Some of
the loops of Henle dip all the way to the tips of the
renal papillae that project from the medulla into the
renal pelvis. Paralleling the long loops of Henle are
the vasa recta, which also loop down into the medulla
before returning to the renal cortex. And finally, the
collecting ducts, which carry urine through the hyperosmotic renal medulla before it is excreted, also play
a critical role in the countercurrent mechanism.
Countercurrent Mechanism
Produces a Hyperosmotic Renal
Medullary Interstitium
The osmolarity of interstitial fluid in almost all parts
of the body is about 300 mOsm/L, which is similar to
the plasma osmolarity. (As discussed in Chapter 25,
the corrected osmolar activity, which accounts for
intermolecular attraction and repulsion, is about
282 mOsm/L.) The osmolarity of the interstitial fluid
in the medulla of the kidney is much higher, increasing progressively to about 1200 to 1400 mOsm/L in the
pelvic tip of the medulla. This means that the renal
medullary interstitium has accumulated solutes in
great excess of water. Once the high solute concentration in the medulla is achieved, it is maintained by a
balanced inflow and outflow of solutes and water in
the medulla.
The major factors that contribute to the buildup of
solute concentration into the renal medulla are as
follows:
1. Active transport of sodium ions and co-transport
of potassium, chloride, and other ions out of the
thick portion of the ascending limb of the loop
of Henle into the medullary interstitium
2. Active transport of ions from the collecting ducts
into the medullary interstitium
3. Facilitated diffusion of large amounts of urea
from the inner medullary collecting ducts into the
medullary interstitium
4. Diffusion of only small amounts of water from the
medullary tubules into the medullary interstitium,
Table 28–1
Summary of Tubule Characteristics—Urine Concentration
Proximal tubule
Thin descending limb
Thin ascending limb
Thick ascending limb
Distal tubule
Cortical collecting
tubule
Inner medullary
collecting duct
Permeability
NaCl
Urea
Active NaCl
Transport
H2O
++
0
0
++
+
+
++
++
0
0
+ADH
+ADH
+
+
+
0
0
0
+
+
+
0
0
0
+
+ADH
0
++ADH
0, minimal level of active transport or permeability; +, moderate level of active
transport or permeability; ++, high level of active transport or permeability;
+ADH, permeability to water or urea is increased by ADH.
far less than the reabsorption of solutes into the
medullary interstitium
Special Characteristics of Loop of Henle That Cause Solutes to
Be Trapped in the Renal Medulla. The transport character-
istics of the loops of Henle are summarized in Table
28–1, along with the characteristics of the proximal
tubules, distal tubules, cortical collecting tubules, and
inner medullary collecting ducts.
The most important cause of the high medullary
osmolarity is active transport of sodium and cotransport of potassium, chloride, and other ions from
the thick ascending loop of Henle into the interstitium.
This pump is capable of establishing about a 200milliosmole concentration gradient between the
tubular lumen and the interstitial fluid. Because the
thick ascending limb is virtually impermeable to water,
the solutes pumped out are not followed by osmotic
flow of water into the interstitium. Thus, the active
transport of sodium and other ions out of the thick
ascending loop adds solutes in excess of water to the
renal medullary interstitium. There is some passive
reabsorption of sodium chloride from the thin ascending limb of Henle’s loop, which is also impermeable to
water, adding further to the high solute concentration
of the renal medullary interstitium.
The descending limb of Henle’s loop, in contrast to
the ascending limb, is very permeable to water, and the
tubular fluid osmolarity quickly becomes equal to the
renal medullary osmolarity. Therefore, water diffuses
out of the descending limb of Henle’s loop into the
interstitium, and the tubular fluid osmolarity gradually
rises as it flows toward the tip of the loop of Henle.
Steps Involved in Causing Hyperosmotic Renal Medullary Interstitium. With these characteristics of the loop of Henle
in mind, let us now discuss how the renal medulla
becomes hyperosmotic. First, assume that the loop of
Henle is filled with fluid with a concentration of
300 mOsm/L, the same as that leaving the proximal
tubule (Figure 28–3, step 1). Next, the active pump of
the thick ascending limb on the loop of Henle is turned
on, reducing the concentration inside the tubule and
352
1
Unit V
300
300
300
300
300
300
300
300
300
300
300
300
5
150
400
500
300
400
500
300
200
300
400
200
400
300
150
350
400
300
300
300
300
6
200
3
400
400
200
400
400
200
400
400
4
350
150
500
500
300
500
300
Repeat Steps 4-6
300
300
200
300
400
200
400
400
400
400
200
150
350
500
200
300
200
300
2
The Body Fluids and Kidneys
7
400
400
300
300
100
700
700
500
1000
1000
800
1200 1200 1000
Figure 28–3
Countercurrent multiplier system in the loop of Henle for producing a hyperosmotic renal medulla. (Numerical values are in milliosmoles
per liter.)
raising the interstitial concentration; this pump establishes a 200-mOsm/L concentration gradient between
the tubular fluid and the interstitial fluid (step 2). The
limit to the gradient is about 200 mOsm/L because
paracellular diffusion of ions back into the tubule
eventually counterbalances transport of ions out of the
lumen when the 200-mOsm/L concentration gradient
is achieved.
Step 3 is that the tubular fluid in the descending limb
of the loop of Henle and the interstitial fluid quickly
reach osmotic equilibrium because of osmosis of water
out of the descending limb. The interstitial osmolarity
is maintained at 400 mOsm/L because of continued
transport of ions out of the thick ascending loop of
Henle. Thus, by itself, the active transport of sodium
chloride out of the thick ascending limb is capable of
establishing only a 200-mOsm/L concentration gradient, much less than that achieved by the countercurrent system.
Step 4 is additional flow of fluid into the loop of
Henle from the proximal tubule, which causes the
hyperosmotic fluid previously formed in the descending limb to flow into the ascending limb. Once this fluid
is in the ascending limb, additional ions are pumped
into the interstitium, with water remaining behind,
until a 200-mOsm/L osmotic gradient is established,
with the interstitial fluid osmolarity rising to
500 mOsm/L (step 5). Then, once again, the fluid in the
descending limb reaches equilibrium with the hyperosmotic medullary interstitial fluid (step 6), and as the
hyperosmotic tubular fluid from the descending limb
of the loop of Henle flows into the ascending limb, still
more solute is continuously pumped out of the tubules
and deposited into the medullary interstitium.
These steps are repeated over and over, with the net
effect of adding more and more solute to the medulla
in excess of water; with sufficient time, this process
gradually traps solutes in the medulla and multiplies
the concentration gradient established by the active
pumping of ions out of the thick ascending loop of
Henle, eventually raising the interstitial fluid osmolarity to 1200 to 1400 mOsm/L as shown in step 7.
Thus, the repetitive reabsorption of sodium chloride
by the thick ascending loop of Henle and continued
inflow of new sodium chloride from the proximal
tubule into the loop of Henle is called the countercurrent multiplier. The sodium chloride reabsorbed from
the ascending loop of Henle keeps adding to the newly
arrived sodium chloride, thus “multiplying” its concentration in the medullary interstitium.
Role of Distal Tubule and
Collecting Ducts in Excreting a
Concentrated Urine
When the tubular fluid leaves the loop of Henle and
flows into the distal convoluted tubule in the renal
cortex, the fluid is dilute, with an osmolarity of only
about 100 mOsm/L (Figure 28–4). The early distal
Chapter 28
NaCl H2O
Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
300
100
300
Urea Contributes to Hyperosmotic
Renal Medullary Interstitium and to a
Concentrated Urine
H2O NaCl
Urea
300
Cortex
NaCl
600
NaCl
H2O
1200
1200
600
600
H2O
NaCl
Urea
1200
1200
Medulla
600
353
Figure 28–4
Formation of a concentrated urine when antidiuretic hormone
(ADH) levels are high. Note that the fluid leaving the loop of Henle
is dilute but becomes concentrated as water is absorbed from the
distal tubules and collecting tubules. With high ADH levels, the
osmolarity of the urine is about the same as the osmolarity of
the renal medullary interstitial fluid in the papilla, which is about
1200 mOsm/L. (Numerical values are in milliosmoles per liter.)
tubule further dilutes the tubular fluid because this
segment, like the ascending loop of Henle, actively
transports sodium chloride out of the tubule but is relatively impermeable to water.
As fluid flows into the cortical collecting tubule, the
amount of water reabsorbed is critically dependent on
the plasma concentration of ADH. In the absence of
ADH, this segment is almost impermeable to water
and fails to reabsorb water but continues to reabsorb
solutes and further dilutes the urine. When there is a
high concentration of ADH, the cortical collecting
tubule becomes highly permeable to water, so that
large amounts of water are now reabsorbed from the
tubule into the cortex interstitium, where it is swept
away by the rapidly flowing peritubular capillaries.
The fact that these large amounts of water are reabsorbed into the cortex, rather than into the renal
medulla, helps to preserve the high medullary interstitial fluid osmolarity.
As the tubular fluid flows along the medullary collecting ducts, there is further water reabsorption from
the tubular fluid into the interstitium, but the total
amount of water is relatively small compared with that
added to the cortex interstitium. The reabsorbed water
is quickly carried away by the vasa recta into the
venous blood. When high levels of ADH are present,
the collecting ducts become permeable to water, so
that the fluid at the end of the collecting ducts has
essentially the same osmolarity as the interstitial fluid
of the renal medulla—about 1200 mOsm/L (see Figure
28–3). Thus, by reabsorbing as much water as possible,
the kidneys form a highly concentrated urine, excreting normal amounts of solutes in the urine while
adding water back to the extracellular fluid and compensating for deficits of body water.
Thus far, we have considered only the contribution of
sodium chloride to the hyperosmotic renal medullary
interstitium. However, urea contributes about 40
to 50 per cent of the osmolarity (500-600 mOsm/L) of
the renal medullary interstitium when the kidney
is forming a maximally concentrated urine. Unlike
sodium chloride, urea is passively reabsorbed from the
tubule. When there is water deficit and blood concentrations of ADH are high, large amounts of urea are
passively reabsorbed from the inner medullary collecting ducts into the interstitium.
The mechanism for reabsorption of urea into the
renal medulla is as follows: As water flows up the
ascending loop of Henle and into the distal and cortical collecting tubules, little urea is reabsorbed because
these segments are impermeable to urea (see Table
28–1). In the presence of high concentrations of ADH,
water is reabsorbed rapidly from the cortical collecting tubule and the urea concentration increases
rapidly because urea is not very permeant in this part
of the tubule. Then, as the tubular fluid flows into the
inner medullary collecting ducts, still more water reabsorption takes place, causing an even higher concentration of urea in the fluid. This high concentration of
urea in the tubular fluid of the inner medullary collecting duct causes urea to diffuse out of the tubule
into the renal interstitium. This diffusion is greatly
facilitated by specific urea transporters. One of these
urea transporters, UT-AI, is activated by ADH,
increasing transport of urea out of the inner medullary
collecting duct even more when ADH levels are elevated. The simultaneous movement of water and urea
out of the inner medullary collecting ducts maintains
a high concentration of urea in the tubular fluid and,
eventually, in the urine, even though urea is being
reabsorbed.
The fundamental role of urea in contributing to
urine concentrating ability is evidenced by the fact that
people who ingest a high-protein diet, yielding large
amounts of urea as a nitrogenous “waste” product,
can concentrate their urine much better than people
whose protein intake and urea production are low.
Malnutrition is associated with a low urea concentration in the medullary interstitium and considerable
impairment of urine concentrating ability.
Recirculation of Urea from Collecting Duct to Loop of Henle
Contributes to Hyperosmotic Renal Medulla. A person
usually excretes about 20 to 50 per cent of the filtered
load of urea. In general, the rate of urea excretion is
determined mainly by two factors: (1) the concentration of urea in the plasma and (2) the glomerular filtration rate (GFR). In patients with renal disease who
have large reductions of GFR, the plasma urea concentration increases markedly, returning the filtered
urea load and urea excretion rate to the normal level
(equal to the rate of urea production), despite the
reduced GFR.
354
Unit V
The Body Fluids and Kidneys
100% remaining
4.5
Urea
Urea 4.5
Cortex
50% remaining
Outer H2O
medulla
100%
remaining
7
30
30
15
Urea
Inner
medulla
Urea
300
300
500
550
20% remaining
Figure 28–5
Recirculation of urea absorbed from the medullary collecting duct
into the interstitial fluid. This urea diffuses into the thin loop of
Henle, and then passes through the distal tubules, and finally
passes back into the collecting duct. The recirculation of urea
helps to trap urea in the renal medulla and contributes to the
hyperosmolarity of the renal medulla. The heavy dark lines, from
the thick ascending loop of Henle to the medullary collecting
ducts, indicate that these segments are not very permeable to
urea. (Numerical values are in milliosmoles per liter of urea during
antidiuresis, when large amounts of antidiuretic hormone are
present. Percentages of the filtered load of urea that remain in the
tubules are indicated in the boxes.)
In the proximal tubule, 40 to 50 per cent of the filtered urea is reabsorbed, but even so, the tubular fluid
urea concentration increases because urea is not
nearly as permeant as water.The concentration of urea
continues to rise as the tubular fluid flows into the thin
segments of the loop of Henle, partly because of water
reabsorption out of the descending loop of Henle but
also because of some secretion of urea into the thin
loop of Henle from the medullary interstitium (Figure
28–5).
The thick limb of the loop of Henle, the distal
tubule, and the cortical collecting tubule are all relatively impermeable to urea, and very little urea reabsorption occurs in these tubular segments. When the
kidney is forming a concentrated urine and high levels
of ADH are present, the reabsorption of water from
the distal tubule and cortical collecting tubule further
raises the tubular fluid concentration of urea. And as
this urea flows into the inner medullary collecting
duct, the high tubular fluid concentration of urea
and specific urea transporters cause urea to diffuse
into the medullary interstitium. A moderate share of
the urea that moves into the medullary interstitium
eventually diffuses into the thin loop of Henle, so that
it passes upward through the ascending loop of Henle,
the distal tubule, the cortical collecting tubule, and
back down into the medullary collecting duct again. In
this way, urea can recirculate through these terminal
parts of the tubular system several times before it is
excreted. Each time around the circuit contributes to
a higher concentration of urea.
This urea recirculation provides an additional mechanism for forming a hyperosmotic renal medulla.
Because urea is one of the most abundant waste products that must be excreted by the kidneys, this mechanism for concentrating urea before it is excreted is
essential to the economy of the body fluid when water
is in short supply.
When there is excess water in the body and low
levels of ADH, the inner medullary collecting ducts
have a much lower permeability to both water and
urea, and more urea is excreted in the urine.
Countercurrent Exchange in the Vasa
Recta Preserves Hyperosmolarity
of the Renal Medulla
Blood flow must be provided to the renal medulla to
supply the metabolic needs of the cells in this part of
the kidney. Without a special medullary blood flow
system, the solutes pumped into the renal medulla by
the countercurrent multiplier system would be rapidly
dissipated.
There are two special features of the renal
medullary blood flow that contribute to the preservation of the high solute concentrations:
1. The medullary blood flow is low, accounting for
less than 5 per cent of the total renal blood flow.
This sluggish blood flow is sufficient to supply
the metabolic needs of the tissues but helps to
minimize solute loss from the medullary
interstitium.
2. The vasa recta serve as countercurrent exchangers,
minimizing washout of solutes from the medullary
interstitium.
The countercurrent exchange mechanism operates as
follows (Figure 28–6): Blood enters and leaves the
medulla by way of the vasa recta at the boundary
of the cortex and renal medulla. The vasa recta, like
other capillaries, are highly permeable to solutes in
the blood, except for the plasma proteins. As blood
descends into the medulla toward the papillae, it
becomes progressively more concentrated, partly
by solute entry from the interstitium and partly by
loss of water into the interstitium. By the time the
blood reaches the tips of the vasa recta, it has a concentration of about 1200 mOsm/L, the same as that of
the medullary interstitium. As blood ascends back
toward the cortex, it becomes progressively less
concentrated as solutes diffuse back out into the
medullary interstitium and as water moves into the
vasa recta.
Thus, although there is a large amount of fluid and
solute exchange across the vasa recta, there is little net
dilution of the concentration of the interstitial fluid at
each level of the renal medulla because of the U shape
Chapter 28
Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
Vasa recta
mOsm/L
Interstitium
mOsm/L
300
350
Solute
600
H2O
600
600
300
Solute
600
Solute
800
H2O
800
800
Solute
H2O
1000
1000
Solute
900
Solute
1000
1200
1200
355
Summary of Urine Concentrating
Mechanism and Changes in
Osmolarity in Different Segments
of the Tubules
The changes in osmolarity and volume of the tubular
fluid as it passes through the different parts of the
nephron are shown in Figure 28–7.
Proximal Tubule. About 65 per cent of the filtered
electrolytes are reabsorbed in the proximal tubule.
However, the tubular membranes are highly permeable to water, so that whenever solutes are reabsorbed,
water also diffuses through the tubular membrane by
osmosis. Therefore, the osmolarity of the fluid remains
about the same as the glomerular filtrate, 300 mOsm/L.
Figure 28–6
Countercurrent exchange in the vasa recta. Plasma flowing down
the descending limb of the vasa recta becomes more hyperosmotic because of diffusion of water out of the blood and diffusion
of solutes from the renal interstitial fluid into the blood. In the
ascending limb of the vasa recta, solutes diffuse back into the
interstitial fluid and water diffuses back into the vasa recta. Large
amounts of solutes would be lost from the renal medulla without
the U shape of the vasa recta capillaries. (Numerical values are
in milliosmoles per liter.)
of the vasa recta capillaries, which act as countercurrent exchangers. Thus, the vasa recta do not create the
medullary hyperosmolarity, but they do prevent it from
being dissipated.
The U-shaped structure of the vessels minimizes
loss of solute from the interstitium but does not
prevent the bulk flow of fluid and solutes into the
blood through the usual colloid osmotic and hydrostatic pressures that favor reabsorption in these capillaries. Thus, under steady-state conditions, the vasa
recta carry away only as much solute and water as is
absorbed from the medullary tubules, and the high
concentration of solutes established by the countercurrent mechanism is maintained.
Increased Medullary Blood Flow Can Reduce Urine Concentrating Ability. Certain vasodilators can markedly increase
renal medullary blood flow, thereby “washing out”
some of the solutes from the renal medulla and reducing maximum urine concentrating ability. Large
increases in arterial pressure can also increase the
blood flow of the renal medulla to a greater extent
than in other regions of the kidney and tend to wash
out the hyperosmotic interstitium, thereby reducing
urine concentrating ability. As discussed earlier,
maximum concentrating ability of the kidney is determined not only by the level of ADH but also by the
osmolarity of the renal medulla interstitial fluid. Even
with maximal levels of ADH, urine concentrating
ability will be reduced if medullary blood flow
increases enough to reduce the hyperosmolarity in the
renal medulla.
Descending Loop of Henle. As fluid flows down the
descending loop of Henle, water is absorbed into the
medulla. The descending limb is highly permeable to
water but much less permeable to sodium chloride and
urea. Therefore, the osmolarity of the fluid flowing
through the descending loop gradually increases until
it is equal to that of the surrounding interstitial fluid,
which is about 1200 mOsm/L when the blood concentration of ADH is high. When a dilute urine is being
formed, owing to low ADH concentrations, the medullary interstitial osmolarity is less than 1200 mOsm/L;
consequently, the descending loop tubular fluid osmolarity also becomes less concentrated. This is due
partly to the fact that less urea is absorbed into the
medullary interstitium from the collecting ducts when
ADH levels are low and the kidney is forming a large
volume of dilute urine.
Thin Ascending Loop of Henle. The thin ascending limb is
essentially impermeable to water but reabsorbs some
sodium chloride. Because of the high concentration of
sodium chloride in the tubular fluid, owing to water
removal from the descending loop of Henle, there is
some passive diffusion of sodium chloride from the
thin ascending limb into the medullary interstitium.
Thus, the tubular fluid becomes more dilute as the
sodium chloride diffuses out of the tubule and water
remains in the tubule. Some of the urea absorbed into
the medullary interstitium from the collecting ducts
also diffuses into the ascending limb, thereby returning the urea to the tubular system and helping to
prevent its washout from the renal medulla. This urea
recycling is an additional mechanism that contributes
to the hyperosmotic renal medulla.
Thick Ascending Loop of Henle. The thick part of the
ascending loop of Henle is also virtually impermeable
to water, but large amounts of sodium, chloride, potassium, and other ions are actively transported from
the tubule into the medullary interstitium. Therefore,
fluid in the thick ascending limb of the loop of Henle
becomes very dilute, falling to a concentration of
about 100 mOsm/L.
356
Unit V
600
Medullary
Cortical
Late distal
Diluting segment
900
Effect of ADH
0.2 ml
25 ml
1200
Osmolarity (mOsm/L)
The Body Fluids and Kidneys
Figure 28–7
8 ml
300
125 ml 44 ml
200
100
0
25 ml
Proximal
tubule
Loop of Henle
20 ml
Distal
tubule
Collecting
tubule
and duct
Early Distal Tubule. The early distal tubule has properties similar to those of the thick ascending loop of
Henle, so that further dilution of the tubular fluid
occurs as solutes are reabsorbed while water remains
in the tubule.
Late Distal Tubule and Cortical Collecting Tubules. In the late
distal tubule and cortical collecting tubules, the osmolarity of the fluid depends on the level of ADH. With
high levels of ADH, these tubules are highly permeable to water, and significant amounts of water are
reabsorbed. Urea, however, is not very permeant in
this part of the nephron, resulting in increased urea
concentration as water is reabsorbed. This allows most
of the urea delivered to the distal tubule and collecting tubule to pass into the inner medullary collecting
ducts, from which it is eventually reabsorbed or
excreted in the urine. In the absence of ADH, little
water is reabsorbed in the late distal tubule and cortical collecting tubule; therefore, osmolarity decreases
even further because of continued active reabsorption
of ions from these segments.
Inner Medullary Collecting Ducts. The concentration of
fluid in the inner medullary collecting ducts also
depends on (1) ADH and (2) the osmolarity of the
medullary interstitium established by the countercurrent mechanism. In the presence of large amounts of
ADH, these ducts are highly permeable to water, and
water diffuses from the tubule into the interstitium
until osmotic equilibrium is reached, with the tubular
fluid having about the same concentration as the renal
medullary interstitium (1200 to 1400 mOsm/L). Thus,
a very concentrated but small volume of urine is produced when ADH levels are high. Because water reabsorption increases urea concentration in the tubular
Urine
Changes in osmolarity of the
tubular fluid as it passes through
the different tubular segments in
the presence of high levels of
antidiuretic hormone (ADH) and
in the absence of ADH. (Numerical values indicate the approximate volumes in milliliters per
minute or in osmolarities in milliosmoles per liter of fluid flowing along the different tubular
segments.)
fluid, and because the inner medullary collecting ducts
have specific urea transporters that greatly facilitate
diffusion, much of the highly concentrated urea in
the ducts diffuses out of the tubular lumen into the
medullary interstitium. This absorption of the urea
into the renal medulla contributes to the high osmolarity of the medullary interstitium and the high concentrating ability of the kidney.
There are several important points to consider that
may not be obvious from this discussion. First,
although sodium chloride is one of the principal
solutes that contributes to the hyperosmolarity of the
medullary interstitium, the kidney can, when needed,
excrete a highly concentrated urine that contains little
sodium chloride. The hyperosmolarity of the urine in
these circumstances is due to high concentrations of
other solutes, especially of waste products such as urea
and creatinine. One condition in which this occurs is
dehydration accompanied by low sodium intake. As
discussed in Chapter 29, low sodium intake stimulates
formation of the hormones angiotensin II and aldosterone, which together cause avid sodium reabsorption
from the tubules while leaving the urea and other
solutes to maintain the highly concentrated urine.
Second, large quantities of dilute urine can be
excreted without increasing the excretion of sodium.
This is accomplished by decreasing ADH secretion,
which reduces water reabsorption in the more distal
tubular segments without significantly altering sodium
reabsorption.
And finally, we should keep in mind that there is
an obligatory urine volume, which is dictated by the
maximum concentrating ability of the kidney and the
amount of solute that must be excreted. Therefore, if
large amounts of solute must be excreted, they must
be accompanied by the minimal amount of water
Chapter 28
Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
necessary to excrete them. For example, if 1200 milliosmoles of solute must be excreted each day, this
requires at least 1 liter of urine if maximal urine concentrating ability is 1200 mOsm/L.
Quantifying Renal Urine
Concentration and Dilution:
“Free Water” and
Osmolar Clearances
The process of concentrating or diluting the urine
requires the kidneys to excrete water and solutes somewhat independently. When the urine is dilute, water is
excreted in excess of solutes. Conversely, when the urine
is concentrated, solutes are excreted in excess of water.
The total clearance of solutes from the blood can be
expressed as the osmolar clearance (Cosm); this is the
volume of plasma cleared of solutes each minute, in
the same way that clearance of a single substance is calculated:
Cosm =
Uosm ¥ V
Posm
where Uosm is the urine osmolarity, V is the urine flow
rate, and Posm is the plasma osmolarity. For example, if
plasma osmolarity is 300 mOsm/L, urine osmolarity is
600 mOsm/L, and urine flow rate is 1 ml/min (0.001 L/
min), the rate of osmolar excretion is 0.6 mOsm/min
(600 mOsm/L ¥ 0.001 L/min) and osmolar clearance is
0.6 mOsm/min divided by 300 mOsm/L, or 0.002 L/min
(2.0 ml/min). This means that 2 milliliters of plasma are
being cleared of solute each minute.
Relative Rates at Which Solutes and Water Are Excreted Can Be
Assessed Using the Concept of “Free-Water Clearance.” Free-
water clearance (CH2O) is calculated as the difference
between water excretion (urine flow rate) and osmolar
clearance:
CH2O = V - Cosm = V -
( Uosm ¥ V )
(Posm )
Thus, the rate of free-water clearance represents the
rate at which solute-free water is excreted by the
kidneys. When free-water clearance is positive, excess
water is being excreted by the kidneys; when free-water
clearance is negative, excess solutes are being removed
from the blood by the kidneys and water is being
conserved.
Using the example discussed earlier, if urine flow
rate is 1 ml/min and osmolar clearance is 2 ml/min,
free-water clearance would be -1 ml/min. This means
that instead of water being cleared from the kidneys in
excess of solutes, the kidneys are actually returning
water back to the systemic circulation, as occurs during
water deficits. Thus, whenever urine osmolarity is greater
than plasma osmolarity, free-water clearance will be negative, indicating water conservation.
When the kidneys are forming a dilute urine (that is,
urine osmolarity is less than plasma osmolarity), freewater clearance will be a positive value, denoting that
water is being removed from the plasma by the kidneys
in excess of solutes. Thus, water free of solutes, called
“free water,” is being lost from the body and the plasma
is being concentrated when free-water clearance is
positive.
357
Disorders of Urinary
Concentrating Ability
An impairment in the ability of the kidneys to concentrate or dilute the urine appropriately can occur with
one or more of the following abnormalities:
1. Inappropriate secretion of ADH. Either too much
or too little ADH secretion results in abnormal
fluid handling by the kidneys.
2. Impairment of the countercurrent mechanism. A
hyperosmotic medullary interstitium is required for
maximal urine concentrating ability. No matter how
much ADH is present, maximal urine concentration
is limited by the degree of hyperosmolarity of the
medullary interstitium.
3. Inability of the distal tubule, collecting tubule, and
collecting ducts to respond to ADH.
Failure to Produce ADH: “Central” Diabetes Insipidus. An
inability to produce or release ADH from the posterior
pituitary can be caused by head injuries or infections, or
it can be congenital. Because the distal tubular segments
cannot reabsorb water in the absence of ADH, this condition, called “central” diabetes insipidus, results in the
formation of a large volume of dilute urine, with urine
volumes that can exceed 15 L/day. The thirst mechanisms, discussed later in this chapter, are activated when
excessive water is lost from the body; therefore, as long
as the person drinks enough water, large decreases in
body fluid water do not occur. The primary abnormality observed clinically in people with this condition is
the large volume of dilute urine. However, if water
intake is restricted, as can occur in a hospital setting
when fluid intake is restricted or the patient is unconscious (for example, because of a head injury), severe
dehydration can rapidly occur.
The treatment for central diabetes insipidus is administration of a synthetic analog of ADH, desmopressin,
which acts selectively on V2 receptors to increase water
permeability in the late distal and collecting tubules.
Desmopressin can be given by injection, as a nasal spray,
or orally, and rapidly restores urine output toward
normal.
Inability of the Kidneys to Respond to ADH: “Nephrogenic”
Diabetes Insipidus. There are circumstances in which
normal or elevated levels of ADH are present but the
renal tubular segments cannot respond appropriately.
This condition is referred to as “nephrogenic” diabetes
insipidus because the abnormality resides in the
kidneys. This abnormality can be due to either failure of
the countercurrent mechanism to form a hyperosmotic
renal medullary interstitium or failure of the distal and
collecting tubules and collecting ducts to respond to
ADH. In either case, large volumes of dilute urine are
formed, which tends to cause dehydration unless fluid
intake is increased by the same amount as urine volume
is increased.
Many types of renal diseases can impair the concentrating mechanism, especially those that damage the
renal medulla. Also, impairment of the function of the
loop of Henle, as occurs with diuretics that inhibit electrolyte reabsorption by this segment, can compromise
urine concentrating ability. And certain drugs, such as
lithium (used to treat manic-depressive disorders)
and tetracyclines (used as antibiotics), can impair the
ability of the distal nephron segments to respond to
ADH.
358
Unit V
The Body Fluids and Kidneys
Nephrogenic diabetes insipidus can be distinguished
from central diabetes insipidus by administration of
desmopressin, the synthetic analog of ADH. Lack of a
prompt decrease in urine volume and an increase in
urine osmolarity within 2 hours after injection of
desmopressin is strongly suggestive of nephrogenic diabetes insipidus. The treatment for nephrogenic diabetes
insipidus is to correct, if possible, the underlying renal
disorder. The hypernatremia can also be attenuated by
a low-sodium diet and administration of a diuretic that
enhances renal sodium excretion, such as a thiazide
diuretic.
Control of Extracellular Fluid
Osmolarity and Sodium
Concentration
Regulation of extracellular fluid osmolarity and
sodium concentration are closely linked because
sodium is the most abundant ion in the extracellular
compartment. Plasma sodium concentration is normally regulated within close limits of 140 to 145 mEq/
L, with an average concentration of about 142 mEq/L.
Osmolarity averages about 300 mOsm/L (about
282 mOsm/L when corrected for interionic attraction)
and seldom changes more than ±2 to 3 per cent. As
discussed in Chapter 25, these variables must be precisely controlled because they determine the distribution of fluid between the intracellular and extracellular
compartments.
Although multiple mechanisms control the amount
of sodium and water excretion by the kidneys, two
primary systems are especially involved in regulating
the concentration of sodium and osmolarity of extracellular fluid: (1) the osmoreceptor-ADH system and
(2) the thirst mechanism.
Osmoreceptor-ADH
Feedback System
Figure 28–8 shows the basic components of the
osmoreceptor-ADH feedback system for control of
extracellular fluid sodium concentration and osmolarity. When osmolarity (plasma sodium concentration)
increases above normal because of water deficit, for
example, this feedback system operates as follows:
1. An increase in extracellular fluid osmolarity
(which in practical terms means an increase in
plasma sodium concentration) causes the special
nerve cells called osmoreceptor cells, located in
the anterior hypothalamus near the supraoptic
nuclei, to shrink.
2. Shrinkage of the osmoreceptor cells causes them
to fire, sending nerve signals to additional nerve
cells in the supraoptic nuclei, which then relay
Water deficit
Estimating Plasma Osmolarity from
Plasma Sodium Concentration
Extracellular osmolarity
In most clinical laboratories, plasma osmolarity is not
routinely measured. However, because sodium and its
associated anions account for about 94 per cent of the
solute in the extracellular compartment, plasma osmolarity (Posm) can be roughly approximated as
Osmoreceptors
ADH secretion
(posterior pituitary)
Posm = 2.1 ¥ Plasma sodium concentration
For instance, with a plasma sodium concentration of
142 mEq/L, the plasma osmolarity would be estimated
from the formula above to be about 298 mOsm/L. To
be more exact, especially in conditions associated with
renal disease, the contribution of two other solutes,
glucose and urea, should be included. Such estimates
of plasma osmolarity are usually accurate within a few
percentage points of those measured directly.
Normally, sodium ions and associated anions (primarily bicarbonate and chloride) represent about 94
per cent of the extracellular osmoles, with glucose and
urea contributing about 3 to 5 per cent of the total
osmoles. However, because urea easily permeates
most cell membranes, it exerts little effective osmotic
pressure under steady-state conditions. Therefore, the
sodium ions in the extracellular fluid and associated
anions are the principal determinants of fluid movement across the cell membrane. Consequently, we can
discuss the control of osmolarity and control of sodium
ion concentration at the same time.
Plasma ADH
H2O permeability in
distal tubules,
collecting ducts
H2O reabsorption
H2O excreted
Figure 28–8
Osmoreceptor-antidiuretic hormone (ADH) feedback mechanism
for regulating extracellular fluid osmolarity in response to a water
deficit.
Chapter 28
359
Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
these signals down the stalk of the pituitary gland
to the posterior pituitary.
3. These action potentials conducted to the posterior
pituitary stimulate the release of ADH, which is
stored in secretory granules (or vesicles) in the
nerve endings.
4. ADH enters the blood stream and is transported
to the kidneys, where it increases the water
permeability of the late distal tubules, cortical
collecting tubules, and medullary collecting ducts.
5. The increased water permeability in the distal
nephron segments causes increased water
reabsorption and excretion of a small volume of
concentrated urine.
Thus, water is conserved in the body while sodium and
other solutes continue to be excreted in the urine. This
causes dilution of the solutes in the extracellular fluid,
thereby correcting the initial excessively concentrated
extracellular fluid.
The opposite sequence of events occurs when the
extracellular fluid becomes too dilute (hypo-osmotic).
For example, with excess water ingestion and a
decrease in extracellular fluid osmolarity, less ADH is
formed, the renal tubules decrease their permeability
for water, less water is reabsorbed, and a large volume
of dilute urine is formed. This in turn concentrates the
body fluids and returns plasma osmolarity toward
normal.
ADH Synthesis in Supraoptic and
Paraventricular Nuclei of the
Hypothalamus and ADH Release
from the Posterior Pituitary
Figure 28–9 shows the neuroanatomy of the hypothalamus and the pituitary gland, where ADH is synthesized and released. The hypothalamus contains two
types of magnocellular (large) neurons that synthesize
ADH in the supraoptic and paraventricular nuclei of
the hypothalamus, about five sixths in the supraoptic
nuclei and about one sixth in the paraventricular
nuclei. Both of these nuclei have axonal extensions to
the posterior pituitary. Once ADH is synthesized, it is
transported down the axons of the neurons to their
tips, terminating in the posterior pituitary gland. When
the supraoptic and paraventricular nuclei are stimulated by increased osmolarity or other factors, nerve
impulses pass down these nerve endings, changing
their membrane permeability and increasing calcium
entry. ADH stored in the secretory granules (also
called vesicles) of the nerve endings is released in
response to increased calcium entry. The released
ADH is then carried away in the capillary blood of the
posterior pituitary into the systemic circulation.
Secretion of ADH in response to an osmotic stimulus is rapid, so that plasma ADH levels can increase
severalfold within minutes, thereby providing a rapid
means for altering renal excretion of water.
A second neuronal area important in controlling
osmolarity and ADH secretion is located along the
Pituitary
Osmoreceptors
Baroreceptors
Cardiopulmonary
receptors
Supraoptic
neuron
Paraventricular
neuron
Anterior
lobe
Posterior
lobe
ADH
Urine:
decreased flow
and concentrated
Figure 28–9
Neuroanatomy of the hypothalamus, where antidiuretic hormone
(ADH) is synthesized, and the posterior pituitary gland, where
ADH is released.
anteroventral region of the third ventricle, called the
AV3V region. At the upper part of this region is a
structure called the subfornical organ, and at the inferior part is another structure called the organum vasculosum of the lamina terminalis. Between these two
organs is the median preoptic nucleus, which has multiple nerve connections with the two organs as well as
with the supraoptic nuclei and the blood pressure
control centers in the medulla of the brain. Lesions of
the AV3V region cause multiple deficits in the control
of ADH secretion, thirst, sodium appetite, and blood
pressure. Electrical stimulation of this region or stimulation by angiotensin II can alter ADH secretion,
thirst, and sodium appetite.
In the vicinity of the AV3V region and the supraoptic nuclei are neuronal cells that are excited by small
increases in extracellular fluid osmolarity; hence, the
term osmoreceptors has been used to describe these
neurons. These cells send nerve signals to the supraoptic nuclei to control their firing and secretion of ADH.
It is also likely that they induce thirst in response to
increased extracellular fluid osmolarity.
360
Unit V
The Body Fluids and Kidneys
Both the subfornical organ and the organum vasculosum of the lamina terminalis have vascular supplies
that lack the typical blood-brain barrier that impedes
the diffusion of most ions from the blood into the brain
tissue. This makes it possible for ions and other solutes
to cross between the blood and the local interstitial
fluid in this region. As a result, the osmoreceptors
rapidly respond to changes in osmolarity of the extracellular fluid, exerting powerful control over the secretion of ADH and over thirst, as discussed later.
ADH release is also controlled by cardiovascular
reflexes that respond to decreases in blood pressure
and/or blood volume, including (1) the arterial baroreceptor reflexes and (2) the cardiopulmonary reflexes,
both of which are discussed in Chapter 18. These reflex
pathways originate in high-pressure regions of the circulation, such as the aortic arch and carotid sinus, and
in the low-pressure regions, especially in the cardiac
atria. Afferent stimuli are carried by the vagus and
glossopharyngeal nerves with synapses in the nuclei of
the tractus solitarius. Projections from these nuclei
relay signals to the hypothalamic nuclei that control
ADH synthesis and secretion.
Thus, in addition to increased osmolarity, two other
stimuli increase ADH secretion: (1) decreased arterial
pressure and (2) decreased blood volume. Whenever
blood pressure and blood volume are reduced, such as
occurs during hemorrhage, increased ADH secretion
causes increased fluid reabsorption by the kidneys,
helping to restore blood pressure and blood volume
toward normal.
Quantitative Importance of
Cardiovascular Reflexes and
Osmolarity in Stimulating
ADH Secretion
As shown in Figure 28–10, either a decrease in effective blood volume or an increase in extracellular fluid
osmolarity stimulates ADH secretion. However, ADH
is considerably more sensitive to small changes in
osmolarity than to similar changes in blood volume.
For example, a change in plasma osmolarity of only
1 per cent is sufficient to increase ADH levels. By
contrast, after blood loss, plasma ADH levels do not
change appreciably until blood volume is reduced by
about 10 per cent. With further decreases in blood
volume, ADH levels rapidly increase. Thus, with severe
decreases in blood volume, the cardiovascular reflexes
play a major role in stimulating ADH secretion. The
usual day-to-day regulation of ADH secretion during
simple dehydration is effected mainly by changes in
plasma osmolarity. Decreased blood volume, however,
PAVP = 1.3 e–0.17 vol.
50
45
40
Plasma ADH (pg/ml)
Cardiovascular Reflex Stimulation
of ADH Release by Decreased
Arterial Pressure and/or Decreased
Blood Volume
Isotonic volume depletion
Isovolemic osmotic increase
35
30
25
20
PAVP = 2.5 D Osm + 2.0
15
10
5
0
0
5
10
15
20
Per cent change
Figure 28–10
The effect of increased plasma osmolarity or decreased blood
volume on the level of plasma (P) antidiuretic hormone (ADH), also
called arginine vasopressin (AVP). (Redrawn from Dunn FL,
Brennan TJ, Nelson AE, Robertson GL: The role of blood osmolality and volume in regulating vasopressin secretion in the rat. J
Clin Invest 52(12):3212, 1973. By copyright permission of the
American Society of Clinical Investigation.)
Table 28–2
Regulation of ADH Secretion
Increase ADH
Decrease ADH
≠ Plasma osmolarity
Ø Blood volume
Ø Blood pressure
Ø Plasma osmolarity
≠ Blood volume
≠ Blood pressure
Nausea
Hypoxia
Drugs:
Morphine
Nicotine
Cyclophosphamide
Drugs:
Alcohol
Clonidine (antihypertensive drug)
Haloperidol (dopamine blocker)
greatly enhances the ADH response to increased
osmolarity.
Other Stimuli for ADH Secretion
ADH secretion can also be increased or decreased by
other stimuli to the central nervous system as well as
by various drugs and hormones, as shown in Table
28–2. For example, nausea is a potent stimulus for
Chapter 28
Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
ADH release, which may increase to as much as 100
times normal after vomiting. Also, drugs such as nicotine and morphine stimulate ADH release, whereas
some drugs, such as alcohol, inhibit ADH release. The
marked diuresis that occurs after ingestion of alcohol
is due in part to inhibition of ADH release.
Role of Thirst in Controlling
Extracellular Fluid Osmolarity
and Sodium Concentration
The kidneys minimize fluid loss during water deficits
through the osmoreceptor-ADH feedback system.
Adequate fluid intake, however, is necessary to counterbalance whatever fluid loss does occur through
sweating and breathing and through the gastrointestinal tract. Fluid intake is regulated by the thirst mechanism, which, together with the osmoreceptor-ADH
mechanism, maintains precise control of extracellular
fluid osmolarity and sodium concentration.
Many of the same factors that stimulate ADH secretion also increase thirst, which is defined as the conscious desire for water.
Central Nervous System Centers
for Thirst
Referring again to Figure 28–9, the same area along
the anteroventral wall of the third ventricle that promotes ADH release also stimulates thirst. Located
anterolaterally in the preoptic nucleus is another small
area that, when stimulated electrically, causes immediate drinking that continues as long as the stimulation
lasts. All these areas together are called the thirst
center.
The neurons of the thirst center respond to injections of hypertonic salt solutions by stimulating drinking behavior. These cells almost certainly function as
osmoreceptors to activate the thirst mechanism, in
the same way that the osmoreceptors stimulate ADH
release.
Increased osmolarity of the cerebrospinal fluid in
the third ventricle has essentially the same effect to
promote drinking. It is likely that the organum vasculosum of the lamina terminalis, which lies immediately
beneath the ventricular surface at the inferior end of
the AV3V region, is intimately involved in mediating
this response.
Stimuli for Thirst
Table 28–3 summarizes some of the known stimuli for
thirst. One of the most important is increased extracellular fluid osmolarity, which causes intracellular
dehydration in the thirst centers, thereby stimulating
the sensation of thirst. The value of this response is
obvious: it helps to dilute extracellular fluids and
returns osmolarity toward normal.
361
Table 28–3
Control of Thirst
Increase Thirst
Decrease Thirst
≠ Osmolarity
Ø Blood volume
Ø Blood pressure
≠ Angiotensin
Ø Osmolarity
≠ Blood volume
≠ Blood pressure
Ø Angiotensin II
Dryness of mouth
Gastric distention
Decreases in extracellular fluid volume and arterial
pressure also stimulate thirst by a pathway that is independent of the one stimulated by increased plasma
osmolarity. Thus, blood volume loss by hemorrhage
stimulates thirst even though there might be no change
in plasma osmolarity. This probably occurs because of
neutral input from cardiopulmonary and systemic
arterial baroreceptors in the circulation.
A third important stimulus for thirst is angiotensin II.
Studies in animals have shown that angiotensin II acts
on the subfornical organ and on the organum vasculosum of the lamina terminalis. These regions are
outside the blood-brain barrier, and peptides such as
angiotensin II diffuse into the tissues. Because
angiotensin II is also stimulated by factors associated
with hypovolemia and low blood pressure, its effect on
thirst helps to restore blood volume and blood pressure toward normal, along with the other actions
of angiotensin II on the kidneys to decrease fluid
excretion.
Dryness of the mouth and mucous membranes of the
esophagus can elicit the sensation of thirst. As a result,
a thirsty person may receive relief from thirst almost
immediately after drinking water, even though the
water has not been absorbed from the gastrointestinal
tract and has not yet had an effect on extracellular
fluid osmolarity.
Gastrointestinal and pharyngeal stimuli influence
thirst. For example, in animals that have an esophageal
opening to the exterior so that water is never absorbed
into the blood, partial relief of thirst occurs after
drinking, although the relief is only temporary. Also,
gastrointestinal distention may partially alleviate
thirst; for instance, simple inflation of a balloon in the
stomach can relieve thirst. However, relief of thirst
sensations through gastrointestinal or pharyngeal
mechanisms is short-lived; the desire to drink is completely satisfied only when plasma osmolarity and/or
blood volume returns to normal.
The ability of animals and humans to “meter” fluid
intake is important because it prevents overhydration.
After a person drinks water, 30 to 60 minutes may be
required for the water to be reabsorbed and distributed throughout the body. If the thirst sensation
were not temporarily relieved after drinking water, the
person would continue to drink more and more, eventually leading to overhydration and excess dilution of
362
Unit V
The Body Fluids and Kidneys
the body fluids. Experimental studies have repeatedly
shown that animals drink almost exactly the amount
necessary to return plasma osmolarity and volume to
normal.
The kidneys must continually excrete at least some
fluid, even in a dehydrated person, to rid the body of
excess solutes that are ingested or produced by metabolism. Water is also lost by evaporation from the lungs
and the gastrointestinal tract and by evaporation and
sweating from the skin. Therefore, there is always a
tendency for dehydration, with resultant increased
extracellular fluid sodium concentration and osmolarity. When the sodium concentration increases only
about 2 mEq/L above normal, the thirst mechanism is
activated, causing a desire to drink water. This is called
the threshold for drinking. Thus, even small increases
in plasma osmolarity are normally followed by water
intake, which restores extracellular fluid osmolarity
and volume toward normal. In this way, the extracellular fluid osmolarity and sodium concentration are
precisely controlled.
Integrated Responses of
Osmoreceptor-ADH and Thirst
Mechanisms in Controlling
Extracellular Fluid Osmolarity and
Sodium Concentration
In a healthy person, the osmoreceptor-ADH and thirst
mechanisms work in parallel to precisely regulate
extracellular fluid osmolarity and sodium concentration, despite the constant challenges of dehydration.
Even with additional challenges, such as high salt
intake, these feedback systems are able to keep plasma
osmolarity reasonably constant. Figure 28–11 shows
that an increase in sodium intake to as high as six times
normal has only a small effect on plasma sodium concentration as long as the ADH and thirst mechanisms
are both functioning normally.
When either the ADH or the thirst mechanism fails,
the other ordinarily can still control extracellular
osmolarity and sodium concentration with reasonable
effectiveness, as long as there is enough fluid intake
to balance the daily obligatory urine volume and
water losses caused by respiration, sweating, or
gastrointestinal losses. However, if both the ADH
and thirst mechanisms fail simultaneously, plasma
sodium concentration and osmolarity are very poorly
controlled; thus, when sodium intake is increased
after blocking the total ADH-thirst system, relatively
large changes in plasma sodium concentration do
occur. In the absence of the ADH-thirst mechanisms,
no other feedback mechanism is capable of adequately regulating plasma sodium concentration and
osmolarity.
Plasma sodium concentration (mEq/L)
Threshold for Osmolar Stimulus
of Drinking
152
ADH and
thirst
systems
blocked
148
144
Normal
140
136
0
30
60
90
120 150
Sodium intake (mEq/day)
180
Figure 28–11
Effect of large changes in sodium intake on extracellular fluid
sodium concentration in dogs under normal conditions (red line)
and after the antidiuretic hormone (ADH) and thirst feedback
systems had been blocked (blue line). Note that control of extracellular fluid sodium concentration is poor in the absence of these
feedback systems. (Courtesy Dr. David B. Young.)
Role of Angiotensin II and
Aldosterone in Controlling
Extracellular Fluid Osmolarity and
Sodium Concentration
As discussed in Chapter 27, both angiotensin II and
aldosterone play an important role in regulating sodium
reabsorption by the renal tubules. When sodium intake
is low, increased levels of these hormones stimulate
sodium reabsorption by the kidneys and, therefore,
prevent large sodium losses, even though sodium intake
may be reduced to as low as 10 per cent of normal. Conversely, with high sodium intake, decreased formation of
these hormones permits the kidneys to excrete large
amounts of sodium.
Because of the importance of angiotensin II and
aldosterone in regulating sodium excretion by the
kidneys, one might mistakenly infer that they also
play an important role in regulating extracellular fluid
sodium concentration. Although these hormones
increase the amount of sodium in the extracellular fluid,
they also increase the extracellular fluid volume by
increasing reabsorption of water along with the sodium.
Therefore, angiotensin II and aldosterone have little
effect on sodium concentration, except under extreme
conditions.
This relative unimportance of aldosterone in regulating extracellular fluid sodium concentration is shown by
the experiment of Figure 28–12. This figure shows the
effect on plasma sodium concentration of changing
Plasma sodium concentration
(mEq/L)
Chapter 28
150
Regulation of Extracellular Fluid Osmolarity and Sodium Concentration
Normal
140
Aldosterone system blocked
130
120
110
363
reasons for this is that large losses of sodium eventually
cause severe volume depletion and decreased blood
pressure, which can activate the thirst mechanism
through the cardiovascular reflexes. This leads to a
further dilution of the plasma sodium concentration,
even though the increased water intake helps to minimize the decrease in body fluid volumes under these
conditions.
Thus, there are extreme situations in which plasma
sodium concentration may change significantly, even
with a functional ADH-thirst mechanism. Even so, the
ADH-thirst mechanism is by far the most powerful
feedback system in the body for controlling extracellular fluid osmolarity and sodium concentration.
100
0
30
60 90 120 150 180 210
Sodium intake (mEq/L)
Salt-Appetite Mechanism
for Controlling Extracellular
Fluid Sodium Concentration
and Volume
Figure 28–12
Effect of large changes in sodium intake on extracellular fluid
sodium concentration in dogs under normal conditions (red line)
and after the aldosterone feedback system had been blocked
(blue line). Note that sodium concentration is maintained relatively
constant over this wide range of sodium intakes, with or without
aldosterone feedback control. (Courtesy Dr. David B. Young.)
sodium intake more than sixfold under two conditions:
(1) under normal conditions and (2) after the aldosterone feedback system has been blocked by removing
the adrenal glands and infusing the animals with aldosterone at a constant rate so that plasma levels could not
change upward or downward. Note that when sodium
intake was increased sixfold, plasma concentration
changed only about 1 to 2 per cent in either case. This
indicates that even without a functional aldosterone
feedback system, plasma sodium concentration can be
well regulated. The same type of experiment has been
conducted after blocking angiotensin II formation, with
the same result.
There are two primary reasons why changes in
angiotensin II and aldosterone do not have a major
effect on plasma sodium concentration. First, as discussed earlier, angiotensin II and aldosterone increase
both sodium and water reabsorption by the renal
tubules, leading to increases in extracellular fluid
volume and sodium quantity but little change in sodium
concentrations. Second, as long as the ADH-thirst mechanism is functional, any tendency toward increased
plasma sodium concentration is compensated for by
increased water intake or increased plasma ADH secretion, which tends to dilute the extracellular fluid back
toward normal. The ADH-thirst system far overshadows the angiotensin II and aldosterone systems for regulating sodium concentration under normal conditions.
Even in patients with primary aldosteronism, who have
extremely high levels of aldosterone, the plasma sodium
concentration usually increases only about 3 to 5 mEq/L
above normal.
Under extreme conditions, caused by complete loss of
aldosterone secretion because of adrenalectomy or in
patients with Addison’s disease (severely impaired
secretion or total lack of aldosterone), there is tremendous loss of sodium by the kidneys, which can lead to
reductions in plasma sodium concentration. One of the
Maintenance of normal extracellular fluid volume and
sodium concentration requires a balance between
sodium excretion and sodium intake. In modern civilizations, sodium intake is almost always greater than
necessary for homeostasis. In fact, the average sodium
intake for individuals in industrialized cultures eating
processed foods usually ranges between 100 and
200 mEq/day, even though humans can survive and
function normally on 10 to 20 mEq/day. Thus, most
people eat far more sodium than is necessary for homeostasis, and there is evidence that our usual high sodium
intake may contribute to cardiovascular disorders such
as hypertension.
Salt appetite is due in part to the fact that animals and
humans like salt and eat it regardless of whether they
are salt-deficient. There is also a regulatory component
to salt appetite in which there is a behavioral drive to
obtain salt when there is sodium deficiency in the body.
This is particularly important in herbivores, which naturally eat a low-sodium diet, but salt craving may also
be important in humans who have extreme deficiency
of sodium, such as occurs in Addison’s disease. In this
instance, there is deficiency of aldosterone secretion,
which causes excessive loss of sodium in the urine and
leads to decreased extracellular fluid volume and
decreased sodium concentration; both of these changes
elicit the desire for salt.
In general, the two primary stimuli that are believed to
increase salt appetite are (1) decreased extracellular fluid
sodium concentration and (2) decreased blood volume
or blood pressure, associated with circulatory insufficiency. These are the same major stimuli that elicit thirst.
The neuronal mechanism for salt appetite is analogous to that of the thirst mechanism. Some of the same
neuronal centers in the AV3V region of the brain seem
to be involved, because lesions in this region frequently
affect both thirst and salt appetite simultaneously in
animals. Also, circulatory reflexes elicited by low blood
pressure or decreased blood volume affect both thirst
and salt appetite at the same time.
References
Cowley AW Jr, Mori T, Mattson D, Zou AP: Role of renal
NO production in the regulation of medullary blood flow.
Am J Physiol Regul Integr Comp Physiol 284:R1355, 2003.
364
Unit V
The Body Fluids and Kidneys
Dwyer TM, Schmidt-Nielsen B: The renal pelvis: machinery
that concentrates urine in the papilla. News Physiol Sci
18:1, 2003.
Edwards A, Delong MJ, Pallone TL: Interstitial water and
solute recovery by inner medullary vasa recta. Am J
Physiol Renal Physiol 278:F257, 2000.
Fitzsimons JT: Angiotensin, thirst, and sodium appetite.
Physiol Rev 78:583, 1998.
Knepper MA, Saidel GM, Hascall VC, Dwyer T: Concentration of solutes in the renal inner medulla: interstitial
hyaluronan as a mechano-osmotic transducer. Am J
Physiol Renal Physiol 284:F433, 2003.
Kozono D, Yasui M, King LS, Agre P: Aquaporin water channels: atomic structure molecular dynamics meet clinical
medicine. J Clin Invest 109:1395, 2002.
McKinley MJ, Johnson AK: The physiological regulation of
thirst and fluid intake. News Physiol Sci 19:1, 2004.
Morello JP, Bichet DG: Nephrogenic diabetes insipidus.
Annu Rev Physiol 63:607, 2001.
Nielsen S, Frokiaer J, Marples D, et al: Aquaporins in the
kidney: from molecules to medicine. Physiol Rev 82:205,
2002.
Pallone TL, Turner MR, Edwards A, Jamison RL: Countercurrent exchange in the renal medulla. Am J Physiol
Regul Integr Comp Physiol 284:R1153, 2003.
Robertson GL: Vasopressin. In Seldin DW, Giebisch G (eds):
The Kidney—Physiology and Pathophysiology, 3rd ed.
New York: Raven Press, 2000.
Sands JM, Layton HE: Urine concentrating mechanism and
its regulation. In Seldin DW, Giebisch G (eds): The
Kidney—Physiology and Pathophysiology, 3rd ed. New
York: Raven Press, 2000.
Sands JM: Molecular mechanisms of urea transport. J Membrane Biol 191:149, 2003.
Stricker EM, Sved AF: Controls of vasopressin secretion and
thirst: similarities and dissimilarities in signals. Physiol
Behav 77:731, 2002.
Verbalis JG: Diabetes insipidus. Rev Endocr Metab Disord
4:177, 2003.
C
H
A
P
T
E
R
2
9
Renal Regulation of Potassium,
Calcium, Phosphate, and
Magnesium; Integration
of Renal Mechanisms for
Control of Blood Volume and
Extracellular Fluid Volume
Regulation of Potassium
Excretion and Potassium
Concentration in
Extracellular Fluid
Extracellular fluid potassium concentration normally is regulated precisely at about 4.2 mEq/L,
seldom rising or falling more than ± 0.3 mEq/L. This precise control is necessary because many cell functions are very sensitive to changes in extracellular
fluid potassium concentration. For instance, an increase in plasma potassium
concentration of only 3 to 4 mEq/L can cause cardiac arrhythmias, and higher
concentrations can lead to cardiac arrest or fibrillation.
A special difficulty in regulating extracellular potassium concentration is the
fact that over 98 per cent of the total body potassium is contained in the cells
and only 2 per cent in the extracellular fluid (Figure 29–1). For a 70-kilogram
adult, who has about 28 liters of intracellular fluid (40 per cent of body weight)
and 14 liters of extracellular fluid (20 per cent of body weight), about 3920 milliequivalents of potassium are inside the cells and only about 59 milliequivalents are in the extracellular fluid. Also, the potassium contained in a single meal
is often as high as 50 milliequivalents, and the daily intake usually ranges
between 50 and 200 mEq/day; therefore, failure to rapidly rid the extracellular
fluid of the ingested potassium could cause life-threatening hyperkalemia
(increased plasma potassium concentration). Likewise, a small loss of potassium
from the extracellular fluid could cause severe hypokalemia (low plasma potassium concentration) in the absence of rapid and appropriate compensatory
responses.
Maintenance of potassium balance depends primarily on excretion by the
kidneys because the amount excreted in the feces is only about 5 to 10 per cent
of the potassium intake. Thus, the maintenance of normal potassium balance
requires the kidneys to adjust their potassium excretion rapidly and precisely
to wide variations in intake, as is also true for most other electrolytes.
Control of potassium distribution between the extracellular and intracellular
compartments also plays an important role in potassium homeostasis. Because
over 98 per cent of the total body potassium is contained in the cells, they can
serve as an overflow site for excess extracellular fluid potassium during hyperkalemia or as a source of potassium during hypokalemia. Thus, redistribution
of potassium between the intra- and extracellular fluid compartments provides a first line of defense against changes in extracellular fluid potassium
concentration.
365
366
Unit V
The Body Fluids and Kidneys
plasma potassium concentration after eating a meal is
much greater than normal. Injections of insulin,
however, can help to correct the hyperkalemia.
K+ intake
100 mEq/day
Extracellular
fluid K+
Intracellular
fluid K+
4.2 mEq/L
x 14 L
140 mEq/L
x 28 L
59 m Eq
3920 mEq
K+ output
Urine 92 mEq/day
Feces 8 mEq/day
100 mEq/day
Aldosterone Increases Potassium Uptake into Cells. Increased
potassium intake also stimulates secretion of aldosterone, which increases cell potassium uptake. Excess
aldosterone secretion (Conn’s syndrome) is almost
invariably associated with hypokalemia, due in part to
movement of extracellular potassium into the cells.
Conversely, patients with deficient aldosterone production (Addison’s disease) often have clinically significant
hyperkalemia due to accumulation of potassium in
the extracellular space as well as to renal retention of
potassium.
b-Adrenergic Stimulation Increases Cellular Uptake of Potassium.
Figure 29–1
Normal potassium intake, distribution of potassium in the body
fluids, and potassium output from the body.
Table 29–1
Factors That Can Alter Potassium Distribution Between the
Intra- and Extracellular Fluid
+
+
Factors That Shift K into Cells
(Decrease Extracellular [K+])
Factors That Shift K Out of Cells
(Increase Extracellular [K+])
•
•
•
•
• Insulin deficiency (diabetes
mellitus)
• Aldosterone deficiency
(Addison’s disease)
• b-adrenergic blockade
• Acidosis
• Cell lysis
• Strenuous exercise
• Increased extracellular fluid
osmolarity
Insulin
Aldosterone
b-adrenergic stimulation
Alkalosis
Regulation of Internal Potassium
Distribution
After ingestion of a normal meal, extracellular fluid
potassium concentration would rise to a lethal level if
the ingested potassium did not rapidly move into the
cells. For example, absorption of 40 milliequivalents of
potassium (the amount contained in a meal rich in vegetables and fruit) into an extracellular fluid volume of
14 liters would raise plasma potassium concentration
by about 2.9 mEq/L if all the potassium remained in
the extracellular compartment. Fortunately, most of
the ingested potassium rapidly moves into the cells
until the kidneys can eliminate the excess. Table 29–1
summarizes some of the factors that can influence the
distribution of potassium between the intra- and extracellular compartments.
Insulin Stimulates Potassium Uptake into Cells. One of the
most important factors that increase cell potassium
uptake after a meal is insulin. In people who have
insulin deficiency owing to diabetes mellitus, the rise in
Increased secretion of catecholamines, especially epinephrine, can cause movement of potassium from the
extracellular to the intracellular fluid, mainly by activation of b2-adrenergic receptors. Treatment of hypertension with b-adrenergic receptor blockers, such as
propranolol, causes potassium to move out of the cells
and creates a tendency toward hyperkalemia.
Acid-Base Abnormalities Can Cause Changes in Potassium Distribution. Metabolic acidosis increases extracellular potas-
sium concentration, in part by causing loss of potassium
from the cells, whereas metabolic alkalosis decreases
extracellular fluid potassium concentration. Although
the mechanisms responsible for the effect of hydrogen
ion concentration on potassium internal distribution are
not completely understood, one effect of increased
hydrogen ion concentration is to reduce the activity
of the sodium-potassium adenosine triphosphatase
(ATPase) pump. This in turn decreases cellular uptake
of potassium and raises extracellular potassium
concentration.
Cell Lysis Causes Increased Extracellular Potassium Concentration. As cells are destroyed, the large amounts of potas-
sium contained in the cells are released into the
extracellular compartment. This can cause significant
hyperkalemia if large amounts of tissue are destroyed,
as occurs with severe muscle injury or with red blood
cell lysis.
Strenuous Exercise Can Cause Hyperkalemia by Releasing Potassium from Skeletal Muscle. During prolonged exercise,
potassium is released from skeletal muscle into the
extracellular fluid. Usually the hyperkalemia is mild, but
it may be clinically significant after heavy exercise in
patients treated with b-adrenergic blockers or in individuals with insulin deficiency. In rare instances, hyperkalemia after exercise may be severe enough to cause
cardiac arrhythmias and sudden death.
Increased Extracellular Fluid Osmolarity Causes Redistribution of
Potassium from the Cells to Extracellular Fluid. Increased
extracellular fluid osmolarity causes osmotic flow of
water out of the cells. The cellular dehydration increases
intracellular potassium concentration, thereby promoting diffusion of potassium out of the cells and
increasing extracellular fluid potassium concentration.
Decreased extracellular fluid osmolarity has the opposite effect. In diabetes mellitus, large increases in plasma
glucose raise extracellular osmolarity, causing cell
Chapter 29
367
Renal Regulation; Integration of Renal Mechanisms
dehydration and movement of potassium from the cells
into the extracellular fluid.
Overview of Renal Potassium
Excretion
Potassium excretion is determined by the sum of three
renal processes: (1) the rate of potassium filtration
(GFR multiplied by the plasma potassium concentration), (2) the rate of potassium reabsorption by the
tubules, and (3) the rate of potassium secretion by
the tubules. The normal rate of potassium filtration is
about 756 mEq/day (GFR, 180 L/day multiplied by
plasma potassium, 4.2 mEq/L); this rate of filtration is
usually relatively constant because of the autoregulatory mechanisms for GFR discussed previously and
the precision with which plasma potassium concentration is regulated. Severe decreases in GFR in certain
renal diseases, however, can cause serious potassium
accumulation and hyperkalemia.
Figure 29–2 summarizes the tubular handling of
potassium under normal conditions. About 65 per cent
of the filtered potassium is reabsorbed in the proximal
tubule. Another 25 to 30 per cent of the filtered potassium is reabsorbed in the loop of Henle, especially in
the thick ascending part where potassium is actively
co-transported along with sodium and chloride. In
both the proximal tubule and the loop of Henle, a relatively constant fraction of the filtered potassium load
is reabsorbed. Changes in potassium reabsorption in
these segments can influence potassium excretion, but
most of the day-to-day variation of potassium excretion is not due to changes in reabsorption in the proximal tubule or loop of Henle.
Most Daily Variation in Potassium Excretion Is Caused by
Changes in Potassium Secretion in Distal and Collecting
Tubules. The most important sites for regulating potas-
sium excretion are the principal cells of the late distal
tubules and cortical collecting tubules. In these tubular
segments, potassium can at times be reabsorbed or at
other times be secreted, depending on the needs of the
body. With a normal potassium intake of 100 mEq/day,
the kidneys must excrete about 92 mEq/day (the
remaining 8 milliequivalents are lost in the feces).
About one third (31 mEq/day) of this amount of
potassium is secreted into the distal and collecting
tubules.
With high potassium intakes, the required extra
excretion of potassium is achieved almost entirely by
increasing the secretion of potassium into the distal
and collecting tubules. In fact, with extremely high
potassium diets, the rate of potassium excretion can
exceed the amount of potassium in the glomerular filtrate, indicating a powerful mechanism for secreting
potassium.
When potassium intake is reduced below normal,
the secretion rate of potassium in the distal and
collecting tubules decreases, causing a reduction in
urinary potassium secretion. With extreme reductions
in potassium intake, there is net reabsorption of potassium in the distal segments of the nephron, and potassium excretion can fall to 1 per cent of the potassium
in the glomerular filtrate (to less than 10 mEq/day).
With potassium intakes below this level, severe
hypokalemia can develop.
Thus, most of the day-to-day regulation of potassium excretion occurs in the late distal and cortical
collecting tubules, where potassium can be either
reabsorbed or secreted, depending on the needs of the
body. In the next section, we consider the basic mechanisms of potassium secretion and the factors that regulate this process.
Potassium Secretion by Principal
Cells of Late Distal and Cortical
Collecting Tubules
The cells in the late distal and cortical collecting
tubules that secrete potassium are called principal cells
65%
(491 mEq/day)
4%
(31 mEq/day)
756 mEq/day
(180 L/day x 4.2 mEq/L)
Figure 29–2
Renal tubular sites of potassium reabsorption and secretion. Potassium is reabsorbed in the proximal tubule and in the
ascending loop of Henle, so that only about
8 per cent of the filtered load is delivered to
the distal tubule. Secretion of potassium
into the late distal tubules and collecting
ducts adds to the amount delivered, so that
the daily excretion is about 12 per cent of
the potassium filtered at the glomerular
capillaries. The percentages indicate how
much of the filtered load is reabsorbed or
secreted into the different tubular segments.
27%
(204 mEq/day)
12%
(92 mEq/day)
368
Unit V
Renal
interstitial
fluid
Principal
cells
The Body Fluids and Kidneys
Tubular
lumen
Na+
Na+
Na+
K+
ATP
K+
K+
0 mV
-70 mV
-50 mV
Figure 29–3
Mechanisms of potassium secretion and sodium reabsorption by
the principal cells of the late distal and collecting tubules.
and make up about 90 per cent of the epithelial cells
in these regions. Figure 29–3 shows the basic cellular
mechanisms of potassium secretion by the principal
cells.
Secretion of potassium from the blood into the
tubular lumen is a two-step process, beginning with
uptake from the interstitium into the cell by the
sodium-potassium ATPase pump in the basolateral
membrane of the cell; this pump moves sodium out of
the cell into the interstitium and at the same time
moves potassium to the interior of the cell. The second
step of the process is passive diffusion of potassium
from the interior of the cell into the tubular fluid. The
sodium-potassium ATPase pump creates a high intracellular potassium concentration, which provides the
driving force for passive diffusion of potassium from
the cell into the tubular lumen. The luminal membrane
of the principal cells is highly permeable to potassium.
One reason for this high permeability is that there are
special channels that are specifically permeable to
potassium ions, thus allowing these ions to diffuse
across the membrane.
Control of Potassium Secretion by Principal Cells. The
primary factors that control potassium secretion by the
principal cells of the late distal and cortical collecting
tubules are (1) the activity of the sodium-potassium
ATPase pump, (2) the electrochemical gradient for
potassium secretion from the blood to the tubular
lumen, and (3) the permeability of the luminal membrane for potassium. These three determinants of
potassium secretion are in turn regulated by the
factors discussed later.
Intercalated Cells Can Reabsorb Potassium During Potassium
Depletion. In circumstances associated with severe
potassium depletion, there is a cessation of potassium
secretion and actually a net reabsorption of potassium in the late distal and collecting tubules. This
reabsorption occurs through the intercalated cells;
although this reabsorptive process is not completely
understood, one mechanism believed to contribute
is a hydrogen-potassium ATPase transport mechanism
located in the luminal membrane. This transporter reabsorbs potassium in exchange for hydrogen ions secreted
into the tubular lumen, and the potassium then diffuses
through the basolateral membrane of the cell into the
blood. This transporter is necessary to allow potassium
reabsorption during extracellular fluid potassium depletion, but under normal conditions it plays a small role
in controlling the excretion of potassium.
Summary of Factors That Regulate
Potassium Secretion: Plasma
Potassium Concentration,
Aldosterone, Tubular Flow Rate, and
Hydrogen Ion Concentration
Because normal regulation of potassium excretion
occurs mainly as a result of changes in potassium
secretion by the principal cells of the late distal and
collecting tubules, we will discuss the primary factors
that influence secretion by these cells. The most important factors that stimulate potassium secretion by the
principal cells include (1) increased extracellular fluid
potassium concentration, (2) increased aldosterone,
and (3) increased tubular flow rate.
One factor that decreases potassium secretion is
increased hydrogen ion concentration (acidosis).
Increased Extracellular Fluid Potassium Concentration Stimulates Potassium Secretion. The rate of potassium secre-
tion in the late distal and cortical collecting tubules is
directly stimulated by increased extracellular fluid
potassium concentration, leading to increases in potassium excretion, as shown in Figure 29–4. This effect
is especially pronounced when extracellular fluid
potassium concentration rises above about 4.1 mEq/L,
slightly less than the normal concentration. Increased
plasma potassium concentration, therefore, serves as
one of the most important mechanisms for increasing
potassium secretion and regulating extracellular fluid
potassium ion concentration.
There are three mechanisms by which increased
extracellular fluid potassium concentration raises
potassium secretion: (1) Increased extracellular fluid
potassium concentration stimulates the sodiumpotassium ATPase pump, thereby increasing potassium uptake across the basolateral membrane. This in
turn increases intracellular potassium ion concentration, causing potassium to diffuse across the luminal
membrane into the tubule. (2) Increased extracellular
potassium concentration increases the potassium gradient from the renal interstitial fluid to the interior of
the epithelial cell; this reduces backleakage of potassium ions from inside the cells through the basolateral
membrane. (3) Increased potassium concentration
stimulates aldosterone secretion by the adrenal cortex,
which further stimulates potassium secretion, as discussed next.
Urinary potassium excretion
(times normal)
4
369
Renal Regulation; Integration of Renal Mechanisms
Effect of aldosterone
Effect of extracellular
K+ concentration
3
2
1
Approximate plasma aldosterone
concentration (ng/100 ml plasma)
Chapter 29
70
60
50
40
30
20
10
0
0
0
3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
Serum potassium concentration (mEq/L)
1
2
3
4
5
Plasma aldosterone (times normal)
1
2
3
4
5
Extracellular potassium concentration
(mEq/L)
Figure 29–4
Figure 29–5
Effect of extracellular fluid potassium ion concentration on plasma
aldosterone concentration. Note that small changes in potassium
concentration cause large changes in aldosterone concentration.
Effect of plasma aldosterone concentration (red line) and extracellular potassium ion concentration (black line) on the rate of
urinary potassium excretion. These factors stimulate potassium
secretion by the principal cells of the cortical collecting tubules.
(Drawn from data in Young DB, Paulsen AW: Interrelated effects
of aldosterone and plasma potassium on potassium excretion. Am
J Physiol 244:F28, 1983.)
1
Ald.
K+
concentration
K+
4
Aldosterone Stimulates Potassium Secretion. In Chapter 27,
we discuss the fact that aldosterone stimulates active
reabsorption of sodium ions by the principal cells of
the late distal tubules and collecting ducts. This effect
is mediated through a sodium-potassium ATPase
pump that transports sodium outward through the
basolateral membrane of the cell and into the blood at
the same time that it pumps potassium into the cell.
Thus, aldosterone also has a powerful effect to control
the rate at which the principal cells secrete potassium.
A second effect of aldosterone is to increase the permeability of the luminal membrane for potassium,
further adding to the effectiveness of aldosterone in
stimulating potassium secretion. Therefore, aldosterone has a powerful effect to increase potassium
excretion, as shown in Figure 29–4.
Increased Extracellular Potassium Ion Concentration Stimulates Aldosterone Secretion. In negative feedback control
systems, the factor that is controlled usually has a
feedback effect on the controller. In the case of the
aldosterone-potassium control system, the rate of
aldosterone secretion by the adrenal gland is controlled strongly by extracellular fluid potassium ion
concentration. Figure 29–5 shows that an increase in
plasma potassium concentration of about 3 mEq/L can
increase plasma aldosterone concentration from
nearly 0 to as high as 60 ng/100 ml, a concentration
almost 10 times normal.
Aldosterone
concentration
K+ excretion
3
+
-
K+ intake
K+
excretion
Ald.
2
Figure 29–6
Basic feedback mechanism for control of extracellular fluid potassium concentration by aldosterone (Ald.).
The effect of potassium ion concentration to stimulate aldosterone secretion is part of a powerful feedback system for regulating potassium excretion, as
shown in Figure 29–6. In this feedback system, an
increase in plasma potassium concentration stimulates
aldosterone secretion and, therefore, increases the
blood level of aldosterone (block 1). The increase in
blood aldosterone then causes a marked increase in
potassium excretion by the kidneys (block 2). The
increased potassium excretion then reduces the extracellular fluid potassium concentration back toward
normal (blocks 3 and 4). Thus, this feedback mechanism acts synergistically with the direct effect of
370
Unit V
The Body Fluids and Kidneys
K+ intake
Plasma K+
concentration
Aldosterone
K+ secretion
Cortical collecting
tubules
K+ excretion
Plasma potassium concentration
(mEq/day)
4.8
4.6
4.4
Normal
4.2
Aldosterone
system
blocked
4.0
3.8
Figure 29–7
0
Primary mechanisms by which high potassium intake raises
potassium excretion. Note that increased plasma potassium concentration directly raises potassium secretion by the cortical collecting tubules and indirectly increases potassium secretion by
raising plasma aldosterone concentration.
30 60 90 120 150 180 210
Potassium intake (mEq/day)
Figure 29–8
increased extracellular potassium concentration to
elevate potassium excretion when potassium intake is
raised (Figure 29–7).
Effect of large changes in potassium intake on extracellular fluid
potassium concentration under normal conditions (red line) and
after the aldosterone feedback had been blocked (blue line). Note
that after blockade of the aldosterone system, regulation of potassium concentration was greatly impaired. (Courtesy Dr. David B.
Young.)
Blockade of Aldosterone Feedback System Greatly Impairs
Control of Potassium Concentration. In the absence of
aldosterone secretion, as occurs in patients with
Addison’s disease, renal secretion of potassium is
impaired, thus causing extracellular fluid potassium
concentration to rise to dangerously high levels. Conversely, with excess aldosterone secretion (primary
aldosteronism), potassium secretion becomes greatly
increased, causing potassium loss by the kidneys and
thus leading to hypokalemia.
The special quantitative importance of the aldosterone feedback system in controlling potassium concentration is shown in Figure 29–8. In this experiment,
potassium intake was increased almost sevenfold in
dogs under two conditions: (1) under normal conditions and (2) after the aldosterone feedback system
had been blocked by removing the adrenal glands and
placing the animals on a fixed rate of aldosterone infusion so that plasma aldosterone concentration could
neither increase nor decrease.
Note that in the normal animals, a sevenfold
increase in potassium intake caused only a slight increase in potassium concentration, from 4.2 to 4.3 mEq/
L. Thus, when the aldosterone feedback system is
functioning normally, potassium concentration is precisely controlled, despite large changes in potassium
intake.
When the aldosterone feedback system was
blocked, the same increases in potassium intake
caused a much larger increase in potassium concentration, from 3.8 to almost 4.7 mEq/L. Thus, control of
potassium concentration is greatly impaired when the
aldosterone feedback system is blocked. A similar
impairment of potassium regulation is observed in
humans with poorly functioning aldosterone feedback
systems, such as occurs in patients with either primary
aldosteronism (too much aldosterone) or Addison’s
disease (too little aldosterone).
Increased Distal Tubular Flow Rate Stimulates Potassium
Secretion. A rise in distal tubular flow rate, as occurs
with volume expansion, high sodium intake, or diuretic
drug treatment, stimulates potassium secretion. Conversely, a decrease in distal tubular flow rate, as caused
by sodium depletion, reduces potassium secretion.
The mechanism for the effect of high-volume flow
rate is as follows: When potassium is secreted into the
tubular fluid, the luminal concentration of potassium
increases, thereby reducing the driving force for potassium diffusion across the luminal membrane. With
increased tubular flow rate, the secreted potassium is
continuously flushed down the tubule, so that the rise
in tubular potassium concentration becomes minimized. Therefore, net potassium secretion is stimulated by increased tubular flow rate.
The effect of increased tubular flow rate is especially
important in helping to preserve normal potassium
excretion during changes in sodium intake. For
example, with a high sodium intake, there is decreased
aldosterone secretion, which by itself would tend to
decrease the rate of potassium secretion and, therefore, reduce urinary excretion of potassium. However,
the high distal tubular flow rate that occurs with a high
sodium intake tends to increase potassium secretion
Chapter 29
Renal Regulation; Integration of Renal Mechanisms
to a loss of potassium, whereas acute acidosis leads to
decreased potassium excretion.
Na+ intake
Aldosterone
Proximal
tubular Na+
reabsorption
GFR
Distal tubular
flow rate
-
371
K+ secretion
Cortical collecting
ducts
+
Unchanged K+
excretion
Figure 29–9
Effect of high sodium intake on renal excretion of potassium. Note
that a high-sodium diet decreases plasma aldosterone, which
tends to decrease potassium secretion by the cortical collecting
tubules. However, the high-sodium diet simultaneously increases
fluid delivery to the cortical collecting duct, which tends to
increase potassium secretion. The opposing effects of a highsodium diet counterbalance each other, so that there is little
change in potassium excretion.
(Figure 29–9), as discussed in the previous paragraph.
Therefore, the two effects of high sodium intake,
decreased aldosterone secretion and the high tubular
flow rate, counterbalance each other, so that there is
little change in potassium excretion. Likewise, with a
low sodium intake, there is little change in potassium
excretion because of the counterbalancing effects of
increased aldosterone secretion and decreased tubular
flow rate on potassium secretion.
Acute Acidosis Decreases Potassium Secretion. Acute
increases in hydrogen ion concentration of the extracellular fluid (acidosis) reduce potassium secretion,
whereas decreased hydrogen ion concentration (alkalosis) increases potassium secretion. The primary
mechanism by which increased hydrogen ion concentration inhibits potassium secretion is by reducing the
activity of the sodium-potassium ATPase pump. This
in turn decreases intracellular potassium concentration and subsequent passive diffusion of potassium
across the luminal membrane into the tubule.
With more prolonged acidosis, lasting over a period of
several days, there is an increase in urinary potassium
excretion. The mechanism for this effect is due in part
to an effect of chronic acidosis to inhibit proximal
tubular sodium chloride and water reabsorption, which
increases distal volume delivery, thereby stimulating
the secretion of potassium. This effect overrides
the inhibitory effect of hydrogen ions on the sodiumpotassium ATPase pump. Thus, chronic acidosis leads
Control of Renal Calcium
Excretion and Extracellular
Calcium Ion Concentration
The mechanisms for regulating calcium ion concentration are discussed in detail in Chapter 79, along with
the endocrinology of the calcium-regulating hormones
parathyroid hormone (PTH) and calcitonin. Therefore, calcium ion regulation is discussed only briefly in
this chapter.
Extracellular fluid calcium ion concentration
normally remains tightly controlled within a few
percentage points of its normal level, 2.4 mEq/L.
When calcium ion concentration falls to low levels
(hypocalcemia), the excitability of nerve and muscle
cells increases markedly and can in extreme cases
result in hypocalcemic tetany. This is characterized by
spastic skeletal muscle contractions. Hypercalcemia
(increased calcium concentration) depresses neuromuscular excitability and can lead to cardiac arrhythmias.
About 50 per cent of the total calcium in the plasma
(5 mEq/L) exists in the ionized form, which is the form
that has biological activity at cell membranes. The
remainder is either bound to the plasma proteins
(about 40 per cent) or complexed in the non-ionized
form with anions such as phosphate and citrate (about
10 per cent).
Changes in plasma hydrogen ion concentration can
influence the degree of calcium binding to plasma proteins. With acidosis, less calcium is bound to the plasma
proteins. Conversely, in alkalosis, a greater amount of
calcium is bound to the plasma proteins. Therefore,
patients with alkalosis are more susceptible to hypocalcemic tetany.
As with other substances in the body, the intake of
calcium must be balanced with the net loss of calcium
over the long term. Unlike ions such as sodium and
chloride, however, a large share of calcium excretion
occurs in the feces. The usual rate of dietary calcium
intake is about 1000 mg/day, with about 900 mg/day of
calcium excreted in the feces. Under certain conditions, fecal calcium excretion can exceed calcium
ingestion because calcium can also be secreted into
the intestinal lumen. Therefore, the gastrointestinal
tract and the regulatory mechanisms that influence
intestinal calcium absorption and secretion play a
major role in calcium homeostasis, as discussed in
Chapter 79.
Almost all the calcium in the body (99 per cent) is
stored in the bone, with only about 1 per cent in the
extracellular fluid and 0.1 per cent in the intracellular
fluid. The bone, therefore, acts as a large reservoir
for storing calcium and as a source of calcium when
extracellular fluid calcium concentration tends to
decrease.
One of the most important regulators of bone uptake
and release of calcium is PTH. When extracellular fluid
372
Unit V
The Body Fluids and Kidneys
[Ca++]
Vitamin D3
activation
PTH
Intestinal Ca++
reabsorption
Renal Ca++
reabsorption
Ca++ release
from bones
Figure 29–10
Compensatory responses to decreased plasma ionized calcium
concentration mediated by parathyroid hormone (PTH) and
vitamin D.
calcium concentration falls below normal, the parathyroid glands are directly stimulated by the low calcium
levels to promote increased secretion of PTH. This
hormone then acts directly on the bones to increase
the resorption of bone salts (release of salts from the
bones) and, therefore, to release large amounts of
calcium into the extracellular fluid, thereby returning
calcium levels back toward normal. When calcium ion
concentration is elevated, PTH secretion decreases, so
that almost no bone resorption now occurs; instead,
excess calcium is deposited in the bones because of
new bone formation. Thus, the day-to-day regulation
of calcium ion concentration is mediated in large part
by the effect of PTH on bone resorption.
The bones, however, do not have an inexhaustible
supply of calcium. Therefore, over the long term, the
intake of calcium must be balanced with calcium
excretion by the gastrointestinal tract and the kidneys.
The most important regulator of calcium reabsorption
at both of these sites is PTH. Thus, PTH regulates
plasma calcium concentration through three main
effects: (1) by stimulating bone resorption; (2) by stimulating activation of vitamin D, which then increases
intestinal reabsorption of calcium; and (3) by directly
increasing renal tubular calcium reabsorption (Figure
29–10). The control of gastrointestinal calcium reabsorption and calcium exchange in the bones is discussed elsewhere, and the remainder of this section
focuses on the mechanisms that control renal calcium
excretion.
Control of Calcium Excretion
by the Kidneys
Because calcium is both filtered and reabsorbed in the
kidneys but not secreted, the rate of renal calcium
excretion is calculated as
Renal calcium excretion = Calcium filtered
- Calcium reabsorbed
Only about 50 per cent of the plasma calcium is
ionized, with the remainder being bound to the plasma
proteins or complexed with anions such as phosphate.
Therefore, only about 50 per cent of the plasma
calcium can be filtered at the glomerulus. Normally,
about 99 per cent of the filtered calcium is reabsorbed
by the tubules, with only about 1 per cent of the filtered calcium being excreted. About 65 per cent of the
filtered calcium is reabsorbed in the proximal tubule,
25 to 30 per cent is reabsorbed in the loop of Henle,
and 4 to 9 per cent is reabsorbed in the distal and collecting tubules. This pattern of reabsorption is similar
to that for sodium.
As is true with the other ions, calcium excretion is
adjusted to meet the body’s needs. With an increase in
calcium intake, there is also increased renal calcium
excretion, although much of the increase of calcium
intake is eliminated in the feces. With calcium depletion, calcium excretion by the kidneys decreases as a
result of enhanced tubular reabsorption.
One of the primary controllers of renal tubular
calcium reabsorption is PTH. With increased levels of
PTH, there is increased calcium reabsorption in the
thick ascending loops of Henle and distal tubules,
which reduces urinary excretion of calcium. Conversely, reduction of PTH promotes calcium excretion
by decreasing reabsorption in the loops of Henle and
distal tubules.
In the proximal tubule, calcium reabsorption usually
parallels sodium and water reabsorption. Therefore,
in instances of extracellular volume expansion or
increased arterial pressure—both of which decrease
proximal sodium and water reabsorption—there is
also reduction in calcium reabsorption and, consequently, increased urinary excretion of calcium.
Conversely, with extracellular volume contraction or
decreased blood pressure, calcium excretion decreases
primarily because of increased proximal tubular
reabsorption.
Another factor that influences calcium reabsorption
is the plasma concentration of phosphate. An increase
in plasma phosphate stimulates PTH, which increases
calcium reabsorption by the renal tubules, thereby
reducing calcium excretion. The opposite occurs with
reduction in plasma phosphate concentration.
Calcium reabsorption is also stimulated by metabolic acidosis and inhibited by metabolic alkalosis.
Most of the effect of hydrogen ion concentration on
calcium excretion results from changes in calcium
reabsorption in the distal tubule.
A summary of the factors that are known to influence calcium excretion by the renal tubules is shown
in Table 29–2.
Regulation of Renal Phosphate
Excretion
Phosphate excretion by the kidneys is controlled
primarily by an overflow mechanism that can be
explained as follows: The renal tubules have a normal
transport maximum for reabsorbing phosphate of
about 0.1 mM/min. When less than this amount
of phosphate is present in the glomerular filtrate,
Chapter 29
Renal Regulation; Integration of Renal Mechanisms
Table 29–2
Factors That Alter Renal Calcium Excretion
Ø Calcium Excretion
≠ Calcium Excretion
≠ Parathyroid hormone (PTH)
Ø Extracellular fluid volume
Ø Blood pressure
≠ Plasma phosphate
Metabolic acidosis
Vitamin D3
Ø PTH
≠ Extracellular fluid volume
≠ Blood pressure
Ø Plasma phosphate
Metabolic alkalosis
essentially all the filtered phosphate is reabsorbed.
When more than this is present, the excess is excreted.
Therefore, phosphate normally begins to spill into the
urine when its concentration in the extracellular fluid
rises above a threshold of about 0.8 mM/L, which gives
a tubular load of phosphate of about 0.1 mM/min,
assuming a GFR of 125 ml/min. Because most people
ingest large quantities of phosphate in milk products
and meat, the concentration of phosphate is usually
maintained above 1 mM/L, a level at which there is
continual excretion of phosphate into the urine.
Changes in tubular phosphate reabsorption can also
influence phosphate excretion. For instance, a diet low
in phosphate can, over time, increase the reabsorptive
transport maximum for phosphate, thereby reducing
the tendency for phosphate to spill over into the urine.
PTH can play a significant role in regulating phosphate concentration through two effects: (1) PTH
promotes bone resorption, thereby dumping large
amounts of phosphate ions into the extracellular fluid
from the bone salts, and (2) PTH decreases the transport maximum for phosphate by the renal tubules,
so that a greater proportion of the tubular phosphate
is lost in the urine. Thus, whenever plasma PTH is
increased, tubular phosphate reabsorption is decreased
and more phosphate is excreted. These interrelations
among phosphate, PTH, and calcium are discussed in
more detail in Chapter 79.
Control of Renal Magnesium
Excretion and Extracellular
Magnesium Ion Concentration
More than one half of the body’s magnesium is stored
in the bones. Most of the rest resides within the cells,
with less than 1 per cent located in the extracellular
fluid. Although the total plasma magnesium concentration is about 1.8 mEq/L, more than one half of this
is bound to plasma proteins. Therefore, the free
ionized concentration of magnesium is only about
0.8 mEq/L.
The normal daily intake of magnesium is about 250
to 300 mg/day, but only about one half of this intake is
absorbed by the gastrointestinal tract. To maintain
magnesium balance, the kidneys must excrete this
absorbed magnesium, about one half the daily intake
of magnesium, or 125 to 150 mg/day. The kidneys
373
normally excrete about 10 to 15 per cent of the magnesium in the glomerular filtrate.
Renal excretion of magnesium can increase
markedly during magnesium excess or can decrease to
almost nil during magnesium depletion. Because magnesium is involved in many biochemical processes in
the body, including activation of many enzymes, its
concentration must be closely regulated.
Regulation of magnesium excretion is achieved
mainly by changing tubular reabsorption. The proximal tubule usually reabsorbs only about 25 per cent of
the filtered magnesium. The primary site of reabsorption is the loop of Henle, where about 65 per cent of
the filtered load of magnesium is reabsorbed. Only a
small amount (usually less than 5 per cent) of the
filtered magnesium is reabsorbed in the distal and
collecting tubules.
The mechanisms that regulate magnesium excretion
are not well understood, but the following disturbances lead to increased magnesium excretion: (1)
increased extracellular fluid magnesium concentration, (2) extracellular volume expansion, and (3)
increased extracellular fluid calcium concentration.
Integration of Renal
Mechanisms for Control
of Extracellular Fluid
Extracellular fluid volume is determined mainly by the
balance between intake and output of water and salt.
In most cases, salt and fluid intakes are dictated by a
person’s habits rather than by physiologic control
mechanisms. Therefore, the burden of extracellular
volume regulation is usually placed on the kidneys,
which must adapt their excretion of salt and water to
match intake of salt and water under steady-state
conditions.
In discussing the regulation of extracellular fluid
volume, we also consider the factors that regulate the
amount of sodium chloride in the extracellular fluid,
because changes in extracellular fluid sodium chloride
content usually cause parallel changes in extracellular
fluid volume, provided the antidiuretic hormone
(ADH)-thirst mechanisms are also operative. When
the ADH-thirst mechanisms are functioning normally,
a change in the amount of sodium chloride in the
extracellular fluid is matched by a similar change in
the amount of extracellular water, so that osmolality
and sodium concentration are maintained relatively
constant.
Sodium Excretion Is Precisely
Matched to Intake Under Steady-State
Conditions
An important consideration in overall control of
sodium excretion—or excretion of any electrolyte, for
that matter—is that under steady-state conditions,
excretion by the kidneys is determined by intake. To
374
Unit V
The Body Fluids and Kidneys
maintain life, a person must, over the long term,
excrete almost precisely the amount of sodium
ingested. Therefore, even with disturbances that cause
major changes in kidney function, balance between
intake and output of sodium usually is restored within
a few days.
If disturbances of kidney function are not too
severe, sodium balance may be achieved mainly by
intrarenal adjustments with minimal changes in extracellular fluid volume or other systemic adjustments.
But when perturbations to the kidneys are severe and
intrarenal compensations are exhausted, systemic
adjustments must be invoked, such as changes in blood
pressure, changes in circulating hormones, and alterations of sympathetic nervous system activity. These
adjustments are costly in terms of overall homeostasis
because they cause other changes throughout the
body that may, in the long run, be damaging. These
compensations, however, are necessary because a
sustained imbalance between fluid and electrolyte
intake and excretion would quickly lead to accumulation or loss of electrolytes and fluid, causing cardiovascular collapse within a few days. Thus, one can view
the systemic adjustments that occur in response to
abnormalities of kidney function as a necessary tradeoff that brings electrolyte and fluid excretion back in
balance with intake.
Sodium Excretion Is Controlled by
Altering Glomerular Filtration or
Tubular Sodium Reabsorption Rates
The two variables that influence sodium and water
excretion are the rates of filtration and the rates of
reabsorption:
Excretion = Glomerular filtration - Tubular
reabsorption
GFR normally is about 180 L/day, tubular reabsorption is 178.5 L/day, and urine excretion is 1.5
L/day. Thus, small changes in GFR or tubular reabsorption potentially can cause large changes in renal
excretion. For example, a 5 per cent increase in GFR
(to 189 L/day) would cause a 9 L/day increase in urine
volume, if tubular compensations did not occur; this
would quickly cause catastrophic changes in body fluid
volumes. Similarly, small changes in tubular reabsorption, in the absence of compensatory adjustments of
GFR, would also lead to dramatic changes in urine
volume and sodium excretion. Tubular reabsorption
and GFR usually are regulated precisely, so that excretion by the kidneys can be exactly matched to intake
of water and electrolytes.
Even with disturbances that alter GFR or tubular
reabsorption, changes in urinary excretion are minimized by various buffering mechanisms. For example,
if the kidneys become greatly vasodilated and GFR
increases (as can occur with certain drugs or high
fever), this raises sodium chloride delivery to the
tubules, which in turn leads to at least two intrarenal
compensations: (1) increased tubular reabsorption of
much of the extra sodium chloride filtered, called
glomerulotubular balance, and (2) macula densa feedback, in which increased sodium chloride delivery to
the distal tubule causes afferent arteriolar constriction
and return of GFR toward normal. Likewise, abnormalities of tubular reabsorption in the proximal tubule
or loop of Henle are partially compensated for by
these same intrarenal feedbacks.
Because neither of these two mechanisms operates
perfectly to restore distal sodium chloride delivery all
the way back to normal, changes in either GFR or
tubular reabsorption can lead to significant changes in
urine sodium and water excretion. When this happens,
other feedback mechanisms come into play, such as
changes in blood pressure and changes in various
hormones, that eventually return sodium excretion to
equal sodium intake. In the next few sections, we
review how these mechanisms operate together to
control sodium and water balance and in so doing act
also to control extracellular fluid volume. We should
keep in mind, however, that all these feedback mechanisms control renal excretion of sodium and water by
altering either GFR or tubular reabsorption.
Importance of Pressure
Natriuresis and Pressure
Diuresis in Maintaining Body
Sodium and Fluid Balance
One of the most basic and powerful mechanisms for
control of blood volume and extracellular fluid
volume, as well as for the maintenance of sodium and
fluid balance, is the effect of blood pressure on sodium
and water excretion—called the pressure natriuresis
and pressure diuresis mechanisms, respectively. As
discussed in Chapter 19, this feedback between the
kidneys and the circulatory system also plays a dominant role in long-term blood pressure regulation.
Pressure diuresis refers to the effect of increased
blood pressure to raise urinary volume excretion,
whereas pressure natriuresis refers to the rise in
sodium excretion that occurs with elevated blood pressure. Because pressure diuresis and natriuresis usually
occur in parallel, we refer to these mechanisms simply
as “pressure natriuresis” in the following discussion.
Figure 29–11 shows the effect of arterial pressure on
urinary sodium output. Note that acute increases in
blood pressure of 30 to 50 mm Hg cause a twofold to
threefold increase in urinary sodium output. This
effect is independent of changes in activity of the sympathetic nervous system or of various hormones, such
as angiotensin II, ADH, or aldosterone, because pressure natriuresis can be demonstrated in an isolated
kidney that has been removed from the influence of
these factors. With chronic increases in blood pressure,
the effectiveness of pressure natriuresis is greatly
enhanced because the increased blood pressure also,
after a short time delay, suppresses renin release and,
therefore, decreases formation of angiotensin II and
Chapter 29
aldosterone. As discussed previously, decreased levels
of angiotensin II and aldosterone inhibit renal tubular
reabsorption of sodium, thereby amplifying the direct
effects of increased blood pressure to raise sodium and
water excretion.
Pressure Natriuresis and Diuresis Are
Key Components of a Renal-Body
Fluid Feedback for Regulating Body
Fluid Volumes and Arterial Pressure
Urinary sodium or volume
output (times normal)
The effect of increased blood pressure to raise urine
output is part of a powerful feedback system that oper-
8
Chronic
Acute
6
4
2
0
0
375
Renal Regulation; Integration of Renal Mechanisms
20 40 60 80 100 120 140 160 180 200
Arterial pressure (mm Hg)
Figure 29–11
Acute and chronic effects of arterial pressure on sodium output
by the kidneys (pressure natriuresis). Note that chronic increases
in arterial pressure cause much greater increases in sodium
output than those measured during acute increases in arterial
pressure.
ates to maintain balance between fluid intake and
output, as shown in Figure 29–12. This is the same
mechanism that is discussed in Chapter 19 for arterial
pressure control. The extracellular fluid volume, blood
volume, cardiac output, arterial pressure, and urine
output are all controlled at the same time as separate
parts of this basic feedback mechanism.
During changes in sodium and fluid intake, this feedback mechanism helps to maintain fluid balance and
to minimize changes in blood volume, extracellular
fluid volume, and arterial pressure as follows:
1. An increase in fluid intake (assuming that sodium
accompanies the fluid intake) above the level of
urine output causes a temporary accumulation of
fluid in the body.
2. As long as fluid intake exceeds urine output, fluid
accumulates in the blood and interstitial spaces,
causing parallel increases in blood volume and
extracellular fluid volume. As discussed later, the
actual increases in these variables are usually
small because of the effectiveness of this
feedback.
3. An increase in blood volume raises mean
circulatory filling pressure.
4. An increase in mean circulatory filling pressure
raises the pressure gradient for venous return.
5. An increased pressure gradient for venous return
elevates cardiac output.
6. An increased cardiac output raises arterial
pressure.
7. An increased arterial pressure increases urine
output by way of pressure diuresis. The steepness
of the normal pressure natriuresis relation
indicates that only a slight increase in blood
pressure is required to raise urinary excretion
severalfold.
8. The increased fluid excretion balances the
increased intake, and further accumulation of fluid
is prevented.
Nonrenal
fluid loss
Total peripheral
resistance
Rate of change of
extracellular
fluid volume
Renal fluid
excretion
Arterial
pressure
Fluid
intake
Arterial pressure
Extracellular
fluid volume
Blood
volume
Figure 29–12
Basic renal–body fluid feedback
mechanism
for
control
of
blood volume, extracellular fluid
volume, and arterial pressure.
Solid lines indicate positive
effects, and dashed lines indicate
negative effects.
Cardiac
output
Heart strength
Venous
return
Mean circulatory
filling pressure
Vascular
capacity
376
Unit V
The Body Fluids and Kidneys
6
Blood volume (liters)
Blood volume
5
Normal range
4
Death
3
2
1
0
0
1
2
3
4
5
6
7
Daily fluid intake
(water and electrolytes) (L/day)
8
Figure 29–13
Approximate effect of changes in daily fluid intake on blood
volume. Note that blood volume remains relatively constant in the
normal range of daily fluid intakes.
Thus, the renal-body fluid feedback mechanism
operates to prevent continuous accumulation of salt
and water in the body during increased salt and water
intake. As long as kidney function is normal and the
pressure diuresis mechanism is operating effectively,
large changes in salt and water intake can be accommodated with only slight changes in blood volume,
extracellular fluid volume, cardiac output, and arterial
pressure.
The opposite sequence of events occurs when fluid
intake falls below normal. In this case, there is a tendency toward decreased blood volume and extracellular fluid volume, as well as reduced arterial pressure.
Even a small decrease in blood pressure causes a
large decrease in urine output, thereby allowing fluid
balance to be maintained with minimal changes in
blood pressure, blood volume, or extracellular fluid
volume. The effectiveness of this mechanism in preventing large changes in blood volume is demonstrated in Figure 29–13, which shows that changes in
blood volume are almost imperceptible despite large
variations in daily intake of water and electrolytes,
except when intake becomes so low that it is not sufficient to make up for fluid losses caused by evaporation or other inescapable losses.
Precision of Blood Volume and
Extracellular Fluid Volume Regulation
By studying Figure 29–12, one can see why the blood
volume remains almost exactly constant despite
extreme changes in daily fluid intake. The reason for
this is the following: (1) a slight change in blood
volume causes a marked change in cardiac output, (2)
a slight change in cardiac output causes a large change
in blood pressure, and (3) a slight change in blood
pressure causes a large change in urine output. These
factors work together to provide effective feedback
control of blood volume.
The same control mechanisms operate whenever
there is a loss of whole blood because of hemorrhage.
In this case, fluid is retained by the kidneys, and other
parallel processes occur to reconstitute the red blood
cells and plasma proteins in the blood. If abnormalities of red blood cell volume remain, such as occurs
when there is deficiency of erythropoietin or other
factors needed to stimulate red blood cell production,
the plasma volume will simply make up the difference,
and the overall blood volume will return essentially to
normal despite the low red blood cell mass.
Distribution of Extracellular
Fluid Between the Interstitial
Spaces and Vascular System
From Figure 29–12 it is apparent that blood volume
and extracellular fluid volume are usually controlled
in parallel with each other. Ingested fluid initially goes
into the blood, but it rapidly becomes distributed
between the interstitial spaces and the plasma. Therefore, blood volume and extracellular fluid volume
usually are controlled simultaneously.
There are circumstances, however, in which the distribution of extracellular fluid between the interstitial
spaces and blood can vary greatly. As discussed in
Chapter 25, the principal factors that can cause accumulation of fluid in the interstitial spaces include (1)
increased capillary hydrostatic pressure, (2) decreased
plasma colloid osmotic pressure, (3) increased permeability of the capillaries, and (4) obstruction of lymphatic vessels. In all these conditions, an unusually high
proportion of the extracellular fluid becomes distributed to the interstitial spaces.
Figure 29–14 shows the normal distribution of fluid
between the interstitial spaces and the vascular system
and the distribution that occurs in edema states. When
small amounts of fluid accumulate in the blood as a
result of either too much fluid intake or a decrease in
renal output of fluid, about 20 to 30 per cent of it stays
in the blood and increases the blood volume. The
remainder is distributed to the interstitial spaces.
When the extracellular fluid volume rises more than
30 to 50 per cent above normal, almost all the additional fluid goes into the interstitial spaces and little
remains in the blood. This occurs because once the
interstitial fluid pressure rises from its normally negative value to become positive, the tissue interstitial
spaces become compliant, and large amounts of fluid
then pour into the tissues without interstitial fluid
pressure rising much more. In other words, the safety
factor against edema, owing to a rising interstitial
fluid pressure that counteracts fluid accumulation in
the tissues, is lost once the tissues become highly
compliant.
Thus, under normal conditions, the interstitial
spaces act as an “overflow” reservoir for excess fluid,
Chapter 29
8
Renal Regulation; Integration of Renal Mechanisms
Sympathetic Nervous System
Control of Renal Excretion: Arterial
Baroreceptor and Low-Pressure
Stretch Receptor Reflexes
Edema
Blood volume (liters)
7
6
5
Normal value
4
3
Death
2
1
0
0
5
10 15 20 25 30 35
Extracellular fluid volume (liters)
377
40
Figure 29–14
Approximate relation between extracellular fluid volume and blood
volume, showing a nearly linear relation in the normal range but
also showing the failure of blood volume to continue rising when
the extracellular fluid volume becomes excessive. When this
occurs, the additional extracellular fluid volume resides in the
interstitial spaces, and edema results.
sometimes increasing in volume 10 to 30 liters. This
causes edema, as explained in Chapter 25, but it also
acts as an important overflow release valve for the circulation, protecting the cardiovascular system against
dangerous overload that could lead to pulmonary
edema and cardiac failure.
To summarize, extracellular fluid volume and blood
volume are controlled simultaneously, but the quantitative amounts of fluid distribution between the
interstitium and the blood depend on the physical
properties of the circulation and the interstitial spaces
as well as on the dynamics of fluid exchange through
the capillary membranes.
Nervous and Hormonal
Factors Increase the
Effectiveness of Renal-Body
Fluid Feedback Control
In Chapter 27, we discuss the nervous and hormonal
factors that influence GFR and tubular reabsorption
and, therefore, renal excretion of salt and water. These
nervous and hormonal mechanisms usually act in
concert with the pressure natriuresis and pressure
diuresis mechanisms, making them more effective in
minimizing the changes in blood volume, extracellular
fluid volume, and arterial pressure that occur in
response to day-to-day challenges. However, abnormalities of kidney function or of the various nervous
and hormonal factors that influence the kidneys can
lead to serious changes in blood pressure and body
fluid volumes, as discussed later.
Because the kidneys receive extensive sympathetic
innervation, changes in sympathetic activity can alter
renal sodium and water excretion as well as regulation
of extracellular fluid volume under some conditions.
For example, when blood volume is reduced by hemorrhage, the pressures in the pulmonary blood vessels
and other low-pressure regions of the thorax decrease,
causing reflex activation of the sympathetic nervous
system. This in turn increases renal sympathetic nerve
activity, which has several effects to decrease sodium
and water excretion: (1) constriction of the renal arterioles, with resultant decreased GFR; (2) increased
tubular reabsorption of salt and water; and (3) stimulation of renin release and increased angiotensin II
and aldosterone formation, both of which further
increase tubular reabsorption. And if the reduction in
blood volume is great enough to lower systemic arterial pressure, further activation of the sympathetic
nervous system occurs because of decreased stretch of
the arterial baroreceptors located in the carotid sinus
and aortic arch. All these reflexes together play an
important role in the rapid restitution of blood volume
that occurs in acute conditions such as hemorrhage.
Also, reflex inhibition of renal sympathetic activity
may contribute to the rapid elimination of excess fluid
in the circulation that occurs after eating a meal that
contains large amounts of salt and water.
Role of Angiotensin II In Controlling
Renal Excretion
One of the body’s most powerful controllers of sodium
excretion is angiotensin II. Changes in sodium and
fluid intake are associated with reciprocal changes in
angiotensin II formation, and this in turn contributes
greatly to the maintenance of body sodium and fluid
balances. That is, when sodium intake is elevated
above normal, renin secretion is decreased, causing
decreased angiotensin II formation. Because
angiotensin II has several important effects in increasing tubular reabsorption of sodium, as explained in
Chapter 27, a reduced level of angiotensin II decreases
tubular reabsorption of sodium and water, thus
increasing the kidneys’ excretion of sodium and water.
The net result is to minimize the rise in extracellular
fluid volume and arterial pressure that would otherwise occur when sodium intake increases.
Conversely, when sodium intake is reduced below
normal, increased levels of angiotensin II cause
sodium and water retention and oppose reductions in
arterial blood pressure that would otherwise occur.
Thus, changes in activity of the renin-angiotensin
system act as a powerful amplifier of the pressure
natriuresis mechanism for maintaining stable blood
pressures and body fluid volumes.
378
Unit V
Angiotensin blockade
12
Sodium intake and output
(times normal)
The Body Fluids and Kidneys
Normal
10
High angiotensin II
8
6
Figure 29–15
4
2
0
60
80
100
120
Arterial pressure (mm Hg)
Importance of Angiotensin II in Increasing Effectiveness of
Pressure Natriuresis. The importance of angiotensin II
in making the pressure natriuresis mechanism more
effective is shown in Figure 29–15. Note that when the
angiotensin control of natriuresis is fully functional,
the pressure natriuresis curve is steep (normal curve),
indicating that only minor changes in blood pressure
are needed to increase sodium excretion when sodium
intake is raised.
In contrast, when angiotensin levels cannot be
decreased in response to increased sodium intake
(high angiotensin II curve), as occurs in some hypertensive patients who have impaired ability to decrease
renin secretion, the pressure natriuresis curve is not
nearly as steep. Therefore, when sodium intake is
raised, much greater increases in arterial pressure are
needed to increase sodium excretion and maintain
sodium balance. For example, in most people, a 10-fold
increase in sodium intake causes an increase of only a
few millimeters of mercury in arterial pressure,
whereas in subjects who cannot suppress angiotensin
II formation appropriately in response to excess
sodium, the same rise in sodium intake causes blood
pressure to rise as much as 50 mm Hg. Thus, the inability to suppress angiotensin II formation when there is
excess sodium reduces the slope of pressure natriuresis and makes arterial pressure very salt sensitive, as
discussed in Chapter 19.
The use of drugs to block the effects of angiotensin
II has proved to be important clinically for improving the kidneys’ ability to excrete salt and water.
When angiotensin II formation is blocked with an
angiotensin-converting enzyme inhibitor (see Figure
29–15) or an angiotensin II receptor antagonist, the
renal–pressure natriuresis curve is shifted to lower
pressures; this indicates an enhanced ability of the
kidneys to excrete sodium because normal levels of
sodium excretion can now be maintained at reduced
arterial pressures. This shift of pressure natriuresis
provides the basis for the chronic blood pressure–
140
160
Effect of excessive angiotensin II
formation and effect of blocking
angiotensin II formation on the
renal–pressure natriuresis curve. Note
that high levels of angiotensin II formation decrease the slope of pressure
natriuresis, making blood pressure
very sensitive to changes in sodium
intake. Blockade of angiotensin II formation shifts pressure natriuresis to
lower blood pressures.
lowering effects in hypertensive patients of the
angiotensin-converting enzyme inhibitors and
angiotensin II receptor antagonists.
Excessive Angiotensin II Does Not Cause Large Increases in
Extracellular Fluid Volume Because Increased Arterial Pressure
Counterbalances Angiotensin-Mediated Sodium Retention.
Although angiotensin II is one of the most powerful
sodium- and water-retaining hormones in the body,
neither a decrease nor an increase in circulating
angiotensin II has a large effect on extracellular fluid
volume or blood volume. The reason for this is
that with large increases in angiotensin II levels, as
occurs with a renin-secreting tumor of the kidney,
the high angiotensin II levels initially cause sodium
and water retention by the kidneys and a small
increase in extracellular fluid volume. This also initiates a rise in arterial pressure that quickly increases
kidney output of sodium and water, thereby overcoming the sodium- and water-retaining effects of the
angiotensin II and re-establishing a balance between
intake and output of sodium at a higher blood pressure. Conversely, after blockade of angiotensin II formation, as occurs when an angiotensin-converting
enzyme inhibitor is administered, there is initial loss of
sodium and water, but the fall in blood pressure offsets
this effect, and sodium excretion is once again restored
to normal.
Role of Aldosterone in Controlling
Renal Excretion
Aldosterone increases sodium reabsorption, especially
in the cortical collecting tubules. The increased sodium
reabsorption is also associated with increased water
reabsorption and potassium secretion. Therefore, the
net effect of aldosterone is to make the kidneys retain
sodium and water but to increase potassium excretion
in the urine.
Chapter 29
Renal Regulation; Integration of Renal Mechanisms
The function of aldosterone in regulating sodium
balance is closely related to that described for
angiotensin II.That is, with reduction in sodium intake,
the increased angiotensin II levels that occur stimulate
aldosterone secretion, which in turn contributes to the
reduction in urinary sodium excretion and, therefore,
to the maintenance of sodium balance. Conversely,
with high sodium intake, suppression of aldosterone
formation decreases tubular reabsorption, allowing
the kidneys to excrete larger amounts of sodium. Thus,
changes in aldosterone formation also aid the pressure
natriuresis mechanism in maintaining sodium balance
during variations in salt intake.
During Chronic Oversecretion of Aldosterone, Kidneys
“Escape” from Sodium Retention as Arterial Pressure Rises.
Although aldosterone has powerful effects on sodium
reabsorption, when there is excessive infusion of
aldosterone or excessive formation of aldosterone, as
occurs in patients with tumors of the adrenal gland
(Conn’s syndrome), the increased sodium reabsorption and decreased sodium excretion by the kidneys
are transient. After 1 to 3 days of sodium and water
retention, the extracellular fluid volume rises by about
10 to 15 per cent and there is a simultaneous increase
in arterial blood pressure. When the arterial pressure
rises sufficiently, the kidneys “escape” from the sodium
and water retention and thereafter excrete amounts of
sodium equal to the daily intake, despite continued
presence of high levels of aldosterone. The primary
reason for the escape is the pressure natriuresis and
diuresis that occur when the arterial pressure rises.
In patients with adrenal insufficiency who do not
secrete enough aldosterone (Addison’s disease), there
is increased excretion of sodium and water, reduction
in extracellular fluid volume, and a tendency toward
low blood pressure. In the complete absence of aldosterone, the volume depletion may be severe unless the
person is allowed to eat large amounts of salt and
drink large amounts of water to balance the increased
urine output of salt and water.
Role of ADH in Controlling Renal
Water Excretion
As discussed in Chapter 28, ADH plays an important
role in allowing the kidneys to form a small volume of
concentrated urine while excreting normal amounts of
salt. This effect is especially important during water
deprivation, which strongly elevates plasma levels of
ADH that in turn increase water reabsorption by the
kidneys and help to minimize the decreases in extracellular fluid volume and arterial pressure that would
otherwise occur. Water deprivation for 24 to 48 hours
normally causes only a small decrease in extracellular
fluid volume and arterial pressure. However, if the
effects of ADH are blocked with a drug that antagonizes the action of ADH to promote water reabsorption in the distal and collecting tubules, the same
period of water deprivation causes a substantial fall in
both extracellular fluid volume and arterial pressure.
379
Conversely, when there is excess extracellular volume,
decreased ADH levels reduce reabsorption of water by
the kidneys, thus helping to rid the body of the excess
volume.
Excess ADH Secretion Usually Causes Only Small Increases in
Extracellular Fluid Volume but Large Decreases in Sodium Concentration. Although ADH is important in regulating
extracellular fluid volume, excessive levels of ADH
seldom cause large increases in arterial pressure or
extracellular fluid volume. Infusion of large amounts
of ADH into animals initially causes renal retention of
water and a 10 to 15 per cent increase in extracellular
fluid volume. As the arterial pressure rises in response
to this increased volume, much of the excess volume
is excreted because of the pressure diuresis mechanism. After several days of ADH infusion, the blood
volume and extracellular fluid volume are elevated no
more than 5 to 10 per cent, and the arterial pressure is
also elevated by less than 10 mm Hg. The same is true
for patients with inappropriate ADH syndrome, in
which ADH levels may be elevated severalfold.
Thus, high levels of ADH do not cause major
increases of either body fluid volume or arterial pressure, although high ADH levels can cause severe reductions in extracellular sodium ion concentration. The
reason for this is that increased water reabsorption by
the kidneys dilutes the extracellular sodium, and at the
same time, the small increase in blood pressure that
does occur causes loss of sodium from the extracellular fluid in the urine through pressure natriuresis.
In patients who have lost their ability to secrete
ADH because of destruction of the supraoptic nuclei,
the urine volume may become 5 to 10 times normal.
This is almost always compensated for by ingestion of
enough water to maintain fluid balance. If free access
to water is prevented, the inability to secrete ADH
may lead to marked reductions in blood volume and
arterial pressure.
Role of Atrial Natriuretic Peptide
in Controlling Renal Excretion
Thus far, we have discussed mainly the role of sodiumand water-retaining hormones in controlling extracellular fluid volume. However, several different natriuretic hormones may also contribute to volume
regulation. One of the most important of the natriuretic hormones is a peptide referred to as atrial natriuretic peptide (ANP), released by the cardiac atrial
muscle fibers. The stimulus for release of this peptide
appears to be overstretch of the atria, which can result
from excess blood volume. Once released by the
cardiac atria, ANP enters the circulation and acts on
the kidneys to cause small increases in GFR and
decreases in sodium reabsorption by the collecting
ducts. These combined actions of ANP lead to
increased excretion of salt and water, which helps to
compensate for the excess blood volume.
Changes in ANP levels probably help to minimize
changes in blood volume during various disturbances,
380
Unit V
The Body Fluids and Kidneys
such as increased salt and water intake. However,
excessive production of ANP or even complete lack of
ANP does not cause major changes in blood volume
because these effects can easily be overcome by small
changes in blood pressure, acting through pressure
natriuresis. For example, infusions of large amounts of
ANP initially raise urine output of salt and water and
cause slight decreases in blood volume. In less than 24
hours, this effect is overcome by a slight decrease in
blood pressure that returns urine output toward
normal, despite continued excess of ANP.
Integrated Responses to
Changes in Sodium Intake
The integration of the different control systems that
regulate sodium and fluid excretion under normal
conditions can be summarized by examining the
homeostatic responses to progressive increases in
dietary sodium intake. As discussed previously, the
kidneys have an amazing capability to match their
excretion of salt and water to intakes that can range
from as low as one tenth of normal to as high as 10
times normal.
High Sodium Intake Suppresses Antinatriuretic Systems and
Activates Natriuretic Systems. As sodium intake is
increased, sodium output initially lags slightly behind
intake. The time delay results in a small increase in the
cumulative sodium balance, which causes a slight
increase in extracellular fluid volume. It is mainly this
small increase in extracellular fluid volume that triggers various mechanisms in the body to increase
sodium excretion. These mechanisms include the
following:
1. Activation of low pressure receptor reflexes that
originate from the stretch receptors of the right
atrium and the pulmonary blood vessels. Signals
from the stretch receptors go to the brain stem
and there inhibit sympathetic nerve activity to the
kidneys to decrease tubular sodium reabsorption.
This mechanism is most important in the first few
hours—or perhaps the first day—after a large
increase in salt and water intake.
2. Small increases in arterial pressure, caused by
volume expansion, raise sodium excretion through
pressure natriuresis.
3. Suppression of angiotensin II formation, caused by
increased arterial pressure and extracellular fluid
volume expansion, decreases tubular sodium
reabsorption by eliminating the normal effect of
angiotensin II to increase sodium reabsorption.
Also, reduced angiotensin II decreases
aldosterone secretion, thus further reducing
tubular sodium reabsorption.
4. Stimulation of natriuretic systems, especially ANP,
contributes further to increased sodium excretion.
Thus, the combined activation of natriuretic systems
and suppression of sodium- and water-retaining
systems leads to an increase in sodium excretion when
sodium intake is increased. The opposite changes take
place when sodium intake is reduced below normal
levels.
Conditions That Cause
Large Increases in Blood
Volume and Extracellular
Fluid Volume
Despite the powerful regulatory mechanisms that maintain blood volume and extracellular fluid volume reasonably constant, there are abnormal conditions that
can cause large increases in both of these variables.
Almost all of these conditions result from circulatory
abnormalities.
Increased Blood Volume and
Extracellular Fluid Volume Caused
by Heart Diseases
In congestive heart failure, blood volume may increase
15 to 20 per cent, and extracellular fluid volume sometimes increases by 200 per cent or more. The reason for
this can be understood by re-examination of Figure
29–12. Initially, heart failure reduces cardiac output and,
consequently, decreases arterial pressure. This in turn
activates multiple sodium-retaining systems, especially
the renin-angiotensin, aldosterone, and sympathetic
nervous systems. In addition, the low blood pressure
itself causes the kidneys to retain salt and water. Therefore, the kidneys retain volume in an attempt to return
the arterial pressure and cardiac output toward normal.
Indeed, if the heart failure is not too severe, the rise in
blood volume can often return cardiac output and arterial pressure virtually all the way to normal, and sodium
excretion will eventually increase back to normal,
although there will remain excess extracellular fluid
volume and blood volume to keep the weakened heart
pumping adequately. However, if the heart is greatly
weakened, arterial pressure will not be able to increase
enough to restore urine output to normal. When this
occurs, the kidneys continue to retain volume until the
person develops severe circulatory congestion and
eventually dies of pulmonary edema.
In myocardial failure, heart valvular disease, and
congenital abnormalities of the heart, an important
circulatory compensation is an increase in blood
volume, which helps to return cardiac output and blood
pressure to normal. This allows even the weakened
heart to maintain a life-sustaining level of cardiac
output.
Increased Blood Volume Caused by
Increased Capacity of Circulation
Any condition that increases vascular capacity will also
cause the blood volume to increase to fill this extra
capacity. An increase in vascular capacity initially
reduces mean circulatory filling pressure (see Figure
29–12), which leads to decreased cardiac output and
decreased arterial pressure. The fall in pressure causes
salt and water retention by the kidneys until the blood
Chapter 29
Renal Regulation; Integration of Renal Mechanisms
volume increases sufficiently to fill the extra capacity.
For example, in pregnancy the increased vascular capacity of the uterus, placenta, and other enlarged organs
of the woman’s body regularly increases the blood
volume 15 to 25 per cent. Similarly, in patients who
have large varicose veins of the legs, which in rare
instances may hold up to an extra liter of blood,
the blood volume simply increases to fill the extra
vascular capacity. In these cases, salt and water are
retained by the kidneys until the total vascular bed is
filled enough to raise blood pressure to the level
required to balance renal output of fluid with daily
intake of fluid.
Conditions That Cause
Large Increases in
Extracellular Fluid
Volume but with Normal
Blood Volume
There are several conditions in which extracellular fluid
volume becomes markedly increased but blood volume
remains normal or even slightly reduced. These conditions are usually initiated by leakage of fluid and protein
into the interstitium, which tends to decrease the blood
volume. The kidneys’ response to these conditions is
similar to the response after hemorrhage. That is, the
kidneys retain salt and water in an attempt to restore
blood volume toward normal. Much of the extra fluid,
however, leaks into the interstitium, causing further
edema.
381
into the tissues of the body. The net result is massive
fluid retention by the kidneys until tremendous extracellular edema occurs unless treatment is instituted to
restore the plasma proteins.
Liver Cirrhosis—Decreased
Synthesis of Plasma Proteins
by the Liver and Sodium Retention
by the Kidneys
A similar sequence of events occurs in cirrhosis of the
liver as in nephrotic syndrome, except that in liver cirrhosis, the reduction in plasma protein concentration
results from destruction of the liver cells, thus reducing
the ability of the liver to synthesize enough plasma
proteins. Cirrhosis is also associated with large amounts
of fibrous tissue in the liver structure, which greatly
impedes the flow of portal blood through the liver.
This in turn raises capillary pressure throughout the
portal vascular bed, which also contributes to the
leakage of fluid and proteins into the peritoneal cavity,
a condition called ascites. Once fluid and protein are lost
from the circulation, the renal responses are similar to
those observed in other conditions associated with
decreased plasma volume. That is, the kidneys continue
to retain salt and water until plasma volume and arterial pressure are restored to normal. In some cases,
plasma volume may actually increase above normal
because of increased vascular capacity in cirrhosis;
the high pressures in the portal circulation can
greatly distend veins and therefore increase vascular
capacity.
References
Nephrotic Syndrome—Loss of
Plasma Proteins in Urine and
Sodium Retention by the Kidneys
The general mechanisms that lead to extracellular
edema are reviewed in Chapter 25. One of the most
important clinical causes of edema is the so-called
nephrotic syndrome. In nephrotic syndrome, the
glomerular capillaries leak large amounts of protein
into the filtrate and the urine because of an increased
permeability of the glomerulus. Thirty to 50 grams of
plasma protein can be lost in the urine each day, sometimes causing the plasma protein concentration to fall
to less than one-third normal. As a consequence of the
decreased plasma protein concentration, the plasma
colloid osmotic pressure falls to low levels. This causes
the capillaries all over the body to filter large amounts
of fluid into the various tissues, which in turn causes
edema and decreases the plasma volume.
Renal sodium retention in nephrotic syndrome
occurs through multiple mechanisms activated by
leakage of protein and fluid from the plasma into the
interstitial fluid, including activation of various sodiumretaining systems such as the renin-angiotensin system,
aldosterone, and possibly the sympathetic nervous
system. The kidneys continue to retain sodium and
water until plasma volume is restored nearly to normal.
However, because of the large amount of sodium and
water retention, the plasma protein concentration
becomes further diluted, causing still more fluid to leak
Antunes-Rodrigues J, de Castro M, Elias LL, et al: Neuroendocrine control of body fluid metabolism. Physiol Rev
84:169, 2004.
Cowley AW Jr: Long-term control of arterial pressure.
Physiol Rev 72:231, 1992.
Fitzsimmons JT: Physiology and pathophysiology of thirst
and salt appetite. In Seldin DW, Giebisch G (eds): The
Kidney—Physiology and Pathophysiology, 3rd ed. New
York: Raven Press, 2000, pp 1153-1174.
Giebisch GH: A trail of research on potassium. Kidney Int
62:1498, 2002.
Giebisch G, Hebert SC, Wang WH: New aspects of renal
potassium transport. Pflugers Arch 446:289, 2003.
Granger JP, Alexander BT, Llinas M: Mechanisms of pressure natriuresis. Curr Hypertens Rep 4:15, 2002.
Guise TA, Mundy GR: Disorders of calcium metabolism. In
Seldin DW, Giebisch G (eds): The Kidney—Physiology
and Pathophysiology, 3rd ed. New York: Raven Press,
2000, pp 1811-1840.
Guyton AC: Blood pressure control—special role of the
kidneys and body fluids. Science 252:1813, 1991.
Hall JE, Brands MW: The renin-angiotensin-aldosterone
system: renal mechanisms and circulatory homeostasis. In
Seldin DW, Giebisch G (eds): The Kidney—Physiology
and Pathophysiology, 3rd ed. New York: Raven Press,
2000, pp 1009-1046.
Hall JE: Angiotensin II and long-term arterial pressure regulation: the overriding dominance of the kidney. J Am Soc
Nephrol 10(Suppl 12):s258, 1999.
382
Unit V
The Body Fluids and Kidneys
Murer H, Hernando N, Forster I, Biber J: Regulation of
Na/Pi transporter in the proximal tubule. Annu Rev
Physiol 65:531, 2003.
Rosa R, Epstein FH: Extrarenal potassium metabolism. In
Seldin DW, Giebisch G (eds): The Kidney—Physiology
and Pathophysiology, 3rd ed. New York: Raven Press,
2000, pp 1551-1574.
Suki WN, Lederer ED, Rouse D: Renal transport of calcium
magnesium and phosphate. In: Brenner BM (ed): The
Kidney, 6th ed. Philadelphia: WB Saunders, 2000, pp 520574.
Warnock DG. Renal genetic disorders related to K+ and
Mg2+. Annu Rev Physiol 64:845, 2002.
Young DB: Quantitative analysis of aldosterone’s role in
potassium regulation. Am J Physiol 255: F811, 1988.
Young DB: Analysis of long-term potassium regulation.
Endocr Rev 6:24, 1985.
C
H
A
P
T
E
R
3
0
Regulation of Acid-Base Balance
Regulation of hydrogen ion (H+) balance is similar
in some ways to the regulation of other ions in the
body. For instance, to achieve homeostasis, there
must be a balance between the intake or production
of H+ and the net removal of H+ from the body.And,
as is true for other ions, the kidneys play a key role
in regulating H+ removal. However, precise control
of extracellular fluid H+ concentration involves
much more than simple elimination of H+ by the kidneys. There are also multiple acid-base buffering mechanisms involving the blood, cells, and lungs that are
essential in maintaining normal H+ concentrations in both the extracellular and
the intracellular fluid.
In this chapter, the various mechanisms that contribute to the regulation of
H+ concentration are discussed, with special emphasis on the control of renal
H+ secretion and renal reabsorption, production, and excretion of bicarbonate
ions (HCO3–), one of the key components of acid-base control systems in the
body fluids.
Hydrogen Ion Concentration Is
Precisely Regulated
Precise H+ regulation is essential because the activities of almost all enzyme
systems in the body are influenced by H+ concentration. Therefore, changes in
hydrogen concentration alter virtually all cell and body functions.
Compared with other ions, the H+ concentration of the body fluids normally
is kept at a low level. For example, the concentration of sodium in extracellular fluid (142 mEq/L) is about 3.5 million times as great as the normal concentration of H+, which averages only 0.00004 mEq/L. Equally important, the
normal variation in H+ concentration in extracellular fluid is only about one millionth as great as the normal variation in sodium ion (Na+) concentration. Thus,
the precision with which H+ is regulated emphasizes its importance to the
various cell functions.
Acids and Bases—Their Definitions
and Meanings
A hydrogen ion is a single free proton released from a hydrogen atom. Molecules
containing hydrogen atoms that can release hydrogen ions in solutions are referred
to as acids. An example is hydrochloric acid (HCl), which ionizes in water to form
hydrogen ions (H+) and chloride ions (Cl–). Likewise, carbonic acid (H2CO3) ionizes
in water to form H+ and bicarbonate ions (HCO3–).
A base is an ion or a molecule that can accept an H+. For example, HCO3– is a
base because it can combine with H+ to form H2CO3. Likewise, HPO4= is a base
because it can accept an H+ to form H2PO4–. The proteins in the body also function
as bases, because some of the amino acids that make up proteins have net negative
charges that readily accept H+. The protein hemoglobin in the red blood cells and
proteins in the other cells of the body are among the most important of the body’s
bases.
The terms base and alkali are often used synonymously. An alkali is a molecule
formed by the combination of one or more of the alkaline metals—sodium, potassium, lithium, and so forth—with a highly basic ion such as a hydroxyl ion (OH–).
383
384
Unit V
The Body Fluids and Kidneys
The base portion of these molecules reacts quickly with
H+ to remove it from solution; they are, therefore,
typical bases. For similar reasons, the term alkalosis
refers to excess removal of H+ from the body fluids, in
contrast to the excess addition of H+, which is referred
to as acidosis.
Strong and Weak Acids and Bases. A strong acid is one that
rapidly dissociates and releases especially large
amounts of H+ in solution. An example is HCl. Weak
acids have less tendency to dissociate their ions and,
therefore, release H+ with less vigor. An example is
H2CO3. A strong base is one that reacts rapidly and
strongly with H+ and, therefore, quickly removes these
from a solution. A typical example is OH–, which reacts
with H+ to form water (H2O). A typical weak base is
HCO3– because it binds with H+ much more weakly than
does OH–. Most of the acids and bases in the extracellular fluid that are involved in normal acid-base regulation are weak acids and bases. The most important ones
that we discuss in detail are H2CO3 and bicarbonate
base.
Normal Hydrogen Ion Concentration and pH of Body Fluids
and Changes That Occur in Acidosis and Alkalosis. As dis-
cussed earlier, the blood H+ concentration is normally
maintained within tight limits around a normal value
of about 0.00004 mEq/L (40 nEq/L). Normal variations
are only about 3 to 5 nEq/L, but under extreme
conditions, the H+ concentration can vary from as low
as 10 nEq/L to as high as 160 nEq/L without causing
death.
Because H+ concentration normally is low, and
because these small numbers are cumbersome, it is customary to express H+ concentration on a logarithm
scale, using pH units. pH is related to the actual H+ concentration by the following formula (H+ concentration
[H+] is expressed in equivalents per liter):
pH = log
1
[H+ ]
= - log [H+ ]
For example, the normal [H+] is 40 nEq/L
(0.00000004 Eq/L). Therefore, the normal pH is
pH = –log [0.00000004]
pH = 7.4
From this formula, one can see that pH is inversely
related to the H+ concentration; therefore, a low pH
corresponds to a high H+ concentration, and a high pH
corresponds to a low H+ concentration.
The normal pH of arterial blood is 7.4, whereas the
pH of venous blood and interstitial fluids is about 7.35
because of the extra amounts of carbon dioxide (CO2)
released from the tissues to form H2CO3 in these fluids
(Table 30–1). Because the normal pH of arterial blood
is 7.4, a person is considered to have acidosis when the
pH falls below this value and to have alkalosis when the
pH rises above 7.4. The lower limit of pH at which a
person can live more than a few hours is about 6.8, and
the upper limit is about 8.0.
Intracellular pH usually is slightly lower than plasma
pH because the metabolism of the cells produces acid,
especially H2CO3. Depending on the type of cells, the
pH of intracellular fluid has been estimated to range
between 6.0 and 7.4. Hypoxia of the tissues and poor
blood flow to the tissues can cause acid accumulation
and decreased intracellular pH.
Table 30–1
pH and H+ Concentration of Body Fluids
H+ Concentration (mEq/L)
pH
Extracellular fluid
Arterial blood
Venous blood
Interstitial fluid
4.0 ¥ 10–5
4.5 ¥ 10–5
4.5 ¥ 10–5
7.40
7.35
7.35
Intracellular fluid
1 ¥ 10–3 to 4 ¥ 10–5
6.0 to 7.4
Urine
3 ¥ 10–2 to 1 ¥ 10–5
4.5 to 8.0
Gastric HCl
160
0.8
The pH of urine can range from 4.5 to 8.0, depending
on the acid-base status of the extracellular fluid. As discussed later, the kidneys play a major role in correcting
abnormalities of extracellular fluid H+ concentration by
excreting acids or bases at variable rates.
An extreme example of an acidic body fluid is the
HCl secreted into the stomach by the oxyntic (parietal)
cells of the stomach mucosa, as discussed in Chapter 64.
The H+ concentration in these cells is about 4 million
times greater than the hydrogen concentration in blood,
with a pH of 0.8.
In the remainder of this chapter, we discuss the regulation of extracellular fluid H+ concentration.
Defenses Against Changes in
Hydrogen Ion Concentration:
Buffers, Lungs, and Kidneys
There are three primary systems that regulate the H+
concentration in the body fluids to prevent acidosis or
alkalosis: (1) the chemical acid-base buffer systems of
the body fluids, which immediately combine with acid
or base to prevent excessive changes in H+ concentration; (2) the respiratory center, which regulates the
removal of CO2 (and, therefore, H2CO3) from the
extracellular fluid; and (3) the kidneys, which can
excrete either acid or alkaline urine, thereby readjusting the extracellular fluid H+ concentration toward
normal during acidosis or alkalosis.
When there is a change in H+ concentration, the
buffer systems of the body fluids react within a fraction
of a second to minimize these changes. Buffer systems
do not eliminate H+ from or add them to the body
but only keep them tied up until balance can be reestablished.
The second line of defense, the respiratory system,
also acts within a few minutes to eliminate CO2 and,
therefore, H2CO3 from the body.
These first two lines of defense keep the H+ concentration from changing too much until the more
slowly responding third line of defense, the kidneys,
can eliminate the excess acid or base from the body.
Although the kidneys are relatively slow to respond
compared with the other defenses, over a period of
hours to several days, they are by far the most powerful of the acid-base regulatory systems.
385
Regulation of Acid-Base Balance
Now, putting the entire system together, we have the
following:
ææ
æ
æÆ H 2CO3 ¨ Æ H+ + HCO3–
CO 2 + H 2O ¨
+
Na+
æææÆ
A buffer is any substance that can reversibly bind H+.
The general form of the buffering reaction is
æææÆ H Buffer
Buffer + H + ¨
æææÆ
In this example, a free H+ combines with the buffer to
form a weak acid (H buffer) that can either remain as
an unassociated molecule or dissociate back to buffer
and H+. When the H+ concentration increases, the
reaction is forced to the right, and more H+ binds to
the buffer, as long as buffer is available. Conversely,
when the H+ concentration decreases, the reaction
shifts toward the left, and H+ is released from the
buffer. In this way, changes in H+ concentration are
minimized.
The importance of the body fluid buffers can be
quickly realized if one considers the low concentration
of H+ in the body fluids and the relatively large
amounts of acids produced by the body each day. For
example, about 80 milliequivalents of hydrogen is
either ingested or produced each day by metabolism,
whereas the H+ concentration of the body fluids normally is only about 0.00004 mEq/L. Without buffering,
the daily production and ingestion of acids would
cause huge changes in body fluid H+ concentration.
The action of acid-base buffers can perhaps best be
explained by considering the buffer system that is
quantitatively the most important in the extracellular
fluid—the bicarbonate buffer system.
Bicarbonate Buffer System
The bicarbonate buffer system consists of a water solution that contains two ingredients: (1) a weak acid,
H2CO3, and (2) a bicarbonate salt, such as NaHCO3.
H2CO3 is formed in the body by the reaction of CO2
with H2O.
Ï
Ì
Ó
Buffering of Hydrogen Ions
in the Body Fluids
æææÆ
Chapter 30
Because of the weak dissociation of H2CO3, the H+
concentration is extremely small.
When a strong acid such as HCl is added to the
bicarbonate buffer solution, the increased H+ released
from the acid (HCl Æ H+ + Cl–) is buffered by HCO3–.
≠H+ + HCO3– Æ H2CO3 Æ CO2 + H2O
As a result, more H2CO3 is formed, causing increased
CO2 and H2O production. From these reactions, one
can see that H+ from the strong acid HCl reacts with
HCO3– to form the very weak acid H2CO3, which in
turn forms CO2 and H2O. The excess CO2 greatly stimulates respiration, which eliminates the CO2 from the
extracellular fluid.
The opposite reactions take place when a strong
base, such as sodium hydroxide (NaOH), is added to
the bicarbonate buffer solution.
NaOH + H2CO3 Æ NaHCO3 + H2O
In this case, the OH– from the NaOH combines with
H2CO3 to form additional HCO3–. Thus, the weak base
NaHCO3 replaces the strong base NaOH. At the same
time, the concentration of H2CO3 decreases (because
it reacts with NaOH), causing more CO2 to combine
with H2O to replace the H2CO3.
CO 2 + H 2O æææÆ H 2CO3 æææÆ ≠HCO3 - + H +
+
+
NaOH
Na
The net result, therefore, is a tendency for the CO2
levels in the blood to decrease, but the decreased CO2
in the blood inhibits respiration and decreases the rate
of CO2 expiration. The rise in blood HCO3– that occurs
is compensated for by increased renal excretion of
HCO3–.
carbonic
anhydrase
This reaction is slow, and exceedingly small amounts
of H2CO3 are formed unless the enzyme carbonic
anhydrase is present. This enzyme is especially abundant in the walls of the lung alveoli, where CO2 is
released; carbonic anhydrase is also present in the
epithelial cells of the renal tubules, where CO2 reacts
with H2O to form H2CO3.
H2CO3 ionizes weakly to form small amounts of H+
and HCO3–.
H 2 CO 3 ¨ Æ
H+ + HCO3 -
æææÆ
The second component of the system, bicarbonate
salt, occurs predominantly as sodium bicarbonate
(NaHCO3) in the extracellular fluid. NaHCO3
ionizes almost completely to form HCO3– and Na+, as
follows:
æÆ Na + + HCO3 NaHCO3 ææ
¨
Quantitative Dynamics of the
Bicarbonate Buffer System
All acids, including H2CO3, are ionized to some extent.
From mass balance considerations, the concentrations
of H+ and HCO3– are proportional to the concentration
of H2CO3.
H 2CO3 ¨ Æ
æææÆ
CO 2 + H 2O ¨ææ
ææ H CO3
æ
ææ
æÆ 2
H + + HCO3 -
For any acid, the concentration of the acid relative
to its dissociated ions is defined by the dissociation
constant K¢.
K¢ =
H+ ¥ HCO3 H 2CO3
(1)
This equation indicates that in an H2CO3 solution, the
amount of free H+ is equal to
H+ = K ¢ ¥
H 2CO3
HCO3 -
(2)
386
Unit V
The Body Fluids and Kidneys
The concentration of undissociated H2CO3 cannot be
measured in solution because it rapidly dissociates into
CO2 and H2O or to H+ and HCO3–. However, the CO2
dissolved in the blood is directly proportional to the
amount of undissociated H2CO3. Therefore, equation 2
can be rewritten as
H+ = K ¥
CO2
HCO3 -
(3)
The dissociation constant (K) for equation 3 is only
about 1/400 of the dissociation constant (K¢) of equation
2 because the proportionality ratio between H2CO3 and
CO2 is 1:400.
Equation 3 is written in terms of the total amount of
CO2 dissolved in solution. However, most clinical laboratories measure the blood CO2 tension (PCO2) rather
than the actual amount of CO2. Fortunately, the amount
of CO2 in the blood is a linear function of PCO2 times
the solubility coefficient for CO2; under physiologic
conditions, the solubility coefficient for CO2 is
0.03 mmol/mm Hg at body temperature.This means that
0.03 millimole of H2CO3 is present in the blood for each
millimeter of mercury PCO2 measured. Therefore, equation 3 can be rewritten as
H+ = K ¥
(0.03 ¥ Pco 2 )
HCO3 -
(4)
Henderson-Hasselbalch Equation. As discussed earlier, it is
customary to express H+ concentration in pH units
rather than in actual concentrations. Recall that pH is
defined as pH = –log H+.
The dissociation constant can be expressed in a
similar manner.
pK = –log K
Therefore, we can express the H+ concentration in
equation 4 in pH units by taking the negative logarithm
of that equation, which yields
- log H+ = - log pK - log
(0.03 ¥ Pco 2 )
HCO3 -
(5)
toward alkalosis. An increase in PCO2 causes the pH to
decrease, shifting the acid-base balance toward acidosis.
The Henderson-Hasselbalch equation, in addition to
defining the determinants of normal pH regulation and
acid-base balance in the extracellular fluid, provides
insight into the physiologic control of acid and base
composition of the extracellular fluid. As discussed
later, the bicarbonate concentration is regulated mainly
by the kidneys, whereas the PCO2 in extracellular fluid is
controlled by the rate of respiration. By increasing the
rate of respiration, the lungs remove CO2 from the
plasma, and by decreasing respiration, the lungs elevate
PCO2. Normal physiologic acid-base homeostasis results
from the coordinated efforts of both of these organs, the
lungs and the kidneys, and acid-base disorders occur
when one or both of these control mechanisms are
impaired, thus altering either the bicarbonate concentration or the PCO2 of extracellular fluid.
When disturbances of acid-base balance result from
a primary change in extracellular fluid bicarbonate concentration, they are referred to as metabolic acid-base
disorders. Therefore, acidosis caused by a primary
decrease in bicarbonate concentration is termed metabolic acidosis, whereas alkalosis caused by a primary
increase in bicarbonate concentration is called metabolic alkalosis. Acidosis caused by an increase in PCO2
is called respiratory acidosis, whereas alkalosis caused
by a decrease in PCO2 is termed respiratory alkalosis.
Bicarbonate Buffer System Titration Curve. Figure 30–1 shows
the changes in pH of the extracellular fluid when the
ratio of HCO3– to CO2 in extracellular fluid is altered.
When the concentrations of these two components are
equal, the right-hand portion of equation 8 becomes the
log of 1, which is equal to 0. Therefore, when the two
components of the buffer system are equal, the pH of
the solution is the same as the pK (6.1) of the bicarbonate buffer system. When base is added to the system,
part of the dissolved CO2 is converted into HCO3–,
causing an increase in the ratio of HCO3– to CO2 and
increasing the pH, as is evident from the HendersonHasselbalch equation. When acid is added, it is buffered
Therefore,
HCO3 -
(6)
Rather than work with a negative logarithm, we can
change the sign of the logarithm and invert the numerator and denominator in the last term, using the law of
logarithms to yield
pH = pK + log
HCO3 (0.03 ¥ Pco 2 )
(7)
For the bicarbonate buffer system, the pK is 6.1, and
equation 7 can be written as
pH = 6.1 + log
HCO3 0.03 ¥ Pco 2
(8)
Equation 8 is the Henderson-Hasselbalch equation,
and with it, one can calculate the pH of a solution if
the molar concentration of HCO3– and the PCO2 are
known.
From the Henderson-Hasselbalch equation, it is
apparent that an increase in HCO3– concentration
causes the pH to rise, shifting the acid-base balance
0
100
Normal
operating
point in body
25
75
50
50
pK
25
75
Per cent of buffer in form of
HCO3Base added
(0.03 ¥ Pco 2 )
Acid added
Per cent of buffer in form of
H2CO3 and CO2
pH = pK - log
0
100
4
5
6
pH
7
8
Figure 30–1
Titration curve for bicarbonate buffer system showing the pH of
extracellular fluid when the percentages of buffer in the form of
HCO3– and CO2 (or H2CO3) are altered.
Chapter 30
Regulation of Acid-Base Balance
by HCO3–, which is then converted into dissolved CO2,
decreasing the ratio of HCO3– to CO2 and decreasing
the pH of the extracellular fluid.
“Buffer Power” Is Determined by the Amount and Relative Concentrations of the Buffer Components. From the titration
curve in Figure 30–1, several points are apparent. First,
the pH of the system is the same as the pK when each
of the components (HCO3– and CO2) constitutes 50 per
cent of the total concentration of the buffer system.
Second, the buffer system is most effective in the central
part of the curve, where the pH is near the pK of the
system. This means that the change in pH for any given
amount of acid or base added to the system is least when
the pH is near the pK of the system. The buffer system
is still reasonably effective for 1.0 pH unit on either side
of the pK, which for the bicarbonate buffer system
extends from a pH of about 5.1 to 7.1 units. Beyond
these limits, the buffering power rapidly diminishes.
And when all the CO2 has been converted into HCO3–
or when all the HCO3– has been converted into CO2, the
system has no more buffering power.
The absolute concentration of the buffers is also an
important factor in determining the buffer power of a
system. With low concentrations of the buffers, only a
small amount of acid or base added to the solution
changes the pH considerably.
Bicarbonate Buffer System Is the Most Important Extracellular
Buffer. From the titration curve shown in Figure 30–1,
one would not expect the bicarbonate buffer system to
be powerful, for two reasons: First, the pH of the extracellular fluid is about 7.4, whereas the pK of the bicarbonate buffer system is 6.1. This means that there is
about 20 times as much of the bicarbonate buffer
system in the form of HCO3– as in the form of dissolved CO2. For this reason, this system operates on
the portion of the buffering curve where the slope is
low and the buffering power is poor. Second, the concentrations of the two elements of the bicarbonate
system, CO2 and HCO3–, are not great.
Despite these characteristics, the bicarbonate buffer
system is the most powerful extracellular buffer in the
body. This apparent paradox is due mainly to the fact
that the two elements of the buffer system, HCO3– and
CO2, are regulated, respectively, by the kidneys and the
lungs, as discussed later. As a result of this regulation,
the pH of the extracellular fluid can be precisely controlled by the relative rate of removal and addition of
HCO3– by the kidneys and the rate of removal of CO2
by the lungs.
Phosphate Buffer System
Although the phosphate buffer system is not important as an extracellular fluid buffer, it plays a major
role in buffering renal tubular fluid and intracellular
fluids.
The main elements of the phosphate buffer system
are H2PO4– and HPO4=. When a strong acid such as
HCl is added to a mixture of these two substances, the
hydrogen is accepted by the base HPO4= and converted to H2PO4–.
387
HCl + Na2HPO4 Æ NaH2PO4 + NaCl
The result of this reaction is that the strong acid, HCl,
is replaced by an additional amount of a weak acid,
NaH2PO4, and the decrease in pH is minimized.
When a strong base, such as NaOH, is added to the
buffer system, the OH– is buffered by the H2PO4– to
form additional amounts of HPO4= + H2O.
NaOH + NaH2PO4 Æ Na2HPO4 + H2O
In this case, a strong base, NaOH, is traded for a weak
base, NaH2PO4, causing only a slight increase in
pH.
The phosphate buffer system has a pK of 6.8, which
is not far from the normal pH of 7.4 in the body fluids;
this allows the system to operate near its maximum
buffering power. However, its concentration in the
extracellular fluid is low, only about 8 per cent of the
concentration of the bicarbonate buffer. Therefore,
the total buffering power of the phosphate system in
the extracellular fluid is much less than that of the
bicarbonate buffering system.
In contrast to its rather insignificant role as an extracellular buffer, the phosphate buffer is especially
important in the tubular fluids of the kidneys, for two
reasons: (1) phosphate usually becomes greatly concentrated in the tubules, thereby increasing the buffering power of the phosphate system, and (2) the tubular
fluid usually has a considerably lower pH than
the extracellular fluid does, bringing the operating
range of the buffer closer to the pK (6.8) of the
system.
The phosphate buffer system is also important in
buffering intracellular fluid because the concentration
of phosphate in this fluid is many times that in the
extracellular fluid. Also, the pH of intracellular fluid is
lower than that of extracellular fluid and therefore is
usually closer to the pK of the phosphate buffer
system compared with the extracellular fluid.
Proteins: Important
Intracellular Buffers
Proteins are among the most plentiful buffers in the
body because of their high concentrations, especially
within the cells.
The pH of the cells, although slightly lower than in
the extracellular fluid, nevertheless changes approximately in proportion to extracellular fluid pH changes.
There is a slight amount of diffusion of H+ and HCO3–
through the cell membrane, although these ions
require several hours to come to equilibrium with the
extracellular fluid, except for rapid equilibrium that
occurs in the red blood cells. CO2, however, can rapidly
diffuse through all the cell membranes. This diffusion
of the elements of the bicarbonate buffer system causes
the pH in intracellular fluid to change when there are
changes in extracellular pH. For this reason, the buffer
systems within the cells help prevent changes in the
pH of extracellular fluid but may take several hours to
become maximally effective.
388
Unit V
The Body Fluids and Kidneys
In the red blood cell, hemoglobin (Hb) is an important buffer, as follows:
æææÆ HHb
H+ + Hb ¨
æææÆ
¨
Approximately 60 to 70 per cent of the total chemical buffering of the body fluids is inside the cells, and
most of this results from the intracellular proteins.
However, except for the red blood cells, the slowness
with which H+ and HCO3– move through the cell membranes often delays for several hours the maximum
ability of the intracellular proteins to buffer extracellular acid-base abnormalities.
In addition to the high concentration of proteins in
the cells, another factor that contributes to their
buffering power is the fact that the pKs of many of
these protein systems are fairly close to 7.4.
Isohydric Principle: All Buffers
in a Common Solution Are
in Equilibrium with the Same
Hydrogen Ion Concentration
We have been discussing buffer systems as though they
operated individually in the body fluids. However, they
all work together, because H+ is common to the reactions of all the systems. Therefore, whenever there is a
change in H+ concentration in the extracellular fluid, the
balance of all the buffer systems changes at the same
time. This phenomenon is called the isohydric principle
and is illustrated by the following formula:
H+ = K1 ¥
HA1
HA 2
HA 3
= K2 ¥
= K3 ¥
A1
A2
A3
K1, K2, K3 are the dissociation constants of three
respective acids, HA1, HA2, HA3, and A1, A2, A3 are the
concentrations of the free negative ions that constitute
the bases of the three buffer systems.
The implication of this principle is that any condition
that changes the balance of one of the buffer systems
also changes the balance of all the others because the
buffer systems actually buffer one another by shifting
H+ back and forth between them.
flowing blood transports it to the lungs, where it diffuses into the alveoli and then is transferred to the
atmosphere by pulmonary ventilation. About 1.2 mol/
L of dissolved CO2 normally is in the extracellular
fluid, corresponding to a Pco2 of 40 mm Hg.
If the rate of metabolic formation of CO2 increases,
the Pco2 of the extracellular fluid is likewise increased.
Conversely, a decreased metabolic rate lowers the
Pco2. If the rate of pulmonary ventilation is increased,
CO2 is blown off from the lungs, and the Pco2 in the
extracellular fluid decreases. Therefore, changes in
either pulmonary ventilation or the rate of CO2 formation by the tissues can change the extracellular fluid
Pco2.
Increasing Alveolar Ventilation
Decreases Extracellular Fluid
Hydrogen Ion Concentration and
Raises pH
If the metabolic formation of CO2 remains constant,
the only other factor that affects Pco2 in extracellular
fluid is the rate of alveolar ventilation. The higher the
alveolar ventilation, the lower the Pco2; conversely, the
lower the alveolar ventilation rate, the higher the Pco2.
As discussed previously, when CO2 concentration
increases, the H2CO3 concentration and H+ concentration also increase, thereby lowering extracellular
fluid pH.
Figure 30–2 shows the approximate changes in
blood pH that are caused by increasing or decreasing
the rate of alveolar ventilation. Note that increasing
alveolar ventilation to about twice normal raises the
pH of the extracellular fluid by about 0.23. If the pH
of the body fluids is 7.40 with normal alveolar ventilation, doubling the ventilation rate raises the pH to
Respiratory Regulation
of Acid-Base Balance
The second line of defense against acid-base disturbances is control of extracellular fluid CO2 concentration by the lungs. An increase in ventilation eliminates
CO2 from extracellular fluid, which, by mass action,
reduces the H+ concentration. Conversely, decreased
ventilation increases CO2, thus also increasing H+ concentration in the extracellular fluid.
pH change in body fluids
+0.3
+0.2
+0.1
0
Normal
-0.1
-0.2
-0.3
-0.4
-0.5
0.5
1.0
1.5
2.0
Rate of alveolar ventilation
(normal = 1)
2.5
Pulmonary Expiration of CO2
Balances Metabolic Formation of CO2
CO2 is formed continually in the body by intracellular
metabolic processes. After it is formed, it diffuses from
the cells into the interstitial fluids and blood, and the
Figure 30–2
Change in extracellular fluid pH caused by increased or
decreased rate of alveolar ventilation, expressed as times normal.
Chapter 30
Regulation of Acid-Base Balance
389
about 7.63. Conversely, a decrease in alveolar ventilation to one fourth normal reduces the pH by 0.45. That
is, if the pH is 7.4 at a normal alveolar ventilation,
reducing the ventilation to one fourth normal reduces
the pH to 6.95. Because the alveolar ventilation rate
can change markedly, from as low as 0 to as high as 15
times normal, one can easily understand how much the
pH of the body fluids can be changed by the respiratory system.
Feedback Control of Hydrogen Ion Concentration by the Respiratory System. Because increased H+ concentration
Increased Hydrogen Ion
Concentration Stimulates
Alveolar Ventilation
That is, whenever the H+ concentration increases
above normal, the respiratory system is stimulated,
and alveolar ventilation increases. This decreases the
PCO2 in extracellular fluid and reduces H+ concentration back toward normal. Conversely, if H+ concentration falls below normal, the respiratory center
becomes depressed, alveolar ventilation decreases, and
H+ concentration increases back toward normal.
Alveolar ventilation (normal = 1)
Not only does the alveolar ventilation rate influence
H+ concentration by changing the Pco2 of the body
fluids, but the H+ concentration affects the rate of
alveolar ventilation. Thus, Figure 30–3 shows that the
alveolar ventilation rate increases four to five times
normal as the pH decreases from the normal value of
7.4 to the strongly acidic value of 7.0. Conversely, when
plasma pH rises above 7.4, this causes a decrease in the
ventilation rate. As one can see from the graph, the
change in ventilation rate per unit pH change is much
greater at reduced levels of pH (corresponding to elevated H+ concentration) compared with increased
levels of pH. The reason for this is that as the alveolar
ventilation rate decreases, owing to an increase in pH
(decreased H+ concentration), the amount of oxygen
added to the blood decreases and the partial pressure
of oxygen (PO2) in the blood also decreases, which
stimulates the ventilation rate. Therefore, the respiratory compensation for an increase in pH is not nearly
as effective as the response to a marked reduction in
pH.
4
≠[H+] Æ ≠Alveolar ventilation
Ø
–
ØPCO2
Efficiency of Respiratory Control of Hydrogen Ion Concentration. Respiratory control cannot return the H+
concentration all the way back to normal when a disturbance outside the respiratory system has altered
pH. Ordinarily, the respiratory mechanism for controlling H+ concentration has an effectiveness between
50 and 75 per cent, corresponding to a feedback gain
of 1 to 3. That is, if the H+ concentration is suddenly
increased by adding acid to the extracellular fluid and
pH falls from 7.4 to 7.0, the respiratory system can
return the pH to a value of about 7.2 to 7.3. This
response occurs within 3 to 12 minutes.
Buffering Power of the Respiratory System. Respiratory reg-
ulation of acid-base balance is a physiologic type of
buffer system because it acts rapidly and keeps the H+
concentration from changing too much until the slowly
responding kidneys can eliminate the imbalance. In
general, the overall buffering power of the respiratory
system is one to two times as great as the buffering
power of all other chemical buffers in the extracellular fluid combined. That is, one to two times as much
acid or base can normally be buffered by this mechanism as by the chemical buffers.
Impairment of Lung Function Can Cause Respiratory Acidosis.
3
2
1
0
7.0
stimulates respiration, and because increased alveolar
ventilation decreases the H+ concentration, the respiratory system acts as a typical negative feedback controller of H+ concentration.
7.1
7.2
7.3
7.4
pH of arterial blood
7.5
7.6
Figure 30–3
Effect of blood pH on the rate of alveolar ventilation.
We have discussed thus far the role of the normal respiratory mechanism as a means of buffering changes
in H+ concentration. However, abnormalities of respiration can also cause changes in H+ concentration. For
example, an impairment of lung function, such as
severe emphysema, decreases the ability of the lungs
to eliminate CO2; this causes a buildup of CO2 in the
extracellular fluid and a tendency toward respiratory
acidosis. Also, the ability to respond to metabolic acidosis is impaired because the compensatory reductions in PCO2 that would normally occur by means of
increased ventilation are blunted. In these circumstances, the kidneys represent the sole remaining physiologic mechanism for returning pH toward normal
after the initial chemical buffering in the extracellular
fluid has occurred.
390
Unit V
The Body Fluids and Kidneys
Renal Control of
Acid-Base Balance
body of the nonvolatile acids produced each day, for a
total of 4400 milliequivalents of H+ secreted into the
tubular fluid each day.
When there is a reduction in the extracellular fluid
H+ concentration (alkalosis), the kidneys fail to reabsorb all the filtered bicarbonate, thereby increasing the
excretion of bicarbonate. Because HCO3– normally
buffers hydrogen in the extracellular fluid, this loss of
bicarbonate is the same as adding an H+ to the extracellular fluid. Therefore, in alkalosis, the removal of
HCO3– raises the extracellular fluid H+ concentration
back toward normal.
In acidosis, the kidneys do not excrete bicarbonate
into the urine but reabsorb all the filtered bicarbonate
and produce new bicarbonate, which is added back to
the extracellular fluid. This reduces the extracellular
fluid H+ concentration back toward normal.
Thus, the kidneys regulate extracellular fluid H + concentration through three fundamental mechanisms: (1)
secretion of H +, (2) reabsorption of filtered HCO3-, and
(3) production of new HCO3-. All these processes are
accomplished through the same basic mechanism, as
discussed in the next few sections.
The kidneys control acid-base balance by excreting
either an acidic or a basic urine. Excreting an acidic
urine reduces the amount of acid in extracellular fluid,
whereas excreting a basic urine removes base from the
extracellular fluid.
The overall mechanism by which the kidneys
excrete acidic or basic urine is as follows: Large
numbers of HCO3– are filtered continuously into the
tubules, and if they are excreted into the urine, this
removes base from the blood. Large numbers of H+ are
also secreted into the tubular lumen by the tubular
epithelial cells, thus removing acid from the blood. If
more H+ is secreted than HCO3– is filtered, there will
be a net loss of acid from the extracellular fluid. Conversely, if more HCO3– is filtered than H+ is secreted,
there will be a net loss of base.
As discussed previously, each day the body produces
about 80 milliequivalents of nonvolatile acids, mainly
from the metabolism of proteins.These acids are called
nonvolatile because they are not H2CO3 and, therefore, cannot be excreted by the lungs. The primary
mechanism for removal of these acids from the body
is renal excretion. The kidneys must also prevent the
loss of bicarbonate in the urine, a task that is quantitatively more important than the excretion of nonvolatile acids. Each day the kidneys filter about 4320
milliequivalents of bicarbonate (180 L/day ¥ 24 mEq/
L); under normal conditions, almost all this is reabsorbed from the tubules, thereby conserving the
primary buffer system of the extracellular fluid.
As discussed later, both the reabsorption of bicarbonate and the excretion of H+ are accomplished
through the process of H+ secretion by the tubules.
Because the HCO3– must react with a secreted H+ to
form H2CO3 before it can be reabsorbed, 4320 milliequivalents of H+ must be secreted each day just to
reabsorb the filtered bicarbonate. Then an additional
80 milliequivalents of H+ must be secreted to rid the
Secretion of Hydrogen Ions
and Reabsorption of
Bicarbonate Ions by the
Renal Tubules
Hydrogen ion secretion and bicarbonate reabsorption
occur in virtually all parts of the tubules except the
descending and ascending thin limbs of the loop of
Henle. Figure 30–4 summarizes bicarbonate reabsorption along the tubule. Keep in mind that for each bicarbonate reabsorbed, an H+ must be secreted.
About 80 to 90 per cent of the bicarbonate reabsorption (and H+ secretion) occurs in the proximal
tubule, so that only a small amount of bicarbonate
flows into the distal tubules and collecting ducts. In the
85%
(3672 mEq/day)
4320 mEq/day
10%
(432 mEq/day)
>4.9%
(215 mEq/day)
Figure 30–4
(1 mEq/day)
Reabsorption of bicarbonate in different segments of the renal tubule. The
percentages of the filtered load of
bicarbonate absorbed by the various
tubular segments are shown, as well
as the number of milliequivalents
reabsorbed per day under normal
conditions.
Regulation of Acid-Base Balance
Chapter 30
thick ascending loop of Henle, another 10 per cent of
the filtered bicarbonate is reabsorbed, and the remainder of the reabsorption takes place in the distal tubule
and collecting duct. As discussed previously, the mechanism by which bicarbonate is reabsorbed also
involves tubular secretion of H+, but different tubular
segments accomplish this task differently.
Hydrogen Ions Are Secreted
by Secondary Active Transport
in the Early Tubular Segments
The epithelial cells of the proximal tubule, the thick
segment of the ascending loop of Henle, and the early
distal tubule all secrete H+ into the tubular fluid
by sodium-hydrogen counter-transport, as shown in
Figure 30–5. This secondary active secretion of H+
is coupled with the transport of Na+ into the cell at
the luminal membrane by the sodium-hydrogen
exchanger protein, and the energy for H+ secretion
against a concentration gradient is derived from the
sodium gradient favoring Na+ movement into the cell.
This gradient is established by the sodium-potassium
adenosine triphosphatase (ATPase) pump in the basolateral membrane. More than 90 per cent of the bicarbonate is reabsorbed in this manner, requiring about
3900 milliequivalents of H+ to be secreted each day by
the tubules. This mechanism, however, does not establish a very high H+ concentration in the tubular fluid;
the tubular fluid becomes very acidic only in the collecting tubules and collecting ducts.
Figure 30–5 shows how the process of H+ secretion
achieves bicarbonate reabsorption. The secretory
Renal
interstitial
fluid
Tubular cells
Na+
K+
Tubular
lumen
Na+ + HCO3Na+
ATP
HCO3- + H+
H+
H2CO3
H2CO3
CO2
H2O
+
CO2
Carbonic
anhydrase
CO2 + H2O
Figure 30–5
Cellular mechanisms for (1) active secretion of hydrogen ions into
the renal tubule; (2) tubular reabsorption of bicarbonate ions by
combination with hydrogen ions to form carbonic acid, which dissociates to form carbon dioxide and water; and (3) sodium ion
reabsorption in exchange for hydrogen ions secreted. This pattern
of hydrogen ion secretion occurs in the proximal tubule, the thick
ascending segment of the loop of Henle, and the early distal
tubule.
391
process begins when CO2 either diffuses into the
tubular cells or is formed by metabolism in the tubular
epithelial cells. CO2, under the influence of the enzyme
carbonic anhydrase, combines with H2O to form
H2CO3, which dissociates into HCO3– and H+. The H+
is secreted from the cell into the tubular lumen by
sodium-hydrogen counter-transport. That is, when an
Na+ moves from the lumen of the tubule to the interior of the cell, it first combines with a carrier protein
in the luminal border of the cell membrane; at the
same time, an H+ in the interior of the cells combines
with the carrier protein. The Na+ moves into the cell
down a concentration gradient that has been established by the sodium-potassium ATPase pump in the
basolateral membrane. The gradient for Na+ movement into the cell then provides the energy for moving
H+ in the opposite direction from the interior of the
cell to the tubular lumen.
The HCO3– generated in the cell (when H+ dissociates from H2CO3) then moves downhill across the
basolateral membrane into the renal interstitial fluid
and the peritubular capillary blood. The net result is
that for every H+ secreted into the tubular lumen, an
HCO3– enters the blood.
Filtered Bicarbonate Ions Are
Reabsorbed by Interaction with
Hydrogen Ions in the Tubules
Bicarbonate ions do not readily permeate the luminal
membranes of the renal tubular cells; therefore,
HCO3– that is filtered by the glomerulus cannot be
directly reabsorbed. Instead, HCO3– is reabsorbed by
a special process in which it first combines with H+ to
form H2CO3, which eventually becomes CO2 and H2O,
as shown in Figure 30–5.
This reabsorption of HCO3– is initiated by a reaction
in the tubules between HCO3– filtered at the glomerulus and H+ secreted by the tubular cells. The H2CO3
formed then dissociates into CO2 and H2O. The CO2
can move easily across the tubular membrane; therefore, it instantly diffuses into the tubular cell, where it
recombines with H2O, under the influence of carbonic
anhydrase, to generate a new H2CO3 molecule. This
H2CO3 in turn dissociates to form HCO3– and H+; the
HCO3– then diffuses through the basolateral membrane into the interstitial fluid and is taken up into the
peritubular capillary blood. The transport of HCO3
across the basolateral membrane is facilitated by
two mechanisms: (1) Na+-HCO3– co-transport and (2)
Cl–-HCO3– exchange.
Thus, each time an H + is formed in the tubular epithelial cells, an HCO3- is also formed and released back
into the blood. The net effect of these reactions is
“reabsorption” of HCO3– from the tubules, although
the HCO3– that actually enters the extracellular fluid
is not the same as that filtered into the tubules. The
reabsorption of filtered HCO3– does not result in net
secretion of H+ because the secreted H+ combines with
the filtered HCO3– and is therefore not excreted.
392
Unit V
The Body Fluids and Kidneys
Bicarbonate Ions Are “Titrated” Against Hydrogen Ions in the
Tubules. Under normal conditions, the rate of tubular
H+ secretion is about 4400 mEq/day, and the rate of filtration by HCO3– is about 4320 mEq/day. Thus, the
quantities of these two ions entering the tubules are
almost equal, and they combine with each other to
form CO2 and H2O. Therefore, it is said that HCO3–
and H+ normally “titrate” each other in the tubules.
The titration process is not quite exact because there
is usually a slight excess of H+ in the tubules to be
excreted in the urine. This excess H+ (about 80 mEq/
day) rids the body of nonvolatile acids produced by
metabolism. As discussed later, most of this H+ is
not excreted as free H+ but rather in combination with
other urinary buffers, especially phosphate and
ammonia.
When there is an excess of HCO3– over H+ in the
urine, as occurs in metabolic alkalosis, the excess
HCO3– cannot be reabsorbed; therefore, the excess
HCO3– is left in the tubules and eventually excreted
into the urine, which helps correct the metabolic
alkalosis.
In acidosis, there is excess H+ relative to HCO3–,
causing complete reabsorption of the bicarbonate;
the excess H+ passes into the urine. The excess H+ is
buffered in the tubules by phosphate and ammonia
and eventually excreted as salts. Thus, the basic mechanism by which the kidneys correct either acidosis or
alkalosis is incomplete titration of H+ against HCO3–,
leaving one or the other to pass into the urine and be
removed from the extracellular fluid.
Primary Active Secretion of Hydrogen
Ions in the Intercalated Cells of Late
Distal and Collecting Tubules
Beginning in the late distal tubules and continuing
through the remainder of the tubular system, the
tubular epithelium secretes H+ by primary active
transport. The characteristics of this transport are different from those discussed for the proximal tubule,
loop of Henle, and early distal tubule.
The mechanism for primary active H+ secretion is
shown in Figure 30–6. It occurs at the luminal membrane of the tubular cell, where H+ is transported
directly by a specific protein, a hydrogen-transporting
ATPase. The energy required for pumping the H+ is
derived from the breakdown of ATP to adenosine
diphosphate.
Primary active secretion of H+ occurs in a special
type of cell called the intercalated cells of the late distal
tubule and in the collecting tubules. Hydrogen ion
secretion in these cells is accomplished in two steps:
(1) the dissolved CO2 in this cell combines with H2O
to form H2CO3, and (2) the H2CO3 then dissociates
into HCO3–, which is reabsorbed into the blood, plus
H+, which is secreted into the tubule by means of the
hydrogen-ATPase mechanism. For each H+ secreted,
an HCO3– is reabsorbed, similar to the process in the
proximal tubules. The main difference is that H+ moves
across the luminal membrane by an active H+ pump
Renal
interstitial
fluid
Tubular cells
Cl-
Cl-
HCO3 +
Tubular
lumen
ClH+
H+
ATP
H2CO3
CO2
H2O
+
CO2
Carbonic
anhydrase
Figure 30–6
Primary active secretion of hydrogen ions through the luminal
membrane of the intercalated epithelial cells of the late distal and
collecting tubules. Note that one bicarbonate ion is absorbed for
each hydrogen ion secreted, and a chloride ion is passively
secreted along with the hydrogen ion.
instead of by counter-transport, as occurs in the early
parts of the nephron.
Although the secretion of H+ in the late distal tubule
and collecting tubules accounts for only about 5 per
cent of the total H+ secreted, this mechanism is important in forming a maximally acidic urine. In the proximal tubules, H+ concentration can be increased only
about threefold to fourfold, and the tubular fluid pH
can be reduced to only about 6.7, although large
amounts of H+ are secreted by this nephron segment.
However, H+ concentration can be increased as much
as 900-fold in the collecting tubules. This decreases the
pH of the tubular fluid to about 4.5, which is the lower
limit of pH that can be achieved in normal kidneys.
Combination of Excess
Hydrogen Ions with Phosphate
and Ammonia Buffers in
the Tubule—A Mechanism
for Generating “New”
Bicarbonate Ions
When H+ is secreted in excess of the bicarbonate filtered into the tubular fluid, only a small part of the
excess H+ can be excreted in the ionic form (H+) in the
urine. The reason for this is that the minimal urine pH
is about 4.5, corresponding to an H+ concentration
of 10–4.5 mEq/L, or 0.03 mEq/L. Thus, for each liter of
urine formed, a maximum of only about 0.03 milliequivalent of free H+ can be excreted. To excrete the
80 milliequivalents of nonvolatile acid formed by
metabolism each day, about 2667 liters of urine would
have to be excreted if the H+ remained free in
solution.
Chapter 30
393
Regulation of Acid-Base Balance
The excretion of large amounts of H+ (on occasion
as much as 500 mEq/day) in the urine is accomplished
primarily by combining the H+ with buffers in the
tubular fluid. The most important buffers are phosphate buffer and ammonia buffer. There are other
weak buffer systems, such as urate and citrate, that are
much less important.
When H+ is titrated in the tubular fluid with HCO3–,
this results in the reabsorption of one HCO3– for each
H+ secreted, as discussed earlier. But when there are
excess H+ in the urine, they combine with buffers other
than HCO3–, and this results in the generation of new
HCO3– that can also enter the blood. Thus, when there
is excess H+ in the extracellular fluid, the kidneys not
only reabsorb all the filtered HCO3– but also generate
new HCO3–, thereby helping to replenish the HCO3–
lost from the extracellular fluid in acidosis. In the next
two sections, we discuss the mechanisms by which
phosphate and ammonia buffers contribute to the generation of new HCO3–.
Renal
interstitial
fluid
Phosphate Buffer System Carries
Excess Hydrogen Ions into the Urine
and Generates New Bicarbonate
HCO3- to the blood. This demonstrates one of the
mechanisms by which the kidneys are able to replenish the extracellular fluid stores of HCO3–.
Under normal conditions, much of the filtered phosphate is reabsorbed, and only about 30 to 40 mEq/day
is available for buffering H+. Therefore, much of the
buffering of excess H+ in the tubular fluid in acidosis
occurs through the ammonia buffer system.
The phosphate buffer system is composed of HPO4=
and H2PO4–. Both become concentrated in the tubular
fluid because of their relatively poor reabsorption and
because of the reabsorption of water from the tubular
fluid. Therefore, although phosphate is not an important extracellular fluid buffer, it is much more effective
as a buffer in the tubular fluid.
Another factor that makes phosphate important as
a tubular buffer is the fact that the pK of this system
is about 6.8. Under normal conditions, the urine is
slightly acidic, and the urine pH is near the pK of the
phosphate buffer system. Therefore, in the tubules, the
phosphate buffer system normally functions near its
most effective range of pH.
Figure 30–7 shows the sequence of events by which
H+ is excreted in combination with phosphate buffer
and the mechanism by which new bicarbonate is added
to the blood. The process of H+ secretion into the
tubules is the same as described earlier. As long as
there is excess HCO3– in the tubular fluid, most of the
secreted H+ combines with HCO3–. However, once all
the HCO3– has been reabsorbed and is no longer available to combine with H+, any excess H+ can combine
with HPO4= and other tubular buffers. After the H+
combines with HPO4= to form H2PO4–, it can be
excreted as a sodium salt (NaH2PO4), carrying with it
the excess hydrogen.
There is one important difference in this sequence
of H+ excretion from that discussed previously. In this
case, the HCO3– that is generated in the tubular cell
and enters the peritubular blood represents a net gain
of HCO3– by the blood, rather than merely a replacement of filtered HCO3–. Therefore, whenever an H +
secreted into the tubular lumen combines with a buffer
other than HCO3-, the net effect is addition of a new
Na+
K+
HCO3-
Tubular
lumen
Na+ + NaHPO4-
Tubular cells
Na+
ATP
HCO3- + H+
H2CO3
CO2
H2O
+
CO2
Na+
H+ + NaHPO4-
NaH2PO4
Carbonic
anhydrase
Figure 30–7
Buffering of secreted hydrogen ions by filtered phosphate
(NaHPO4–). Note that a new bicarbonate ion is returned to the
blood for each NaHPO4– that reacts with a secreted hydrogen ion.
Excretion of Excess Hydrogen Ions
and Generation of New Bicarbonate
by the Ammonia Buffer System
A second buffer system in the tubular fluid that is even
more important quantitatively than the phosphate
buffer system is composed of ammonia (NH3) and the
ammonium ion (NH4+). Ammonium ion is synthesized
from glutamine, which comes mainly from the metabolism of amino acids in the liver. The glutamine delivered to the kidneys is transported into the epithelial
cells of the proximal tubules, thick ascending limb of
the loop of Henle, and distal tubules (Figure 30–8).
Once inside the cell, each molecule of glutamine is
metabolized in a series of reactions to ultimately form
two NH4+ and two HCO3–. The NH4+ is secreted into
the tubular lumen by a counter-transport mechanism
in exchange for sodium, which is reabsorbed. The
HCO3– is transported across the basolateral membrane, along with the reabsorbed Na+, into the interstitial fluid and is taken up by the peritubular
capillaries. Thus, for each molecule of glutamine
metabolized in the proximal tubules, two NH4+ are
secreted into the urine and two HCO3– are reabsorbed
into the blood. The HCO3– generated by this process
constitutes new bicarbonate.
In the collecting tubules, the addition of NH4+ to the
tubular fluids occurs through a different mechanism
(Figure 30–9). Here, H+ is secreted by the tubular
394
Unit V
Renal
interstitial
fluid
Glutamine
The Body Fluids and Kidneys
Proximal
tubular cells
Tubular
lumen
Glutamine
Glutamine
Cl-
2HCO3-
2NH4+
NH4+ + Cl-
NH4+
Na+
Na+
An increase in extracellular fluid H+ concentration
stimulates renal glutamine metabolism and, therefore,
increases the formation of NH4+ and new HCO3– to be
used in H+ buffering; a decrease in H+ concentration
has the opposite effect.
Under normal conditions, the amount of H+ eliminated by the ammonia buffer system accounts for
about 50 per cent of the acid excreted and 50 per cent
of the new HCO3– generated by the kidneys. However,
with chronic acidosis, the rate of NH4+ excretion can
increase to as much as 500 mEq/day. Therefore, with
chronic acidosis, the dominant mechanism by which
acid is eliminated is excretion of NH4+. This also provides the most important mechanism for generating
new bicarbonate during chronic acidosis.
Figure 30–8
Production and secretion of ammonium ion (NH4+) by proximal
tubular cells. Glutamine is metabolized in the cell, yielding NH4+
and bicarbonate. The NH4+ is secreted into the lumen by a
sodium-NH4+ pump. For each glutamine molecule metabolized,
two NH4+ are produced and secreted and two HCO3– are returned
to the blood.
Renal
interstitial
fluid
Tubular
lumen
Collecting
tubular cells
Na+
ATP
ClHCO3- + H+
H2CO3
CO2
NH3
NH3
K+
H2O
+
CO2
Carbonic
anhydrase
ATP
H+
NH4+ + Cl-
Figure 30–9
Buffering of hydrogen ion secretion by ammonia (NH3) in the collecting tubules. Ammonia diffuses into the tubular lumen, where it
reacts with secreted hydrogen ions to form NH4+, which is then
excreted. For each NH4+ excreted, a new HCO3– is formed in the
tubular cells and returned to the blood.
membrane into the lumen, where it combines with
NH3 to form NH4+, which is then excreted. The collecting ducts are permeable to NH3, which can easily
diffuse into the tubular lumen. However, the luminal
membrane of this part of the tubules is much less permeable to NH4+; therefore, once the H+ has reacted
with NH3 to form NH4+, the NH4+ is trapped in the
tubular lumen and eliminated in the urine. For each
NH4+ excreted, a new HCO3- is generated and added to
the blood.
Chronic Acidosis Increases NH4+ Excretion. One of the most
important features of the renal ammonium-ammonia
buffer system is that it is subject to physiologic control.
Quantifying Renal
Acid-Base Excretion
Based on the principles discussed earlier, we can quantitate the kidneys’ net excretion of acid or net addition
or elimination of bicarbonate from the blood as
follows.
Bicarbonate excretion is calculated as the urine flow
rate multiplied by urinary bicarbonate concentration.
This number indicates how rapidly the kidneys are
removing HCO3– from the blood (which is the same as
adding an H+ to the blood). In alkalosis, the loss of
HCO3– helps return the plasma pH toward normal.
The amount of new bicarbonate contributed to
the blood at any given time is equal to the amount of
H+ secreted that ends up in the tubular lumen with
nonbicarbonate urinary buffers. As discussed previously, the primary sources of nonbicarbonate urinary
buffers are NH4+ and phosphate. Therefore, the
amount of HCO3– added to the blood (and H+ excreted
by NH4+) is calculated by measuring NH4+
excretion (urine flow rate multiplied by urinary NH4+
concentration).
The rest of the nonbicarbonate, non-NH4+ buffer
excreted in the urine is measured by determining a
value known as titratable acid. The amount of titratable acid in the urine is measured by titrating the urine
with a strong base, such as NaOH, to a pH of 7.4, the
pH of normal plasma, and the pH of the glomerular
filtrate. This titration reverses the events that occurred
in the tubular lumen when the tubular fluid was
titrated by excreted H+. Therefore, the number of milliequivalents of NaOH required to return the urinary
pH to 7.4 equals the number of milliequivalents of H+
added to the tubular fluid that combined with phosphate and other organic buffers. The titratable acid
measurement does not include H+ in association with
NH4+, because the pK of the ammonia-ammonium
reaction is 9.2, and titration with NaOH to a pH of 7.4
does not remove the H+ from NH4+.
Thus, the net acid excretion by the kidneys can be
assessed as
Net acid excretion = NH4+ excretion + Urinary
titratable acid – Bicarbonate excretion
Chapter 30
395
Regulation of Acid-Base Balance
The reason we subtract bicarbonate excretion is that
the loss of HCO3– is the same as the addition of H+ to
the blood. To maintain acid-base balance, the net acid
excretion must equal the nonvolatile acid production
in the body. In acidosis, the net acid excretion increases
markedly, especially because of increased NH4+ excretion, thereby removing acid from the blood. The net
acid excretion also equals the rate of net HCO3– addition to the blood. Therefore, in acidosis, there is a net
addition of HCO3– back to the blood as more NH4+ and
urinary titratable acid are excreted.
In alkalosis, titratable acid and NH4+ excretion drop
to 0, whereas HCO3– excretion increases. Therefore, in
alkalosis, there is a negative net acid secretion. This
means that there is a net loss of HCO3– from the blood
(which is the same as adding H+ to the blood) and that
no new HCO3– is generated by the kidneys.
Regulation of Renal Tubular Hydrogen
Ion Secretion
As discussed earlier, H+ secretion by the tubular
epithelium is necessary for both HCO3– reabsorption
and generation of new HCO3– associated with titratable acid formation. Therefore, the rate of H+ secretion must be carefully regulated if the kidneys are to
effectively perform their functions in acid-base homeostasis. Under normal conditions, the kidney tubules
must secrete at least enough H+ to reabsorb almost all
the HCO3– that is filtered, and there must be enough
H+ left over to be excreted as titratable acid or NH4+
to rid the body of the nonvolatile acids produced each
day from metabolism.
In alkalosis, tubular secretion of H+ must be reduced
to a level that is too low to achieve complete HCO3–
reabsorption, enabling the kidneys to increase HCO3–
excretion. In this condition, titratable acid and
ammonia are not excreted because there is no excess
H+ available to combine with nonbicarbonate buffers;
therefore, there is no new HCO3– added to the
urine in alkalosis. During acidosis, the tubular H+
secretion must be increased sufficiently to reabsorb
all the filtered HCO3– and still have enough H+ left
over to excrete large amounts of NH4+ and titratable
acid, thereby contributing large amounts of new
HCO3– to the total body extracellular fluid. The
most important stimuli for increasing H+ secretion by
the tubules in acidosis are (1) an increase in PCO2
of the extracellular fluid and (2) an increase in H+
concentration of the extracellular fluid (decreased
pH).
The tubular cells respond directly to an increase in
PCO2 of the blood, as occurs in respiratory acidosis,
with an increase in the rate of H+ secretion as follows:
The increased PCO2 raises the PCO2 of the tubular cells,
causing increased formation of H+ in the tubular
cells, which in turn stimulates the secretion of H+. The
second factor that stimulates H+ secretion is an
increase in extracellular fluid H+ concentration
(decreased pH).
Table 30–2
Factors That Increase or Decrease H+ Secretion and HCO3Reabsorption by the Renal Tubules
Increase H+ Secretion and
HCO3– Reabsorption
Decrease H+ Secretion and
HCO3– Reabsorption
≠ PCO2
Ø PCO2
≠ H+, Ø HCO3–
Ø H+, ≠ HCO3–
Ø Extracellular fluid volume
≠ Extracellular fluid volume
≠ Angiotensin II
Ø Angiotensin II
≠ Aldosterone
Ø Aldosterone
Hypokalemia
Hyperkalemia
A special factor that can increase H+ secretion under
some pathophysiologic conditions is excessive aldosterone secretion. Aldosterone stimulates the secretion
of H+ by the intercalated cells of the collecting duct.
Therefore, oversecretion of aldosterone, as occurs in
Conn’s syndrome, can cause excessive secretion of H+
into the tubular fluid and, consequently, increased
amounts of bicarbonate added back to the blood. This
usually causes alkalosis in patients with excessive
aldosterone secretion.
The tubular cells usually respond to a decrease in H+
concentration (alkalosis) by reducing H+ secretion.
The decreased H+ secretion results from decreased
extracellular PCO2, as occurs in respiratory alkalosis, or
from a decrease in H+ concentration per se, as occurs
in both respiratory and metabolic alkalosis.
Table 30–2 summarizes the major factors that influence H+ secretion and HCO3– reabsorption. Some of
these are not directly related to the regulation of acidbase balance. For example, H+ secretion is coupled
to Na+ reabsorption by the Na+-H+ exchanger in the
proximal tubule and thick ascending loop of Henle.
Therefore, factors that stimulate Na+ reabsorption,
such as decreased extracellular fluid volume, may also
secondarily increase H+ secretion.
Extracellular fluid volume depletion stimulates
sodium reabsorption by the renal tubules and
increases H+ secretion and HCO3– reabsorption
through multiple mechanisms, including (1) increased
angiotensin II levels, which directly stimulate the activity of the Na+-H+ exchanger in the renal tubules, and
(2) increased aldosterone levels, which stimulate H+
secretion by the intercalated cells of the cortical collecting tubules. Therefore, extracellular fluid volume
depletion tends to cause alkalosis due to excess H+
secretion and HCO3– reabsorption.
Changes in plasma potassium concentration can
also influence H+ secretion, with hypokalemia stimulating and hyperkalemia inhibiting H+ secretion in the
proximal tubule. A decreased plasma potassium concentration tends to increase the H+ concentration in
the renal tubular cells. This, in turn, stimulates H+
secretion and HCO3– reabsorption and leads to alkalosis. Hyperkalemia decreases H+ secretion and HCO3–
reabsorption and tends to cause acidosis.
396
Unit V
The Body Fluids and Kidneys
Renal Correction of Acidosis—
Increased Excretion of
Hydrogen Ions and Addition
of Bicarbonate Ions to the
Extracellular Fluid
Now that we have described the mechanisms by which
the kidneys secrete H+ and reabsorb HCO3–, we can
explain how the kidneys readjust the pH of the extracellular fluid when it becomes abnormal.
Referring to equation 8, the HendersonHasselbalch equation, we can see that acidosis occurs
when the ratio of HCO3– to CO2 in the extracellular
fluid decreases, thereby decreasing pH. If this ratio
decreases because of a fall in HCO3–, the acidosis is
referred to as metabolic acidosis. If the pH falls
because of an increase in PCO2, the acidosis is referred
to as respiratory acidosis.
Acidosis Decreases the Ratio of
HCO3-/H+ in Renal Tubular Fluid
Both respiratory and metabolic acidosis cause a
decrease in the ratio of HCO3– to H+ in the renal
tubular fluid. As a result, there is an excess of H+ in the
renal tubules, causing complete reabsorption of HCO3–
and still leaving additional H+ available to combine
with the urinary buffers NH4+ and HPO4=. Thus, in acidosis, the kidneys reabsorb all the filtered HCO3– and
contribute new HCO3– through the formation of NH4+
and titratable acid.
In metabolic acidosis, an excess of H + over HCO3–
occurs in the tubular fluid primarily because of
decreased filtration of HCO3-. This decreased filtration
of HCO3– is caused mainly by a decrease in the extracellular fluid concentration of HCO3–.
In respiratory acidosis, the excess H+ in the tubular
fluid is due mainly to the rise in extracellular fluid
PCO2, which stimulates H+ secretion.
As discussed previously, with chronic acidosis,
regardless of whether it is respiratory or metabolic,
there is an increase in the production of NH4+, which
further contributes to the excretion of H+ and the addition of new HCO3– to the extracellular fluid. With
severe chronic acidosis, as much as 500 mEq/day of H+
can be excreted in the urine, mainly in the form of
NH4+; this, in turn, contributes up to 500 mEq/day of
new HCO3– that is added to the blood.
Thus, with chronic acidosis, the increased secretion
of H+ by the tubules helps eliminate excess H+ from
the body and increases the quantity of HCO3– in the
extracellular fluid. This increases the bicarbonate part
of the bicarbonate buffer system, which, in accordance
with the Henderson-Hasselbalch equation, helps raise
the extracellular pH and corrects the acidosis. If the
acidosis is metabolically mediated, additional compensation by the lungs causes a reduction in PCO2, also
helping to correct the acidosis.
Table 30–3 summarizes the characteristics associated with respiratory and metabolic acidosis as well as
Table 30–3
Characteristics of Primary Acid-Base Disturbances
pH
H+
PCO2
HCO3–
Normal
7.4
40 mEq/L
40 mm Hg
24 mEq/L
Respiratory acidosis
Ø
≠
≠≠
≠
Respiratory alkalosis
≠
Ø
ØØ
Ø
Metabolic acidosis
Ø
≠
Ø
ØØ
Metabolic alkalosis
≠
Ø
≠
≠≠
The primary event is indicated by the double arrows (≠≠ or ØØ). Note that
respiratory acid-base disorders are initiated by an increase or decrease in
PCO2, whereas metabolic disorders are initiated by an increase or decrease in
HCO3–.
respiratory and metabolic alkalosis, which are discussed in the next section. Note that in respiratory
acidosis, there is a reduction in pH, an increase in
extracellular fluid H+ concentration, and an increase in
PCO2, which is the initial cause of the acidosis. The
compensatory response is an increase in plasma
HCO3-, caused by the addition of new bicarbonate to
the extracellular fluid by the kidneys. The rise in HCO3–
helps offset the increase in PCO2, thereby returning the
plasma pH toward normal.
In metabolic acidosis, there is also a decrease in pH
and a rise in extracellular fluid H+ concentration.
However, in this case, the primary abnormality is a
decrease in plasma HCO3–. The primary compensations include increased ventilation rate, which reduces
PCO2 , and renal compensation, which, by adding new
bicarbonate to the extracellular fluid, helps minimize
the initial fall in extracellular HCO3- concentration.
Renal Correction of
Alkalosis—Decreased Tubular
Secretion of Hydrogen Ions
and Increased Excretion
of Bicarbonate Ions
The compensatory responses to alkalosis are basically
opposite to those that occur in acidosis. In alkalosis,
the ratio of HCO3– to CO2 in the extracellular fluid
increases, causing a rise in pH (a decrease in H+
concentration), as is evident from the HendersonHasselbalch equation.
Alkalosis Increases the Ratio of
HCO3-/H+ in Renal Tubular Fluid
Regardless of whether the alkalosis is caused by metabolic or respiratory abnormalities, there is still an
increase in the ratio of HCO3– to H+ in the renal
tubular fluid. The net effect of this is an excess of
HCO3– that cannot be reabsorbed from the tubules
and is, therefore, excreted in the urine. Thus, in alkalosis, HCO3– is removed from the extracellular fluid by
Chapter 30
Regulation of Acid-Base Balance
renal excretion, which has the same effect as adding an
H+ to the extracellular fluid. This helps return the H+
concentration and pH back toward normal.
Table 30–3 shows the overall characteristics of respiratory and metabolic alkalosis. In respiratory alkalosis, there is an increase in extracellular fluid pH and a
decrease in H+ concentration. The cause of the alkalosis is a decrease in plasma PCO2 , caused by hyperventilation. The reduction in PCO2 then leads to a decrease
in the rate of H+ secretion by the renal tubules. The
decrease in H+ secretion reduces the amount of H+ in
the renal tubular fluid. Consequently, there is not
enough H+ to react with all the HCO3– that is filtered.
Therefore, the HCO3– that cannot react with H+ is not
reabsorbed and is excreted in the urine. This results in
a decrease in plasma HCO3– concentration and correction of the alkalosis. Therefore, the compensatory
response to a primary reduction in PCO2 in respiratory
alkalosis is a reduction in plasma HCO3- concentration,
caused by increased renal excretion of HCO3-.
In metabolic alkalosis, there is also an increase in
plasma pH and a decrease in H+ concentration.
The cause of metabolic alkalosis, however, is a rise in
the extracellular fluid HCO3– concentration. This is
partly compensated for by a reduction in the respiration rate, which increases PCO2 and helps return the
extracellular fluid pH toward normal. In addition, the
increase in HCO3– concentration in the extracellular
fluid leads to an increase in the filtered load of HCO3–,
which in turn causes an excess of HCO3– over H+
secreted in the renal tubular fluid. The excess HCO3–
in the tubular fluid fails to be reabsorbed because
there is no H+ to react with, and it is excreted in the
urine. In metabolic alkalosis, the primary compensations are decreased ventilation, which raises PCO2 , and
increased renal HCO3- excretion, which helps compensate for the initial rise in extracellular fluid HCO3concentration.
Clinical Causes of
Acid-Base Disorders
Respiratory Acidosis Is Caused
by Decreased Ventilation and
Increased PCO2
From the previous discussion, it is obvious that any
factor that decreases the rate of pulmonary ventilation
also increases the PCO2 of extracellular fluid. This causes
an increase in H2CO3 and H+ concentration, thus resulting in acidosis. Because the acidosis is caused by
an abnormality in respiration, it is called respiratory
acidosis.
Respiratory acidosis can occur from pathological
conditions that damage the respiratory centers or
that decrease the ability of the lungs to eliminate CO2.
For example, damage to the respiratory center in the
medulla oblongata can lead to respiratory acidosis.Also,
obstruction of the passageways of the respiratory tract,
pneumonia, emphysema, or decreased pulmonary membrane surface area, as well as any factor that interferes
with the exchange of gases between the blood and the
alveolar air, can cause respiratory acidosis.
397
In respiratory acidosis, the compensatory responses
available are (1) the buffers of the body fluids and (2)
the kidneys, which require several days to compensate
for the disorder.
Respiratory Alkalosis Results
from Increased Ventilation and
Decreased PCO2
Respiratory alkalosis is caused by overventilation by
the lungs. Rarely does this occur because of physical
pathological conditions. However, a psychoneurosis can
occasionally cause overbreathing to the extent that a
person becomes alkalotic.
A physiologic type of respiratory alkalosis occurs
when a person ascends to high altitude. The low oxygen
content of the air stimulates respiration, which causes
excess loss of CO2 and development of mild respiratory
alkalosis. Again, the major means for compensation are
the chemical buffers of the body fluids and the ability
of the kidneys to increase HCO3– excretion.
Metabolic Acidosis Results from
Decreased Extracellular Fluid
Bicarbonate Concentration
The term metabolic acidosis refers to all other types of
acidosis besides those caused by excess CO2 in the body
fluids. Metabolic acidosis can result from several general
causes: (1) failure of the kidneys to excrete metabolic
acids normally formed in the body, (2) formation of
excess quantities of metabolic acids in the body, (3)
addition of metabolic acids to the body by ingestion or
infusion of acids, and (4) loss of base from the body
fluids, which has the same effect as adding an acid to the
body fluids. Some specific conditions that cause metabolic acidosis are the following.
Renal Tubular Acidosis. This type of acidosis results from
a defect in renal secretion of H+ or in reabsorption of
HCO3–, or both. These disorders are generally of two
types: (1) impairment of renal tubular HCO3– reabsorption, causing loss of HCO3– in the urine, or (2)
inability of the renal tubular H+ secretory mechanism to
establish a normal acidic urine, causing the excretion of
an alkaline urine. In these cases, inadequate amounts of
titratable acid and NH4+ are excreted, so that there is
net accumulation of acid in the body fluids. Some causes
of renal tubular acidosis include chronic renal failure,
insufficient aldosterone secretion (Addison’s disease),
and several hereditary and acquired disorders that
impair tubular function, such as Fanconi’s syndrome.
Diarrhea. Severe diarrhea is probably the most frequent
cause of metabolic acidosis. The cause of this acidosis is
the loss of large amounts of sodium bicarbonate into the
feces. The gastrointestinal secretions normally contain
large amounts of bicarbonate, and diarrhea results in
the loss of HCO3– from the body, which has the same
effect as losing large amounts of bicarbonate in the
urine. This form of metabolic acidosis is particularly
serious and can cause death, especially in young
children.
Vomiting of Intestinal Contents. Vomiting of gastric contents alone would cause loss of acid and a tendency
398
Unit V
The Body Fluids and Kidneys
toward alkalosis because the stomach secretions are
highly acidic. However, vomiting large amounts from
deeper in the gastrointestinal tract, which sometimes
occurs, causes loss of bicarbonate and results in metabolic acidosis in the same way that diarrhea causes
acidosis.
Diabetes Mellitus. Diabetes mellitus is caused by lack of
insulin secretion by the pancreas (type I diabetes) or
by insufficient insulin secretion to compensate for
decreased sensitivity to the effects of insulin (type II
diabetes). In the absence of sufficient insulin, the normal
use of glucose for metabolism is prevented. Instead,
some of the fats are split into acetoacetic acid, and this
is metabolized by the tissues for energy in place of
glucose.With severe diabetes mellitus, blood acetoacetic
acid levels can rise very high, causing severe metabolic
acidosis. In an attempt to compensate for this acidosis,
large amounts of acid are excreted in the urine, sometimes as much as 500 mmol/day.
Ingestion of Acids. Rarely are large amounts of acids
ingested in normal foods. However, severe metabolic
acidosis occasionally results from the ingestion of
certain acidic poisons. Some of these include acetylsalicylics (aspirin) and methyl alcohol (which forms formic
acid when it is metabolized).
Chronic Renal Failure. When kidney function declines
markedly, there is a buildup of the anions of weak acids
in the body fluids that are not being excreted by the
kidneys. In addition, the decreased glomerular filtration
rate reduces the excretion of phosphates and NH4+,
which reduces the amount of bicarbonate added back
to the body fluids. Thus, chronic renal failure can be
associated with severe metabolic acidosis.
Metabolic Alkalosis Is Caused
by Increased Extracellular Fluid
Bicarbonate Concentration
When there is excess retention of HCO3– or loss of H+
from the body, this results in metabolic alkalosis. Metabolic alkalosis is not nearly as common as metabolic acidosis, but some of the causes of metabolic alkalosis are
as follows.
Administration of Diuretics (Except the Carbonic Anhydrase
Inhibitors). All diuretics cause increased flow of fluid
along the tubules, usually causing increased flow in the
distal and collecting tubules. This leads to increased
reabsorption of Na+ from these parts of the nephrons.
Because the sodium reabsorption here is coupled with
H+ secretion, the enhanced sodium reabsorption also
leads to an increase in H+ secretion and an increase in
bicarbonate reabsorption. These changes lead to the
development of alkalosis, characterized by increased
extracellular fluid bicarbonate concentration.
Excess Aldosterone. When large amounts of aldosterone
are secreted by the adrenal glands, a mild metabolic
alkalosis develops. As discussed previously, aldosterone
promotes extensive reabsorption of Na+ from the distal
and collecting tubules and at the same time stimulates
the secretion of H+ by the intercalated cells of the collecting tubules. This increased secretion of H+ leads to
its increased excretion by the kidneys and, therefore,
metabolic alkalosis.
Vomiting of Gastric Contents. Vomiting of the gastric con-
tents alone, without vomiting of the lower gastrointestinal contents, causes loss of the HCl secreted by the
stomach mucosa. The net result is a loss of acid from the
extracellular fluid and development of metabolic alkalosis. This type of alkalosis occurs especially in neonates
who have pyloric obstruction caused by hypertrophied
pyloric sphincter muscles.
Ingestion of Alkaline Drugs. A common cause of metabolic
alkalosis is ingestion of alkaline drugs, such as sodium
bicarbonate, for the treatment of gastritis or peptic
ulcer.
Treatment of Acidosis
or Alkalosis
The best treatment for acidosis or alkalosis is to correct
the condition that caused the abnormality. This is often
difficult, especially in chronic diseases that cause
impaired lung function or kidney failure. In these circumstances, various agents can be used to neutralize the
excess acid or base in the extracellular fluid.
To neutralize excess acid, large amounts of sodium
bicarbonate can be ingested by mouth. The sodium
bicarbonate is absorbed from the gastrointestinal tract
into the blood and increases the bicarbonate portion of
the bicarbonate buffer system, thereby increasing pH
toward normal. Sodium bicarbonate can also be infused
intravenously, but because of the potentially dangerous
physiologic effects of such treatment, other substances
are often used instead, such as sodium lactate and
sodium gluconate. The lactate and gluconate portions of
the molecules are metabolized in the body, leaving the
sodium in the extracellular fluid in the form of sodium
bicarbonate and thereby increasing the pH of the fluid
toward normal.
For the treatment of alkalosis, ammonium chloride
can be administered by mouth. When the ammonium
chloride is absorbed into the blood, the ammonia
portion is converted by the liver into urea. This
reaction liberates HCl, which immediately reacts with
the buffers of the body fluids to shift the H+ concentration in the acidic direction. Ammonium chloride
occasionally is infused intravenously, but NH4+ is
highly toxic, and this procedure can be dangerous. Another substance used occasionally is lysine
monohydrochloride.
Clinical Measurements and
Analysis of Acid-Base
Disorders
Appropriate therapy of acid-base disorders requires
proper diagnosis. The simple acid-base disorders
described previously can be diagnosed by analyzing
three measurements from an arterial blood sample: pH,
plasma bicarbonate concentration, and PCO2.
The diagnosis of simple acid-base disorders involves
several steps, as shown in Figure 30–10. By examining the pH, one can determine whether the disorder
is acidosis or alkalosis. A pH less than 7.4 indicates
acidosis, whereas a pH greater than 7.4 indicates
alkalosis.
Chapter 30
Regulation of Acid-Base Balance
Complex Acid-Base Disorders and
Use of the Acid-Base Nomogram
for Diagnosis
Arterial blood sample
<7.4
pH?
>7.4
Acidosis
-
HCO3
<24 mEq/L
Pco2
>40 mm Hg
Alkalosis
-
HCO3
>24 mEq/L
Pco2
<40 mm Hg
Respiratory
Metabolic
Respiratory
Respiratory
compensation
Renal
compensation
Respiratory
compensation
Renal
compensation
Pco2
<40 mm Hg
HCO3>24 mEq/L
Pco2
>40 mm Hg
HCO3<24 mEq/L
Metabolic
399
Figure 30–10
Analysis of simple acid-base disorders. If the compensatory
responses are markedly different from those shown at the bottom
of the figure, one should suspect a mixed acid-base disorder.
The second step is to examine the plasma PCO2 and
HCO3– concentration. The normal value for PCO2 is
about 40 mm Hg, and for HCO3–, it is 24 mEq/L. If the
disorder has been characterized as acidosis and the
plasma PCO2 is increased, there must be a respiratory
component to the acidosis. After renal compensation,
the plasma HCO3– concentration in respiratory acidosis
would tend to increase above normal. Therefore, the
expected values for a simple respiratory acidosis would
be reduced plasma pH, increased PCO2 , and increased
plasma HCO3– concentration after partial renal
compensation.
For metabolic acidosis, there would also be a decrease
in plasma pH. However, with metabolic acidosis, the
primary abnormality is a decrease in plasma HCO3–
concentration. Therefore, if a low pH is associated with
a low HCO3– concentration, there must be a metabolic
component to the acidosis. In simple metabolic acidosis,
the PCO2 is reduced because of partial respiratory compensation, in contrast to respiratory acidosis, in which
PCO2 is increased. Therefore, in simple metabolic acidosis, one would expect to find a low pH, a low plasma
HCO3- concentration, and a reduction in PCO2 after
partial respiratory compensation.
The procedures for categorizing the types of alkalosis involve the same basic steps. First, alkalosis implies
that there is an increase in plasma pH. If the increase in
pH is associated with decreased PCO2, there must be a
respiratory component to the alkalosis. If the rise in pH
is associated with increased HCO3–, there must be a
metabolic component to the alkalosis. Therefore, in
simple respiratory alkalosis, one would expect to find
increased pH, decreased PCO2 , and decreased HCO3concentration in the plasma. In simple metabolic alkalosis, one would expect to find increased pH, increased
plasma HCO3-, and increased PCO2.
In some instances, acid-base disorders are not accompanied by appropriate compensatory responses. When
this occurs, the abnormality is referred to as a mixed
acid-base disorder. This means that there are two or
more underlying causes for the acid-base disturbance.
For example, a patient with low pH would be categorized as acidotic. If the disorder was metabolically mediated, this would also be accompanied by a low plasma
HCO3– concentration and, after appropriate respiratory
compensation, a low PCO2. However, if the low plasma
pH and low HCO3– concentration are associated with
elevated PCO2, one would suspect a respiratory component to the acidosis as well as a metabolic component.
Therefore, this disorder would be categorized as a
mixed acidosis. This could occur, for example, in a
patient with acute HCO3– loss from the gastrointestinal
tract because of diarrhea (metabolic acidosis) who also
has emphysema (respiratory acidosis).
A convenient way to diagnose acid-base disorders is
to use an acid-base nomogram, as shown in Figure
30–11. This diagram can be used to determine the type
of acidosis or alkalosis, as well as its severity. In this
acid-base diagram, pH, HCO3– concentration, and PCO2
values intersect according to the Henderson-Hasselbalch equation. The central open circle shows normal
values and the deviations that can still be considered
within the normal range. The shaded areas of the
diagram show the 95 per cent confidence limits for the
normal compensations to simple metabolic and respiratory disorders.
When using this diagram, one must assume that sufficient time has elapsed for a full compensatory
response, which is 6 to 12 hours for the ventilatory compensations in primary metabolic disorders and 3 to 5
days for the metabolic compensations in primary respiratory disorders. If a value is within the shaded area, this
suggests that there is a simple acid-base disturbance.
Conversely, if the values for pH, bicarbonate, or PCO2
lie outside the shaded area, this suggests that there may
be a mixed acid-base disorder.
It is important to recognize that an acid-base value
within the shaded area does not always mean that there
is a simple acid-base disorder. With this reservation in
mind, the acid-base diagrams can be used as a quick
means of determining the specific type and severity of
an acid-base disorder.
For example, assume that the arterial plasma from a
patient yields the following values: pH 7.30, plasma
HCO3– concentration 12.0 mEq/L, and plasma PCO2
25 mm Hg. With these values, one can look at the
diagram and find that this represents a simple metabolic
acidosis, with appropriate respiratory compensation
that reduces the PCO2 from its normal value of 40 mm
Hg to 25 mm Hg.
A second example would be a patient with the following values: pH 7.15, plasma HCO3– concentration
7 mEq/L, and plasma PCO2 50 mm Hg. In this example,
the patient is acidotic, and there appears to be a metabolic component because the plasma HCO3– concentration is lower than the normal value of 24 mEq/L.
However, the respiratory compensation that would
normally reduce PCO2 is absent, and PCO2 is slightly
increased above the normal value of 40 mm Hg. This is
400
Unit V
60
The Body Fluids and Kidneys
120 100 90 80 70
110
56
60
50
Arterial plasma [HCO3-] (mEq/L)
52
48
35
Pco2 (mm Hg)
44
32
Metabolic
alkalosis
Chronic
respiratory
acidosis
40
36
Acute
respiratory
acidosis
30
25
20
28
Normal
24
20
16
Acute
respiratory
alkalosis
15
10
12
Metabolic
acidosis
8
4
0
7.00
40
7.10
7.20
Chronic
respiratory
alkalosis
7.30
7.40
7.50
Arterial blood pH
consistent with a mixed acid-base disturbance consisting of metabolic acidosis as well as a respiratory
component.
The acid-base diagram serves as a quick way to assess
the type and severity of disorders that may be contributing to abnormal pH, PCO2 , and plasma bicarbonate concentrations. In a clinical setting, the patient’s
history and other physical findings also provide important clues concerning causes and treatment of the acidbase disorders.
Use of Anion Gap to Diagnose
Acid-Base Disorders
The concentrations of anions and cations in plasma
must be equal to maintain electrical neutrality. Therefore, there is no real “anion gap” in the plasma.
However, only certain cations and anions are routinely
measured in the clinical laboratory. The cation normally
measured is Na+, and the anions are usually Cl– and
HCO3–. The “anion gap” (which is only a diagnostic
concept) is the difference between unmeasured anions
and unmeasured cations, and is estimated as
Plasma anion gap = [Na+] – [HCO3–] – [Cl–]
= 144 – 24 – 108 = 10 mEq/L
The anion gap will increase if unmeasured anions rise
or if unmeasured cations fall. The most important
unmeasured cations include calcium, magnesium, and
potassium, and the major unmeasured anions are
albumin, phosphate, sulfate, and other organic anions.
Usually the unmeasured anions exceed the unmeasured
cations, and the anion gap ranges between 8 and
16 mEq/L.
The plasma anion gap is used mainly in diagnosing
different causes of metabolic acidosis. In metabolic aci-
Pco2 (mm Hg)
7.60
7.70
7.80
Figure 30–11
Acid-base nomogram showing
arterial blood pH, arterial plasma
HCO3–, and PCO2 values. The
central open circle shows the
approximate limits for acid-base
status in normal people. The
shaded areas in the nomogram
show the approximate limits
for the normal compensations
caused by simple metabolic and
respiratory disorders. For values
lying outside the shaded areas,
one should suspect a mixed acidbase disorder. (Adapted from
Cogan MG, Rector FC Jr: AcidBase Disorders in the Kidney, 3rd
ed. Philadelphia: WB Saunders,
1986.)
Table 30–4
Metabolic Acidosis Associated with Normal or Increased
Plasma Anion Gap
Increased Anion Gap
(Normochloremia)
Normal Anion Gap
(Hyperchloremia)
Diabetes mellitus (ketoacidosis)
Lactic acidosis
Chronic renal failure
Aspirin (acetylsalicylic acid)
poisoning
Methanol poisoning
Ethylene glycol poisoning
Starvation
Diarrhea
Renal tubular acidosis
Carbonic anhydrase inhibitors
Addison’s disease
dosis, the plasma HCO3– is reduced. If the plasma
sodium concentration is unchanged, the concentration
of anions (either Cl– or an unmeasured anion) must
increase to maintain electroneutrality. If plasma Cl–
increases in proportion to the fall in plasma HCO3–, the
anion gap will remain normal, and this is often referred
to as hyperchloremic metabolic acidosis.
If the decrease in plasma HCO3– is not accompanied
by increased Cl–, there must be increased levels of
unmeasured anions and therefore an increase in the calculated anion gap. Metabolic acidosis caused by excess
nonvolatile acids (besides HCl), such as lactic acid or
ketoacids, is associated with an increased plasma anion
gap because the fall in HCO3– is not matched by an
equal increase in Cl–. Some examples of metabolic acidosis associated with a normal or increased anion gap
are shown in Table 30–4. By calculating the anion gap,
one can narrow some of the potential causes of metabolic acidosis.
Chapter 30
Regulation of Acid-Base Balance
References
Alpern RJ: Renal acidification mechanisms. In Brenner BM
(ed): The Kidney, 6th ed. Philadelphia: WB Saunders, 2000,
pp 455-519.
Capasso G, Unwin R, Rizzo M, et al: Bicarbonate transport
along the loop of Henle: molecular mechanisms and regulation. J Nephrol 15(Suppl 5):S88, 2002.
Decoursey TE: Voltage-gated proton channels and other
proton transfer pathways. Physiol Rev 83:475, 2003.
Gennari FJ, Maddox DA: Renal regulation of acid-base
homeostasis. In Seldin DW, Giebisch G (eds): The
Kidney—Physiology and Pathophysiology, 3rd ed. New
York: Raven Press, 2000, pp 2015-2054.
Good DW: Ammonium transport by the thick ascending
limb of Henle’s loop. Ann Rev Physiol 56:623, 1994.
Igarashi I, Sekine T, Inatomi J, Seki G: Unraveling the molecular pathogenesis of isolated proximal renal tubular acidosis. J Am Soc Nephrol 13:2171, 2002.
401
Karet FE: Inherited distal renal tubular acidosis. J Am Soc
Nephrol 13:2178, 2002.
Laffey JG, Kavanagh BP: Hypocapnia. N Engl J Med 347:43,
2002.
Lemann J Jr, Bushinsky DA, Hamm LL: Bone buffering of
acid and base in humans. Am J Physiol Renal Physiol
285:F811, 2003.
Madias NE, Adrogue HJ: Cross-talk between two organs:
how the kidney responds to disruption of acid-base
balance by the lung. Nephron Physiol 93:61, 2003.
Wagner CA, Geibel JP: Acid-base transport in the collecting
duct. J Nephrol 15(Suppl 5):S112, 2002.
Wesson DE, Alpern RJ, Seldin DW: Clinical syndromes of
metabolic alkalosis. In Seldin DW, Giebisch G (eds): The
Kidney—Physiology and Pathophysiology, 3rd ed. New
York: Raven Press, 2000, pp 2055-2072.
White NH: Management of diabetic ketoacidosis. Rev
Endocr Metab Disord 4:343, 2003.
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3
Kidney Diseases and Diuretics
Diuretics and Their
Mechanisms of Action
A diuretic is a substance that increases the rate of urine
volume output, as the name implies. Most diuretics also
increase urinary excretion of solutes, especially sodium
and chloride. In fact, most diuretics that are used clinically act by decreasing the rate of sodium reabsorption
from the tubules, which causes natriuresis (increased
sodium output), which in turn causes diuresis (increased water output). That is, in
most cases, increased water output occurs secondary to inhibition of tubular
sodium reabsorption, because sodium remaining in the tubules acts osmotically to
decrease water reabsorption. Because the renal tubular reabsorption of many
solutes, such as potassium, chloride, magnesium, and calcium, is also influenced secondarily by sodium reabsorption, many diuretics raise renal output of these solutes
as well.
The most common clinical use of diuretics is to reduce extracellular fluid volume,
especially in diseases associated with edema and hypertension. As discussed in
Chapter 25, loss of sodium from the body mainly decreases extracellular fluid
volume; therefore, diuretics are most often administered in clinical conditions in
which extracellular fluid volume is expanded.
Some diuretics can increase urine output more than 20-fold within a few minutes
after they are administered. However, the effect of most diuretics on renal output
of salt and water subsides within a few days (Figure 31–1). This is due to activation
of other compensatory mechanisms initiated by decreased extracellular fluid
volume. For example, a decrease in extracellular fluid volume often reduces arterial
pressure and glomerular filtration rate (GFR) and increases renin secretion and
angiotensin II formation; all these responses, together, eventually override the
chronic effects of the diuretic on urine output. Thus, in the steady state, urine output
becomes equal to intake, but only after reductions in arterial pressure and extracellular fluid volume have occurred, relieving the hypertension or edema that
prompted the use of diuretics in the first place.
The many diuretics available for clinical use have different mechanisms of action
and, therefore, inhibit tubular reabsorption at different sites along the renal
nephron. The general classes of diuretics and their mechanisms of action are shown
in Table 31–1.
Osmotic Diuretics Decrease Water Reabsorption
by Increasing Osmotic Pressure of Tubular Fluid
Injection into the blood stream of substances that are not easily reabsorbed by the
renal tubules, such as urea, mannitol, and sucrose, causes a marked increase in the
concentration of osmotically active molecules in the tubules. The osmotic pressure
of these solutes then greatly reduces water reabsorption, flushing large amounts of
tubular fluid into the urine.
Large volumes of urine are also formed in certain diseases associated with excess
solutes that fail to be reabsorbed from the tubular fluid. For example, when the
blood glucose concentration rises to high levels in diabetes mellitus, the increased
filtered load of glucose into the tubules exceeds their capacity to reabsorb glucose
(i.e., exceeds their transport maximum for glucose). Above a plasma glucose concentration of about 250 mg/dl, little of the extra glucose is reabsorbed by the tubules;
instead, the excess glucose remains in the tubules, acts as an osmotic diuretic, and
causes rapid loss of fluid into the urine. In patients with diabetes mellitus, the high
402
1
Chapter 31
403
Kidney Diseases and Diuretics
urine output is balanced by a high level of fluid intake
owing to activation of the thirst mechanism.
Extracellular fluid
volume (liters)
Sodium excretion or
sodium intake (mEq/day)
Diuretic therapy
200
“Loop” Diuretics Decrease Active
Sodium-Chloride-Potassium
Reabsorption in the Thick
Ascending Loop of Henle
Excretion
100
Intake
15.0
14.0
13.0
-4
-2
0
2
4
Time (days)
6
8
Figure 31–1
Sodium excretion and extracellular fluid volume during diuretic
administration. The immediate increase in sodium excretion is
accompanied by a decrease in extracellular fluid volume. If
sodium intake is held constant, compensatory mechanisms will
eventually return sodium excretion to equal sodium intake, thus
re-establishing sodium balance.
Furosemide, ethacrynic acid, and bumetanide are powerful diuretics that decrease active reabsorption in the
thick ascending limb of the loop of Henle by blocking
the 1-sodium, 2-chloride, 1-potassium co-transporter
located in the luminal membrane of the epithelial cells.
These diuretics are among the most powerful of the clinically used diuretics.
By blocking active sodium-chloride-potassium cotransport in the luminal membrane of the loop of Henle,
the loop diuretics raise urine output of sodium, chloride,
potassium, and other electrolytes, as well as water, for
two reasons: (1) they greatly increase the quantities of
solutes delivered to the distal parts of the nephrons, and
these act as osmotic agents to prevent water reabsorption as well; and (2) they disrupt the countercurrent
multiplier system by decreasing absorption of ions from
the loop of Henle into the medullary interstitium,
thereby decreasing the osmolarity of the medullary
interstitial fluid. Because of this effect, loop diuretics
impair the ability of the kidneys to either concentrate
or dilute the urine. Urinary dilution is impaired because
the inhibition of sodium and chloride reabsorption in
the loop of Henle causes more of these ions to be
excreted along with increased water excretion. Urinary
concentration is impaired because the renal medullary
interstitial fluid concentration of these ions, and therefore renal medullary osmolarity, is reduced. Consequently, reabsorption of fluid from the collecting ducts
is decreased, so that the maximal concentrating ability
of the kidneys is also greatly reduced. In addition,
decreased renal medullary interstitial fluid osmolarity
reduces absorption of water from the descending loop
of Henle. Because of these multiple effects, 20 to 30 per
cent of the glomerular filtrate may be delivered into
the urine, causing, under acute conditions, urine output
to be as great as 25 times normal for at least a few
minutes.
Table 31–1
Classes of Diuretics, Their Mechanisms of Action, and Tubular Sites of Action
Class of Diuretic
Mechanism of Action
Tubular Site of Action
Osmotic diuretics (mannitol)
Inhibit water and solute reabsorption by increasing
osmolarity of tubular fluid
Inhibit Na+-K+-Cl- co-transport in luminal membrane
Inhibit Na+-Cl- co-transport in luminal membrane
Mainly proximal tubules
Inhibit H+ secretion and HCO3– reabsorption, which
reduces Na+ reabsorption
Inhibit action of aldosterone on tubular receptor,
decrease Na+ reabsorption, and decrease
K+ secretion
Block entry of Na+ into Na+ channels of luminal
membrane, decrease Na+ reabsorption, and
decrease K+ secretion
Proximal tubules
Loop diuretics (furosemide, bumetanide)
Thiazide diuretics (hydrochlorothiazide,
chlorthalidone)
Carbonic anhydrase inhibitors
(acetazolamide)
Aldosterone antagonists
(spironolactone, eplerenone)
Sodium channel blockers
(triamterene, amiloride)
Thick ascending loop of Henle
Early distal tubules
Collecting tubules
Collecting tubules
404
Unit V
The Body Fluids and Kidneys
Thiazide Diuretics Inhibit SodiumChloride Reabsorption in the Early
Distal Tubule
Diuretics That Block Sodium
Channels in the Collecting Tubules
Decrease Sodium Reabsorption
The thiazide derivatives, such as chlorothiazide, act
mainly on the early distal tubules to block the sodiumchloride co-transporter in the luminal membrane of the
tubular cells. Under favorable conditions, these agents
cause 5 to 10 per cent of the glomerular filtrate to pass
into the urine. This is about the same amount of sodium
normally reabsorbed by the distal tubules.
Amiloride and triamterene also inhibit sodium reabsorption and potassium secretion in the collecting
tubules, similar to the effects of spironolactone.
However, at the cellular level, these drugs act directly
to block the entry of sodium into the sodium channels
of the luminal membrane of the collecting tubule
epithelial cells. Because of this decreased sodium entry
into the epithelial cells, there is also decreased sodium
transport across the cells’ basolateral membranes and,
therefore, decreased activity of the sodium-potassiumadenosine triphosphatase pump. This decreased activity
reduces the transport of potassium into the cells and
ultimately decreases the secretion of potassium into the
tubular fluid. For this reason, the sodium channel blockers are also potassium-sparing diuretics and decrease
the urinary excretion rate of potassium.
Carbonic Anhydrase Inhibitors
Block Sodium-Bicarbonate
Reabsorption in the Proximal
Tubules
Acetazolamide inhibits the enzyme carbonic anhydrase,
which is critical for the reabsorption of bicarbonate in
the proximal tubule, as discussed in Chapter 30. Carbonic anhydrase is abundant in the proximal tubule, the
primary site of action of carbonic anhydrase inhibitors.
Some carbonic anhydrase is also present in other
tubular cells, such as in the intercalated cells of the collecting tubule.
Because hydrogen ion secretion and bicarbonate
reabsorption in the proximal tubules are coupled to
sodium reabsorption through the sodium-hydrogen ion
counter-transport mechanism in the luminal membrane,
decreasing bicarbonate reabsorption also reduces
sodium reabsorption. The blockage of sodium and
bicarbonate reabsorption from the tubular fluid causes
these ions to remain in the tubules and act as an osmotic
diuretic. Predictably, a disadvantage of the carbonic
anhydrase inhibitors is that they cause some degree of
acidosis because of the excessive loss of bicarbonate
ions in the urine.
Competitive Inhibitors of
Aldosterone Decrease Sodium
Reabsorption from and Potassium
Secretion into the Cortical
Collecting Tubule
Spironolactone and eplerenone are aldosterone antagonists that compete with aldosterone for receptor sites
in the cortical collecting tubule epithelial cells and,
therefore, can decrease the reabsorption of sodium and
secretion of potassium in this tubular segment.As a consequence, sodium remains in the tubules and acts as an
osmotic diuretic, causing increased excretion of water as
well as sodium. Because these drugs also block the
effect of aldosterone to promote potassium secretion in
the tubules, they decrease the excretion of potassium.
Aldosterone antagonists also cause movement of potassium from the cells to the extracellular fluid. In some
instances, this causes extracellular fluid potassium concentration to increase excessively. For this reason,
spironolactone and other aldosterone inhibitors are
referred to as potassium-sparing diuretics. Many of the
other diuretics cause loss of potassium in the urine, in
contrast to the aldosterone antagonists, which “spare”
the loss of potassium.
Kidney Diseases
Diseases of the kidneys are among the most important
causes of death and disability in many countries
throughout the world. For example, in 2004, more than
20 million adults in the United States were estimated
to have chronic kidney disease.
Severe kidney diseases can be divided into two main
categories: (1) acute renal failure, in which the kidneys
abruptly stop working entirely or almost entirely but
may eventually recover nearly normal function, and
(2) chronic renal failure, in which there is progressive
loss of function of more and more nephrons that gradually decreases overall kidney function. Within these
two general categories, there are many specific kidney
diseases that can affect the kidney blood vessels,
glomeruli, tubules, renal interstitium, and parts of the
urinary tract outside the kidney, including the ureters
and bladder. In this chapter, we discuss specific physiologic abnormalities that occur in a few of the more
important types of kidney diseases.
Acute Renal Failure
The causes of acute renal failure can be divided into
three main categories:
1. Acute renal failure resulting from decreased
blood supply to the kidneys; this condition is
often referred to as prerenal acute renal failure
to reflect the fact that the abnormality occurs
in a system before the kidneys. This can be a
consequence of heart failure with reduced cardiac
output and low blood pressure or conditions
associated with diminished blood volume and low
blood pressure, such as severe hemorrhage.
2. Intrarenal acute renal failure resulting from
abnormalities within the kidney itself, including
those that affect the blood vessels, glomeruli, or
tubules.
3. Postrenal acute renal failure, resulting from
obstruction of the urinary collecting system
Chapter 31
Kidney Diseases and Diuretics
anywhere from the calyces to the outflow from
the bladder. The most common causes of
obstruction of the urinary tract outside the kidney
are kidney stones, caused by precipitation of
calcium, urate, or cystine.
Prerenal Acute Renal Failure
Caused by Decreased Blood Flow
to the Kidney
The kidneys normally receive an abundant blood supply
of about 1100 ml/min, or about 20 to 25 per cent of the
cardiac output.The main purpose of this high blood flow
to the kidneys is to provide enough plasma for the high
rates of glomerular filtration needed for effective regulation of body fluid volumes and solute concentrations.
Therefore, decreased renal blood flow is usually accompanied by decreased GFR and decreased urine output
of water and solutes. Consequently, conditions that
acutely diminish blood flow to the kidneys usually cause
oliguria, which refers to diminished urine output below
the level of intake of water and solutes.This causes accumulation of water and solutes in the body fluids. If renal
blood flow is markedly reduced, total cessation of urine
output can occur, a condition referred to as anuria.
As long as renal blood flow does not fall below about
20 to 25 per cent of normal, acute renal failure can
usually be reversed if the cause of the ischemia is corrected before damage to the renal cells has occurred.
Unlike some tissues, the kidney can endure a relatively
large reduction in blood flow before actual damage to
the renal cells occurs. The reason for this is that as renal
blood flow is reduced, the GFR and the amount of
sodium chloride filtered by the glomeruli (as well as
the filtration rate of water and other electrolytes) are
reduced. This decreases the amount of sodium chloride
that must be reabsorbed by the tubules, which uses most
of the energy and oxygen consumed by the normal
kidney. Therefore, as renal blood flow and GFR fall,
the requirement for renal oxygen consumption is also
reduced. As the GFR approaches zero, oxygen consumption of the kidney approaches the rate that is
required to keep the renal tubular cells alive even when
they are not reabsorbing sodium. When blood flow is
reduced below this basal requirement, which is usually
less than 20 to 25 per cent of the normal renal blood
flow, the renal cells start to become hypoxic, and further
decreases in renal blood flow, if prolonged, will cause
damage or even death of the renal cells, especially the
tubular epithelial cells. If the cause of prerenal acute
renal failure is not corrected and ischemia of the kidney
persists longer than a few hours, this type of renal failure
can evolve into intrarenal acute renal failure, as discussed later. Acute reduction of renal blood flow is a
common cause of acute renal failure in hospitalized
patients. Table 31–2 shows some of the common causes
of decreased renal blood flow and prerenal acute renal
failure.
Intrarenal Acute Renal Failure
Caused by Abnormalities
Within the Kidney
Abnormalities that originate within the kidney and that
abruptly diminish urine output fall into the general category of intrarenal acute renal failure. This category of
405
Table 31–2
Some Causes of Prerenal Acute Renal Failure
Intravascular volume depletion
Hemorrhage (trauma, surgery, postpartum, gastrointestinal)
Diarrhea or vomiting
Burns
Cardiac failure
Myocardial infarction
Valvular damage
Peripheral vasodilation and resultant hypotension
Anaphylactic shock
Anesthesia
Sepsis, severe infections
Primary renal hemodynamic abnormalities
Renal artery stenosis, embolism, or thrombosis of renal artery
or vein
Table 31–3
Some Causes of Intrarenal Acute Renal Failure
Small vessel and/or glomerular injury
Vasculitis (polyarteritis nodosa)
Cholesterol emboli
Malignant hypertension
Acute glomerulonephritis
Tubular epithelial injury (tubular necrosis)
Acute tubular necrosis due to ischemia
Acute tubular necrosis due to toxins (heavy metals, ethylene
glycol, insecticides, poison mushrooms, carbon tetrachloride)
Renal interstitial injury
Acute pyelonephritis
Acute allergic interstitial nephritis
acute renal failure can be further divided into (1)
conditions that injure the glomerular capillaries or
other small renal vessels, (2) conditions that damage the
renal tubular epithelium, and (3) conditions that cause
damage to the renal interstitium. This type of classification refers to the primary site of injury, but because the
renal vasculature and tubular system are functionally
interdependent, damage to the renal blood vessels can
lead to tubular damage, and primary tubular damage
can lead to damage of the renal blood vessels. Causes
of intrarenal acute renal failure are listed in Table 31–3.
Acute Renal Failure Caused by Glomerulonephritis. Acute
glomerulonephritis is a type of intrarenal acute renal
failure usually caused by an abnormal immune reaction
that damages the glomeruli. In about 95 per cent of the
patients with this disease, damage to the glomeruli
occurs 1 to 3 weeks after an infection elsewhere in the
body, usually caused by certain types of group A beta
streptococci. The infection may have been a streptococcal sore throat, streptococcal tonsillitis, or even streptococcal infection of the skin. It is not the infection itself
that damages the kidneys. Instead, over a few weeks, as
antibodies develop against the streptococcal antigen,
the antibodies and antigen react with each other to form
an insoluble immune complex that becomes entrapped
in the glomeruli, especially in the basement membrane
portion of the glomeruli.
Once the immune complex has deposited in the
glomeruli, many of the cells of the glomeruli begin to
proliferate, but mainly the mesangial cells that lie
406
Unit V
The Body Fluids and Kidneys
between the endothelium and the epithelium. In addition, large numbers of white blood cells become
entrapped in the glomeruli. Many of the glomeruli
become blocked by this inflammatory reaction, and
those that are not blocked usually become excessively
permeable, allowing both protein and red blood cells to
leak from the blood of the glomerular capillaries into
the glomerular filtrate. In severe cases, either total or
almost complete renal shutdown occurs.
The acute inflammation of the glomeruli usually subsides in about 2 weeks, and in most patients, the kidneys
return to almost normal function within the next few
weeks to few months. Sometimes, however, many of the
glomeruli are destroyed beyond repair, and in a small
percentage of patients, progressive renal deterioration
continues indefinitely, leading to chronic renal failure, as
described in a subsequent section of this chapter.
acute renal failure even when the kidneys’ blood supply
and other functions are initially normal. If the urine
output of only one kidney is diminished, no major
change in body fluid composition will occur because the
contralateral kidney can increase its urine output sufficiently to maintain relatively normal levels of extracellular electrolytes and solutes as well as normal
extracellular fluid volume. With this type of renal
failure, normal kidney function can be restored if the
basic cause of the problem is corrected within a few
hours. But chronic obstruction of the urinary tract,
lasting for several days or weeks, can lead to irreversible
kidney damage. Some of the causes of postrenal acute
failure include (1) bilateral obstruction of the ureters or
renal pelvises caused by large stones or blood clots, (2)
bladder obstruction, and (3) obstruction of the urethra.
Tubular Necrosis as a Cause of Acute Renal Failure. Another
cause of intrarenal acute renal failure is tubular necrosis, which means destruction of epithelial cells in the
tubules. Some common causes of tubular necrosis are
(1) severe ischemia and inadequate supply of oxygen
and nutrients to the tubular epithelial cells and (2)
poisons, toxins, or medications that destroy the tubular
epithelial cells.
Acute Tubular Necrosis Caused by Severe Renal
Ischemia. Severe ischemia of the kidney can result from
circulatory shock or any other disturbance that severely
impairs the blood supply to the kidney. If the ischemia
is severe enough to seriously impair the delivery of
nutrients and oxygen to the renal tubular epithelial
cells, and if the insult is prolonged, damage or eventual
destruction of the epithelial cells can occur. When this
happens, tubular cells “slough off” and plug many of the
nephrons, so that there is no urine output from the
blocked nephrons; the affected nephrons often fail to
excrete urine even when renal blood flow is restored to
normal, as long as the tubules remain plugged. The most
common causes of ischemic damage to the tubular
epithelium are the prerenal causes of acute renal failure
associated with circulatory shock, as discussed earlier in
this chapter.
Acute Tubular Necrosis Caused by Toxins or Medications.
There is a long list of renal poisons and medications that
can damage the tubular epithelium and cause acute renal
failure. Some of these are carbon tetrachloride, heavy
metals (such as mercury and lead), ethylene glycol (which
is a major component in antifreeze), various insecticides,
various medications (such as tetracyclines) used as
antibiotics, and cis-platinum, which is used in treating
certain cancers. Each of these substances has a specific
toxic action on the renal tubular epithelial cells, causing
death of many of them. As a result, the epithelial cells
slough away from the basement membrane and plug the
tubules. In some instances, the basement membrane also
is destroyed. If the basement membrane remains intact,
new tubular epithelial cells can grow along the surface
of the membrane, so that the tubule repairs itself within
10 to 20 days.
Postrenal Acute Renal Failure
Caused by Abnormalities of the
Lower Urinary Tract
Multiple abnormalities in the lower urinary tract can
block or partially block urine flow and therefore lead to
Physiologic Effects of Acute
Renal Failure
A major physiologic effect of acute renal failure is
retention in the blood and extracellular fluid of water,
waste products of metabolism, and electrolytes. This
can lead to water and salt overload, which in turn can
lead to edema and hypertension. Excessive retention
of potassium, however, is often a more serious threat
to patients with acute renal failure, because increases
in plasma potassium concentration (hyperkalemia) to
more than about 8 mEq/L (only twice normal) can be
fatal. Because the kidneys are also unable to excrete
sufficient hydrogen ions, patients with acute renal
failure develop metabolic acidosis, which in itself can
be lethal or can aggravate the hyperkalemia.
In the most severe cases of acute renal failure, complete anuria occurs. The patient will die in 8 to 14 days
unless kidney function is restored or unless an artificial kidney is used to rid the body of the excessive
retained water, electrolytes, and waste products of
metabolism. Other effects of diminished urine output,
as well as treatment with an artificial kidney, are discussed in the next section in relation to chronic renal
failure.
Chronic Renal Failure:
An Irreversible Decrease
in the Number of Functional
Nephrons
Chronic renal failure results from progressive and irreversible loss of large numbers of functioning nephrons.
Serious clinical symptoms often do not occur until the
number of functional nephrons falls to at least 70 to
75 per cent below normal. In fact, relatively normal
blood concentrations of most electrolytes and normal
body fluid volumes can still be maintained until the
number of functioning nephrons decreases below 20
to 25 per cent of normal.
Table 31–4 gives some of the most important causes
of chronic renal failure. In general, chronic renal
Chapter 31
407
Kidney Diseases and Diuretics
Table 31–4
Primary
kidney disease
Some Causes of Chronic Renal Failure
Metabolic disorders
Diabetes mellitus
Obesity
Amyloidosis
Hypertension
Renal vascular disorders
Atherosclerosis
Nephrosclerosis-hypertension
Immunologic disorders
Glomerulonephritis
Polyarteritis nodosa
Lupus erythematosus
Infections
Pyelonephritis
Tuberculosis
Primary tubular disorders
Nephrotoxins (analgesics, heavy metals)
Urinary tract obstruction
Renal calculi
Hypertrophy of prostate
Urethral constriction
Congenital disorders
Polycystic disease
Congenital absence of kidney tissue (renal hypoplasia)
failure, like acute renal failure, can occur because of
disorders of the blood vessels, glomeruli, tubules, renal
interstitium, and lower urinary tract. Despite the wide
variety of diseases that can lead to chronic renal
failure, the end result is essentially the same—a
decrease in the number of functional nephrons.
Vicious Circle of Chronic Renal
Failure Leading to End-Stage
Renal Disease
In many cases, an initial insult to the kidney leads to
progressive deterioration of kidney function and
further loss of nephrons to the point where the person
must be placed on dialysis treatment or transplanted
with a functional kidney to survive. This condition is
referred to as end-stage renal disease.
Studies in laboratory animals have shown that surgical removal of large portions of the kidney initially
causes adaptive changes in the remaining nephrons
that lead to increased blood flow, increased GFR, and
increased urine output in the surviving nephrons. The
exact mechanisms responsible for these changes are
not well understood but involve hypertrophy (growth
of the various structures of the surviving nephrons) as
well as functional changes that decrease vascular
resistance and tubular reabsorption in the surviving
nephrons. These adaptive changes permit a person to
excrete normal amounts of water and solutes even
when kidney mass is reduced to 20 to 25 per cent of
normal. Over a period of several years, however, the
renal functional changes may lead to further injury of
the remaining nephrons, particularly to the glomeruli
of these nephrons.
+
Nephron
number
Hypertrophy
and vasodilation
of surviving
nephrons
Glomerular
sclerosis
Arterial
pressure
Glomerular
pressure
and/or
filtration
Figure 31–2
Vicious circle that can occur with primary kidney disease. Loss of
nephrons because of disease may increase pressure and flow in
the surviving glomerular capillaries, which in turn may eventually
injure these “normal” capillaries as well, thus causing progressive
sclerosis and eventual loss of these glomeruli.
The cause of this additional injury is not known, but
some investigators believe that it may be related in
part to increased pressure or stretch of the remaining
glomeruli, which occurs as a result of functional
vasodilation or increased blood pressure; the chronic
increase in pressure and stretch of the small arterioles
and glomeruli are believed to cause sclerosis of these
vessels (replacement of normal tissue with connective
tissue). These sclerotic lesions can eventually obliterate the glomerulus, leading to further reduction in
kidney function, further adaptive changes in the
remaining nephrons, and a slowly progressing vicious
circle that eventually terminates in end-stage renal
disease (Figure 31–2). The only proven method
of slowing down this progressive loss of kidney function is to lower arterial pressure and glomerular
hydrostatic pressure, especially by using drugs such
as angiotensin-converting enzyme inhibitors or
angiotensin II antagonists.
Table 31–5 gives the most common causes of endstage renal disease. In the early 1980s, glomerulonephritis in all its various forms was believed to be
the most common initiating cause of end-stage renal
disease. In recent years, diabetes mellitus and hypertension have become recognized as the leading causes
of end-stage renal disease, together accounting for
approximately 70 per cent of all chronic renal failure.
Excessive weight gain (obesity) appears to be the
most important risk factor for the two main causes of
end-stage renal disease—diabetes and hypertension.
As discussed in Chapter 78, type II diabetes, which is
408
Unit V
The Body Fluids and Kidneys
Table 31–5
Most Common Causes of End-Stage Renal Disease (ESRD)
2.5
Diabetes mellitus
Hypertension
Glomerulonephritis
Polycystic kidney disease
Other/unknown
Percentage of Total ESRD Patients
44
26
8
2
20
closely linked to obesity, accounts for approximately
90 per cent of all diabetes mellitus. Excess weight gain
is also a major cause of essential hypertension,
accounting for as much as 65 to 75 per cent of the risk
for developing hypertension in adults. In addition to
causing renal injury through diabetes and hypertension, obesity may have additive or synergistic effects
to worsen renal function in patients with pre-existing
kidney disease.
Injury to the Renal Vasculature as a
Cause of Chronic Renal Failure
Many types of vascular lesions can lead to renal
ischemia and death of kidney tissue. The most common
of these are (1) atherosclerosis of the larger renal arteries, with progressive sclerotic constriction of the vessels;
(2) fibromuscular hyperplasia of one or more of the
large arteries, which also causes occlusion of the vessels;
and (3) nephrosclerosis, caused by sclerotic lesions of
the smaller arteries, arterioles, and glomeruli.
Atherosclerotic or hyperplastic lesions of the large
arteries frequently affect one kidney more than the
other and, therefore, cause unilaterally diminished
kidney function. As discussed in Chapter 19, hypertension often occurs when the artery of one kidney is
constricted while the artery of the other kidney is
still normal, a condition analogous to “two-kidney”
Goldblatt hypertension.
Benign nephrosclerosis, the most common form of
kidney disease, is seen to at least some extent in about
70 per cent of postmortem examinations in people who
die after the age of 60. This type of vascular lesion
occurs in the smaller interlobular arteries and in the
afferent arterioles of the kidney. It is believed to begin
with leakage of plasma through the intimal membrane
of these vessels. This causes fibrinoid deposits to
develop in the medial layers of these vessels, followed
by progressive thickening of the vessel wall that eventually constricts the vessels and, in some cases, occludes
them. Because there is essentially no collateral circulation among the smaller renal arteries, occlusion of one
or more of them causes destruction of a comparable
number of nephrons. Therefore, much of the kidney
tissue becomes replaced by small amounts of fibrous
tissue. When sclerosis occurs in the glomeruli, the injury
is referred to as glomerulosclerosis.
Nephrosclerosis and glomerulosclerosis occur to
some extent in most people after the fourth decade of
life, causing about a 10 per cent decrease in the number
of functional nephrons each 10 years after age 40
(Figure 31–3). This loss of glomeruli and overall
nephron function is reflected by a progressive decrease
Glomeruli (x 106)
Cause
2.0
1.5
1.0
0.5
0.0
0
20
40
Age (years)
60
80
Figure 31–3
Effect of aging on the number of functional glomeruli.
in both renal blood flow and GFR. Even in “normal”
people, kidney plasma flow and GFR decrease by 40 to
50 per cent by age 80.
The frequency and severity of nephrosclerosis and
glomerulosclerosis are greatly increased by concurrent
hypertension or diabetes mellitus. In fact, diabetes mellitus and hypertension are the two most important
causes of end-stage renal disease, as discussed previously. Thus, benign nephrosclerosis in association with
severe hypertension can lead to a rapidly progressing
malignant nephrosclerosis. The characteristic histological features of malignant nephrosclerosis include large
amounts of fibrinoid deposits in the arterioles and progressive thickening of the vessels, with severe ischemia
occurring in the affected nephrons. For unknown
reasons, the incidence of malignant nephrosclerosis and
severe glomerulosclerosis is significantly higher in
blacks than in whites of similar ages who have similar
degrees of severity of hypertension or diabetes.
Injury to the Glomeruli as a Cause
of Chronic Renal Failure—
Glomerulonephritis
Chronic glomerulonephritis can be caused by several
diseases that cause inflammation and damage to the
capillary loops in the glomeruli of the kidneys. In contrast to the acute form of this disease, chronic glomerulonephritis is a slowly progressive disease that often
leads to irreversible renal failure. It may be a primary
kidney disease, following acute glomerulonephritis, or it
may be secondary to systemic diseases, such as lupus
erythematosus.
In most cases, chronic glomerulonephritis begins with
accumulation of precipitated antigen-antibody complexes in the glomerular membrane. In contrast to acute
glomerulonephritis, streptococcal infections account for
only a small percentage of patients with the chronic
form of glomerulonephritis. The results of the accumulation of antigen-antibody complex in the glomerular
membranes are inflammation, progressive thickening of
the membranes, and eventual invasion of the glomeruli
Chapter 31
Kidney Diseases and Diuretics
by fibrous tissue. In the later stages of the disease, the
glomerular capillary filtration coefficient becomes
greatly reduced because of decreased numbers of filtering capillaries in the glomerular tufts and because of
thickened glomerular membranes. In the final stages of
the disease, many glomeruli are replaced by fibrous
tissue and are, therefore, unable to filter fluid.
Injury to the Renal Interstitium as a
Cause of Chronic Renal Failure—
Pyelonephritis
Primary or secondary disease of the renal interstitium
is referred to as interstitial nephritis. In general, this can
result from vascular, glomerular, or tubular damage that
destroys individual nephrons, or it can involve primary
damage to the renal interstitium by poisons, drugs, and
bacterial infections.
Renal interstitial injury caused by bacterial infection is called pyelonephritis. The infection can result
from different types of bacteria but especially from
Escherichia coli that originate from fecal contamination
of the urinary tract. These bacteria reach the kidneys
either by way of the blood stream or, more commonly,
by ascension from the lower urinary tract by way of the
ureters to the kidneys.
Although the normal bladder is able to clear bacteria
readily, there are two general clinical conditions that
may interfere with the normal flushing of bacteria from
the bladder: (1) the inability of the bladder to empty
completely, leaving residual urine in the bladder, and
(2) the existence of obstruction of urine outflow. With
impaired ability to flush bacteria from the bladder, the
bacteria multiply and the bladder becomes inflamed, a
condition termed cystitis. Once cystitis has occurred, it
may remain localized without ascending to the kidney,
or in some people, bacteria may reach the renal pelvis
because of a pathological condition in which urine is
propelled up one or both of the ureters during micturition. This condition is called vesicoureteral reflux and is
due to the failure of the bladder wall to occlude the
ureter during micturition; as a result, some of the urine
is propelled upward toward the kidney, carrying with it
bacteria that can reach the renal pelvis and renal
medulla, where they can initiate the infection and
inflammation associated with pyelonephritis.
Pyelonephritis begins in the renal medulla and therefore usually affects the function of the medulla more
than it affects the cortex, at least in the initial stages.
Because one of the primary functions of the medulla is
to provide the countercurrent mechanism for concentrating urine, patients with pyelonephritis frequently
have markedly impaired ability to concentrate the urine.
With long-standing pyelonephritis, invasion of the
kidneys by bacteria not only causes damage to the renal
medulla interstitium but also results in progressive
damage of renal tubules, glomeruli, and other structures
throughout the kidney. Consequently, large parts of
functional renal tissue are lost, and chronic renal failure
can develop.
Nephrotic Syndrome—Excretion
of Protein in the Urine Because of
Increased Glomerular Permeability
Many patients with kidney disease develop the
nephrotic syndrome, which is characterized by loss of
409
large quantities of plasma proteins into the urine. In
some instances, this occurs without evidence of other
major abnormalities of kidney function, but more often
it is associated with some degree of renal failure.
The cause of the protein loss in the urine is increased
permeability of the glomerular membrane. Therefore,
any disease that increases the permeability of this membrane can cause the nephrotic syndrome. Such diseases
include (1) chronic glomerulonephritis, which affects
primarily the glomeruli and often causes greatly
increased permeability of the glomerular membrane;
(2) amyloidosis, which results from deposition of an
abnormal proteinoid substance in the walls of the blood
vessels and seriously damages the basement membrane
of the glomeruli; and (3) minimal change nephrotic syndrome, which is associated with no major abnormality
in the glomerular capillary membrane that can be
detected with light microscopy. As discussed in Chapter
26, minimal change nephropathy has been found to
be associated with loss of the negative charges that are
normally present in the glomerular capillary basement
membrane. Immunologic studies have also shown
abnormal immune reactions in some cases, suggesting
that the loss of the negative charges may have resulted
from antibody attack on the membrane. Loss of normal
negative charges in the basement membrane of the
glomerular capillaries allows proteins, especially
albumin, to pass through the glomerular membrane
with ease because the negative charges in the basement
membrane normally repel the negatively charged
plasma proteins.
Minimal change nephropathy can occur in adults, but
more frequently it occurs in children between the ages
of 2 and 6 years. Increased permeability of the glomerular capillary membrane occasionally allows as much as
40 grams of plasma protein loss into the urine each day,
which is an extreme amount for a young child. Therefore, the child’s plasma protein concentration often falls
below 2 g/dl, and the colloid osmotic pressure falls from
a normal value of 28 to less than 10 mm Hg. As a consequence of this low colloid osmotic pressure in the
plasma, large amounts of fluid leak from the capillaries
all over the body into most of the tissues, causing severe
edema, as discussed in Chapter 25.
Nephron Function in Chronic
Renal Failure
Loss of Functional Nephrons Requires the Surviving Nephrons
to Excrete More Water and Solutes. It would be reasonable
to suspect that decreasing the number of functional
nephrons, which reduces the GFR, would also cause
major decreases in renal excretion of water and
solutes. Yet patients who have lost as much as 75 per
cent of their nephrons are able to excrete normal
amounts of water and electrolytes without serious
accumulation of any of these in the body fluids.
Further reduction in the number of nephrons,
however, leads to electrolyte and fluid retention, and
death usually ensues when the number of nephrons
falls below 5 to 10 per cent of normal.
In contrast to the electrolytes, many of the waste
products of metabolism, such as urea and creatinine,
accumulate almost in proportion to the number of
nephrons that have been destroyed.The reason for this
410
Unit V
The Body Fluids and Kidneys
50
Plasma concentration
GFR (ml/min)
100
Creatinine production and
renal excretion (g/day)
Serum creatinine
concentration (mg/dl)
0
2
A
B
1
C
0
0
Positive balance
25
50
75
Glomerular filtration rate
(percentage of normal)
100
Production
2
Excretion
GFR x PCreatinine
1
0
0
1
2
Days
3
4
Figure 31–4
Effect of reducing glomerular filtration rate (GFR) by 50 per cent
on serum creatinine concentration and on creatinine excretion rate
when the production rate of creatinine remains constant.
is that substances such as creatinine and urea depend
largely on glomerular filtration for their excretion, and
they are not reabsorbed as avidly as the electrolytes.
Creatinine, for example, is not reabsorbed at all, and
the excretion rate is equal to the rate at which it is
filtered.
Creatinine filtration rate
= GFR ¥ Plasma creatinine concentration
= Creatinine excretion rate
Therefore, if GFR decreases, the creatinine excretion rate also transiently decreases, causing accumulation of creatinine in the body fluids and raising plasma
concentration until the excretion rate of creatinine
returns to normal—the same rate at which creatinine
is produced in the body (Figure 31–4). Thus, under
steady-state conditions, the creatinine excretion rate
equals the rate of creatinine production, despite reductions in GFR; however, this normal rate of creatinine
excretion occurs at the expense of elevated plasma
creatinine concentration, as shown in curve A of
Figure 31–5.
Figure 31–5
Representative patterns of adaptation for different types of solutes
in chronic renal failure. Curve A shows the approximate changes
in the plasma concentrations of solutes such as creatinine and
urea that are filtered and poorly reabsorbed. Curve B shows the
approximate concentrations for solutes such as phosphate and
urate. Curve C shows the approximate concentrations for solutes
such as sodium and chloride.
Some solutes, such as phosphate, urate, and hydrogen ions, are often maintained near the normal range
until GFR falls below 20 to 30 per cent of normal.
Thereafter, the plasma concentrations of these substances rise, but not in proportion to the fall in GFR,
as shown in curve B of Figure 31–5. Maintenance of
relatively constant plasma concentrations of these
solutes as GFR declines is accomplished by excreting
progressively larger fractions of the amounts of these
solutes that are filtered at the glomerular capillaries;
this occurs by decreasing the rate of tubular reabsorption or, in some instances, by increasing tubular
secretion rates.
In the case of sodium and chloride ions, their plasma
concentrations are maintained virtually constant
even with severe decreases in GFR (see curve C of
Figure 31–5). This is accomplished by greatly decreasing tubular reabsorption of these electrolytes.
For example, with a 75 per cent loss of functional
nephrons, each surviving nephron must excrete four
times as much sodium and four times as much volume
as under normal conditions (Table 31–6).
Part of this adaptation occurs because of increased
blood flow and increased GFR in each of the surviving nephrons, owing to hypertrophy of the blood
vessels and glomeruli, as well as functional changes
that cause the blood vessels to vasodilate. Even with
large decreases in the total GFR, normal rates of renal
Chapter 31
411
Kidney Diseases and Diuretics
Table 31–6
Total Kidney Excretion and Excretion per Nephron in
Renal Failure
3.0
GFR, glomerular filtration rate.
excretion can still be maintained by decreasing the
rate at which the tubules reabsorb water and solutes.
Isosthenuria—Inability of the Kidney to Concentrate or Dilute
the Urine. One important effect of the rapid rate of
tubular flow that occurs in the remaining nephrons of
diseased kidneys is that the renal tubules lose their
ability to concentrate or dilute the urine. The concentrating ability of the kidney is impaired mainly because
(1) the rapid flow of tubular fluid through the collecting ducts prevents adequate water reabsorption, and
(2) the rapid flow through both the loop of Henle and
the collecting ducts prevents the countercurrent mechanism from operating effectively to concentrate the
medullary interstitial fluid solutes. Therefore, as progressively more nephrons are destroyed, the maximum
concentrating ability of the kidney declines, and urine
osmolarity and specific gravity (a measure of the total
solute concentration) approach the osmolarity and
specific gravity of the glomerular filtrate, as shown in
Figure 31–6.
The diluting mechanism in the kidney is also
impaired when the number of nephrons decreases
because the rapid flushing of fluid through the loops
of Henle and the high load of solutes such as urea
cause a relatively high solute concentration in the
tubular fluid of this part of the nephron. As a consequence, the diluting capacity of the kidney is impaired,
and the minimal urine osmolality and specific gravity
approach those of the glomerular filtrate. Because the
concentrating mechanism becomes impaired to a
greater extent than does the diluting mechanism in
chronic renal failure, an important clinical test of renal
function is to determine how well the kidneys can
concentrate urine when a person’s water intake is
restricted for 12 or more hours.
Effects of Renal Failure on the Body
Fluids—Uremia
The effect of complete renal failure on the body fluids
depends on (1) water and food intake and (2) the
degree of impairment of renal function. Assuming that
a person with complete renal failure continues to ingest
the same amounts of water and food, the concentrations
Specific gravity of urine
0.75
500,000
40
80
1.5
1.030
Isosthenuria
1.020
1.010
Glomerular filtrate specific gravity
1.000
2,000,000 1,500,000 1,000,000 500,000
0
Number of nephrons in both kidneys
Figure 31–6
Development of isosthenuria in a patient with decreased numbers
of functional nephrons.
Increase
2,000,000
125
62.5
1.5
1.040
ter
Wa +
Na
H+
Phenols
N
NP K+
Normal
HPO4=
HCO
Decrease
75% Loss of
Nephrons
Normal
Number of nephrons
Total GFR (ml/min)
Single nephron GFR (nl/min)
Volume excreted for all
nephrons (ml/min)
Volume excreted per nephron
(nl/min)
1.050
3
SO4=
-
Kidney shutdown
0
3
6
Days
9
12
Figure 31–7
Effect of kidney failure on extracellular fluid constituents. NPN,
nonprotein nitrogens.
of different substances in the extracellular fluid are
approximately those shown in Figure 31–7. Important
effects include (1) generalized edema resulting from
water and salt retention, (2) acidosis resulting from
failure of the kidneys to rid the body of normal acidic
products, (3) high concentration of the nonprotein nitrogens—especially urea, creatinine, and uric acid—resulting from failure of the body to excrete the metabolic
end products of proteins, and (4) high concentrations of
other substances excreted by the kidney, including
phenols, sulfates, phosphates, potassium, and guanidine
bases. This total condition is called uremia because of
the high concentration of urea in the body fluids.
412
Unit V
The Body Fluids and Kidneys
Water Retention and Development of Edema in Renal Failure. If
water intake is restricted immediately after acute renal
failure begins, the total body fluid content may become
only slightly increased. If fluid intake is not limited and
the patient drinks in response to the normal thirst mechanisms, the body fluids begin to increase immediately
and rapidly.
With chronic partial kidney failure, accumulation of
fluid may not be severe, as long as salt and fluid intake
are not excessive, until kidney function falls to 25 per
cent of normal or lower. The reason for this, as discussed
previously, is that the surviving nephrons excrete larger
amounts of salt and water. Even the small fluid retention that does occur, along with increased secretion of
renin and angiotensin II that usually occurs in ischemic
kidney disease, often causes severe hypertension in
chronic renal failure. Almost all patients with kidney
function so reduced as to require dialysis to preserve life
develop hypertension. In most of these patients, severe
reduction of salt intake or removal of extracellular fluid
by dialysis can control the hypertension. The remaining
patients continue to have hypertension even after
excess sodium has been removed by dialysis. In this
group, removal of the ischemic kidneys usually corrects
the hypertension (as long as fluid retention is prevented
by dialysis) because it removes the source of excessive
renin secretion and subsequent increased angiotensin II
formation.
Uremia—Increase in Urea and Other Nonprotein Nitrogens
(Azotemia). The nonprotein nitrogens include urea, uric
acid, creatinine, and a few less important compounds.
These, in general, are the end products of protein
metabolism and must be removed from the body to
ensure continued normal protein metabolism in the
cells. The concentrations of these, particularly of urea,
can rise to as high as 10 times normal during 1 to 2
weeks of total renal failure. With chronic renal failure,
the concentrations rise approximately in proportion to
the degree of reduction in functional nephrons. For this
reason, measuring the concentrations of these substances, especially of urea and creatinine, provides an
important means for assessing the degree of renal
failure.
Osteomalacia in Chronic Renal Failure Caused by Decreased
Production of Active Vitamin D and by Phosphate Retention by
the Kidneys. Prolonged renal failure also causes osteo-
malacia, a condition in which the bones are partially
absorbed and, therefore, become greatly weakened. An
important cause of this condition is the following:
Vitamin D must be converted by a two-stage process,
first in the liver and then in the kidneys, into 1,25–
dihydroxycholecalciferol before it is able to promote
calcium absorption from the intestine. Therefore,
serious damage to the kidney greatly reduces the blood
concentration of active vitamin D, which in turn
decreases intestinal absorption of calcium and the availability of calcium to the bones.
Another important cause of demineralization of the
skeleton in chronic renal failure is the rise in serum
phosphate concentration that occurs as a result of
decreased GFR. This rise in serum phosphate causes
increased binding of phosphate with calcium in the
plasma, thus decreasing the plasma serum ionized
calcium concentration, which in turn stimulates parathyroid hormone secretion. This secondary hyperparathyroidism then stimulates the release of calcium from
bones, causing further demineralization of the bones.
Hypertension and Kidney Disease
As discussed earlier in this chapter, hypertension can
exacerbate injury to the glomeruli and blood vessels of
the kidneys and is a major cause of end-stage renal
disease. Conversely, abnormalities of kidney function
can cause hypertension, as discussed in detail in Chapter
19. Thus, the relation between hypertension and kidney
disease can, in some instances, propagate a vicious
circle: primary kidney damage leads to increased blood
pressure, which in turn causes further damage to the
kidneys, further increases in blood pressure, and so
forth, until end-stage renal disease develops.
Not all types of kidney disease cause hypertension,
because damage to certain portions of the kidney cause
uremia without hypertension. Nevertheless, some types
of renal damage are particularly prone to cause hypertension. A classification of kidney disease relative
to hypertensive or nonhypertensive effects is the
following.
Acidosis in Renal Failure. Each day the body normally pro-
duces about 50 to 80 millimoles more metabolic acid
than metabolic alkali. Therefore, when the kidneys fail
to function, acid accumulates in the body fluids. The
buffers of the body fluids normally can buffer 500 to
1000 millimoles of acid without lethal increases in extracellular fluid hydrogen ion concentration, and the phosphate compounds in the bones can buffer an additional
few thousand millimoles of hydrogen ion. However,
when this buffering power is used up, the blood pH falls
drastically, and the patient will become comatose and
die if the pH falls below about 6.8.
Anemia in Chronic Renal Failure Caused by Decreased Erythropoietin Secretion. Patients with severe chronic renal
failure almost always develop anemia. The most important cause of this is decreased renal secretion of
erythropoietin, which stimulates the bone marrow to
produce red blood cells. If the kidneys are seriously
damaged, they are unable to form adequate quantities
of erythropoietin, which leads to diminished red blood
cell production and consequent anemia.
Renal Lesions That Reduce the Ability of the Kidneys to Excrete
Sodium and Water Promote Hypertension. Renal lesions that
decrease the ability of the kidneys to excrete sodium
and water almost invariably cause hypertension. Therefore, lesions that either decrease GFR or increase
tubular reabsorption usually lead to hypertension of
varying degrees. Some specific types of renal abnormalities that can cause hypertension are as follows:
1. Increased renal vascular resistance, which reduces
renal blood flow and GFR. An example is
hypertension caused by renal artery stenosis.
2. Decreased glomerular capillary filtration coefficient,
which reduces GFR. An example of this is chronic
glomerulonephritis, which causes inflammation and
thickening of the glomerular capillary membranes,
thereby reducing the glomerular capillary filtration
coefficient.
3. Excessive tubular sodium reabsorption. An example
is hypertension caused by excessive aldosterone
secretion, which increases sodium reabsorption
mainly in the cortical collecting tubules.
Chapter 31
Kidney Diseases and Diuretics
Once hypertension has developed, renal excretion of
sodium and water returns to normal because the high
arterial pressure causes pressure natriuresis and pressure diuresis, so that intake and output of sodium
and water become balanced once again. Even when
there are large increases in renal vascular resistance or
decreases in the glomerular capillary coefficient, the
GFR may still return to nearly normal levels after the
arterial blood pressure rises. Likewise, when tubular
reabsorption is increased, as occurs with excessive
aldosterone secretion, the urinary excretion rate is initially reduced but then returns to normal as arterial
pressure rises. Thus, after hypertension develops, there
may be no sign of impaired excretion of sodium and
water other than the hypertension. As explained in
Chapter 19, normal excretion of sodium and water at an
elevated arterial pressure means that pressure natriuresis and pressure diuresis have been reset to a higher
arterial pressure.
Hypertension Caused by Patchy Renal Damage and Increased
Renal Secretion of Renin. If one part of the kidney is
ischemic and the remainder is not ischemic, such as
occurs when one renal artery is severely constricted, the
ischemic renal tissue secretes large quantities of renin.
This secretion leads to the formation of angiotensin II,
which can cause hypertension. The most likely sequence
of events in causing this hypertension, as discussed in
Chapter 19, is (1) the ischemic kidney tissue itself
excretes less than normal amounts of water and salt; (2)
the renin secreted by the ischemic kidney, and subsequent increased angiotensin II formation, affects the
nonischemic kidney tissue, causing it also to retain salt
and water; and (3) excess salt and water cause hypertension in the usual manner.
A similar type of hypertension can result when patchy
areas of one or both kidneys become ischemic as a
result of arteriosclerosis or vascular injury in specific
portions of the kidneys. When this occurs, the ischemic
nephrons excrete less salt and water but secrete greater
amounts of renin, which causes increased angiotensin
II formation. The high levels of angiotensin II then
impair the ability of the surrounding otherwise normal
nephrons to excrete sodium and water. As a result,
hypertension develops, which restores the overall
excretion of sodium and water by the kidney, so that
balance between intake and output of salt and water
is maintained, but at the expense of high blood
pressure.
Kidney Diseases That Cause Loss of Entire Nephrons Lead to Renal
Failure But May Not Cause Hypertension. Loss of large
numbers of whole nephrons, such as occurs with the loss
of one kidney and part of another kidney, almost always
leads to renal failure if the amount of kidney tissue lost
is great enough. If the remaining nephrons are normal
and the salt intake is not excessive, this condition might
not cause clinically significant hypertension, because
even a slight rise in blood pressure will raise the GFR
and decrease tubular sodium reabsorption sufficiently
to promote enough water and salt excretion in the urine,
even with the few nephrons that remain intact.
However, a patient with this type of abnormality may
become severely hypertensive if additional stresses are
imposed, such as eating a large amount of salt. In this
case, the kidneys simply cannot clear adequate quantities of salt with the small number of functioning
nephrons that remain.
413
Specific Tubular Disorders
In Chapter 27, we point out that several mechanisms are
responsible for transporting different individual substances across the tubular epithelial membranes. In
Chapter 3, we also point out that each cellular enzyme
and each carrier protein is formed in response to a
respective gene in the nucleus. If any required gene
happens to be absent or abnormal, the tubules may be
deficient in one of the appropriate carrier proteins or
one of the enzymes needed for solute transport by the
renal tubular epithelial cells. For this reason, many
hereditary tubular disorders occur because of the transport of individual substances or groups of substances
through the tubular membrane. In addition, damage to
the tubular epithelial membrane by toxins or ischemia
can cause important renal tubular disorders.
Renal Glycosuria—Failure of the Kidneys to Reabsorb Glucose.
In this condition, the blood glucose concentration may
be normal, but the transport mechanism for tubular
reabsorption of glucose is greatly limited or absent.
Consequently, despite a normal blood glucose level,
large amounts of glucose pass into the urine each day.
Because diabetes mellitus is also associated with the
presence of glucose in the urine, renal glycosuria, which
is a relatively benign condition, must be ruled out before
making a diagnosis of diabetes mellitus.
Aminoaciduria—Failure of the Kidneys to Reabsorb Amino Acids.
Some amino acids share mutual transport systems for
reabsorption, whereas other amino acids have their own
distinct transport systems. Rarely, a condition called
generalized aminoaciduria results from deficient reabsorption of all amino acids; more frequently, deficiencies
of specific carrier systems may result in (1) essential
cystinuria, in which large amounts of cystine fail to be
reabsorbed and often crystallize in the urine to form
renal stones; (2) simple glycinuria, in which glycine fails
to be reabsorbed; or (3) beta-aminoisobutyricaciduria,
which occurs in about 5 per cent of all people but apparently has no major clinical significance.
Renal Hypophosphatemia—Failure of the Kidneys to Reabsorb
Phosphate. In renal hypophosphatemia, the renal
tubules fail to reabsorb large enough quantities of phosphate ions when the phosphate concentration of the
body fluids falls very low. This condition usually does
not cause serious immediate abnormalities, because
the phosphate concentration of the extracellular fluid
can vary widely without causing major cellular dysfunction. Over a long period, a low phosphate level
causes diminished calcification of the bones, causing the
person to develop rickets. This type of rickets is refractory to vitamin D therapy, in contrast to the rapid
response of the usual type of rickets, as discussed in
Chapter 79.
Renal Tubular Acidosis—Failure of the Tubules to Secrete Hydrogen Ions. In this condition, the renal tubules are unable
to secrete adequate amounts of hydrogen ions. As a
result, large amounts of sodium bicarbonate are continually lost in the urine. This causes a continued state of
metabolic acidosis, as discussed in Chapter 30. This type
of renal abnormality can be caused by hereditary disorders, or it can occur as a result of widespread injury
to the renal tubules.
414
Unit V
The Body Fluids and Kidneys
Nephrogenic Diabetes Insipidus—Failure of the Kidneys to
Respond to Antidiuretic Hormone. Occasionally, the renal
tubules do not respond to antidiuretic hormone, causing
large quantities of dilute urine to be excreted. As long
as the person is supplied with plenty of water, this condition seldom causes severe difficulty. However, when
adequate quantities of water are not available, the
person rapidly becomes dehydrated.
Semipermeable Flowing
membrane
blood
Fanconi’s Syndrome—A Generalized Reabsorptive Defect of the
Renal Tubules. Fanconi’s syndrome is usually associated
with increased urinary excretion of virtually all amino
acids, glucose, and phosphate. In severe cases, other
manifestations are also observed, such as (1) failure
to reabsorb sodium bicarbonate, which results in
metabolic acidosis; (2) increased excretion of potassium
and sometimes calcium; and (3) nephrogenic diabetes
insipidus.
There are multiple causes of Fanconi’s syndrome,
which results from a generalized inability of the renal
tubular cells to transport various substances. Some of
these causes include (1) hereditary defects in cell transport mechanisms, (2) toxins or drugs that injure the
renal tubular epithelial cells, and (3) injury to the renal
tubular cells as a result of ischemia. The proximal
tubular cells are especially affected in Fanconi’s syndrome caused by tubular injury, because these cells
reabsorb and secrete many of the drugs and toxins that
can cause damage.
Treatment of Renal Failure
by Dialysis with an
Artificial Kidney
Severe loss of kidney function, either acutely or chronically, is a threat to life and requires removal of toxic
waste products and restoration of body fluid volume
and composition toward normal. This can be accomplished by dialysis with an artificial kidney. In certain
types of acute renal failure, an artificial kidney may be
used to tide the patient over until the kidneys resume
their function. If the loss of kidney function is irreversible, it is necessary to perform dialysis chronically
to maintain life. In the United States alone, nearly
300,000 people with irreversible renal failure or even
total kidney removal are being maintained by dialysis
with artificial kidneys. Because dialysis cannot maintain
completely normal body fluid composition and cannot
replace all the multiple functions performed by the
kidneys, the health of patients maintained on artificial
kidneys usually remains significantly impaired. A better
treatment for permanent loss of kidney function is to
restore functional kidney tissue by means of a kidney
transplant.
Basic Principles of Dialysis. The basic principle of the arti-
ficial kidney is to pass blood through minute blood
channels bounded by a thin membrane. On the other
side of the membrane is a dialyzing fluid into which
unwanted substances in the blood pass by diffusion.
Figure 31–8 shows the components of one type of artificial kidney in which blood flows continually between
two thin membranes of cellophane; outside the membrane is a dialyzing fluid. The cellophane is porous
enough to allow the constituents of the plasma, except
the plasma proteins, to diffuse in both directions—from
plasma into the dialyzing fluid or from the dialyzing
Flowing
dialysate
Waste Water
products
Blood out
Bubble
trap
Dialyzer
Blood in
Dialysate
in
Fresh dialyzing
solution
Constant
temperature
bath
Dialysate
out
Used dialyzing
solution
Figure 31–8
Principles of dialysis with an artificial kidney.
fluid back into the plasma. If the concentration of a substance is greater in the plasma than in the dialyzing
fluid, there will be a net transfer of the substance from
the plasma into the dialyzing fluid.
The rate of movement of solute across the dialyzing
membrane depends on (1) the concentration gradient
of the solute between the two solutions, (2) the permeability of the membrane to the solute, (3) the surface
area of the membrane, and (4) the length of time
that the blood and fluid remain in contact with the
membrane.
Thus, the maximum rate of solute transfer occurs initially when the concentration gradient is greatest (when
dialysis is begun) and slows down as the concentration
gradient is dissipated. In a flowing system, as is the case
with “hemodialysis,” in which blood and dialysate fluid
flow through the artificial kidney, the dissipation of the
concentration gradient can be reduced and diffusion of
solute across the membrane can be optimized by
increasing the flow rate of the blood, the dialyzing fluid,
or both.
In normal operation of the artificial kidney, blood
flows continually or intermittently back into the vein.
The total amount of blood in the artificial kidney at any
one time is usually less than 500 milliliters, the rate of
flow may be several hundred milliliters per minute, and
Chapter 31
Kidney Diseases and Diuretics
Table 31–7
Comparison of Dialyzing Fluid with Normal and Uremic
Plasma
Constituent
Electrolytes (mEq/L)
Na+
K+
Ca++
Mg++
Cl–
HCO3–
Lactate–
HPO4=
Urate–
Sulfate=
Nonelectrolytes
Glucose
Urea
Creatinine
Normal
Plasma
Dialyzing
Fluid
Uremic
Plasma
142
5
3
1.5
107
24
1.2
3
0.3
0.5
133
1.0
3.0
1.5
105
35.7
1.2
0
0
0
142
7
2
1.5
107
14
1.2
9
2
3
100
26
1
125
0
0
100
200
6
the total diffusion surface area is between 0.6 and 2.5
square meters. To prevent coagulation of the blood in
the artificial kidney, a small amount of heparin is infused
into the blood as it enters the artificial kidney. In addition to diffusion of solutes, mass transfer of solutes and
water can be produced by applying a hydrostatic pressure to force the fluid and solutes across the membranes
of the dialyzer; such filtration is called bulk flow.
Dialyzing Fluid. Table 31–7 compares the constituents in
a typical dialyzing fluid with those in normal plasma and
uremic plasma. Note that the concentrations of ions and
other substances in dialyzing fluid are not the same as
the concentrations in normal plasma or in uremic
plasma. Instead, they are adjusted to levels that are
needed to cause appropriate movement of water and
solutes through the membrane during dialysis.
Note that there is no phosphate, urea, urate, sulfate,
or creatinine in the dialyzing fluid; however, these are
present in high concentrations in the uremic blood.
Therefore, when a uremic patient is dialyzed, these substances are lost in large quantities into the dialyzing
fluid.
The effectiveness of the artificial kidney can be
expressed in terms of the amount of plasma that is
cleared of different substances each minute, which, as
discussed in Chapter 27, is the primary means for
expressing the functional effectiveness of the kidneys
themselves to rid the body of unwanted substances.
Most artificial kidneys can clear urea from the plasma
415
at a rate of 100 to 225 ml/min, which shows that at least
for the excretion of urea, the artificial kidney can function about twice as rapidly as two normal kidneys
together, whose urea clearance is only 70 ml/min. Yet
the artificial kidney is used for only 4 to 6 hours per day,
three times a week. Therefore, the overall plasma clearance is still considerably limited when the artificial
kidney replaces the normal kidneys. Also, it is important
to keep in mind that the artificial kidney cannot replace
some of the other functions of the kidneys, such as
secretion of erythropoietin, which is necessary for red
blood cell production.
References
Andreoli TE (ed): Cecil’s Essentials of Medicine, 6th ed.
Philadelphia: WB Saunders, 2004.
Fishbane SA, Scribner BH: Blood pressure control in dialysis patients. Semin Dial 15:144, 2002.
Hall JE:The kidney, hypertension, and obesity. Hypertension
41:625, 2003.
Hall JE, Henegar JR, Dwyer TM, et al: Is obesity a major
cause of chronic renal disease? Adv Ren Replace Ther
11:41, 2004.
Hostetter TH: Prevention of the development and progression of renal disease. J Am Soc Nephrol 14(Suppl 2):S144,
2003.
Levey AS, Beto JA, Coronado BE, et al: Controlling the epidemic of cardiovascular disease in chronic renal disease.
What do we know? What do we need to learn? Where do
we go from here? National Kidney Foundation Task Force
on Cardiovascular Disease. Am J Kidney Dis 32:853, 1998.
Luke RG: Chronic renal failure. In: Goldman F, Bennett JC
(eds): Cecil Textbook of Medicine, 21st ed. Philadelphia:
WB Saunders, 2000, pp 571-578.
Mitch WE: Acute renal failure. In: Goldman F, Bennett JC
(eds): Cecil Textbook of Medicine, 21st ed. Philadelphia:
WB Saunders, 2000, pp 567-570.
Molitoris BA: Transitioning to therapy in ischemic acute
renal failure. J Am Soc Nephrol 14:265, 2003.
Sarnak MJ, Levey AS, Schoolwerth AC, et al: Kidney disease
as a risk factor for development of cardiovascular disease.
Hypertension 42:1050, 2003.
Schrier RW: Atlas of Diseases of the Kidney. http://
www.kidneyatlas.org/.
Shankar SS, Brater DC: Loop diuretics: from the Na-K-2Cl
transporter to clinical use. Am J Physiol Renal Physiol
284:F11, 2003.
Singri N, Ahya SN, Levin ML: Acute renal failure. JAMA
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United States Renal Data System. http://www.usrds.org/.
Wilcox CS: New insights into diuretic use in patients with
chronic renal disease. J Am Soc Nephrol 13:798, 2002.
U
N
I
Blood Cells,
Immunity, and
Blood Clotting
32. Red Blood Cells, Anemia, and Polycythemia
33. Resistance of the Body to Infection: I. Leukocytes,
Granulocytes, the Monocyte-Macrophage System,
and Inflammation
34. Resistance of the Body to Infection: II. Immunity
and Allergy
35. Blood Types; Transfusion; Tissue and Organ
Transplantation
36. Hemostasis and Blood Coagulation
T
VI
C
H
A
P
T
E
R
3
2
Red Blood Cells, Anemia,
and Polycythemia
With this chapter we begin discussing the blood cells
and cells of the macrophage system and lymphatic
system. We first present the functions of red blood
cells, which are the most abundant cells of the blood
and are necessary for the delivery of oxygen to the
tissues.
Red Blood Cells (Erythrocytes)
The major function of red blood cells, also known as erythrocytes, is to transport hemoglobin, which in turn carries oxygen from the lungs to the tissues. In
some lower animals, hemoglobin circulates as free protein in the plasma, not
enclosed in red blood cells. When it is free in the plasma of the human being,
about 3 per cent of it leaks through the capillary membrane into the tissue
spaces or through the glomerular membrane of the kidney into the glomerular
filtrate each time the blood passes through the capillaries. Therefore, for hemoglobin to remain in the human blood stream, it must exist inside red blood cells.
The red blood cells have other functions besides transport of hemoglobin.
For instance, they contain a large quantity of carbonic anhydrase, an enzyme
that catalyzes the reversible reaction between carbon dioxide (CO2) and water
to form carbonic acid (H2CO3), increasing the rate of this reaction several thousandfold. The rapidity of this reaction makes it possible for the water of the
blood to transport enormous quantities of CO2 in the form of bicarbonate ion
(HCO3–) from the tissues to the lungs, where it is reconverted to CO2 and
expelled into the atmosphere as a body waste product. The hemoglobin in the
cells is an excellent acid-base buffer (as is true of most proteins), so that the red
blood cells are responsible for most of the acid-base buffering power of whole
blood.
Shape and Size of Red Blood Cells. Normal red blood cells, shown in Figure 32–3,
are biconcave discs having a mean diameter of about 7.8 micrometers and
a thickness of 2.5 micrometers at the thickest point and 1 micrometer or less
in the center. The average volume of the red blood cell is 90 to 95 cubic
micrometers.
The shapes of red blood cells can change remarkably as the cells squeeze
through capillaries. Actually, the red blood cell is a “bag” that can be deformed
into almost any shape. Furthermore, because the normal cell has a great excess
of cell membrane for the quantity of material inside, deformation does not
stretch the membrane greatly and, consequently, does not rupture the cell, as
would be the case with many other cells.
Concentration of Red Blood Cells in the Blood. In normal men, the average number
of red blood cells per cubic millimeter is 5,200,000 (±300,000); in normal women,
it is 4,700,000 (±300,000). Persons living at high altitudes have greater numbers
of red blood cells. This is discussed later.
Quantity of Hemoglobin in the Cells. Red blood cells have the ability to concentrate
hemoglobin in the cell fluid up to about 34 grams in each 100 milliliters of cells.
419
420
Unit VI
Blood Cells, Immunity, and Blood Clotting
The concentration does not rise above this value,
because this is the metabolic limit of the cell’s hemoglobin-forming mechanism. Furthermore, in normal
people, the percentage of hemoglobin is almost always
near the maximum in each cell. However, when hemoglobin formation is deficient, the percentage of hemoglobin in the cells may fall considerably below this
value, and the volume of the red cell may also decrease
because of diminished hemoglobin to fill the cell.
When the hematocrit (the percentage of blood that
is cells—normally, 40 to 45 per cent) and the quantity
of hemoglobin in each respective cell are normal, the
whole blood of men contains an average of 15 grams
of hemoglobin per 100 milliliters of cells; for women,
it contains an average of 14 grams per 100 milliliters.
As discussed in connection with blood transport of
oxygen in Chapter 40, each gram of pure hemoglobin
is capable of combining with 1.34 milliliters of oxygen.
Therefore, in a normal man, a maximum of about 20
milliliters of oxygen can be carried in combination
with hemoglobin in each 100 milliliters of blood, and
in a normal woman, 19 milliliters of oxygen can be
carried.
Production of Red Blood Cells
100
Vertebra
50
Sternu
m
0
sh
(
)
aft
(sh
25
r
mu
Fe
75
Tibia
Cellularity (per cent)
Areas of the Body That Produce Red Blood Cells. In the early
weeks of embryonic life, primitive, nucleated red blood
cells are produced in the yolk sac. During the middle
trimester of gestation, the liver is the main organ for
production of red blood cells, but reasonable numbers
are also produced in the spleen and lymph nodes. Then,
during the last month or so of gestation and after birth,
red blood cells are produced exclusively in the bone
marrow.
As demonstrated in Figure 32–1, the bone marrow
of essentially all bones produces red blood cells until
a person is 5 years old. The marrow of the long bones,
except for the proximal portions of the humeri and
tibiae, becomes quite fatty and produces no more red
blood cells after about age 20 years. Beyond this age,
0 5 10 15 20
Rib
a ft )
30
40
Age (years)
50
60
70
Figure 32–1
Relative rates of red blood cell production in the bone marrow of
different bones at different ages.
most red cells continue to be produced in the marrow
of the membranous bones, such as the vertebrae,
sternum, ribs, and ilia. Even in these bones, the marrow
becomes less productive as age increases.
Genesis of Blood Cells
Pluripotential Hematopoietic Stem Cells, Growth Inducers, and
Differentiation Inducers. The blood cells begin their lives
in the bone marrow from a single type of cell called
the pluripotential hematopoietic stem cell, from which
all the cells of the circulating blood are eventually
derived. Figure 32–2 shows the successive divisions of
the pluripotential cells to form the different circulating blood cells. As these cells reproduce, a small
portion of them remains exactly like the original
pluripotential cells and is retained in the bone marrow
to maintain a supply of these, although their numbers
diminish with age. Most of the reproduced cells,
however, differentiate to form the other cell types
shown to the right in Figure 32–2. The intermediatestage cells are very much like the pluripotential
stem cells, even though they have already become
committed to a particular line of cells and are called
committed stem cells.
The different committed stem cells, when grown in
culture, will produce colonies of specific types of blood
cells. A committed stem cell that produces erythrocytes is called a colony-forming unit–erythrocyte, and
the abbreviation CFU-E is used to designate this type
of stem cell. Likewise, colony-forming units that form
granulocytes and monocytes have the designation
CFU-GM, and so forth.
Growth and reproduction of the different stem
cells are controlled by multiple proteins called growth
inducers. Four major growth inducers have been
described, each having different characteristics. One of
these, interleukin-3, promotes growth and reproduction of virtually all the different types of committed
stem cells, whereas the others induce growth of only
specific types of cells.
The growth inducers promote growth but not differentiation of the cells. This is the function of another
set of proteins called differentiation inducers. Each of
these causes one type of committed stem cell to differentiate one or more steps toward a final adult blood
cell.
Formation of the growth inducers and differentiation inducers is itself controlled by factors outside the
bone marrow. For instance, in the case of erythrocytes
(red blood cells), exposure of the blood to low oxygen
for a long time results in growth induction, differentiation, and production of greatly increased numbers of
erythrocytes, as discussed later in the chapter. In the
case of some of the white blood cells, infectious
diseases cause growth, differentiation, and eventual
formation of specific types of white blood cells that are
needed to combat each infection.
Stages of Differentiation of Red Blood Cells
The first cell that can be identified as belonging to the
red blood cell series is the proerythroblast, shown at
the starting point in Figure 32–3. Under appropriate
Chapter 32
421
Red Blood Cells, Anemia, and Polycythemia
Erythrocytes
CFU-B
(Colony-forming
unit–blast)
CFU-E
(Colony-forming
unit–erythrocytes)
Granulocytes
(Neutrophils)
(Eosinophils)
(Basophils)
Monocytes
PHSC
(Pluripotent
hematopoietic
stem cell)
CFU-S
(Colony-forming
unit–spleen)
CFU-GM
(Colony-forming unit–
granulocytes, monocytes) Macrocytes
Megakaryocytes
CFU-M
(Colony-forming unit–
megakaryocytes)
Platelets
T lymphocytes
Figure 32–2
Formation of the multiple different
blood cells from the original pluripotent hematopoietic stem cell (PHSC)
in the bone marrow.
PHSC
LSC
(Lymphoid stem cell)
B lymphocytes
GENESIS OF RBC
Proerythroblast
Basophil
erythroblast
Microcytic,
hypochromic anemia
Sickle cell anemia
Megaloblastic anemia
Erythroblastosis fetalis
Polychromatophil
erythroblast
Orthochromatic
erythroblast
Reticulocyte
Erythrocytes
Figure 32–3
Genesis of normal red blood cells (RBCs) and characteristics of RBCs in different types of anemias.
422
Unit VI
Blood Cells, Immunity, and Blood Clotting
stimulation, large numbers of these cells are formed
from the CFU-E stem cells.
Once the proerythroblast has been formed, it
divides multiple times, eventually forming many
mature red blood cells. The first-generation cells are
called basophil erythroblasts because they stain with
basic dyes; the cell at this time has accumulated very
little hemoglobin. In the succeeding generations, as
shown in Figure 32–3, the cells become filled with
hemoglobin to a concentration of about 34 per cent,
the nucleus condenses to a small size, and its final
remnant is absorbed or extruded from the cell. At the
same time, the endoplasmic reticulum is also reabsorbed. The cell at this stage is called a reticulocyte
because it still contains a small amount of basophilic
material, consisting of remnants of the Golgi apparatus, mitochondria, and a few other cytoplasmic
organelles. During this reticulocyte stage, the cells pass
from the bone marrow into the blood capillaries by
diapedesis (squeezing through the pores of the capillary membrane).
The remaining basophilic material in the reticulocyte normally disappears within 1 to 2 days, and the
cell is then a mature erythrocyte. Because of the short
life of the reticulocytes, their concentration among all
the red cells of the blood is normally slightly less than
1 per cent.
Regulation of Red Blood Cell Production—Role
of Erythropoietin
Hematopoietic Stem Cells
Kidney
Proerythroblasts
Erythropoietin
Red Blood Cells
Decreases
Tissue Oxygenation
Decreases
Factors that decrease
oxygenation
1. Low blood volume
2. Anemia
3. Low hemoglobin
4. Poor blood flow
5. Pulmonary disease
Figure 32–4
Function of the erythropoietin mechanism to increase production
of red blood cells when tissue oxygenation decreases.
The total mass of red blood cells in the circulatory
system is regulated within narrow limits, so that (1) an
adequate number of red cells is always available to
provide sufficient transport of oxygen from the lungs
to the tissues, yet (2) the cells do not become so numerous that they impede blood flow. What we know about
this control mechanism is diagrammed in Figure 32–4
and is as follows.
can also increase the rate of red cell production. This
is especially apparent in prolonged cardiac failure and
in many lung diseases, because the tissue hypoxia
resulting from these conditions increases red cell production, with a resultant increase in hematocrit and
usually total blood volume as well.
Tissue Oxygenation Is the Most Essential Regulator of Red
Blood Cell Production. Any condition that causes the
Erythropoietin Stimulates Red Cell Production, and Its Formation Increases in Response to Hypoxia. The principal stim-
quantity of oxygen transported to the tissues to
decrease ordinarily increases the rate of red blood cell
production. Thus, when a person becomes extremely
anemic as a result of hemorrhage or any other condition, the bone marrow immediately begins to produce
large quantities of red blood cells. Also, destruction of
major portions of the bone marrow by any means,
especially by x-ray therapy, causes hyperplasia of the
remaining bone marrow, thereby attempting to supply
the demand for red blood cells in the body.
At very high altitudes, where the quantity of oxygen
in the air is greatly decreased, insufficient oxygen is
transported to the tissues, and red cell production is
greatly increased. In this case, it is not the concentration of red blood cells in the blood that controls red
cell production but the amount of oxygen transported
to the tissues in relation to tissue demand for oxygen.
Various diseases of the circulation that cause
decreased blood flow through the peripheral vessels,
and particularly those that cause failure of oxygen
absorption by the blood as it passes through the lungs,
ulus for red blood cell production in low oxygen states
is a circulating hormone called erythropoietin, a glycoprotein with a molecular weight of about 34,000. In
the absence of erythropoietin, hypoxia has little or no
effect in stimulating red blood cell production. But
when the erythropoietin system is functional, hypoxia
causes a marked increase in erythropoietin production, and the erythropoietin in turn enhances red
blood cell production until the hypoxia is relieved.
Role of the Kidneys in Formation of Erythropoietin. In
the normal person, about 90 per cent of all erythropoietin is formed in the kidneys; the remainder is
formed mainly in the liver. It is not known exactly
where in the kidneys the erythropoietin is formed.
One likely possibility is that the renal tubular epithelial cells secrete the erythropoietin, because anemic
blood is unable to deliver enough oxygen from the
peritubular capillaries to the highly oxygen-consuming
tubular cells, thus stimulating erythropoietin production.
Chapter 32
Red Blood Cells, Anemia, and Polycythemia
At times, hypoxia in other parts of the body, but
not in the kidneys, stimulates kidney erythropoietin
secretion, which suggests that there might be some
nonrenal sensor that sends an additional signal to the
kidneys to produce this hormone. In particular, both
norepinephrine and epinephrine and several of the
prostaglandins stimulate erythropoietin production.
When both kidneys are removed from a person or
when the kidneys are destroyed by renal disease, the
person invariably becomes very anemic because the 10
per cent of the normal erythropoietin formed in other
tissues (mainly in the liver) is sufficient to cause only
one third to one half the red blood cell formation
needed by the body.
423
building blocks of DNA. Therefore, lack of either
vitamin B12 or folic acid causes abnormal and diminished DNA and, consequently, failure of nuclear
maturation and cell division. Furthermore, the erythroblastic cells of the bone marrow, in addition to
failing to proliferate rapidly, produce mainly larger
than normal red cells called macrocytes, and the cell
itself has a flimsy membrane and is often irregular,
large, and oval instead of the usual biconcave disc.
These poorly formed cells, after entering the circulating blood, are capable of carrying oxygen normally, but
their fragility causes them to have a short life, one half
to one third normal. Therefore, it is said that deficiency
of either vitamin B12 or folic acid causes maturation
failure in the process of erythropoiesis.
Effect of Erythropoietin in Erythrogenesis. When an
animal or a person is placed in an atmosphere of low
oxygen, erythropoietin begins to be formed within
minutes to hours, and it reaches maximum production
within 24 hours. Yet almost no new red blood cells
appear in the circulating blood until about 5 days later.
From this fact, as well as other studies, it has been
determined that the important effect of erythropoietin
is to stimulate the production of proerythroblasts from
hematopoietic stem cells in the bone marrow. In addition, once the proerythroblasts are formed, the
erythropoietin causes these cells to pass more rapidly
through the different erythroblastic stages than they
normally do, further speeding up the production of
new red blood cells. The rapid production of cells continues as long as the person remains in a low oxygen
state or until enough red blood cells have been produced to carry adequate amounts of oxygen to the
tissues despite the low oxygen; at this time, the rate of
erythropoietin production decreases to a level that will
maintain the required number of red cells but not an
excess.
In the absence of erythropoietin, few red blood
cells are formed by the bone marrow. At the other
extreme, when large quantities of erythropoietin
are formed available, and if there is plenty of iron and
other required nutrients available, the rate of red
blood cell production can rise to perhaps 10 or more
times normal. Therefore, the erythropoietin mechanism for controlling red blood cell production is a
powerful one.
Maturation of Red Blood Cells—Requirement
for Vitamin B12 (Cyanocobalamin) and
Folic Acid
Because of the continuing need to replenish red blood
cells, the erythropoietic cells of the bone marrow are
among the most rapidly growing and reproducing cells
in the entire body. Therefore, as would be expected,
their maturation and rate of production are affected
greatly by a person’s nutritional status.
Especially important for final maturation of the red
blood cells are two vitamins, vitamin B12 and folic acid.
Both of these are essential for the synthesis of DNA,
because each in a different way is required for the formation of thymidine triphosphate, one of the essential
Maturation Failure Caused by Poor Absorption of Vitamin
B12 from the Gastrointestinal Tract—Pernicious Anemia. A
common cause of red blood cell maturation failure is
failure to absorb vitamin B12 from the gastrointestinal
tract. This often occurs in the disease pernicious
anemia, in which the basic abnormality is an atrophic
gastric mucosa that fails to produce normal gastric
secretions. The parietal cells of the gastric glands
secrete a glycoprotein called intrinsic factor, which
combines with vitamin B12 in food and makes the B12
available for absorption by the gut. It does this in the
following way: (1) Intrinsic factor binds tightly with
the vitamin B12. In this bound state, the B12 is protected
from digestion by the gastrointestinal secretions. (2)
Still in the bound state, intrinsic factor binds to specific
receptor sites on the brush border membranes of the
mucosal cells in the ileum. (3) Then, vitamin B12 is
transported into the blood during the next few hours
by the process of pinocytosis, carrying intrinsic factor
and the vitamin together through the membrane. Lack
of intrinsic factor, therefore, causes diminished availability of vitamin B12 because of faulty absorption of
the vitamin.
Once vitamin B12 has been absorbed from the gastrointestinal tract, it is first stored in large quantities in
the liver, then released slowly as needed by the bone
marrow. The minimum amount of vitamin B12 required
each day to maintain normal red cell maturation is
only 1 to 3 micrograms, and the normal storage in the
liver and other body tissues is about 1000 times this
amount.Therefore, 3 to 4 years of defective B12 absorption are usually required to cause maturation failure
anemia.
Failure of Maturation Caused by Deficiency of Folic Acid
(Pteroylglutamic Acid). Folic acid is a normal constituent
of green vegetables, some fruits, and meats (especially
liver). However, it is easily destroyed during cooking.
Also, people with gastrointestinal absorption abnormalities, such as the frequently occurring small intestinal disease called sprue, often have serious difficulty
absorbing both folic acid and vitamin B12. Therefore,
in many instances of maturation failure, the cause is
deficiency of intestinal absorption of both folic acid
and vitamin B12.
424
Unit VI
I. 2 succinyl-CoA + 2 glycine
II.
III.
IV.
V.
Blood Cells, Immunity, and Blood Clotting
A
P
C
C
HC
CH
N
H
4 pyrrole
protoporphyrin IX
(pyrrole)
protoporphyrin IX + Fe++
heme
heme + polypeptide
hemoglobin chain (a or b)
2 a chains + 2 b chains
hemoglobin A
Figure 32–5
Formation of hemoglobin.
Formation of Hemoglobin
Synthesis of hemoglobin begins in the proerythroblasts and continues even into the reticulocyte stage of
the red blood cells. Therefore, when reticulocytes leave
the bone marrow and pass into the blood stream, they
continue to form minute quantities of hemoglobin for
another day or so until they become mature erythrocytes.
Figure 32–5 shows the basic chemical steps in the
formation of hemoglobin. First, succinyl-CoA, formed
in the Krebs metabolic cycle (as explained in Chapter
67), binds with glycine to form a pyrrole molecule. In
turn, four pyrroles combine to form protoporphyrin
IX, which then combines with iron to form the heme
molecule. Finally, each heme molecule combines with
a long polypeptide chain, a globin synthesized by
ribosomes, forming a subunit of hemoglobin called a
hemoglobin chain (Figure 32–6). Each chain has a
molecular weight of about 16,000; four of these in turn
bind together loosely to form the whole hemoglobin
molecule.
There are several slight variations in the different
subunit hemoglobin chains, depending on the amino
acid composition of the polypeptide portion. The different types of chains are designated alpha chains, beta
chains, gamma chains, and delta chains. The most
common form of hemoglobin in the adult human
being, hemoglobin A, is a combination of two alpha
chains and two beta chains. Hemoglobin A has a
molecular weight of 64,458.
Because each hemoglobin chain has a heme prosthetic group containing an atom of iron, and because
there are four hemoglobin chains in each hemoglobin
molecule, one finds four iron atoms in each hemoglobin molecule; each of these can bind loosely with one
molecule of oxygen, making a total of four molecules
of oxygen (or eight oxygen atoms) that can be transported by each hemoglobin molecule.
The types of hemoglobin chains in the hemoglobin
molecule determine the binding affinity of the hemoglobin for oxygen. Abnormalities of the chains can
alter the physical characteristics of the hemoglobin
molecule as well. For instance, in sickle cell anemia, the
amino acid valine is substituted for glutamic acid at
CH2
H
C
CH
H3C
CH3
A
B
Fe
HC
CH2
(–)N
N
N(–)
H3C
CH
O2
N
C
D
CH2
CH
C
H
CH3
CH2
CH2
CH2
COOH
COOH
Polypepitide
(hemoglobin chain–a or b)
Figure 32–6
Basic structure of the hemoglobin molecule, showing one of the
four heme chains that bind together to form the hemoglobin
molecule.
one point in each of the two beta chains. When this
type of hemoglobin is exposed to low oxygen, it forms
elongated crystals inside the red blood cells that are
sometimes 15 micrometers in length. These make it
almost impossible for the cells to pass through many
small capillaries, and the spiked ends of the crystals are
likely to rupture the cell membranes, leading to sickle
cell anemia.
Combination of Hemoglobin with Oxygen. The most impor-
tant feature of the hemoglobin molecule is its ability
to combine loosely and reversibly with oxygen. This
ability is discussed in detail in Chapter 40 in relation
Chapter 32
425
Red Blood Cells, Anemia, and Polycythemia
to respiration, because the primary function of hemoglobin in the body is to combine with oxygen in the
lungs and then to release this oxygen readily in the
peripheral tissue capillaries, where the gaseous tension
of oxygen is much lower than in the lungs.
Oxygen does not combine with the two positive
bonds of the iron in the hemoglobin molecule. Instead,
it binds loosely with one of the so-called coordination
bonds of the iron atom. This is an extremely loose
bond, so that the combination is easily reversible. Furthermore, the oxygen does not become ionic oxygen
but is carried as molecular oxygen (composed of two
oxygen atoms) to the tissues, where, because of the
loose, readily reversible combination, it is released into
the tissue fluids still in the form of molecular oxygen
rather than ionic oxygen.
Iron Metabolism
Because iron is important for the formation not only
of hemoglobin but also of other essential elements in
the body (e.g., myoglobin, cytochromes, cytochrome
oxidase, peroxidase, catalase), it is important to understand the means by which iron is utilized in the body.
The total quantity of iron in the body averages 4 to 5
grams, about 65 per cent of which is in the form of
hemoglobin. About 4 per cent is in the form of myoglobin, 1 per cent is in the form of the various heme
compounds that promote intracellular oxidation, 0.1
per cent is combined with the protein transferrin in the
blood plasma, and 15 to 30 per cent is stored for later
use, mainly in the reticuloendothelial system and liver
parenchymal cells, principally in the form of ferritin.
Transport and Storage of Iron. Transport, storage, and
metabolism of iron in the body are diagrammed in
Figure 32–7 and can be explained as follows: When
iron is absorbed from the small intestine, it immediately combines in the blood plasma with a beta
globulin, apotransferrin, to form transferrin, which is
then transported in the plasma. The iron is loosely
bound in the transferrin and, consequently, can be
released to any tissue cell at any point in the body.
Excess iron in the blood is deposited especially in the
liver hepatocytes and less in the reticuloendothelial
cells of the bone marrow.
In the cell cytoplasm, iron combines mainly with
a protein, apoferritin, to form ferritin. Apoferritin
has a molecular weight of about 460,000, and
varying quantities of iron can combine in clusters of
iron radicals with this large molecule; therefore,
ferritin may contain only a small amount of iron or a
large amount. This iron stored as ferritin is called
storage iron.
Smaller quantities of the iron in the storage pool are
in an extremely insoluble form called hemosiderin.
This is especially true when the total quantity of iron
in the body is more than the apoferritin storage pool
can accommodate. Hemosiderin collects in cells in the
form of large clusters that can be observed microscopically as large particles. In contrast, ferritin particles are so small and dispersed that they usually can
be seen in the cell cytoplasm only with the electron
microscope.
When the quantity of iron in the plasma falls low,
some of the iron in the ferritin storage pool is removed
easily and transported in the form of transferrin in the
plasma to the areas of the body where it is needed. A
unique characteristic of the transferrin molecule is that
it binds strongly with receptors in the cell membranes
of erythroblasts in the bone marrow. Then, along with
its bound iron, it is ingested into the erythroblasts by
endocytosis. There the transferrin delivers the iron
directly to the mitochondria, where heme is synthesized. In people who do not have adequate quantities
of transferrin in their blood, failure to transport iron
to the erythroblasts in this manner can cause severe
hypochromic anemia—that is, red cells that contain
much less hemoglobin than normal.
Bilirubin (excreted)
Tissues
Ferritin
Hemosiderin
Macrophages
Degrading hemoglobin
Hemoglobin
Red Cells
Heme
Free
iron
Free iron
Enzymes
Transferrin–Fe
Plasma
Blood loss – 0.7 mg Fe Fe++ absorbed Fe excreted–0.6 mg daily
(small intestine)
daily in menses
Figure 32–7
Iron transport and metabolism.
426
Unit VI
Blood Cells, Immunity, and Blood Clotting
When red blood cells have lived their life span and
are destroyed, the hemoglobin released from the cells
is ingested by monocyte-macrophage cells. There,
iron is liberated and is stored mainly in the ferritin
pool to be used as needed for the formation of new
hemoglobin.
Daily Loss of Iron. A man excretes about 0.6 milligram
of iron each day, mainly into the feces. Additional
quantities of iron are lost when bleeding occurs. For a
woman, additional menstrual loss of blood brings longterm iron loss to an average of about 1.3 mg/day.
Absorption of Iron from the Intestinal Tract
Iron is absorbed from all parts of the small intestine,
mostly by the following mechanism. The liver secretes
moderate amounts of apotransferrin into the bile,
which flows through the bile duct into the duodenum.
Here, the apotransferrin binds with free iron and also
with certain iron compounds, such as hemoglobin and
myoglobin from meat, two of the most important
sources of iron in the diet. This combination is called
transferrin. It, in turn, is attracted to and binds with
receptors in the membranes of the intestinal epithelial
cells. Then, by pinocytosis, the transferrin molecule,
carrying its iron store, is absorbed into the epithelial
cells and later released into the blood capillaries
beneath these cells in the form of plasma transferrin.
Iron absorption from the intestines is extremely
slow, at a maximum rate of only a few milligrams per
day. This means that even when tremendous quantities
of iron are present in the food, only small proportions
can be absorbed.
Regulation of Total Body Iron by Controlling Rate of Absorption.
When the body has become saturated with iron so that
essentially all apoferritin in the iron storage areas is
already combined with iron, the rate of additional iron
absorption from the intestinal tract becomes greatly
decreased. Conversely, when the iron stores have
become depleted, the rate of absorption can accelerate probably five or more times normal. Thus, total
body iron is regulated mainly by altering the rate of
absorption.
Life Span and Destruction of Red
Blood Cells
When red blood cells are delivered from the bone
marrow into the circulatory system, they normally circulate an average of 120 days before being destroyed.
Even though mature red cells do not have a nucleus,
mitochondria, or endoplasmic reticulum, they do have
cytoplasmic enzymes that are capable of metabolizing
glucose and forming small amounts of adenosine
triphosphate. These enzymes also (1) maintain pliability of the cell membrane, (2) maintain membrane
transport of ions, (3) keep the iron of the cells’ hemoglobin in the ferrous form rather than ferric form, and
(4) prevent oxidation of the proteins in the red cells.
Even so, the metabolic systems of old red cells become
progressively less active, and the cells become more
and more fragile, presumably because their life
processes wear out.
Once the red cell membrane becomes fragile, the
cell ruptures during passage through some tight spot
of the circulation. Many of the red cells self-destruct
in the spleen, where they squeeze through the red pulp
of the spleen. There, the spaces between the structural
trabeculae of the red pulp, through which most of the
cells must pass, are only 3 micrometers wide, in comparison with the 8-micrometer diameter of the red
cell. When the spleen is removed, the number of old
abnormal red cells circulating in the blood increases
considerably.
Destruction of Hemoglobin. When red blood cells burst
and release their hemoglobin, the hemoglobin is
phagocytized almost immediately by macrophages in
many parts of the body, but especially by the Kupffer
cells of the liver and macrophages of the spleen and
bone marrow. During the next few hours to days, the
macrophages release iron from the hemoglobin and
pass it back into the blood, to be carried by transferrin either to the bone marrow for the production of
new red blood cells or to the liver and other tissues for
storage in the form of ferritin. The porphyrin portion
of the hemoglobin molecule is converted by the
macrophages, through a series of stages, into the bile
pigment bilirubin, which is released into the blood and
later removed from the body by secretion through the
liver into the bile; this is discussed in relation to liver
function in Chapter 70.
Anemias
Anemia means deficiency of hemoglobin in the blood,
which can be caused by either too few red blood cells
or too little hemoglobin in the cells. Some types of
anemia and their physiologic causes are the following.
Blood Loss Anemia. After rapid hemorrhage, the body
replaces the fluid portion of the plasma in 1 to 3 days,
but this leaves a low concentration of red blood cells.
If a second hemorrhage does not occur, the red blood
cell concentration usually returns to normal within 3
to 6 weeks.
In chronic blood loss, a person frequently cannot
absorb enough iron from the intestines to form hemoglobin as rapidly as it is lost. Red cells are then produced that are much smaller than normal and have too
little hemoglobin inside them, giving rise to microcytic,
hypochromic anemia, which is shown in Figure 32–3.
Aplastic Anemia. Bone marrow aplasia means lack of
functioning bone marrow. For instance, a person
exposed to gamma ray radiation from a nuclear bomb
blast can sustain complete destruction of bone
marrow, followed in a few weeks by lethal anemia.
Likewise, excessive x-ray treatment, certain industrial
chemicals, and even drugs to which the person might
be sensitive can cause the same effect.
Chapter 32
Red Blood Cells, Anemia, and Polycythemia
Megaloblastic Anemia. Based on the earlier discussions
of vitamin B12, folic acid, and intrinsic factor from the
stomach mucosa, one can readily understand that loss
of any one of these can lead to slow reproduction of
erythroblasts in the bone marrow. As a result, the red
cells grow too large, with odd shapes, and are called
megaloblasts. Thus, atrophy of the stomach mucosa,
as occurs in pernicious anemia, or loss of the entire
stomach after surgical total gastrectomy can lead to
megaloblastic anemia. Also, patients who have intestinal sprue, in which folic acid, vitamin B12, and other
vitamin B compounds are poorly absorbed, often
develop megaloblastic anemia. Because in these states
the erythroblasts cannot proliferate rapidly enough to
form normal numbers of red blood cells, those red cells
that are formed are mostly oversized, have bizarre
shapes, and have fragile membranes. These cells
rupture easily, leaving the person in dire need of an
adequate number of red cells.
Hemolytic Anemia. Different abnormalities of the red
blood cells, many of which are hereditarily acquired,
make the cells fragile, so that they rupture easily as
they go through the capillaries, especially through the
spleen. Even though the number of red blood cells
formed may be normal, or even much greater than
normal in some hemolytic diseases, the life span of the
fragile red cell is so short that the cells are destroyed
faster than they can be formed, and serious anemia
results. Some of these types of anemia are the
following.
In hereditary spherocytosis, the red cells are very
small and spherical rather than being biconcave discs.
These cells cannot withstand compression forces
because they do not have the normal loose, baglike cell
membrane structure of the biconcave discs. On passing
through the splenic pulp and some other tight vascular beds, they are easily ruptured by even slight
compression.
In sickle cell anemia, which is present in 0.3 to 1.0
per cent of West African and American blacks, the cells
have an abnormal type of hemoglobin called hemoglobin S, containing faulty beta chains in the hemoglobin molecule, as explained earlier in the chapter.
When this hemoglobin is exposed to low concentrations of oxygen, it precipitates into long crystals inside
the red blood cell. These crystals elongate the cell and
give it the appearance of a sickle rather than a biconcave disc. The precipitated hemoglobin also damages
the cell membrane, so that the cells become highly
fragile, leading to serious anemia. Such patients frequently experience a vicious circle of events called a
sickle cell disease “crisis,” in which low oxygen tension
in the tissues causes sickling, which leads to ruptured
red cells, which causes a further decrease in oxygen
tension and still more sickling and red cell destruction.
Once the process starts, it progresses rapidly, eventuating in a serious decrease in red blood cells within a
few hours and, often, death.
In erythroblastosis fetalis, Rh-positive red blood
cells in the fetus are attacked by antibodies from
an Rh-negative mother. These antibodies make the
427
Rh-positive cells fragile, leading to rapid rupture and
causing the child to be born with serious anemia. This
is discussed in Chapter 35 in relation to the Rh factor
of blood. The extremely rapid formation of new red
cells to make up for the destroyed cells in erythroblastosis fetalis causes a large number of early blast
forms of red cells to be released from the bone marrow
into the blood.
Effects of Anemia on Function of the
Circulatory System
The viscosity of the blood, which was discussed in
Chapter 14, depends almost entirely on the blood concentration of red blood cells. In severe anemia, the
blood viscosity may fall to as low as 1.5 times that of
water rather than the normal value of about 3. This
decreases the resistance to blood flow in the peripheral blood vessels, so that far greater than normal
quantities of blood flow through the tissues and return
to the heart, thereby greatly increasing cardiac output.
Moreover, hypoxia resulting from diminished transport of oxygen by the blood causes the peripheral
tissue blood vessels to dilate, allowing a further
increase in the return of blood to the heart and
increasing the cardiac output to a still higher level—
sometimes three to four times normal. Thus, one of the
major effects of anemia is greatly increased cardiac
output, as well as increased pumping workload on the
heart.
The increased cardiac output in anemia partially
offsets the reduced oxygen-carrying effect of the
anemia, because even though each unit quantity of
blood carries only small quantities of oxygen, the rate
of blood flow may be increased enough so that almost
normal quantities of oxygen are actually delivered to
the tissues. However, when a person with anemia
begins to exercise, the heart is not capable of pumping
much greater quantities of blood than it is already
pumping. Consequently, during exercise, which
greatly increases tissue demand for oxygen, extreme
tissue hypoxia results, and acute cardiac failure
ensues.
Polycythemia
Secondary Polycythemia. Whenever the tissues become
hypoxic because of too little oxygen in the breathed
air, such as at high altitudes, or because of failure of
oxygen delivery to the tissues, such as in cardiac
failure, the blood-forming organs automatically
produce large quantities of extra red blood cells. This
condition is called secondary polycythemia, and the
red cell count commonly rises to 6 to 7 million/mm3,
about 30 per cent above normal.
A common type of secondary polycythemia, called
physiologic polycythemia, occurs in natives who live at
altitudes of 14,000 to 17,000 feet, where the atmospheric oxygen is very low. The blood count is generally
6 to 7 million/mm3; this allows these people to perform
428
Unit VI
Blood Cells, Immunity, and Blood Clotting
reasonably high levels of continuous work even in a
rarefied atmosphere.
Polycythemia Vera (Erythremia). In addition to those
people who have physiologic polycythemia, others
have a pathological condition known as polycythemia
vera, in which the red blood cell count may be 7 to 8
million/mm3 and the hematocrit may be 60 to 70 per
cent instead of the normal 40 to 45 per cent. Polycythemia vera is caused by a genetic aberration in the
hemocytoblastic cells that produce the blood cells. The
blast cells no longer stop producing red cells when too
many cells are already present. This causes excess production of red blood cells in the same manner that a
breast tumor causes excess production of a specific
type of breast cell. It usually causes excess production
of white blood cells and platelets as well.
In polycythemia vera, not only does the hematocrit
increase, but the total blood volume also increases, on
some occasions to almost twice normal. As a result, the
entire vascular system becomes intensely engorged. In
addition, many blood capillaries become plugged by
the viscous blood; the viscosity of the blood in polycythemia vera sometimes increases from the normal of
3 times the viscosity of water to 10 times that of water.
Effect of Polycythemia on Function of
the Circulatory System
Because of the greatly increased viscosity of the blood
in polycythemia, blood flow through the peripheral
blood vessels is often very sluggish. In accordance with
the factors that regulate return of blood to the heart,
as discussed in Chapter 20, increasing blood viscosity
decreases the rate of venous return to the heart. Conversely, the blood volume is greatly increased in polycythemia, which tends to increase venous return.
Actually, the cardiac output in polycythemia is not far
from normal, because these two factors more or less
neutralize each other.
The arterial pressure is also normal in most people
with polycythemia, although in about one third of
them, the arterial pressure is elevated. This means that
the blood pressure–regulating mechanisms can usually
offset the tendency for increased blood viscosity to
increase peripheral resistance and, thereby, increase
arterial pressure. Beyond certain limits, however, these
regulations fail, and hypertension develops.
The color of the skin depends to a great extent on
the quantity of blood in the skin subpapillary venous
plexus. In polycythemia vera, the quantity of blood in
this plexus is greatly increased. Further, because the
blood passes sluggishly through the skin capillaries
before entering the venous plexus, a larger than
normal quantity of hemoglobin is deoxygenated. The
blue color of all this deoxygenated hemoglobin masks
the red color of the oxygenated hemoglobin. Therefore, a person with polycythemia vera ordinarily has a
ruddy complexion with a bluish (cyanotic) tint to the
skin.
References
Alayash AI: Oxygen therapeutics: can we tame haemoglobin? Nat Rev Drug Discov 3:152, 2004.
Brissot P, Troadec MB, Loreal O: The clinical relevance of
new insights in iron transport and metabolism. Curr
Hematol Rep 3:107, 2004.
Claster S, Vichinsky EP: Managing sickle cell disease. BMJ
327:1151, 2003.
Fandrey J: Oxygen-dependent and tissue-specific regulation
of erythropoietin gene expression. Am J Physiol Regul
Integr Comp Physiol 286:R977, 2004.
Hallberg L: Perspectives on nutritional iron deficiency. Annu
Rev Nutr 21:1, 2001.
Hentze MW, Muckenthaler MU, Andrews NC: Balancing
acts: molecular control of mammalian iron metabolism.
Cell 117:285, 2004.
Lappin T: The cellular biology of erythropoietin receptors.
Oncologist 8(Suppl 1):15, 2003.
Maxwell P: HIF-1: an oxygen response system with special
relevance to the kidney. J Am Soc Nephrol 14:2712, 2003.
Persons DA: Update on gene therapy for hemoglobin disorders. Curr Opin Mol Ther 5:508, 2003.
Pietrangelo A: Hereditary hemochromatosis—a new look at
an old disease. N Engl J Med 350:2383, 2004.
Shah S, Vega R: Hereditary spherocytosis. Pediatr Rev
25:168, 2004.
Tefferi A: A contemporary approach to the diagnosis and
management of polycythemia vera. Curr Hematol Rep
2:237, 2003.
Trigg ME: Hematopoietic stem cells. Pediatrics 113(4
Suppl):1051, 2004.
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3
3
Resistance of the Body
to Infection: I. Leukocytes,
Granulocytes, the
Monocyte-Macrophage System,
and Inflammation
Our bodies are exposed continually to bacteria,
viruses, fungi, and parasites, all of which occur
normally and to varying degrees in the skin, the
mouth, the respiratory passageways, the intestinal
tract, the lining membranes of the eyes, and even the
urinary tract. Many of these infectious agents are
capable of causing serious abnormal physiologic
function or even death if they invade the deeper
tissues. In addition, we are exposed intermittently to other highly infectious bacteria and viruses besides those that are normally present, and these can cause
acute lethal diseases such as pneumonia, streptococcal infection, and typhoid
fever.
Our bodies have a special system for combating the different infectious and
toxic agents. This is comprised of blood leukocytes (white blood cells) and tissue
cells derived from leukocytes. These cells work together in two ways to prevent
disease: (1) by actually destroying invading bacteria or viruses by phagocytosis,
and (2) by forming antibodies and sensitized lymphocytes, one or both of which
may destroy or inactivate the invader. This chapter is concerned with the first
of these methods, and Chapter 34 with the second.
Leukocytes (White Blood Cells)
The leukocytes, also called white blood cells, are the mobile units of the body’s
protective system. They are formed partially in the bone marrow (granulocytes
and monocytes and a few lymphocytes) and partially in the lymph tissue (lymphocytes and plasma cells). After formation, they are transported in the blood
to different parts of the body where they are needed.
The real value of the white blood cells is that most of them are specifically
transported to areas of serious infection and inflammation, thereby providing a
rapid and potent defense against infectious agents. As we see later, the granulocytes and monocytes have a special ability to “seek out and destroy” a foreign
invader.
General Characteristics of Leukocytes
Types of White Blood Cells. Six types of white blood cells are normally present in
the blood. They are polymorphonuclear neutrophils, polymorphonuclear
eosinophils, polymorphonuclear basophils, monocytes, lymphocytes, and, occasionally, plasma cells. In addition, there are large numbers of platelets, which are
fragments of another type of cell similar to the white blood cells found in the
bone marrow, the megakaryocyte. The first three types of cells, the polymorphonuclear cells, all have a granular appearance, as shown in cell numbers 7,
429
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Unit VI
Blood Cells, Immunity, and Blood Clotting
Genesis of Myelocytes
Genesis of Lymphocytes
1
3
2
13
4
8
11
Figure 33–1
14
Genesis of white blood cells. The
different cells of the myelocyte
series are 1, myeloblast; 2,
promyelocyte; 3, megakaryocyte;
4, neutrophil myelocyte; 5, young
neutrophil metamyelocyte; 6,
“band” neutrophil metamyelocyte;
7, polymorphonuclear neutrophil;
8, eosinophil myelocyte; 9,
eosinophil metamyelocyte; 10,
polymorphonuclear
eosinophil;
11, basophil myelocyte; 12, polymorphonuclear basophil; 13–16,
stages of monocyte formation.
5
9
15
6
7
10
12
16
10, and 12 in Figure 33–1, for which reason they are
called granulocytes, or, in clinical terminology, “polys,”
because of the multiple nuclei.
The granulocytes and monocytes protect the body
against invading organisms mainly by ingesting
them—that is, by phagocytosis. The lymphocytes and
plasma cells function mainly in connection with the
immune system; this is discussed in Chapter 34. Finally,
the function of platelets is specifically to activate the
blood clotting mechanism, which is discussed in
Chapter 36.
Diapedesis
Chemotaxis
source
Concentrations of the Different White Blood Cells in the Blood.
The adult human being has about 7000 white blood
cells per microliter of blood (in comparison with 5
million red blood cells). Of the total white blood cells,
the normal percentages of the different types are
approximately the following:
Polymorphonuclear neutrophils
Polymorphonuclear eosinophils
Polymorphonuclear basophils
Monocytes
Lymphocytes
62.0%
2.3%
0.4%
5.3%
30.0%
The number of platelets, which are only cell fragments, in each microliter of blood is normally about
300,000.
Genesis of the White Blood Cells
Early differentiation of the pluripotential hematopoietic stem cell into the different types of committed
stem cells is shown in Figure 32–2 in the previous
Increased
permeability
Margination
Chemotactic
substance
Figure 33–2
Movement of neutrophils by diapedesis through capillary pores
and by chemotaxis toward an area of tissue damage.
chapter. Aside from those cells committed to form red
blood cells, two major lineages of white blood cells are
formed, the myelocytic and the lymphocytic lineages.
The left side of Figure 33–1 shows the myelocytic
lineage, beginning with the myeloblast; the right
shows the lymphocytic lineage, beginning with the
lymphoblast.
The granulocytes and monocytes are formed only
in the bone marrow. Lymphocytes and plasma cells
are produced mainly in the various lymphogenous
tissues—especially the lymph glands, spleen, thymus,
tonsils, and various pockets of lymphoid tissue
Chapter 33
Resistance of the Body to Infection: I. Leukocytes, Granulocytes, the Monocyte-Macrophage System
elsewhere in the body, such as in the bone marrow and
in so-called Peyer’s patches underneath the epithelium
in the gut wall.
The white blood cells formed in the bone marrow
are stored within the marrow until they are needed in
the circulatory system. Then, when the need arises,
various factors cause them to be released (these
factors are discussed later). Normally, about three
times as many white blood cells are stored in the
marrow as circulate in the entire blood.This represents
about a 6-day supply of these cells.
The lymphocytes are mostly stored in the various
lymphoid tissues, except for a small number that are
temporarily being transported in the blood.
As shown in Figure 33–1, megakaryocytes (cell 3)
are also formed in the bone marrow. These megakaryocytes fragment in the bone marrow; the small fragments, known as platelets (or thrombocytes), then pass
into the blood. They are very important in the initiation of blood clotting.
Life Span of the White Blood Cells
The life of the granulocytes after being released from
the bone marrow is normally 4 to 8 hours circulating
in the blood and another 4 to 5 days in tissues where
they are needed. In times of serious tissue infection,
this total life span is often shortened to only a few
hours because the granulocytes proceed even more
rapidly to the infected area, perform their functions,
and, in the process, are themselves destroyed.
The monocytes also have a short transit time, 10 to
20 hours in the blood, before wandering through the
capillary membranes into the tissues. Once in the
tissues, they swell to much larger sizes to become tissue
macrophages, and, in this form, can live for months
unless destroyed while performing phagocytic functions. These tissue macrophages are the basis of the
tissue macrophage system, discussed in greater detail
later, which provides continuing defense against
infection.
Lymphocytes enter the circulatory system continually, along with drainage of lymph from the lymph
nodes and other lymphoid tissue. After a few hours,
they pass out of the blood back into the tissues by diapedesis. Then, still later, they re-enter the lymph and
return to the blood again and again; thus, there is continual circulation of lymphocytes through the body.
The lymphocytes have life spans of weeks or months;
this life span depends on the body’s need for these
cells.
The platelets in the blood are replaced about once
every 10 days; in other words, about 30,000 platelets
are formed each day for each microliter of blood.
431
other injurious agents. The neutrophils are mature
cells that can attack and destroy bacteria even in the
circulating blood. Conversely, the tissue macrophages
begin life as blood monocytes, which are immature
cells while still in the blood and have little ability to
fight infectious agents at that time. However, once they
enter the tissues, they begin to swell—sometimes
increasing their diameters as much as fivefold—to as
great as 60 to 80 micrometers, a size that can barely be
seen with the naked eye. These cells are now called
macrophages, and they are extremely capable of combating intratissue disease agents.
White Blood Cells Enter the Tissue Spaces by Diapedesis.
Neutrophils and monocytes can squeeze through the
pores of the blood capillaries by diapedesis. That is,
even though a pore is much smaller than a cell, a small
portion of the cell slides through the pore at a time;
the portion sliding through is momentarily constricted
to the size of the pore, as shown in Figure 33–2.
White Blood Cells Move Through Tissue Spaces by Ameboid
Motion. Both neutrophils and macrophages can move
through the tissues by ameboid motion, described in
Chapter 2. Some cells move at velocities as great as
40 mm/min, a distance as great as their own length
each minute.
White Blood Cells Are Attracted to Inflamed Tissue Areas by
Chemotaxis. Many different chemical substances in the
tissues cause both neutrophils and macrophages to
move toward the source of the chemical. This phenomenon, shown in Figure 33–2, is known as chemotaxis. When a tissue becomes inflamed, at least a dozen
different products are formed that can cause chemotaxis toward the inflamed area. They include (1) some
of the bacterial or viral toxins, (2) degenerative products of the inflamed tissues themselves, (3) several
reaction products of the “complement complex”
(discussed in Chapter 34) activated in inflamed tissues,
and (4) several reaction products caused by plasma
clotting in the inflamed area, as well as other
substances.
As shown in Figure 33–2, chemotaxis depends on
the concentration gradient of the chemotactic substance. The concentration is greatest near the source,
which directs the unidirectional movement of the
white cells. Chemotaxis is effective up to 100 micrometers away from an inflamed tissue. Therefore, because
almost no tissue area is more than 50 micrometers
away from a capillary, the chemotactic signal can easily
move hordes of white cells from the capillaries into the
inflamed area.
Phagocytosis
Neutrophils and Macrophages
Defend Against Infections
It is mainly the neutrophils and tissue macrophages
that attack and destroy invading bacteria, viruses, and
The most important function of the neutrophils and
macrophages is phagocytosis, which means cellular
ingestion of the offending agent. Phagocytes must be
selective of the material that is phagocytized; otherwise, normal cells and structures of the body might be
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Unit VI
Blood Cells, Immunity, and Blood Clotting
ingested. Whether phagocytosis will occur depends
especially on three selective procedures.
First, most natural structures in the tissues have
smooth surfaces, which resist phagocytosis. But if the
surface is rough, the likelihood of phagocytosis is
increased.
Second, most natural substances of the body have
protective protein coats that repel the phagocytes.
Conversely, most dead tissues and foreign particles
have no protective coats, which makes them subject to
phagocytosis.
Third, the immune system of the body (described in
detail in Chapter 34) develops antibodies against infectious agents such as bacteria. The antibodies then
adhere to the bacterial membranes and thereby make
the bacteria especially susceptible to phagocytosis. To
do this, the antibody molecule also combines with the
C3 product of the complement cascade, which is an
additional part of the immune system discussed in the
next chapter. The C3 molecules, in turn, attach to
receptors on the phagocyte membrane, thus initiating
phagocytosis. This selection and phagocytosis process
is called opsonization.
Phagocytosis by Neutrophils. The neutrophils entering
the tissues are already mature cells that can immediately begin phagocytosis. On approaching a particle to
be phagocytized, the neutrophil first attaches itself to
the particle and then projects pseudopodia in all directions around the particle. The pseudopodia meet one
another on the opposite side and fuse. This creates an
enclosed chamber that contains the phagocytized particle. Then the chamber invaginates to the inside of the
cytoplasmic cavity and breaks away from the outer cell
membrane to form a free-floating phagocytic vesicle
(also called a phagosome) inside the cytoplasm. A
single neutrophil can usually phagocytize 3 to 20 bacteria before the neutrophil itself becomes inactivated
and dies.
Phagocytosis by Macrophages. Macrophages are the end-
stage product of monocytes that enter the tissues from
the blood. When activated by the immune system as
described in Chapter 34, they are much more powerful phagocytes than neutrophils, often capable of
phagocytizing as many as 100 bacteria. They also have
the ability to engulf much larger particles, even whole
red blood cells or, occasionally, malarial parasites,
whereas neutrophils are not capable of phagocytizing
particles much larger than bacteria. Also, after digesting particles, macrophages can extrude the residual
products and often survive and function for many
more months.
Once Phagocytized, Most Particles Are Digested by Intracellular Enzymes. Once a foreign particle has been phagocy-
tized, lysosomes and other cytoplasmic granules in
the neutrophil or macrophage immediately come in
contact with the phagocytic vesicle, and their membranes fuse, thereby dumping many digestive enzymes
and bactericidal agents into the vesicle. Thus, the
phagocytic vesicle now becomes a digestive vesicle,
and digestion of the phagocytized particle begins
immediately.
Both neutrophils and macrophages contain an
abundance of lysosomes filled with proteolytic
enzymes especially geared for digesting bacteria
and other foreign protein matter. The lysosomes of
macrophages (but not of neutrophils) also contain
large amounts of lipases, which digest the thick lipid
membranes possessed by some bacteria such as the
tuberculosis bacillus.
Both Neutrophils and Macrophages Can Kill Bacteria. In
addition to the digestion of ingested bacteria in
phagosomes, neutrophils and macrophages contain
bactericidal agents that kill most bacteria even when
the lysosomal enzymes fail to digest them. This is especially important, because some bacteria have protective coats or other factors that prevent their
destruction by digestive enzymes. Much of the killing
effect results from several powerful oxidizing agents
formed by enzymes in the membrane of the phagosome or by a special organelle called the peroxisome.
These oxidizing agents include large quantities of
superoxide (O2–), hydrogen peroxide (H2O2), and
hydroxyl ions (–OH–), all of which are lethal to most
bacteria, even in small quantities. Also, one of the
lysosomal enzymes, myeloperoxidase, catalyzes the
reaction between H2O2 and chloride ions to form
hypochlorite, which is exceedingly bactericidal.
Some bacteria, however, notably the tuberculosis
bacillus, have coats that are resistant to lysosomal
digestion and also secrete substances that partially
resist the killing effects of the neutrophils and
macrophages. These bacteria are responsible for
many of the chronic diseases, an example of which is
tuberculosis.
Monocyte-Macrophage Cell
System (Reticuloendothelial
System)
In the preceding paragraphs, we described the
macrophages mainly as mobile cells that are capable
of wandering through the tissues. However, after
entering the tissues and becoming macrophages,
another large portion of monocytes becomes attached
to the tissues and remains attached for months or even
years until they are called on to perform specific local
protective functions. They have the same capabilities
as the mobile macrophages to phagocytize large
quantities of bacteria, viruses, necrotic tissue, or other
foreign particles in the tissue. And, when appropriately
stimulated, they can break away from their attachments and once again become mobile macrophages
that respond to chemotaxis and all the other stimuli
related to the inflammatory process.Thus, the body has
a widespread “monocyte-macrophage system” in virtually all tissue areas.
The total combination of monocytes, mobile
macrophages, fixed tissue macrophages, and a few
Chapter 33
Resistance of the Body to Infection: I. Leukocytes, Granulocytes, the Monocyte-Macrophage System
specialized endothelial cells in the bone marrow,
spleen, and lymph nodes is called the reticuloendothelial system. However, all or almost all these cells originate from monocytic stem cells; therefore, the
reticuloendothelial system is almost synonymous with
the monocyte-macrophage system. Because the term
reticuloendothelial system is much better known
in medical literature than the term monocytemacrophage system, it should be remembered as a
generalized phagocytic system located in all tissues,
especially in those tissue areas where large quantities
of particles, toxins, and other unwanted substances
must be destroyed.
Tissue Macrophages in the Skin and Subcutaneous Tissues (Histiocytes). Although the skin is mainly impregnable to
infectious agents, this is no longer true when the skin
is broken. When infection begins in a subcutaneous
tissue and local inflammation ensues, local tissue
macrophages can divide in situ and form still more
macrophages. Then they perform the usual functions
of attacking and destroying the infectious agents, as
described earlier.
Macrophages in the Lymph Nodes. Essentially no particulate matter that enters the tissues, such as bacteria, can
be absorbed directly through the capillary membranes
into the blood. Instead, if the particles are not
destroyed locally in the tissues, they enter the lymph
and flow to the lymph nodes located intermittently
along the course of the lymph flow. The foreign particles are then trapped in these nodes in a meshwork of
sinuses lined by tissue macrophages.
Figure 33–3 illustrates the general organization of
the lymph node, showing lymph entering through the
lymph node capsule by way of afferent lymphatics,
then flowing through the nodal medullary sinuses, and
finally passing out the hilus into efferent lymphatics
that eventually empty into the venous blood.
433
Large numbers of macrophages line the lymph
sinuses, and if any particles enter the sinuses by way
of the lymph, the macrophages phagocytize them and
prevent general dissemination throughout the body.
Alveolar Macrophages in the Lungs. Another route by
which invading organisms frequently enter the body
is through the lungs. Large numbers of tissue
macrophages are present as integral components of
the alveolar walls. They can phagocytize particles that
become entrapped in the alveoli. If the particles are
digestible, the macrophages can also digest them and
release the digestive products into the lymph. If the
particle is not digestible, the macrophages often form
a “giant cell” capsule around the particle until such
time—if ever—that it can be slowly dissolved. Such
capsules are frequently formed around tuberculosis
bacilli, silica dust particles, and even carbon particles.
Macrophages (Kupffer Cells) in the Liver Sinusoids. Still
another favorite route by which bacteria invade the
body is through the gastrointestinal tract. Large
numbers of bacteria from ingested food constantly
pass through the gastrointestinal mucosa into the
portal blood. Before this blood enters the general circulation, it passes through the sinusoids of the liver;
these sinusoids are lined with tissue macrophages
called Kupffer cells, shown in Figure 33–4. These cells
form such an effective particulate filtration system that
almost none of the bacteria from the gastrointestinal
tract succeeds in passing from the portal blood into the
general systemic circulation. Indeed, motion pictures
of phagocytosis by Kupffer cells have demonstrated
phagocytosis of a single bacterium in less than 1/100 of a
second.
Afferent lymphatics
Primary
nodule
Capsule
Valve
Subcapsular
sinus
Lymph in
medullary
sinuses
Hilus
Medullary cord
Germinal
center
Kupffer cells
Efferent lymphatics
Figure 33–3
Functional diagram of a lymph node. (Redrawn from Ham AW:
Histology, 6th ed. Philadelphia: JB Lippincott, 1969.) (Modified
from Gartner LP, Hiatt JL: Color Textbook of Histology, 2nd ed.
Philadelphia, WB Saunders, 2001.)
Figure 33–4
Kupffer cells lining the liver sinusoids, showing phagocytosis of
India ink particles into the cytoplasm of the Kupffer cells.
(Redrawn from Copenhaver WM, et al: Bailey’s Textbook of Histology, 10th ed. Baltimore: Williams & Wilkins, 1971.)
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Unit VI
Blood Cells, Immunity, and Blood Clotting
Macrophages of the Spleen and Bone Marrow. If an invad-
ing organism succeeds in entering the general circulation, there are other lines of defense by the tissue
macrophage system, especially by macrophages of the
spleen and bone marrow. In both these tissues,
macrophages have become entrapped by the reticular
meshwork of the two organs, and when foreign particles come in contact with these macrophages, they are
phagocytized.
The spleen is similar to the lymph nodes, except that
blood, instead of lymph, flows through the tissue
spaces of the spleen. Figure 33–5 shows a small peripheral segment of spleen tissue. Note that a small artery
penetrates from the splenic capsule into the splenic
pulp and terminates in small capillaries. The capillaries are highly porous, allowing whole blood to pass out
of the capillaries into cords of red pulp. The blood then
gradually squeezes through the trabecular meshwork
of these cords and eventually returns to the circulation
through the endothelial walls of the venous sinuses.
The trabeculae of the red pulp are lined with vast
numbers of macrophages, and the venous sinuses are
also lined with macrophages. This peculiar passage of
blood through the cords of the red pulp provides an
exceptional means of phagocytizing unwanted debris
in the blood, including especially old and abnormal red
blood cells.
Inflammation: Role of
Neutrophils and Macrophages
Inflammation
When tissue injury occurs, whether caused by bacteria,
trauma, chemicals, heat, or any other phenomenon,
multiple substances are released by the injured tissues
Pulp
Capillaries
Venous sinuses
Vein
Artery
and cause dramatic secondary changes in the surrounding uninjured tissues. This entire complex of
tissue changes is called inflammation.
Inflammation is characterized by (1) vasodilation of
the local blood vessels, with consequent excess local
blood flow; (2) increased permeability of the capillaries, allowing leakage of large quantities of fluid into
the interstitial spaces; (3) often clotting of the fluid in
the interstitial spaces because of excessive amounts of
fibrinogen and other proteins leaking from the capillaries; (4) migration of large numbers of granulocytes
and monocytes into the tissue; and (5) swelling of the
tissue cells. Some of the many tissue products that
cause these reactions are histamine, bradykinin, serotonin, prostaglandins, several different reaction products of the complement system (described in Chapter
34), reaction products of the blood clotting system, and
multiple substances called lymphokines that are
released by sensitized T cells (part of the immune
system; also discussed in Chapter 34). Several of these
substances strongly activate the macrophage system,
and within a few hours, the macrophages begin to
devour the destroyed tissues. But at times, the
macrophages also further injure the still-living tissue
cells.
“Walling-Off” Effect of Inflammation. One of the first
results of inflammation is to “wall off ” the area of
injury from the remaining tissues. The tissue spaces
and the lymphatics in the inflamed area are blocked
by fibrinogen clots so that after a while, fluid barely
flows through the spaces. This walling-off process
delays the spread of bacteria or toxic products.
The intensity of the inflammatory process is usually
proportional to the degree of tissue injury. For
instance, when staphylococci invade tissues, they
release extremely lethal cellular toxins. As a result,
inflammation develops rapidly—indeed, much more
rapidly than the staphylococci themselves can multiply and spread. Therefore, local staphylococcal infection is characteristically walled off rapidly and
prevented from spreading through the body. Streptococci, in contrast, do not cause such intense local tissue
destruction. Therefore, the walling-off process develops slowly over many hours, while many streptococci
reproduce and migrate. As a result, streptococci often
have a far greater tendency to spread through the
body and cause death than do staphylococci, even
though staphylococci are far more destructive to the
tissues.
Macrophage and Neutrophil
Responses During Inflammation
Tissue Macrophage Is a First Line of Defense Against Infection.
Figure 33–5
Functional structures of the spleen. (Modified from Bloom W,
Fawcett DW: A Textbook of Histology, 10th ed. Philadelphia: WB
Saunders, 1975.)
Within minutes after inflammation begins, the
macrophages already present in the tissues, whether
histiocytes in the subcutaneous tissues, alveolar
macrophages in the lungs, microglia in the brain, or
others, immediately begin their phagocytic actions.
Chapter 33
Resistance of the Body to Infection: I. Leukocytes, Granulocytes, the Monocyte-Macrophage System
When activated by the products of infection and
inflammation, the first effect is rapid enlargement of
each of these cells. Next, many of the previously sessile
macrophages break loose from their attachments
and become mobile, forming the first line of defense
against infection during the first hour or so. The
numbers of these early mobilized macrophages often
are not great, but they are lifesaving.
Neutrophil Invasion of the Inflamed Area Is a Second Line of
Defense. Within the first hour or so after inflammation
begins, large numbers of neutrophils begin to invade
the inflamed area from the blood. This is caused by
products from the inflamed tissues that initiate the following reactions: (1) They alter the inside surface of
the capillary endothelium, causing neutrophils to stick
to the capillary walls in the inflamed area. This effect
is called margination and is shown in Figure 33–2. (2)
They cause the intercellular attachments between the
endothelial cells of the capillaries and small venules
to loosen, allowing openings large enough for neutrophils to pass by diapedesis directly from the blood
into the tissue spaces. (3) Other products of inflammation then cause chemotaxis of the neutrophils
toward the injured tissues, as explained earlier.
Thus, within several hours after tissue damage
begins, the area becomes well supplied with neutrophils. Because the blood neutrophils are already
mature cells, they are ready to immediately begin their
scavenger functions for killing bacteria and removing
foreign matter.
Acute Increase in Number of Neutrophils in the Blood—“Neutrophilia.” Also within a few hours after the onset of
acute, severe inflammation, the number of neutrophils
in the blood sometimes increases fourfold to fivefold—from a normal of 4000 to 5000 to 15,000 to
25,000 neutrophils per microliter. This is called neutrophilia, which means an increase in the number of
neutrophils in the blood. Neutrophilia is caused by
products of inflammation that enter the blood stream,
are transported to the bone marrow, and there act on
the stored neutrophils of the marrow to mobilize these
into the circulating blood. This makes even more neutrophils available to the inflamed tissue area.
Second Macrophage Invasion into the Inflamed Tissue Is a Third
Line of Defense. Along with the invasion of neutrophils,
monocytes from the blood enter the inflamed tissue
and enlarge to become macrophages. However, the
number of monocytes in the circulating blood is low:
also, the storage pool of monocytes in the bone
marrow is much less than that of neutrophils. Therefore, the buildup of macrophages in the inflamed tissue
area is much slower than that of neutrophils, requiring
several days to become effective. Furthermore, even
after invading the inflamed tissue, monocytes are still
immature cells, requiring 8 hours or more to swell to
much larger sizes and develop tremendous quantities
of lysosomes; only then do they acquire the full capacity of tissue macrophages for phagocytosis. Yet, after
several days to several weeks, the macrophages finally
435
come to dominate the phagocytic cells of the inflamed
area because of greatly increased bone marrow production of new monocytes, as explained later.
As already pointed out, macrophages can phagocytize far more bacteria (about five times as many) and
far larger particles, including even neutrophils themselves and large quantities of necrotic tissue, than can
neutrophils. Also, the macrophages play an important
role in initiating the development of antibodies, as we
discuss in Chapter 34.
Increased Production of Granulocytes and Monocytes by the
Bone Marrow Is a Fourth Line of Defense. The fourth line of
defense is greatly increased production of both granulocytes and monocytes by the bone marrow. This
results from stimulation of the granulocytic and monocytic progenitor cells of the marrow. However, it
takes 3 to 4 days before newly formed granulocytes
and monocytes reach the stage of leaving the bone
marrow. If the stimulus from the inflamed tissue continues, the bone marrow can continue to produce these
cells in tremendous quantities for months and even
years, sometimes at a rate 20 to 50 times normal.
Feedback Control of the Macrophage and
Neutrophil Responses
Although more than two dozen factors have been
implicated in control of the macrophage response to
inflammation, five of these are believed to play dominant roles. They are shown in Figure 33–6 and consist
of (1) tumor necrosis factor (TNF), (2) interleukin-1
(IL-1), (3) granulocyte-monocyte colony-stimulating
factor (GM-CSF), (4) granulocyte colony-stimulating
factor (G-CSF), and (5) monocyte colony-stimulating
factor (M-CSF). These factors are formed by activated
macrophage cells in the inflamed tissues and in smaller
quantities by other inflamed tissue cells.
The cause of the increased production of granulocytes and monocytes by the bone marrow is mainly
the three colony-stimulating factors, one of which,
GM-CSF, stimulates both granulocyte and monocyte
production; the other two, G-CSF and M-CSF, stimulate granulocyte and monocyte production, respectively. This combination of TNF, IL-1, and colony-stimulating factors provides a powerful feedback mechanism that begins with tissue inflammation and
proceeds to formation of large numbers of defensive
white blood cells that help remove the cause of the
inflammation.
Formation of Pus
When neutrophils and macrophages engulf large
numbers of bacteria and necrotic tissue, essentially
all the neutrophils and many, if not most, of the
macrophages eventually die. After several days, a
cavity is often excavated in the inflamed tissues that
contains varying portions of necrotic tissue, dead neutrophils, dead macrophages, and tissue fluid. This
mixture is commonly known as pus. After the infection has been suppressed, the dead cells and necrotic
tissue in the pus gradually autolyze over a period of
days, and the end products are eventually absorbed
436
Unit VI
Blood Cells, Immunity, and Blood Clotting
INFLAMMATION
Activated
macrophage
TNF
IL-1
Endothelial cells,
fibroblasts,
lymphocytes
TNF
IL-1
GM-CSF
G-CSF
M-CSF
GM-CSF
G-CSF
M-CSF
Bone marrow
Granulocytes
Monocytes/macrophages
Figure 33–6
Control of bone marrow production of granulocytes and monocyte-macrophages in response to multiple growth factors released
from activated macrophages in an inflamed tissue. G-CSF, granulocyte colony-stimulating factor; GM-CSF, granulocyte-monocyte
colony-stimulating factor; IL-1, interleukin-1; M-CSF, monocyte
colony-stimulating factor; TNF, tumor necrosis factor.
into the surrounding tissues and lymph until most of
the evidence of tissue damage is gone.
Eosinophils
The eosinophils normally constitute about 2 per cent
of all the blood leukocytes. Eosinophils are weak
phagocytes, and they exhibit chemotaxis, but in comparison with the neutrophils, it is doubtful that the
eosinophils are significant in protecting against the
usual types of infection.
Eosinophils, however, are often produced in large
numbers in people with parasitic infections, and they
migrate in large numbers into tissues diseased by parasites. Although most parasites are too large to be
phagocytized by eosinophils or any other phagocytic
cells, eosinophils attach themselves to the parasites by
way of special surface molecules and release substances that kill many of the parasites. For instance,
one of the most widespread infections is schistosomiasis, a parasitic infection found in as many as one third
of the population of some Third World countries; the
parasite can invade any part of the body. Eosinophils
attach themselves to the juvenile forms of the parasite
and kill many of them. They do so in several ways: (1)
by releasing hydrolytic enzymes from their granules,
which are modified lysosomes; (2) probably by also
releasing highly reactive forms of oxygen that are
especially lethal to parasites; and (3) by releasing from
the granules a highly larvacidal polypeptide called
major basic protein.
In a few areas of the world, another parasitic disease
that causes eosinophilia is trichinosis. This results from
invasion of the body’s muscles by the Trichinella parasite (“pork worm”) after a person eats undercooked
infested pork.
Eosinophils also have a special propensity to collect
in tissues in which allergic reactions occur, such as in
the peribronchial tissues of the lungs in people with
asthma and in the skin after allergic skin reactions.
This is caused at least partly by the fact that many mast
cells and basophils participate in allergic reactions, as
we discuss in the next paragraph. The mast cells and
basophils release an eosinophil chemotactic factor that
causes eosinophils to migrate toward the inflamed
allergic tissue. The eosinophils are believed to detoxify some of the inflammation-inducing substances
released by the mast cells and basophils and probably
also to phagocytize and destroy allergen-antibody
complexes, thus preventing excess spread of the local
inflammatory process.
Basophils
The basophils in the circulating blood are similar to
the large tissue mast cells located immediately outside
many of the capillaries in the body. Both mast cells and
basophils liberate heparin into the blood, a substance
that can prevent blood coagulation.
The mast cells and basophils also release histamine,
as well as smaller quantities of bradykinin and
serotonin. Indeed, it is mainly the mast cells in
inflamed tissues that release these substances during
inflammation.
The mast cells and basophils play an exceedingly
important role in some types of allergic reactions
because the type of antibody that causes allergic reactions, the immunoglobulin E (IgE) type (see Chapter
34), has a special propensity to become attached to
mast cells and basophils. Then, when the specific
antigen for the specific IgE antibody subsequently
reacts with the antibody, the resulting attachment of
antigen to antibody causes the mast cell or basophil
to rupture and release exceedingly large quantities of
histamine, bradykinin, serotonin, heparin, slow-reacting
substance of anaphylaxis, and a number of lysosomal
enzymes. These cause local vascular and tissue reactions that cause many, if not most, of the allergic manifestations. These reactions are discussed in greater
detail in Chapter 34.
Leukopenia
A clinical condition known as leukopenia occasionally
occurs in which the bone marrow produces very few
Chapter 33
Resistance of the Body to Infection: I. Leukocytes, Granulocytes, the Monocyte-Macrophage System
white blood cells, leaving the body unprotected against
many bacteria and other agents that might invade the
tissues.
Normally, the human body lives in symbiosis with
many bacteria, because all the mucous membranes of
the body are constantly exposed to large numbers of
bacteria. The mouth almost always contains various
spirochetal, pneumococcal, and streptococcal bacteria,
and these same bacteria are present to a lesser extent
in the entire respiratory tract. The distal gastrointestinal tract is especially loaded with colon bacilli. Furthermore, one can always find bacteria on the surfaces
of the eyes, urethra, and vagina. Any decrease in the
number of white blood cells immediately allows invasion of adjacent tissues by bacteria that are already
present.
Within 2 days after the bone marrow stops producing white blood cells, ulcers may appear in the mouth
and colon, or the person might develop some form of
severe respiratory infection. Bacteria from the ulcers
rapidly invade surrounding tissues and the blood.
Without treatment, death often ensues in less than a
week after acute total leukopenia begins.
Irradiation of the body by x-rays or gamma rays, or
exposure to drugs and chemicals that contain benzene
or anthracene nuclei, is likely to cause aplasia of the
bone marrow. Indeed, some common drugs, such as
chloramphenicol (an antibiotic), thiouracil (used to
treat thyrotoxicosis), and even various barbiturate
hypnotics, on very rare occasions cause leukopenia,
thus setting off the entire infectious sequence of this
malady.
After moderate irradiation injury to the bone
marrow, some stem cells, myeloblasts, and hemocytoblasts may remain undestroyed in the marrow and are
capable of regenerating the bone marrow, provided
sufficient time is available. A patient properly treated
with transfusions, plus antibiotics and other drugs to
ward off infection, usually develops enough new bone
marrow within weeks to months for blood cell concentrations to return to normal.
The Leukemias
Uncontrolled production of white blood cells can be
caused by cancerous mutation of a myelogenous or
lymphogenous cell. This causes leukemia, which is
usually characterized by greatly increased numbers
of abnormal white blood cells in the circulating
blood.
437
extramedullary tissues—especially in the lymph nodes,
spleen, and liver.
In myelogenous leukemia, the cancerous process
occasionally produces partially differentiated cells,
resulting in what might be called neutrophilic
leukemia, eosinophilic leukemia, basophilic leukemia,
or monocytic leukemia. More frequently, however, the
leukemia cells are bizarre and undifferentiated and
not identical to any of the normal white blood cells.
Usually, the more undifferentiated the cell, the more
acute is the leukemia, often leading to death within a
few months if untreated. With some of the more differentiated cells, the process can be chronic, sometimes developing slowly over 10 to 20 years. Leukemic
cells, especially the very undifferentiated cells, are
usually nonfunctional for providing the normal protection against infection.
Effects of Leukemia on the Body
The first effect of leukemia is metastatic growth of
leukemic cells in abnormal areas of the body.
Leukemic cells from the bone marrow may reproduce
so greatly that they invade the surrounding bone,
causing pain and, eventually, a tendency for bones to
fracture easily.
Almost all leukemias eventually spread to the
spleen, lymph nodes, liver, and other vascular regions,
regardless of whether the origin of the leukemia is in
the bone marrow or the lymph nodes. Common effects
in leukemia are the development of infection, severe
anemia, and a bleeding tendency caused by thrombocytopenia (lack of platelets). These effects result
mainly from displacement of the normal bone marrow
and lymphoid cells by the nonfunctional leukemic
cells.
Finally, perhaps the most important effect of
leukemia on the body is excessive use of metabolic
substrates by the growing cancerous cells. The
leukemic tissues reproduce new cells so rapidly that
tremendous demands are made on the body reserves
for foodstuffs, specific amino acids, and vitamins. Consequently, the energy of the patient is greatly depleted,
and excessive utilization of amino acids by the
leukemic cells causes especially rapid deterioration of
the normal protein tissues of the body. Thus, while the
leukemic tissues grow, other tissues become debilitated. After metabolic starvation has continued long
enough, this alone is sufficient to cause death.
Types of Leukemia. Leukemias are divided into two
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Blood Cells, Immunity, and Blood Clotting
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4
Resistance of the Body to
Infection: II. Immunity and Allergy
Innate Immunity
The human body has the ability to resist almost all
types of organisms or toxins that tend to damage the
tissues and organs. This capability is called immunity. Much of immunity is acquired immunity that
does not develop until after the body is first
attacked by a bacterium, virus, or toxin, often
requiring weeks or months to develop the immunity. An additional portion of
immunity results from general processes, rather than from processes directed
at specific disease organisms. This is called innate immunity. It includes the
following:
1. Phagocytosis of bacteria and other invaders by white blood cells and cells
of the tissue macrophage system, as described in Chapter 33.
2. Destruction of swallowed organisms by the acid secretions of the stomach
and the digestive enzymes.
3. Resistance of the skin to invasion by organisms.
4. Presence in the blood of certain chemical compounds that attach to
foreign organisms or toxins and destroy them. Some of these compounds
are (1) lysozyme, a mucolytic polysaccharide that attacks bacteria and
causes them to dissolute; (2) basic polypeptides, which react with and
inactivate certain types of gram-positive bacteria; (3) the complement
complex that is described later, a system of about 20 proteins that can be
activated in various ways to destroy bacteria; and (4) natural killer
lymphocytes that can recognize and destroy foreign cells, tumor cells, and
even some infected cells.
This innate immunity makes the human body resistant to such diseases as
some paralytic viral infections of animals, hog cholera, cattle plague, and distemper—a viral disease that kills a large percentage of dogs that become
afflicted with it. Conversely, many lower animals are resistant or even immune
to many human diseases, such as poliomyelitis, mumps, human cholera, measles,
and syphilis, which are very damaging or even lethal to human beings.
Acquired (Adaptive) Immunity
In addition to its generalized innate immunity, the human body has the ability
to develop extremely powerful specific immunity against individual invading
agents such as lethal bacteria, viruses, toxins, and even foreign tissues from other
animals. This is called acquired or adaptive immunity. Acquired immunity is
caused by a special immune system that forms antibodies and/or activated lymphocytes that attack and destroy the specific invading organism or toxin. It is
with this acquired immunity mechanism and some of its associated reactions—
especially the allergies—that this chapter is concerned.
Acquired immunity can often bestow extreme protection. For instance,
certain toxins, such as the paralytic botulinum toxin or the tetanizing toxin of
tetanus, can be protected against in doses as high as 100,000 times the amount
that would be lethal without immunity. This is the reason the treatment process
known as immunization is so important in protecting human beings against
439
440
Unit VI
Blood Cells, Immunity, and Blood Clotting
disease and against toxins, as explained in the course
of this chapter.
Basic Types of Acquired Immunity
Two basic but closely allied types of acquired immunity occur in the body. In one of these the body
develops circulating antibodies, which are globulin
molecules in the blood plasma that are capable of
attacking the invading agent. This type of immunity is
called humoral immunity or B-cell immunity (because
B lymphocytes produce the antibodies). The second
type of acquired immunity is achieved through the
formation of large numbers of activated T lymphocytes
that are specifically crafted in the lymph nodes to
destroy the foreign agent. This type of immunity is
called cell-mediated immunity or T-cell immunity
(because the activated lymphocytes are T lymphocytes). We shall see shortly that both the antibodies
and the activated lymphocytes are formed in the lymphoid tissues of the body. Let us discuss the initiation
of the immune process by antigens.
Both Types of Acquired Immunity Are
Initiated by Antigens
Because acquired immunity does not develop until
after invasion by a foreign organism or toxin, it is clear
that the body must have some mechanism for recognizing this invasion. Each toxin or each type of organism almost always contains one or more specific
chemical compounds in its makeup that are different
from all other compounds. In general, these are proteins or large polysaccharides, and it is they that initiate the acquired immunity. These substances are called
antigens (antibody generations).
For a substance to be antigenic, it usually must have
a high molecular weight, 8000 or greater. Furthermore,
the process of antigenicity usually depends on regularly recurring molecular groups, called epitopes, on
the surface of the large molecule. This also explains
why proteins and large polysaccharides are almost
always antigenic, because both of these have this stereochemical characteristic.
Lymphocytes Are Responsible for
Acquired Immunity
Acquired immunity is the product of the body’s lymphocytes. In people who have a genetic lack of lymphocytes or whose lymphocytes have been destroyed
by radiation or chemicals, no acquired immunity can
develop. And within days after birth, such a person
dies of fulminating bacterial infection unless treated
by heroic measures. Therefore, it is clear that the lymphocytes are essential to survival of the human being.
The lymphocytes are located most extensively in the
lymph nodes, but they are also found in special lymphoid tissues such as the spleen, submucosal areas of
the gastrointestinal tract, thymus, and bone marrow.
The lymphoid tissue is distributed advantageously in
the body to intercept invading organisms or toxins
before they can spread too widely.
In most instances, the invading agent first enters the
tissue fluids and then is carried by way of lymph vessels
to the lymph node or other lymphoid tissue. For
instance, the lymphoid tissue of the gastrointestinal
walls is exposed immediately to antigens invading
from the gut. The lymphoid tissue of the throat and
pharynx (the tonsils and adenoids) is well located to
intercept antigens that enter by way of the upper respiratory tract. The lymphoid tissue in the lymph nodes
is exposed to antigens that invade the peripheral
tissues of the body. And, finally, the lymphoid tissue of
the spleen, thymus, and bone marrow plays the specific
role of intercepting antigenic agents that have succeeded in reaching the circulating blood.
Two Types of Lymphocytes Promote “Cell-Mediated” Immunity
or “Humoral” Immunity—the T and the B Lymphocytes.
Although most lymphocytes in normal lymphoid
tissue look alike when studied under the microscope,
these cells are distinctly divided into two major
populations. One of the populations, the T lymphocytes, is responsible for forming the activated lymphocytes that provide “cell-mediated” immunity, and the
other population, the B lymphocytes, is responsible
for forming antibodies that provide “humoral”
immunity.
Both types of lymphocytes are derived originally in
the embryo from pluripotent hematopoietic stem cells
that form lymphocytes as one of their most important
offspring as they differentiate. Almost all of the lymphocytes that are formed eventually end up in the
lymphoid tissue, but before doing so, they are further
differentiated or “preprocessed” in the following ways.
The lymphocytes that are destined to eventually
form activated T lymphocytes first migrate to and are
preprocessed in the thymus gland, and thus they are
called “T” lymphocytes to designate the role of
the thymus. They are responsible for cell-mediated
immunity.
The other population of lymphocytes—the B lymphocytes that are destined to form antibodies—are
preprocessed in the liver during midfetal life and in the
bone marrow in late fetal life and after birth. This population of cells was first discovered in birds, which have
a special preprocessing organ called the bursa of Fabricius. For this reason, these lymphocytes are called “B”
lymphocytes to designate the role of the bursa, and
they are responsible for humoral immunity. Figure
34–1 shows the two lymphocyte systems for the formation, respectively, of (1) the activated T lymphocytes and (2) the antibodies.
Preprocessing of the T and B
Lymphocytes
Although all lymphocytes in the body originate from
lymphocyte-committed stem cells of the embryo, these
Chapter 34
441
Resistance of the Body to Infection: II. Immunity and Allergy
Cell-Mediated Immunity
Thymus
Activated T
lymphocytes
Antigen
T lymphocytes
Lymph node
Figure 34–1
Formation of antibodies and sensitized lymphocytes by a lymph
node in response to antigens.
This figure also shows the origin
of thymic (T) and bursal (B) lymphocytes that respectively are
responsible for the cell-mediated
and humoral immune processes.
B lymphocytes
Stem cell
Antigen
Plasma
cells
Antibodies
Fetal liver,
bone marrow
stem cells themselves are incapable of forming directly
either activated T lymphocytes or antibodies. Before
they can do so, they must be further differentiated in
appropriate processing areas as follows.
Thymus Gland Preprocesses the T Lymphocytes. The T lym-
phocytes, after origination in the bone marrow, first
migrate to the thymus gland. Here they divide rapidly
and at the same time develop extreme diversity for
reacting against different specific antigens. That is, one
thymic lymphocyte develops specific reactivity against
one antigen. Then the next lymphocyte develops specificity against another antigen. This continues until
there are thousands of different types of thymic lymphocytes with specific reactivities against many thousands of different antigens. These different types of
preprocessed T lymphocytes now leave the thymus
and spread by way of the blood throughout the body
to lodge in lymphoid tissue everywhere.
The thymus also makes certain that any T lymphocytes leaving the thymus will not react against proteins
or other antigens that are present in the body’s
own tissues; otherwise, the T lymphocytes would be
lethal to the person’s own body in only a few days.
The thymus selects which T lymphocytes will be
released by first mixing them with virtually all the specific “self-antigens” from the body’s own tissues. If a T
lymphocyte reacts, it is destroyed and phagocytized
instead of being released. This happens to as many as
90 per cent of the cells. Thus, the only cells that are
finally released are those that are nonreactive against
the body’s own antigens—they react only against antigens from an outside source, such as from a bacterium,
a toxin, or even transplanted tissue from another
person.
Humoral Immunity
Most of the preprocessing of T lymphocytes in the
thymus occurs shortly before birth of a baby and for a
few months after birth. Beyond this period, removal of
the thymus gland diminishes (but does not eliminate)
the T-lymphocytic immune system. However, removal
of the thymus several months before birth can prevent
development of all cell-mediated immunity. Because
this cellular type of immunity is mainly responsible for
rejection of transplanted organs, such as hearts and
kidneys, one can transplant organs with much less likelihood of rejection if the thymus is removed from an
animal a reasonable time before its birth.
Liver and Bone Marrow Preprocess the B Lymphocytes. Much
less is known about the details for preprocessing B
lymphocytes than for preprocessing T lymphocytes. In
the human being, B lymphocytes are known to be preprocessed in the liver during midfetal life and in the
bone marrow during late fetal life and after birth.
B lymphocytes are different from T lymphocytes in
two ways: First, instead of the whole cell developing
reactivity against the antigen, as occurs for the T lymphocytes, the B lymphocytes actively secrete antibodies that are the reactive agents. These agents are large
protein molecules that are capable of combining with
and destroying the antigenic substance, which is
explained elsewhere in this chapter and in Chapter
33. Second, the B lymphocytes have even greater
diversity than the T lymphocytes, thus forming many
millions of types of B-lymphocyte antibodies with different specific reactivities. After preprocessing, the B
lymphocytes, like the T lymphocytes, migrate to lymphoid tissue throughout the body, where they lodge
near but slightly removed from the T-lymphocyte
areas.
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T Lymphocytes and B-Lymphocyte
Antibodies React Highly Specifically
Against Specific Antigens—Role of
Lymphocyte Clones
When specific antigens come in contact with T and B
lymphocytes in the lymphoid tissue, certain of the T
lymphocytes become activated to form activated T
cells, and certain of the B lymphocytes become activated to form antibodies. The activated T cells and
antibodies in turn react highly specifically against the
particular types of antigens that initiated their development. The mechanism of this specificity is the
following.
Millions of Specific Types of Lymphocytes Are Stored in the
Lymphoid Tissue. Millions of different types of pre-
formed B lymphocytes and preformed T lymphocytes
that are capable of forming highly specific types of
antibodies or T cells have been stored in the lymph
tissue, as explained earlier. Each of these preformed
lymphocytes is capable of forming only one type of
antibody or one type of T cell with a single type of
specificity. And only the specific type of antigen with
which it can react can activate it. Once the specific lymphocyte is activated by its antigen, it reproduces wildly,
forming tremendous numbers of duplicate lymphocytes. If it is a B lymphocyte, its progeny will eventually secrete the specific type of antibody that then
circulates throughout the body. If it is a T lymphocyte,
its progeny are specific sensitized T cells that are
released into the lymph and then carried to the blood
and circulated through all the tissue fluids and back
into the lymph, sometimes circulating around and
around in this circuit for months or years.
All the different lymphocytes that are capable of
forming one specificity of antibody or T cell are called
a clone of lymphocytes. That is, the lymphocytes in
each clone are alike and are derived originally from
one or a few early lymphocytes of its specific type.
Origin of the Many Clones
of Lymphocytes
Only several hundred to a few thousand genes code
for the millions of different types of antibodies and T
lymphocytes. At first, it was a mystery how it was possible for so few genes to code for the millions of different specificities of antibody molecules or T cells that
can be produced by the lymphoid tissue, especially
when one considers that a single gene is usually necessary for the formation of each different type of
protein. This mystery has now been solved.
The whole gene for forming each type of T cell or B
cell is never present in the original stem cells from
which the functional immune cells are formed. Instead,
there are only “gene segments”—actually, hundreds of
such segments—but not whole genes. During preprocessing of the respective T- and B-cell lymphocytes,
these gene segments become mixed with one another
in random combinations, in this way finally forming
whole genes.
Because there are several hundred types of gene
segments, as well as millions of different combinations
in which the segments can be arranged in single cells,
one can understand the millions of different cell gene
types that can occur. For each functional T or B lymphocyte that is finally formed, the gene structure codes
for only a single antigen specificity. These mature cells
then become the highly specific T and B cells that
spread to and populate the lymphoid tissue.
Mechanism for Activating a Clone
of Lymphocytes
Each clone of lymphocytes is responsive to only a
single type of antigen (or to several similar antigens
that have almost exactly the same stereochemical
characteristics). The reason for this is the following: In
the case of the B lymphocytes, each of these has on the
surface of its cell membrane about 100,000 antibody
molecules that will react highly specifically with
only one specific type of antigen. Therefore, when
the appropriate antigen comes along, it immediately
attaches to the antibody in the cell membrane;
this leads to the activation process, which we describe
in more detail subsequently. In the case of the T
lymphocytes, molecules similar to antibodies, called
surface receptor proteins (or T-cell markers), are on
the surface of the T-cell membrane, and these, too,
are highly specific for one specified activating
antigen.
Role of Macrophages in the Activation Process. Aside from
the lymphocytes in lymphoid tissue, literally millions
of macrophages are also present in the same tissue.
These line the sinusoids of the lymph nodes, spleen,
and other lymphoid tissue, and they lie in apposition
to many of the lymph node lymphocytes. Most invading organisms are first phagocytized and partially
digested by the macrophages, and the antigenic products are liberated into the macrophage cytosol. The
macrophages then pass these antigens by cell-to-cell
contact directly to the lymphocytes, thus leading to
activation of the specified lymphocytic clones. The
macrophages, in addition, secrete a special activating
substance that promotes still further growth and
reproduction of the specific lymphocytes. This substance is called interleukin-1.
Role of the T Cells in Activation of the B Lymphocytes. Most
antigens activate both T lymphocytes and B lymphocytes at the same time. Some of the T cells that are
formed, called helper cells, secrete specific substances
(collectively called lymphokines) that activate the
specific B lymphocytes. Indeed, without the aid of
these helper T cells, the quantity of antibodies formed
by the B lymphocytes is usually slight. We will discuss
this cooperative relationship between helper T cells
and B cells again after we have an opportunity
to describe the mechanisms of the T-cell system of
immunity.
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Resistance of the Body to Infection: II. Immunity and Allergy
Specific Attributes of the
B-Lymphocyte System—Humoral
Immunity and the Antibodies
Formation of Antibodies by Plasma Cells. Before exposure
to a specific antigen, the clones of B lymphocytes
remain dormant in the lymphoid tissue. On entry of a
foreign antigen, macrophages in the lymphoid tissue
phagocytize the antigen and then present it to adjacent
B lymphocytes. In addition, the antigen is presented to
T cells at the same time, and activated helper T cells
are formed. These helper cells also contribute to
extreme activation of the B lymphocytes, as we discuss
more fully later.
Those B lymphocytes specific for the antigen immediately enlarge and take on the appearance of
lymphoblasts. Some of the lymphoblasts further differentiate to form plasmablasts, which are precursors
of plasma cells. In the plasmablasts, the cytoplasm
expands and the rough endoplasmic reticulum vastly
proliferates. The plasmablasts then begin to divide at
a rate of about once every 10 hours for about nine divisions, giving in 4 days a total population of about 500
cells for each original plasmablast. The mature plasma
cell then produces gamma globulin antibodies at an
extremely rapid rate—about 2000 molecules per
second for each plasma cell. In turn, the antibodies are
secreted into the lymph and carried to the circulating
blood.This process continues for several days or weeks
until finally exhaustion and death of the plasma cells
occur.
Formation of “Memory” Cells—Difference Between Primary
Response and Secondary Response. A few of the lym-
phoblasts formed by activation of a clone of B lymphocytes do not go on to form plasma cells but instead
form moderate numbers of new B lymphocytes similar
to those of the original clone. In other words, the Bcell population of the specifically activated clone
becomes greatly enhanced, and the new B lymphocytes are added to the original lymphocytes of the
same clone. They also circulate throughout the body to
populate all the lymphoid tissue; immunologically,
however, they remain dormant until activated once
again by a new quantity of the same antigen. These
lymphocytes are called memory cells. Subsequent
exposure to the same antigen will cause a much more
rapid and much more potent antibody response this
second time around, because there are many more
memory cells than there were original B lymphocytes
of the specific clone.
Figure 34–2 shows the differences between the
primary response for forming antibodies that occurs on
first exposure to a specific antigen and the secondary
response that occurs after second exposure to the same
antigen. Note the 1-week delay in the appearance of
the primary response, its weak potency, and its short
life. The secondary response, by contrast, begins
rapidly after exposure to the antigen (often within
hours), is far more potent, and forms antibodies for
many months rather than for only a few weeks. The
128
Concentration of antibody
Chapter 34
Secondary
Primary
64
32
16
8
0
0
2
4
Weeks
6
8
Figure 34–2
Time course of the antibody response in the circulating blood to
a primary injection of antigen and to a secondary injection several
months later.
increased potency and duration of the secondary
response explain why immunization is usually accomplished by injecting antigen in multiple doses with
periods of several weeks or several months between
injections.
Nature of the Antibodies
The antibodies are gamma globulins called immunoglobulins (abbreviated as Ig), and they have molecular weights between 160,000 and 970,000. They usually
constitute about 20 per cent of all the plasma proteins.
All the immunoglobulins are composed of combinations of light and heavy polypeptide chains. Most are
a combination of two light and two heavy chains,
as shown in Figure 34–3. However, some of the
immunoglobulins have combinations of as many as
10 heavy and 10 light chains, which gives rise to
high-molecular-weight immunoglobulins. Yet, in all
immunoglobulins, each heavy chain is paralleled by a
light chain at one of its ends, thus forming a heavy-light
pair, and there are always at least 2 and as many as 10
such pairs in each immunoglobulin molecule.
Figure 34–3 shows by the circled area a designated
end of each light and heavy chain, called the variable
portion; the remainder of each chain is called the constant portion. The variable portion is different for each
specificity of antibody, and it is this portion that
attaches specifically to a particular type of antigen. The
constant portion of the antibody determines other
properties of the antibody, establishing such factors as
diffusivity of the antibody in the tissues, adherence of
the antibody to specific structures within the tissues,
attachment to the complement complex, the ease with
which the antibodies pass through membranes, and
other biological properties of the antibody.
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Antigen-binding sites
Antigen
Variable portion
Antigen
S
Light
chain
•S
S
S•S
S•S
Antibodies
•S
Constant portion
Heavy
chain
Figure 34–3
Structure of the typical IgG antibody, showing it to be composed
of two heavy polypeptide chains and two light polypeptide chains.
The antigen binds at two different sites on the variable portions of
the chains.
Specificity of Antibodies. Each antibody is specific for a
particular antigen; this is caused by its unique structural organization of amino acids in the variable portions of both the light and heavy chains. The amino
acid organization has a different steric shape for each
antigen specificity, so that when an antigen comes in
contact with it, multiple prosthetic groups of the
antigen fit as a mirror image with those of the antibody, thus allowing rapid and tight bonding between
the antibody and the antigen. When the antibody is
highly specific, there are so many bonding sites that the
antibody-antigen coupling is exceedingly strong, held
together by (1) hydrophobic bonding, (2) hydrogen
bonding, (3) ionic attractions, and (4) van der Waals
forces. It also obeys the thermodynamic mass action
law.
Concentration of bound antibody-antigen
Ka =
Concentration of antibody
¥ Concentration of antigen
Ka is called the affinity constant and is a measure of
how tightly the antibody binds with the antigen.
Note, especially, in Figure 34–3 that there are two
variable sites on the illustrated antibody for attachment of antigens, making this type of antibody bivalent. A small proportion of the antibodies, which
consist of combinations of up to 10 light and 10 heavy
chains, have as many as 10 binding sites.
Classes of Antibodies. There are five general classes of
antibodies, respectively named IgM, IgG, IgA, IgD,
and IgE. Ig stands for immunoglobulin, and the other
five respective letters designate the respective classes.
For the purpose of our present limited discussion,
two of these classes of antibodies are of particular
importance: IgG, which is a bivalent antibody and constitutes about 75 per cent of the antibodies of the
normal person, and IgE, which constitutes only a small
percentage of the antibodies but is especially involved
Figure 34–4
Binding of antigen molecules to one another by bivalent
antibodies.
in allergy. The IgM class is also interesting because
a large share of the antibodies formed during the
primary response are of this type. These antibodies
have 10 binding sites that make them exceedingly
effective in protecting the body against invaders, even
though there are not many IgM antibodies.
Mechanisms of Action of Antibodies
Antibodies act mainly in two ways to protect the body
against invading agents: (1) by direct attack on the
invader and (2) by activation of the “complement
system” that then has multiple means of its own for
destroying the invader.
Direct Action of Antibodies on Invading Agents. Figure 34–4
shows antibodies (designated by the red Y-shaped
bars) reacting with antigens (designated by the shaded
objects). Because of the bivalent nature of the antibodies and the multiple antigen sites on most invading agents, the antibodies can inactivate the invading
agent in one of several ways, as follows:
1. Agglutination, in which multiple large particles
with antigens on their surfaces, such as bacteria or
red cells, are bound together into a clump
2. Precipitation, in which the molecular complex of
soluble antigen (such as tetanus toxin) and
antibody becomes so large that it is rendered
insoluble and precipitates
3. Neutralization, in which the antibodies cover the
toxic sites of the antigenic agent
4. Lysis, in which some potent antibodies are
occasionally capable of directly attacking
membranes of cellular agents and thereby cause
rupture of the agent
These direct actions of antibodies attacking the antigenic invaders often are not strong enough to play a
major role in protecting the body against the invader.
Most of the protection comes through the amplifying
effects of the complement system described next.
Chapter 34
445
Resistance of the Body to Infection: II. Immunity and Allergy
Antigen–antibody complex
Opsonization of bacteria
C1
C1
Activate mast
cells and basophils
C4 + C2
C42 + C4a
Chemotaxis of
white blood cells
C3b + C3a
C3
C5b + C5a
C5
Micro-organism +
B and D
C6 + C7
C5b67
Figure 34–5
Cascade of reactions during
activation of the classic pathway
of complement. (Modified from
Alexander JW, Good RA: Fundamentals of Clinical Immunology.
Philadelphia:
WB
Saunders,
1977.)
C8 + C9
Lysis of cells
Complement System for Antibody Action
“Complement” is a collective term that describes a
system of about 20 proteins, many of which are enzyme
precursors. The principal actors in this system are 11
proteins designated C1 through C9, B, and D, shown
in Figure 34–5. All these are present normally among
the plasma proteins in the blood as well as among the
proteins that leak out of the capillaries into the tissue
spaces. The enzyme precursors are normally inactive,
but they can be activated mainly by the so-called
classic pathway.
Classic Pathway. The classic pathway is initiated by an
antigen-antibody reaction. That is, when an antibody
binds with an antigen, a specific reactive site on the
“constant” portion of the antibody becomes uncovered, or “activated,” and this in turn binds directly with
the C1 molecule of the complement system, setting
into motion a “cascade” of sequential reactions, shown
in Figure 34–5, beginning with activation of the proenzyme C1 itself. The C1 enzymes that are formed then
activate successively increasing quantities of enzymes in
the later stages of the system, so that from a small
beginning, an extremely large “amplified” reaction
occurs. Multiple end products are formed, as shown to
the right in the figure, and several of these cause
important effects that help to prevent damage to the
body’s tissues caused by the invading organism or
toxin. Among the more important effects are the
following:
1. Opsonization and phagocytosis. One of the
products of the complement cascade, C3b,
strongly activates phagocytosis by both
neutrophils and macrophages, causing these cells
C5b6789
2.
3.
4.
5.
6.
to engulf the bacteria to which the antigenantibody complexes are attached. This process is
called opsonization. It often enhances the number
of bacteria that can be destroyed by many
hundredfold.
Lysis. One of the most important of all the
products of the complement cascade is the lytic
complex, which is a combination of multiple
complement factors and designated C5b6789.
This has a direct effect of rupturing the cell
membranes of bacteria or other invading
organisms.
Agglutination. The complement products also
change the surfaces of the invading organisms,
causing them to adhere to one another, thus
promoting agglutination.
Neutralization of viruses. The complement
enzymes and other complement products can
attack the structures of some viruses and thereby
render them nonvirulent.
Chemotaxis. Fragment C5a initiates chemotaxis of
neutrophils and macrophages, thus causing large
numbers of these phagocytes to migrate into the
tissue area adjacent to the antigenic agent.
Activation of mast cells and basophils. Fragments
C3a, C4a, and C5a activate mast cells and
basophils, causing them to release histamine,
heparin, and several other substances into the
local fluids. These substances in turn cause
increased local blood flow, increased leakage of
fluid and plasma protein into the tissue, and other
local tissue reactions that help inactivate or
immobilize the antigenic agent. The same factors
play a major role in inflammation (which was
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Unit VI
Blood Cells, Immunity, and Blood Clotting
discussed in Chapter 33) and in allergy, as we
discuss later.
7. Inflammatory effects. In addition to inflammatory
effects caused by activation of the mast cells and
basophils, several other complement products
contribute to local inflammation. These products
cause (1) the already increased blood flow to
increase still further, (2) the capillary leakage of
proteins to be increased, and (3) the interstitial
fluid proteins to coagulate in the tissue spaces,
thus preventing movement of the invading
organism through the tissues.
T cell
Cell-cell
adhesion
proteins
T-cell receptor
Foreign protein
MHC protein
Antigenpresenting
cell
Special Attributes of the
T-Lymphocyte System–Activated
T Cells and Cell-Mediated Immunity
Figure 34–6
Release of Activated T Cells from Lymphoid Tissue and Formation of Memory Cells. On exposure to the proper antigen,
as presented by adjacent macrophages, the T lymphocytes of a specific lymphocyte clone proliferate and
release large numbers of activated, specifically reacting T cells in ways that parallel antibody release by
activated B cells. The principal difference is that
instead of releasing antibodies, whole activated T cells
are formed and released into the lymph. These then
pass into the circulation and are distributed throughout the body, passing through the capillary walls into
the tissue spaces, back into the lymph and blood once
again, and circulating again and again throughout the
body, sometimes lasting for months or even years.
Also, T-lymphocyte memory cells are formed in the
same way that B memory cells are formed in the antibody system. That is, when a clone of T lymphocytes is
activated by an antigen, many of the newly formed
lymphocytes are preserved in the lymphoid tissue to
become additional T lymphocytes of that specific
clone; in fact, these memory cells even spread throughout the lymphoid tissue of the entire body. Therefore,
on subsequent exposure to the same antigen anywhere
in the body, release of activated T cells occurs far more
rapidly and much more powerfully than had occurred
during first exposure.
Antigen-Presenting Cells, MHC Proteins, and Antigen Receptors on the T Lymphocytes. T-cell responses are extremely
antigen specific, like the antibody responses of B cells,
and are at least as important as antibodies in defending against infection. In fact, acquired immune
responses usually require assistance from T cells to
begin the process, and T cells play a major role in actually helping to eliminate invading pathogens.
Although B lymphocytes recognize intact antigens,
T lymphocytes respond to antigens only when they are
bound to specific molecules called MHC proteins on
the surface of antigen-presenting cells in the lymphoid
tissues (Figure 34–6). The three major types of
antigen-presenting cells are macrophages, B lymphocytes, and dendritic cells. The dendritic cells, the most
potent of the antigen-presenting cells, are located
throughout the body, and their only known function is
Activation of T cells requires interaction of T-cell receptors with an
antigen (foreign protein) that is transported to the surface of the
antigen-presenting cell by a major histocompatibility complex
(MHC) protein. Cell-to-cell adhesion proteins enable the T cell
to bind to the antigen-presenting cell long enough to become
activated.
to present antigens to T cells. Interaction of cell adhesion proteins is critical in permitting the T cells to bind
to antigen-presenting cells long enough to become
activated.
The MHC proteins are encoded by a large group of
genes called the major histocompatibility complex
(MHC). The MHC proteins bind peptide fragments
of antigen proteins that are degraded inside antigenpresenting cells and then transport them to the cell
surface. There are two types of MHC proteins: (1)
MHC I proteins, which present antigens to cytotoxic T
cells, and (2) MHC II proteins, which present antigens
to T helper cells. The specific functions of cytotoxic and
helper T cells are discussed later.
The antigens on the surface of antigen-presenting
cells bind with receptor molecules on the surfaces of T
cells in the same way that they bind with plasma
protein antibodies. These receptor molecules are composed of a variable unit similar to the variable portion
of the humoral antibody, but its stem section is firmly
bound to the cell membrane of the T lymphocyte.
There are as many as 100,000 receptor sites on a single
T cell.
Several Types of T Cells and Their
Different Functions
It has become clear that there are multiple types of T
cells. They are classified into three major groups: (1)
helper T cells, (2) cytotoxic T cells, and (3) suppressor
T cells. The functions of each of these are distinct.
Helper T Cells—Their Role in Overall
Regulation of Immunity
The helper T cells are by far the most numerous of the
T cells, usually constituting more than three quarters
Chapter 34
Resistance of the Body to Infection: II. Immunity and Allergy
lethal effects of AIDS. Some of the specific regulatory
functions are the following.
Preprocessor
Preprocessor
areas
areas
Processed
antigen
Antigen
Stimulation of Growth and Proliferation of Cytotoxic T
Cells and Suppressor T Cells. In the absence of helper
MHC
Interleukin-1
Antigen-specific receptor
Cytotoxic
T cells
Helper
T cells
Suppressor
T cells
Differentiation
Plasma
cells
IgM
IgG
Antigen
IgA
B cell
T cells, the clones for producing cytotoxic T cells and
suppressor T cells are activated only slightly by most
antigens. The lymphokine interleukin-2 has an especially strong stimulatory effect in causing growth and
proliferation of both cytotoxic and suppressor T cells.
In addition, several of the other lymphokines have less
potent effects.
Stimulation of B-Cell Growth and Differentiation to
Form Plasma Cells and Antibodies. The direct actions
Lymphokines!!
Proliferation
447
IgE
Figure 34–7
Regulation of the immune system, emphasizing a pivotal role of
the helper T cells. MHC, major histocompatibility complex.
of antigen to cause B-cell growth, proliferation, formation of plasma cells, and secretion of antibodies are
also slight without the “help” of the helper T cells.
Almost all the interleukins participate in the B-cell
response, but especially interleukins 4, 5, and 6. In fact,
these three interleukins have such potent effects on
the B cells that they have been called B-cell stimulating factors or B-cell growth factors.
Activation of the Macrophage System. The lymphokines also affect the macrophages. First, they slow
or stop the migration of the macrophages after they
have been chemotactically attracted into the inflamed
tissue area, thus causing great accumulation of
macrophages. Second, they activate the macrophages
to cause far more efficient phagocytosis, allowing them
to attack and destroy increasing numbers of invading
bacteria or other tissue-destroying agents.
Feedback Stimulatory Effect on the Helper Cells Themselves. Some of the lymphokines, especially inter-
of all of them. As their name implies, they help in the
functions of the immune system, and they do so in
many ways. In fact, they serve as the major regulator
of virtually all immune functions, as shown in Figure
34–7. They do this by forming a series of protein mediators, called lymphokines, that act on other cells of the
immune system as well as on bone marrow cells.
Among the important lymphokines secreted by the
helper T cells are the following:
Interleukin-2
Interleukin-3
Interleukin-4
Interleukin-5
Interleukin-6
Granulocyte-monocyte colony-stimulating factor
Interferon-g
Specific Regulatory Functions of the Lymphokines. In the
absence of the lymphokines from the helper T cells,
the remainder of the immune system is almost paralyzed. In fact, it is the helper T cells that are inactivated
or destroyed by the acquired immunodeficiency syndrome (AIDS) virus, which leaves the body almost
totally unprotected against infectious disease, therefore leading to the now well-known debilitating and
leukin-2, have a direct positive feedback effect in
stimulating activation of the helper T cells themselves.
This acts as an amplifier by further enhancing the
helper cell response as well as the entire immune
response to an invading antigen.
Cytotoxic T Cells
The cytotoxic T cell is a direct-attack cell that is
capable of killing micro-organisms and, at times, even
some of the body’s own cells. For this reason, these
cells are called killer cells. The receptor proteins on the
surfaces of the cytotoxic cells cause them to bind
tightly to those organisms or cells that contain the
appropriate binding-specific antigen. Then, they kill
the attacked cell in the manner shown in Figure 34–8.
After binding, the cytotoxic T cell secretes holeforming proteins, called perforins, that literally punch
round holes in the membrane of the attacked cell.
Then fluid flows rapidly into the cell from the interstitial space. In addition, the cytotoxic T cell releases
cytotoxic substances directly into the attacked cell.
Almost immediately, the attacked cell becomes greatly
swollen, and it usually dissolves shortly thereafter.
Especially important, these cytotoxic killer cells can
pull away from the victim cells after they have
punched holes and delivered cytotoxic substances and
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Unit VI
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Cytotoxic
T cells
(killer cells)
Cytotoxic
and
digestive
enzymes
Antigen
receptors
as being distinctive from bacteria or viruses, and the
person’s immunity system forms few antibodies or
activated T cells against his or her own antigens.
Most Tolerance Results from Clone Selection During Preprocessing. It is believed that most tolerance develops
Attacked
cell
Specific
binding
Antigen
Figure 34–8
Direct destruction of an invading cell by sensitized lymphocytes
(cytotoxic T cells).
then move on to kill more cells. Indeed, some of these
cells persist for months in the tissues.
Some of the cytotoxic T cells are especially lethal to
tissue cells that have been invaded by viruses because
many virus particles become entrapped in the membranes of the tissue cells and attract T cells in response
to the viral antigenicity. The cytotoxic cells also play
an important role in destroying cancer cells, heart
transplant cells, or other types of cells that are foreign
to the person’s own body.
Suppressor T Cells
Much less is known about the suppressor T cells than
about the others, but they are capable of suppressing
the functions of both cytotoxic and helper T cells. It is
believed that these suppressor functions serve the
purpose of preventing the cytotoxic cells from causing
excessive immune reactions that might be damaging to
the body’s own tissues. For this reason, the suppressor
cells are classified, along with the helper T cells, as regulatory T cells. It is probable that the suppressor T-cell
system plays an important role in limiting the ability
of the immune system to attack a person’s own body
tissues, called immune tolerance, as we discuss in the
next section.
Tolerance of the Acquired Immunity
System to One’s Own Tissues—Role
of Preprocessing in the Thymus and
Bone Marrow
If a person should become immune to his or her own
tissues, the process of acquired immunity would
destroy the individual’s own body. The immune mechanism normally “recognizes” a person’s own tissues
during preprocessing of T lymphocytes in the thymus
and of B lymphocytes in the bone marrow. The reason
for this belief is that injecting a strong antigen into a
fetus while the lymphocytes are being preprocessed in
these two areas prevents development of clones of
lymphocytes in the lymphoid tissue that are specific for
the injected antigen. Experiments have shown that
specific immature lymphocytes in the thymus, when
exposed to a strong antigen, become lymphoblastic,
proliferate considerably, and then combine with the
stimulating antigen—an effect that is believed to cause
the cells themselves to be destroyed by the thymic
epithelial cells before they can migrate to and colonize
the total body lymphoid tissue.
It is believed that during the preprocessing of lymphocytes in the thymus and bone marrow, all or most
of those clones of lymphocytes that are specific to
damage the body’s own tissues are self-destroyed
because of their continual exposure to the body’s
antigens.
Failure of the Tolerance Mechanism Causes Autoimmune Diseases. Sometimes people lose their immune tolerance
of their own tissues. This occurs to a greater extent the
older a person becomes. It usually occurs after destruction of some of the body’s own tissues, which releases
considerable quantities of “self-antigens” that circulate in the body and presumably cause acquired
immunity in the form of either activated T cells or
antibodies.
Several specific diseases that result from autoimmunity include (1) rheumatic fever, in which the body
becomes immunized against tissues in the joints and
heart, especially the heart valves, after exposure to a
specific type of streptococcal toxin that has an epitope
in its molecular structure similar to the structure of
some of the body’s own self-antigens; (2) one type of
glomerulonephritis, in which the person becomes
immunized against the basement membranes of
glomeruli; (3) myasthenia gravis, in which immunity
develops against the acetylcholine receptor proteins of
the neuromuscular junction, causing paralysis; and (4)
lupus erythematosus, in which the person becomes
immunized against many different body tissues at the
same time, a disease that causes extensive damage and
often rapid death.
Immunization by Injection of Antigens
Immunization has been used for many years to
produce acquired immunity against specific diseases.A
person can be immunized by injecting dead organisms
that are no longer capable of causing disease but that
still have some of their chemical antigens. This type of
immunization is used to protect against typhoid fever,
Chapter 34
Resistance of the Body to Infection: II. Immunity and Allergy
449
whooping cough, diphtheria, and many other types of
bacterial diseases.
Immunity can be achieved against toxins that have
been treated with chemicals so that their toxic nature
has been destroyed even though their antigens for
causing immunity are still intact. This procedure is
used in immunizing against tetanus, botulism, and
other similar toxic diseases.
And, finally, a person can be immunized by being
infected with live organisms that have been “attenuated.” That is, these organisms either have been grown
in special culture media or have been passed through
a series of animals until they have mutated enough
that they will not cause disease but do still carry specific antigens required for immunization. This procedure is used to protect against poliomyelitis, yellow
fever, measles, smallpox, and many other viral diseases.
diffuse from the circulating blood in large numbers
into the skin to respond to the poison ivy toxin. And,
at the same time, these T cells elicit a cell-mediated
type of immune reaction. Remembering that this type
of immunity can cause release of many toxic substances from the activated T cells as well as extensive
invasion of the tissues by macrophages along with
their subsequent effects, one can well understand that
the eventual result of some delayed-reaction allergies
can be serious tissue damage. The damage normally
occurs in the tissue area where the instigating antigen
is present, such as in the skin in the case of poison ivy,
or in the lungs to cause lung edema or asthmatic
attacks in the case of some airborne antigens.
Passive Immunity
Some people have an “allergic” tendency. Their allergies are called atopic allergies because they are caused
by a nonordinary response of the immune system. The
allergic tendency is genetically passed from parent to
child and is characterized by the presence of large
quantities of IgE antibodies in the blood. These antibodies are called reagins or sensitizing antibodies to
distinguish them from the more common IgG antibodies. When an allergen (defined as an antigen
that reacts specifically with a specific type of IgE
reagin antibody) enters the body, an allergen-reagin
reaction lakes place, and a subsequent allergic reaction
occurs.
A special characteristic of the IgE antibodies (the
reagins) is a strong propensity to attach to mast cells
and basophils. Indeed, a single mast cell or basophil
can bind as many as half a million molecules of IgE
antibodies. Then, when an antigen (an allergen) that
has multiple binding sites binds with several IgE antibodies that are already attached to a mast cell or
basophil, this causes immediate change in the membrane of the mast cell or basophil, perhaps resulting
from a physical effect of the antibody molecules to
contort the cell membrane. At any rate, many of the
mast cells and basophils rupture; others release special
agents immediately or shortly thereafter, including histamine, protease, slow-reacting substance of anaphylaxis (which is a mixture of toxic leukotrienes),
eosinophil chemotactic substance, neutrophil chemotactic substance, heparin, and platelet activating factors.
These substances cause such effects as dilation of the
local blood vessels; attraction of eosinophils and neutrophils to the reactive site; increased permeability of
the capillaries with loss of fluid into the tissues; and
contraction of local smooth muscle cells. Therefore,
several different tissue responses can occur, depending on the type of tissue in which the allergen-reagin
reaction occurs. Among the different types of allergic
reactions caused in this manner are the following.
Thus far, all the acquired immunity we have discussed
has been active immunity. That is, the person’s own
body develops either antibodies or activated T cells in
response to invasion of the body by a foreign antigen.
However, temporary immunity can be achieved in a
person without injecting any antigen. This is done by
infusing antibodies, activated T cells, or both obtained
from the blood of someone else or from some other
animal that has been actively immunized against the
antigen.
Antibodies last in the body of the recipient for 2 to
3 weeks, and during that time, the person is protected
against the invading disease. Activated T cells last for
a few weeks if transfused from another person but
only for a few hours to a few days if transfused
from an animal. Such transfusion of antibodies or T
lymphocytes to confer immunity is called passive
immunity.
ALLERGY AND
HYPERSENSITIVITY
An important undesirable side effect of immunity is
the development, under some conditions, of allergy or
other types of immune hypersensitivity. There are
several types of allergy and other hypersensitivities,
some of which occur only in people who have a specific allergic tendency.
Allergy Caused by Activated T Cells:
Delayed-Reaction Allergy
Delayed-reaction allergy is caused by activated T cells
and not by antibodies. In the case of poison ivy, the
toxin of poison ivy in itself does not cause much harm
to the tissues. However, on repeated exposure, it does
cause the formation of activated helper and cytotoxic
T cells. Then, after subsequent exposure to the poison
ivy toxin, within a day or so, the activated T cells
Allergies in the “Allergic” Person,
Who Has Excess IgE Antibodies
Anaphylaxis. When a specific allergen is injected
directly into the circulation, the allergen can react with
basophils of the blood and mast cells in the tissues
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Blood Cells, Immunity, and Blood Clotting
located immediately outside the small blood vessels
if the basophils and mast cells have been sensitized
by attachment of IgE reagins. Therefore, a widespread
allergic reaction occurs throughout the vascular
system and closely associated tissues. This is called
anaphylaxis. Histamine is released into the circulation
and causes body-wide vasodilation as well as increased
permeability of the capillaries with resultant marked
loss of plasma from the circulation. An occasional
person who experiences this reaction dies of circulatory shock within a few minutes unless treated with
epinephrine to oppose the effects of the histamine.
Also released from the activated basophils and mast
cells is a mixture of leukotrienes called slow-reacting
substance of anaphylaxis. These leukotrienes can cause
spasm of the smooth muscle of the bronchioles, eliciting an asthma-like attack, sometimes causing death by
suffocation.
Urticaria. Urticaria results from antigen entering specific skin areas and causing localized anaphylactoid
reactions. Histamine released locally causes (1) vasodilation that induces an immediate red flare and (2)
increased local permeability of the capillaries that
leads to local circumscribed areas of swelling of the
skin within another few minutes. The swellings are
commonly called hives. Administration of antihistamine drugs to a person before exposure will prevent
the hives.
Hay Fever. In hay fever, the allergen-reagin reaction
occurs in the nose. Histamine released in response to
the reaction causes local intranasal vascular dilation,
with resultant increased capillary pressure as well as
increased capillary permeability. Both these effects
cause rapid fluid leakage into the nasal cavities and
into associated deeper tissues of the nose; and the
nasal linings become swollen and secretory. Here
again, use of antihistamine drugs can prevent this
swelling reaction. But other products of the allergenreagin reaction can still cause irritation of the nose,
eliciting the typical sneezing syndrome.
Asthma. Asthma often occurs in the “allergic” type of
person. In such a person, the allergen-reagin reaction
occurs in the bronchioles of the lungs. Here, an important product released from the mast cells is believed
to be the slow-reacting substance of anaphylaxis, which
causes spasm of the bronchiolar smooth muscle. Consequently, the person has difficulty breathing until the
reactive products of the allergic reaction have been
removed. Administration of antihistaminics has less
effect on the course of asthma because histamine does
not appear to be the major factor eliciting the asthmatic reaction.
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Alberts B, Johnson A, Lewis J, et al: Molecular Biology of
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Cheroutre H, Madakamutil L: Acquired and natural
memory T cells join forces at the mucosal front line. Nat
Rev Immunol 4:290, 2004.
Cooper MA, Pommering TL, Koranyi K: Primary immunodeficiencies. Am Fam Physician 68:2001, 2003.
Cossart P, Sansonetti PJ: Bacterial invasion: the paradigms
of enteroinvasive pathogens. Science 304:242, 2004.
Eisenbarth GS, Gottlieb PA: Autoimmune polyendocrine
syndromes. N Engl J Med. 350:2068, 2004.
Figdor CG, de Vries IJ, Lesterhuis WJ, Melief CJ: Dendritic
cell immunotherapy: mapping the way. Nat Med 10:475,
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Grossman Z, Min B, Meier-Schellersheim M, Paul WE: Concomitant regulation of T-cell activation and homeostasis.
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Kupper TS, Fuhlbrigge RC: Immune surveillance in the skin:
mechanisms and clinical consequences. Nat Rev Immunol
4:211, 2004.
La Cava A, Matarese G: The weight of leptin in immunity.
Nat Rev Immunol 4:371, 2004.
Linton PJ, Dorshkind K: Age-related changes in lymphocyte
development and function. Nat Immunol 5:133, 2004.
MacGlashan D Jr: Histamine: a mediator of inflammation. J
Allergy Clin Immunol 112(4 Suppl):S53, 2003.
McGeady SJ: Immunocompetence and allergy. Pediatrics
113(4 Suppl):1107, 2004.
Nikolich-Zugich J, Slifka MK, Messaoudi I: The many important facets of T-cell repertoire diversity. Nat Rev Immunol
4:123, 2004 .
Petrie HT: Cell migration and the control of post-natal T-cell
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Scott CC, Botelho RJ, Grinstein S: Phagosome maturation:
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5
Blood Types; Transfusion; Tissue
and Organ Transplantation
Antigenicity Causes Immune
Reactions of Blood
When blood transfusions from one person to
another were first attempted, immediate or delayed
agglutination and hemolysis of the red blood cells
often occurred, resulting in typical transfusion reactions that frequently led to death. Soon it was discovered that the bloods of different people have different antigenic and immune
properties, so that antibodies in the plasma of one blood will react with antigens on the surfaces of the red cells of another blood type. If proper precautions are taken, one can determine ahead of time whether the antibodies
and antigens present in the donor and recipient bloods will cause a transfusion
reaction.
Multiplicity of Antigens in the Blood Cells. At least 30 commonly occurring antigens
and hundreds of other rare antigens, each of which can at times cause antigenantibody reactions, have been found in human blood cells, especially on the surfaces of the cell membranes. Most of the antigens are weak and therefore are
of importance principally for studying the inheritance of genes to establish
parentage.
Two particular types of antigens are much more likely than the others to
cause blood transfusion reactions. They are the O-A-B system of antigens and
the Rh system.
O-A-B Blood Types
A and B Antigens—Agglutinogens
Two antigens—type A and type B—occur on the surfaces of the red blood cells
in a large proportion of human beings. It is these antigens (also called agglutinogens because they often cause blood cell agglutination) that cause most
blood transfusion reactions. Because of the way these agglutinogens are inherited, people may have neither of them on their cells, they may have one, or they
may have both simultaneously.
Major O-A-B Blood Types. In transfusing blood from one person to another, the
bloods of donors and recipients are normally classified into four major O-A-B
blood types, as shown in Table 35–1, depending on the presence or absence of
the two agglutinogens, the A and B agglutinogens. When neither A nor B agglutinogen is present, the blood is type O. When only type A agglutinogen is
present, the blood is type A.When only type B agglutinogen is present, the blood
is type B. When both A and B agglutinogens are present, the blood is type AB.
Genetic Determination of the Agglutinogens. Two genes, one on each of two paired
chromosomes, determine the O-A-B blood type. These genes can be any one of
three types but only one type on each of the two chromosomes: type O, type A,
451
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Table 35–1
Blood Types with Their Genotypes and Their Constituent
Agglutinogens and Agglutinins
Blood Types
Agglutinogens
Agglutinins
OO
O
—
OA or AA
OB or BB
AB
A
B
AB
A
B
A and B
Anti-A and
Anti-B
Anti-B
Anti-A
—
or type B. The type O gene is either functionless or
almost functionless, so that it causes no significant type
O agglutinogen on the cells. Conversely, the type A
and type B genes do cause strong agglutinogens on the
cells.
The six possible combinations of genes, as shown in
Table 35–1, are OO, OA, OB, AA, BB, and AB. These
combinations of genes are known as the genotypes,
and each person is one of the six genotypes.
One can also observe from Table 35–1 that a person
with genotype OO produces no agglutinogens, and
therefore the blood type is O. A person with genotype
OA or AA produces type A agglutinogens and therefore has blood type A. Genotypes OB and BB give
type B blood, and genotype AB gives type AB blood.
Relative Frequencies of the Different Blood Types.
The prevalence of the different blood types among one
group of persons studied was approximately:
O
A
B
AB
47%
41%
9%
3%
It is obvious from these percentages that the O and
A genes occur frequently, whereas the B gene is
infrequent.
Agglutinins
When type A agglutinogen is not present in a person’s
red blood cells, antibodies known as anti-A agglutinins
develop in the plasma. Also, when type B agglutinogen is not present in the red blood cells, antibodies
known as anti-B agglutinins develop in the plasma.
Thus, referring once again to Table 35–1, note that
type O blood, although containing no agglutinogens,
does contain both anti-A and anti-B agglutinins; type
A blood contains type A agglutinogens and anti-B
agglutinins; type B blood contains type B agglutinogens and anti-A agglutinins. Finally, type AB blood
contains both A and B agglutinogens but no
agglutinins.
Titer of the Agglutinins at Different Ages. Immediately after
birth, the quantity of agglutinins in the plasma is
almost zero. Two to 8 months after birth, an infant
begins to produce agglutinins—anti-A agglutinins
Average titer of agglutinins
Genotypes
Anti-A agglutinins in
groups B and O blood
400
Anti-B agglutinins in
groups A and O blood
300
200
100
0
0 10 20 30 40 50 60 70 80 90 100
Age of person (years)
Figure 35–1
Average titers of anti-A and anti-B agglutinins in the plasmas of
people with different blood types.
when type A agglutinogens are not present in the cells,
and anti-B agglutinins when type B agglutinogens are
not in the cells. Figure 35–1 shows the changing titers
of the anti-A and anti-B agglutinins at different ages.
A maximum titer is usually reached at 8 to 10 years
of age, and this gradually declines throughout the
remaining years of life.
Origin of Agglutinins in the Plasma. The agglutinins are
gamma globulins, as are almost all antibodies, and they
are produced by the same bone marrow and lymph
gland cells that produce antibodies to any other antigens. Most of them are IgM and IgG immunoglobulin
molecules.
But why are these agglutinins produced in people
who do not have the respective agglutinogens in their
red blood cells? The answer to this is that small
amounts of type A and B antigens enter the body in
food, in bacteria, and in other ways, and these substances initiate the development of the anti-A and
anti-B agglutinins.
For instance, infusion of group A antigen into a
recipient having a non-A blood type causes a typical
immune response with formation of greater quantities
of anti-A agglutinins than ever. Also, the neonate has
few, if any, agglutinins, showing that agglutinin formation occurs almost entirely after birth.
Agglutination Process In Transfusion
Reactions
When bloods are mismatched so that anti-A or anti-B
plasma agglutinins are mixed with red blood cells that
contain A or B agglutinogens, respectively, the red
Chapter 35
Blood Types; Transfusion; Tissue and Organ Transplantation
cells agglutinate as a result of the agglutinins’ attaching themselves to the red blood cells. Because the
agglutinins have two binding sites (IgG type) or 10
binding sites (IgM type), a single agglutinin can attach
to two or more red blood cells at the same time,
thereby causing the cells to be bound together by the
agglutinin. This causes the cells to clump, which is the
process of “agglutination.” Then these clumps plug
small blood vessels throughout the circulatory system.
During ensuing hours to days, either physical distortion of the cells or attack by phagocytic white blood
cells destroys the membranes of the agglutinated cells,
releasing hemoglobin into the plasma, which is called
“hemolysis” of the red blood cells.
Acute Hemolysis Occurs in Some Transfusion Reactions.
Sometimes, when recipient and donor bloods are mismatched, immediate hemolysis of red cells occurs in
the circulating blood. In this case, the antibodies cause
lysis of the red blood cells by activating the complement system, which releases proteolytic enzymes (the
lytic complex) that rupture the cell membranes, as
described in Chapter 34. Immediate intravascular
hemolysis is far less common than agglutination followed by delayed hemolysis, because not only does
there have to be a high titer of antibodies for lysis to
occur, but also a different type of antibody seems to
be required, mainly the IgM antibodies; these antibodies are called hemolysins.
Blood Typing
Before giving a transfusion to a person, it is necessary
to determine the blood type of the recipient’s blood
and the blood type of the donor blood so that the
bloods can be appropriately matched. This is called
blood typing and blood matching, and these are performed in the following way: The red blood cells are
first separated from the plasma and diluted with saline.
One portion is then mixed with anti-A agglutinin and
another portion with anti-B agglutinin. After several
minutes, the mixtures are observed under a microscope. If the red blood cells have become clumped—
that is, “agglutinated”—one knows that an antibodyantigen reaction has resulted.
Table 35–2 lists the presence (+) or absence (-) of
agglutination of the four types of red blood cells. Type
Table 35–2
O red blood cells have no agglutinogens and therefore
do not react with either the anti-A or the anti-B agglutinins. Type A blood has A agglutinogens and therefore agglutinates with anti-A agglutinins. Type B
blood has B agglutinogens and agglutinates with
anti-B agglutinins. Type AB blood has both A and B
agglutinogens and agglutinates with both types of
agglutinins.
Rh Blood Types
Along with the O-A-B blood type system, the Rh
blood type system is also important when transfusing
blood. The major difference between the O-A-B
system and the Rh system is the following: In the
O-A-B system, the plasma agglutinins responsible for
causing transfusion reactions develop spontaneously,
whereas in the Rh system, spontaneous agglutinins
almost never occur. Instead, the person must first be
massively exposed to an Rh antigen, such as by transfusion of blood containing the Rh antigen, before
enough agglutinins to cause a significant transfusion
reaction will develop.
Rh Antigens—“Rh-Positive” and “Rh-Negative” People.
There are six common types of Rh antigens, each of
which is called an Rh factor. These types are designated C, D, E, c, d, and e. A person who has a C antigen
does not have the c antigen, but the person missing the
C antigen always has the c antigen. The same is true
for the D-d and E-e antigens. Also, because of the
manner of inheritance of these factors, each person has
one of each of the three pairs of antigens.
The type D antigen is widely prevalent in the
population and considerably more antigenic than
the other Rh antigens. Anyone who has this type of
antigen is said to be Rh positive, whereas a person who
does not have type D antigen is said to be Rh negative.
However, it must be noted that even in Rh-negative
people, some of the other Rh antigens can still cause
transfusion reactions, although the reactions are
usually much milder.
About 85 per cent of all white people are Rh positive and 15 per cent, Rh negative. In American blacks,
the percentage of Rh-positives is about 95, whereas in
African blacks, it is virtually 100 per cent.
Rh Immune Response
Blood Typing, Showing Agglutination of Cells of the
Different Blood Types with Anti-A or Anti-B Agglutinins
in the Sera
Red Blood Cell Types
O
A
B
AB
453
Formation of Anti-Rh Agglutinins. When red blood cells
Sera
Anti-A
Anti-B
+
+
+
+
containing Rh factor are injected into a person whose
blood does not contain the Rh factor—that is, into
an Rh-negative person—anti-Rh agglutinins develop
slowly, reaching maximum concentration of agglutinins about 2 to 4 months later. This immune response
occurs to a much greater extent in some people than
in others. With multiple exposures to the Rh factor,
an Rh-negative person eventually becomes strongly
“sensitized” to Rh factor.
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Characteristics of Rh Transfusion Reactions. If an Rhnegative person has never before been exposed to Rhpositive blood, transfusion of Rh-positive blood into
that person will likely cause no immediate reaction.
However, anti-Rh antibodies can develop in sufficient
quantities during the next 2 to 4 weeks to cause agglutination of those transfused cells that are still circulating in the blood. These cells are then hemolyzed by the
tissue macrophage system. Thus, a delayed transfusion
reaction occurs, although it is usually mild. On subsequent transfusion of Rh-positive blood into the same
person, who is now already immunized against the Rh
factor, the transfusion reaction is greatly enhanced and
can be immediate and as severe as a transfusion reaction caused by mismatched type A or B blood.
Erythroblastosis Fetalis (“Hemolytic Disease
of the Newborn”)
Erythroblastosis fetalis is a disease of the fetus and
newborn child characterized by agglutination and
phagocytosis of the fetus’s red blood cells. In most
instances of erythroblastosis fetalis, the mother is Rh
negative and the father Rh positive. The baby has
inherited the Rh-positive antigen from the father, and
the mother develops anti-Rh agglutinins from exposure to the fetus’s Rh antigen. In turn, the mother’s
agglutinins diffuse through the placenta into the fetus
and cause red blood cell agglutination.
Incidence of the Disease. An Rh-negative mother having
her first Rh-positive child usually does not develop
sufficient anti-Rh agglutinins to cause any harm.
However, about 3 per cent of second Rh-positive
babies exhibit some signs of erythroblastosis fetalis;
about 10 per cent of third babies exhibit the disease;
and the incidence rises progressively with subsequent
pregnancies.
Effect of the Mother’s Antibodies on the Fetus. After antiRh antibodies have formed in the mother, they diffuse
slowly through the placental membrane into the
fetus’s blood. There they cause agglutination of the
fetus’s blood. The agglutinated red blood cells subsequently hemolyze, releasing hemoglobin into the
blood. The fetus’s macrophages then convert the
hemoglobin into bilirubin, which causes the baby’s
skin to become yellow (jaundiced). The antibodies can
also attack and damage other cells of the body.
passed from the baby’s bone marrow into the circulatory system, and it is because of the presence of these
nucleated blastic red blood cells that the disease is
called erythroblastosis fetalis.
Although the severe anemia of erythroblastosis
fetalis is usually the cause of death, many children who
barely survive the anemia exhibit permanent mental
impairment or damage to motor areas of the brain
because of precipitation of bilirubin in the neuronal
cells, causing destruction of many, a condition called
kernicterus.
Treatment of the Erythroblastotic Neonate. One treatment
for erythroblastosis fetalis is to replace the neonate’s
blood with Rh-negative blood. About 400 milliliters of
Rh-negative blood is infused over a period of 1.5 or
more hours while the neonate’s own Rh-positive blood
is being removed. This procedure may be repeated
several times during the first few weeks of life, mainly
to keep the bilirubin level low and thereby prevent
kernicterus. By the time these transfused Rh-negative
cells are replaced with the infant’s own Rh-positive
cells, a process that requires 6 or more weeks, the antiRh agglutinins that had come from the mother will
have been destroyed.
Prevention of Erythroblastosis Fetalis. The D antigen of
the Rh blood group system is the primary culprit in
causing immunization of an Rh-negative mother to
an Rh-positive fetus. In the 1970’s, a dramatic reduction in the incidence of erythroblastosis fetalis was
achieved with the development of Rh immunoglobulin globin, an anti-D antibody that is administered to
the expectant mother starting at 28 to 30 weeks of
gestation. The anti-D antibody is also administered to
Rh-negative women who deliver Rh-positive babies to
prevent sensitization of the mothers to the D antigen.
This greatly reduces the risk of developing large
amounts of D antibodies during the second pregnancy.
The mechanism by which Rh immunoglobulin
globin prevents sensitization of the D antigen is not
completely understood, but one effect of the anti-D
antibody is to inhibit antigen-induced B lymphocyte
antibody production in the expectant mother. The
administered anti-D antibody also attaches to Dantigen sites on Rh-positive fetal red blood cells that
may cross the placenta and enter the circulation of
the expectant mother, thereby interfering with the
immune response to the D antigen.
Clinical Picture of Erythroblastosis. The jaundiced, ery-
throblastotic newborn baby is usually anemic at birth,
and the anti-Rh agglutinins from the mother usually
circulate in the infant’s blood for another 1 to 2
months after birth, destroying more and more red
blood cells.
The hematopoietic tissues of the infant attempt to
replace the hemolyzed red blood cells. The liver and
spleen become greatly enlarged and produce red
blood cells in the same manner that they normally do
during the middle of gestation. Because of the rapid
production of red cells, many early forms of red blood
cells, including many nucleated blastic forms, are
Transfusion Reactions Resulting
from Mismatched Blood Types
If donor blood of one blood type is transfused into a
recipient who has another blood type, a transfusion
reaction is likely to occur in which the red blood cells
of the donor blood are agglutinated. It is rare that the
transfused blood causes agglutination of the recipient’s
cells, for the following reason: The plasma portion of
the donor blood immediately becomes diluted by all
the plasma of the recipient, thereby decreasing the
Chapter 35
Blood Types; Transfusion; Tissue and Organ Transplantation
titer of the infused agglutinins to a level usually too
low to cause agglutination. Conversely, the small
amount of infused blood does not significantly dilute
the agglutinins in the recipient’s plasma. Therefore, the
recipient’s agglutinins can still agglutinate the mismatched donor cells.
As explained earlier, all transfusion reactions eventually cause either immediate hemolysis resulting from
hemolysins or later hemolysis resulting from phagocytosis of agglutinated cells. The hemoglobin released
from the red cells is then converted by the phagocytes
into bilirubin and later excreted in the bile by the liver,
as discussed in Chapter 70. The concentration of bilirubin in the body fluids often rises high enough to cause
jaundice—that is, the person’s internal tissues and skin
become colored with yellow bile pigment. But if liver
function is normal, the bile pigment will be excreted
into the intestines by way of the liver bile, so that jaundice usually does not appear in an adult person unless
more than 400 milliliters of blood is hemolyzed in less
than a day.
Acute Kidney Shutdown After Transfusion Reactions. One of
the most lethal effects of transfusion reactions is
kidney failure, which can begin within a few minutes
to few hours and continue until the person dies of
renal failure.
The kidney shutdown seems to result from three
causes: First, the antigen-antibody reaction of the
transfusion reaction releases toxic substances from the
hemolyzing blood that cause powerful renal vasoconstriction. Second, loss of circulating red cells in the
recipient, along with production of toxic substances
from the hemolyzed cells and from the immune reaction, often causes circulatory shock. The arterial blood
pressure falls very low, and renal blood flow and urine
output decrease. Third, if the total amount of free
hemoglobin released into the circulating blood is
greater than the quantity that can bind with “haptoglobin” (a plasma protein that binds small amounts of
hemoglobin), much of the excess leaks through the
glomerular membranes into the kidney tubules. If this
amount is still slight, it can be reabsorbed through the
tubular epithelium into the blood and will cause no
harm; if it is great, then only a small percentage is reabsorbed. Yet water continues to be reabsorbed, causing
the tubular hemoglobin concentration to rise so high
that the hemoglobin precipitates and blocks many of
the kidney tubules. Thus, renal vasoconstriction, circulatory shock, and renal tubular blockage together
cause acute renal shutdown. If the shutdown is complete and fails to resolve, the patient dies within a week
to 12 days, as explained in Chapter 31, unless treated
with an artificial kidney.
Transplantation of Tissues
and Organs
Most of the different antigens of red blood cells that
cause transfusion reactions are also widely present
in other cells of the body, and each bodily tissue has
455
its own additional complement of antigens. Consequently, foreign cells transplanted anywhere into the
body of a recipient can produce immune reactions. In
other words, most recipients are just as able to resist
invasion by foreign tissue cells as to resist invasion by
foreign bacteria or red cells.
Autografts, Isografts, Allografts, and Xenografts. A trans-
plant of a tissue or whole organ from one part of the
same animal to another part is called an autograft;
from one identical twin to another, an isograft; from
one human being to another or from any animal to
another animal of the same species, an allograft; and
from a lower animal to a human being or from an
animal of one species to one of another species, a
xenograft.
Transplantation of Cellular Tissues. In the case of auto-
grafts and isografts, cells in the transplant contain virtually the same types of antigens as in the tissues of
the recipient and will almost always continue to live
normally and indefinitely if an adequate blood supply
is provided.
At the other extreme, in the case of xenografts,
immune reactions almost always occur, causing death
of the cells in the graft within 1 day to 5 weeks after
transplantation unless some specific therapy is used to
prevent the immune reactions.
Some of the different cellular tissues and organs that
have been transplanted as allografts, either experimentally or for therapeutic purposes, from one person
to another are skin, kidney, heart, liver, glandular
tissue, bone marrow, and lung. With proper “matching”
of tissues between persons, many kidney allografts
have been successful for at least 5 to 15 years, and allograft liver and heart transplants for 1 to 15 years.
Attempts to Overcome Immune
Reactions in Transplanted Tissue
Because of the extreme potential importance of transplanting certain tissues and organs, serious attempts
have been made to prevent antigen-antibody reactions
associated with transplantation. The following specific
procedures have met with some degrees of clinical or
experimental success.
Tissue Typing—The HLA Complex of Antigens
The most important antigens for causing graft rejection are a complex called the HLA antigens. Six of
these antigens are present on the tissue cell membranes of each person, but there are about 150 different HLA antigens to choose from. Therefore, this
represents more than a trillion possible combinations.
Consequently, it is virtually impossible for two
persons, except in the case of identical twins, to have
the same six HLA antigens. Development of significant immunity against any one of these antigens can
cause graft rejection.
The HLA antigens occur on the white blood cells as
well as on the tissue cells. Therefore, tissue typing for
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Unit VI
Blood Cells, Immunity, and Blood Clotting
these antigens is done on the membranes of lymphocytes that have been separated from the person’s
blood. The lymphocytes are mixed with appropriate
antisera and complement; after incubation, the cells
are tested for membrane damage, usually by testing
the rate of trans-membrane uptake by the lymphocytic
cells of a special dye.
Some of the HLA antigens are not severely antigenic, for which reason a precise match of some
antigens between donor and recipient is not always
essential to allow allograft acceptance. Therefore, by
obtaining the best possible match between donor and
recipient, the grafting procedure has become far less
hazardous. The best success has been with tissue-type
matches between siblings and between parent and
child. The match in identical twins is exact, so that
transplants between identical twins are almost never
rejected because of immune reactions.
Prevention of Graft Rejection by Suppressing
the Immune System
If the immune system were completely suppressed,
graft rejection would not occur. In fact, in an occasional person who has serious depression of the
immune system, grafts can be successful without the
use of significant therapy to prevent rejection. But in
the normal person, even with the best possible tissue
typing, allografts seldom resist rejection for more than
a few days or weeks without use of specific therapy to
suppress the immune system. Furthermore, because
the T cells are mainly the portion of the immune
system important for killing grafted cells, their suppression is much more important than suppression of
plasma antibodies. Some of the therapeutic agents that
have been used for this purpose include the following:
1. Glucocorticoid hormones isolated from adrenal
cortex glands (or drugs with glucocorticoid-like
activity), which suppress the growth of all
lymphoid tissue and, therefore, decrease
formation of antibodies and T cells.
2. Various drugs that have a toxic effect on the
lymphoid system and, therefore, block formation
of antibodies and T cells, especially the drug
azathioprine.
3. Cyclosporine, which has a specific inhibitory effect
on the formation of helper T cells and, therefore,
is especially efficacious in blocking the T-cell
rejection reaction. This has proved to be one of
the most valuable of all the drugs because it does
not depress some other portions of the immune
system.
Use of these agents often leaves the person unprotected from infectious disease; therefore, sometimes
bacterial and viral infections become rampant. In addition, the incidence of cancer is several times as great
in an immunosuppressed person, presumably because
the immune system is important in destroying many
early cancer cells before they can begin to proliferate.
To summarize, transplantation of living tissues in
human beings has had very limited but important
success. When someone does finally succeed in blocking the immune response of the recipient without at
the same time destroying the recipient’s specific immunity for disease, the story will change overnight.
References
Alayash AI: Oxygen therapeutics: can we tame haemoglobin? Nat Rev Drug Discov 3:152, 2004.
Altomonte M, Fonsatti E, Visintin A, Maio M: Targeted
therapy of solid malignancies via HLA class II antigens: a
new biotherapeutic approach? Oncogene 22:6564, 2003.
Avent ND, Reid ME: The Rh blood group system: a review.
Blood 95:375, 2000.
Bowman J: Thirty-five years of Rh prophylaxis. Transfusion
43:1661, 2003.
Goodnough LT, Shander A: Evolution in alternatives to
blood transfusion. Hematol J 4:87, 2003.
Gottstein R, Cooke RW: Systematic review of intravenous
immunoglobulin in haemolytic disease of the newborn.
Arch Dis Child Fetal Neonatal Ed 88:F6, 2003.
Heeger PS: T-cell allorecognition and transplant rejection: a
summary and update. Am J Transplant 3:525, 2003.
Horn KD: The classification, recognition and significance
of polyagglutination in transfusion medicine. Blood Rev
13:36, 1999.
Miller J, Mathew JM, Esquenazi V: Toward tolerance to
human organ transplants: a few additional corollaries and
questions. Transplantation 77:940, 2004.
Ricordi C, Strom TB: Clinical islet transplantation: advances
and immunological challenges. Nat Rev Immunol 4:259,
2004.
Schroeder RA, Marroquin CE, Kuo PC: Tolerance and
the “Holy Grail” of transplantation. J Surg Res 111:109,
2003.
Schulak JA: Steroid immunosuppression in kidney transplantation: a passing era. J Surg Res 117:154, 2004.
Spahn DR, Pasch T: Physiological properties of blood substitutes. News Physiol Sci 16:38, 2001.
Strober S, Lowsky RJ, Shizuru JA, et al: Approaches to transplantation tolerance in humans. Transplantation 77:932,
2004.
Sumpter TL, Wilkes DS: Role of autoimmunity in organ allograft rejection: a focus on immunity to type V collagen
in the pathogenesis of lung transplant rejection. Am J
Physiol Lung Cell Mol Physiol 286:L1129, 2004.
Telen MJ: Red blood cell surface adhesion molecules: their
possible roles in normal human physiology and disease.
Semin Hematol 37:130, 2000.
Trigg ME: Hematopoietic stem cells. Pediatrics 113(4
Suppl):1051, 2004.
Triulzi DJ: Specialized transfusion support for solid organ
transplantation. Curr Opin Hematol 9:527, 2002.
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3
6
Hemostasis and Blood Coagulation
Events in Hemostasis
The term hemostasis means prevention of blood
loss. Whenever a vessel is severed or ruptured,
hemostasis is achieved by several mechanisms: (1)
vascular constriction, (2) formation of a platelet
plug, (3) formation of a blood clot as a result of
blood coagulation, and (4) eventual growth of
fibrous tissue into the blood clot to close the hole in the vessel permanently.
Vascular Constriction
Immediately after a blood vessel has been cut or ruptured, the trauma to the
vessel wall itself causes the smooth muscle in the wall to contract; this instantaneously reduces the flow of blood from the ruptured vessel. The contraction
results from (1) local myogenic spasm, (2) local autacoid factors from the traumatized tissues and blood platelets, and (3) nervous reflexes. The nervous
reflexes are initiated by pain nerve impulses or other sensory impulses that originate from the traumatized vessel or nearby tissues. However, even more vasoconstriction probably results from local myogenic contraction of the blood
vessels initiated by direct damage to the vascular wall. And, for the smaller
vessels, the platelets are responsible for much of the vasoconstriction by releasing a vasoconstrictor substance, thromboxane A2.
The more severely a vessel is traumatized, the greater the degree of vascular
spasm. The spasm can last for many minutes or even hours, during which time
the processes of platelet plugging and blood coagulation can take place.
Formation of the Platelet Plug
If the cut in the blood vessel is very small—indeed, many very small vascular
holes do develop throughout the body each day—the cut is often sealed by a
platelet plug, rather than by a blood clot. To understand this, it is important that
we first discuss the nature of platelets themselves.
Physical and Chemical Characteristics of Platelets
Platelets (also called thrombocytes) are minute discs 1 to 4 micrometers in diameter. They are formed in the bone marrow from megakaryocytes, which are
extremely large cells of the hematopoietic series in the marrow; the megakaryocytes fragment into the minute platelets either in the bone marrow or soon
after entering the blood, especially as they squeeze through capillaries. The
normal concentration of platelets in the blood is between 150,000 and 300,000
per microliter.
Platelets have many functional characteristics of whole cells, even though
they do not have nuclei and cannot reproduce. In their cytoplasm are such active
factors as (1) actin and myosin molecules, which are contractile proteins similar
to those found in muscle cells, and still another contractile protein, thrombosthenin, that can cause the platelets to contract; (2) residuals of both the
endoplasmic reticulum and the Golgi apparatus that synthesize various enzymes
and especially store large quantities of calcium ions; (3) mitochondria and
457
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Unit VI
Blood Cells, Immunity, and Blood Clotting
enzyme systems that are capable of forming adenosine
triphosphate (ATP) and adenosine diphosphate
(ADP); (4) enzyme systems that synthesize
prostaglandins, which are local hormones that cause
many vascular and other local tissue reactions; (5) an
important protein called fibrin-stabilizing factor, which
we discuss later in relation to blood coagulation; and
(6) a growth factor that causes vascular endothelial
cells, vascular smooth muscle cells, and fibroblasts to
multiply and grow, thus causing cellular growth that
eventually helps repair damaged vascular walls.
The cell membrane of the platelets is also important.
On its surface is a coat of glycoproteins that repulses
adherence to normal endothelium and yet causes
adherence to injured areas of the vessel wall, especially
to injured endothelial cells and even more so to any
exposed collagen from deep within the vessel wall. In
addition, the platelet membrane contains large
amounts of phospholipids that activate multiple stages
in the blood-clotting process, as we discuss later.
Thus, the platelet is an active structure. It has a halflife in the blood of 8 to 12 days, so that over several
weeks its functional processes run out. Then it is
eliminated from the circulation mainly by the tissue
macrophage system. More than one half of the
platelets are removed by macrophages in the spleen,
where the blood passes through a latticework of tight
trabeculae.
Mechanism of the Platelet Plug
Platelet repair of vascular openings is based on several
important functions of the platelet itself. When
platelets come in contact with a damaged vascular
surface, especially with collagen fibers in the vascular
wall, the platelets themselves immediately change
their own characteristics drastically. They begin to
swell; they assume irregular forms with numerous irradiating pseudopods protruding from their surfaces;
their contractile proteins contract forcefully and cause
the release of granules that contain multiple active
factors; they become sticky so that they adhere to collagen in the tissues and to a protein called von Willebrand factor that leaks into the traumatized tissue
from the plasma; they secrete large quantities of ADP;
and their enzymes form thromboxane A2. The ADP
and thromboxane in turn act on nearby platelets to
activate them as well, and the stickiness of these additional platelets causes them to adhere to the original
activated platelets.
Therefore, at the site of any opening in a blood
vessel wall, the damaged vascular wall activates successively increasing numbers of platelets that themselves attract more and more additional platelets, thus
forming a platelet plug. This is at first a loose plug, but
it is usually successful in blocking blood loss if the vascular opening is small. Then, during the subsequent
process of blood coagulation, fibrin threads form.
These attach tightly to the platelets, thus constructing
an unyielding plug.
Importance of the Platelet Mechanism for Closing Vascular
Holes. The platelet-plugging mechanism is extremely
important for closing minute ruptures in very small
blood vessels that occur many thousands of times
daily. Indeed, multiple small holes through the
endothelial cells themselves are often closed by
platelets actually fusing with the endothelial cells to
form additional endothelial cell membrane. A person
who has few blood platelets develops each day literally thousands of small hemorrhagic areas under the
skin and throughout the internal tissues, but this does
not occur in the normal person.
Blood Coagulation in the
Ruptured Vessel
The third mechanism for hemostasis is formation of
the blood clot. The clot begins to develop in 15 to 20
seconds if the trauma to the vascular wall has been
severe, and in 1 to 2 minutes if the trauma has been
minor. Activator substances from the traumatized vascular wall, from platelets, and from blood proteins
adhering to the traumatized vascular wall initiate the
clotting process. The physical events of this process are
shown in Figure 36–1, and Table 36–1 lists the most
important of the clotting factors.
Within 3 to 6 minutes after rupture of a vessel, if the
vessel opening is not too large, the entire opening or
broken end of the vessel is filled with clot. After 20
minutes to an hour, the clot retracts; this closes the
vessel still further. Platelets also play an important role
in this clot retraction, as is discussed later.
Fibrous Organization or Dissolution
of the Blood Clot
Once a blood clot has formed, it can follow one of two
courses: (1) It can become invaded by fibroblasts,
1. Severed vessel
2. Platelets agglutinate
3. Fibrin appears
4. Fibrin clot forms
5. Clot retraction occurs
Figure 36–1
Clotting process in a traumatized blood vessel. (Modified from
Seegers WH: Hemostatic Agents, 1948. Courtesy of Charles C
Thomas, Publisher, Ltd., Springfield, IL.)
Chapter 36
459
Hemostasis and Blood Coagulation
Table 36–1
Prothrombin
Clotting Factors in Blood and Their Synonyms
Clotting Factor
Synonyms
Fibrinogen
Prothrombin
Tissue factor
Calcium
Factor V
Factor I
Factor II
Factor III; tissue thromboplastin
Factor IV
Proaccelerin; labile factor; Ac-globulin
(Ac-G)
Serum prothrombin conversion
accelerator (SPCA); proconvertin;
stable factor
Antihemophilic factor (AHF);
antihemophilic globulin (AHG);
antihemophilic factor A
Plasma thromboplastin component
(PTC); Christmas factor;
antihemophilic factor B
Stuart factor; Stuart-Prower factor
Plasma thromboplastin antecedent
(PTA); antihemophilic factor C
Hageman factor
Fibrin-stabilizing factor
Fletcher factor
Fitzgerald factor; HMWK
(high-molecular-weight) kininogen
Factor VII
Factor VIII
Factor IX
Factor X
Factor XI
Factor XII
Factor XIII
Prekallikrein
High-molecular-weight
kininogen
Platelets
Prothrombin
activator
Ca++
Thrombin
Fibrinogen
Fibrinogen monomer
Ca++
Fibrin fibers
Thrombin
activated
fibrin-stabilizing
factor
Cross-linked fibrin fibers
Figure 36–2
Schema for conversion of prothrombin to thrombin and polymerization of fibrinogen to form fibrin fibers.
which subsequently form connective tissue all through
the clot, or (2) it can dissolve. The usual course for a
clot that forms in a small hole of a vessel wall is invasion by fibroblasts, beginning within a few hours after
the clot is formed (which is promoted at least partially
by growth factor secreted by platelets). This continues
to complete organization of the clot into fibrous tissue
within about 1 to 2 weeks.
Conversely, when excess blood has leaked into the
tissues and tissue clots have occurred where they are
not needed, special substances within the clot itself
usually become activated. These function as enzymes
to dissolve the clot, as discussed later in the chapter.
three essential steps: (1) In response to rupture of the
vessel or damage to the blood itself, a complex cascade
of chemical reactions occurs in the blood involving
more than a dozen blood coagulation factors. The net
result is formation of a complex of activated substances collectively called prothrombin activator. (2)
The prothrombin activator catalyzes conversion of
prothrombin into thrombin. (3) The thrombin acts as
an enzyme to convert fibrinogen into fibrin fibers that
enmesh platelets, blood cells, and plasma to form the
clot.
Let us discuss first the mechanism by which the
blood clot itself is formed, beginning with conversion
of prothrombin to thrombin; then we will come back
to the initiating stages in the clotting process by which
prothrombin activator is formed.
Mechanism of Blood
Coagulation
Conversion of Prothrombin
to Thrombin
Basic Theory. More than 50 important substances that
cause or affect blood coagulation have been found in
the blood and in the tissues—some that promote coagulation, called procoagulants, and others that inhibit
coagulation, called anticoagulants. Whether blood will
coagulate depends on the balance between these two
groups of substances. In the blood stream, the anticoagulants normally predominate, so that the blood
does not coagulate while it is circulating in the blood
vessels. But when a vessel is ruptured, procoagulants
from the area of tissue damage become “activated”
and override the anticoagulants, and then a clot does
develop.
First, prothrombin activator is formed as a result of
rupture of a blood vessel or as a result of damage
to special substances in the blood. Second, the prothrombin activator, in the presence of sufficient
amounts of ionic Ca++, causes conversion of prothrombin to thrombin (Figure 36–2). Third, the thrombin causes polymerization of fibrinogen molecules into
fibrin fibers within another 10 to 15 seconds. Thus, the
rate-limiting factor in causing blood coagulation is
usually the formation of prothrombin activator and
not the subsequent reactions beyond that point,
because these terminal steps normally occur rapidly to
form the clot itself.
Platelets also play an important role in the conversion of prothrombin to thrombin because much
of the prothrombin first attaches to prothrombin
General Mechanism. All research workers in the field of
blood coagulation agree that clotting takes place in
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Unit VI
Blood Cells, Immunity, and Blood Clotting
receptors on the platelets already bound to the
damaged tissue.
Prothrombin and Thrombin. Prothrombin is a plasma
protein, an alpha2-globulin, having a molecular weight
of 68,700. It is present in normal plasma in a concentration of about 15 mg/dl. It is an unstable protein that
can split easily into smaller compounds, one of which
is thrombin, which has a molecular weight of 33,700,
almost exactly one half that of prothrombin.
Prothrombin is formed continually by the liver, and
it is continually being used throughout the body for
blood clotting. If the liver fails to produce prothrombin, in a day or so prothrombin concentration in
the plasma falls too low to provide normal blood
coagulation.
Vitamin K is required by the liver for normal formation of prothrombin as well as for formation of a
few other clotting factors. Therefore, either lack of
vitamin K or the presence of liver disease that prevents normal prothrombin formation can decrease the
prothrombin level so low that a bleeding tendency
results.
Conversion of Fibrinogen to Fibrin—
Formation of the Clot
Fibrinogen. Fibrinogen is a high-molecular-weight
protein (MW = 340,000) that occurs in the plasma in
quantities of 100 to 700 mg/dl. Fibrinogen is formed in
the liver, and liver disease can decrease the concentration of circulating fibrinogen, as it does the concentration of prothrombin, pointed out above.
Because of its large molecular size, little fibrinogen
normally leaks from the blood vessels into the interstitial fluids, and because fibrinogen is one of the essential factors in the coagulation process, interstitial fluids
ordinarily do not coagulate. Yet, when the permeability of the capillaries becomes pathologically increased,
fibrinogen does then leak into the tissue fluids in sufficient quantities to allow clotting of these fluids in
much the same way that plasma and whole blood can
clot.
Action of Thrombin on Fibrinogen to Form Fibrin. Thrombin
is a protein enzyme with weak proteolytic capabilities.
It acts on fibrinogen to remove four low-molecularweight peptides from each molecule of fibrinogen,
forming one molecule of fibrin monomer that has the
automatic capability to polymerize with other fibrin
monomer molecules to form fibrin fibers. Therefore,
many fibrin monomer molecules polymerize within
seconds into long fibrin fibers that constitute the reticulum of the blood clot.
In the early stages of polymerization, the fibrin
monomer molecules are held together by weak noncovalent hydrogen bonding, and the newly forming
fibers are not cross-linked with one another; therefore,
the resultant clot is weak and can be broken apart with
ease. But another process occurs during the next few
minutes that greatly strengthens the fibrin reticulum.
This involves a substance called fibrin-stabilizing
factor that is present in small amounts in normal
plasma globulins but is also released from platelets
entrapped in the clot. Before fibrin-stabilizing factor
can have an effect on the fibrin fibers, it must itself be
activated. The same thrombin that causes fibrin formation also activates the fibrin-stabilizing factor.
Then this activated substance operates as an enzyme
to cause covalent bonds between more and more of the
fibrin monomer molecules, as well as multiple crosslinkages between adjacent fibrin fibers, thus adding
tremendously to the three-dimensional strength of the
fibrin meshwork.
Blood Clot. The clot is composed of a meshwork of
fibrin fibers running in all directions and entrapping
blood cells, platelets, and plasma. The fibrin fibers also
adhere to damaged surfaces of blood vessels; therefore, the blood clot becomes adherent to any vascular
opening and thereby prevents further blood loss.
Clot Retraction—Serum. Within a few minutes after a
clot is formed, it begins to contract and usually
expresses most of the fluid from the clot within 20 to
60 minutes. The fluid expressed is called serum because
all its fibrinogen and most of the other clotting factors
have been removed; in this way, serum differs from
plasma. Serum cannot clot because it lacks these
factors.
Platelets are necessary for clot retraction to occur.
Therefore, failure of clot retraction is an indication
that the number of platelets in the circulating blood
might be low. Electron micrographs of platelets in
blood clots show that they become attached to the
fibrin fibers in such a way that they actually bond
different fibers together. Furthermore, platelets
entrapped in the clot continue to release procoagulant
substances, one of the most important of which is
fibrin-stabilizing factor, which causes more and more
cross-linking bonds between adjacent fibrin fibers. In
addition, the platelets themselves contribute directly
to clot contraction by activating platelet thrombosthenin, actin, and myosin molecules, which are all
contractile proteins in the platelets and cause strong
contraction of the platelet spicules attached to the
fibrin. This also helps compress the fibrin meshwork
into a smaller mass. The contraction is activated and
accelerated by thrombin as well as by calcium ions
released from calcium stores in the mitochondria,
endoplasmic reticulum, and Golgi apparatus of the
platelets.
As the clot retracts, the edges of the broken blood
vessel are pulled together, thus contributing still
further to the ultimate state of hemostasis.
Vicious Circle of Clot Formation
Once a blood clot has started to develop, it normally
extends within minutes into the surrounding blood.
That is, the clot itself initiates a vicious circle (positive
feedback) to promote more clotting. One of the most
Chapter 36
461
Hemostasis and Blood Coagulation
Prothrombin activator is generally considered to be
formed in two ways, although, in reality, the two ways
interact constantly with each other: (1) by the extrinsic pathway that begins with trauma to the vascular
wall and surrounding tissues and (2) by the intrinsic
pathway that begins in the blood itself.
In both the extrinsic and the intrinsic pathways, a
series of different plasma proteins called bloodclotting factors play major roles. Most of these are inactive forms of proteolytic enzymes. When converted to
the active forms, their enzymatic actions cause the successive, cascading reactions of the clotting process.
Most of the clotting factors, which are listed in Table
36–1, are designated by Roman numerals. To indicate
the activated form of the factor, a small letter “a” is
added after the Roman numeral, such as Factor VIIIa
to indicate the activated state of Factor VIII.
important causes of this is the fact that the proteolytic
action of thrombin allows it to act on many of the
other blood-clotting factors in addition to fibrinogen.
For instance, thrombin has a direct proteolytic effect
on prothrombin itself, tending to convert this into still
more thrombin, and it acts on some of the blood-clotting factors responsible for formation of prothrombin
activator. (These effects, discussed in subsequent paragraphs, include acceleration of the actions of Factors
VIII, IX, X, XI, and XII and aggregation of platelets.)
Once a critical amount of thrombin is formed, a vicious
circle develops that causes still more blood clotting
and more and more thrombin to be formed; thus,
the blood clot continues to grow until blood leakage
ceases.
Initiation of Coagulation: Formation
of Prothrombin Activator
Extrinsic Pathway for Initiating Clotting
The extrinsic pathway for initiating the formation of
prothrombin activator begins with a traumatized vascular wall or traumatized extravascular tissues that
come in contact with the blood. This leads to the following steps, as shown in Figure 36–3:
1. Release of tissue factor. Traumatized tissue
releases a complex of several factors called tissue
factor or tissue thromboplastin. This factor is
composed especially of phospholipids from the
membranes of the tissue plus a lipoprotein
complex that functions mainly as a proteolytic
enzyme.
Now that we have discussed the clotting process itself,
we must turn to the more complex mechanisms that
initiate clotting in the first place. These mechanisms
are set into play by (1) trauma to the vascular wall and
adjacent tissues, (2) trauma to the blood, or (3) contact
of the blood with damaged endothelial cells or with
collagen and other tissue elements outside the blood
vessel. In each instance, this leads to the formation of
prothrombin activator, which then causes prothrombin
conversion to thrombin and all the subsequent clotting
steps.
(1)
Tissue trauma
Tissue factor
Vll
(2)
VIIa
Activated X (Xa)
X
Ca++
V
(3)
Platelet
phospholipids
Prothrombin
Activator
Prothrombin
Figure 36–3
Extrinsic pathway for initiating blood clotting.
Ca++
Thrombin
Ca++
462
Unit VI
Blood Cells, Immunity, and Blood Clotting
2. Activation of Factor X—role of Factor VII and
tissue factor. The lipoprotein complex of tissue
factor further complexes with blood coagulation
Factor VII and, in the presence of calcium ions,
acts enzymatically on Factor X to form activated
Factor X (Xa).
3. Effect of activated Factor X (Xa) to form
prothrombin activator—role of Factor V. The
activated Factor X combines immediately with
tissue phospholipids that are part of tissue factor
or with additional phospholipids released from
platelets as well as with Factor V to form the
complex called prothrombin activator. Within a
few seconds, in the presence of calcium ions
(Ca++), this splits prothrombin to form thrombin,
and the clotting process proceeds as already
explained. At first, the Factor V in the
prothrombin activator complex is inactive, but
once clotting begins and thrombin begins to form,
the proteolytic action of thrombin activates Factor
V. This then becomes an additional strong
accelerator of prothrombin activation. Thus, in the
final prothrombin activator complex, activated
Factor X is the actual protease that causes
splitting of prothrombin to form thrombin;
activated Factor V greatly accelerates this
protease activity, and platelet phospholipids act as
a vehicle that further accelerates the process. Note
especially the positive feedback effect of thrombin,
acting through Factor V, to accelerate the entire
process once it begins.
Intrinsic Pathway for Initiating Clotting
The second mechanism for initiating formation of prothrombin activator, and therefore for initiating clotting, begins with trauma to the blood itself or exposure
of the blood to collagen from a traumatized blood
vessel wall. Then the process continues through the
series of cascading reactions shown in Figure 36–4.
1. Blood trauma causes (1) activation of Factor XII
and (2) release of platelet phospholipids. Trauma
to the blood or exposure of the blood to vascular
Blood trauma or
contact with collagen
(1)
XII
Activated XII (XIIa)
(HMW kininogen, prekallikrein)
(2)
(3)
XI
Activated XI (XIa)
Ca++
IX
Activated IX (IXa)
VIII
Thrombin
VIIIa
(4)
(5)
X
Ca++
Activated X (Xa)
Platelet
phospholipids
Thrombin
Ca++
V
Prothrombin
Activator
Platelet
phospholipids
Prothrombin
Thrombin
Ca++
Figure 36–4
Intrinsic pathway for initiating
blood clotting.
Chapter 36
2.
3.
4.
5.
Hemostasis and Blood Coagulation
wall collagen alters two important clotting factors
in the blood: Factor XII and the platelets. When
Factor XII is disturbed, such as by coming into
contact with collagen or with a wettable surface
such as glass, it takes on a new molecular
configuration that converts it into a proteolytic
enzyme called “activated Factor XII.”
Simultaneously, the blood trauma also damages
the platelets because of adherence to either
collagen or a wettable surface (or by damage
in other ways), and this releases platelet
phospholipids that contain the lipoprotein called
platelet factor 3, which also plays a role in
subsequent clotting reactions.
Activation of Factor XI. The activated Factor XII
acts enzymatically on Factor XI to activate this
factor as well, which is the second step in the
intrinsic pathway. This reaction also requires
HMW (high-molecular-weight) kininogen and is
accelerated by prekallikrein.
Activation of Factor IX by activated Factor XI.
The activated Factor XI then acts enzymatically
on Factor IX to activate this factor also.
Activation of Factor X—role of Factor VIII.
The activated Factor IX, acting in concert with
activated Factor VIII and with the platelet
phospholipids and factor 3 from the traumatized
platelets, activates Factor X. It is clear that when
either Factor VIII or platelets are in short supply,
this step is deficient. Factor VIII is the factor that
is missing in a person who has classic hemophilia,
for which reason it is called antihemophilic factor.
Platelets are the clotting factor that is lacking in
the bleeding disease called thrombocytopenia.
Action of activated Factor X to form prothrombin
activator—role of Factor V. This step in the
intrinsic pathway is the same as the last step in
the extrinsic pathway. That is, activated Factor X
combines with Factor V and platelet or tissue
phospholipids to form the complex called
prothrombin activator. The prothrombin activator
in turn initiates within seconds the cleavage of
prothrombin to form thrombin, thereby setting
into motion the final clotting process, as described
earlier.
Role of Calcium Ions in the Intrinsic and
Extrinsic Pathways
Except for the first two steps in the intrinsic pathway,
calcium ions are required for promotion or acceleration of all the blood-clotting reactions. Therefore, in
the absence of calcium ions, blood clotting by either
pathway does not occur.
In the living body, the calcium ion concentration
seldom falls low enough to significantly affect the
kinetics of blood clotting. But, when blood is removed
from a person, it can be prevented from clotting by
reducing the calcium ion concentration below the
threshold level for clotting, either by deionizing the
calcium by causing it to react with substances such as
citrate ion or by precipitating the calcium with substances such as oxalate ion.
463
Interaction Between the Extrinsic
and Intrinsic Pathways—Summary
of Blood-Clotting Initiation
It is clear from the schemas of the intrinsic and extrinsic systems that after blood vessels rupture, clotting
occurs by both pathways simultaneously. Tissue factor
initiates the extrinsic pathway, whereas contact of
Factor XII and platelets with collagen in the vascular
wall initiates the intrinsic pathway.
An especially important difference between the
extrinsic and intrinsic pathways is that the extrinsic
pathway can be explosive; once initiated, its speed of
completion to the final clot is limited only by the
amount of tissue factor released from the traumatized
tissues and by the quantities of Factors X, VII, and V
in the blood. With severe tissue trauma, clotting can
occur in as little as 15 seconds. The intrinsic pathway
is much slower to proceed, usually requiring 1 to 6
minutes to cause clotting.
Prevention of Blood Clotting
in the Normal Vascular System—
Intravascular Anticoagulants
Endothelial Surface Factors. Probably the most important
factors for preventing clotting in the normal vascular
system are (1) the smoothness of the endothelial cell
surface, which prevents contact activation of the intrinsic clotting system; (2) a layer of glycocalyx on the
endothelium (glycocalyx is a mucopolysaccharide
adsorbed to the surfaces of the endothelial cells),
which repels clotting factors and platelets, thereby preventing activation of clotting; and (3) a protein bound
with the endothelial membrane, thrombomodulin,
which binds thrombin. Not only does the binding of
thrombin with thrombomodulin slow the clotting
process by removing thrombin, but the thrombomodulin-thrombin complex also activates a plasma protein,
protein C, that acts as an anticoagulant by inactivating
activated Factors V and VIII.
When the endothelial wall is damaged, its smoothness and its glycocalyx-thrombomodulin layer are lost,
which activates both Factor XII and the platelets,
thus setting off the intrinsic pathway of clotting. If
Factor XII and platelets come in contact with the
subendothelial collagen, the activation is even more
powerful.
Antithrombin Action of Fibrin and Antithrombin III. Among
the most important anticoagulants in the blood itself
are those that remove thrombin from the blood. The
most powerful of these are (1) the fibrin fibers that
themselves are formed during the process of clotting
and (2) an alpha-globulin called antithrombin III or
antithrombin-heparin cofactor.
While a clot is forming, about 85 to 90 per cent of
the thrombin formed from the prothrombin becomes
adsorbed to the fibrin fibers as they develop. This helps
prevent the spread of thrombin into the remaining
blood and, therefore, prevents excessive spread of the
clot.
464
Unit VI
Blood Cells, Immunity, and Blood Clotting
The thrombin that does not adsorb to the fibrin
fibers soon combines with antithrombin III, which
further blocks the effect of the thrombin on the fibrinogen and then also inactivates the thrombin itself
during the next 12 to 20 minutes.
Heparin. Heparin is another powerful anticoagulant,
but its concentration in the blood is normally low, so
that only under special physiologic conditions does
it have significant anticoagulant effects. However,
heparin is used widely as a pharmacological agent in
medical practice in much higher concentrations to
prevent intravascular clotting.
The heparin molecule is a highly negatively charged
conjugated polysaccharide. By itself, it has little or no
anticoagulant properties, but when it combines with
antithrombin III, the effectiveness of antithrombin III
for removing thrombin increases by a hundredfold to
a thousandfold, and thus it acts as an anticoagulant.
Therefore, in the presence of excess heparin, removal
of free thrombin from the circulating blood by
antithrombin III is almost instantaneous.
The complex of heparin and antithrombin III
removes several other activated coagulation factors in
addition to thrombin, further enhancing the effectiveness of anticoagulation. The others include activated
Factors XII, XI, X, and IX.
Heparin is produced by many different cells of the
body, but especially large quantities are formed by the
basophilic mast cells located in the pericapillary connective tissue throughout the body. These cells continually secrete small quantities of heparin that diffuse
into the circulatory system. The basophil cells of the
blood, which are functionally almost identical to the
mast cells, release small quantities of heparin into
the plasma.
Mast cells are abundant in tissue surrounding the
capillaries of the lungs and to a lesser extent capillaries of the liver. It is easy to understand why large quantities of heparin might be needed in these areas
because the capillaries of the lungs and liver receive
many embolic clots formed in slowly flowing venous
blood; sufficient formation of heparin prevents further
growth of the clots.
Lysis of Blood Clots—Plasmin
The plasma proteins contain a euglobulin called
plasminogen (or profibrinolysin) that, when activated,
becomes a substance called plasmin (or fibrinolysin).
Plasmin is a proteolytic enzyme that resembles trypsin,
the most important proteolytic digestive enzyme of
pancreatic secretion. Plasmin digests fibrin fibers and
some other protein coagulants such as fibrinogen,
Factor V, Factor VIII, prothrombin, and Factor XII.
Therefore, whenever plasmin is formed, it can cause
lysis of a clot by destroying many of the clotting
factors, thereby sometimes even causing hypocoagulability of the blood.
Activation of Plasminogen to Form Plasmin: Then Lysis of Clots.
When a clot is formed, a large amount of plasminogen
is trapped in the clot along with other plasma proteins.
This will not become plasmin or cause lysis of the clot
until it is activated. The injured tissues and vascular
endothelium very slowly release a powerful activator
called tissue plasminogen activator (t-PA) that a few
days later, after the clot has stopped the bleeding,
eventually converts plasminogen to plasmin, which in
turn removes the remaining unnecessary blood clot. In
fact, many small blood vessels in which blood flow has
been blocked by clots are reopened by this mechanism. Thus, an especially important function of the
plasmin system is to remove minute clots from millions
of tiny peripheral vessels that eventually would
become occluded were there no way to clear them.
Conditions That Cause
Excessive Bleeding in
Human Beings
Excessive bleeding can result from deficiency of any
one of the many blood-clotting factors. Three particular types of bleeding tendencies that have been studied
to the greatest extent are discussed here: bleeding
caused by (1) vitamin K deficiency, (2) hemophilia, and
(3) thrombocytopenia (platelet deficiency).
Decreased Prothrombin, Factor VII,
Factor IX, and Factor X Caused
by Vitamin K Deficiency
With few exceptions, almost all the blood-clotting
factors are formed by the liver. Therefore, diseases of
the liver such as hepatitis, cirrhosis, and acute yellow
atrophy can sometimes depress the clotting system so
greatly that the patient develops a severe tendency to
bleed.
Another cause of depressed formation of clotting
factors by the liver is vitamin K deficiency. Vitamin K
is necessary for liver formation of five of the important clotting factors: prothrombin, Factor VII, Factor
IX, Factor X, and protein C. In the absence of vitamin
K, subsequent insufficiency of these coagulation
factors in the blood can lead to serious bleeding tendencies.
Vitamin K is continually synthesized in the intestinal tract by bacteria, so that vitamin K deficiency
seldom occurs in the normal person as a result of
vitamin K absence from the diet (except in neonates
before they establish their intestinal bacterial flora).
However, in gastrointestinal disease, vitamin K deficiency often occurs as a result of poor absorption of
fats from the gastrointestinal tract. The reason is that
vitamin K is fat-soluble and ordinarily is absorbed into
the blood along with the fats.
One of the most prevalent causes of vitamin K deficiency is failure of the liver to secrete bile into the gastrointestinal tract (which occurs either as a result of
obstruction of the bile ducts or as a result of liver
disease). Lack of bile prevents adequate fat digestion
Chapter 36
Hemostasis and Blood Coagulation
and absorption and, therefore, depresses vitamin K
absorption as well. Thus, liver disease often causes
decreased production of prothrombin and some other
clotting factors both because of poor vitamin K
absorption and because of the diseased liver cells.
Because of this, vitamin K is injected into all surgical
patients with liver disease or with obstructed bile ducts
before performing the surgical procedure. Ordinarily,
if vitamin K is given to a deficient patient 4 to 8 hours
before the operation and the liver parenchymal cells
are at least one-half normal in function, sufficient
clotting factors will be produced to prevent excessive
bleeding during the operation.
Hemophilia
Hemophilia is a bleeding disease that occurs almost
exclusively in males. In 85 per cent of cases, it is caused
by an abnormality or deficiency of Factor VIII; this
type of hemophilia is called hemophilia A or classic
hemophilia. About 1 of every 10,000 males in the
United States has classic hemophilia. In the other 15
per cent of hemophilia patients, the bleeding tendency
is caused by deficiency of Factor IX. Both of these
factors are transmitted genetically by way of the
female chromosome. Therefore, almost never will a
woman have hemophilia because at least one of her
two X chromosomes will have the appropriate genes.
If one of her X chromosomes is deficient, she will be
a hemophilia carrier, transmitting the disease to half of
her male offspring and transmitting the carrier state to
half of her female offspring.
The bleeding trait in hemophilia can have various
degrees of severity, depending on the character of the
genetic deficiency. Bleeding usually does not occur
except after trauma, but in some patients, the degree
of trauma required to cause severe and prolonged
bleeding may be so mild that it is hardly noticeable.
For instance, bleeding can often last for days after
extraction of a tooth.
Factor VIII has two active components, a large component with a molecular weight in the millions and a
smaller component with a molecular weight of about
230,000. The smaller component is most important in
the intrinsic pathway for clotting, and it is deficiency
of this part of Factor VIII that causes classic hemophilia. Another bleeding disease with somewhat different characteristics, called von Willebrand’s disease,
results from loss of the large component.
When a person with classic hemophilia experiences
severe prolonged bleeding, almost the only therapy
that is truly effective is injection of purified Factor
VIII. The cost of Factor VIII is high, and its availability is limited because it can be gathered only from
human blood and only in extremely small quantities.
Thrombocytopenia
Thrombocytopenia means the presence of very low
numbers of platelets in the circulating blood. People
465
with thrombocytopenia have a tendency to bleed, as
do hemophiliacs, except that the bleeding is usually
from many small venules or capillaries, rather than
from larger vessels as in hemophilia. As a result, small
punctate hemorrhages occur throughout all the body
tissues. The skin of such a person displays many small,
purplish blotches, giving the disease the name thrombocytopenic purpura. As stated earlier, platelets are
especially important for repair of minute breaks in
capillaries and other small vessels.
Ordinarily, bleeding will not occur until the number
of platelets in the blood falls below 50,000/ml, rather
than the normal 150,000 to 300,000. Levels as low as
10,000/ml are frequently lethal.
Even without making specific platelet counts in the
blood, sometimes one can suspect the existence of
thrombocytopenia if the person’s blood fails to retract,
because, as pointed out earlier, clot retraction is normally dependent on release of multiple coagulation
factors from the large numbers of platelets entrapped
in the fibrin mesh of the clot.
Most people with thrombocytopenia have the
disease known as idiopathic thrombocytopenia, which
means thrombocytopenia of unknown cause. In most
of these people, it has been discovered that for
unknown reasons, specific antibodies have formed and
react against the platelets themselves to destroy them.
Relief from bleeding for 1 to 4 days can often be
effected in a patient with thrombocytopenia by
giving fresh whole blood transfusions that contain
large numbers of platelets. Also, splenectomy is often
helpful, sometimes effecting almost complete cure
because the spleen normally removes large numbers
of platelets from the blood.
Thromboembolic Conditions
in the Human Being
Thrombi and Emboli. An abnormal clot that develops in
a blood vessel is called a thrombus. Once a clot has
developed, continued flow of blood past the clot is
likely to break it away from its attachment and cause
the clot to flow with the blood; such freely flowing clots
are known as emboli. Also, emboli that originate in
large arteries or in the left side of the heart can flow
peripherally and plug arteries or arterioles in the
brain, kidneys, or elsewhere. Emboli that originate in
the venous system or in the right side of the heart generally flow into the lungs to cause pulmonary arterial
embolism.
Cause of Thromboembolic Conditions. The causes of throm-
boembolic conditions in the human being are usually
twofold: (1) Any roughened endothelial surface of a
vessel—as may be caused by arteriosclerosis, infection,
or trauma—is likely to initiate the clotting process. (2)
Blood often clots when it flows very slowly through
blood vessels, where small quantities of thrombin and
other procoagulants are always being formed.
466
Unit VI
Blood Cells, Immunity, and Blood Clotting
Use of t-PA in Treating Intravascular Clots. Genetically
engineered t-PA (tissue plasminogen activator) is
available. When delivered directly to a thrombosed
area through a catheter, it is effective in activating
plasminogen to plasmin, which in turn can dissolve
some intravascular clots. For instance, if used within
the first hour or so after thrombotic occlusion of a
coronary artery, the heart is often spared serious
damage.
Femoral Venous Thrombosis and
Massive Pulmonary Embolism
Because clotting almost always occurs when blood
flow is blocked for many hours in any vessel of the
body, the immobility of patients confined to bed plus
the practice of propping the knees with pillows often
causes intravascular clotting because of blood stasis in
one or more of the leg veins for hours at a time. Then
the clot grows, mainly in the direction of the slowly
moving venous blood, sometimes growing the entire
length of the leg veins and occasionally even up into
the common iliac vein and inferior vena cava. Then,
about 1 time out of every 10, a large part of the clot
disengages from its attachments to the vessel wall and
flows freely with the venous blood through the right
side of the heart and into the pulmonary arteries to
cause massive blockage of the pulmonary arteries,
called massive pulmonary embolism. If the clot is large
enough to occlude both of the pulmonary arteries at
the same time, immediate death ensues. If only one
pulmonary artery is blocked, death may not occur, or
the embolism may lead to death a few hours to several
days later because of further growth of the clot within
the pulmonary vessels. But, again, t-PA therapy can be
a lifesaver.
Disseminated Intravascular
Coagulation
Occasionally the clotting mechanism becomes activated in widespread areas of the circulation, giving rise
to the condition called disseminated intravascular
coagulation. This often results from the presence of
large amounts of traumatized or dying tissue in the
body that releases great quantities of tissue factor into
the blood. Frequently, the clots are small but numerous, and they plug a large share of the small peripheral blood vessels. This occurs especially in patients
with widespread septicemia, in which either circulating bacteria or bacterial toxins—especially endotoxins—activate the clotting mechanisms. Plugging of
small peripheral vessels greatly diminishes delivery of
oxygen and other nutrients to the tissues—a situation
that leads to or exacerbates circulatory shock. It is
partly for this reason that septicemic shock is lethal in
85 per cent or more of patients.
A peculiar effect of disseminated intravascular
coagulation is that the patient on occasion begins
to bleed. The reason for this is that so many of the
clotting factors are removed by the widespread clotting that too few procoagulants remain to allow
normal hemostasis of the remaining blood.
Anticoagulants for
Clinical Use
In some thromboembolic conditions, it is desirable to
delay the coagulation process. Various anticoagulants
have been developed for this purpose. The ones most
useful clinically are heparin and the coumarins.
Heparin as an Intravenous
Anticoagulant
Commercial heparin is extracted from several different animal tissues and prepared in almost pure form.
Injection of relatively small quantities, about 0.5 to
1 mg/kg of body weight, causes the blood-clotting time
to increase from a normal of about 6 minutes to 30 or
more minutes. Furthermore, this change in clotting
time occurs instantaneously, thereby immediately preventing or slowing further development of a thromboembolic condition.
The action of heparin lasts about 1.5 to 4 hours. The
injected heparin is destroyed by an enzyme in the
blood known as heparinase.
Coumarins as Anticoagulants
When a coumarin, such as warfarin, is given to a
patient, the plasma levels of prothrombin and Factors
VII, IX, and X, all formed by the liver, begin to fall,
indicating that warfarin has a potent depressant effect
on liver formation of these compounds. Warfarin
causes this effect by competing with vitamin K for
reactive sites in the enzymatic processes for formation
of prothrombin and the other three clotting factors,
thereby blocking the action of vitamin K.
After administration of an effective dose of warfarin, the coagulant activity of the blood decreases to
about 50 per cent of normal by the end of 12 hours and
to about 20 per cent of normal by the end of 24 hours.
In other words, the coagulation process is not blocked
immediately but must await the natural consumption
of the prothrombin and the other affected coagulation
factors already present in the plasma. Normal coagulation usually returns 1 to 3 days after discontinuing
coumarin therapy.
Prevention of Blood Coagulation
Outside the Body
Although blood removed from the body and held in a
glass test tube normally clots in about 6 minutes, blood
collected in siliconized containers often does not clot
for 1 hour or more. The reason for this delay is that
preparing the surfaces of the containers with silicone
467
Hemostasis and Blood Coagulation
prevents contact activation of platelets and Factor XII,
the two principal factors that initiate the intrinsic clotting mechanism. Conversely, untreated glass containers allow contact activation of the platelets and Factor
XII, with rapid development of clots.
Heparin can be used for preventing coagulation of
blood outside the body as well as in the body. Heparin
is especially used in surgical procedures in which the
blood must be passed through a heart-lung machine
or artificial kidney machine and then back into the
person.
Various substances that decrease the concentration
of calcium ions in the blood can also be used for preventing blood coagulation outside the body. For
instance, a soluble oxalate compound mixed in a very
small quantity with a sample of blood causes precipitation of calcium oxalate from the plasma and thereby
decreases the ionic calcium level so much that blood
coagulation is blocked.
Any substance that deionizes the blood calcium will
prevent coagulation. The negatively charged citrate ion
is especially valuable for this purpose, mixed with
blood usually in the form of sodium, ammonium, or
potassium citrate. The citrate ion combines with
calcium in the blood to cause an un-ionized calcium
compound, and the lack of ionic calcium prevents
coagulation. Citrate anticoagulants have an important
advantage over the oxalate anticoagulants because
oxalate is toxic to the body, whereas moderate quantities of citrate can be injected intravenously. After
injection, the citrate ion is removed from the blood
within a few minutes by the liver and is polymerized
into glucose or metabolized directly for energy.
Consequently, 500 milliliters of blood that has been
rendered incoagulable by citrate can ordinarily be
transfused into a recipient within a few minutes
without dire consequences. But if the liver is damaged
or if large quantities of citrated blood or plasma are
given too rapidly (within fractions of a minute), the
citrate ion may not be removed quickly enough, and
the citrate can, under these conditions, greatly depress
the level of calcium ion in the blood, which can result
in tetany and convulsive death.
Blood Coagulation Tests
Bleeding Time
When a sharp-pointed knife is used to pierce the tip
of the finger or lobe of the ear, bleeding ordinarily lasts
for 1 to 6 minutes. The time depends largely on the
depth of the wound and the degree of hyperemia in
the finger or ear lobe at the time of the test. Lack of
any one of several of the clotting factors can prolong
the bleeding time, but it is especially prolonged by lack
of platelets.
Clotting Time
Many methods have been devised for determining
blood clotting times. The one most widely used is to
collect blood in a chemically clean glass test tube and
then to tip the tube back and forth about every 30
seconds until the blood has clotted. By this method,
the normal clotting time is 6 to 10 minutes. Procedures
using multiple test tubes have also been devised for
determining clotting time more accurately.
Unfortunately, the clotting time varies widely,
depending on the method used for measuring it, so it
is no longer used in many clinics. Instead, measurements of the clotting factors themselves are made,
using sophisticated chemical procedures.
Prothrombin Time
Prothrombin time gives an indication of the concentration of prothrombin in the blood. Figure 36–5 shows
the relation of prothrombin concentration to prothrombin time. The method for determining prothrombin time is the following.
Blood removed from the patient is immediately
oxalated so that none of the prothrombin can change
into thrombin. Then, a large excess of calcium ion and
tissue factor is quickly mixed with the oxalated blood.
The excess calcium nullifies the effect of the oxalate,
and the tissue factor activates the prothrombin-tothrombin reaction by means of the extrinsic clotting
pathway. The time required for coagulation to take
place is known as the prothrombin time. The shortness
of the time is determined mainly by prothrombin concentration. The normal prothrombin time is about 12
seconds. In each laboratory, a curve relating prothrombin concentration to prothrombin time, such as
100
Concentration (per cent of normal)
Chapter 36
50.0
25.0
12.5
6.25
0
0
10
20
30
40
50
60
Prothrombin time
(seconds)
Figure 36–5
Relation of prothrombin concentration in the blood to “prothrombin time.”
468
Unit VI
Blood Cells, Immunity, and Blood Clotting
that shown in Figure 36–5, is drawn for the method
used so that the prothrombin in the blood can be quantified.
Tests similar to that for prothrombin time have been
devised to determine the quantities of other blood
clotting factors. In each of these tests, excesses of
calcium ions and all the other factors besides the one
being tested are added to oxalated blood all at once.
Then the time required for coagulation is determined
in the same manner as for prothrombin time. If the
factor being tested is deficient, the coagulation time is
prolonged. The time itself can then be used to quantitate the concentration of the factor.
References
Brass LF: Thrombin and platelet activation. Chest 124(3
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Caprini JA, Glase CJ, Anderson CB, Hathaway K: Laboratory markers in the diagnosis of venous thromboembolism. Circulation 109(12 Suppl 1):I4, 2004.
Dorsam RT, Kunapuli SP: Central role of the P2Y12 receptor in platelet activation. J Clin Invest 113:340, 2004.
Fisher M, Brott TG: Emerging therapies for acute ischemic
stroke: new therapies on trial. Stroke 34:359, 2003.
Geddis AE, Kaushansky K: Inherited thrombocytopenias:
toward a molecular understanding of disorders of platelet
production. Curr Opin Pediatr 16:15, 2004.
Kahn SR, Ginsberg JS: Relationship between deep venous
thrombosis and the postthrombotic syndrome. Arch
Intern Med 164:17, 2004.
Koreth R, Weinert C, Weisdorf DJ, Key NS: Measurement of
bleeding severity: a critical review. Transfusion 44:605,
2004.
Levi M: Current understanding of disseminated intravascular coagulation. Br J Haematol 124:567, 2004.
Lindsberg PJ, Kaste M: Thrombolysis for acute stroke. Curr
Opin Neurol 16:73, 2003.
Moake JL: Thrombotic microangiopathies. N Engl J Med
347:589, 2002.
Roberts HR, Monroe DM, Escobar MA: Current concepts
of hemostasis: implications for therapy. Anesthesiology
100:722, 2004.
Saenko EL, Ananyeva NM, Shima M, et al: The future of
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1:922, 2003.
Solum NO: Procoagulant expression in platelets and defects
leading to clinical disorders. Arterioscler Thromb Vasc
Biol 19:2841, 1999.
Toh CH, Dennis M: Disseminated intravascular coagulation:
old disease, new hope. BMJ 327:974, 2003.
Tsai HM: Advances in the pathogenesis, diagnosis, and treatment of thrombotic thrombocytopenic purpura. J Am Soc
Nephrol 14:1072, 2003.
Tsai HM: Platelet activation and the formation of the
platelet plug: deficiency of ADAMTS13 causes thrombotic
thrombocytopenic purpura. Arterioscler Thromb Vasc
Biol 23:388, 2003.
VandenDriessche T, Collen D, Chuah MK: Gene therapy for
the hemophilias. J Thromb Haemost 1:1550, 2003.
Vesely SK, Perdue JJ, Rizvi MA, et al: Management of adult
patients with persistent idiopathic thrombocytopenic
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White RH, Ginsberg JS: Low-molecular-weight heparins: are
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U
N
Respiration
37. Pulmonary Ventilation
38. Pulmonary Circulation, Pulmonary Edema,
Pleural Fluid
39. Physical Principles of Gas Exchange; Diffusion
of Oxygen and Carbon Dioxide Through the
Respiratory Membrane
40. Transport of Oxygen and Carbon Dioxide in
Blood and Tissue Fluids
41. Regulation of Respiration
42. Respiratory Insufficiency—Pathophysiology,
Diagnosis, Oxygen Therapy
I
T
VII
C
H
A
P
T
E
R
3
7
Pulmonary Ventilation
The goals of respiration are to provide oxygen to
the tissues and to remove carbon dioxide. To
achieve these goals, respiration can be divided into
four major functions: (1) pulmonary ventilation,
which means the inflow and outflow of air between
the atmosphere and the lung alveoli; (2) diffusion
of oxygen and carbon dioxide between the alveoli
and the blood; (3) transport of oxygen and carbon
dioxide in the blood and body fluids to and from the body’s tissue cells; and
(4) regulation of ventilation and other facets of respiration. This chapter is a
discussion of pulmonary ventilation, and the subsequent five chapters
cover other respiratory functions plus the physiology of special respiratory
abnormalities.
Mechanics of Pulmonary Ventilation
Muscles That Cause Lung Expansion and Contraction
The lungs can be expanded and contracted in two ways: (1) by downward and
upward movement of the diaphragm to lengthen or shorten the chest cavity,
and (2) by elevation and depression of the ribs to increase and decrease the
anteroposterior diameter of the chest cavity. Figure 37–1 shows these two
methods.
Normal quiet breathing is accomplished almost entirely by the first method,
that is, by movement of the diaphragm. During inspiration, contraction of the
diaphragm pulls the lower surfaces of the lungs downward. Then, during expiration, the diaphragm simply relaxes, and the elastic recoil of the lungs, chest
wall, and abdominal structures compresses the lungs and expels the air. During
heavy breathing, however, the elastic forces are not powerful enough to cause
the necessary rapid expiration, so that extra force is achieved mainly by contraction of the abdominal muscles, which pushes the abdominal contents
upward against the bottom of the diaphragm, thereby compressing the lungs.
The second method for expanding the lungs is to raise the rib cage. This
expands the lungs because, in the natural resting position, the ribs slant downward, as shown on the left side of Figure 37–1, thus allowing the sternum to fall
backward toward the vertebral column. But when the rib cage is elevated, the
ribs project almost directly forward, so that the sternum also moves forward,
away from the spine, making the anteroposterior thickness of the chest about
20 per cent greater during maximum inspiration than during expiration. Therefore, all the muscles that elevate the chest cage are classified as muscles of inspiration, and those muscles that depress the chest cage are classified as muscles
of expiration. The most important muscles that raise the rib cage are the external intercostals, but others that help are the (1) sternocleidomastoid muscles,
which lift upward on the sternum; (2) anterior serrati, which lift many of the
ribs; and (3) scaleni, which lift the first two ribs.
The muscles that pull the rib cage downward during expiration are mainly
the (1) abdominal recti, which have the powerful effect of pulling downward on
the lower ribs at the same time that they and other abdominal muscles also compress the abdominal contents upward against the diaphragm, and (2) internal
intercostals.
471
472
Unit VII
Respiration
Increased
A–P diameter
Elevated
rib cage
External
intercostals
contracted
Internal
intercostals
relaxed
Diaphragmatic
contraction
Abdominals
contracted
0.25
Contraction and expansion of the thoracic cage during expiration
and inspiration, demonstrating diaphragmatic contraction, function of the intercostal muscles, and elevation and depression of
the rib cage.
Figure 37–1 also shows the mechanism by which the
external and internal intercostals act to cause inspiration and expiration. To the left, the ribs during
expiration are angled downward, and the external
intercostals are elongated forward and downward. As
they contract, they pull the upper ribs forward in relation to the lower ribs, and this causes leverage on the
ribs to raise them upward, thereby causing inspiration.
The internal intercostals function exactly in the opposite manner, functioning as expiratory muscles because
they angle between the ribs in the opposite direction
and cause opposite leverage.
Movement of Air In and Out of the
Lungs and the Pressures
That Cause the Movement
The lung is an elastic structure that collapses like a
balloon and expels all its air through the trachea whenever there is no force to keep it inflated. Also, there
are no attachments between the lung and the walls of
the chest cage, except where it is suspended at its hilum
from the mediastinum. Instead, the lung “floats” in the
thoracic cavity, surrounded by a thin layer of pleural
fluid that lubricates movement of the lungs within the
cavity. Further, continual suction of excess fluid into
lymphatic channels maintains a slight suction between
the visceral surface of the lung pleura and the parietal
pleural surface of the thoracic cavity. Therefore, the
lungs are held to the thoracic wall as if glued there,
except that they are well lubricated and can slide
freely as the chest expands and contracts.
Pleural Pressure and Its Changes
During Respiration
Pleural pressure is the pressure of the fluid in the thin
space between the lung pleura and the chest wall
pleura. As noted earlier, this is normally a slight
suction, which means a slightly negative pressure. The
normal pleural pressure at the beginning of inspiration
0
Alveolar pressure
+2
INSPIRATION
Figure 37–1
Lung volume
0.50
0
Pressure (cm H2O)
EXPIRATION
Volume change (liters)
Increased
vertical diameter
–2
Transpulmonary pressure
–4
–6
Pleural pressure
–8
Inspiration
Expiration
Figure 37–2
Changes in lung volume, alveolar pressure, pleural pressure, and
transpulmonary pressure during normal breathing.
is about –5 centimeters of water, which is the amount
of suction required to hold the lungs open to their
resting level. Then, during normal inspiration, expansion of the chest cage pulls outward on the lungs with
greater force and creates more negative pressure, to an
average of about –7.5 centimeters of water.
These relationships between pleural pressure and
changing lung volume are demonstrated in Figure
37–2, showing in the lower panel the increasing negativity of the pleural pressure from –5 to –7.5 during
inspiration and in the upper panel an increase in lung
volume of 0.5 liter. Then, during expiration, the events
are essentially reversed.
Alveolar Pressure
Alveolar pressure is the pressure of the air inside the
lung alveoli. When the glottis is open and no air is
flowing into or out of the lungs, the pressures in all
parts of the respiratory tree, all the way to the alveoli,
are equal to atmospheric pressure, which is considered
to be zero reference pressure in the airways—that is,
0 centimeters water pressure. To cause inward flow of
air into the alveoli during inspiration, the pressure in
the alveoli must fall to a value slightly below atmospheric pressure (below 0). The second curve (labeled
“alveolar pressure”) of Figure 37–2 demonstrates that
during normal inspiration, alveolar pressure decreases
to about –1 centimeter of water. This slight negative
Chapter 37
pressure is enough to pull 0.5 liter of air into the lungs
in the 2 seconds required for normal quiet inspiration.
During expiration, opposite pressures occur: The
alveolar pressure rises to about +1 centimeter of water,
and this forces the 0.5 liter of inspired air out of the
lungs during the 2 to 3 seconds of expiration.
Transpulmonary Pressure. Finally, note in Figure 37–2 the
difference between the alveolar pressure and the
pleural pressure. This is called the transpulmonary
pressure. It is the pressure difference between that in
the alveoli and that on the outer surfaces of the lungs,
and it is a measure of the elastic forces in the lungs
that tend to collapse the lungs at each instant of respiration, called the recoil pressure.
Compliance of the Lungs
The extent to which the lungs will expand for each unit
increase in transpulmonary pressure (if enough time is
allowed to reach equilibrium) is called the lung compliance. The total compliance of both lungs together in
the normal adult human being averages about 200 milliliters of air per centimeter of water transpulmonary
pressure. That is, every time the transpulmonary pressure increases 1 centimeter of water, the lung volume,
after 10 to 20 seconds, will expand 200 milliliters.
Compliance Diagram of the Lungs. Figure 37–3 is a diagram
relating lung volume changes to changes in transpulmonary pressure. Note that the relation is different for
inspiration and expiration. Each curve is recorded by
changing the transpulmonary pressure in small steps
and allowing the lung volume to come to a steady level
between successive steps. The two curves are called,
respectively, the inspiratory compliance curve and the
expiratory compliance curve, and the entire diagram is
called the compliance diagram of the lungs.
The characteristics of the compliance diagram are
determined by the elastic forces of the lungs. These can
be divided into two parts: (1) elastic forces of the lung
tissue itself and (2) elastic forces caused by surface
tension of the fluid that lines the inside walls of the
alveoli and other lung air spaces.
The elastic forces of the lung tissue are determined
mainly by elastin and collagen fibers interwoven
among the lung parenchyma. In deflated lungs, these
fibers are in an elastically contracted and kinked state;
then, when the lungs expand, the fibers become
stretched and unkinked, thereby elongating and exerting even more elastic force.
The elastic forces caused by surface tension are
much more complex. The significance of surface
tension is shown in Figure 37–4, which compares the
compliance diagram of the lungs when filled with
saline solution and when filled with air.When the lungs
are filled with air, there is an interface between the
alveolar fluid and the air in the alveoli. In the case of
the saline solution–filled lungs, there is no air-fluid
interface; therefore, the surface tension effect is not
present—only tissue elastic forces are operative in the
saline solution–filled lung.
Note that transpleural pressures required to expand
air-filled lungs are about three times as great as those
required to expand saline solution–filled lungs. Thus,
one can conclude that the tissue elastic forces tending
to cause collapse of the air-filled lung represent only
about one third of the total lung elasticity, whereas the
Saline-filled
Lung volume change (liters)
Lung volume change (liters)
473
Pulmonary Ventilation
0.50
Expiration
0.25
Inspiration
Expiration
0.25
Inspiration
0
0
0
–4
–5
Pleural pressure (cm H2O)
Air-filled
0.50
–6
–2
–4
–6
Pleural pressure (cm H2O)
–8
Figure 37–4
Figure 37–3
Compliance diagram in a healthy person. This diagram shows
compliance of the lungs alone.
Comparison of the compliance diagrams of saline-filled and airfilled lungs when the alveolar pressure is maintained at atmospheric pressure (0 cm H2O) and pleural pressure is changed.
474
Unit VII
fluid-air surface tension forces in the alveoli represent
about two thirds.
The fluid-air surface tension elastic forces of the
lungs also increase tremendously when the substance
called surfactant is not present in the alveolar fluid. Let
us now discuss surfactant and its relation to the surface
tension forces.
Surfactant, Surface Tension, and Collapse
of the Alveoli
Principle of Surface Tension. When water forms a surface
with air, the water molecules on the surface of the
water have an especially strong attraction for one
another. As a result, the water surface is always
attempting to contract. This is what holds raindrops
together: that is, there is a tight contractile membrane
of water molecules around the entire surface of the
raindrop. Now let us reverse these principles and see
what happens on the inner surfaces of the alveoli.
Here, the water surface is also attempting to contract.
This results in an attempt to force the air out of the
alveoli through the bronchi and, in doing so, causes the
alveoli to try to collapse. The net effect is to cause an
elastic contractile force of the entire lungs, which is
called the surface tension elastic force.
Surfactant and Its Effect on Surface Tension. Surfactant is a
surface active agent in water, which means that it
greatly reduces the surface tension of water. It is
secreted by special surfactant-secreting epithelial cells
called type II alveolar epithelial cells, which constitute
about 10 per cent of the surface area of the alveoli.
These cells are granular, containing lipid inclusions
that are secreted in the surfactant into the alveoli.
Surfactant is a complex mixture of several
phospholipids, proteins, and ions. The most important
components are the phospholipid dipalmitoylphosphatidylcholine, surfactant apoproteins, and calcium
ions. The dipalmitoylphosphatidylcholine, along with
several less important phospholipids, is responsible
for reducing the surface tension. It does this by not
dissolving uniformly in the fluid lining the alveolar
surface. Instead, part of the molecule dissolves,
while the remainder spreads over the surface of the
water in the alveoli. This surface has from one
twelfth to one half the surface tension of a pure water
surface.
In quantitative terms, the surface tension of different water fluids is approximately the following: pure
water, 72 dynes/cm; normal fluids lining the alveoli but
without surfactant, 50 dynes/cm; normal fluids lining
the alveoli and with normal amounts of surfactant
included, between 5 and 30 dynes/cm.
Pressure in Occluded Alveoli Caused by Surface Tension. If the
air passages leading from the alveoli of the lungs are
blocked, the surface tension in the alveoli tends to collapse the alveoli. This creates positive pressure in the
alveoli, attempting to push the air out. The amount of
pressure generated in this way in an alveolus can be calculated from the following formula:
Respiration
Pressure =
2 ¥ Surface tension
Radius of alveolus
For the average-sized alveolus with a radius of about
100 micrometers and lined with normal surfactant, this
calculates to be about 4 centimeters of water pressure
(3 mm Hg). If the alveoli were lined with pure water
without any surfactant, the pressure would calculate to
be about 18 centimeters of water pressure, 4.5 times as
great. Thus, one sees how important surfactant is in
reducing alveolar surface tension and therefore also
reducing the effort required by the respiratory muscles
to expand the lungs.
Effect of Alveolar Radius on the Pressure Caused by Surface
Tension. Note from the preceding formula that the pres-
sure generated as a result of surface tension in the
alveoli is inversely affected by the radius of the alveolus, which means that the smaller the alveolus, the
greater the alveolar pressure caused by the surface
tension. Thus, when the alveoli have half the normal
radius (50 instead of 100 micrometers), the pressures
noted earlier are doubled. This is especially significant
in small premature babies, many of whom have alveoli
with radii less than one quarter that of an adult person.
Further, surfactant does not normally begin to be
secreted into the alveoli until between the sixth and
seventh months of gestation, and in some cases, even
later than that. Therefore, many premature babies have
little or no surfactant in the alveoli when they are born,
and their lungs have an extreme tendency to collapse,
sometimes as great as six to eight times that in a normal
adult person. This causes the condition called respiratory distress syndrome of the newborn. It is fatal if
not treated with strong measures, especially properly
applied continuous positive pressure breathing.
Effect of the Thoracic Cage
on Lung Expansibility
Thus far, we have discussed the expansibility of the
lungs alone, without considering the thoracic cage. The
thoracic cage has its own elastic and viscous characteristics, similar to those of the lungs; even if the lungs
were not present in the thorax, muscular effort would
still be required to expand the thoracic cage.
Compliance of the Thorax and the
Lungs Together
The compliance of the entire pulmonary system (the
lungs and thoracic cage together) is measured while
expanding the lungs of a totally relaxed or paralyzed
person. To do this, air is forced into the lungs a little at
a time while recording lung pressures and volumes. To
inflate this total pulmonary system, almost twice as
much pressure is needed as to inflate the same lungs
after removal from the chest cage. Therefore, the compliance of the combined lung-thorax system is almost
exactly one half that of the lungs alone—110 milliliters
of volume per centimeter of water pressure for
the combined system, compared with 200 ml/cm for
the lungs alone. Furthermore, when the lungs are
expanded to high volumes or compressed to low
volumes, the limitations of the chest become extreme;
Chapter 37
475
Pulmonary Ventilation
when near these limits, the compliance of the combined lung-thorax system can be less than one fifth
that of the lungs alone.
“Work” of Breathing
We have already pointed out that during normal quiet
breathing, all respiratory muscle contraction occurs
during inspiration; expiration is almost entirely a
passive process caused by elastic recoil of the lungs and
chest cage. Thus, under resting conditions, the respiratory muscles normally perform “work” to cause inspiration but not to cause expiration.
The work of inspiration can be divided into three
fractions: (1) that required to expand the lungs against
the lung and chest elastic forces, called compliance work
or elastic work; (2) that required to overcome the viscosity of the lung and chest wall structures, called tissue
resistance work; and (3) that required to overcome
airway resistance to movement of air into the lungs,
called airway resistance work.
Energy Required for Respiration. During normal quiet respi-
ration, only 3 to 5 per cent of the total energy expended
by the body is required for pulmonary ventilation. But
during heavy exercise, the amount of energy required
can increase as much as 50-fold, especially if the person
has any degree of increased airway resistance or decreased pulmonary compliance. Therefore, one of the
major limitations on the intensity of exercise that can
be performed is the person’s ability to provide enough
muscle energy for the respiratory process alone.
Pulmonary Volumes
and Capacities
Recording Changes in Pulmonary
Volume—Spirometry
A simple method for studying pulmonary ventilation
is to record the volume movement of air into and out
of the lungs, a process called spirometry. A typical
basic spirometer is shown in Figure 37–5. It consists of
a drum inverted over a chamber of water, with the
drum counterbalanced by a weight. In the drum is a
breathing gas, usually air or oxygen; a tube connects
the mouth with the gas chamber. When one breathes
into and out of the chamber, the drum rises and falls,
and an appropriate recording is made on a moving
sheet of paper.
Figure 37–6 shows a spirogram indicating changes in
lung volume under different conditions of breathing.
For ease in describing the events of pulmonary ventilation, the air in the lungs has been subdivided in this
diagram into four volumes and four capacities, which
are the average for a young adult man.
Pulmonary Volumes
To the left in Figure 37–6 are listed four pulmonary
lung volumes that, when added together, equal the
maximum volume to which the lungs can be expanded.
The significance of each of these volumes is the
following:
Floating
drum
Oxygen
chamber
Recording
drum
Water
Mouthpiece
Counterbalancing
weight
Figure 37–5
Spirometer.
1. The tidal volume is the volume of air inspired or
expired with each normal breath; it amounts to
about 500 milliliters in the adult male.
2. The inspiratory reserve volume is the extra volume
of air that can be inspired over and above the
normal tidal volume when the person inspires
with full force; it is usually equal to about
3000 milliliters.
3. The expiratory reserve volume is the maximum
extra volume of air that can be expired by
forceful expiration after the end of a normal
tidal expiration; this normally amounts to about
1100 milliliters.
4. The residual volume is the volume of air
remaining in the lungs after the most forceful
expiration; this volume averages about
1200 milliliters.
Pulmonary Capacities
In describing events in the pulmonary cycle, it is
sometimes desirable to consider two or more of the
volumes together. Such combinations are called pulmonary capacities. To the right in Figure 37–6 are listed
the important pulmonary capacities, which can be
described as follows:
1. The inspiratory capacity equals the tidal volume
plus the inspiratory reserve volume. This is the
amount of air (about 3500 milliliters) a person can
breathe in, beginning at the normal expiratory
level and distending the lungs to the maximum
amount.
2. The functional residual capacity equals the
expiratory reserve volume plus the residual
volume. This is the amount of air that remains in
the lungs at the end of normal expiration (about
2300 milliliters).
3. The vital capacity equals the inspiratory reserve
volume plus the tidal volume plus the expiratory
reserve volume. This is the maximum amount of
air a person can expel from the lungs after first
filling the lungs to their maximum extent and then
476
Unit VII
6000
Respiration
Inspiration
Lung volume (ml)
5000
4000
Vital
capacity
Total lung
capacity
Tidal
volume
3000
2000
Inspiratory
capacity
Inspiratory
reserve
volume
Functional
residual
capacity
Expiratory
reserve volume
1000
Residual
volume
Expiration
Figure 37–6
Time
expiring to the maximum extent (about
4600 milliliters).
4. The total lung capacity is the maximum volume to
which the lungs can be expanded with the greatest
possible effort (about 5800 milliliters); it is equal
to the vital capacity plus the residual volume.
All pulmonary volumes and capacities are about 20
to 25 per cent less in women than in men, and they are
greater in large and athletic people than in small and
asthenic people.
Abbreviations and Symbols Used
in Pulmonary Function Studies
Spirometry is only one of many measurement procedures that the pulmonary physician uses daily. Many of
these measurement procedures depend heavily on
mathematical computations. To simplify these calculations as well as the presentation of pulmonary function
data, a number of abbreviations and symbols have
become standardized. The more important of these are
given in Table 37–1. Using these symbols, we present
here a few simple algebraic exercises showing some of
the interrelations among the pulmonary volumes and
capacities; the student should think through and verify
these interrelations.
VC = IRV + VT + ERV
VC = IC + ERV
TLC = VC + RV
TLC = IC + FRC
FRC = ERV + RV
Diagram showing respiratory excursions during
normal breathing and during maximal inspiration
and maximal expiration.
Determination of Functional
Residual Capacity, Residual
Volume, and Total Lung Capacity—
Helium Dilution Method
The functional residual capacity (FRC), which is the
volume of air that remains in the lungs at the end of
each normal expiration, is important to lung function.
Because its value changes markedly in some types of
pulmonary disease, it is often desirable to measure this
capacity. The spirometer cannot be used in a direct way
to measure the functional residual capacity, because the
air in the residual volume of the lungs cannot be expired
into the spirometer, and this volume constitutes about
one half of the functional residual capacity. To measure
functional residual capacity, the spirometer must be
used in an indirect manner, usually by means of a
helium dilution method, as follows.
A spirometer of known volume is filled with air mixed
with helium at a known concentration. Before breathing from the spirometer, the person expires normally. At
the end of this expiration, the remaining volume in the
lungs is equal to the functional residual capacity. At this
point, the subject immediately begins to breathe from
the spirometer, and the gases of the spirometer mix with
the gases of the lungs. As a result, the helium becomes
diluted by the functional residual capacity gases, and the
volume of the functional residual capacity can be calculated from the degree of dilution of the helium, using
the following formula:
Ci He
FRC = Ê
- 1ˆ Vi Spir
Ë CfHe
¯
Chapter 37
477
Pulmonary Ventilation
Table 37–1
Abbreviations and Symbols for Pulmonary Function
VT
FRC
ERV
RV
IC
IRV
TLC
VC
Raw
C
VD
V
.A
V
.I
V
.E
V
.s
V
.A
V
. o2
Vco2
.
Vco
Dlo2
DlCO
tidal volume
functional residual capacity
expiratory reserve volume
residual volume
inspiratory capacity
inspiratory reserve volume
total lung capacity
vital capacity
resistance of the airways to flow of air into
the lung
compliance
volume of dead space gas
volume of alveolar gas
inspired volume of ventilation per minute
expired volume of ventilation per minute
shunt flow
alveolar ventilation per minute
rate of oxygen uptake per minute
amount of carbon dioxide eliminated per
minute
rate of carbon monoxide uptake per minute
PB
Palv
Ppl
PO2
PCO2
PN2
PaO2
PaCO2
PAO2
atmospheric pressure
alveolar pressure
pleural pressure
partial pressure of oxygen
partial pressure of carbon dioxide
partial pressure of nitrogen
partial pressure of oxygen in arterial blood
partial pressure of carbon dioxide in arterial blood
partial pressure of oxygen in alveolar gas
PACO2
PAH2O
R
Q̇
partial pressure of carbon dioxide in alveolar gas
partial pressure of water in alveolar gas
respiratory exchange ratio
cardiac output
CaO2
Cv̄o2
SO2
concentration of oxygen in arterial blood
concentration of oxygen in mixed venous blood
percentage saturation of hemoglobin with oxygen
SaO2
percentage saturation of hemoglobin with oxygen
in arterial blood
diffusing capacity of the lungs for oxygen
diffusing capacity of the lungs for carbon
monoxide
where FRC is functional residual capacity, CiHe is initial
concentration of helium in the spirometer, CfHe is final
concentration of helium in the spirometer, and ViSpir is
initial volume of the spirometer.
Once the FRC has been determined, the residual
volume (RV) can be determined by subtracting expiratory reserve volume (ERV), as measured by normal
spirometry, from the FRC. Also, the total lung capacity
(TLC) can be determined by adding the inspiratory
capacity (IC) to the FRC. That is,
RV = FRC – ERV
and
TLC = FRC + IC
Minute Respiratory Volume
Equals Respiratory Rate
Times Tidal Volume
The minute respiratory volume is the total amount of
new air moved into the respiratory passages each
minute; this is equal to the tidal volume times the respiratory rate per minute. The normal tidal volume is
about 500 milliliters, and the normal respiratory rate
is about 12 breaths per minute. Therefore, the minute
respiratory volume averages about 6 L/min. A person
can live for a short period with a minute respiratory
volume as low as 1.5 L/min and a respiratory rate of
only 2 to 4 breaths per minute.
The respiratory rate occasionally rises to 40 to 50
per minute, and the tidal volume can become as great
as the vital capacity, about 4600 milliliters in a young
adult man. This can give a minute respiratory volume
greater than 200 L/min, or more than 30 times normal.
Most people cannot sustain more than one half to two
thirds these values for longer than 1 minute.
Alveolar Ventilation
The ultimate importance of pulmonary ventilation is
to continually renew the air in the gas exchange areas
of the lungs, where air is in proximity to the pulmonary
blood. These areas include the alveoli, alveolar sacs,
alveolar ducts, and respiratory bronchioles. The rate at
which new air reaches these areas is called alveolar
ventilation.
“Dead Space” and Its Effect
on Alveolar Ventilation
Some of the air a person breathes never reaches the
gas exchange areas but simply fills respiratory passages where gas exchange does not occur, such as the
nose, pharynx, and trachea.This air is called dead space
air because it is not useful for gas exchange.
On expiration, the air in the dead space is expired
first, before any of the air from the alveoli reaches the
atmosphere. Therefore, the dead space is very disadvantageous for removing the expiratory gases from the
lungs.
Measurement of the Dead Space Volume. A simple method
for measuring dead space volume is demonstrated by
the graph in Figure 37–7. In making this measurement,
the subject suddenly takes a deep breath of oxygen. This
478
Unit VII
Respiration
Anatomic Versus Physiologic Dead Space. The method just
40
20
nitro
concentration
gen
Reco
rded
60
Inspiration of pure oxygen
Per cent nitrogen
80
0
0
100
200
300
400
Air expired (ml)
500
Figure 37–7
Record of the changes in nitrogen concentration in the expired air
after a single previous inspiration of pure oxygen. This record can
be used to calculate dead space, as discussed in the text.
fills the entire dead space with pure oxygen. Some
oxygen also mixes with the alveolar air but does not
completely replace this air. Then the person expires
through a rapidly recording nitrogen meter, which
makes the record shown in the figure. The first portion
of the expired air comes from the dead space regions of
the respiratory passageways, where the air has been
completely replaced by oxygen. Therefore, in the early
part of the record, only oxygen appears, and the nitrogen concentration is zero. Then, when alveolar air
begins to reach the nitrogen meter, the nitrogen concentration rises rapidly, because alveolar air containing
large amounts of nitrogen begins to mix with the dead
space air. After still more air has been expired, all the
dead space air has been washed from the passages, and
only alveolar air remains. Therefore, the recorded nitrogen concentration reaches a plateau level equal to its
concentration in the alveoli, as shown to the right in the
figure. With a little thought, the student can see that the
gray area represents the air that has no nitrogen in it;
this area is a measure of the volume of dead space air.
For exact quantification, the following equation is used:
Gray area ¥ VE
VD =
Pink area + Gray area
where VD is dead space air and VE is the total volume
of expired air.
Let us assume, for instance, that the gray area on the
graph is 30 square centimeters, the pink area is 70
square centimeters, and the total volume expired is 500
milliliters. The dead space would be
30
¥ 500, or 150 ml
30 + 70
Normal Dead Space Volume. The normal dead space air in
a young adult man is about 150 milliliters. This increases
slightly with age.
described for measuring the dead space measures the
volume of all the space of the respiratory system other
than the alveoli and their other closely related gas
exchange areas; this space is called the anatomic dead
space. On occasion, some of the alveoli themselves are
nonfunctional or only partially functional because of
absent or poor blood flow through the adjacent pulmonary capillaries. Therefore, from a functional point of
view, these alveoli must also be considered dead space.
When the alveolar dead space is included in the total
measurement of dead space, this is called the physiologic dead space, in contradistinction to the anatomic
dead space. In a normal person, the anatomic and physiologic dead spaces are nearly equal because all alveoli
are functional in the normal lung, but in a person with
partially functional or nonfunctional alveoli in some
parts of the lungs, the physiologic dead space may be as
much as 10 times the volume of the anatomic dead
space, or 1 to 2 liters. These problems are discussed
further in Chapter 39 in relation to pulmonary gaseous
exchange and in Chapter 42 in relation to certain pulmonary diseases.
Rate of Alveolar Ventilation
Alveolar ventilation per minute is the total volume of
new air entering the alveoli and adjacent gas exchange
areas each minute. It is equal to the respiratory rate
times the amount of new air that enters these areas
with each breath.
.
V A = Freq • (VT – VD)
.
where V A is the volume of alveolar ventilation per
minute, Freq is the frequency of respiration per
minute, VT is the tidal volume, and VD is the physiologic dead space volume.
Thus, with a normal tidal volume of 500 milliliters, a
normal dead space of 150 milliliters, and a respiratory
rate of 12 breaths per minute, alveolar ventilation
equals 12 ¥ (500 – 150), or 4200 ml/min.
Alveolar ventilation is one of the major factors
determining the concentrations of oxygen and carbon
dioxide in the alveoli. Therefore, almost all discussions
of gaseous exchange in the following chapters on the
respiratory system emphasize alveolar ventilation.
Functions of the Respiratory
Passageways
Trachea, Bronchi, and Bronchioles
Figure 37–8 shows the respiratory system, demonstrating especially the respiratory passageways.The air is distributed to the lungs by way of the trachea, bronchi, and
bronchioles.
One of the most important problems in all the respiratory passageways is to keep them open and allow easy
passage of air to and from the alveoli. To keep the
trachea from collapsing, multiple cartilage rings extend
about five sixths of the way around the trachea. In the
walls of the bronchi, less extensive curved cartilage
plates also maintain a reasonable amount of rigidity yet
allow sufficient motion for the lungs to expand and
Chapter 37
479
Pulmonary Ventilation
Conchae
CO2
O2
Alveolus
Glottis
Larynx, vocal
cords
Trachea
Epiglottis
Pharynx
Esophagus
O2
O2
CO2
CO2
Pulmonary
capillary
Pulmonary arteries
Pulmonary veins
Alveoli
Figure 37–8
Respiratory passages.
contract. These plates become progressively less extensive in the later generations of bronchi and are gone in
the bronchioles, which usually have diameters less than
1.5 millimeters. The bronchioles are not prevented from
collapsing by the rigidity of their walls. Instead, they are
kept expanded mainly by the same transpulmonary
pressures that expand the alveoli. That is, as the alveoli
enlarge, the bronchioles also enlarge, but not as much.
Muscular Wall of the Bronchi and Bronchioles and Its Control. In
all areas of the trachea and bronchi not occupied by cartilage plates, the walls are composed mainly of smooth
muscle. Also, the walls of the bronchioles are almost
entirely smooth muscle, with the exception of the most
terminal bronchiole, called the respiratory bronchiole,
which is mainly pulmonary epithelium and underlying
fibrous tissue plus a few smooth muscle fibers. Many
obstructive diseases of the lung result from narrowing
of the smaller bronchi and larger bronchioles, often
because of excessive contraction of the smooth muscle
itself.
Resistance to Airflow in the Bronchial Tree. Under normal
respiratory conditions, air flows through the respiratory
passageways so easily that less than 1 centimeter of
water pressure gradient from the alveoli to the atmosphere is sufficient to cause enough airflow for quiet
breathing. The greatest amount of resistance to airflow
occurs not in the minute air passages of the terminal
bronchioles but in some of the larger bronchioles and
bronchi near the trachea. The reason for this high resistance is that there are relatively few of these larger
bronchi in comparison with the approximately 65,000
parallel terminal bronchioles, through each of which
only a minute amount of air must pass.
Yet in disease conditions, the smaller bronchioles
often play a far greater role in determining airflow
resistance because of their small size and because they
are easily occluded by (1) muscle contraction in their
walls, (2) edema occurring in the walls, or (3) mucus collecting in the lumens of the bronchioles.
Nervous and Local Control of the Bronchiolar Musculature—
“Sympathetic” Dilation of the Bronchioles. Direct control of
the bronchioles by sympathetic nerve fibers is relatively
weak because few of these fibers penetrate to the
central portions of the lung. However, the bronchial tree
is very much exposed to norepinephrine and epinephrine released into the blood by sympathetic stimulation
of the adrenal gland medullae. Both these hormones—
especially epinephrine, because of its greater stimulation of beta-adrenergic receptors—cause dilation of the
bronchial tree.
Parasympathetic Constriction of the Bronchioles. A few
parasympathetic nerve fibers derived from the vagus
nerves penetrate the lung parenchyma. These nerves
secrete acetylcholine and, when activated, cause mild
to moderate constriction of the bronchioles. When a
disease process such as asthma has already caused some
bronchiolar constriction, superimposed parasympathetic nervous stimulation often worsens the condition.
When this occurs, administration of drugs that block the
effects of acetylcholine, such as atropine, can sometimes
relax the respiratory passages enough to relieve the
obstruction.
Sometimes the parasympathetic nerves are also activated by reflexes that originate in the lungs. Most of
these begin with irritation of the epithelial membrane
of the respiratory passageways themselves, initiated by
noxious gases, dust, cigarette smoke, or bronchial infection. Also, a bronchiolar constrictor reflex often occurs
when microemboli occlude small pulmonary arteries.
Local Secretory Factors Often Cause Bronchiolar Constriction.
Several substances formed in the lungs themselves are
480
Unit VII
often quite active in causing bronchiolar constriction.
Two of the most important of these are histamine and
slow reactive substance of anaphylaxis. Both of these are
released in the lung tissues by mast cells during allergic
reactions, especially those caused by pollen in the air.
Therefore, they play key roles in causing the airway
obstruction that occurs in allergic asthma; this is especially true of the slow reactive substance of anaphylaxis.
The same irritants that cause parasympathetic constrictor reflexes of the airways—smoke, dust, sulfur
dioxide, and some of the acidic elements in smog—often
act directly on the lung tissues to initiate local, nonnervous reactions that cause obstructive constriction of
the airways.
Mucus Lining the Respiratory Passageways, and Action of Cilia
to Clear the Passageways
All the respiratory passages, from the nose to the terminal bronchioles, are kept moist by a layer of mucus
that coats the entire surface. The mucus is secreted
partly by individual mucous goblet cells in the epithelial lining of the passages and partly by small submucosal glands. In addition to keeping the surfaces moist,
the mucus traps small particles out of the inspired air
and keeps most of these from ever reaching the alveoli.
The mucus itself is removed from the passages in the
following manner.
The entire surface of the respiratory passages, both in
the nose and in the lower passages down as far as the
terminal bronchioles, is lined with ciliated epithelium,
with about 200 cilia on each epithelial cell. These cilia
beat continually at a rate of 10 to 20 times per second
by the mechanism explained in Chapter 2, and the direction of their “power stroke” is always toward the
pharynx. That is, the cilia in the lungs beat upward,
whereas those in the nose beat downward. This continual beating causes the coat of mucus to flow slowly, at a
velocity of a few millimeters per minute, toward the
pharynx. Then the mucus and its entrapped particles are
either swallowed or coughed to the exterior.
Cough Reflex
The bronchi and trachea are so sensitive to light touch
that very slight amounts of foreign matter or other
causes of irritation initiate the cough reflex. The larynx
and carina (the point where the trachea divides into the
bronchi) are especially sensitive, and the terminal bronchioles and even the alveoli are sensitive to corrosive
chemical stimuli such as sulfur dioxide gas or chlorine
gas. Afferent nerve impulses pass from the respiratory
passages mainly through the vagus nerves to the
medulla of the brain. There, an automatic sequence of
events is triggered by the neuronal circuits of the
medulla, causing the following effect.
First, up to 2.5 liters of air are rapidly inspired.
Second, the epiglottis closes, and the vocal cords shut
tightly to entrap the air within the lungs. Third, the
abdominal muscles contract forcefully, pushing against
the diaphragm while other expiratory muscles, such as
the internal intercostals, also contract forcefully. Consequently, the pressure in the lungs rises rapidly to as
much as 100 mm Hg or more. Fourth, the vocal cords
and the epiglottis suddenly open widely, so that air
under this high pressure in the lungs explodes outward.
Indeed, sometimes this air is expelled at velocities
ranging from 75 to 100 miles per hour. Importantly, the
strong compression of the lungs collapses the bronchi
and trachea by causing their noncartilaginous parts to
Respiration
invaginate inward, so that the exploding air actually
passes through bronchial and tracheal slits. The rapidly
moving air usually carries with it any foreign matter that
is present in the bronchi or trachea.
Sneeze Reflex
The sneeze reflex is very much like the cough reflex,
except that it applies to the nasal passageways instead
of the lower respiratory passages. The initiating stimulus of the sneeze reflex is irritation in the nasal passageways; the afferent impulses pass in the fifth cranial
nerve to the medulla, where the reflex is triggered. A
series of reactions similar to those for the cough reflex
takes place; however, the uvula is depressed, so that
large amounts of air pass rapidly through the nose, thus
helping to clear the nasal passages of foreign matter.
Normal Respiratory Functions
of the Nose
As air passes through the nose, three distinct normal
respiratory functions are performed by the nasal cavities: (1) the air is warmed by the extensive surfaces of
the conchae and septum, a total area of about 160
square centimeters (see Figure 37–8); (2) the air is
almost completely humidified even before it passes
beyond the nose; and (3) the air is partially filtered.
These functions together are called the air conditioning
function of the upper respiratory passageways. Ordinarily, the temperature of the inspired air rises to within
1°F of body temperature and to within 2 to 3 per cent
of full saturation with water vapor before it reaches the
trachea. When a person breathes air through a tube
directly into the trachea (as through a tracheostomy),
the cooling and especially the drying effect in the lower
lung can lead to serious lung crusting and infection.
Filtration Function of the Nose. The hairs at the entrance to
the nostrils are important for filtering out large particles. Much more important, though, is the removal
of particles by turbulent precipitation. That is, the air
passing through the nasal passageways hits many
obstructing vanes: the conchae (also called turbinates,
because they cause turbulence of the air), the septum,
and the pharyngeal wall. Each time air hits one of these
obstructions, it must change its direction of movement.
The particles suspended in the air, having far more mass
and momentum than air, cannot change their direction
of travel as rapidly as the air can. Therefore, they continue forward, striking the surfaces of the obstructions,
and are entrapped in the mucous coating and transported by the cilia to the pharynx to be swallowed.
Size of Particles Entrapped in the Respiratory Passages.
The nasal turbulence mechanism for removing particles
from air is so effective that almost no particles larger
than 6 micrometers in diameter enter the lungs through
the nose. This size is smaller than the size of red blood
cells.
Of the remaining particles, many that are between 1
and 5 micrometers settle in the smaller bronchioles as a
result of gravitational precipitation. For instance, terminal bronchiolar disease is common in coal miners
because of settled dust particles. Some of the still
smaller particles (smaller than 1 micrometer in diameter) diffuse against the walls of the alveoli and adhere
to the alveolar fluid. But many particles smaller than 0.5
Chapter 37
481
Pulmonary Ventilation
Thyroid
cartilage
Thyroarytenoid
muscle
Vocal
ligament
Lateral
cricoarytenoid
muscle
Figure 37–9
A, Anatomy of the larynx.
B, Laryngeal function in phonation, showing the positions of the
vocal cords during different
types of phonation. (Modified
from Greene MC: The Voice and
Its Disorders, 4th ed. Philadelphia: JB Lippincott, 1980.)
Arytenoid
cartilage
A
Transverse
arytenoid
muscle
micrometer in diameter remain suspended in the alveolar air and are expelled by expiration. For instance, the
particles of cigarette smoke are about 0.3 micrometer.
Almost none of these particles are precipitated in the
respiratory passageways before they reach the alveoli.
Unfortunately, up to one third of them do precipitate in
the alveoli by the diffusion process, with the balance
remaining suspended and expelled in the expired air.
Many of the particles that become entrapped in
the alveoli are removed by alveolar macrophages, as
explained in Chapter 33, and others are carried away by
the lung lymphatics. An excess of particles can cause
growth of fibrous tissue in the alveolar septa, leading to
permanent debility.
Vocalization
Speech involves not only the respiratory system but
also (1) specific speech nervous control centers in the
cerebral cortex, which are discussed in Chapter 57; (2)
respiratory control centers of the brain; and (3) the
articulation and resonance structures of the mouth and
nasal cavities. Speech is composed of two mechanical
functions: (1) phonation, which is achieved by the
larynx, and (2) articulation, which is achieved by the
structures of the mouth.
Phonation. The larynx, shown in Figure 37–9A, is especially adapted to act as a vibrator. The vibrating element
is the vocal folds, commonly called the vocal cords. The
vocal cords protrude from the lateral walls of the larynx
toward the center of the glottis; they are stretched and
positioned by several specific muscles of the larynx
itself.
Figure 37–9B shows the vocal cords as they are seen
when looking into the glottis with a laryngoscope.
During normal breathing, the cords are wide open to
allow easy passage of air. During phonation, the cords
move together so that passage of air between them will
cause vibration. The pitch of the vibration is determined
mainly by the degree of stretch of the cords, but also by
how tightly the cords are approximated to one another
and by the mass of their edges.
Figure 37–9A shows a dissected view of the vocal
folds after removal of the mucous epithelial lining.
Immediately inside each cord is a strong elastic ligament
called the vocal ligament. This is attached anteriorly to
the large thyroid cartilage, which is the cartilage that
projects forward from the anterior surface of the neck
and is called the “Adam’s apple.” Posteriorly, the vocal
Full
abduction
Posterior
cricoarytenoid
muscle
Gentle Intermediate position–
abduction
loud whisper
Stage
whisper
Phonation
B
ligament is attached to the vocal processes of two arytenoid cartilages. The thyroid cartilage and the arytenoid
cartilages articulate from below with another cartilage
not shown in Figure 37–9, the cricoid cartilage.
The vocal cords can be stretched by either forward
rotation of the thyroid cartilage or posterior rotation of
the arytenoid cartilages, activated by muscles stretching
from the thyroid cartilage and arytenoid cartilages to
the cricoid cartilage. Muscles located within the vocal
cords lateral to the vocal ligaments, the thyroarytenoid
muscles, can pull the arytenoid cartilages toward the
thyroid cartilage and, therefore, loosen the vocal cords.
Also, slips of these muscles within the vocal cords can
change the shapes and masses of the vocal cord edges,
sharpening them to emit high-pitched sounds and blunting them for the more bass sounds.
Finally, several other sets of small laryngeal muscles
lie between the arytenoid cartilages and the cricoid cartilage and can rotate these cartilages inward or outward
or pull their bases together or apart to give the various
configurations of the vocal cords shown in Figure 37–9B.
Articulation and Resonance. The three major organs of
articulation are the lips, tongue, and soft palate. They
need not be discussed in detail because we are all familiar with their movements during speech and other
vocalizations.
The resonators include the mouth, the nose and associated nasal sinuses, the pharynx, and even the chest
cavity. Again, we are all familiar with the resonating
qualities of these structures. For instance, the function
of the nasal resonators is demonstrated by the change
in voice quality when a person has a severe cold that
blocks the air passages to these resonators.
References
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Foster WM: Mucociliary transport and cough in humans.
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Haitsma JJ, Papadakos PJ, Lachmann B: Surfactant therapy
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Hilaire G, Duron B: Maturation of the mammalian respiratory system. Physiol Rev 79:325, 1999.
482
Unit VII
Lai-Fook SJ: Pleural mechanics and fluid exchange. Physiol
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McConnell AK, Romer LM: Dyspnoea in health and
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Respiration
Powell FL, Hopkins SR: Comparative physiology of lung
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Sant’Ambrogio G, Widdicombe J: Reflexes from airway
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Uhlig S, Taylor AE: Methods in Pulmonary Research. Basel:
Birkhauser Verlag, 1998.
West JB: Respiratory Physiology. New York: Oxford University Press, 1996.
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Widdicombe J: Neuroregulation of cough: implications for
drug therapy. Curr Opin Pharmacol 2:256, 2002.
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3
8
Pulmonary Circulation,
Pulmonary Edema, Pleural Fluid
Some aspects of blood flow distribution and other
hemodynamics are particular to the pulmonary circulation and are especially important for gas
exchange in the lungs. The present discussion is concerned with these special features of the pulmonary
circulation.
Physiologic Anatomy of the Pulmonary
Circulatory System
Pulmonary Vessels. The pulmonary artery extends only 5 centimeters beyond the apex
of the right ventricle and then divides into right and left main branches that supply
blood to the two respective lungs.
The pulmonary artery is thin, with a wall thickness one third that of the aorta.
The pulmonary arterial branches are very short, and all the pulmonary arteries, even
the smaller arteries and arterioles, have larger diameters than their counterpart systemic arteries. This, combined with the fact that the vessels are thin and distensible,
gives the pulmonary arterial tree a large compliance, averaging almost 7 ml/mm Hg,
which is similar to that of the entire systemic arterial tree. This large compliance
allows the pulmonary arteries to accommodate the stroke volume output of the
right ventricle.
The pulmonary veins, like the pulmonary arteries, are also short. They immediately empty their effluent blood into the left atrium, to be pumped by the left heart
through the systemic circulation.
Bronchial Vessels. Blood also flows to the lungs through small bronchial arteries that
originate from the systemic circulation, amounting to about 1 to 2 per cent of the
total cardiac output. This bronchial arterial blood is oxygenated blood, in contrast
to the partially deoxygenated blood in the pulmonary arteries. It supplies the supporting tissues of the lungs, including the connective tissue, septa, and large and
small bronchi. After this bronchial and arterial blood has passed through the supporting tissues, it empties into the pulmonary veins and enters the left atrium, rather
than passing back to the right atrium. Therefore, the flow into the left atrium and
the left ventricular output are about 1 to 2 per cent greater than the right ventricular output.
Lymphatics. Lymph vessels are present in all the supportive tissues of the lung, beginning in the connective tissue spaces that surround the terminal bronchioles, coursing to the hilum of the lung, and thence mainly into the right thoracic lymph duct.
Particulate matter entering the alveoli is partly removed by way of these channels,
and plasma protein leaking from the lung capillaries is also removed from the lung
tissues, thereby helping to prevent pulmonary edema.
Pressures in the Pulmonary System
Pressure Pulse Curve in the Right Ventricle. The pressure pulse curves of the right
ventricle and pulmonary artery are shown in the lower portion of Figure 38–1.
These curves are contrasted with the much higher aortic pressure curve shown
in the upper portion of the figure. The systolic pressure in the right ventricle of
the normal human being averages about 25 mm Hg, and the diastolic pressure
483
484
Unit VII
Respiration
normal human being, the diastolic pulmonary arterial
pressure is about 8 mm Hg, and the mean pulmonary
arterial pressure is 15 mm Hg.
Aortic pressure curve
120
Pressure (mm Hg)
Pulmonary Capillary Pressure. The mean pulmonary cap-
illary pressure, as diagrammed in Figure 38–2, is about
7 mm Hg. The importance of this low capillary pressure is discussed in detail later in the chapter in relation to fluid exchange functions of the pulmonary
capillaries.
75
Right ventricular curve
Pulmonary artery curve
25
8
0
0
1
Seconds
2
Figure 38–1
mm Hg
Pressure pulse contours in the right ventricle, pulmonary artery,
and aorta.
25
S
15
M
8
7
D
Pulmonary
capillaries
Left
atrium
2
0
Pulmonary
artery
Pulmonary
capillaries
Left
atrium
Figure 38–2
Pressures in the different vessels of the lungs. D, diastolic; M,
mean; S, systolic; red curve, arterial pulsations.
averages about 0 to 1 mm Hg, values that are only one
fifth those for the left ventricle.
Pressures in the Pulmonary Artery. During systole, the
pressure in the pulmonary artery is essentially equal
to the pressure in the right ventricle, as also shown
in Figure 38–1. However, after the pulmonary valve
closes at the end of systole, the ventricular pressure
falls precipitously, whereas the pulmonary arterial
pressure falls more slowly as blood flows through the
capillaries of the lungs.
As shown in Figure 38–2, the systolic pulmonary
arterial pressure averages about 25 mm Hg in the
Left Atrial and Pulmonary Venous Pressures. The mean
pressure in the left atrium and the major pulmonary
veins averages about 2 mm Hg in the recumbent
human being, varying from as low as 1 mm Hg to as
high as 5 mm Hg. It usually is not feasible to measure
a human being’s left atrial pressure using a direct
measuring device because it is difficult to pass a
catheter through the heart chambers into the left
atrium. However, the left atrial pressure can often be
estimated with moderate accuracy by measuring the
so-called pulmonary wedge pressure. This is achieved
by inserting a catheter first through a peripheral vein
to the right atrium, then through the right side of the
heart and through the pulmonary artery into one of
the small branches of the pulmonary artery, finally
pushing the catheter until it wedges tightly in the small
branch.
The pressure measured through the catheter, called
the “wedge pressure,” is about 5 mm Hg. Because all
blood flow has been stopped in the small wedged
artery, and because the blood vessels extending
beyond this artery make a direct connection with the
pulmonary capillaries, this wedge pressure is usually
only 2 to 3 mm Hg greater than the left atrial pressure.
When the left atrial pressure rises to high values,
the pulmonary wedge pressure also rises. Therefore,
wedge pressure measurements can be used to clinically
study changes in pulmonary capillary pressure and
left atrial pressure in patients with congestive heart
failure.
Blood Volume of the Lungs
The blood volume of the lungs is about 450 milliliters,
about 9 per cent of the total blood volume of the entire
circulatory system. Approximately 70 milliliters of this
pulmonary blood volume is in the pulmonary capillaries, and the remainder is divided about equally
between the pulmonary arteries and the veins.
Lungs as a Blood Reservoir. Under various physiological
and pathological conditions, the quantity of blood in
the lungs can vary from as little as one half normal up
to twice normal. For instance, when a person blows out
air so hard that high pressure is built up in the lungs—
such as when blowing a trumpet—as much as 250 milliliters of blood can be expelled from the pulmonary
circulatory system into the systemic circulation.
Also, loss of blood from the systemic circulation by
hemorrhage can be partly compensated for by the
Chapter 38
485
Pulmonary Circulation, Pulmonary Edema, Pleural Fluid
automatic shift of blood from the lungs into the systemic vessels.
Shift of Blood Between the Pulmonary and Systemic
Circulatory Systems as a Result of Cardiac Pathology.
Failure of the left side of the heart or increased resistance to blood flow through the mitral valve as a result
of mitral stenosis or mitral regurgitation causes blood
to dam up in the pulmonary circulation, sometimes
increasing the pulmonary blood volume as much as
100 per cent and causing large increases in the pulmonary vascular pressures. Because the volume of the
systemic circulation is about nine times that of the pulmonary system, a shift of blood from one system to the
other affects the pulmonary system greatly but usually
has only mild systemic circulatory effects.
Blood Flow Through the Lungs
and Its Distribution
The blood flow through the lungs is essentially equal
to the cardiac output. Therefore, the factors that
control cardiac output—mainly peripheral factors,
as discussed in Chapter 20—also control pulmonary
blood flow. Under most conditions, the pulmonary
vessels act as passive, distensible tubes that enlarge
with increasing pressure and narrow with decreasing
pressure. For adequate aeration of the blood to
occur, it is important for the blood to be distributed to
those segments of the lungs where the alveoli are
best oxygenated. This is achieved by the following
mechanism.
Effect of Hydrostatic
Pressure Gradients in the
Lungs on Regional
Pulmonary Blood Flow
In Chapter 15, it was pointed out that the blood pressure in the foot of a standing person can be as much as
90 mm Hg greater than the pressure at the level of the
heart. This is caused by hydrostatic pressure—that is, by
the weight of the blood itself in the blood vessels. The
same effect, but to a lesser degree, occurs in the lungs.
In the normal, upright adult, the lowest point in the
lungs is about 30 centimeters below the highest point.
This represents a 23 mm Hg pressure difference, about
15 mm Hg of which is above the heart and 8 below. That
is, the pulmonary arterial pressure in the uppermost
portion of the lung of a standing person is about 15 mm
Hg less than the pulmonary arterial pressure at the level
of the heart, and the pressure in the lowest portion of
the lungs is about 8 mm Hg greater. Such pressure differences have profound effects on blood flow through
the different areas of the lungs. This is demonstrated by
the lower curve in Figure 38–3, which depicts blood flow
per unit of lung tissue at different levels of the lung in
the upright person. Note that in the standing position at
rest, there is little flow in the top of the lung but about
five times as much flow in the bottom. To help explain
these differences, one often describes the lung as being
divided into three zones, as shown in Figure 38–4.
In each zone, the patterns of blood flow are quite
different.
Zones 1, 2, and 3 of Pulmonary
Blood Flow
The capillaries in the alveolar walls are distended by the
blood pressure inside them, but simultaneously, they are
When the concentration of oxygen in the air of the
alveoli decreases below normal—especially when it
falls below 70 per cent of normal (below 73 mm Hg
Po2)—the adjacent blood vessels constrict, with the
vascular resistance increasing more than fivefold at
extremely low oxygen levels. This is opposite to the
effect observed in systemic vessels, which dilate rather
than constrict in response to low oxygen. It is believed
that the low oxygen concentration causes some yet
undiscovered vasoconstrictor substance to be released
from the lung tissue; this substance promotes constriction of the small arteries and arterioles. It has been
suggested that this vasoconstrictor might be secreted
by the alveolar epithelial cells when they become
hypoxic.
This effect of low oxygen on pulmonary vascular
resistance has an important function: to distribute
blood flow where it is most effective. That is, if some
alveoli are poorly ventilated so that their oxygen concentration becomes low, the local vessels constrict.This
causes the blood to flow through other areas of the
lungs that are better aerated, thus providing an automatic control system for distributing blood flow to the
pulmonary areas in proportion to their alveolar
oxygen pressures.
Blood flow
(per unit of tissue)
Effect of Diminished Alveolar Oxygen on Local Alveolar Blood
Flow—Automatic Control of Pulmonary Blood Flow Distribution.
Top
Exercise
Standing at rest
Middle
Lung level
Bottom
Figure 38–3
Blood flow at different levels in the lung of an upright person at
rest and during exercise. Note that when the person is at rest, the
blood flow is very low at the top of the lungs; most of the flow is
through the bottom of the lung.
486
Unit VII
ZONE 1
Artery
PALV
Vein
Ppc
ZONE 2
Artery
PALV
Vein
Ppc
ZONE 3
Artery
PALV
Vein
Respiration
during cardiac systole. Conversely, during diastole, the
8 mm Hg diastolic pressure at the level of the heart is
not sufficient to push the blood up the 15 mm Hg hydrostatic pressure gradient required to cause diastolic capillary flow. Therefore, blood flow through the apical part
of the lung is intermittent, with flow during systole but
cessation of flow during diastole; this is called zone 2
blood flow. Zone 2 blood flow begins in the normal
lungs about 10 centimeters above the midlevel of the
heart and extends from there to the top of the lungs.
In the lower regions of the lungs, from about 10 centimeters above the level of the heart all the way to the
bottom of the lungs, the pulmonary arterial pressure
during both systole and diastole remains greater than
the zero alveolar air pressure. Therefore, there is continuous flow through the alveolar capillaries, or zone 3
blood flow. Also, when a person is lying down, no part
of the lung is more than a few centimeters above the
level of the heart. In this case, blood flow in a normal
person is entirely zone 3 blood flow, including the lung
apices.
Zone 1 Blood Flow Occurs Only Under Abnormal Conditions.
Ppc
Figure 38–4
Mechanics of blood flow in the three blood flow zones of the lung:
zone 1, no flow—alveolar air pressure (PALV) is greater than arterial pressure; zone 2, intermittent flow—systolic arterial pressure
rises higher than alveolar air pressure, but diastolic arterial pressure falls below alveolar air pressure; and zone 3, continuous
flow—arterial pressure and pulmonary capillary pressure (Ppc)
remain greater than alveolar air pressure at all times.
compressed by the alveolar air pressure on their outsides. Therefore, any time the lung alveolar air pressure
becomes greater than the capillary blood pressure,
the capillaries close and there is no blood flow. Under
different normal and pathological lung conditions, one
may find any one of three possible zones of pulmonary
blood flow, as follows:
Zone 1: No blood flow during all portions of the
cardiac cycle because the local alveolar capillary
pressure in that area of the lung never rises higher
than the alveolar air pressure during any part of the
cardiac cycle
Zone 2: Intermittent blood flow only during the pulmonary arterial pressure peaks because the systolic
pressure is then greater than the alveolar air pressure, but the diastolic pressure is less than the alveolar air pressure
Zone 3: Continuous blood flow because the alveolar
capillary pressure remains greater than alveolar air
pressure during the entire cardiac cycle
Normally, the lungs have only zones 2 and 3 blood
flow—zone 2 (intermittent flow) in the apices, and zone
3 (continuous flow) in all the lower areas. For example,
when a person is in the upright position, the pulmonary
arterial pressure at the lung apex is about 15 mm Hg
less than the pressure at the level of the heart. Therefore, the apical systolic pressure is only 10 mm Hg
(25 mm Hg at heart level minus 15 mm Hg hydrostatic
pressure difference). This 10 mm Hg apical blood pressure is greater than the zero alveolar air pressure, so that
blood flows through the pulmonary apical capillaries
Zone 1 blood flow, which is blood flow at no time during
the cardiac cycle, occurs when either the pulmonary systolic arterial pressure is too low or the alveolar pressure
is too high to allow flow. For instance, if an upright
person is breathing against a positive air pressure so
that the intra-alveolar air pressure is at least 10 mm Hg
greater than normal but the pulmonary systolic blood
pressure is normal, one would expect zone 1 blood
flow—no blood flow—in the lung apices. Another
instance in which zone 1 blood flow occurs is in an
upright person whose pulmonary systolic arterial pressure is exceedingly low, as might occur after severe
blood loss.
Effect of Exercise on Blood Flow Through the Different Parts of
the Lungs. Referring again to Figure 38–3, one sees that
the blood flow in all parts of the lung increases during
exercise. The increase in flow in the top of the lung may
be 700 to 800 per cent, whereas the increase in the lower
part of the lung may be no more than 200 to 300 per
cent. The reason for these differences is that the pulmonary vascular pressures rise enough during exercise
to convert the lung apices from a zone 2 pattern into a
zone 3 pattern of flow.
Effect of Increased Cardiac Output
on Pulmonary Blood Flow and
Pulmonary Arterial Pressure During
Heavy Exercise
During heavy exercise, blood flow through the lungs
increases fourfold to sevenfold. This extra flow is
accommodated in the lungs in three ways: (1) by
increasing the number of open capillaries, sometimes
as much as threefold; (2) by distending all the capillaries and increasing the rate of flow through each
capillary more than twofold; and (3) by increasing the
pulmonary arterial pressure. In the normal person, the
first two changes decrease pulmonary vascular resistance so much that the pulmonary arterial pressure
rises very little, even during maximum exercise; this
effect is shown in Figure 38–5.
Chapter 38
Pulmonary Circulation, Pulmonary Edema, Pleural Fluid
487
above 30 mm Hg, causing similar increases in capillary
pressure, pulmonary edema is likely to develop, as we
discuss later in the chapter.
30
Pulmonary arterial
pressure (mm Hg)
Pulmonary Capillary Dynamics
Normal value
Exchange of gases between the alveolar air and the
pulmonary capillary blood is discussed in the next
chapter. However, it is important for us to note here
that the alveolar walls are lined with so many capillaries that, in most places, the capillaries almost touch
one another side by side. Therefore, it is often said that
the capillary blood flows in the alveolar walls as a
“sheet of flow,” rather than in individual capillaries.
20
10
0
0
4
8
12
16
20
Cardiac output (L/min)
24
Figure 38–5
Effect on mean pulmonary arterial pressure caused by increasing
the cardiac output during exercise.
The ability of the lungs to accommodate greatly
increased blood flow during exercise without increasing the pulmonary arterial pressure conserves the
energy of the right side of the heart. This ability also
prevents a significant rise in pulmonary capillary pressure, thus also preventing the development of pulmonary edema.
Function of the Pulmonary Circulation
When the Left Atrial Pressure Rises
as a Result of Left-Sided Heart Failure
The left atrial pressure in a healthy person almost
never rises above +6 mm Hg, even during the most
strenuous exercise. These small changes in left atrial
pressure have virtually no effect on pulmonary circulatory function because this merely expands the pulmonary venules and opens up more capillaries so that
blood continues to flow with almost equal ease from
the pulmonary arteries.
When the left side of the heart fails, however, blood
begins to dam up in the left atrium. As a result, the
left atrial pressure can rise on occasion from its normal
value of 1 to 5 mm Hg all the way up to 40 to
50 mm Hg. The initial rise in atrial pressure, up to
about 7 mm Hg, has very little effect on pulmonary circulatory function. But when the left atrial pressure
rises to greater than 7 or 8 mm Hg, further increases
in left atrial pressure above these levels cause almost
equally great increases in pulmonary arterial pressure,
thus causing a concomitant increased load on the right
heart. Any increase in left atrial pressure above 7 or
8 mm Hg increases the capillary pressure almost
equally as much.When the left atrial pressure has risen
Pulmonary Capillary Pressure. No direct measurements
of pulmonary capillary pressure have ever been
made. However, “isogravimetric” measurement of pulmonary capillary pressure, using a technique described
in Chapter 16, has given a value of 7 mm Hg. This is
probably nearly correct, because the mean left atrial
pressure is about 2 mm Hg and the mean pulmonary
arterial pressure is only 15 mm Hg, so the mean
pulmonary capillary pressure must lie somewhere
between these two values.
Length of Time Blood Stays in the Pulmonary Capillaries.
From histological study of the total cross-sectional
area of all the pulmonary capillaries, it can be calculated that when the cardiac output is normal, blood
passes through the pulmonary capillaries in about 0.8
second. When the cardiac output increases, this can
shorten to as little as 0.3 second. The shortening would
be much greater were it not for the fact that additional
capillaries, which normally are collapsed, open up to
accommodate the increased blood flow. Thus, in only
a fraction of a second, blood passing through the
alveolar capillaries becomes oxygenated and loses its
excess carbon dioxide.
Capillary Exchange of Fluid in the
Lungs, and Pulmonary Interstitial
Fluid Dynamics
The dynamics of fluid exchange across the lung capillary membranes are qualitatively the same as for
peripheral tissues. However, quantitatively, there are
important differences, as follows:
1. The pulmonary capillary pressure is low, about
7 mm Hg, in comparison with a considerably
higher functional capillary pressure in the
peripheral tissues of about 17 mm Hg.
2. The interstitial fluid pressure in the lung is
slightly more negative than that in the peripheral
subcutaneous tissue. (This has been measured in
two ways: by a micropipette inserted into the
pulmonary interstitium, giving a value of about
–5 mm Hg, and by measuring the absorption
pressure of fluid from the alveoli, giving a value of
about –8 mm Hg.)
488
Unit VII
Respiration
3. The pulmonary capillaries are relatively leaky to
protein molecules, so that the colloid osmotic
pressure of the pulmonary interstitial fluid is
about 14 mm Hg, in comparison with less than
half this value in the peripheral tissues.
4. The alveolar walls are extremely thin, and the
alveolar epithelium covering the alveolar surfaces
is so weak that it can be ruptured by any positive
pressure in the interstitial spaces greater than
alveolar air pressure (greater than 0 mm Hg),
which allows dumping of fluid from the interstitial
spaces into the alveoli.
Now let us see how these quantitative differences
affect pulmonary fluid dynamics.
pressure at the pulmonary capillary membrane; this
can be calculated as follows:
Interrelations Between Interstitial Fluid Pressure and Other
Pressures in the Lung. Figure 38–6 shows a pulmonary
Negative Pulmonary Interstitial Pressure and the Mechanism
for Keeping the Alveoli “Dry.” One of the most important
capillary, a pulmonary alveolus, and a lymphatic capillary draining the interstitial space between the blood
capillary and the alveolus. Note the balance of forces
at the blood capillary membrane, as follows:
problems in lung function is to understand why the
alveoli do not normally fill with fluid. One’s first inclination is to think that the alveolar epithelium is strong
enough and continuous enough to keep fluid from
leaking out of the interstitial spaces into the alveoli.
This is not true, because experiments have shown
that there are always openings between the alveolar
epithelial cells through which even large protein molecules, as well as water and electrolytes, can pass.
However, if one remembers that the pulmonary capillaries and the pulmonary lymphatic system normally
maintain a slight negative pressure in the interstitial
spaces, it is clear that whenever extra fluid appears
in the alveoli, it will simply be sucked mechanically
into the lung interstitium through the small openings
between the alveolar epithelial cells. Then the excess
fluid is either carried away through the pulmonary
lymphatics or absorbed into the pulmonary capillaries.
Thus, under normal conditions, the alveoli are kept
“dry,” except for a small amount of fluid that seeps
from the epithelium onto the lining surfaces of the
alveoli to keep them moist.
mm Hg
Forces tending to cause movement of fluid outward from the
capillaries and into the pulmonary interstitium:
Capillary pressure
Interstitial fluid colloid osmotic pressure
Negative interstitial fluid pressure
TOTAL OUTWARD FORCE
7
14
8
29
Forces tending to cause absorption of fluid into the capillaries:
Plasma colloid osmotic pressure
28
TOTAL INWARD FORCE
28
Thus, the normal outward forces are slightly greater
than the inward forces, providing a mean filtration
Pressures Causing Fluid Movement
CAPILLARY
mm Hg
Total outward force
Total inward force
MEAN FILTRATION PRESSURE
+29
–28
+1
This filtration pressure causes a slight continual flow
of fluid from the pulmonary capillaries into the interstitial spaces, and except for a small amount that evaporates in the alveoli, this fluid is pumped back to the
circulation through the pulmonary lymphatic system.
ALVEOLUS
Pulmonary Edema
Hydrostatic
pressure
Osmotic
pressure
Net
pressure
+7
- 28
-8
-8
- 8 (Surface
tension
at pore)
- 14
(+ 1)
-5
( 0)
(Evaporation)
-4
Lymphatic pump
Figure 38–6
Hydrostatic and osmotic forces at the capillary (left) and alveolar
membrane (right) of the lungs. Also shown is the tip end of a lymphatic vessel (center) that pumps fluid from the pulmonary interstitial spaces. (Modified from Guyton AC, Taylor AE, Granger HJ:
Circulatory Physiology II: Dynamics and Control of the Body
Fluids. Philadelphia: WB Saunders, 1975.)
Pulmonary edema occurs in the same way that edema
occurs elsewhere in the body. Any factor that causes the
pulmonary interstitial fluid pressure to rise from the
negative range into the positive range will cause rapid
filling of the pulmonary interstitial spaces and alveoli
with large amounts of free fluid.
The most common causes of pulmonary edema are as
follows:
1. Left-sided heart failure or mitral valve disease, with
consequent great increases in pulmonary venous
pressure and pulmonary capillary pressure and
flooding of the interstitial spaces and alveoli.
2. Damage to the pulmonary blood capillary
membranes caused by infections such as
pneumonia or by breathing noxious substances
such as chlorine gas or sulfur dioxide gas. Each of
these causes rapid leakage of both plasma proteins
and fluid out of the capillaries and into both the
lung interstitial spaces and the alveoli.
“Pulmonary Edema Safety Factor.” Experiments in animals
have shown that the pulmonary capillary pressure nor-
489
Pulmonary Circulation, Pulmonary Edema, Pleural Fluid
Figure 38–7
Rate of fluid loss into the lung tissues when the left
atrial pressure (and pulmonary capillary pressure)
is increased. (From Guyton AC, Lindsey AW: Effect
of elevated left atrial pressure and decreased
plasma protein concentration on the development
of pulmonary edema. Circ Res 7:649, 1959.)
mally must rise to a value at least equal to the colloid
osmotic pressure of the plasma inside the capillaries
before significant pulmonary edema will occur. To give
an example, Figure 38–7 shows how different levels of
left atrial pressure increase the rate of pulmonary
edema formation in dogs. Remember that every time
the left atrial pressure rises to high values, the pulmonary capillary pressure rises to a level 1 to 2 mm Hg
greater than the left atrial pressure. In these experiments, as soon as the left atrial pressure rose above
23 mm Hg (causing the pulmonary capillary pressure to
rise above 25 mm Hg), fluid began to accumulate in the
lungs. This fluid accumulation increased even more
rapidly with further increases in capillary pressure. The
plasma colloid osmotic pressure during these experiments was equal to this 25 mm Hg critical pressure
level. Therefore, in the human being, whose normal
plasma colloid osmotic pressure is 28 mm Hg, one can
predict that the pulmonary capillary pressure must
rise from the normal level of 7 mm Hg to more than
28 mm Hg to cause pulmonary edema, giving an acute
safety factor against pulmonary edema of 21 mm Hg.
Safety Factor in Chronic Conditions. When the pulmonary capillary pressure remains elevated chronically
(for at least 2 weeks), the lungs become even more
resistant to pulmonary edema because the lymph
vessels expand greatly, increasing their capability of carrying fluid away from the interstitial spaces perhaps as
much as 10-fold. Therefore, in patients with chronic
mitral stenosis, pulmonary capillary pressures of 40 to
45 mm Hg have been measured without the development of lethal pulmonary edema.
Rapidity of Death in Acute Pulmonary Edema. When the pul-
monary capillary pressure rises even slightly above the
safety factor level, lethal pulmonary edema can occur
within hours, or even within 20 to 30 minutes if the capillary pressure rises 25 to 30 mm Hg above the safety
factor level. Thus, in acute left-sided heart failure, in
Rate of edema formation =
edema fluid per hour
dry weight of lung
Chapter 38
10
9
x
x
8
7
x
6
x
5
4
x
x
x
x
x
10 15 20 25 30 35 40
Left atrial pressure (mm Hg)
45
3
x
2
x
1
0 x
x
0
5
x
x
x
x xx
x
x x
x
x
x
50
which the pulmonary capillary pressure occasionally
does rise to 50 mm Hg, death frequently ensues in less
than 30 minutes from acute pulmonary edema.
Fluid in the Pleural Cavity
When the lungs expand and contract during normal
breathing, they slide back and forth within the pleural
cavity. To facilitate this, a thin layer of mucoid fluid lies
between the parietal and visceral pleurae.
Figure 38–8 shows the dynamics of fluid exchange in
the pleural space. The pleural membrane is a porous,
mesenchymal, serous membrane through which small
amounts of interstitial fluid transude continually into
the pleural space. These fluids carry with them tissue
proteins, giving the pleural fluid a mucoid characteristic, which is what allows extremely easy slippage of the
moving lungs.
The total amount of fluid in each pleural cavity is normally slight, only a few milliliters. Whenever the quantity becomes more than barely enough to begin flowing
in the pleural cavity, the excess fluid is pumped away by
lymphatic vessels opening directly from the pleural
cavity into (1) the mediastinum, (2) the superior surface
of the diaphragm, and (3) the lateral surfaces of the
parietal pleura. Therefore, the pleural space—the space
between the parietal and visceral pleurae—is called a
potential space because it normally is so narrow that it
is not obviously a physical space.
“Negative Pressure” in Pleural Fluid. A negative force is
always required on the outside of the lungs to keep the
lungs expanded. This is provided by negative pressure
in the normal pleural space. The basic cause of this negative pressure is pumping of fluid from the space by the
lymphatics (which is also the basis of the negative pressure found in most tissue spaces of the body). Because
the normal collapse tendency of the lungs is about
–4 mm Hg, the pleural fluid pressure must always be at
490
Unit VII
Venous system
Lymphatics
Artery
Respiration
(3) greatly reduced plasma colloid osmotic pressure,
thus allowing excessive transudation of fluid; and (4)
infection or any other cause of inflammation of the surfaces of the pleural cavity, which breaks down the capillary membranes and allows rapid dumping of both
plasma proteins and fluid into the cavity.
References
Vein
Figure 38–8
Dynamics of fluid exchange in the intrapleural space.
least as negative as –4 mm Hg to keep the lungs
expanded. Actual measurements have shown that the
pressure is usually about –7 mm Hg, which is a few millimeters of mercury more negative than the collapse
pressure of the lungs. Thus, the negativity of the pleural
fluid keeps the normal lungs pulled against the parietal
pleura of the chest cavity, except for an extremely thin
layer of mucoid fluid that acts as a lubricant.
Pleural Effusion. Pleural effusion means the collection of
large amounts of free fluid in the pleural space.The effusion is analogous to edema fluid in the tissues and can
be called “edema of the pleural cavity.” The causes of
the effusion are the same as the causes of edema in
other tissues (discussed in Chapter 25), including (1)
blockage of lymphatic drainage from the pleural cavity;
(2) cardiac failure, which causes excessively high peripheral and pulmonary capillary pressures, leading to
excessive transudation of fluid into the pleural cavity;
Gehlbach BK, Geppert E: The pulmonary manifestations of
left heart failure. Chest 125:669, 2004.
Guazzi M: Alveolar-capillary membrane dysfunction in
heart failure: evidence of a pathophysiologic role. Chest
124:1090, 2003.
Guyton AC, Lindsey AW: Effect of elevated left atrial pressure and decreased plasma protein concentration on the
development of pulmonary edema. Circ Res 7:649, 1959.
Guyton AC, Parker JC, Taylor AE, et al: Forces governing
water movement in the lung. In: Fishman AP, Renkin EM
(eds): Pulmonary Edema. Baltimore: Waverly Press, 1979,
p 65.
Guyton AC, Taylor AE, Granger HJ: Circulatory Physiology.
II. Dynamics and Control of the Body Fluids. Philadelphia: WB Saunders, 1975.
Hoschele S, Mairbaurl H: Alveolar flooding at high altitude:
failure of reabsorption? News Physiol Sci 18:55, 2003.
Lai-Fook SJ: Pleural mechanics and fluid exchange. Physiol
Rev 84:385, 2004.
Matalon S, Hardiman KM, Jain L, et al: Regulation of ion
channel structure and function by reactive oxygennitrogen species. Am J Physiol Lung Cell Mol Physiol
285:L1184, 2003.
Miserocchi G, Negrini D, Passi A, De Luca G: Development
of lung edema: interstitial fluid dynamics and molecular
structure. News Physiol Sci 16:66, 2001.
Schoene RB: Limits of human lung function at high altitude.
J Exp Biol 204:3121, 2001.
Taylor AE, Guyton AC, Bishop VS: Permeability of the alveolar membrane to solutes. Circ Res 16:353, 1965.
Wallace J: Update in pulmonary diseases. Ann Intern Med
139:499, 2003.
West JB: Respiratory Physiology—The Essentials, 5th ed.
Baltimore: Williams & Wilkins, 1994.
West JB: Invited review: pulmonary capillary stress failure.
J Appl Physiol 89:2483, 2000.
C
H
A
P
T
E
R
3
9
Physical Principles of Gas
Exchange; Diffusion of Oxygen
and Carbon Dioxide Through
the Respiratory Membrane
After the alveoli are ventilated with fresh air, the
next step in the respiratory process is diffusion of
oxygen from the alveoli into the pulmonary blood
and diffusion of carbon dioxide in the opposite
direction, out of the blood. The process of diffusion
is simply the random motion of molecules intertwining their way in all directions through
the respiratory membrane and adjacent fluids.
However, in respiratory physiology, one is concerned not only with the basic
mechanism by which diffusion occurs but also with the rate at which it occurs;
this is a much more complex problem, requiring a deeper understanding of the
physics of diffusion and gas exchange.
Physics of Gas Diffusion and Gas
Partial Pressures
Molecular Basis of Gas Diffusion
All the gases of concern in respiratory physiology are simple molecules that are free
to move among one another, which is the process called “diffusion.” This is also true
of gases dissolved in the fluids and tissues of the body.
For diffusion to occur, there must be a source of energy. This is provided by the
kinetic motion of the molecules themselves. Except at absolute zero temperature,
all molecules of all matter are continually undergoing motion. For free molecules
that are not physically attached to others, this means linear movement at high velocity until they strike other molecules. Then they bounce away in new directions and
continue until striking other molecules again. In this way, the molecules move
rapidly and randomly among one another.
Net Diffusion of a Gas in One Direction—Effect of a Concentration Gradient. If a gas chamber
or a solution has a high concentration of a particular gas at one end of the chamber
and a low concentration at the other end, as shown in Figure 39–1, net diffusion of
the gas will occur from the high-concentration area toward the low-concentration
area. The reason is obvious: There are far more molecules at end A of the chamber
to diffuse toward end B than there are molecules to diffuse in the opposite direction. Therefore, the rates of diffusion in each of the two directions are proportionately different, as demonstrated by the lengths of the arrows in the figure.
Gas Pressures in a Mixture of Gases—“Partial Pressures”
of Individual Gases
Pressure is caused by multiple impacts of moving molecules against a surface. Therefore, the pressure of a gas acting on the surfaces of the respiratory passages and
alveoli is proportional to the summated force of impact of all the molecules of that
gas striking the surface at any given instant. This means that the pressure is directly
proportional to the concentration of the gas molecules.
491
492
Unit VII
When partial pressure is expressed in atmospheres
(1 atmosphere pressure equals 760 mm Hg) and concentration is expressed in volume of gas dissolved in
each volume of water, the solubility coefficients for
important respiratory gases at body temperature are the
following:
Dissolved gas molecules
A
B
Figure 39–1
Diffusion of oxygen from one end of a chamber (A) to the other
(B). The difference between the lengths of the arrows represents
net diffusion.
In respiratory physiology, one deals with mixtures of
gases, mainly of oxygen, nitrogen, and carbon dioxide.
The rate of diffusion of each of these gases is directly
proportional to the pressure caused by that gas alone,
which is called the partial pressure of that gas. The
concept of partial pressure can be explained as follows.
Consider air, which has an approximate composition
of 79 per cent nitrogen and 21 per cent oxygen. The
total pressure of this mixture at sea level averages
760 mm Hg. It is clear from the preceding description of
the molecular basis of pressure that each gas contributes
to the total pressure in direct proportion to its concentration. Therefore, 79 per cent of the 760 mm Hg is
caused by nitrogen (600 mm Hg) and 21 per cent by
oxygen (160 mm Hg). Thus, the “partial pressure” of
nitrogen in the mixture is 600 mm Hg, and the “partial
pressure” of oxygen is 160 mm Hg; the total pressure is
760 mm Hg, the sum of the individual partial pressures.
The partial pressures of individual gases in a mixture are
designated by the symbols Po2, Pco2, Pn2, Ph2o, Phe, and
so forth.
Pressures of Gases Dissolved in
Water and Tissues
Gases dissolved in water or in body tissues also exert
pressure, because the dissolved gas molecules are
moving randomly and have kinetic energy. Further,
when the gas dissolved in fluid encounters a surface,
such as the membrane of a cell, it exerts its own partial
pressure in the same way that a gas in the gas phase
does. The partial pressures of the separate dissolved
gases are designated the same as the partial pressures
in the gas state, that is, Po2, Pco2, Pn2, Phe, and so forth.
Factors That Determine the Partial Pressure of a Gas Dissolved in
a Fluid. The partial pressure of a gas in a solution is
determined not only by its concentration but also by the
solubility coefficient of the gas. That is, some types of
molecules, especially carbon dioxide, are physically or
chemically attracted to water molecules, whereas others
are repelled. When molecules are attracted, far more of
them can be dissolved without building up excess partial
pressure within the solution. Conversely, in the case of
those that are repelled, high partial pressure will
develop with fewer dissolved molecules. These relations
are expressed by the following formula, which is Henry’s
law:
Partial pressure =
Respiration
Concentration of dissolved gas
Solubility coefficient
Oxygen
Carbon dioxide
Carbon monoxide
Nitrogen
Helium
0.024
0.57
0.018
0.012
0.008
From this table, one can see that carbon dioxide is
more than 20 times as soluble as oxygen. Therefore, the
partial pressure of carbon dioxide (for a given concentration) is less than one twentieth that exerted by
oxygen.
Diffusion of Gases Between the Gas Phase in the Alveoli and the
Dissolved Phase in the Pulmonary Blood. The partial pressure
of each gas in the alveolar respiratory gas mixture tends
to force molecules of that gas into solution in the blood
of the alveolar capillaries. Conversely, the molecules of
the same gas that are already dissolved in the blood
are bouncing randomly in the fluid of the blood, and
some of these bouncing molecules escape back into the
alveoli. The rate at which they escape is directly proportional to their partial pressure in the blood.
But in which direction will net diffusion of the gas
occur? The answer is that net diffusion is determined by
the difference between the two partial pressures. If the
partial pressure is greater in the gas phase in the alveoli,
as is normally true for oxygen, then more molecules will
diffuse into the blood than in the other direction. Alternatively, if the partial pressure of the gas is greater in
the dissolved state in the blood, which is normally true
for carbon dioxide, then net diffusion will occur toward
the gas phase in the alveoli.
Vapor Pressure of Water
When nonhumidified air is breathed into the respiratory
passageways, water immediately evaporates from the
surfaces of these passages and humidifies the air. This
results from the fact that water molecules, like the different dissolved gas molecules, are continually escaping
from the water surface into the gas phase. The partial
pressure that the water molecules exert to escape
through the surface is called the vapor pressure of the
water. At normal body temperature, 37°C, this vapor
pressure is 47 mm Hg. Therefore, once the gas mixture
has become fully humidified—that is, once it is in “equilibrium” with the water—the partial pressure of the
water vapor in the gas mixture is 47 mm Hg. This partial
pressure, like the other partial pressures, is designated
Ph2o.
The vapor pressure of water depends entirely on the
temperature of the water. The greater the temperature,
the greater the kinetic activity of the molecules and,
therefore, the greater the likelihood that the water molecules will escape from the surface of the water into the
gas phase. For instance, the water vapor pressure at
0°C is 5 mm Hg, and at 100°C it is 760 mm Hg. But the
most important value to remember is the vapor pressure at body temperature, 47 mm Hg; this value appears
in many of our subsequent discussions.
Chapter 39
Physical Principles of Gas Exchange; Diffusion of Oxygen and Carbon Dioxide
Diffusion of Gases Through
Fluids—Pressure Difference Causes
Net Diffusion
Now, let us return to the problem of diffusion. From the
preceding discussion, it is clear that when the partial
pressure of a gas is greater in one area than in another
area, there will be net diffusion from the high-pressure
area toward the low-pressure area. For instance, returning to Figure 39–1, one can readily see that the molecules in the area of high pressure, because of their
greater number, have a greater statistical chance of
moving randomly into the area of low pressure than
do molecules attempting to go in the other direction.
However, some molecules do bounce randomly from
the area of low pressure toward the area of high pressure. Therefore, the net diffusion of gas from the area of
high pressure to the area of low pressure is equal to the
number of molecules bouncing in this forward direction
minus the number bouncing in the opposite direction;
this is proportional to the gas partial pressure difference
between the two areas, called simply the pressure difference for causing diffusion.
Quantifying the Net Rate of Diffusion in Fluids. In addition to
the pressure difference, several other factors affect the
rate of gas diffusion in a fluid. They are (1) the solubility of the gas in the fluid, (2) the cross-sectional area of
the fluid, (3) the distance through which the gas must
diffuse, (4) the molecular weight of the gas, and (5) the
temperature of the fluid. In the body, the last of these
factors, the temperature, remains reasonably constant
and usually need not be considered.
The greater the solubility of the gas, the greater the
number of molecules available to diffuse for any given
partial pressure difference. The greater the crosssectional area of the diffusion pathway, the greater the
total number of molecules that diffuse. Conversely,
the greater the distance the molecules must diffuse, the
longer it will take the molecules to diffuse the entire distance. Finally, the greater the velocity of kinetic movement of the molecules, which is inversely proportional
to the square root of the molecular weight, the greater
the rate of diffusion of the gas. All these factors can be
expressed in a single formula, as follows:
Dµ
DP ¥ A ¥ S
,
d ¥ MW
in which D is the diffusion rate, DP is the partial pressure difference between the two ends of the diffusion
493
pathway, A is the cross-sectional area of the pathway, S
is the solubility of the gas, d is the distance of diffusion,
and MW is the molecular weight of the gas.
It is obvious from this formula that the characteristics
of the gas itself determine two factors of the formula:
solubility and molecular weight. Together, these two
factors determine the diffusion coefficient of the gas,
which is proportional to S/ MW . That is, the relative
rates at which different gases at the same partial pressure levels will diffuse are proportional to their diffusion coefficients. Assuming that the diffusion coefficient
for oxygen is 1, the relative diffusion coefficients for different gases of respiratory importance in the body fluids
are as follows:
Oxygen
Carbon dioxide
Carbon monoxide
Nitrogen
Helium
1.0
20.3
0.81
0.53
0.95
Diffusion of Gases Through Tissues
The gases that are of respiratory importance are all
highly soluble in lipids and, consequently, are highly
soluble in cell membranes. Because of this, the major
limitation to the movement of gases in tissues is the rate
at which the gases can diffuse through the tissue water
instead of through the cell membranes. Therefore, diffusion of gases through the tissues, including through
the respiratory membrane, is almost equal to the diffusion of gases in water, as given in the preceding list.
Composition of Alveolar
Air—Its Relation to
Atmospheric Air
Alveolar air does not have the same concentrations of
gases as atmospheric air by any means, which can
readily be seen by comparing the alveolar air composition in Table 39–1 with that of atmospheric air. There
are several reasons for the differences. First, the alveolar air is only partially replaced by atmospheric air
with each breath. Second, oxygen is constantly being
absorbed into the pulmonary blood from the alveolar
air. Third, carbon dioxide is constantly diffusing from
the pulmonary blood into the alveoli. And fourth, dry
Table 39–1
Partial Pressures of Respiratory Gases as They Enter and Leave the Lungs (at Sea Level)
Atmospheric Air*
(mm Hg)
N2
O2
CO2
H2O
TOTAL
597.0
159.0
0.3
3.7
760.0
* On an average cool, clear day.
Humidified Air
(mm Hg)
(78.62%)
(20.84%)
(0.04%)
(0.50%)
(100.0%)
563.4
149.3
0.3
47.0
760.0
Alveolar Air
(mm Hg)
(74.09%)
(19.67%)
(0.04%)
(6.20%)
(100.0%)
569.0
104.0
40.0
47.0
760.0
Expired Air
(mm Hg)
(74.9%)
(13.6%)
(5.3%)
(6.2%)
(100.0%)
566.0
120.0
27.0
47.0
760.0
(74.5%)
(15.7%)
(3.6%)
(6.2%)
(100.0%)
Unit VII
atmospheric air that enters the respiratory passages is
humidified even before it reaches the alveoli.
Humidification of the Air in the Respiratory Passages. Table
39–1 shows that atmospheric air is composed almost
entirely of nitrogen and oxygen; it normally contains
almost no carbon dioxide and little water vapor.
However, as soon as the atmospheric air enters the respiratory passages, it is exposed to the fluids that cover
the respiratory surfaces. Even before the air enters the
alveoli, it becomes (for all practical purposes) totally
humidified.
The partial pressure of water vapor at a normal
body temperature of 37°C is 47 mm Hg, which is therefore the partial pressure of water vapor in the alveolar air. Because the total pressure in the alveoli
cannot rise to more than the atmospheric pressure
(760 mm Hg at sea level), this water vapor simply
dilutes all the other gases in the inspired air. Table 39–1
also shows that humidification of the air dilutes the
oxygen partial pressure at sea level from an average
of 159 mm Hg in atmospheric air to 149 mm Hg in the
humidified air, and it dilutes the nitrogen partial pressure from 597 to 563 mm Hg.
Rate at Which Alveolar Air Is Renewed
by Atmospheric Air
In Chapter 37, it was pointed out that the average male
functional residual capacity of the lungs (the volume
of air remaining in the lungs at the end of normal expiration) measures about 2300 milliliters. Yet only 350
milliliters of new air is brought into the alveoli with
each normal inspiration, and this same amount of old
alveolar air is expired. Therefore, the volume of alveolar air replaced by new atmospheric air with each
breath is only one seventh of the total, so that multiple breaths are required to exchange most of the alveolar air. Figure 39–2 shows this slow rate of renewal of
the alveolar air. In the first alveolus of the figure, an
excess amount of a gas is present in the alveoli, but
note that even at the end of 16 breaths, the excess
Oxygen is continually being absorbed from the alveoli
into the blood of the lungs, and new oxygen is continually being breathed into the alveoli from the atmosphere. The more rapidly oxygen is absorbed, the lower
its concentration in the alveoli becomes; conversely,
the more rapidly new oxygen is breathed into the
alveoli from the atmosphere, the higher its concentration becomes. Therefore, oxygen concentration in the
alveoli, and its partial pressure as well, is controlled by
(1) the rate of absorption of oxygen into the blood and
(2) the rate of entry of new oxygen into the lungs by
the ventilatory process.
100
1/2
80
no
60
rm
40
al
al
rm
no
al
ve
a lv
o la
al
20
ve
eo
rv
al
12th breath
Oxygen Concentration and Partial
Pressure in the Alveoli
2¥
8th breath
3rd breath
Importance of the Slow Replacement of Alveolar Air. The slow
replacement of alveolar air is of particular importance
in preventing sudden changes in gas concentrations in
the blood. This makes the respiratory control mechanism much more stable than it would be otherwise, and
it helps prevent excessive increases and decreases in
tissue oxygenation, tissue carbon dioxide concentration, and tissue pH when respiration is temporarily
interrupted.
rm
4th breath
2nd breath
gas still has not been completely removed from the
alveoli.
Figure 39–3 demonstrates graphically the rate at
which excess gas in the alveoli is normally removed,
showing that with normal alveolar ventilation, about
one half the gas is removed in 17 seconds. When a
person’s rate of alveolar ventilation is only one half
normal, one half the gas is removed in 34 seconds, and
when the rate of ventilation is twice normal, one half
is removed in about 8 seconds.
No
1st breath
Respiration
Concentration of gas
(per cent of original concentration)
494
ola
la r
en
ven
t ila
r ven
tilat
io
n
ti o n
tilation
0
0
10
20
30
40
Time (seconds)
50
16th breath
Figure 39–2
Expiration of a gas from an alveolus with successive breaths.
Figure 39–3
Rate of removal of excess gas from alveoli.
60
Chapter 39
495
Physical Principles of Gas Exchange; Diffusion of Oxygen and Carbon Dioxide
Upper limit at maximum ventilation
Alveolar partial pressure
of oxygen (mm Hg)
150
Figure 39–4
250 ml O2/min
125
100
A
Normal alveolar PO2
75
50
1000 ml O2/min
25
0
0
5
Effect of alveolar ventilation on the alveolar PO2 at
two rates of oxygen absorption from the alveoli—
250 ml/min and 1000 ml/min. Point A is the normal
operating point.
35
40
175
Alveolar partial pressure
of CO2 (mm Hg)
Figure 39–4 shows the effect of both alveolar ventilation and rate of oxygen absorption into the blood on
the alveolar partial pressure of oxygen (Po2). One
curve represents oxygen absorption at a rate of
250 ml/min, and the other curve represents a rate of
1000 ml/min. At a normal ventilatory rate of 4.2 L/min
and an oxygen consumption of 250 ml/min, the normal
operating point in Figure 39–4 is point A. The figure
also shows that when 1000 milliliters of oxygen is being
absorbed each minute, as occurs during moderate
exercise, the rate of alveolar ventilation must increase
fourfold to maintain the alveolar Po2 at the normal
value of 104 mm Hg.
Another effect shown in Figure 39–4 is that an
extremely marked increase in alveolar ventilation can
never increase the alveolar Po2 above 149 mm Hg as
long as the person is breathing normal atmospheric
air at sea level pressure, because this is the maximum
Po2 in humidified air at this pressure. If the person
breathes gases that contain partial pressures of oxygen
higher than 149 mm Hg, the alveolar Po2 can approach
these higher pressures at high rates of ventilation.
10
15
20
25
30
Alveolar ventilation (L/min)
150
125
800 ml CO2/min
100
75
50
Normal alveolar PCO2
A
25
200 ml CO2/min
0
0
5
10 15 20 25 30 35
Alveolar ventilation (L/min)
40
Figure 39–5
Effect of alveolar ventilation on the alveolar PCO2 at two rates of
carbon dioxide excretion from the blood—800 ml/min and 200 ml/
min. Point A is the normal operating point.
CO2 Concentration and Partial
Pressure in the Alveoli
Carbon dioxide is continually being formed in the
body and then carried in the blood to the alveoli; it is
continually being removed from the alveoli by ventilation. Figure 39–5 shows the effects on the alveolar
partial pressure of carbon dioxide (Pco2) of both alveolar ventilation and two rates of carbon dioxide excretion, 200 and 800 ml/min. One curve represents a
normal rate of carbon dioxide excretion of 200 ml/min.
At the normal rate of alveolar ventilation of 4.2 L/min,
the operating point for alveolar Pco2 is at point A in
Figure 39–5—that is, 40 mm Hg.
Two other facts are also evident from Figure 39–5:
First, the alveolar PCO2 increases directly in proportion
to the rate of carbon dioxide excretion, as represented
by the fourfold elevation of the curve (when 800
milliliters of CO2 are excreted per minute). Second,
the alveolar PCO2 decreases in inverse proportion to
alveolar ventilation. Therefore, the concentrations and
partial pressures of both oxygen and carbon dioxide in
the alveoli are determined by the rates of absorption
or excretion of the two gases and by the amount of
alveolar ventilation.
Expired Air
Expired air is a combination of dead space air and alveolar air; its overall composition is therefore determined
496
Unit VII
Respiration
160
Pressures of O2 and CO2
(mm Hg)
140
120
Oxygen (PO2)
100
Alveolar air
and dead
space air
Dead
space
air
80
60
Alveolar air
Carbon dioxide (PCO2)
40
20
0
0
100
300
200
Milliliters of air expired
400
500
Figure 39–6
Oxygen and carbon dioxide partial pressures in the
various portions of normal expired air.
by (1) the amount of the expired air that is dead space
air and (2) the amount that is alveolar air. Figure 39–6
shows the progressive changes in oxygen and carbon
dioxide partial pressures in the expired air during the
course of expiration. The first portion of this air, the
dead space air from the respiratory passageways, is
typical humidified air, as shown in Table 39–1.Then, progressively more and more alveolar air becomes mixed
with the dead space air until all the dead space air has
finally been washed out and nothing but alveolar air is
expired at the end of expiration. Therefore, the method
of collecting alveolar air for study is simply to collect
a sample of the last portion of the expired air after
forceful expiration has removed all the dead space
air.
Normal expired air, containing both dead space air
and alveolar air, has gas concentrations and partial
pressures approximately as shown in Table 39–1—that
is, concentrations between those of alveolar air and
humidified atmospheric air.
Terminal bronchiole
Respiratory bronchiole
Atrium
Alveolar
duct
Alveoli
Diffusion of Gases Through
the Respiratory Membrane
Alveolar sacs
Respiratory Unit. Figure 39–7 shows the respiratory unit
(also called “respiratory lobule”), which is composed
of a respiratory bronchiole, alveolar ducts, atria, and
alveoli. There are about 300 million alveoli in the two
lungs, and each alveolus has an average diameter of
about 0.2 millimeter. The alveolar walls are extremely
thin, and between the alveoli is an almost solid
network of interconnecting capillaries, shown in
Figure 39–8. Indeed, because of the extensiveness of
the capillary plexus, the flow of blood in the alveolar
wall has been described as a “sheet” of flowing blood.
Thus, it is obvious that the alveolar gases are in very
close proximity to the blood of the pulmonary capillaries. Further, gas exchange between the alveolar air
Figure 39–7
Respiratory unit. (Redrawn from Miller WS: The Lung. Springfield,
Ill: Charles C Thomas, 1947.)
and the pulmonary blood occurs through the membranes of all the terminal portions of the lungs, not
merely in the alveoli themselves. All these membranes
are collectively known as the respiratory membrane,
also called the pulmonary membrane.
Chapter 39
Physical Principles of Gas Exchange; Diffusion of Oxygen and Carbon Dioxide
497
Epithelial
basement
Alveolar
epithelium membrane
Fluid and
surfactant
layer
Alveolus
Capillary
Diffusion
Oxygen
Diffusion
Carbon dioxide
A
Alveolus
Alveolus
Red blood
cell
Interstitial space
Capillaries
Lymphatic
vessel
Alveolus
Interstitial space
Capillary endothelium
Capillary basement membrane
Figure 39–9
Vein
Artery
Perivascular
interstitial space
Alveolus
B
Figure 39–8
A, Surface view of capillaries in an alveolar wall. B, Cross-sectional view of alveolar walls and their vascular supply. (A, From
Maloney JE, Castle BL: Pressure-diameter relations of capillaries
and small blood vessels in frog lung. Respir Physiol 7:150, 1969.
Reproduced by permission of ASP Biological and Medical Press,
North-Holland Division.)
Respiratory Membrane. Figure 39–9 shows the ultrastructure of the respiratory membrane drawn in cross
section on the left and a red blood cell on the right. It
also shows the diffusion of oxygen from the alveolus
into the red blood cell and diffusion of carbon dioxide
in the opposite direction. Note the following different
layers of the respiratory membrane:
1. A layer of fluid lining the alveolus and containing
surfactant that reduces the surface tension of the
alveolar fluid
2. The alveolar epithelium composed of thin
epithelial cells
3. An epithelial basement membrane
4. A thin interstitial space between the alveolar
epithelium and the capillary membrane
Ultrastructure of the alveolar respiratory membrane, shown in
cross section.
5. A capillary basement membrane that in many
places fuses with the alveolar epithelial basement
membrane
6. The capillary endothelial membrane
Despite the large number of layers, the overall
thickness of the respiratory membrane in some areas
is as little as 0.2 micrometer, and it averages about 0.6
micrometer, except where there are cell nuclei. From
histological studies, it has been estimated that the total
surface area of the respiratory membrane is about 70
square meters in the normal adult human male. This is
equivalent to the floor area of a 25–by-30–foot room.
The total quantity of blood in the capillaries of the
lungs at any given instant is 60 to 140 milliliters. Now
imagine this small amount of blood spread over the
entire surface of a 25–by-30–foot floor, and it is easy
to understand the rapidity of the respiratory exchange
of oxygen and carbon dioxide.
The average diameter of the pulmonary capillaries
is only about 5 micrometers, which means that red
blood cells must squeeze through them. The red blood
cell membrane usually touches the capillary wall, so
that oxygen and carbon dioxide need not pass through
significant amounts of plasma as they diffuse between
the alveolus and the red cell. This, too, increases the
rapidity of diffusion.
498
Unit VII
Factors That Affect the Rate
of Gas Diffusion Through the
Respiratory Membrane
Referring to the earlier discussion of diffusion of gases
in water, one can apply the same principles and mathematical formulas to diffusion of gases through the
respiratory membrane. Thus, the factors that determine how rapidly a gas will pass through the membrane are (1) the thickness of the membrane, (2) the
surface area of the membrane, (3) the diffusion coefficient of the gas in the substance of the membrane, and
(4) the partial pressure difference of the gas between
the two sides of the membrane.
The thickness of the respiratory membrane occasionally increases—for instance, as a result of edema
fluid in the interstitial space of the membrane and in
the alveoli—so that the respiratory gases must then
diffuse not only through the membrane but also
through this fluid. Also, some pulmonary diseases
cause fibrosis of the lungs, which can increase the
thickness of some portions of the respiratory membrane. Because the rate of diffusion through the membrane is inversely proportional to the thickness of the
membrane, any factor that increases the thickness to
more than two to three times normal can interfere significantly with normal respiratory exchange of gases.
The surface area of the respiratory membrane can be
greatly decreased by many conditions. For instance,
removal of an entire lung decreases the total surface
area to one half normal. Also, in emphysema, many of
the alveoli coalesce, with dissolution of many alveolar
walls. Therefore, the new alveolar chambers are much
larger than the original alveoli, but the total surface
area of the respiratory membrane is often decreased
as much as fivefold because of loss of the alveolar
walls. When the total surface area is decreased to
about one third to one fourth normal, exchange of
gases through the membrane is impeded to a significant degree, even under resting conditions, and during
competitive sports and other strenuous exercise, even
the slightest decrease in surface area of the lungs can
be a serious detriment to respiratory exchange of
gases.
The diffusion coefficient for transfer of each gas
through the respiratory membrane depends on the
gas’s solubility in the membrane and, inversely, on the
square root of the gas’s molecular weight. The rate
of diffusion in the respiratory membrane is almost
exactly the same as that in water, for reasons explained
earlier. Therefore, for a given pressure difference,
carbon dioxide diffuses about 20 times as rapidly as
oxygen. Oxygen diffuses about twice as rapidly as
nitrogen.
The pressure difference across the respiratory membrane is the difference between the partial pressure of
the gas in the alveoli and the partial pressure of the
gas in the pulmonary capillary blood. The partial pressure represents a measure of the total number of molecules of a particular gas striking a unit area of the
alveolar surface of the membrane in unit time, and the
Respiration
pressure of the gas in the blood represents the number
of molecules that attempt to escape from the blood
in the opposite direction. Therefore, the difference
between these two pressures is a measure of the net
tendency for the gas molecules to move through the
membrane. When the partial pressure of a gas in the
alveoli is greater than the pressure of the gas in
the blood, as is true for oxygen, net diffusion from the
alveoli into the blood occurs; when the pressure of the
gas in the blood is greater than the partial pressure in
the alveoli, as is true for carbon dioxide, net diffusion
from the blood into the alveoli occurs.
Diffusing Capacity of the
Respiratory Membrane
The ability of the respiratory membrane to exchange
a gas between the alveoli and the pulmonary blood
is expressed in quantitative terms by the respiratory
membrane’s diffusing capacity, which is defined as
the volume of a gas that will diffuse through the membrane each minute for a partial pressure difference of
1 mm Hg. All the factors discussed earlier that affect
diffusion through the respiratory membrane can affect
this diffusing capacity.
Diffusing Capacity for Oxygen. In the average young man,
the diffusing capacity for oxygen under resting conditions averages 21 ml/min/mm Hg. In functional terms,
what does this mean? The mean oxygen pressure
difference across the respiratory membrane during
normal, quiet breathing is about 11 mm Hg. Multiplication of this pressure by the diffusing capacity (11 ¥
21) gives a total of about 230 milliliters of oxygen diffusing through the respiratory membrane each minute;
this is equal to the rate at which the resting body uses
oxygen.
Change in Oxygen Diffusing Capacity During Exercise.
During strenuous exercise or other conditions that
greatly increase pulmonary blood flow and alveolar
ventilation, the diffusing capacity for oxygen increases
in young men to a maximum of about 65 ml/min/
mm Hg, which is three times the diffusing capacity
under resting conditions. This increase is caused by
several factors, among which are (1) opening up of
many previously dormant pulmonary capillaries or
extra dilation of already open capillaries, thereby
increasing the surface area of the blood into which the
oxygen can diffuse; and (2) a better match between
the ventilation of the alveoli and the perfusion of the
alveolar capillaries with blood, called the ventilationperfusion ratio, which is explained in detail later in
this chapter. Therefore, during exercise, oxygenation of
the blood is increased not only by increased alveolar
ventilation but also by greater diffusing capacity of
the respiratory membrane for transporting oxygen into
the blood.
Diffusing Capacity for Carbon Dioxide. The diffusing capacity for carbon dioxide has never been measured
Chapter 39
Physical Principles of Gas Exchange; Diffusion of Oxygen and Carbon Dioxide
because of the following technical difficulty: Carbon
dioxide diffuses through the respiratory membrane so
rapidly that the average Pco2 in the pulmonary blood
is not far different from the Pco2 in the alveoli—the
average difference is less than 1 mm Hg—and with the
available techniques, this difference is too small to be
measured.
Nevertheless, measurements of diffusion of other
gases have shown that the diffusing capacity varies
directly with the diffusion coefficient of the particular
gas. Because the diffusion coefficient of carbon dioxide
is slightly more than 20 times that of oxygen, one
would expect a diffusing capacity for carbon dioxide
under resting conditions of about 400 to 450 ml/min/
mm Hg and during exercise of about 1200 to 1300 ml/
min/mm Hg. Figure 39–10 compares the measured or
calculated diffusing capacities of carbon monoxide,
oxygen, and carbon dioxide at rest and during exercise,
showing the extreme diffusing capacity of carbon
dioxide and the effect of exercise on the diffusing
capacity of each of these gases.
Measurement of Diffusing Capacity—The Carbon Monoxide
Method. The oxygen diffusing capacity can be calculated
from measurements of (1) alveolar Po2, (2) Po2 in the
pulmonary capillary blood, and (3) the rate of oxygen
uptake by the blood. However, measuring the Po2 in the
pulmonary capillary blood is so difficult and so imprecise that it is not practical to measure oxygen diffusing
1300
1200
Resting
Exercise
Diffusing capacity (ml/min/mm Hg)
1100
1000
900
800
700
600
500
400
300
200
100
0
CO
O2
CO2
Figure 39–10
Diffusing capacities for carbon monoxide, oxygen, and carbon
dioxide in the normal lungs under resting conditions and during
exercise.
499
capacity by such a direct procedure, except on an experimental basis.
To obviate the difficulties encountered in measuring
oxygen diffusing capacity directly, physiologists usually
measure carbon monoxide diffusing capacity instead
and then calculate the oxygen diffusing capacity from
this. The principle of the carbon monoxide method is
the following: A small amount of carbon monoxide is
breathed into the alveoli, and the partial pressure of the
carbon monoxide in the alveoli is measured from appropriate alveolar air samples. The carbon monoxide pressure in the blood is essentially zero, because hemoglobin
combines with this gas so rapidly that its pressure never
has time to build up. Therefore, the pressure difference
of carbon monoxide across the respiratory membrane is
equal to its partial pressure in the alveolar air sample.
Then, by measuring the volume of carbon monoxide
absorbed in a short period and dividing this by the
alveolar carbon monoxide partial pressure, one can
determine accurately the carbon monoxide diffusing
capacity.
To convert carbon monoxide diffusing capacity to
oxygen diffusing capacity, the value is multiplied by a
factor of 1.23 because the diffusion coefficient for
oxygen is 1.23 times that for carbon monoxide. Thus,
the average diffusing capacity for carbon monoxide
in young men at rest is 17 ml/min/mm Hg, and the
diffusing capacity for oxygen is 1.23 times this, or
21 ml/min/mm Hg.
Effect of the VentilationPerfusion Ratio on Alveolar
Gas Concentration
In the early part of this chapter, we learned that two
factors determine the Po2 and the Pco2 in the alveoli: (1)
the rate of alveolar ventilation and (2) the rate of transfer of oxygen and carbon dioxide through the respiratory membrane. These earlier discussions made the
assumption that all the alveoli are ventilated equally
and that blood flow through the alveolar capillaries is
the same for each alveolus. However, even normally to
some extent, and especially in many lung diseases, some
areas of the lungs are well ventilated but have almost
no blood flow, whereas other areas may have excellent
blood flow but little or no ventilation. In either of these
conditions, gas exchange through the respiratory membrane is seriously impaired, and the person may suffer
severe respiratory distress despite both normal total
ventilation and normal total pulmonary blood flow, but
with the ventilation and blood flow going to different
parts of the lungs. Therefore, a highly quantitative
concept has been developed to help us understand respiratory exchange when there is imbalance between
alveolar ventilation and alveolar blood flow. This
concept is called the ventilation-perfusion ratio.
In quantitative. terms,
the ventilation-perfusion
ratio
.
.
is expressed as Va/Q. When Va (alveolar
ventilation) is
.
normal for a given alveolus and Q (blood flow) is also
normal. for. the same alveolus, the ventilation-perfusion
ratio (Va/Q
. ) is also said to be normal. When the
. ventilation (Va) is zero,
. . yet there is still perfusion (Q) of the
alveolus, the Va/Q is zero. Or, at the
. other extreme,
when there
is adequate
.
. .ventilation (Va) but zero perfusion (Q), the ratio Va/Q is infinity. At a ratio of either
zero or infinity, there is no exchange of gases through
the respiratory membrane of the affected alveoli, which
500
Unit VII
Respiration
explains the importance of this concept. Therefore, let
us explain the respiratory consequences of these two
extremes.
any alveolar ventilation—the air in the alveolus
comes to equilibrium with the blood oxygen and carbon
dioxide because these gases diffuse between the blood
and the alveolar air. Because the blood that perfuses
the capillaries is venous blood returning to the lungs
from the systemic circulation, it is the gases in this
blood with which the alveolar gases equilibrate. In
Chapter 40, we will learn that the normal venous blood
(v̄) has a Po2 of 40 mm Hg and a Pco2 of 45 mm Hg.
Therefore, these are also the normal partial pressures of
these two gases in alveoli that have blood flow but no
ventilation.
. .
Alveolar Oxygen and Carbon Dioxide Partial Pressures When VA/Q
Equals Infinity. .The. effect on the alveolar gas partial pres-
sures when Va/Q equals
is entirely different
. infinity
.
from the effect when Va/Q equals zero because now
there is no capillary blood flow to carry oxygen away or
to bring carbon dioxide to the alveoli. Therefore, instead
of the alveolar gases coming to equilibrium with the
venous blood, the alveolar air becomes equal to the
humidified inspired air. That is, the air that is inspired
loses no oxygen to the blood and gains no carbon
dioxide from the blood. And because normal inspired
and humidified air has a Po2 of 149 mm Hg and a Pco2
of 0 mm Hg, these will be the partial pressures of these
two gases in the alveoli.
. .
Gas Exchange and Alveolar Partial Pressures When VA/Q Is
Normal. When there is both normal alveolar ventilation
and normal alveolar capillary blood flow (normal alveolar perfusion), exchange of oxygen and carbon dioxide
through the respiratory membrane is nearly optimal,
and alveolar Po2 is normally at a level of 104 mm Hg,
which lies between that of the inspired air (149 mm Hg)
and that of venous blood (40 mm Hg). Likewise,
alveolar Pco2 lies between two extremes; it is normally
40 mm Hg, in contrast to 45 mm Hg in venous blood and
0 mm Hg in inspired air. Thus, under normal conditions,
the alveolar air Po2 averages 104 mm Hg and the Pco2
averages 40 mm Hg.
. .
PO2-PCO2, VA/Q Diagram
The concepts presented in the preceding sections can be
shown in graphical form, as
in Figure
. demonstrated
.
39–11, called the Po2-Pco2, Va/Q diagram. The curve in
the diagram represents all possible
. Po
. 2 and Pco2 combinations
between the limits of Va/Q equals zero and
. .
Va/Q equals infinity when the gas pressures in the
venous blood are normal and the person is breathing
air at sea-level pressure.
Thus, point v̄ is the plot of Po2
. .
and Pco2 when Va/Q equals zero. At this point, the Po2
is 40 mm Hg and the Pco2 is 45 mm Hg, which are the
values in normal venous blood.
. .
At the other end of the curve, when Va/Q equals infinity, point I represents inspired air, showing Po2 to be 149
mm Hg while Pco2 is zero. Also plotted on the curve
is. the
. point that represents normal alveolar air when
Va/Q is normal. At this point, Po2 is 104 mm Hg and
Pco2 is 40 mm Hg.
50
v
VA /Q = 0
VA /Q = Normal
(PO2 = 40)
(PCO2 = 45)
40
PCO2 (mm Hg)
. .
Alveolar Oxygen and Carbon
. . Dioxide Partial Pressures When VA/Q
Equals Zero. When Va/Q is equal to zero—that is, without
Normal alveolar
air
(PO2 = 104)
(PCO2 = 40)
30
20
10
(PO2 = 149)
(PCO2 = 0) I
0
20
40
VA /Q = ∞
60 80 100 120 140 160
PO2 (mm Hg)
Figure 39–11
. .
Normal PO2-PCO2, VA/Q diagram.
Concept. of. “Physiologic Shunt”
(When VA/Q Is Below Normal)
. .
Whenever Va/Q is below normal, there is inadequate
ventilation to provide the oxygen needed to fully oxygenate the blood flowing through the alveolar capillaries. Therefore, a certain fraction of the venous
blood passing through the pulmonary capillaries does
not become oxygenated. This fraction is called shunted
blood. Also, some additional blood flows through
bronchial vessels rather than through alveolar capillaries, normally about 2 per cent of the cardiac output; this,
too, is unoxygenated, shunted blood.
The total quantitative amount of shunted blood per
minute is called the physiologic shunt. This physiologic
shunt is measured in clinical pulmonary function laboratories by analyzing the concentration of oxygen in
both mixed venous blood and arterial blood, along with
simultaneous measurement of cardiac output. From
these values, the physiologic shunt can be calculated by
the following equation:
.
Q ps Ci O2 - CaO2
. =
.
Qt
Ci O2 - C v O2
.
in which . Qps is the physiologic shunt blood flow per
minute, Qt is cardiac output per minute, CiO is the concentration of oxygen in the arterial blood if there is an
“ideal” ventilation-perfusion ratio, CaO is the measured
concentration of oxygen in the arterial blood, and Cv̄O
is the measured concentration of oxygen in the mixed
venous blood.
The greater the physiologic shunt, the greater the
amount of blood that fails to be oxygenated as it passes
through the lungs.
2
2
2
Concept of the .“Physiologic
Dead
.
Space” (When VA/Q Is Greater
Than Normal)
When ventilation of some of the alveoli is great but
alveolar blood flow is low, there is far more available
oxygen in the alveoli than can be transported away from
Chapter 39
Physical Principles of Gas Exchange; Diffusion of Oxygen and Carbon Dioxide
the alveoli by the flowing blood. Thus, the ventilation of
these alveoli is said to be wasted. The ventilation of the
anatomical dead space areas of the respiratory passageways is also wasted. The sum of these two types of
wasted ventilation is called the physiologic dead space.
This is measured in the clinical pulmonary function laboratory by making appropriate blood and expiratory
gas measurements and using the following equation,
called the Bohr equation:
Vdphys PaCO2 - Pe CO2
=
,
Vt
PaCO2
in which Vdphys is the physiologic dead space, Vt is the
tidal volume, PaCO is the partial pressure of carbon
dioxide in the arterial blood, and Pe CO2 is the average
partial pressure of carbon dioxide in the entire expired
air.
When the physiologic dead space is great, much of the
work of ventilation is wasted effort because so much of
the ventilating air never reaches the blood.
2
Abnormalities of VentilationPerfusion Ratio
. .
Abnormal VA/Q in the Upper and Lower Normal Lung. In a
normal person in the upright position, both pulmonary
capillary blood flow and alveolar ventilation are considerably less in the upper part of the lung than in the
lower part; however, blood flow is decreased considerably more. than
. ventilation is. Therefore, at the top of
the lung, Va/Q is as much as 2.5 times as great as the
ideal value, which causes a moderate degree of physiologic dead space in this area of the lung.
At the other extreme, in the bottom of the lung, there
is slightly
. . too little ventilation in relation to blood flow,
with Va/Q as low as 0.6 times the ideal value. In this
area, a small fraction of the blood fails to become
normally oxygenated, and this represents a physiologic
shunt.
In both extremes, inequalities of ventilation and
perfusion decrease slightly the lung’s effectiveness
for exchanging oxygen and carbon dioxide. However,
during exercise, blood flow to the upper part of the lung
increases markedly, so that far less physiologic dead
space occurs, and the effectiveness of gas exchange now
approaches optimum.
. .
Abnormal VA/Q in Chronic Obstructive Lung Disease. Most
people who smoke for many years develop various
degrees of bronchial obstruction; in a large share of
these persons, this condition eventually becomes so
severe that they develop serious alveolar air trapping
and resultant emphysema. The emphysema in turn
causes many of the alveolar walls to be destroyed. Thus,
501
two
. .abnormalities occur in smokers to cause abnormal
Va/Q. First, because many of the small bronchioles are
obstructed, the alveoli beyond
the obstructions are
. .
unventilated, causing a Va/Q that approaches zero.
Second, in those areas of the lung where the alveolar
walls have been mainly destroyed but there is still
alveolar ventilation, most of the ventilation is wasted
because of inadequate blood flow to transport the blood
gases.
Thus, in chronic obstructive lung disease, some areas
of the lung exhibit serious physiologic shunt, and other
areas exhibit serious physiologic dead space. Both these
conditions tremendously decrease the effectiveness of
the lungs as gas exchange organs, sometimes reducing
their effectiveness to as little as one tenth normal. In
fact, this is the most prevalent cause of pulmonary disability today.
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Rahn H, Farhi EE: Ventilation, perfusion, and gas
exchange—the Va/Q concept. In: Fenn WO, Rahn H (eds):
Handbook of Physiology. Sec 3, Vol 1. Baltimore: Williams
& Wilkins, 1964, p 125.
Uhlig S, Taylor AE: Methods in Pulmonary Research. Basel:
Birkhauser Verlag, 1998.
West JB: Pulmonary Physiology and Pathophysiology: An
Integrated, Case-Based Approach. Philadelphia: Lippincott Williams & Wilkins, 2001.
West JB: Pulmonary Physiology—The Essentials. Baltimore:
Lippincott Williams & Wilkins, 2003.
Williams MC:Alveolar type I cells: molecular phenotype and
development. Annu Rev Physiol 65:669, 2003.
C
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A
P
T
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4
0
Transport of Oxygen and Carbon
Dioxide in Blood and Tissue Fluids
Once oxygen has diffused from the alveoli into
the pulmonary blood, it is transported to the
peripheral tissue capillaries almost entirely in combination with hemoglobin. The presence of hemoglobin in the red blood cells allows the blood to
transport 30 to 100 times as much oxygen as could
be transported in the form of dissolved oxygen in
the water of the blood.
In the body’s tissue cells, oxygen reacts with various foodstuffs to form large
quantities of carbon dioxide. This carbon dioxide enters the tissue capillaries
and is transported back to the lungs. Carbon dioxide, like oxygen, also combines
with chemical substances in the blood that increase carbon dioxide transport
15- to 20-fold.
The purpose of this chapter is to present both qualitatively and quantitatively
the physical and chemical principles of oxygen and carbon dioxide transport in
the blood and tissue fluids.
Transport of Oxygen from the Lungs to the
Body Tissues
In Chapter 39, we pointed out that gases can move from one point to another
by diffusion and that the cause of this movement is always a partial pressure
difference from the first point to the next. Thus, oxygen diffuses from the alveoli
into the pulmonary capillary blood because the oxygen partial pressure (Po2)
in the alveoli is greater than the Po2 in the pulmonary capillary blood. In the
other tissues of the body, a higher Po2 in the capillary blood than in the tissues
causes oxygen to diffuse into the surrounding cells.
Conversely, when oxygen is metabolized in the cells to form carbon dioxide,
the intracellular carbon dioxide pressure (Pco2) rises to a high value, which
causes carbon dioxide to diffuse into the tissue capillaries. After blood flows to
the lungs, the carbon dioxide diffuses out of the blood into the alveoli, because
the Pco2 in the pulmonary capillary blood is greater than that in the alveoli.
Thus, the transport of oxygen and carbon dioxide by the blood depends on both
diffusion and the flow of blood. We now consider quantitatively the factors
responsible for these effects.
Diffusion of Oxygen from the Alveoli to the Pulmonary
Capillary Blood
The top part of Figure 40–1 shows a pulmonary alveolus adjacent to a pulmonary capillary, demonstrating diffusion of oxygen molecules between the
alveolar air and the pulmonary blood. The Po2 of the gaseous oxygen in the
alveolus averages 104 mm Hg, whereas the Po2 of the venous blood entering the
pulmonary capillary at its arterial end averages only 40 mm Hg because a large
amount of oxygen was removed from this blood as it passed through the peripheral tissues. Therefore, the initial pressure difference that causes oxygen to
diffuse into the pulmonary capillary is 104 – 40, or 64 mm Hg. In the graph at
502
Chapter 40
Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
Mixed with
pulmonary
shunt blood
Alveolus PO2 = 104 mm Hg
100
Pulmonary Capillary
PO2 = 104 mm Hg
PO2 = 40 mm Hg
Arterial End
Venous End
Alveolar oxygen partial pressure
Systemic
venous
blood
60
Pulmonary Systemic
capillaries
arterial
blood
Systemic Systemic
capillaries venous
blood
40
O
2
90
80
70
Blo
od
P
Blood PO2 (mm Hg)
100
PO2
110
80
503
20
0
60
50
40
Figure 40–2
Changes in PO2 in the pulmonary capillary blood, systemic arterial blood, and systemic capillary blood, demonstrating the effect
of “venous admixture.”
Figure 40–1
Uptake of oxygen by the pulmonary capillary blood. (The curve in
this figure was constructed from data in Milhorn HT Jr, Pulley PE
Jr: A theoretical study of pulmonary capillary gas exchange and
venous admixture. Biophys J 8:337, 1968.)
Therefore, during exercise, even with a shortened time
of exposure in the capillaries, the blood can still
become fully oxygenated, or nearly so.
Transport of Oxygen in the
Arterial Blood
the bottom of the figure, the curve shows the rapid rise
in blood Po2 as the blood passes through the capillary;
the blood Po2 rises almost to that of the alveolar air by
the time the blood has moved a third of the distance
through the capillary, becoming almost 104 mm Hg.
Uptake of Oxygen by the Pulmonary Blood During Exercise.
During strenuous exercise, a person’s body may
require as much as 20 times the normal amount of
oxygen. Also, because of increased cardiac output
during exercise, the time that the blood remains in the
pulmonary capillary may be reduced to less than one
half normal. Yet, because of the great safety factor for
diffusion of oxygen through the pulmonary membrane, the blood still becomes almost saturated with
oxygen by the time it leaves the pulmonary capillaries.
This can be explained as follows.
First, it was pointed out in Chapter 39 that the diffusing capacity for oxygen increases almost threefold
during exercise; this results mainly from increased
surface area of capillaries participating in the diffusion
and also from a more nearly ideal ventilation-perfusion ratio in the upper part of the lungs.
Second, note in the curve of Figure 40–1 that under
nonexercising conditions, the blood becomes almost
saturated with oxygen by the time it has passed
through one third of the pulmonary capillary, and little
additional oxygen normally enters the blood during
the latter two thirds of its transit. That is, the blood
normally stays in the lung capillaries about three
times as long as necessary to cause full oxygenation.
About 98 per cent of the blood that enters the left
atrium from the lungs has just passed through the alveolar capillaries and has become oxygenated up to a Po2
of about 104 mm Hg. Another 2 per cent of the blood
has passed from the aorta through the bronchial circulation, which supplies mainly the deep tissues of the
lungs and is not exposed to lung air. This blood flow is
called “shunt flow,” meaning that blood is shunted past
the gas exchange areas. On leaving the lungs, the Po2
of the shunt blood is about that of normal systemic
venous blood, about 40 mm Hg. When this blood
combines in the pulmonary veins with the oxygenated
blood from the alveolar capillaries, this so-called
venous admixture of blood causes the Po2 of the blood
entering the left heart and pumped into the aorta to
fall to about 95 mm Hg. These changes in blood Po2 at
different points in the circulatory system are shown in
Figure 40–2.
Diffusion of Oxygen from the
Peripheral Capillaries into the
Tissue Fluid
When the arterial blood reaches the peripheral tissues,
its Po2 in the capillaries is still 95 mm Hg.Yet, as shown
in Figure 40–3, the Po2 in the interstitial fluid that surrounds the tissue cells averages only 40 mm Hg. Thus,
there is a tremendous initial pressure difference that
causes oxygen to diffuse rapidly from the capillary
504
Unit VII
Arterial end
of capillary
Venous end
of capillary
40 mm Hg
PO2 = 95 mm Hg
PO2 = 40 mm Hg
23 mm Hg
Figure 40–3
Diffusion of oxygen from a tissue capillary to the cells. (PO2 in
interstitial fluid = 40 mm Hg, and in tissue cells = 23 mm Hg.)
Upper limit of infinite blood flow
80
p ti o n
sum
con
nor
ma
l
O2
1/
4
60
No
rm
Interstitial fluid PO2 (mm Hg)
100
40
O
al
A
4
2
s
co n
ump
tion
al O 2
orm
n
¥
B
ption
sum
con
20
C
0
0
100 200 300 400 500 600
Blood flow (per cent of normal)
700
Figure 40–4
Effect of blood flow and rate of oxygen consumption on tissue PO2.
blood into the tissues—so rapidly that the capillary Po2
falls almost to equal the 40 mm Hg pressure in the
interstitium. Therefore, the Po2 of the blood leaving
the tissue capillaries and entering the systemic veins is
also about 40 mm Hg.
Effect of Rate of Blood Flow on Interstitial Fluid PO2. If the
blood flow through a particular tissue is increased,
greater quantities of oxygen are transported into the
tissue, and the tissue Po2 becomes correspondingly
higher. This is shown in Figure 40–4. Note that an
increase in flow to 400 per cent of normal increases
the Po2 from 40 mm Hg (at point A in the figure) to
66 mm Hg (at point B). However, the upper limit to
which the Po2 can rise, even with maximal blood flow, is
95 mm Hg, because this is the oxygen pressure in the
arterial blood. Conversely, if blood flow through the
tissue decreases, the tissue Po2 also decreases, as shown
at point C.
Effect of Rate of Tissue Metabolism on Interstitial Fluid PO2. If
the cells use more oxygen for metabolism than normally, this reduces the interstitial fluid Po2. Figure 40–4
also demonstrates this effect, showing reduced interstitial fluid Po2 when the cellular oxygen consumption is
increased, and increased Po2 when consumption is
decreased.
Respiration
In summary, tissue Po2 is determined by a balance
between (1) the rate of oxygen transport to the tissues
in the blood and (2) the rate at which the oxygen is used
by the tissues.
Diffusion of Oxygen from the
Peripheral Capillaries to the
Tissue Cells
Oxygen is always being used by the cells. Therefore,
the intracellular Po2 in the peripheral tissue cells
remains lower than the Po2 in the peripheral capillaries. Also, in many instances, there is considerable physical distance between the capillaries and the cells.
Therefore, the normal intracellular Po2 ranges from
as low as 5 mm Hg to as high as 40 mm Hg, averaging
(by direct measurement in lower animals) 23 mm Hg.
Because only 1 to 3 mm Hg of oxygen pressure is
normally required for full support of the chemical
processes that use oxygen in the cell, one can see that
even this low intracellular Po2 of 23 mm Hg is more
than adequate and provides a large safety factor.
Diffusion of Carbon Dioxide from
the Peripheral Tissue Cells into the
Capillaries and from the Pulmonary
Capillaries into the Alveoli
When oxygen is used by the cells, virtually all of it
becomes carbon dioxide, and this increases the intracellular Pco2; because of this high tissue cell Pco2,
carbon dioxide diffuses from the cells into the tissue
capillaries and is then carried by the blood to the lungs.
In the lungs, it diffuses from the pulmonary capillaries
into the alveoli and is expired.
Thus, at each point in the gas transport chain, carbon
dioxide diffuses in the direction exactly opposite to the
diffusion of oxygen. Yet there is one major difference
between diffusion of carbon dioxide and of oxygen:
carbon dioxide can diffuse about 20 times as rapidly as
oxygen. Therefore, the pressure differences required to
cause carbon dioxide diffusion are, in each instance, far
less than the pressure differences required to cause
oxygen diffusion. The CO2 pressures are approximately the following:
1. Intracellular Pco2, 46 mm Hg; interstitial Pco2,
45 mm Hg. Thus, there is only a 1 mm Hg pressure
differential, as shown in Figure 40–5.
2. Pco2 of the arterial blood entering the tissues,
40 mm Hg; Pco2 of the venous blood leaving the
tissues, 45 mm Hg. Thus, as shown in Figure 40–5,
the tissue capillary blood comes almost exactly
to equilibrium with the interstitial Pco2 of
45 mm Hg.
3. Pco2 of the blood entering the pulmonary
capillaries at the arterial end, 45 mm Hg; Pco2 of
the alveolar air, 40 mm Hg. Thus, only a 5 mm Hg
pressure difference causes all the required carbon
dioxide diffusion out of the pulmonary capillaries
Arterial end
of capillary
PCO2 = 40 mm Hg
Venous end
of capillary
45 mm Hg
46 mm Hg
505
Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
120
PCO2 = 45 mm Hg
Interstitial fluid PCO2 (mm Hg)
Chapter 40
100
Figure 40–5
Uptake of carbon dioxide by the blood in the tissue capillaries.
(PCO2 in tissue cells = 46 mm Hg, and in interstitial fluid =
45 mm Hg.)
Alveolus PCO2 = 40 mm Hg
80
10⫻ normal metabolism
B
60
A Normal metabolism
40
1/4
Lower limit of infinite blood flow
normal metabolism
C
20
0
0
Pulmonary Capillary
PCO2 = 40 mm Hg
PCO2 = 45 mm Hg
Arterial End
Venous End
100
200
300
400
500
Blood flow (per cent of normal)
600
Blood PCO2 (mm Hg)
45
Figure 40–7
44
Effect of blood flow and metabolic rate on peripheral tissue PCO2.
43
42
41
Pulmonary capillary blood
40
Alveolar carbon dioxide partial pressure
Figure 40–6
Diffusion of carbon dioxide from the pulmonary blood into the alveolus. (This curve was constructed from data in Milhorn HT Jr,
Pulley PE Jr: A theoretical study of pulmonary capillary gas
exchange and venous admixture. Biophys J 8:337, 1968.)
Pco2 from the normal value of 45 mm Hg to
41 mm Hg, down to a level almost equal to the
Pco2 in the arterial blood (40 mm Hg) entering
the tissue capillaries.
2. Note also that a 10-fold increase in tissue
metabolic rate greatly elevates the interstitial fluid
Pco2 at all rates of blood flow, whereas decreasing
the metabolism to one quarter normal causes the
interstitial fluid Pco2 to fall to about 41 mm Hg,
closely approaching that of the arterial blood,
40 mm Hg.
Role of Hemoglobin in Oxygen
Transport
into the alveoli. Furthermore, as shown in Figure
40–6, the Pco2 of the pulmonary capillary blood
falls to almost exactly equal the alveolar Pco2 of
40 mm Hg before it has passed more than about
one third the distance through the capillaries. This
is the same effect that was observed earlier for
oxygen diffusion, except that it is in the opposite
direction.
Effect of Rate of Tissue Metabolism and Tissue Blood Flow on
Interstitial PCO2. Tissue capillary blood flow and tissue
metabolism affect the Pco2 in ways exactly opposite to
their effect on tissue Po2. Figure 40–7 shows these
effects, as follows:
1. A decrease in blood flow from normal (point A)
to one quarter normal (point B) increases
peripheral tissue Pco2 from the normal value of
45 mm Hg to an elevated level of 60 mm Hg.
Conversely, increasing the blood flow to six
times normal (point C) decreases the interstitial
Normally, about 97 per cent of the oxygen transported
from the lungs to the tissues is carried in chemical
combination with hemoglobin in the red blood cells.
The remaining 3 per cent is transported in the dissolved state in the water of the plasma and blood cells.
Thus, under normal conditions, oxygen is carried to the
tissues almost entirely by hemoglobin.
Reversible Combination of Oxygen
with Hemoglobin
The chemistry of hemoglobin is presented in Chapter
32, where it was pointed out that the oxygen molecule
combines loosely and reversibly with the heme portion
of hemoglobin. When Po2 is high, as in the pulmonary
capillaries, oxygen binds with the hemoglobin, but
when Po2 is low, as in the tissue capillaries, oxygen is
released from the hemoglobin. This is the basis for
506
Unit VII
Respiration
20
Hemoglobin saturation (%)
90
Oxygenated blood
leaving the lungs
80
18
16
70
14
60
12
10
50
Reduced blood returning
from tissues
40
8
30
6
20
4
10
2
Volumes (%)
100
0
0
0
10 20 30 40 50 60 70 80 90 100 110 120 130 140
Pressure of oxygen in blood (PO2) (mm Hg)
Figure 40–8
Oxygen-hemoglobin dissociation curve.
O2
wi
und
bo
oglobin
th hem
Normal arterial blood
20
18
16
14
12
10
8
6
4
2
0
Normal venous blood
the oxygen-hemoglobin dissociation curve, which
demonstrates a progressive increase in the percentage
of hemoglobin bound with oxygen as blood Po2
increases, which is called the per cent saturation of
hemoglobin. Because the blood leaving the lungs and
entering the systemic arteries usually has a Po2 of
about 95 mm Hg, one can see from the dissociation
curve that the usual oxygen saturation of systemic arterial blood averages 97 per cent. Conversely, in normal
venous blood returning from the peripheral tissues, the
Po2 is about 40 mm Hg, and the saturation of hemoglobin averages 75 per cent.
Venous blood in exercise
Oxygen-Hemoglobin Dissociation Curve. Figure 40–8 shows
Oxygen in blood (volumes %)
almost all oxygen transport from the lungs to the
tissues.
20
40
0
60
80 100 120 140
Pressure of oxygen in blood (PO2) (mm Hg)
Maximum Amount of Oxygen That Can Combine with the
Hemoglobin of the Blood. The blood of a normal person
contains about 15 grams of hemoglobin in each 100
milliliters of blood, and each gram of hemoglobin can
bind with a maximum of 1.34 milliliters of oxygen (1.39
milliliters when the hemoglobin is chemically pure, but
impurities such as methemoglobin reduce this). Therefore, 15 times 1.34 equals 20.1, which means that, on
average, the 15 grams of hemoglobin in 100 milliliters
of blood can combine with a total of almost exactly 20
milliliters of oxygen if the hemoglobin is 100 per cent
saturated. This is usually expressed as 20 volumes per
cent. The oxygen-hemoglobin dissociation curve for
the normal person can also be expressed in terms of
volume per cent of oxygen, as shown by the far right
scale in Figure 40–8, instead of per cent saturation of
hemoglobin.
Amount of Oxygen Released from the Hemoglobin When Systemic Arterial Blood Flows Through the Tissues. The total
quantity of oxygen bound with hemoglobin in normal
Figure 40–9
Effect of blood PO2 on the quantity of oxygen bound with hemoglobin in each 100 milliliters of blood.
systemic arterial blood, which is 97 per cent saturated,
is about 19.4 milliliters per 100 milliliters of blood. This
is shown in Figure 40–9. On passing through the tissue
capillaries, this amount is reduced, on average, to
14.4 milliliters (Po2 of 40 mm Hg, 75 per cent saturated
hemoglobin). Thus, under normal conditions, about 5
milliliters of oxygen are transported from the lungs to
the tissues by each 100 milliliters of blood flow.
Transport of Oxygen During Strenuous Exercise. During
heavy exercise, the muscle cells use oxygen at a rapid
rate, which, in extreme cases, can cause the muscle
interstitial fluid Po2 to fall from the normal 40 mm Hg
Chapter 40
Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
to as low as 15 mm Hg. At this low pressure, only 4.4
milliliters of oxygen remain bound with the hemoglobin in each 100 milliliters of blood, as shown in Figure
40–9. Thus, 19.4 – 4.4, or 15 milliliters, is the quantity
of oxygen actually delivered to the tissues by each 100
milliliters of blood flow. Thus, three times as much
oxygen as normal is delivered in each volume of blood
that passes through the tissues. And keep in mind that
the cardiac output can increase to six to seven times
normal in well-trained marathon runners. Thus, multiplying the increase in cardiac output (six- to sevenfold)
by the increase in oxygen transport in each volume of
blood (threefold) gives a 20-fold increase in oxygen
transport to the tissues. We see later in the chapter
that several other factors facilitate delivery of oxygen
into muscles during exercise, so that muscle tissue Po2
often falls very little below normal even during very
strenuous exercise.
Utilization Coefficient. The percentage of the blood that
gives up its oxygen as it passes through the tissue capillaries is called the utilization coefficient. The normal
value for this is about 25 per cent, as is evident from
the preceding discussion—that is, 25 per cent of the
oxygenated hemoglobin gives its oxygen to the tissues.
During strenuous exercise, the utilization coefficient in
the entire body can increase to 75 to 85 per cent. And
in local tissue areas where blood flow is extremely
slow or the metabolic rate is very high, utilization
coefficients approaching 100 per cent have been
recorded—that is, essentially all the oxygen is given to
the tissues.
Effect of Hemoglobin to “Buffer” the
Tissue PO2
Although hemoglobin is necessary for the transport of
oxygen to the tissues, it performs another function
essential to life. This is its function as a “tissue oxygen
buffer” system. That is, the hemoglobin in the blood is
mainly responsible for stabilizing the oxygen pressure
in the tissues. This can be explained as follows.
Role of Hemoglobin in Maintaining Nearly Constant PO2 in the
Tissues. Under basal conditions, the tissues require
about 5 milliliters of oxygen from each 100 milliliters
of blood passing through the tissue capillaries. Referring back to the oxygen-hemoglobin dissociation curve
in Figure 40–9, one can see that for the normal 5 milliliters of oxygen to be released per 100 milliliters of
blood flow, the Po2 must fall to about 40 mm Hg.
Therefore, the tissue Po2 normally cannot rise above
this 40 mm Hg level, because if it did, the amount of
oxygen needed by the tissues would not be released
from the hemoglobin. In this way, the hemoglobin normally sets an upper limit on the oxygen pressure in the
tissues at about 40 mm Hg.
Conversely, during heavy exercise, extra amounts of
oxygen (as much as 20 times normal) must be delivered from the hemoglobin to the tissues. But this can
be achieved with little further decrease in tissue Po2
507
because of (1) the steep slope of the dissociation curve
and (2) the increase in tissue blood flow caused by the
decreased Po2; that is, a very small fall in Po2 causes
large amounts of extra oxygen to be released from the
hemoglobin. It can be seen, then, that the hemoglobin
in the blood automatically delivers oxygen to the
tissues at a pressure that is held rather tightly between
about 15 and 40 mm Hg.
When Atmospheric Oxygen Concentration Changes Markedly,
the Buffer Effect of Hemoglobin Still Maintains Almost Constant
Tissue PO2. The normal Po2 in the alveoli is about
104 mm Hg, but as one ascends a mountain or ascends
in an airplane, the Po2 can easily fall to less than half
this amount. Alternatively, when one enters areas of
compressed air, such as deep in the sea or in pressurized chambers, the Po2 may rise to 10 times this level.
Even so, the tissue Po2 changes little.
It can be seen from the oxygen-hemoglobin dissociation curve in Figure 40–8 that when the alveolar Po2
is decreased to as low as 60 mm Hg, the arterial hemoglobin is still 89 per cent saturated with oxygen—only
8 per cent below the normal saturation of 97 per cent.
Further, the tissues still remove about 5 milliliters of
oxygen from each 100 milliliters of blood passing
through the tissues; to remove this oxygen, the Po2
of the venous blood falls to 35 mm Hg—only 5 mm Hg
below the normal value of 40 mm Hg. Thus, the tissue
Po2 hardly changes, despite the marked fall in alveolar
Po2 from 104 to 60 mm Hg.
Conversely, when the alveolar Po2 rises as high as
500 mm Hg, the maximum oxygen saturation of
hemoglobin can never rise above 100 per cent, which
is only 3 per cent above the normal level of 97 per cent.
Only a small amount of additional oxygen dissolves in
the fluid of the blood, as will be discussed subsequently. Then, when the blood passes through the
tissue capillaries and loses several milliliters of oxygen
to the tissues, this reduces the Po2 of the capillary
blood to a value only a few millimeters greater than
the normal 40 mm Hg. Consequently, the level of alveolar oxygen may vary greatly—from 60 to more than
500 mm Hg Po2—and still the Po2 in the peripheral
tissues does not vary more than a few millimeters from
normal, demonstrating beautifully the tissue “oxygen
buffer” function of the blood hemoglobin system.
Factors That Shift the OxygenHemoglobin Dissociation Curve—
Their Importance for Oxygen
Transport
The oxygen-hemoglobin dissociation curves of Figures
40–8 and 40–9 are for normal, average blood.
However, a number of factors can displace the dissociation curve in one direction or the other in the
manner shown in Figure 40–10. This figure shows
that when the blood becomes slightly acidic, with the
pH decreasing from the normal value of 7.4 to 7.2, the
oxygen-hemoglobin dissociation curve shifts, on
508
Unit VII
oxygen-hemoglobin dissociation curve to the left and
upward. Therefore, the quantity of oxygen that binds
with the hemoglobin at any given alveolar Po2
becomes considerably increased, thus allowing greater
oxygen transport to the tissues.
100
90
80
pH
70
60
Effect of BPG to Shift the Oxygen-Hemoglobin Dissociation
Curve. The normal BPG in the blood keeps the
7.6
7.
7.2 4
Hemoglobin saturation (%)
Respiration
Shift to right:
(1) Increased hydrogen ions
(2) Increased CO2
(3) Increased temperature
(4) Increased BPG
50
40
30
20
10
0
0 10 20 30 40 50 60 70 80 90 100110 120130 140
Pressure of oxygen in blood (PO2) (mm Hg)
Figure 40–10
Shift of the oxygen-hemoglobin dissociation curve to the right
caused by an increase in hydrogen ion concentration (decrease
in pH). BPG, 2,3-biphosphoglycerate.
average, about 15 per cent to the right. Conversely, an
increase in pH from the normal 7.4 to 7.6 shifts the
curve a similar amount to the left.
In addition to pH changes, several other factors are
known to shift the curve. Three of these, all of which
shift the curve to the right, are (1) increased carbon
dioxide concentration, (2) increased blood temperature, and (3) increased 2,3-biphosphoglycerate (BPG),
a metabolically important phosphate compound
present in the blood in different concentrations under
different metabolic conditions.
Increased Delivery of Oxygen to the Tissues When Carbon
Dioxide and Hydrogen Ions Shift the Oxygen-Hemoglobin Dissociation Curve—The Bohr Effect. A shift of the oxygen-
hemoglobin dissociation curve to the right in response
to increases in blood carbon dioxide and hydrogen
ions has a significant effect by enhancing the release
of oxygen from the blood in the tissues and enhancing
oxygenation of the blood in the lungs. This is called the
Bohr effect, which can be explained as follows: As the
blood passes through the tissues, carbon dioxide diffuses from the tissue cells into the blood.This increases
the blood Po2, which in turn raises the blood H2CO3
(carbonic acid) and the hydrogen ion concentration.
These effects shift the oxygen-hemoglobin dissociation curve to the right and downward, as shown in
Figure 40–10, forcing oxygen away from the hemoglobin and therefore delivering increased amounts of
oxygen to the tissues.
Exactly the opposite effects occur in the lungs,
where carbon dioxide diffuses from the blood into
the alveoli. This reduces the blood Pco2 and decreases
the hydrogen ion concentration, shifting the
oxygen-hemoglobin dissociation curve shifted slightly
to the right all the time. In hypoxic conditions that
last longer than a few hours, the quantity of BPG in
the blood increases considerably, thus shifting the
oxygen-hemoglobin dissociation curve even farther to
the right. This causes oxygen to be released to the
tissues at as much as 10 mm Hg higher tissue oxygen
pressure than would be the case without this increased
BPG. Therefore, under some conditions, the BPG
mechanism can be important for adaptation to
hypoxia, especially to hypoxia caused by poor tissue
blood flow.
Shift of the Dissociation Curve During Exercise. During exercise, several factors shift the dissociation curve considerably to the right, thus delivering extra amounts
of oxygen to the active, exercising muscle fibers. The
exercising muscles, in turn, release large quantities of
carbon dioxide; this and several other acids released
by the muscles increase the hydrogen ion concentration in the muscle capillary blood. In addition, the temperature of the muscle often rises 2° to 3°C, which can
increase oxygen delivery to the muscle fibers even
more.All these factors act together to shift the oxygenhemoglobin dissociation curve of the muscle capillary
blood considerably to the right. This right-hand shift
of the curve forces oxygen to be released from the
blood hemoglobin to the muscle at Po2 levels as great
as 40 mm Hg, even when 70 per cent of the oxygen has
already been removed from the hemoglobin. Then, in
the lungs, the shift occurs in the opposite direction,
allowing the pickup of extra amounts of oxygen from
the alveoli.
Metabolic Use of Oxygen by the Cells
Effect of Intracellular PO2 on Rate of Oxygen Usage. Only
a minute level of oxygen pressure is required in the
cells for normal intracellular chemical reactions to
take place. The reason for this is that the respiratory
enzyme systems of the cell, which are discussed in
Chapter 67, are geared so that when the cellular Po2 is
more than 1 mm Hg, oxygen availability is no longer a
limiting factor in the rates of the chemical reactions.
Instead, the main limiting factor is the concentration
of adenosine diphosphate (ADP) in the cells. This
effect is demonstrated in Figure 40–11, which shows
the relation between intracellular Po2 and the rate of
oxygen usage at different concentrations of ADP. Note
that whenever the intracellular Po2 is above 1 mm Hg,
the rate of oxygen usage becomes constant for any
given concentration of ADP in the cell. Conversely,
when the ADP concentration is altered, the rate of
Chapter 40
Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
oxygen that can be transported to the tissue in each
100 milliliters of blood and (2) the rate of blood flow.
If the rate of blood flow falls to zero, the amount of
available oxygen also falls to zero. Thus, there are
times when the rate of blood flow through a tissue
can be so low that tissue Po2 falls below the critical
1 mm Hg required for intracellular metabolism. Under
these conditions, the rate of tissue usage of oxygen
is blood flow limited. Neither diffusion-limited nor
blood flow–limited oxygen states can continue for
long, because the cells receive less oxygen than is
required to continue the life of the cells.
ADP = 11/2 normal
1.5
Rate of oxygen usage
(times normal resting level)
509
ADP = Normal resting level
1.0
ADP = 1/2 normal
0.5
Transport of Oxygen in the
Dissolved State
0
0
2
3
1
Intracellular PO2 (mm Hg)
4
Figure 40–11
Effect of intracellular adenosine diphosphate (ADP) and PO2 on
rate of oxygen usage by the cells. Note that as long as the intracellular PO2 remains above 1 mm Hg, the controlling factor for the
rate of oxygen usage is the intracellular concentration of ADP.
oxygen usage changes in proportion to the change in
ADP concentration.
As explained in Chapter 3, when adenosine triphosphate (ATP) is used in the cells to provide energy, it is
converted into ADP. The increasing concentration
of ADP increases the metabolic usage of oxygen as
it combines with the various cell nutrients, releasing
energy that reconverts the ADP back to ATP. Under
normal operating conditions, the rate of oxygen usage
by the cells is controlled ultimately by the rate of energy
expenditure within the cells—that is, by the rate at which
ADP is formed from ATP.
Effect of Diffusion Distance from the Capillary to the Cell on
Oxygen Usage. Tissue cells are seldom more than 50
micrometers away from a capillary, and oxygen normally can diffuse readily enough from the capillary to
the cell to supply all the required amounts of oxygen
for metabolism. However, occasionally, cells are
located farther from the capillaries, and the rate of
oxygen diffusion to these cells can become so low that
intracellular Po2 falls below the critical level required
to maintain maximal intracellular metabolism. Thus,
under these conditions, oxygen usage by the cells is
said to be diffusion limited and is no longer determined by the amount of ADP formed in the cells.
But this almost never occurs, except in pathological
states.
Effect of Blood Flow on Metabolic Use of Oxygen. The total
amount of oxygen available each minute for use in
any given tissue is determined by (1) the quantity of
At the normal arterial Po2 of 95 mm Hg, about 0.29 milliliter of oxygen is dissolved in every 100 milliliters of
water in the blood, and when the Po2 of the blood falls
to the normal 40 mm Hg in the tissue capillaries, only
0.12 milliliter of oxygen remains dissolved. In other
words, 0.17 milliliter of oxygen is normally transported
in the dissolved state to the tissues by each 100 milliliters of arterial blood flow. This compares with almost
5 milliliters of oxygen transported by the red cell hemoglobin. Therefore, the amount of oxygen transported to
the tissues in the dissolved state is normally slight, only
about 3 per cent of the total, as compared with 97 per
cent transported by the hemoglobin.
During strenuous exercise, when hemoglobin release
of oxygen to the tissues increases another threefold, the
relative quantity of oxygen transported in the dissolved
state falls to as little as 1.5 per cent. But if a person
breathes oxygen at very high alveolar Po2 levels, the
amount transported in the dissolved state can become
much greater, sometimes so much so that a serious
excess of oxygen occurs in the tissues, and “oxygen poisoning” ensues. This often leads to brain convulsions
and even death, as discussed in detail in Chapter 44
in relation to the high-pressure breathing of oxygen
among deep-sea divers.
Combination of Hemoglobin with
Carbon Monoxide—Displacement
of Oxygen
Carbon monoxide combines with hemoglobin at the
same point on the hemoglobin molecule as does oxygen;
it can therefore displace oxygen from the hemoglobin,
thereby decreasing the oxygen carrying capacity of
blood. Further, it binds with about 250 times as much
tenacity as oxygen, which is demonstrated by the carbon
monoxide–hemoglobin dissociation curve in Figure
40–12. This curve is almost identical to the oxygenhemoglobin dissociation curve, except that the carbon
monoxide partial pressures, shown on the abscissa, are
at a level 1/250 of those for the oxygen-hemoglobin dissociation curve of Figure 40–8. Therefore, a carbon
monoxide partial pressure of only 0.4 mm Hg in the
alveoli, 1/250 that of normal alveolar oxygen (100 mm Hg
Po2), allows the carbon monoxide to compete equally
with the oxygen for combination with the hemoglobin
and causes half the hemoglobin in the blood to become
bound with carbon monoxide instead of with oxygen.
Therefore, a carbon monoxide pressure of only
510
Unit VII
Respiration
Capillary
Hemoglobin saturation (%)
Interstitial
fluid
Red blood cell
100
90
Cell
Hgb • CO2
80
Carbonic
Hgb
anhydrase
+
H2CO3
H2O + CO2
70
60
HCO3– + H+
+
H2O
Hgb
Cl
50
40
30
H2O
20
Cl
10
HHgb
HCO3–
Plasma
0
0.1
0.3
0.4
0
0.2
Gas pressure of carbon monoxide (mm Hg)
CO2
CO2
CO2
CO2 transported as:
1. CO2
= 7%
2. Hgb • CO2 = 23%
3. HCO3ⴚ
= 70%
Figure 40–13
Transport of carbon dioxide in the blood.
Figure 40–12
Carbon monoxide–hemoglobin dissociation curve. Note the
extremely low carbon monoxide pressures at which carbon
monoxide combines with hemoglobin.
0.6 mm Hg (a volume concentration of less than one
part per thousand in air) can be lethal.
Even though the oxygen content of blood is greatly
reduced in carbon monoxide poisoning, the Po2 of the
blood may be normal. This makes exposure to carbon
monoxide especially dangerous, because the blood is
bright red and there are no obvious signs of hypoxemia,
such as a bluish color of the fingertips or lips (cyanosis).
Also, Po2 is not reduced, and the feedback mechanism
that usually stimulates increased respiration rate in
response to lack of oxygen (usually reflected by a low
Po2) is absent. Because the brain is one of the first
organs affected by lack of oxygen, the person may
become disoriented and unconscious before becoming
aware of the danger.
A patient severely poisoned with carbon monoxide
can be treated by administering pure oxygen, because
oxygen at high alveolar pressure can displace carbon
monoxide rapidly from its combination with hemoglobin. The patient can also benefit from simultaneous
administration of 5 per cent carbon dioxide, because
this strongly stimulates the respiratory center, which
increases alveolar ventilation and reduces the alveolar
carbon monoxide. With intensive oxygen and carbon
dioxide therapy, carbon monoxide can be removed from
the blood as much as 10 times as rapidly as without
therapy.
Transport of Carbon Dioxide
in the Blood
Transport of carbon dioxide by the blood is not nearly
as problematical as transport of oxygen is, because
even in the most abnormal conditions, carbon dioxide
can usually be transported in far greater quantities
than oxygen can be. However, the amount of carbon
dioxide in the blood has a lot to do with the acid-base
balance of the body fluids, which is discussed in
Chapter 30. Under normal resting conditions, an
average of 4 milliliters of carbon dioxide is transported
from the tissues to the lungs in each 100 milliliters of
blood.
Chemical Forms in Which Carbon
Dioxide Is Transported
To begin the process of carbon dioxide transport,
carbon dioxide diffuses out of the tissue cells in the
dissolved molecular carbon dioxide form. On entering
the tissue capillaries, the carbon dioxide initiates a host
of almost instantaneous physical and chemical reactions, shown in Figure 40–13, which are essential for
carbon dioxide transport.
Transport of Carbon Dioxide in the Dissolved State. A small
portion of the carbon dioxide is transported in the dissolved state to the lungs. Recall that the Pco2 of venous
blood is 45 mm Hg and that of arterial blood is
40 mm Hg. The amount of carbon dioxide dissolved in
the fluid of the blood at 45 mm Hg is about 2.7 ml/dl
(2.7 volumes per cent). The amount dissolved at
40 mm Hg is about 2.4 milliliters, or a difference of 0.3
milliliter. Therefore, only about 0.3 milliliter of carbon
dioxide is transported in the dissolved form by each
100 milliliters of blood flow. This is about 7 per cent of
all the carbon dioxide normally transported.
Transport of Carbon Dioxide in the Form of Bicarbonate Ion
Reaction of Carbon Dioxide with Water in the Red
Blood Cells—Effect of Carbonic Anhydrase. The dis-
solved carbon dioxide in the blood reacts with water
to form carbonic acid. This reaction would occur
much too slowly to be of importance were it not for
the fact that inside the red blood cells is a protein
enzyme called carbonic anhydrase, which catalyzes the
Chapter 40
Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
reaction between carbon dioxide and water and accelerates its reaction rate about 5000-fold. Therefore,
instead of requiring many seconds or minutes to occur,
as is true in the plasma, the reaction occurs so rapidly
in the red blood cells that it reaches almost complete
equilibrium within a very small fraction of a second.
This allows tremendous amounts of carbon dioxide to
react with the red blood cell water even before the
blood leaves the tissue capillaries.
Dissociation of Carbonic Acid into Bicarbonate and
Hydrogen Ions. In another fraction of a second, the
carbonic acid formed in the red cells (H2CO3) dissociates into hydrogen and bicarbonate ions (H+ and
HCO3–). Most of the hydrogen ions then combine with
the hemoglobin in the red blood cells, because the
hemoglobin protein is a powerful acid-base buffer. In
turn, many of the bicarbonate ions diffuse from the red
cells into the plasma, while chloride ions diffuse into
the red cells to take their place. This is made possible
by the presence of a special bicarbonate-chloride
carrier protein in the red cell membrane that shuttles
these two ions in opposite directions at rapid velocities. Thus, the chloride content of venous red blood
cells is greater than that of arterial red cells, a phenomenon called the chloride shift.
The reversible combination of carbon dioxide with
water in the red blood cells under the influence of carbonic anhydrase accounts for about 70 per cent of the
carbon dioxide transported from the tissues to the
lungs. Thus, this means of transporting carbon dioxide
is by far the most important. Indeed, when a carbonic
anhydrase inhibitor (acetazolamide) is administered
to an animal to block the action of carbonic anhydrase
in the red blood cells, carbon dioxide transport from
the tissues becomes so poor that the tissue Pco2 can be
made to rise to 80 mm Hg instead of the normal
45 mm Hg.
dioxide with water inside the red blood cells, it is
doubtful that under normal conditions this carbamino
mechanism transports more than 20 per cent of the
total carbon dioxide.
Carbon Dioxide Dissociation Curve
The curve shown in Figure 40–14—called the carbon
dioxide dissociation curve—depicts the dependence
of total blood carbon dioxide in all its forms on Pco2.
Note that the normal blood Pco2 ranges between the
limits of 40 mm Hg in arterial blood and 45 mm Hg in
venous blood, which is a very narrow range. Note also
that the normal concentration of carbon dioxide in the
blood in all its different forms is about 50 volumes per
cent, but only 4 volumes per cent of this is exchanged
during normal transport of carbon dioxide from the
tissues to the lungs. That is, the concentration rises to
about 52 volumes per cent as the blood passes through
the tissues and falls to about 48 volumes per cent as it
passes through the lungs.
When Oxygen Binds with
Hemoglobin, Carbon Dioxide Is
Released (the Haldane Effect)
to Increase CO2 Transport
Earlier in the chapter, it was pointed out that an
increase in carbon dioxide in the blood causes oxygen
to be displaced from the hemoglobin (the Bohr effect),
which is an important factor in increasing oxygen
transport. The reverse is also true: binding of oxygen
with hemoglobin tends to displace carbon dioxide
from the blood. Indeed, this effect, called the Haldane
effect, is quantitatively far more important in
CO2 in blood (volumes per cent)
Transport of Carbon Dioxide in Combination with Hemoglobin
and Plasma Proteins—Carbaminohemoglobin. In addition to
80
70
60
50
40
30
20
10
Normal operating range
reacting with water, carbon dioxide reacts directly with
amine radicals of the hemoglobin molecule to form
the compound carbaminohemoglobin (CO2Hgb). This
combination of carbon dioxide and hemoglobin is a
reversible reaction that occurs with a loose bond, so
that the carbon dioxide is easily released into the
alveoli, where the Pco2 is lower than in the pulmonary
capillaries.
A small amount of carbon dioxide also reacts in the
same way with the plasma proteins in the tissue capillaries. This is much less significant for the transport of
carbon dioxide because the quantity of these proteins
in the blood is only one fourth as great as the quantity
of hemoglobin.
The quantity of carbon dioxide that can be carried
from the peripheral tissues to the lungs by carbamino
combination with hemoglobin and plasma proteins is
about 30 per cent of the total quantity transported—
that is, normally about 1.5 milliliters of carbon dioxide
in each 100 milliliters of blood. However, because this
reaction is much slower than the reaction of carbon
511
0
0 10 20 30 40 50 60 70 80 90 100 110 120
Gas pressure of carbon dioxide (mm Hg)
Figure 40–14
Carbon dioxide dissociation curve.
512
Unit VII
falls to 48 volumes per cent (point B). This represents
an additional 2 volumes per cent loss of carbon
dioxide. Thus, the Haldane effect approximately
doubles the amount of carbon dioxide released from
the blood in the lungs and approximately doubles the
pickup of carbon dioxide in the tissues.
55
CO2 in blood (volumes per cent)
Respiration
A
PO2 = 40 mm Hg
Change in Blood Acidity During
Carbon Dioxide Transport
50
PO2 = 100 mm Hg
B
45
35
40
45
50
PCO2
Figure 40–15
Portions of the carbon dioxide dissociation curve when the PO2 is
100 mm Hg or 40 mm Hg. The arrow represents the Haldane
effect on the transport of carbon dioxide, as discussed in the text.
promoting carbon dioxide transport than is the Bohr
effect in promoting oxygen transport.
The Haldane effect results from the simple fact that
the combination of oxygen with hemoglobin in the
lungs causes the hemoglobin to become a stronger
acid. This displaces carbon dioxide from the blood and
into the alveoli in two ways: (1) The more highly acidic
hemoglobin has less tendency to combine with carbon
dioxide to form carbaminohemoglobin, thus displacing
much of the carbon dioxide that is present in the carbamino form from the blood. (2) The increased acidity
of the hemoglobin also causes it to release an excess
of hydrogen ions, and these bind with bicarbonate
ions to form carbonic acid; this then dissociates into
water and carbon dioxide, and the carbon dioxide is
released from the blood into the alveoli and, finally,
into the air.
Figure 40–15 demonstrates quantitatively the significance of the Haldane effect on the transport of carbon
dioxide from the tissues to the lungs. This figure shows
small portions of two carbon dioxide dissociation
curves: (1) when the Po2 is 100 mm Hg, which is the
case in the blood capillaries of the lungs, and (2) when
the Po2 is 40 mm Hg, which is the case in the tissue capillaries. Point A shows that the normal Pco2 of
45 mm Hg in the tissues causes 52 volumes per cent of
carbon dioxide to combine with the blood. On entering the lungs, the Pco2 falls to 40 mm Hg and the Po2
rises to 100 mm Hg. If the carbon dioxide dissociation
curve did not shift because of the Haldane effect, the
carbon dioxide content of the blood would fall only to
50 volumes per cent, which would be a loss of only 2
volumes per cent of carbon dioxide. However, the
increase in Po2 in the lungs lowers the carbon dioxide
dissociation curve from the top curve to the lower
curve of the figure, so that the carbon dioxide content
The carbonic acid formed when carbon dioxide enters
the blood in the peripheral tissues decreases the blood
pH. However, reaction of this acid with the acid-base
buffers of the blood prevents the hydrogen ion concentration from rising greatly (and the pH from falling
greatly). Ordinarily, arterial blood has a pH of about
7.41, and as the blood acquires carbon dioxide in the
tissue capillaries, the pH falls to a venous value of about
7.37. In other words, a pH change of 0.04 unit takes
place. The reverse occurs when carbon dioxide is
released from the blood in the lungs, with the pH rising
to the arterial value of 7.41 once again. In heavy exercise or other conditions of high metabolic activity, or
when blood flow through the tissues is sluggish, the
decrease in pH in the tissue blood (and in the tissues
themselves) can be as much as 0.50, about 12 times
normal, thus causing significant tissue acidosis.
Respiratory Exchange Ratio
The discerning student will have noted that normal
transport of oxygen from the lungs to the tissues by each
100 milliliters of blood is about 5 milliliters, whereas
normal transport of carbon dioxide from the tissues
to the lungs is about 4 milliliters. Thus, under normal
resting conditions, only about 82 per cent as much
carbon dioxide is expired from the lungs as oxygen is
taken up by the lungs. The ratio of carbon dioxide
output to oxygen uptake is called the respiratory
exchange ratio (R). That is,
R=
Rate of carbon dioxide output
Rate of oxygen uptake
The value for R changes under different metabolic
conditions. When a person is using exclusively carbohydrates for body metabolism, R rises to 1.00. Conversely,
when a person is using exclusively fats for metabolic
energy, the R level falls to as low as 0.7. The reason
for this difference is that when oxygen is metabolized
with carbohydrates, one molecule of carbon dioxide is
formed for each molecule of oxygen consumed; when
oxygen reacts with fats, a large share of the oxygen
combines with hydrogen atoms from the fats to form
water instead of carbon dioxide. In other words, when
fats are metabolized, the respiratory quotient of the
chemical reactions in the tissues is about 0.70 instead of
1.00. (The tissue respiratory quotient is discussed in
Chapter 71.) For a person on a normal diet consuming
average amounts of carbohydrates, fats, and proteins,
the average value for R is considered to be 0.825.
References
Albert R, Spiro S, Jett J: Comprehensive Respiratory Medicine. Philadelphia: Mosby, 2002.
Chapter 40
Transport of Oxygen and Carbon Dioxide in Blood and Tissue Fluids
Dempsey JA, Wagner PD: Exercise-induced arterial hypoxemia. J Appl Physiol 87:1997, 1999.
Geers C, Gros G: Carbon dioxide transport and carbonic
anhydrase in blood and muscle. Physiol Rev 80:681,
2000.
Henry RP, Swenson ER: The distribution and physiological
significance of carbonic anhydrase in vertebrate gas
exchange organs. Respir Physiol 121:1, 2000.
Jones AM, Koppo K, Burnley M: Effects of prior exercise on
metabolic and gas exchange responses to exercise. Sports
Med 33:949, 2003.
Nikinmaa M: Membrane transport and control of
hemoglobin-oxygen affinity in nucleated erythrocytes.
Physiol Rev 72:301, 1992.
Piiper J: Perfusion, diffusion and their heterogeneities limiting blood-tissue O2 transfer in muscle. Acta Physiol Scand
168:603, 2000.
513
Richardson RS: Oxygen transport and utilization: an integration of the muscle systems. Adv Physiol Educ 27:183,
2003.
Roy TK, Popel AS: Theoretical predictions of end-capillary
Po2 in muscles of athletic and nonathletic animals at
Vo2max. Am J Physiol 271:H721, 1996.
Spahn DR, Pasch T: Physiological properties of blood substitutes. News Physiol Sci 16:38, 2001.
Tsai AG, Johnson PC, Intaglietta M: Oxygen gradients in the
microcirculation. Physiol Rev 83:933, 2003.
Wagner PD: Diffusive resistance to O2 transport in muscle.
Acta Physiol Scand 168:609, 2000.
West JB: Pulmonary Physiology and Pathophysiology: An
Integrated, Case-Based Approach. Philadelphia: Lippincott Williams & Wilkins, 2001.
West JB: Pulmonary Physiology—The Essentials. Baltimore:
Lippincott Williams & Wilkins, 2003.
C
H
A
P
T
E
R
4
Regulation of Respiration
The nervous system normally adjusts the rate of
alveolar ventilation almost exactly to the demands
of the body so that the oxygen pressure (Po2) and
carbon dioxide pressure (Pco2) in the arterial blood
are hardly altered even during heavy exercise and
most other types of respiratory stress. This chapter
describes the function of this neurogenic system for
regulation of respiration.
Respiratory Center
The respiratory center is composed of several groups of neurons located bilaterally in the medulla oblongata and pons of the brain stem, as shown in Figure
41–1. It is divided into three major collections of neurons: (1) a dorsal respiratory group, located in the dorsal portion of the medulla, which mainly causes
inspiration; (2) a ventral respiratory group, located in the ventrolateral part of
the medulla, which mainly causes expiration; and (3) the pneumotaxic center,
located dorsally in the superior portion of the pons, which mainly controls rate
and depth of breathing. The dorsal respiratory group of neurons plays the most
fundamental role in the control of respiration. Therefore, let us discuss its function first.
Dorsal Respiratory Group of Neurons—Its Control
of Inspiration and of Respiratory Rhythm
The dorsal respiratory group of neurons extends most of the length of the
medulla. Most of its neurons are located within the nucleus of the tractus solitarius, although additional neurons in the adjacent reticular substance of the
medulla also play important roles in respiratory control. The nucleus of the
tractus solitarius is the sensory termination of both the vagal and the glossopharyngeal nerves, which transmit sensory signals into the respiratory center
from (1) peripheral chemoreceptors, (2) baroreceptors, and (3) several types of
receptors in the lungs.
Rhythmical Inspiratory Discharges from the Dorsal Respiratory Group. The basic rhythm
of respiration is generated mainly in the dorsal respiratory group of neurons.
Even when all the peripheral nerves entering the medulla have been sectioned
and the brain stem transected both above and below the medulla, this group of
neurons still emits repetitive bursts of inspiratory neuronal action potentials. The
basic cause of these repetitive discharges is unknown. In primitive animals,
neural networks have been found in which activity of one set of neurons excites
a second set, which in turn inhibits the first. Then, after a period of time, the
mechanism repeats itself, continuing throughout the life of the animal. Therefore, most respiratory physiologists believe that some similar network of
neurons is present in the human being, located entirely within the medulla; it
probably involves not only the dorsal respiratory group but adjacent areas of
the medulla as well, and is responsible for the basic rhythm of respiration.
Inspiratory “Ramp” Signal. The nervous signal that is transmitted to the inspiratory muscles, mainly the diaphragm, is not an instantaneous burst of action
514
1
Chapter 41
Regulation of Respiration
Pneumotaxic center
Fourth ventricle
Inhibits
? Apneustic center
Dorsal respiratory
group (inspiration)
Vagus and
glossopharyngeal
Ventral respiratory
group (expiration
and inspiration)
Respiratory motor
pathways
Figure 41–1
Organization of the respiratory center.
potentials. Instead, in normal respiration, it begins
weakly and increases steadily in a ramp manner for
about 2 seconds. Then it ceases abruptly for approximately the next 3 seconds, which turns off the excitation of the diaphragm and allows elastic recoil of the
lungs and the chest wall to cause expiration. Next, the
inspiratory signal begins again for another cycle; this
cycle repeats again and again, with expiration occurring in between. Thus, the inspiratory signal is a ramp
signal. The obvious advantage of the ramp is that it
causes a steady increase in the volume of the lungs
during inspiration, rather than inspiratory gasps.
There are two qualities of the inspiratory ramp that
are controlled, as follows:
1. Control of the rate of increase of the ramp signal,
so that during heavy respiration, the ramp
increases rapidly and therefore fills the lungs
rapidly.
2. Control of the limiting point at which the ramp
suddenly ceases. This is the usual method for
controlling the rate of respiration; that is, the
earlier the ramp ceases, the shorter the duration
of inspiration. This also shortens the duration of
expiration. Thus, the frequency of respiration is
increased.
A Pneumotaxic Center Limits the
Duration of Inspiration and Increases
the Respiratory Rate
A pneumotaxic center, located dorsally in the nucleus
parabrachialis of the upper pons, transmits signals to
the inspiratory area. The primary effect of this center
is to control the “switch-off” point of the inspiratory
ramp, thus controlling the duration of the filling phase
of the lung cycle. When the pneumotaxic signal is
strong, inspiration might last for as little as 0.5 second,
thus filling the lungs only slightly; when the pneumotaxic signal is weak, inspiration might continue for 5
515
or more seconds, thus filling the lungs with a great
excess of air.
The function of the pneumotaxic center is primarily
to limit inspiration. This has a secondary effect of
increasing the rate of breathing, because limitation
of inspiration also shortens expiration and the entire
period of each respiration. A strong pneumotaxic
signal can increase the rate of breathing to 30 to 40
breaths per minute, whereas a weak pneumotaxic
signal may reduce the rate to only 3 to 5 breaths per
minute.
Ventral Respiratory Group of
Neurons—Functions in Both
Inspiration and Expiration
Located in each side of the medulla, about 5 millimeters anterior and lateral to the dorsal respiratory group
of neurons, is the ventral respiratory group of neurons,
found in the nucleus ambiguus rostrally and the
nucleus retroambiguus caudally. The function of this
neuronal group differs from that of the dorsal respiratory group in several important ways:
1. The neurons of the ventral respiratory group
remain almost totally inactive during normal quiet
respiration. Therefore, normal quiet breathing is
caused only by repetitive inspiratory signals from
the dorsal respiratory group transmitted mainly to
the diaphragm, and expiration results from elastic
recoil of the lungs and thoracic cage.
2. There is no evidence that the ventral respiratory
neurons participate in the basic rhythmical
oscillation that controls respiration.
3. When the respiratory drive for increased
pulmonary ventilation becomes greater than
normal, respiratory signals spill over into the
ventral respiratory neurons from the basic
oscillating mechanism of the dorsal respiratory
area. As a consequence, the ventral respiratory
area contributes extra respiratory drive as well.
4. Electrical stimulation of a few of the neurons in
the ventral group causes inspiration, whereas
stimulation of others causes expiration. Therefore,
these neurons contribute to both inspiration and
expiration. They are especially important in
providing the powerful expiratory signals to the
abdominal muscles during very heavy expiration.
Thus, this area operates more or less as an
overdrive mechanism when high levels of
pulmonary ventilation are required, especially
during heavy exercise.
Lung Inflation Signals Limit
Inspiration—The Hering-Breuer
Inflation Reflex
In addition to the central nervous system respiratory
control mechanisms operating entirely within the
brain stem, sensory nerve signals from the lungs also
516
Unit VII
help control respiration. Most important, located in
the muscular portions of the walls of the bronchi and
bronchioles throughout the lungs are stretch receptors
that transmit signals through the vagi into the dorsal
respiratory group of neurons when the lungs become
overstretched. These signals affect inspiration in much
the same way as signals from the pneumotaxic center;
that is, when the lungs become overly inflated, the
stretch receptors activate an appropriate feedback
response that “switches off” the inspiratory ramp and
thus stops further inspiration. This is called the HeringBreuer inflation reflex. This reflex also increases the
rate of respiration, as is true for signals from the pneumotaxic center.
In human beings, the Hering-Breuer reflex probably
is not activated until the tidal volume increases to
more than three times normal (greater than about 1.5
liters per breath). Therefore, this reflex appears to be
mainly a protective mechanism for preventing excess
lung inflation rather than an important ingredient in
normal control of ventilation.
Control of Overall Respiratory
Center Activity
Up to this point, we have discussed the basic mechanisms for causing inspiration and expiration, but it is
also important to know how the intensity of the respiratory control signals is increased or decreased to
match the ventilatory needs of the body. For example,
during heavy exercise, the rates of oxygen usage and
carbon dioxide formation are often increased to as
much as 20 times normal, requiring commensurate
increases in pulmonary ventilation. The major purpose
of the remainder of this chapter is to discuss this
control of ventilation in accord with the respiratory
needs of the body.
Respiration
Direct Chemical Control of
Respiratory Center Activity by Carbon
Dioxide and Hydrogen Ions
Chemosensitive Area of the Respiratory Center. We have discussed mainly three areas of the respiratory center:
the dorsal respiratory group of neurons, the ventral
respiratory group, and the pneumotaxic center. It
is believed that none of these is affected directly by
changes in blood carbon dioxide concentration or
hydrogen ion concentration. Instead, an additional
neuronal area, a chemosensitive area, shown in Figure
41–2, is located bilaterally, lying only 0.2 millimeter
beneath the ventral surface of the medulla. This area
is highly sensitive to changes in either blood Pco2 or
hydrogen ion concentration, and it in turn excites the
other portions of the respiratory center.
Excitation of the Chemosensitive Neurons by
Hydrogen Ions Is Likely the Primary Stimulus
The sensor neurons in the chemosensitive area are
especially excited by hydrogen ions; in fact, it is
believed that hydrogen ions may be the only important direct stimulus for these neurons. However,
hydrogen ions do not easily cross the blood-brain
barrier. For this reason, changes in hydrogen ion concentration in the blood have considerably less effect
in stimulating the chemosensitive neurons than do
changes in blood carbon dioxide, even though carbon
dioxide is believed to stimulate these neurons secondarily by changing the hydrogen ion concentration, as
explained in the following section.
Carbon Dioxide Stimulates the
Chemosensitive Area
Although carbon dioxide has little direct effect in
stimulating the neurons in the chemosensitive area, it
Chemical Control
of Respiration
The ultimate goal of respiration is to maintain proper
concentrations of oxygen, carbon dioxide, and hydrogen ions in the tissues. It is fortunate, therefore, that
respiratory activity is highly responsive to changes in
each of these.
Excess carbon dioxide or excess hydrogen ions in
the blood mainly act directly on the respiratory center
itself, causing greatly increased strength of both the
inspiratory and the expiratory motor signals to the
respiratory muscles.
Oxygen, in contrast, does not have a significant
direct effect on the respiratory center of the brain in
controlling respiration. Instead, it acts almost entirely
on peripheral chemoreceptors located in the carotid
and aortic bodies, and these in turn transmit appropriate nervous signals to the respiratory center for
control of respiration.
Let us discuss first the stimulation of the respiratory
center itself by carbon dioxide and hydrogen ions.
Chemosensitive
area
Inspiratory area
H+ + HCO3-
H2CO3
CO2 + H2O
Figure 41–2
Stimulation of the brain stem inspiratory area by signals from the
chemosensitive area located bilaterally in the medulla, lying only
a fraction of a millimeter beneath the ventral medullary surface.
Note also that hydrogen ions stimulate the chemosensitive area,
but carbon dioxide in the fluid gives rise to most of the hydrogen
ions.
517
Regulation of Respiration
11
10
9
8
7
Normal
does have a potent indirect effect. It does this by reacting with the water of the tissues to form carbonic acid,
which dissociates into hydrogen and bicarbonate ions;
the hydrogen ions then have a potent direct stimulatory effect on respiration. These reactions are shown
in Figure 41–2.
Why does blood carbon dioxide have a more potent
effect in stimulating the chemosensitive neurons than
do blood hydrogen ions? The answer is that the bloodbrain barrier is not very permeable to hydrogen ions,
but carbon dioxide passes through this barrier almost
as if the barrier did not exist. Consequently, whenever
the blood Pco2 increases, so does the Pco2 of both the
interstitial fluid of the medulla and the cerebrospinal
fluid. In both these fluids, the carbon dioxide immediately reacts with the water to form new hydrogen ions.
Thus, paradoxically, more hydrogen ions are released
into the respiratory chemosensitive sensory area of the
medulla when the blood carbon dioxide concentration
increases than when the blood hydrogen ion concentration increases. For this reason, respiratory center
activity is increased very strongly by changes in blood
carbon dioxide, a fact that we subsequently discuss
quantitatively.
Alveolar ventilation (basal rate = 1)
Chapter 41
6
5
4
3
2
PCO2
pH
1
0
20
30
40
50 60 70 80
PCO2 (mm Hg)
7.6 7.5 7.4 7.3
Decreased Stimulatory Effect of Carbon Dioxide After the First
1 to 2 Days. Excitation of the respiratory center by
carbon dioxide is great the first few hours after the
blood carbon dioxide first increases, but then it gradually declines over the next 1 to 2 days, decreasing to
about one fifth the initial effect. Part of this decline
results from renal readjustment of the hydrogen ion
concentration in the circulating blood back toward
normal after the carbon dioxide first increases the
hydrogen concentration. The kidneys achieve this by
increasing the blood bicarbonate, which binds with the
hydrogen ions in the blood and cerebrospinal fluid to
reduce their concentrations. But even more important,
over a period of hours, the bicarbonate ions also
slowly diffuse through the blood-brain and blood–
cerebrospinal fluid barriers and combine directly with
the hydrogen ions adjacent to the respiratory neurons
as well, thus reducing the hydrogen ions back to near
normal. A change in blood carbon dioxide concentration therefore has a potent acute effect on controlling
respiratory drive but only a weak chronic effect after
a few days’ adaptation.
Quantitative Effects of Blood PCO2
and Hydrogen Ion Concentration on
Alveolar Ventilation
Figure 41–3 shows quantitatively the approximate
effects of blood Pco2 and blood pH (which is an
inverse logarithmic measure of hydrogen ion concentration) on alveolar ventilation. Note especially the
very marked increase in ventilation caused by an
increase in Pco2 in the normal range between 35 and
75 mm Hg. This demonstrates the tremendous effect
that carbon dioxide changes have in controlling respiration. By contrast, the change in respiration in the
normal blood pH range between 7.3 and 7.5 is less than
one tenth as great.
90 100
7.2 7.1 7.0 6.9
pH
Figure 41–3
Effects of increased arterial blood PCO2 and decreased arterial pH
(increased hydrogen ion concentration) on the rate of alveolar
ventilation.
Unimportance of Oxygen for Control of the
Respiratory Center
Changes in oxygen concentration have virtually no
direct effect on the respiratory center itself to alter
respiratory drive (although oxygen changes do have
an indirect effect, acting through the peripheral
chemoreceptors, as explained in the next section).
We learned in Chapter 40 that the hemoglobinoxygen buffer system delivers almost exactly normal
amounts of oxygen to the tissues even when the pulmonary Po2 changes from a value as low as 60 mm Hg
up to a value as high as 1000 mm Hg. Therefore, except
under special conditions, adequate delivery of oxygen
can occur despite changes in lung ventilation ranging
from slightly below one half normal to as high as 20 or
more times normal. This is not true for carbon dioxide,
because both the blood and tissue Pco2 changes
inversely with the rate of pulmonary ventilation; thus,
the processes of animal evolution have made carbon
dioxide the major controller of respiration, not
oxygen.
Yet, for those special conditions in which the tissues
get into trouble for lack of oxygen, the body has a
special mechanism for respiratory control located in
the peripheral chemoreceptors, outside the brain respiratory center; this mechanism responds when the
blood oxygen falls too low, mainly below a Po2 of
70 mm Hg, as explained in the next section.
518
Unit VII
Peripheral Chemoreceptor
System for Control of
Respiratory Activity—Role of
Oxygen in Respiratory Control
In addition to control of respiratory activity by the respiratory center itself, still another mechanism is available for controlling respiration. This is the peripheral
chemoreceptor system, shown in Figure 41–4. Special
nervous chemical receptors, called chemoreceptors, are
located in several areas outside the brain. They are
especially important for detecting changes in oxygen
in the blood, although they also respond to a lesser
extent to changes in carbon dioxide and hydrogen ion
concentrations. The chemoreceptors transmit nervous
signals to the respiratory center in the brain to help
regulate respiratory activity.
Most of the chemoreceptors are in the carotid
bodies. However, a few are also in the aortic bodies,
shown in the lower part of Figure 41–4, and a very few
are located elsewhere in association with other arteries of the thoracic and abdominal regions.
The carotid bodies are located bilaterally in the
bifurcations of the common carotid arteries. Their
afferent nerve fibers pass through Hering’s nerves to
the glossopharyngeal nerves and then to the dorsal
respiratory area of the medulla. The aortic bodies are
located along the arch of the aorta; their afferent
nerve fibers pass through the vagi, also to the dorsal
medullary respiratory area.
Each of the chemoreceptor bodies receives its own
special blood supply through a minute artery directly
from the adjacent arterial trunk. Further, blood flow
through these bodies is extreme, 20 times the weight
of the bodies themselves each minute. Therefore, the
percentage of oxygen removed from the flowing blood
is virtually zero. This means that the chemoreceptors
Respiration
are exposed at all times to arterial blood, not venous
blood, and their Po2s are arterial Po2s.
Stimulation of the Chemoreceptors by Decreased Arterial
Oxygen. When the oxygen concentration in the arterial
blood falls below normal, the chemoreceptors become
strongly stimulated. This is demonstrated in Figure
41–5, which shows the effect of different levels of arterial Po2 on the rate of nerve impulse transmission from
a carotid body. Note that the impulse rate is particularly sensitive to changes in arterial Po2 in the range of
60 down to 30 mm Hg, a range in which hemoglobin
saturation with oxygen decreases rapidly.
Effect of Carbon Dioxide and Hydrogen Ion Concentration on
Chemoreceptor Activity. An increase in either carbon
dioxide concentration or hydrogen ion concentration
also excites the chemoreceptors and, in this way,
indirectly increases respiratory activity. However, the
direct effects of both these factors in the respiratory
center itself are so much more powerful than their
effects mediated through the chemoreceptors (about
seven times as powerful) that, for practical purposes,
the indirect effects of carbon dioxide and hydrogen
ions through the chemoreceptors do not need to be
considered. Yet there is one difference between the
peripheral and central effects of carbon dioxide: the
stimulation by way of the peripheral chemoreceptors
occurs as much as five times as rapidly as central stimulation, so that the peripheral chemoreceptors might
be especially important in increasing the rapidity of
response to carbon dioxide at the onset of exercise.
Basic Mechanism of Stimulation of the Chemoreceptors by
Oxygen Deficiency. The exact means by which low Po2
excites the nerve endings in the carotid and aortic
bodies is still unknown. However, these bodies have
Medulla
Glossopharyngeal nerve
Carotid body
Carotid body nerve
impulses per second
Vagus nerve
800
600
400
200
0
0
Aortic bodies
Figure 41–4
Respiratory control by peripheral chemoreceptors in the carotid
and aortic bodies.
100
200
300
400
Arterial PO2 (mm Hg)
500
Figure 41–5
Effect of arterial PO2 on impulse rate from the carotid body of a
cat.
Chapter 41
519
Regulation of Respiration
multiple highly characteristic glandular-like cells,
called glomus cells, that synapse directly or indirectly
with the nerve endings. Some investigators have suggested that these glomus cells might function as the
chemoreceptors and then stimulate the nerve endings.
But other studies suggest that the nerve endings themselves are directly sensitive to the low Po2.
Effect of Low Arterial PO2 to Stimulate
Alveolar Ventilation When Arterial Carbon
Dioxide and Hydrogen Ion Concentrations
Remain Normal
Figure 41–6 shows the effect of low arterial Po2 on
alveolar ventilation when the Pco2 and the hydrogen
ion concentration are kept constant at their normal
levels. In other words, in this figure, only the ventilatory drive due to the effect of low oxygen on the
chemoreceptors is active. The figure shows almost no
effect on ventilation as long as the arterial Po2 remains
greater than 100 mm Hg. But at pressures lower than
100 mm Hg, ventilation approximately doubles when
the arterial Po2 falls to 60 mm Hg and can increase as
much as fivefold at very low Po2s. Under these conditions, low arterial Po2 obviously drives the ventilatory
process quite strongly.
Chronic Breathing of Low Oxygen Stimulates
Respiration Even More—The Phenomenon
of “Acclimatization”
Mountain climbers have found that when they ascend
a mountain slowly, over a period of days rather than a
period of hours, they breathe much more deeply and
therefore can withstand far lower atmospheric oxygen
concentrations than when they ascend rapidly. This is
called acclimatization.
The reason for acclimatization is that, within 2 to 3
days, the respiratory center in the brain stem loses
about four fifths of its sensitivity to changes in Pco2
and hydrogen ions. Therefore, the excess ventilatory
blow-off of carbon dioxide that normally would inhibit
an increase in respiration fails to occur, and low
oxygen can drive the respiratory system to a much
higher level of alveolar ventilation than under acute
conditions. Instead of the 70 per cent increase in ventilation that might occur after acute exposure to low
oxygen, the alveolar ventilation often increases 400
to 500 per cent after 2 to 3 days of low oxygen; this
helps immensely in supplying additional oxygen to the
mountain climber.
Composite Effects of PCO2, pH, and
PO2 on Alveolar Ventilation
Figure 41–7 gives a quick overview of the manner in
which the chemical factors Po2, Pco2, and pH—
together—affect alveolar ventilation. To understand
this diagram, first observe the four red curves.
These curves were recorded at different levels of
arterial Po2—40 mm Hg, 50 mm Hg, 60 mm Hg, and
100 mm Hg. For each of these curves, the Pco2 was
changed from lower to higher levels. Thus, this
7
4
30
3
2
20
PCO2
Ventilation
0
160 140 120 100 80 60 40
Arterial PO2 (mm Hg)
Alveolar ventilation (L/min)
5
1
60
40
Arterial PCO2 (mm Hg)
Alveolar ventilation (normal = 1)
6
pH = 7.4
pH = 7.3
50
40
PO2 (mm Hg)
40
50
40
50 60
100
60
30
100
20
10
0
20
0
0
0
10
20
30
40
50
Alveolar PCO2 (mm Hg)
60
Figure 41–6
The lower curve demonstrates the effect of different levels of arterial PO2 on alveolar ventilation, showing a sixfold increase in ventilation as the PO2 decreases from the normal level of 100 mm Hg
to 20 mm Hg. The upper line shows that the arterial PCO2 was kept
at a constant level during the measurements of this study; pH also
was kept constant.
Figure 41–7
Composite diagram showing the interrelated effects of PCO2, PO2,
and pH on alveolar ventilation. (Drawn from data in Cunningham
DJC, Lloyd BB: The Regulation of Human Respiration. Oxford:
Blackwell Scientific Publications, 1963.)
520
Unit VII
Respiration
“family” of red curves represents the combined effects
of alveolar Pco2 and Po2 on ventilation.
Now observe the green curves. The red curves were
measured at a blood pH of 7.4; the green curves were
measured at a pH of 7.3. We now have two families of
curves representing the combined effects of Pco2 and
Po2 on ventilation at two different pH values. Still
other families of curves would be displaced to the right
at higher pHs and displaced to the left at lower pHs.
Thus, using this diagram, one can predict the level of
alveolar ventilation for most combinations of alveolar
Pco2, alveolar Po2, and arterial pH.
to transmit at the same time collateral impulses into
the brain stem to excite the respiratory center. This is
analogous to the stimulation of the vasomotor center
of the brain stem during exercise that causes a simultaneous increase in arterial pressure.
Actually, when a person begins to exercise, a large
share of the total increase in ventilation begins immediately on initiation of the exercise, before any blood
chemicals have had time to change. It is likely that
most of the increase in respiration results from neurogenic signals transmitted directly into the brain stem
respiratory center at the same time that signals go to
the body muscles to cause muscle contraction.
Regulation of Respiration
During Exercise
Interrelation Between Chemical Factors and Nervous: Factors
in the Control of Respiration During Exercise. When a
In strenuous exercise, oxygen consumption and carbon
dioxide formation can increase as much as 20-fold.Yet,
as illustrated in Figure 41–8, in the healthy athlete,
alveolar ventilation ordinarily increases almost exactly
in step with the increased level of oxygen metabolism.
The arterial Po2, Pco2, and pH remain almost exactly
normal.
In trying to analyze what causes the increased ventilation during exercise, one is tempted to ascribe this
to increases in blood carbon dioxide and hydrogen
ions, plus a decrease in blood oxygen. However, this is
questionable, because measurements of arterial Pco2,
pH, and Po2 show that none of these values changes
significantly during exercise, so that none of them
becomes abnormal enough to stimulate respiration.
Therefore, the question must be asked: What causes
intense ventilation during exercise? At least one effect
seems to be predominant. The brain, on transmitting
motor impulses to the exercising muscles, is believed
person exercises, direct nervous signals presumably
stimulate the respiratory center almost the proper
amount to supply the extra oxygen required for exercise and to blow off extra carbon dioxide. Occasionally, however, the nervous respiratory control signals
are either too strong or too weak. Then chemical
factors play a significant role in bringing about the
final adjustment of respiration required to keep the
oxygen, carbon dioxide, and hydrogen ion concentrations of the body fluids as nearly normal as possible.
This is demonstrated in Figure 41–9, which shows in
the lower curve changes in alveolar ventilation during
a 1-minute period of exercise and in the upper curve
changes in arterial Pco2. Note that at the onset of exercise, the alveolar ventilation increases instantaneously,
without an initial increase in arterial Pco2. In fact, this
Arterial PCO2
(mm Hg)
44
40
38
36
110
Alveolar ventilation
(L/min)
Total ventilation (L/min)
120
42
100
80
60
40
20
Moderate
exercise
0
0
1.0
Exercise
18
14
10
6
2
Severe
exercise
0
1
Minutes
2
2.0
3.0
4.0
O2 consumption (L/min)
Figure 41–9
Figure 41–8
Effect of exercise on oxygen consumption and ventilatory rate.
(From Gray JS: Pulmonary Ventilation and Its Physiological Regulation. Springfield, Ill: Charles C Thomas, 1950.)
Changes in alveolar ventilation (bottom curve) and arterial PCO2
(top curve) during a 1-minute period of exercise and also after termination of exercise. (Extrapolated to the human being from data
in dogs in Bainton CR: Effect of speed vs grade and shivering on
ventilation in dogs during active exercise. J Appl Physiol 33:778,
1972.)
Chapter 41
Regulation of Respiration
the rate of carbon dioxide release, thus keeping arterial Pco2 near its normal value. The upper curve of
Figure 41–10 also shows that if, during exercise, the
arterial Pco2 does change from its normal value of
40 mm Hg, it has an extra stimulatory effect on ventilation at a Pco2 greater than 40 mm Hg and a depressant effect at a Pco2 less than 40 mm Hg.
140
120
Alveolar ventilation (L/min)
521
Possibility That the Neurogenic Factor for Control of Ventilation During Exercise Is a Learned Response. Many experi-
100
ments suggest that the brain’s ability to shift the
ventilatory response curve during exercise, as shown
in Figure 41–10, is at least partly a learned response.
That is, with repeated periods of exercise, the brain
becomes progressively more able to provide the
proper signals required to keep the blood Pco2 at its
normal level. Also, there is reason to believe that even
the cerebral cortex is involved in this learning, because
experiments that block only the cortex also block the
learned response.
80
60
40
Exercise
Resting
Normal
20
0
20
30
40
50
60
80
Arterial PCO2 (mm Hg)
100
Figure 41–10
Approximate effect of maximum exercise in an athlete to shift the
alveolar PCO2-ventilation response curve to a level much higher
than normal. The shift, believed to be caused by neurogenic
factors, is almost exactly the right amount to maintain arterial PCO2
at the normal level of 40 mm Hg both in the resting state and
during heavy exercise.
increase in ventilation is usually great enough so that
at first it actually decreases arterial Pco2 below normal,
as shown in the figure. The presumed reason that the
ventilation forges ahead of the buildup of blood
carbon dioxide is that the brain provides an “anticipatory” stimulation of respiration at the onset of exercise, causing extra alveolar ventilation even before it
is needed. However, after about 30 to 40 seconds,
the amount of carbon dioxide released into the blood
from the active muscles approximately matches the
increased rate of ventilation, and the arterial Pco2
returns essentially to normal even as the exercise continues, as shown toward the end of the 1-minute period
of exercise in the figure.
Figure 41–10 summarizes the control of respiration
during exercise in still another way, this time more
quantitatively. The lower curve of this figure shows the
effect of different levels of arterial Pco2 on alveolar
ventilation when the body is at rest—that is, not exercising. The upper curve shows the approximate shift of
this ventilatory curve caused by neurogenic drive from
the respiratory center that occurs during heavy exercise. The points indicated on the two curves show the
arterial Pco2 first in the resting state and then in the
exercising state. Note in both instances that the Pco2
is at the normal level of 40 mm Hg. In other words, the
neurogenic factor shifts the curve about 20-fold in the
upward direction, so that ventilation almost matches
Other Factors That
Affect Respiration
Voluntary Control of Respiration. Thus far, we have dis-
cussed the involuntary system for the control of respiration. However, we all know that for short periods of
time, respiration can be controlled voluntarily and that
one can hyperventilate or hypoventilate to such an
extent that serious derangements in Pco2, pH, and Po2
can occur in the blood.
Effect of Irritant Receptors in the Airways. The epithelium of
the trachea, bronchi, and bronchioles is supplied with
sensory nerve endings called pulmonary irritant receptors that are stimulated by many incidents. These cause
coughing and sneezing, as discussed in Chapter 39. They
may also cause bronchial constriction in such diseases
as asthma and emphysema.
Function of Lung “J Receptors.” A few sensory nerve
endings have been described in the alveolar walls in
juxtaposition to the pulmonary capillaries—hence the
name “J receptors.”They are stimulated especially when
the pulmonary capillaries become engorged with blood
or when pulmonary edema occurs in such conditions as
congestive heart failure. Although the functional role of
the J receptors is not clear, their excitation may give the
person a feeling of dyspnea.
Effect of Brain Edema. The activity of the respiratory
center may be depressed or even inactivated by acute
brain edema resulting from brain concussion. For
instance, the head might be struck against some solid
object, after which the damaged brain tissues swell,
compressing the cerebral arteries against the cranial
vault and thus partially blocking cerebral blood supply.
Occasionally, respiratory depression resulting from
brain edema can be relieved temporarily by intravenous
injection of hypertonic solutions such as highly concentrated mannitol solution. These solutions osmotically
remove some of the fluids of the brain, thus relieving
intracranial pressure and sometimes re-establishing respiration within a few minutes.
522
Unit VII
Anesthesia. Perhaps the most prevalent cause of respiratory depression and respiratory arrest is overdosage
with anesthetics or narcotics. For instance, sodium pentobarbital depresses the respiratory center considerably
more than many other anesthetics, such as halothane. At
one time, morphine was used as an anesthetic, but this
drug is now used only as an adjunct to anesthetics
because it greatly depresses the respiratory center while
having less ability to anesthetize the cerebral cortex.
Periodic Breathing
An abnormality of respiration called periodic breathing
occurs in a number of disease conditions. The person
breathes deeply for a short interval and then breathes
slightly or not at all for an additional interval, with the
cycle repeating itself over and over. One type of periodic breathing, Cheyne-Stokes breathing, is characterized by slowly waxing and waning respiration occurring about every 40 to 60 seconds, as illustrated in Figure
41–11.
Basic Mechanism of Cheyne-Stokes Breathing. The basic
cause of Cheyne-Stokes breathing is the following:
When a person overbreathes, thus blowing off too much
carbon dioxide from the pulmonary blood while at the
same time increasing blood oxygen, it takes several
seconds before the changed pulmonary blood can be
transported to the brain and inhibit the excess ventilation. By this time, the person has already overventilated
for an extra few seconds. Therefore, when the overventilated blood finally reaches the brain respiratory
center, the center becomes depressed an excessive
amount. Then the opposite cycle begins. That is, carbon
dioxide increases and oxygen decreases in the alveoli.
Again, it takes a few seconds before the brain can
respond to these new changes. When the brain does
respond, the person breathes hard once again, and the
cycle repeats.
The basic cause of Cheyne-Stokes breathing occurs
in everyone. However, under normal conditions, this
mechanism is highly “damped.” That is, the fluids of the
blood and the respiratory center control areas have
large amounts of dissolved and chemically bound
Depth of
respiration
PCO2 of
respiratory
neurons
Respiration
carbon dioxide and oxygen. Therefore, normally, the
lungs cannot build up enough extra carbon dioxide or
depress the oxygen sufficiently in a few seconds to cause
the next cycle of the periodic breathing. But under two
separate conditions, the damping factors can be overridden, and Cheyne-Stokes breathing does occur:
1. When a long delay occurs for transport of blood
from the lungs to the brain, changes in carbon
dioxide and oxygen in the alveoli can continue
for many more seconds than usual. Under these
conditions, the storage capacities of the alveoli and
pulmonary blood for these gases are exceeded;
then, after a few more seconds, the periodic
respiratory drive becomes extreme, and CheyneStokes breathing begins. This type of CheyneStokes breathing often occurs in patients with
severe cardiac failure because blood flow is slow,
thus delaying the transport of blood gases from the
lungs to the brain. In fact, in patients with chronic
heart failure, Cheyne-Stokes breathing can
sometimes occur on and off for months.
2. A second cause of Cheyne-Stokes breathing is
increased negative feedback gain in the respiratory
control areas. This means that a change in blood
carbon dioxide or oxygen causes a far greater
change in ventilation than normally. For instance,
instead of the normal 2- to 3-fold increase in
ventilation that occurs when the Pco2 rises
3 mm Hg, the same 3 mm Hg rise might increase
ventilation 10- to 20-fold. The brain feedback
tendency for periodic breathing is now strong
enough to cause Cheyne-Stokes breathing without
extra blood flow delay between the lungs and
brain. This type of Cheyne-Stokes breathing occurs
mainly in patients with brain damage. The brain
damage often turns off the respiratory drive
entirely for a few seconds; then an extra intense
increase in blood carbon dioxide turns it back on
with great force. Cheyne-Stokes breathing of this
type is frequently a prelude to death from brain
malfunction.
Typical records of changes in pulmonary and respiratory center Pco2 during Cheyne-Stokes breathing are
shown in Figure 41–11. Note that the Pco2 of the
pulmonary blood changes in advance of the Pco2 of
the respiratory neurons. But the depth of respiration
corresponds with the Pco2 in the brain, not with the
Pco2 in the pulmonary blood where the ventilation is
occurring.
Sleep Apnea
Respiratory
center excited
PCO2 of
lung blood
Figure 41–11
Cheyne-Stokes breathing, showing changing PCO2 in the pulmonary blood (red line) and delayed changes in the PCO2 of the
fluids of the respiratory center (blue line).
The term apnea means absence of spontaneous breathing. Occasional apneas occur during normal sleep, but
in persons with sleep apnea, the frequency and duration
are greatly increased, with episodes of apnea lasting for
10 seconds or longer and occurring 300 to 500 times
each night. Sleep apneas can be caused by obstruction
of the upper airways, especially the pharynx, or by
impaired central nervous system respiratory drive.
Obstructive Sleep Apnea Is Caused by Blockage of the Upper
Airway. The muscles of the pharynx normally keep this
passage open to allow air to flow into the lungs during
inspiration. During sleep, these muscles usually relax,
but the airway passage remains open enough to permit
adequate airflow. Some individuals have an especially
Chapter 41
Regulation of Respiration
narrow passage, and relaxation of these muscles during
sleep causes the pharynx to completely close so that air
cannot flow into the lungs.
In persons with sleep apnea, loud snoring and labored
breathing occur soon after falling asleep. The snoring
proceeds, often becoming louder, and is then interrupted by a long silent period during which no breathing (apnea) occurs. These periods of apnea result
in significant decreases in Po2 and increases in Pco2,
which greatly stimulate respiration. This, in turn, causes
sudden attempts to breathe, which result in loud snorts
and gasps followed by snoring and repeated episodes of
apnea. The periods of apnea and labored breathing are
repeated several hundred times during the night, resulting in fragmented, restless sleep. Therefore, patients
with sleep apnea usually have excessive daytime drowsiness as well as other disorders, including increased sympathetic activity, high heart rates, pulmonary and
systemic hypertension, and a greatly elevated risk for
cardiovascular disease.
Obstructive sleep apnea most commonly occurs in
older, obese persons in whom there is increased fat deposition in the soft tissues of the pharynx or compression
of the pharynx due to excessive fat masses in the neck.
In a few individuals, sleep apnea may be associated with
nasal obstruction, a very large tongue, enlarged tonsils,
or certain shapes of the palate that greatly increase
resistance to the flow of air to the lungs during inspiration. The most common treatments of obstructive sleep
apnea include (1) surgery to remove excess fat tissue
at the back of the throat (a procedure called uvulopalatopharyngoplasty), to remove enlarged tonsils or
adenoids, or to create an opening in the trachea (tracheostomy) to bypass the obstructed airway during
sleep, and (2) nasal ventilation with continuous positive
airway pressure (CPAP).
“Central” Sleep Apnea Occurs When the Neural Drive to Respiratory Muscles Is Transiently Abolished. In a few persons
with sleep apnea, the central nervous system drive to
the ventilatory muscles transiently ceases. Disorders
that can cause cessation of the ventilatory drive during
sleep include damage to the central respiratory centers
or abnormalities of the respiratory neuromuscular apparatus. Patients affected by central sleep apnea may have
decreased ventilation when they are awake, although
they are fully capable of normal voluntary breathing.
During sleep, their breathing disorders usually worsen,
resulting in more frequent episodes of apnea that
decrease Po2 and increase Pco2 until a critical level is
reached that eventually stimulates respiration. These
transient instabilities of respiration cause restless sleep
and clinical features similar to those observed in
obstructive sleep apnea.
In most patients, the cause of central sleep apnea is
unknown, although instability of the respiratory drive
can result from strokes or other disorders that make the
respiratory centers of the brain less responsive to the
523
stimulatory effects of carbon dioxide and hydrogen ions.
Patients with this disease are extremely sensitive to
even small doses of sedatives or narcotics, which further
reduce the responsiveness of the respiratory centers to
the stimulatory effects of carbon dioxide. Medications
that stimulate the respiratory centers can sometimes be
helpful, but ventilation with CPAP at night is usually
necessary.
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Zuperku EJ, McCrimmon DR: Gain modulation of respiratory neurons. Respir Physiol Neurobiol 131:121, 2002.
C
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4
Respiratory Insufficiency—
Pathophysiology, Diagnosis,
Oxygen Therapy
Diagnosis and treatment of most respiratory disorders depend heavily on understanding the basic
physiologic principles of respiration and gas
exchange. Some respiratory diseases result from
inadequate ventilation. Others result from abnormalities of diffusion through the pulmonary membrane or abnormal blood transport of gases
between the lungs and tissues. Therapy is often
entirely different for these diseases, so it is no longer satisfactory simply to make
a diagnosis of “respiratory insufficiency.”
Useful Methods for Studying Respiratory Abnormalities
In the previous few chapters, we have discussed a number of methods for studying respiratory abnormalities, including measuring vital capacity, tidal air, functional residual capacity, dead space, physiologic shunt, and physiologic dead
space. This array of measurements is only part of the armamentarium of the
clinical pulmonary physiologist. Some other interesting tools are described here.
Study of Blood Gases and Blood pH
Among the most fundamental of all tests of pulmonary performance are determinations of the blood Po2, CO2, and pH. It is often important to make these
measurements rapidly as an aid in determining appropriate therapy for acute
respiratory distress or acute abnormalities of acid-base balance. Several simple
and rapid methods have been developed to make these measurements within
minutes, using no more than a few drops of blood. They are the following.
Determination of Blood pH. Blood pH is measured using a glass pH electrode of
the type used in all chemical laboratories. However, the electrodes used for this
purpose are miniaturized. The voltage generated by the glass electrode is a
direct measure of pH, and this is generally read directly from a voltmeter scale,
or it is recorded on a chart.
Determination of Blood CO2. A glass electrode pH meter can also be used to deter-
mine blood CO2 in the following way: When a weak solution of sodium bicarbonate is exposed to carbon dioxide gas, the carbon dioxide dissolves in the
solution until an equilibrium state is established. In this equilibrium state, the
pH of the solution is a function of the carbon dioxide and bicarbonate ion concentrations in accordance with the Henderson-Hasselbalch equation that is
explained in Chapter 30; that is,
pH = 6.1 + log
HCO3 CO 2
When the glass electrode is used to measure CO2 in blood, a miniature glass
electrode is surrounded by a thin plastic membrane. In the space between the
electrode and plastic membrane is a solution of sodium bicarbonate of known
524
2
525
Respiratory Insufficiency—Pathophysiology, Diagnosis, Oxygen Therapy
concentration. Blood is then superfused onto the outer
surface of the plastic membrane, allowing carbon
dioxide to diffuse from the blood into the bicarbonate
solution. Only a drop or so of blood is required. Next,
the pH is measured by the glass electrode, and the CO2
is calculated by use of the above formula.
300
ry
to
ra
pi
ex
200
w
flo
In many respiratory diseases, particularly in asthma,
the resistance to airflow becomes especially great
during expiration, sometimes causing tremendous difficulty in breathing. This has led to the concept called
maximum expiratory flow, which can be defined as
follows: When a person expires with great force, the
expiratory airflow reaches a maximum flow beyond
which the flow cannot be increased any more even
with greatly increased additional force. This is the
maximum expiratory flow. The maximum expiratory
flow is much greater when the lungs are filled with a
large volume of air than when they are almost empty.
These principles can be understood by referring to
Figure 42–1.
Figure 42–1A shows the effect of increased pressure
applied to the outsides of the alveoli and air passageways caused by compressing the chest cage.The arrows
indicate that the same pressure compresses the
outsides of both the alveoli and the bronchioles.Therefore, not only does this pressure force air from
the alveoli toward the bronchioles, but it also tends
to collapse the bronchioles at the same time, which
will oppose movement of air to the exterior. Once
the bronchioles have almost completely collapsed,
further expiratory force can still greatly increase the
alveolar pressure, but it also increases the degree of
bronchiolar collapse and airway resistance by an equal
amount, thus preventing further increase in flow.
um
Measurement of Maximum
Expiratory Flow
400
im
ax
in a fluid can be measured by a technique called
polarography. Electric current is made to flow
between a small negative electrode and the solution.
If the voltage of the electrode is more than -0.6 volt
different from the voltage of the solution, oxygen will
deposit on the electrode. Furthermore, the rate of
current flow through the electrode will be directly proportional to the concentration of oxygen (and therefore to PO2 as well). In practice, a negative platinum
electrode with a surface area of about 1 square millimeter is used, and this is separated from the blood
by a thin plastic membrane that allows diffusion of
oxygen but not diffusion of proteins or other substances that will “poison” the electrode.
Often all three of the measuring devices for pH,
CO2, and Po2 are built into the same apparatus, and all
these measurements can be made within a minute or
so using a single, droplet-size sample of blood. Thus,
changes in the blood gases and pH can be followed
almost moment by moment at the bedside.
500
M
Determination of Blood PO2. The concentration of oxygen
A
Expiratory air flow (L/min)
Chapter 42
Total lung
capacity
100
Residual
volume
0
6
B
5
4
3
2
Lung volume (liters)
1
0
Figure 42–1
A, Collapse of the respiratory passageway during maximum expiratory effort, an effect that limits expiratory flow rate. B, Effect of
lung volume on the maximum expiratory air flow, showing
decreasing maximum expiratory air flow as the lung volume
becomes smaller.
Therefore, beyond a critical degree of expiratory force,
a maximum expiratory flow has been reached.
Figure 42–1B shows the effect of different degrees
of lung collapse (and therefore of bronchiolar collapse
as well) on the maximum expiratory flow. The curve
recorded in this section shows the maximum expiratory flow at all levels of lung volume after a healthy
person first inhales as much air as possible and then
expires with maximum expiratory effort until he or she
can expire at no greater rate. Note that the person
quickly reaches a maximum expiratory airflow of more
than 400 L/min. But regardless of how much additional expiratory effort the person exerts, this is still
the maximum flow rate that he or she can achieve.
Note also that as the lung volume becomes smaller,
the maximum expiratory flow rate also becomes less.
The main reason for this is that in the enlarged lung
the bronchi and bronchioles are held open partially by
way of elastic pull on their outsides by lung structural
elements; however, as the lung becomes smaller, these
structures are relaxed, so that the bronchi and bronchioles are collapsed more easily by external chest
pressure, thus progressively reducing the maximum
expiratory flow rate as well.
Abnormalities of the Maximum Expiratory Flow-Volume Curve.
Figure 42–2 shows the normal maximum expiratory
flow-volume curve, along with two additional flowvolume curves recorded in two types of lung diseases:
constricted lungs and partial airway obstruction. Note
that the constricted lungs have both reduced total lung
capacity (TLC) and reduced residual volume (RV).
526
Unit VII
NORMAL
Normal
Airway obstruction
Constricted lungs
400
A
Maximum
inspiration
4
300
FEV1
3
200
100
TLC
RV
0
7
6
5
4
3
2
Lung volume (liters)
1
0
Figure 42–2
Effect of two respiratory abnormalities—constricted lungs and
airway obstruction—-on the maximum expiratory flow-volume
curve. TLC, total lung capacity; RV, residual volume.
Furthermore, because the lung cannot expand to a
normal maximum volume, even with the greatest possible expiratory effort, the maximal expiratory flow
cannot rise to equal that of the normal curve. Constricted lung diseases include fibrotic diseases of the
lung itself, such as tuberculosis and silicosis, and diseases that constrict the chest cage, such as kyphosis,
scoliosis, and fibrotic pleurisy.
In diseases with airway obstruction, it is usually
much more difficult to expire than to inspire because
the closing tendency of the airways is greatly increased
by the extra positive pressure required in the chest
to cause expiration. By contrast, the extra negative
pleural pressure that occurs during inspiration actually
“pulls” the airways open at the same time that it
expands the alveoli. Therefore, air tends to enter the
lung easily but then becomes trapped in the lungs.
Over a period of months or years, this effect increases
both the TLC and the RV, as shown by the green curve
in Figure 42–2. Also, because of the obstruction of the
airways and because they collapse more easily than
normal airways, the maximum expiratory flow rate is
greatly reduced.
The classic disease that causes severe airway
obstruction is asthma. Serious airway obstruction also
occurs in some stages of emphysema.
Forced Expiratory Vital Capacity and
Forced Expiratory Volume
Another exceedingly useful clinical pulmonary test,
and one that is also simple, is to make a record on a
spirometer of the forced expiratory vital capacity
(FVC). Such a record is shown in Figure 42–3A for a
person with normal lungs and in Figure 42–3B for a
person with partial airway obstruction. In performing
the FVC maneuver, the person first inspires maximally
to the total lung capacity, then exhales into the
Lung volume change (liters)
Expiratory air flow (L/min)
500
Respiration
FVC
2
FEV1/FVC%
= 80%
1
0
0
B
1
2
3
4
5
6
7
AIRWAY OBSTRUCTION
4
3
FEV1
2
FEV1/FVC% FVC
= 47%
1
0
0
1
2 3 4
Seconds
5
6
7
Figure 42–3
Recordings during the forced vital capacity maneuver: A, in a
healthy person and B, in a person with partial airway obstruction.
(The “zero” on the volume scale is residual volume.)
spirometer with maximum expiratory effort as rapidly
and as completely as possible. The total distance of the
downslope of the lung volume record represents the
FVC, as shown in the figure.
Now, study the difference between the two records
(1) for normal lungs and (2) for partial airway obstruction. The total volume changes of the FVCs are not
greatly different, indicating only a moderate difference
in basic lung volumes in the two persons. There is,
however, a major difference in the amounts of air that
these persons can expire each second, especially during
the first second. Therefore, it is customary to compare
the recorded forced expiratory volume during the first
second (FEV1) with the normal. In the normal person
(see Figure 42–3A), the percentage of the FVC that is
expired in the first second divided by the total FVC
(FEV1/FVC%) is 80 per cent. However, note in Figure
42–3B that, with airway obstruction, this value
decreased to only 47 per cent. In serious airway
obstruction, as often occurs in acute asthma, this can
decrease to less than 20 per cent.
Physiologic Peculiarities
of Specific Pulmonary
Abnormalities
Chronic Pulmonary Emphysema
The term pulmonary emphysema literally means
excess air in the lungs. However, this term is usually
Chapter 42
Respiratory Insufficiency—Pathophysiology, Diagnosis, Oxygen Therapy
used to describe complex obstructive and destructive
process of the lungs caused by many years of smoking.
It results from the following major pathophysiologic
changes in the lungs:
1. Chronic infection, caused by inhaling smoke or
other substances that irritate the bronchi and
bronchioles. The chronic infection seriously
deranges the normal protective mechanisms of the
airways, including partial paralysis of the cilia of
the respiratory epithelium, an effect caused by
nicotine. As a result, mucus cannot be moved
easily out of the passageways. Also, stimulation
of excess mucus secretion occurs, which further
exacerbates the condition. Too, inhibition of the
alveolar macrophages occurs, so that they become
less effective in combating infection.
2. The infection, excess mucus, and inflammatory
edema of the bronchiolar epithelium together
cause chronic obstruction of many of the smaller
airways.
3. The obstruction of the airways makes it especially
difficult to expire, thus causing entrapment of
air in the alveoli and overstretching them. This,
combined with the lung infection, causes marked
destruction of as much as 50 to 80 per cent of the
alveolar walls. Therefore, the final picture of the
emphysematous lung is that shown in Figures
42–4 (top) and 42–5.
The physiologic effects of chronic emphysema are
extremely varied, depending on the severity of the
disease and the relative degrees of bronchiolar obstruction versus lung parenchymal destruction.Among
the different abnormalities are the following:
1. The bronchiolar obstruction increases airway
resistance and results in greatly increased work of
breathing. It is especially difficult for the person
to move air through the bronchioles during
expiration because the compressive force on the
outside of the lung not only compresses the
alveoli but also compresses the bronchioles,
which further increases their resistance during
expiration.
2. The marked loss of alveolar walls greatly
decreases the diffusing capacity of the lung, which
reduces the ability of the lungs to oxygenate the
blood and remove carbon dioxide from the blood.
3. The obstructive process is frequently much worse
in some parts of the lungs than in other parts,
so that some portions of the lungs are well
ventilated, while other portions are poorly
ventilated. This often causes extremely abnormal
ventilation-perfusion
ratios, with a very low
. .
Va/Q in some parts (physiologic shunt), resulting
. .
in poor aeration of the blood, and very high Va/Q
in other parts (physiologic dead space), resulting
in wasted ventilation, both effects occurring in the
same lungs.
4. Loss of large portions of the alveolar walls also
decreases the number of pulmonary capillaries
through which blood can pass. As a result, the
pulmonary vascular resistance often increases
markedly, causing pulmonary hypertension. This in
527
Figure 42–4
Contrast of the emphysematous lung (top figure) with the normal
lung (bottom figure), showing extensive alveolar destruction in
emphysema. (Reproduced with permission of Patricia Delaney
and the Department of Anatomy, The Medical College of
Wisconsin.)
turn overloads the right side of the heart and
frequently causes right-sided heart failure.
Chronic emphysema usually progresses slowly over
many years. The person develops both hypoxia and
hypercapnia because of hypoventilation of many
alveoli plus loss of alveolar walls. The net result of
all these effects is severe, prolonged, devastating air
hunger that can last for years until the hypoxia and
hypercapnia cause death—a high penalty to pay for
smoking.
Pneumonia
The term pneumonia includes any inflammatory condition of the lung in which some or all of the alveoli
are filled with fluid and blood cells, as shown in Figure
42–5. A common type of pneumonia is bacterial pneumonia, caused most frequently by pneumococci.
This disease begins with infection in the alveoli; the
pulmonary membrane becomes inflamed and highly
porous so that fluid and even red and white blood cells
528
Unit VII
Respiration
Fluid and blood cells
Confluent alveoli
Edema
Normal
Pneumonia
Emphysema
Figure 42–5
Lung alveolar changes in pneumonia and emphysema.
leak out of the blood into the alveoli.Thus, the infected
alveoli become progressively filled with fluid and cells,
and the infection spreads by extension of bacteria or
virus from alveolus to alveolus. Eventually, large areas
of the lungs, sometimes whole lobes or even a whole
lung, become “consolidated,” which means that they
are filled with fluid and cellular debris.
In pneumonia, the gas exchange functions of the
lungs change in different stages of the disease. In early
stages, the pneumonia process might well be localized
to only one lung, with alveolar ventilation reduced
while blood flow through the lung continues normally.
This results in two major pulmonary abnormalities: (1)
reduction in the total available surface area of the respiratory membrane and (2) decreased ventilationperfusion ratio. Both these effects cause hypoxemia
(low blood oxygen) and hypercapnia (high blood
carbon dioxide).
Figure 42–6 shows the effect of the decreased
ventilation-perfusion ratio in pneumonia, showing that
the blood passing through the aerated lung becomes
97 per cent saturated with oxygen, whereas that
passing through the unaerated lung is about 60 per
cent saturated.Therefore, the average saturation of the
blood pumped by the left heart into the aorta is only
about 78 per cent, which is far below normal.
Atelectasis
Atelectasis means collapse of the alveoli. It can occur
in localized areas of a lung or in an entire lung. Its most
common causes are (1) total obstruction of the airway
or (2) lack of surfactant in the fluids lining the alveoli.
Airway Obstruction. The airway obstruction type of
atelectasis usually results from (1) blockage of many
small bronchi with mucus or (2) obstruction of a major
bronchus by either a large mucus plug or some solid
object such as a tumor. The air entrapped beyond the
block is absorbed within minutes to hours by the blood
flowing in the pulmonary capillaries. If the lung tissue
Pulmonary arterial blood
60% saturated with O2
Pneumonia
Right
pulmonary
vein 97%
saturated
Left
pulmonary
vein 60%
saturated
Aorta:
Blood 1/2 = 97%
1/2 = 60%
Mean
= 78%
Figure 42–6
Effect of pneumonia on percentage saturation of oxygen in the
pulmonary artery, the right and left pulmonary veins, and the aorta.
is pliable enough, this will lead simply to collapse of
the alveoli. However, if the lung is rigid because of
fibrotic tissue and cannot collapse, absorption of air
from the alveoli creates very negative pressures within
the alveoli, which pull fluid out of the pulmonary capillaries into the alveoli, thus causing the alveoli to fill
completely with edema fluid. This almost always is the
effect that occurs when an entire lung becomes
atelectatic, a condition called massive collapse of the
lung.
The effects on overall pulmonary function caused by
massive collapse (atelectasis) of an entire lung are
shown in Figure 42–7. Collapse of the lung tissue not
only occludes the alveoli but also almost always
increases the resistance to blood flow through the pulmonary vessels of the collapsed lung. This resistance
Chapter 42
Respiratory Insufficiency—Pathophysiology, Diagnosis, Oxygen Therapy
Asthma
Pulmonary arterial blood
60% saturated with O2
Atelectasis
Right
pulmonary
vein 97%
saturated
529
Left
pulmonary
vein 60%
saturatedflow 1/5 normal
Aorta:
Blood 5/6 = 97%
1/6 = 60%
Mean saturation
= 91%
Figure 42–7
Effect of atelectasis on aortic blood oxygen saturation.
increase occurs partially because of the lung collapse
itself, which compresses and folds the vessels as the
volume of the lung decreases. In addition, hypoxia in
the collapsed alveoli causes additional vasoconstriction, as explained in Chapter 38.
Because of the vascular constriction, blood flow
through the atelectatic lung becomes slight. Fortunately, most of the blood is routed through the ventilated lung and therefore becomes well aerated. In the
situation shown in Figure 42–7, five sixths of the blood
passes through the aerated lung and only one sixth
through the unaerated lung. As a result, the overall
ventilation-perfusion ratio is only moderately compromised, so that the aortic blood has only mild
oxygen desaturation despite total loss of ventilation in
an entire lung.
Lack of “Surfactant” as a Cause of Lung Collapse. The secretion and function of surfactant in the alveoli were
discussed in Chapter 37. It was pointed out that the
surfactant is secreted by special alveolar epithelial
cells into the fluids that coat the inside surface of the
alveoli. The surfactant in turn decreases the surface
tension in the alveoli 2- to 10-fold, which normally
plays a major role in preventing alveolar collapse.
However, in a number of conditions, such as in hyaline
membrane disease (also called respiratory distress syndrome), which often occurs in newborn premature
babies, the quantity of surfactant secreted by the
alveoli is so greatly depressed that the surface tension
of the alveolar fluid becomes several times normal.
This causes a serious tendency for the lungs of these
babies to collapse or to become filled with fluid. As
explained in Chapter 37, many of these infants die of
suffocation when large portions of the lungs become
atelectatic.
Asthma is characterized by spastic contraction of the
smooth muscle in the bronchioles, which partially
obstructs the bronchioles and causes extremely difficult breathing. It occurs in 3 to 5 per cent of all people
at some time in life.
The usual cause of asthma is contractile hypersensitivity of the bronchioles in response to foreign substances in the air. In about 70 per cent of patients
younger than age 30 years, the asthma is caused by
allergic hypersensitivity, especially sensitivity to plant
pollens. In older people, the cause is almost always
hypersensitivity to nonallergenic types of irritants in
the air, such as irritants in smog.
The allergic reaction that occurs in the allergic type
of asthma is believed to occur in the following way: The
typical allergic person has a tendency to form abnormally large amounts of IgE antibodies, and these antibodies cause allergic reactions when they react with the
specific antigens that have caused them to develop in
the first place, as explained in Chapter 34. In asthma,
these antibodies are mainly attached to mast cells that
are present in the lung interstitium in close association
with the bronchioles and small bronchi. When the asthmatic person breathes in pollen to which he or she is
sensitive (that is, to which the person has developed
IgE antibodies), the pollen reacts with the mast cell–
attached antibodies and causes the mast cells to release
several different substances. Among them are (a)
histamine, (b) slow-reacting substance of anaphylaxis
(which is a mixture of leukotrienes), (c) eosinophilic
chemotactic factor, and (d) bradykinin. The combined
effects of all these factors, especially the slow-reacting
substance of anaphylaxis, are to produce (1) localized
edema in the walls of the small bronchioles, as well as
secretion of thick mucus into the bronchiolar lumens,
and (2) spasm of the bronchiolar smooth muscle.
Therefore, the airway resistance increases greatly.
As discussed earlier in this chapter, the bronchiolar
diameter becomes more reduced during expiration
than during inspiration in asthma, caused by bronchiolar collapse during expiratory effort that compresses
the outsides of the bronchioles. Because the bronchioles of the asthmatic lungs are already partially
occluded, further occlusion resulting from the external
pressure creates especially severe obstruction during
expiration. That is, the asthmatic person often can
inspire quite adequately but has great difficulty expiring. Clinical measurements show (1) greatly reduced
maximum expiratory rate and (2) reduced timed expiratory volume. Also, all of this together results in
dyspnea, or “air hunger,” which is discussed later in
this chapter.
The functional residual capacity and residual volume
of the lung become especially increased during the
acute asthmatic attack because of the difficulty in
expiring air from the lungs. Also, over a period of
years, the chest cage becomes permanently enlarged,
causing a “barrel chest,” and both the functional residual capacity and lung residual volume become permanently increased.
530
Unit VII
Tuberculosis
In tuberculosis, the tubercle bacilli cause a peculiar
tissue reaction in the lungs, including (1) invasion of
the infected tissue by macrophages and (2) “walling
off” of the lesion by fibrous tissue to form the so-called
tubercle. This walling-off process helps to limit further
transmission of the tubercle bacilli in the lungs and
therefore is part of the protective process against
extension of the infection. However, in about 3 per
cent of all people who develop tuberculosis, if
untreated, the walling-off process fails and tubercle
bacilli spread throughout the lungs, often causing
extreme destruction of lung tissue with formation of
large abscess cavities.
Thus, tuberculosis in its late stages is characterized
by many areas of fibrosis throughout the lungs, as well
as reduced total amount of functional lung tissue.
These effects cause (1) increased “work” on the part
of the respiratory muscles to cause pulmonary ventilation and reduced vital capacity and breathing capacity; (2) reduced total respiratory membrane surface area
and increased thickness of the respiratory membrane,
causing progressively diminished pulmonary diffusing
capacity; and (3) abnormal ventilation-perfusion ratio
in the lungs, further reducing overall pulmonary diffusion of oxygen and carbon dioxide.
Hypoxia and Oxygen Therapy
Almost any of the conditions discussed in the past
few sections of this chapter can cause serious degrees
of bodywide cellular hypoxia. Sometimes, oxygen
therapy is of great value; other times, it is of moderate
value; and, at still other times, it is of almost no value.
Therefore, it is important to understand the different
types of hypoxia; then we can discuss the physiologic
principles of oxygen therapy. The following is a
descriptive classification of the causes of hypoxia:
1. Inadequate oxygenation of the blood in the lungs
because of extrinsic reasons
a. Deficiency of oxygen in the atmosphere
b. Hypoventilation (neuromuscular disorders)
2. Pulmonary disease
a. Hypoventilation caused by increased airway
resistance or decreased pulmonary compliance
b. Abnormal alveolar ventilation-perfusion ratio
(including either increased physiologic dead
space or increased physiologic shunt)
c. Diminished respiratory membrane diffusion
3. Venous-to-arterial shunts (“right-to-left” cardiac
shunts)
4. Inadequate oxygen transport to the tissues by the
blood
a. Anemia or abnormal hemoglobin
b. General circulatory deficiency
c. Localized circulatory deficiency (peripheral,
cerebral, coronary vessels)
d. Tissue edema
5. Inadequate tissue capability of using oxygen
a. Poisoning of cellular oxidation enzymes
Respiration
b. Diminished cellular metabolic capacity for using
oxygen, because of toxicity, vitamin deficiency, or
other factors
This classification of the types of hypoxia is mainly
self-evident from the discussions earlier in the chapter.
Only one of the types of hypoxia in the classification
needs further elaboration: this is the hypoxia caused
by inadequate capability of the body’s tissue cells to
use oxygen.
Inadequate Tissue Capability to Use Oxygen. The classic
cause of inability of the tissues to use oxygen is cyanide
poisoning, in which the action of the enzyme
cytochrome oxidase is completely blocked by the
cyanide—to such an extent that the tissues simply
cannot use oxygen even when plenty is available. Also,
deficiencies of some of the tissue cellular oxidative
enzymes or of other elements in the tissue oxidative
system can lead to this type of hypoxia. A special
example occurs in the disease beriberi, in which several
important steps in tissue utilization of oxygen and
formation of carbon dioxide are compromised because
of vitamin B deficiency.
Effects of Hypoxia on the Body. Hypoxia, if severe enough,
can cause death of cells throughout the body, but in
less severe degrees it causes principally (1) depressed
mental activity, sometimes culminating in coma, and
(2) reduced work capacity of the muscles. These effects
are specifically discussed in Chapter 43 in relation to
high-altitude physiology.
Oxygen Therapy in Different Types
of Hypoxia
Oxygen can be administered by (1) placing the
patient’s head in a “tent” that contains air fortified
with oxygen, (2) allowing the patient to breathe either
pure oxygen or high concentrations of oxygen from
a mask, or (3) administering oxygen through an
intranasal tube.
Recalling the basic physiologic principles of the
different types of hypoxia, one can readily decide
when oxygen therapy will be of value and, if so, how
valuable.
In atmospheric hypoxia, oxygen therapy can completely correct the depressed oxygen level in the
inspired gases and, therefore, provide 100 per cent
effective therapy.
In hypoventilation hypoxia, a person breathing 100
per cent oxygen can move five times as much oxygen
into the alveoli with each breath as when breathing
normal air. Therefore, here again oxygen therapy can
be extremely beneficial. (However, this provides no
benefit for the excess blood carbon dioxide also caused
by the hypoventilation.)
In hypoxia caused by impaired alveolar membrane
diffusion, essentially the same result occurs as in
hypoventilation hypoxia, because oxygen therapy can
increase the Po2 in the lung alveoli from the normal
value of about 100 mm Hg to as high as 600 mm Hg.
PO2 in alveoli and blood (mm Hg)
Chapter 42
Respiratory Insufficiency—Pathophysiology, Diagnosis, Oxygen Therapy
300
200
Alveolar PO2 with tent therapy
Normal alveolar PO2
Pulmonary edema + O2 therapy
Pulmonary edema with no therapy
100
Capillary blood
0
Arterial end
Venous end
Blood in pulmonary capillary
Figure 42–8
Absorption of oxygen into the pulmonary capillary blood in pulmonary edema with and without oxygen tent therapy.
This raises the oxygen pressure gradient for diffusion
of oxygen from the alveoli to the blood from the
normal value of 60 mm Hg to as high as 560 mm Hg,
an increase of more than 800 per cent. This highly beneficial effect of oxygen therapy in diffusion hypoxia is
demonstrated in Figure 42–8, which shows that the
pulmonary blood in this patient with pulmonary
edema picks up oxygen three to four times as rapidly
as would occur with no therapy.
In hypoxia caused by anemia, abnormal hemoglobin
transport of oxygen, circulatory deficiency, or physiologic shunt, oxygen therapy is of much less value
because normal oxygen is already available in the
alveoli. The problem instead is that one or more of the
mechanisms for transporting oxygen from the lungs to
the tissues is deficient. Even so, a small amount of
extra oxygen, between 7 and 30 per cent, can be transported in the dissolved state in the blood when alveolar oxygen is increased to maximum even though the
amount transported by the hemoglobin is hardly
altered. This small amount of extra oxygen may be the
difference between life and death.
In the different types of hypoxia caused by inadequate tissue use of oxygen, there is abnormality neither
of oxygen pickup by the lungs nor of transport to the
tissues. Instead, the tissue metabolic enzyme system is
simply incapable of using the oxygen that is delivered.
Therefore, oxygen therapy is of hardly any measurable benefit.
Cyanosis
The term cyanosis means blueness of the skin, and its
cause is excessive amounts of deoxygenated hemoglobin in the skin blood vessels, especially in the capillaries. This deoxygenated hemoglobin has an intense
531
dark blue-purple color that is transmitted through the
skin.
In general, definite cyanosis appears whenever the
arterial blood contains more than 5 grams of deoxygenated hemoglobin in each 100 milliliters of blood. A
person with anemia almost never becomes cyanotic
because there is not enough hemoglobin for 5 grams
to be deoxygenated in 100 milliliters of arterial blood.
Conversely, in a person with excess red blood cells, as
occurs in polycythemia vera, the great excess of available hemoglobin that can become deoxygenated leads
frequently to cyanosis, even under otherwise normal
conditions.
Hypercapnia
Hypercapnia means excess carbon dioxide in the body
fluids.
One might suspect, on first thought, that any respiratory condition that causes hypoxia would also cause
hypercapnia. However, hypercapnia usually occurs
in association with hypoxia only when the hypoxia
is caused by hypoventilation or circulatory deficiency.
The reasons for this are the following.
Hypoxia caused by too little oxygen in the air, too
little hemoglobin, or poisoning of the oxidative
enzymes has to do only with the availability of oxygen
or use of oxygen by the tissues. Therefore, it is readily
understandable that hypercapnia is not a concomitant
of these types of hypoxia.
In hypoxia resulting from poor diffusion through
the pulmonary membrane or through the tissues,
serious hypercapnia usually does not occur at the same
time because carbon dioxide diffuses 20 times as
rapidly as oxygen. If hypercapnia does begin to occur,
this immediately stimulates pulmonary ventilation,
which corrects the hypercapnia but not necessarily the
hypoxia.
Conversely, in hypoxia caused by hypoventilation,
carbon dioxide transfer between the alveoli and the
atmosphere is affected as much as is oxygen transfer.
Hypercapnia then occurs along with the hypoxia. And
in circulatory deficiency, diminished flow of blood
decreases carbon dioxide removal from the tissues,
resulting in tissue hypercapnia in addition to tissue
hypoxia. However, the transport capacity of the blood
for carbon dioxide is more than three times that for
oxygen, so that the resulting tissue hypercapnia is
much less than the tissue hypoxia.
When the alveolar Pco2 rises above about 60 to
75 mm Hg, an otherwise normal person by then is
breathing about as rapidly and deeply as he or she
can, and “air hunger,” also called dyspnea, becomes
severe.
If the Pco2 rises to 80 to 100 mm Hg, the person
becomes lethargic and sometimes even semicomatose.
Anesthesia and death can result when the Pco2 rises
to 120 to 150 mm Hg. At these higher levels of Pco2,
the excess carbon dioxide now begins to depress respiration rather than stimulate it, thus causing a vicious
circle: (1) more carbon dioxide, (2) further decrease in
532
Unit VII
Respiration
respiration, (3) then more carbon dioxide, and so
forth—culminating rapidly in a respiratory death.
A
Mechanism
for applying
positive and
negative
pressure
Dyspnea
Dyspnea means mental anguish associated with inability to ventilate enough to satisfy the demand for air. A
common synonym is air hunger.
At least three factors often enter into the development of the sensation of dyspnea. They are (1) abnormality of respiratory gases in the body fluids, especially
hypercapnia and, to a much less extent, hypoxia; (2)
the amount of work that must be performed by the respiratory muscles to provide adequate ventilation; and
(3) state of mind.
A person becomes very dyspneic especially from
excess buildup of carbon dioxide in the body fluids. At
times, however, the levels of both carbon dioxide and
oxygen in the body fluids are normal, but to attain this
normality of the respiratory gases, the person has to
breathe forcefully. In these instances, the forceful
activity of the respiratory muscles frequently gives the
person a sensation of dyspnea.
Finally, the person’s respiratory functions may be
normal and still dyspnea may be experienced because
of an abnormal state of mind. This is called neurogenic
dyspnea or emotional dyspnea. For instance, almost
anyone momentarily thinking about the act of breathing may suddenly start taking breaths a little more
deeply than ordinarily because of a feeling of mild
dyspnea. This feeling is greatly enhanced in people
who have a psychological fear of not being able to
receive a sufficient quantity of air, such as on entering
small or crowded rooms.
Artificial Respiration
Resuscitator. Many types of respiratory resuscitators
are available, and each has its own characteristic principles of operation. The resuscitator shown in Figure
42–9A consists of a tank supply of oxygen or air; a
mechanism for applying intermittent positive pressure
and, with some machines, negative pressure as well;
and a mask that fits over the face of the patient or a
connector for joining the equipment to an endotracheal tube. This apparatus forces air through the mask
or endotracheal tube into the lungs of the patient
during the positive-pressure cycle of the resuscitator
and then usually allows the air to flow passively out of
the lungs during the remainder of the cycle.
Earlier resuscitators often caused damage to the
lungs because of excessive positive pressure. Their
usage was at one time greatly decried. However, resuscitators now have adjustable positive-pressure limits
that are commonly set at 12 to 15 cm H2O pressure for
normal lungs (but sometimes much higher for noncompliant lungs).
Tank Respirator (the “Iron-Lung”). Figure 42–9B shows the
tank respirator with a patient’s body inside the tank
B
Positive
pressure
valve
Negative
pressure
valve
Leather diaphragm
Figure 42–9
A, Resuscitator. B, Tank respirator.
and the head protruding through a flexible but airtight
collar. At the end of the tank opposite the patient’s
head a motor-driven leather diaphragm moves back
and forth with sufficient excursion to raise and lower
the pressure inside the tank. As the leather diaphragm
moves inward, positive pressure develops around the
body and causes expiration; as the diaphragm moves
outward, negative pressure causes inspiration. Check
valves on the respirator control the positive and
negative pressures. Ordinarily these pressures are
adjusted so that the negative pressure that causes
inspiration falls to -10 to -20 cm H2O and the positive
pressure rises to 0 to +5 cm H2O.
Effect of the Resuscitator and the Tank Respirator on Venous
Return. When air is forced into the lungs under posi-
tive pressure by a resuscitator, or when the pressure
around the patient’s body is reduced by the tank
respirator, the pressure inside the lungs becomes
greater than pressure everywhere else in the body.
Flow of blood into the chest and heart from the
peripheral veins becomes impeded. As a result, use
of excessive pressures with either the resuscitator or
the tank respirator can reduce the cardiac output—
sometimes to lethal levels. For instance, continuous
exposure for more than a few minutes to greater than
30 mm Hg positive pressure in the lungs can cause
death because of inadequate venous return to the
heart.
Chapter 42
Respiratory Insufficiency—Pathophysiology, Diagnosis, Oxygen Therapy
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Barnes PJ, Adcock IM: How do corticosteroids work in
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Basu S, Fenton MJ: Toll-like receptors: function and roles in
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L887, 2004.
Cardoso WV: Molecular regulation of lung development.
Annu Rev Physiol 63:471, 2001.
Carter EP, Garat C, Imamura M: Continual emerging roles
of HO-1: protection against airway inflammation. Am J
Physiol Lung Cell Mol Physiol 287:L24, 2004.
Dwyer TM: Cigarette smoke-induced airway inflammation
as sampled by the expired breath condensate. Am J Med
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Knight DA, Holgate ST: The airway epithelium: structural
and functional properties in health and disease. Respirology 8:432, 2003.
McConnell AK, Romer LM: Dyspnoea in health and
obstructive pulmonary disease: the role of respiratory
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Naureckas ET, Solway J: Clinical practice. Mild asthma.
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Page S, Ammit AJ, Black JL, Armour CL: Human mast cell
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L1313, 2001.
Rodrigo GJ, Rodrigo C, Hall JB: Acute asthma in adults: a
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Schwiebert LM: Cystic fibrosis, gene therapy, and lung
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Mol Physiol 286:L715, 2004.
Sin DD, McAlister FA, Man SF, Anthonisen NR: Contemporary management of chronic obstructive pulmonary
disease: scientific review. JAMA 290:2301, 2003.
Wardlaw AJ, Brightling CE, Green R, et al: New insights
into the relationship between airway inflammation and
asthma. Clin Sci (Lond) 103:201, 2002.
West JB: Pulmonary Physiology and Pathophysiology: An
Integrated, Case-Based Approach. Philadelphia: Lippincott Williams & Wilkins, 2001.
Whitsett JA, Weaver TE: Hydrophobic surfactant proteins
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2002.
Wills-Karp M, Ewart SL: Time to draw breath: asthmasusceptibility genes are identified. Nat Rev Genet 5:376,
2004.
U
N
I
Aviation, Space, and
Deep-Sea Diving
Physiology
43. Aviation, High-Altitude, and Space Physiology
44. Physiology of Deep-Sea Diving and Other
Hyperbaric Conditions
T
VIII
C
H
A
P
T
E
R
4
3
Aviation, High-Altitude,
and Space Physiology
As we have ascended to higher and higher altitudes
in aviation, mountain climbing, and space vehicles,
it has become progressively more important to
understand the effects of altitude and low gas pressures (as well as several other factors—acceleratory
forces, weightlessness, and so forth) on the human
body. This chapter deals with these problems.
Effects of Low Oxygen Pressure on the Body
Barometric Pressures at Different Altitudes. Table 43–1 gives the approximate barometric and oxygen pressures at different altitudes, showing that at sea level, the
barometric pressure is 760 mm Hg; at 10,000 feet, only 523 mm Hg; and at
50,000 feet, 87 mm Hg. This decrease in barometric pressure is the basic cause
of all the hypoxia problems in high-altitude physiology because, as the barometric pressure decreases, the atmospheric oxygen partial pressure decreases
proportionately, remaining at all times slightly less than 21 per cent of the total
barometric pressure—Po2 at sea level about 159 mm Hg, but at 50,000 feet only
18 mm Hg.
Alveolar PO2 at Different Elevations
Carbon Dioxide and Water Vapor Decrease the Alveolar Oxygen. Even at high altitudes,
carbon dioxide is continually excreted from the pulmonary blood into the
alveoli. Also, water vaporizes into the inspired air from the respiratory surfaces.
These two gases dilute the oxygen in the alveoli, thus reducing the oxygen concentration. Water vapor pressure in the alveoli remains 47 mm Hg as long as
the body temperature is normal, regardless of altitude.
In the case of carbon dioxide, during exposure to very high altitudes, the alveolar Pco2 falls from the sea-level value of 40 mm Hg to lower values. In the
acclimatized person, who increases his or her ventilation about fivefold, the Pco2
falls to about 7 mm Hg because of increased respiration.
Now let us see how the pressures of these two gases affect the alveolar
oxygen. For instance, assume that the barometric pressure falls from the normal
sea-level value of 760 mm Hg to 253 mm Hg, which is the usual measured value
at the top of 29,028–foot Mount Everest. Forty-seven millimeters of mercury of
this must be water vapor, leaving only 206 mm Hg for all the other gases. In the
acclimatized person, 7 mm of the 206 mm Hg must be carbon dioxide, leaving
only 199 mm Hg. If there were no use of oxygen by the body, one fifth of this
199 mm Hg would be oxygen and four fifths would be nitrogen; that is, the Po2
in the alveoli would be 40 mm Hg. However, some of this remaining alveolar
oxygen is continually being absorbed into the blood, leaving about 35 mm Hg
oxygen pressure in the alveoli. At the summit of Mount Everest, only the best
of acclimatized people can barely survive when breathing air. But the effect is
very different when the person is breathing pure oxygen, as we see in the following discussions.
537
538
Unit VIII
Aviation, Space, and Deep-Sea Diving Physiology
Table 43–1
Effects of Acute Exposure to Low Atmospheric Pressures on Alveolar Gas Concentrations and Arterial Oxygen Saturation*
Breathing Air
Altitude (ft)
0
10,000
20,000
30,000
40,000
50,000
Barometric
Pressure
(mm Hg)
PO2 in
Air
(mm Hg)
PCO2 in
Alveoli
(mm Hg)
PO2 in
Alveoli
(mm Hg)
760
523
349
226
141
87
159
110
73
47
29
18
40 (40)
36 (23)
24 (10)
24 (7)
104
67
40
18
Breathing Pure Oxygen
Arterial
Oxygen
Saturation
(%)
(104)
(77)
(53)
(30)
97
90
73
24
(97)
(92)
(85)
(38)
PCO2 in
Alveoli
(mm Hg)
PO2 in
Alveoli
(mm Hg)
40
40
40
40
36
24
673
436
262
139
58
16
Arterial
Oxygen
Saturation
(%)
100
100
100
99
84
15
Alveolar PO2 at Different Altitudes. The fifth column of
Table 43–1 shows the approximate Po2s in the alveoli
at different altitudes when one is breathing air for both
the unacclimatized and the acclimatized person. At sea
level, the alveolar Po2 is 104 mm Hg; at 20,000 feet altitude, it falls to about 40 mm Hg in the unacclimatized
person but only to 53 mm Hg in the acclimatized. The
difference between these two is that alveolar ventilation increases much more in the acclimatized person
than in the unacclimatized person, as we discuss later.
Saturation of Hemoglobin with Oxygen at Different Altitudes.
Figure 43–1 shows arterial blood oxygen saturation at
different altitudes while a person is breathing air and
while breathing oxygen. Up to an altitude of about
10,000 feet, even when air is breathed, the arterial
oxygen saturation remains at least as high as 90 per
cent. Above 10,000 feet, the arterial oxygen saturation
falls rapidly, as shown by the blue curve of the figure,
until it is slightly less than 70 per cent at 20,000 feet
and much less at still higher altitudes.
Effect of Breathing Pure Oxygen on
Alveolar PO2 at Different Altitudes
Arterial oxygen saturation (per cent)
* Numbers in parentheses are acclimatized values.
Breathing pure oxygen
Breathing air
100
90
80
70
60
50
0
10
20
30
40
Altitude (thousands of feet)
50
Figure 43–1
Effect of high altitude on arterial oxygen saturation when breathing air and when breathing pure oxygen.
When a person breathes pure oxygen instead of air,
most of the space in the alveoli formerly occupied by
nitrogen becomes occupied by oxygen. At 30,000 feet,
an aviator could have an alveolar Po2 as high as
139 mm Hg instead of the 18 mm Hg when breathing
air (see Table 43–1).
The red curve of Figure 43–1 shows arterial blood
hemoglobin oxygen saturation at different altitudes
when one is breathing pure oxygen. Note that the saturation remains above 90 per cent until the aviator
ascends to about 39,000 feet; then it falls rapidly to
about 50 per cent at about 47,000 feet.
breathing pure oxygen in an unpressurized airplane
can ascend to far higher altitudes than one breathing
air. For instance, the arterial saturation at 47,000 feet
when one is breathing oxygen is about 50 per cent and
is equivalent to the arterial oxygen saturation at 23,000
feet when one is breathing air. In addition, because an
unacclimatized person usually can remain conscious
until the arterial oxygen saturation falls to 50 per cent,
for short exposure times the ceiling for an aviator in
an unpressurized airplane when breathing air is about
23,000 feet and when breathing pure oxygen is about
47,000 feet, provided the oxygen-supplying equipment
operates perfectly.
The “Ceiling” When Breathing Air
and When Breathing Oxygen in an
Unpressurized Airplane
Acute Effects of Hypoxia
Comparing the two arterial blood oxygen saturation curves in Figure 43–1, one notes that an aviator
Some of the important acute effects of hypoxia in the
unacclimatized person breathing air, beginning at an
Chapter 43
Aviation, High-Altitude, and Space Physiology
altitude of about 12,000 feet, are drowsiness, lassitude,
mental and muscle fatigue, sometimes headache, occasionally nausea, and sometimes euphoria.These effects
progress to a stage of twitchings or seizures above
18,000 feet and end, above 23,000 feet in the unacclimatized person, in coma, followed shortly thereafter
by death.
One of the most important effects of hypoxia is
decreased mental proficiency, which decreases judgment, memory, and performance of discrete motor
movements. For instance, if an unacclimatized aviator
stays at 15,000 feet for 1 hour, mental proficiency ordinarily falls to about 50 per cent of normal, and after
18 hours at this level it falls to about 20 per cent of
normal.
539
An important mechanism for the gradual decrease
in bicarbonate concentration is compensation by the
kidneys for the respiratory alkalosis, as discussed in
Chapter 30. The kidneys respond to decreased Pco2
by reducing hydrogen ion secretion and increasing
bicarbonate excretion. This metabolic compensation
for the respiratory alkalosis gradually reduces plasma
and cerebrospinal fluid bicarbonate concentration and
pH toward normal and removes part of the inhibitory
effect on respiration of low hydrogen ion concentration. Thus, the respiratory centers are much more
responsive to the peripheral chemoreceptor stimulus
caused by the hypoxia after the kidneys compensate
for the alkalosis.
Increase in Red Blood Cells and Hemoglobin Concentration
During Acclimatization. As discussed in Chapter 32,
Acclimatization to Low PO2
A person remaining at high altitudes for days, weeks,
or years becomes more and more acclimatized to the
low Po2, so that it causes fewer deleterious effects on
the body. And it becomes possible for the person to
work harder without hypoxic effects or to ascend to
still higher altitudes.
The principal means by which acclimatization comes
about are (1) a great increase in pulmonary ventilation, (2) increased numbers of red blood cells, (3)
increased diffusing capacity of the lungs, (4) increased
vascularity of the peripheral tissues, and (5) increased
ability of the tissue cells to use oxygen despite low Po2.
Increased Pulmonary Ventilation—Role of Arterial Chemoreceptors. Immediate exposure to low Po2 stimulates the
arterial chemoreceptors, and this increases alveolar
ventilation to a maximum of about 1.65 times normal.
Therefore, compensation occurs within seconds for the
high altitude, and it alone allows the person to rise
several thousand feet higher than would be possible
without the increased ventilation. Then, if the person
remains at very high altitude for several days, the
chemoreceptors increase ventilation still more, up to
about five times normal.
The immediate increase in pulmonary ventilation on
rising to a high altitude blows off large quantities of
carbon dioxide, reducing the Pco2 and increasing the
pH of the body fluids. These changes inhibit the brain
stem respiratory center and thereby oppose the effect
of low PO2 to stimulate respiration by way of the
peripheral arterial chemoreceptors in the carotid and
aortic bodies. But during the ensuing 2 to 5 days, this
inhibition fades away, allowing the respiratory center
to respond with full force to the peripheral chemoreceptor stimulus from hypoxia, and ventilation increases to about five times normal.
The cause of this fading inhibition is believed to be
mainly a reduction of bicarbonate ion concentration in
the cerebrospinal fluid as well as in the brain tissues.
This in turn decreases the pH in the fluids surrounding the chemosensitive neurons of the respiratory
center, thus increasing the respiratory stimulatory
activity of the center.
hypoxia is the principal stimulus for causing an
increase in red blood cell production. Ordinarily, when
a person remains exposed to low oxygen for weeks at
a time, the hematocrit rises slowly from a normal value
of 40 to 45 to an average of about 60, with an average
increase in whole blood hemoglobin concentration
from normal of 15 g/dl to about 20 g/dl.
In addition, the blood volume also increases, often
by 20 to 30 per cent, and this increase times the
increased blood hemoglobin concentration gives an
increase in total body hemoglobin of 50 or more per
cent.
Increased Diffusing Capacity After Acclimatization. It will
be recalled that the normal diffusing capacity for
oxygen through the pulmonary membrane is about
21 ml/mm Hg/min, and this diffusing capacity can
increase as much as threefold during exercise. A
similar increase in diffusing capacity occurs at high
altitude.
Part of the increase results from increased pulmonary capillary blood volume, which expands the
capillaries and increases the surface area through
which oxygen can diffuse into the blood. Another part
results from an increase in lung air volume, which
expands the surface area of the alveolar-capillary
interface still more. A final part results from an
increase in pulmonary arterial blood pressure; this
forces blood into greater numbers of alveolar capillaries than normally—especially in the upper parts
of the lungs, which are poorly perfused under usual
conditions.
Peripheral Circulatory System Changes During Acclimatization—Increased Tissue Capillarity. The cardiac output
often increases as much as 30 per cent immediately
after a person ascends to high altitude but then
decreases back toward normal over a period of weeks
as the blood hematocrit increases, so that the amount
of oxygen transported to the peripheral body tissues
remains about normal.
Another circulatory adaptation is growth of
increased numbers of systemic circulatory capillaries
in the nonpulmonary tissues, which is called increased tissue capillarity (or angiogenesis). This occurs
Unit VIII
Aviation, Space, and Deep-Sea Diving Physiology
especially in animals born and bred at high altitudes
but less so in animals that later in life become exposed
to high altitude.
In active tissues exposed to chronic hypoxia, the
increase in capillarity is especially marked. For
instance, capillary density in right ventricular muscle
increases markedly because of the combined effects
of hypoxia and excess workload on the right ventricle
caused by pulmonary hypertension at high altitude.
Cellular Acclimatization. In animals native to altitudes of
13,000 to 17,000 feet, cell mitochondria and cellular
oxidative enzyme systems are slightly more plentiful
than in sea-level inhabitants. Therefore, it is presumed
that the tissue cells of high altitude–acclimatized
human beings also can use oxygen more effectively
than can their sea-level counterparts.
Natural Acclimatization
of Native Human Beings Living
at High Altitudes
Many native human beings in the Andes and in the
Himalayas live at altitudes above 13,000 feet—one
group in the Peruvian Andes lives at an altitude of
17,500 feet and works a mine at an altitude of 19,000
feet. Many of these natives are born at these altitudes
and live there all their lives. In all aspects of acclimatization, the natives are superior to even the bestacclimatized lowlanders, even though the lowlanders
might also have lived at high altitudes for 10 or more
years. Acclimatization of the natives begins in infancy.
The chest size, especially, is greatly increased, whereas
the body size is somewhat decreased, giving a high
ratio of ventilatory capacity to body mass. In addition,
their hearts, which from birth onward pump extra
amounts of cardiac output, are considerably larger
than the hearts of lowlanders.
Delivery of oxygen by the blood to the tissues is also
highly facilitated in these natives. For instance, Figure
43–2 shows oxygen-hemoglobin dissociation curves for
natives who live at sea level and for their counterparts
who live at 15,000 feet. Note that the arterial oxygen
Po2 in the natives at high altitude is only 40 mm Hg,
but because of the greater quantity of hemoglobin,
the quantity of oxygen in their arterial blood is greater
than that in the blood of the natives at the lower
altitude. Note also that the venous Po2 in the highaltitude natives is only 15 mm Hg less than the venous
Po2 for the lowlanders, despite the very low arterial
Po2, indicating that oxygen transport to the tissues is
exceedingly effective in the naturally acclimatized
high-altitude natives.
Reduced Work Capacity at
High Altitudes and Positive Effect
of Acclimatization
In addition to the mental depression caused by
hypoxia, as discussed earlier, the work capacity of all
muscles is greatly decreased in hypoxia. This includes
Quantity of oxygen in blood (vol %)
540
Mountain dwellers
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
(15,000 ft)
(Arterial values)
X
X
X
X
Sea-level dwellers
(Venous values)
0
20 40 60 80 100 120 140
Pressure of oxygen in blood (PO2) (mm Hg)
Figure 43–2
Oxygen-hemoglobin dissociation curves for blood of high-altitude
residents (red curve) and sea-level residents (blue curve),
showing the respective arterial and venous PO2 levels and oxygen
contents as recorded in their native surroundings. (Data from
Oxygen-dissociation curves for bloods of high-altitude and sealevel residents. PAHO Scientific Publication No. 140, Life at High
Altitudes, 1966.)
not only skeletal muscles but also cardiac muscles.
In general, work capacity is reduced in direct proportion to the decrease in maximum rate of oxygen
uptake that the body can achieve.
To give an idea of the importance of acclimatization
in increasing work capacity, consider this: The work
capacities as per cent of normal for unacclimatized and
acclimatized people at an altitude of 17,000 feet are as
follows:
Work capacity
(per cent of normal)
Unacclimatized
Acclimatized for 2 months
Native living at 13,200 feet but working at
17,000 feet
Thus, naturally acclimatized native
achieve a daily work output even at
almost equal to that of a lowlander at
even well-acclimatized lowlanders can
achieve this result.
50
68
87
persons can
high altitude
sea level, but
almost never
Acute Mountain Sickness and
High-Altitude Pulmonary Edema
A small percentage of people who ascend rapidly to
high altitudes become acutely sick and can die if not
Chapter 43
541
Aviation, High-Altitude, and Space Physiology
given oxygen or removed to a low altitude. The sickness begins from a few hours up to about 2 days after
ascent. Two events frequently occur:
1. Acute cerebral edema. This is believed to result
from local vasodilation of the cerebral blood
vessels, caused by the hypoxia. Dilation of the
arterioles increases blood flow into the capillaries,
thus increasing capillary pressure, which in turn
causes fluid to leak into the cerebral tissues.
The cerebral edema can then lead to severe
disorientation and other effects related to cerebral
dysfunction.
2. Acute pulmonary edema. The cause of this is still
unknown, but a suggested answer is the following:
The severe hypoxia causes the pulmonary arterioles
to constrict potently, but the constriction is much
greater in some parts of the lungs than in other
parts, so that more and more of the pulmonary
blood flow is forced through fewer and fewer still
unconstricted pulmonary vessels. The postulated
result is that the capillary pressure in these areas of
the lungs becomes especially high and local edema
occurs. Extension of the process to progressively
more areas of the lungs leads to spreading
pulmonary edema and severe pulmonary
dysfunction that can be lethal. Allowing the person
to breathe oxygen usually reverses the process
within hours.
Effects of Acceleratory Forces
on the Body in Aviation and
Space Physiology
Because of rapid changes in velocity and direction of
motion in airplanes or spacecraft, several types of
acceleratory forces affect the body during flight. At the
beginning of flight, simple linear acceleration occurs;
at the end of flight, deceleration; and every time the
vehicle turns, centrifugal acceleration.
Centrifugal Acceleratory Forces
When an airplane makes a turn, the force of centrifugal acceleration is determined by the following
relation:
f=
mv 2
r
in which f is centrifugal acceleratory force, m is the
mass of the object, v is velocity of travel, and r is radius
of curvature of the turn. From this formula, it is
obvious that as the velocity increases, the force of
centrifugal acceleration increases in proportion to the
square of the velocity. It is also obvious that the force
of acceleration is directly proportional to the sharpness
of the turn (the less the radius).
Chronic Mountain Sickness
of Acceleratory Force—“G.” When an
aviator is simply sitting in his seat, the force with which
he is pressing against the seat results from the pull of
gravity and is equal to his weight. The intensity of this
force is said to be +1 G because it is equal to the pull
of gravity. If the force with which he presses against
the seat becomes five times his normal weight during
pull-out from a dive, the force acting on the seat is
+5 G.
If the airplane goes through an outside loop so that
the person is held down by his seat belt, negative G is
applied to his body; if the force with which he is held
down by his belt is equal to the weight of his body, the
negative force is -1 G.
Measurement
Occasionally, a person who remains at high altitude
too long develops chronic mountain sickness, in which
the following effects occur: (1) the red cell mass and
hematocrit become exceptionally high, (2) the pulmonary arterial pressure becomes elevated even more
than the normal elevation that occurs during acclimatization, (3) the right side of the heart becomes greatly
enlarged, (4) the peripheral arterial pressure begins to
fall, (5) congestive heart failure ensues, and (6) death
often follows unless the person is removed to a lower
altitude.
The causes of this sequence of events are probably
threefold: First, the red cell mass becomes so great that
the blood viscosity increases severalfold; this increased
viscosity tends to decrease tissue blood flow so that
oxygen delivery also begins to decrease. Second, the
pulmonary arterioles become vasoconstricted because
of the lung hypoxia. This results from the hypoxic
vascular constrictor effect that normally operates to
divert blood flow from low-oxygen to high-oxygen
alveoli, as explained in Chapter 38. But because all the
alveoli are now in the low-oxygen state, all the arterioles become constricted, the pulmonary arterial pressure rises excessively, and the right side of the heart
fails. Third, the alveolar arteriolar spasm diverts much
of the blood flow through nonalveolar pulmonary
vessels, thus causing an excess of pulmonary shunt
blood flow where the blood is poorly oxygenated; this
further compounds the problem. Most of these people
recover within days or weeks when they are moved to
a lower altitude.
Effects of Centrifugal Acceleratory Force on the Body—
(Positive G)
Effects on the Circulatory System. The most impor-
tant effect of centrifugal acceleration is on the circulatory system, because blood is mobile and can be
translocated by centrifugal forces.
When an aviator is subjected to positive G, blood is
centrifuged toward the lowermost part of the body.
Thus, if the centrifugal acceleratory force is +5 G and
the person is in an immobilized standing position,
the pressure in the veins of the feet becomes greatly
increased (to about 450 mm Hg). In the sitting position, the pressure becomes nearly 300 mm Hg. And, as
pressure in the vessels of the lower body increases,
these vessels passively dilate so that a major portion
of the blood from the upper body is translocated into
the lower vessels. Because the heart cannot pump
542
Arterial pressure
(mm Hg)
Unit VIII
Aviation, Space, and Deep-Sea Diving Physiology
100
50
0
0
5
10
15
20
25
30
Time from start of G to symptoms
(sec)
Figure 43–3
Changes in systolic (top of curve) and diastolic (bottom of curve)
arterial pressures after abrupt and continuing exposure of a sitting
person to an acceleratory force from top to bottom of 3.3. G. (Data
from Martin EE, Henry JP: Effects of time and temperature upon
tolerance to positive acceleration. J Aviation Med 22:382, 1951.)
unless blood returns to it, the greater the quantity of
blood “pooled” in this way in the lower body, the less
that is available for the cardiac output.
Figure 43–3 shows the changes in systolic and
diastolic arterial pressures (top and bottom curves,
respectively) in the upper body when a centrifugal
acceleratory force of +3.3 G is suddenly applied to a
sitting person. Note that both these pressures fall
below 22 mm Hg for the first few seconds after the
acceleration begins but then return to a systolic pressure of about 55 mm Hg and a diastolic pressure of
20 mm Hg within another 10 to 15 seconds. This secondary recovery is caused mainly by activation of the
baroreceptor reflexes.
Acceleration greater than 4 to 6 G causes “blackout” of vision within a few seconds and unconsciousness shortly thereafter. If this great degree of
acceleration is continued, the person will die.
cranium show less tendency for rupture than would be
expected for the following reason: The cerebrospinal
fluid is centrifuged toward the head at the same time
that blood is centrifuged toward the cranial vessels,
and the greatly increased pressure of the cerebrospinal
fluid acts as a cushioning buffer on the outside of the
brain to prevent intracerebral vascular rupture.
Because the eyes are not protected by the cranium,
intense hyperemia occurs in them during strong negative G. As a result, the eyes often become temporarily
blinded with “red-out.”
Protection of the Body Against Centrifugal Acceleratory
Forces. Specific procedures and apparatus have been
developed to protect aviators against the circulatory
collapse that might occur during positive G. First, if
the aviator tightens his or her abdominal muscles to
an extreme degree and leans forward to compress the
abdomen, some of the pooling of blood in the large
vessels of the abdomen can be prevented, thereby delaying the onset of blackout. Also, special “antiG” suits have been devised to prevent pooling of blood
in the lower abdomen and legs. The simplest of these
applies positive pressure to the legs and abdomen by
inflating compression bags as the G increases. Theoretically, a pilot submerged in a tank or suit of water
might experience little effect of G forces on the circulation because the pressures developed in the water
pressing on the outside of the body during centrifugal
acceleration would almost exactly balance the forces
acting in the body. However, the presence of air in
the lungs still allows displacement of the heart, lung
tissues, and diaphragm into seriously abnormal positions despite submersion in water. Therefore, even if
this procedure were used, the limit of safety almost
certainly would still be less than 10 G.
Effects of Linear Acceleratory Forces
on the Body
Effects on the Vertebrae. Extremely high acceleratory
forces for even a fraction of a second can fracture the
vertebrae. The degree of positive acceleration that the
average person can withstand in the sitting position
before vertebral fracture occurs is about 20 G.
Negative G. The effects of negative G on the body are
less dramatic acutely but possibly more damaging permanently than the effects of positive G. An aviator can
usually go through outside loops up to negative acceleratory forces of -4 to -5 G without causing permanent harm, although causing intense momentary
hyperemia of the head. Occasionally, psychotic disturbances lasting for 15 to 20 minutes occur as a result of
brain edema.
Occasionally, negative G forces can be so great
(-20 G, for instance) and centrifugation of the blood
into the head is so great that the cerebral blood pressure reaches 300 to 400 mm Hg, sometimes causing
small vessels on the surface of the head and in the
brain to rupture. However, the vessels inside the
Acceleratory Forces in Space Travel. Unlike an airplane, a
spacecraft cannot make rapid turns; therefore, centrifugal acceleration is of little importance except
when the spacecraft goes into abnormal gyrations.
However, blast-off acceleration and landing deceleration can be tremendous; both of these are types
of linear acceleration, one positive and the other
negative.
Figure 43–4 shows an approximate profile of acceleration during blast-off in a three-stage spacecraft,
demonstrating that the first-stage booster causes acceleration as high as 9 G, and the second-stage booster as
high as 8 G. In the standing position, the human body
could not withstand this much acceleration, but in a
semireclining position transverse to the axis of acceleration, this amount of acceleration can be withstood
with ease despite the fact that the acceleratory forces
continue for as long as several minutes at a time.
Therefore, we see the reason for the reclining seats
used by astronauts.
Chapter 43
Aviation, High-Altitude, and Space Physiology
other words, the speed of landing is about 20 feet per
second, and the force of impact against the earth is
1/81 the impact force without a parachute. Even so,
the force of impact is still great enough to cause considerable damage to the body unless the parachutist
is properly trained in landing. Actually, the force of
impact with the earth is about the same as that which
would be experienced by jumping without a parachute
from a height of about 6 feet. Unless forewarned, the
parachutist will be tricked by his senses into striking
the earth with extended legs, and this will result in
tremendous deceleratory forces along the skeletal axis
of the body, resulting in fracture of his pelvis, vertebrae, or leg. Consequently, the trained parachutist
strikes the earth with knees bent but muscles taut to
cushion the shock of landing.
10
Acceleration (G)
8
6
4
2
First
booster
0
0
1
Second
booster
2
3
Minutes
Space
ship
4
543
5
“Artificial Climate” in the
Sealed Spacecraft
Figure 43–4
Acceleratory forces during takeoff of a spacecraft.
Problems also occur during deceleration when the
spacecraft re-enters the atmosphere. A person traveling at Mach 1 (the speed of sound and of fast airplanes) can be safely decelerated in a distance of about
0.12 mile, whereas a person traveling at a speed of
Mach 100 (a speed possible in interplanetary space
travel) would require a distance of about 10,000 miles
for safe deceleration. The principal reason for this difference is that the total amount of energy that must
be dispelled during deceleration is proportional to
the square of the velocity, which alone increases the
required distance for decelerations between Mach 1
versus Mach 100 about 10,000-fold. But in addition to
this, a human being can withstand far less deceleration
if the period of deceleration lasts for a long time than
for a short time. Therefore, deceleration must be
accomplished much more slowly from high velocities
than is necessary at lower velocities.
Deceleratory Forces Associated with Parachute Jumps.
When the parachuting aviator leaves the airplane, his
velocity of fall is at first exactly 0 feet per second.
However, because of the acceleratory force of gravity,
within 1 second his velocity of fall is 32 feet per second
(if there is no air resistance); in 2 seconds it is 64 feet
per second; and so on. As the velocity of fall increases,
the air resistance tending to slow the fall also increases.
Finally, the deceleratory force of the air resistance
exactly balances the acceleratory force of gravity, so
that after falling for about 12 seconds, the person will
be falling at a “terminal velocity” of 109 to 119 miles
per hour (175 feet per second). If the parachutist has
already reached terminal velocity before opening his
parachute, an “opening shock load” of up to 1200
pounds can occur on the parachute shrouds.
The usual-sized parachute slows the fall of the parachutist to about one ninth the terminal velocity. In
Because there is no atmosphere in outer space, an artificial atmosphere and climate must be produced in a
spacecraft. Most important, the oxygen concentration
must remain high enough and the carbon dioxide concentration low enough to prevent suffocation. In some
earlier space missions, a capsule atmosphere containing pure oxygen at about 260 mm Hg pressure was
used, but in the modern space shuttle, gases about
equal to those in normal air are used, with four times
as much nitrogen as oxygen and a total pressure of
760 mm Hg. The presence of nitrogen in the mixture
greatly diminishes the likelihood of fire and explosion.
It also protects against development of local patches
of lung atelectasis that often occur when breathing
pure oxygen because oxygen is absorbed rapidly when
small bronchi are temporarily blocked by mucous
plugs.
For space travel lasting more than several months,
it is impractical to carry along an adequate oxygen
supply. For this reason, recycling techniques have been
proposed for use of the same oxygen over and over
again. Some recycling processes depend on purely
physical procedures, such as electrolysis of water to
release oxygen. Others depend on biological methods,
such as use of algae with their large store of chlorophyll to release oxygen from carbon dioxide by the
process of photosynthesis. A completely satisfactory
system for recycling has yet to be achieved.
Weightlessness in Space
A person in an orbiting satellite or a nonpropelled
spacecraft experiences weightlessness, or a state of
near-zero G force, which is sometimes called microgravity. That is, the person is not drawn toward the
bottom, sides, or top of the spacecraft but simply floats
inside its chambers. The cause of this is not failure of
gravity to pull on the body, because gravity from any
nearby heavenly body is still active. However, the
gravity acts on both the spacecraft and the person at
544
Unit VIII
Aviation, Space, and Deep-Sea Diving Physiology
the same time, so that both are pulled with exactly the
same acceleratory forces and in the same direction.
For this reason, the person simply is not attracted
toward any specific wall of the spacecraft.
Physiologic Problems of Weightlessness (Microgravity). The
physiologic problems of weightlessness have not
proved to be of much significance, as long as the period
of weightlessness is not too long. Most of the problems
that do occur are related to three effects of the weightlessness: (1) motion sickness during the first few days
of travel, (2) translocation of fluids within the body
because of failure of gravity to cause normal hydrostatic pressures, and (3) diminished physical activity
because no strength of muscle contraction is required
to oppose the force of gravity.
Almost 50 per cent of astronauts experience motion
sickness, with nausea and sometimes vomiting, during
the first 2 to 5 days of space travel. This probably
results from an unfamiliar pattern of motion signals
arriving in the equilibrium centers of the brain, and at
the same time lack of gravitational signals.
The observed effects of prolonged stay in space
are the following: (1) decrease in blood volume, (2)
decrease in red blood cell mass, (3) decrease in muscle
strength and work capacity, (4) decrease in maximum
cardiac output, and (5) loss of calcium and phosphate
from the bones, as well as loss of bone mass. Most of
these same effects also occur in people who lie in bed
for an extended period of time. For this reason, exercise programs are carried out by astronauts during
prolonged space missions.
In previous space laboratory expeditions in which
the exercise program had been less vigorous, the astronauts had severely decreased work capacities for the
first few days after returning to earth. They also had a
tendency to faint (and still do, to some extent) when
they stood up during the first day or so after return to
gravity because of diminished blood volume and
diminished responses of the arterial pressure control
mechanisms.
Cardiovascular, Muscle, and Bone “Deconditioning” During
Prolonged Exposure to Weightlessness. During very long
space flights and prolonged exposure to microgravity,
gradual “deconditioning” effects occur on the cardiovascular system, skeletal muscles, and bone despite rigorous exercise during the flight. Studies of astronauts
on space flights lasting several months have shown that
they may lose as much 1.0 percent of their bone mass
each month even though they continue to exercise.
Substantial atrophy of cardiac and skeletal muscles
also occurs during prolonged exposure to a microgravity environment.
One of the most serious effects is cardiovascular
“deconditioning”, which includes decreased work
capacity, reduced blood volume, impaired baroreceptor reflexes, and reduced orthostatic tolerance. These
changes greatly limit the astronauts’ ability to stand
upright or perform normal daily activities after returning to the full gravity of Earth. Astronauts returning
from space flights lasting 4 to 6 months are also susceptible to bone fractures and may require several
weeks before they return to pre-flight cardiovascular,
bone, and muscle fitness. As space flights become
longer in preparation for possible human exploration
of other planets, such as Mars, the effects of prolonged
microgravity could pose a very serious threat to astronauts after they land, especially in the event of an
emergency landing. Therefore, considerable research
effort has been directed toward developing countermeasures, in addition to exercise, that can prevent or
more effectively attenuate these changes. One such
countermeasure that is being tested is the application
of intermittent “artificial gravity” caused by short
periods (e.g., 1 hour each day) of centrifugal acceleration of the astronauts while they sit in specially
designed short-arm centrifuges that create forces of up
to 2 to 3 G.
References
Adams GR, Caiozzo VJ, Baldwin KM: Skeletal muscle
unweighting: spaceflight and ground-based models. J Appl
Physiol 95:2185, 2003.
Alfrey CP, Udden MM, Leach-Huntoon C, et al: Control of
red blood cell mass in spaceflight. J Appl Physiol 81:98,
1996.
Basnyat B, Murdoch DR: High-altitude illness. Lancet
361:1967, 2003.
Convertino VA: Mechanisms of microgravity induced orthostatic intolerance: implications for effective countermeasures. J Gravit Physiol 9:1, 2002.
Eckberg DL: Bursting into space: alterations of sympathetic
control by space travel. Acta Physiol Scand 177:299, 2003.
Hackett PH, Roach RC: High-altitude illness. N Engl J Med
345:107, 2001.
Harm DL, Jennings RT, Meck JV, et al: Gender issues related
to spaceflight: a NASA perspective. J Appl Physiol
91:2374, 2001.
Hochachka PW, Beatty CL, Burelle Y, et al: The lactate
paradox in human high-altitude physiological performance. News Physiol Sci 17:122, 2002.
Hoschele S, Mairbaurl H: Alveolar flooding at high altitude:
failure of reabsorption? News Physiol Sci 18:55, 2003.
Hultgren HN: High-altitude pulmonary edema: current concepts. Annu Rev Med 47:267, 1996.
Rupert JL, Hochachka PW: Genetic approaches to understanding human adaptation to altitude in the Andes. J Exp
Biol 204(Pt 18):3151, 2001.
Smith SM, Heer M: Calcium and bone metabolism during
space flight. Nutrition 18:849, 2002.
West JB: Climbing Mount Everest without oxygen. News
Physiol Sci 1:25, 1986.
West JB: Man in space. News Physiol Sci 1:198, 1986.
West JB: High Life—History of High-Altitude Physiology
and Medicine. Bethesda, MD: American Physiological
Society, 1998.
Zhang LF: Vascular adaptation to microgravity: what have
we learned? J Appl Physiol 91:2415, 2001.
C
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4
4
Physiology of Deep-Sea Diving
and Other Hyperbaric Conditions
When human beings descend beneath the sea, the
pressure around them increases tremendously. To
keep the lungs from collapsing, air must be supplied
at very high pressure to keep them inflated. This
exposes the blood in the lungs also to extremely
high alveolar gas pressure, a condition called hyperbarism. Beyond certain limits, these high pressures
can cause tremendous alterations in body physiology and can be lethal.
Relationship of Pressure to Sea Depth. A column of seawater 33 feet deep exerts the
same pressure at its bottom as the pressure of the atmosphere above the sea.
Therefore, a person 33 feet beneath the ocean surface is exposed to 2 atmospheres pressure, 1 atmosphere of pressure caused by the weight of the air above
the water and the second atmosphere by the weight of the water itself. At 66
feet the pressure is 3 atmospheres, and so forth, in accord with the table in
Figure 44–1.
Effect of Sea Depth on the Volume of Gases-Boyle’s Law. Another important effect of
depth is compression of gases to smaller and smaller volumes. The lower part
of Figure 44–1 shows a bell jar at sea level containing 1 liter of air. At 33 feet
beneath the sea, where the pressure is 2 atmospheres, the volume has been compressed to only one-half liter, and at 8 atmospheres (233 feet) to one-eighth
liter.Thus, the volume to which a given quantity of gas is compressed is inversely
proportional to the pressure. This is a principle of physics called Boyle’s law,
which is extremely important in diving physiology because increased pressure
can collapse the air chambers of the diver’s body, especially the lungs, and often
causes serious damage.
Many times in this chapter it is necessary to refer to actual volume versus
sea-level volume. For instance, we might speak of an actual volume of 1 liter at
a depth of 300 feet; this is the same quantity of air as a sea-level volume of
10 liters.
Effect of High Partial Pressures of Individual
Gases on the Body
The individual gases to which a diver is exposed when breathing air are nitrogen, oxygen, and carbon dioxide; each of these at times can cause significant
physiologic effects at high pressures.
Nitrogen Narcosis at High Nitrogen Pressures
About four fifths of the air is nitrogen. At sea-level pressure, the nitrogen has
no significant effect on bodily function, but at high pressures it can cause varying
degrees of narcosis. When the diver remains beneath the sea for an hour or
more and is breathing compressed air, the depth at which the first symptoms of
mild narcosis appear is about 120 feet. At this level the diver begins to exhibit
545
Unit VIII
Depth (feet)
Sea level
33
66
100
133
166
200
300
400
500
Aviation, Space, and Deep-Sea Diving Physiology
Atmosphere(s)
1
2
3
4
5
6
7
10
13
16
1 liter
Sea level
1/2
1/4
liter
liter
33 ft
100 ft
30
A
Oxygen in blood (volumes per cent)
546
25
B
20
Oxygen-hemoglobin dissociation curve
Total O2 in blood
Combined with
hemoglobin
Dissolved in
water of blood
Normal alveolar
oxygen pressure
15
10
Oxygen
poisoning
5
0
0
1560
2280
3040
760
Oxygen partial pressure in lungs (mm Hg)
Figure 44–2
Quantity of oxygen dissolved in the fluid of the blood and in combination with hemoglobin at very high PO2s.
Oxygen Toxicity at High Pressures
1/8
liter
233 ft
Figure 44–1
Effect of sea depth on pressure (top table) and on gas volume
(bottom).
joviality and to lose many of his or her cares. At 150
to 200 feet, the diver becomes drowsy. At 200 to 250
feet, his or her strength wanes considerably, and the
diver often becomes too clumsy to perform the work
required. Beyond 250 feet (8.5 atmospheres pressure),
the diver usually becomes almost useless as a result of
nitrogen narcosis if he or she remains at these depths
too long.
Nitrogen narcosis has characteristics similar to those
of alcohol intoxication, and for this reason it has
frequently been called “raptures of the depths.” The
mechanism of the narcotic effect is believed to be the
same as that of most other gas anesthetics. That is, it
dissolves in the fatty substances in neuronal membranes and, because of its physical effect on altering
ionic conductance through the membranes, reduces
neuronal excitability.
Effect of Very High PO2 on Blood Oxygen Transport. When the
Po2 in the blood rises above 100 mm Hg, the amount
of oxygen dissolved in the water of the blood increases
markedly. This is shown in Figure 44–2, which depicts
the same oxygen-hemoglobin dissociation curve as
that shown in Chapter 40 but with the alveolar Po2
extended to more than 3000 mm Hg. Also depicted by
the lowest curve in the figure is the volume of oxygen
dissolved in the fluid of the blood at each Po2 level.
Note that in the normal range of alveolar Po2 (below
120 mm Hg), almost none of the total oxygen in the
blood is accounted for by dissolved oxygen, but as the
oxygen pressure rises into the thousands of millimeters of mercury, a large portion of the total oxygen is
then dissolved in the water of the blood, in addition to
that bound with hemoglobin.
Effect of High Alveolar PO2 on Tissue PO2. Let us assume that
the Po2 in the lungs is about 3000 mm Hg (4 atmospheres pressure). Referring to Figure 44–2, one finds
that this represents a total oxygen content in each 100
milliliters of blood of about 29 volumes per cent, as
demonstrated by point A in the figure—this means
20 volumes per cent bound with hemoglobin and 9
volumes per cent dissolved in the blood water. As this
blood passes through the tissue capillaries and the
tissues use their normal amount of oxygen, about
5 milliliters from each 100 milliliters of blood, the
Chapter 44
Physiology of Deep-Sea Diving and Other Hyperbaric Conditions
oxygen content on leaving the tissue capillaries is still
24 volumes per cent (point B in the figure). At this
point, the Po2 is approximately 1200 mm Hg, which
means that oxygen is delivered to the tissues at this
extremely high pressure instead of at the normal value
of 40 mm Hg. Thus, once the alveolar Po2 rises above
a critical level, the hemoglobin-oxygen buffer mechanism (discussed in Chapter 40) is no longer capable
of keeping the tissue Po2 in the normal, safe range
between 20 and 60 mm Hg.
Acute Oxygen Poisoning. The extremely high tissue Po2
that occurs when oxygen is breathed at very high alveolar oxygen pressure can be detrimental to many of
the body’s tissues. For instance, breathing oxygen at
4 atmospheres pressure of oxygen (Po2 = 3040 mm Hg)
will cause brain seizures followed by coma in most
people within 30 to 60 minutes. The seizures often
occur without warning and, for obvious reasons, are
likely to be lethal to divers submerged beneath the
sea.
Other symptoms encountered in acute oxygen poisoning include nausea, muscle twitchings, dizziness,
disturbances of vision, irritability, and disorientation.
Exercise greatly increases the diver’s susceptibility to
oxygen toxicity, causing symptoms to appear much
earlier and with far greater severity than in the resting
person.
Excessive Intracellular Oxidation as a Cause of
Nervous System Oxygen Toxicity—“Oxidizing Free
Radicals.” Molecular oxygen (O2) has little capability
of oxidizing other chemical compounds. Instead, it
must first be converted into an “active” form of
oxygen. There are several forms of active oxygen
called oxygen free radicals. One of the most
important
_
of these is the superoxide free radical O2 , and another
is the peroxide radical in the form of hydrogen peroxide. Even when the tissue Po2 is normal at the level of
40 mm Hg, small amounts of free radicals are continually being formed from the dissolved molecular
oxygen. Fortunately, the tissues also contain multiple
enzymes that rapidly remove these free radicals,
including peroxidases, catalases, and superoxide dismutases. Therefore, so long as the hemoglobin-oxygen
buffering mechanism maintains a normal tissue Po2,
the oxidizing free radicals are removed rapidly enough
that they have little or no effect in the tissues.
Above a critical alveolar Po2 (above about 2
atmospheres Po2), the hemoglobin-oxygen buffering
mechanism fails, and the tissue Po2 can then rise to
hundreds or thousands of millimeters of mercury. At
these high levels, the amounts of oxidizing free radicals literally swamp the enzyme systems designed to
remove them, and now they can have serious destructive and even lethal effects on the cells. One of the
principal effects is to oxidize the polyunsaturated fatty
acids that are essential components of many of the cell
membranes. Another effect is to oxidize some of the
cellular enzymes, thus damaging severely the cellular
metabolic systems. The nervous tissues are especially
susceptible because of their high lipid content.
547
Therefore, most of the acute lethal effects of acute
oxygen toxicity are caused by brain dysfunction.
Chronic Oxygen Poisoning Causes Pulmonary Disability. A person can be exposed to only 1 atmosphere
pressure of oxygen almost indefinitely without developing the acute oxygen toxicity of the nervous system
just described. However, after only about 12 hours of
1 atmosphere oxygen exposure, lung passageway congestion, pulmonary edema, and atelectasis caused by
damage to the linings of the bronchi and alveoli begin
to develop. The reason for this effect in the lungs but
not in other tissues is that the air spaces of the lungs
are directly exposed to the high oxygen pressure, but
oxygen is delivered to the other body tissues at almost
normal Po2 because of the hemoglobin-oxygen buffer
system.
Carbon Dioxide Toxicity at Great
Depths in the Sea
If the diving gear is properly designed and functions
properly, the diver has no problem due to carbon
dioxide toxicity because depth alone does not increase
the carbon dioxide partial pressure in the alveoli. This
is true because depth does not increase the rate of
carbon dioxide production in the body, and as long as
the diver continues to breathe a normal tidal volume
and expires the carbon dioxide as it is formed, alveolar carbon dioxide pressure will be maintained at a
normal value.
In certain types of diving gear, however, such as the
diving helmet and some types of rebreathing apparatuses, carbon dioxide can build up in the dead space
air of the apparatus and be rebreathed by the diver.
Up to an alveolar carbon dioxide pressure (Pco2) of
about 80 mm Hg, twice that in normal alveoli, the
diver usually tolerates this buildup by increasing the
minute respiratory volume a maximum of 8- to 11-fold
to compensate for the increased carbon dioxide.
Beyond 80-mm Hg alveolar Pco2, the situation
becomes intolerable, and eventually the respiratory
center begins to be depressed, rather than excited,
because of the negative tissue metabolic effects of high
Pco2. The diver’s respiration then begins to fail rather
than to compensate. In addition, the diver develops
severe respiratory acidosis, and varying degrees of
lethargy, narcosis, and finally even anesthesia, as discussed in Chapter 42.
Decompression of the Diver After
Excess Exposure to High Pressure
When a person breathes air under high pressure for a
long time, the amount of nitrogen dissolved in the
body fluids increases. The reason for this is the following: Blood flowing through the pulmonary capillaries becomes saturated with nitrogen to the same
high pressure as that in the alveolar breathing mixture.
And over several more hours, enough nitrogen is
548
Unit VIII
Aviation, Space, and Deep-Sea Diving Physiology
carried to all the tissues of the body to raise their tissue
Pn2 also to equal the Pn2 in the breathing air.
Because nitrogen is not metabolized by the body, it
remains dissolved in all the body tissues until the nitrogen pressure in the lungs is decreased back to some
lower level, at which time the nitrogen can be removed
by the reverse respiratory process; however, this
removal often takes hours to occur and is the source
of multiple problems collectively called decompression
sickness.
Pressure Outside Body
Before
decompression
After sudden
decompression
O2 = 1044 mm Hg
N2 = 3956
O2 = 159 mm Hg
N2 = 601
Total = 5000 mm Hg
Total = 760 mm Hg
Body
Gaseous pressure
in the body fluids
H2O = 47 mm Hg
CO2 = 40
O2 = 60
N2 = 3918
Body
Gaseous pressure
in the body fluids
H2O = 47 mm Hg
CO2 = 40
O2 = 60
N2 = 3918
Volume of Nitrogen Dissolved in the Body Fluids at Different
Depths. At sea level, almost exactly 1 liter of nitrogen
is dissolved in the entire body. Slightly less than one
half of this is dissolved in the water of the body and a
little more than one half in the fat of the body. This is
true because nitrogen is five times as soluble in fat as
in water.
After the diver has become saturated with nitrogen,
the sea-level volume of nitrogen dissolved in the body
at different depths is as follows:
Total = 4065
A
Feet
Liters
0
33
100
200
300
1
2
4
7
10
Several hours are required for the gas pressures of
nitrogen in all the body tissues to come nearly to equilibrium with the gas pressure of nitrogen in the alveoli.
The reason for this is that the blood does not flow
rapidly enough and the nitrogen does not diffuse
rapidly enough to cause instantaneous equilibrium.
The nitrogen dissolved in the water of the body comes
to almost complete equilibrium in less than 1 hour, but
the fat tissue, requiring five times as much transport of
nitrogen and having a relatively poor blood supply,
reaches equilibrium only after several hours. For this
reason, if a person remains at deep levels for only a
few minutes, not much nitrogen dissolves in the body
fluids and tissues, whereas if the person remains at a
deep level for several hours, both the body water and
body fat become saturated with nitrogen.
Decompression Sickness (Synonyms: Bends, Compressed Air
Sickness, Caisson Disease, Diver’s Paralysis, Dysbarism). If a
diver has been beneath the sea long enough that large
amounts of nitrogen have dissolved in his or her body
and the diver then suddenly comes back to the surface
of the sea, significant quantities of nitrogen bubbles
can develop in the body fluids either intracellularly or
extracellularly and can cause minor or serious damage
in almost any area of the body, depending on the
number and sizes of bubbles formed; this is called
decompression sickness.
The principles underlying bubble formation are
shown in Figure 44–3. In Figure 44–3A, the diver’s
tissues have become equilibrated to a high dissolved
Total = 4065
B
Figure 44–3
Gaseous pressures both inside and outside the body, showing (A)
saturation of the body to high gas pressures when breathing air
at a total pressure of 5000 mm Hg, and (B) the great excesses of
intra-body pressures that are responsible for bubble formation in
the tissues when the lung intra-alveolar pressure body is
suddenly returned from 5000 mm Hg to normal pressure of
760 mm Hg.
nitrogen pressure (Pn2 = 3918 mm Hg), about 6.5
times the normal amount of nitrogen in the tissues.
As long as the diver remains deep beneath the sea, the
pressure against the outside of his or her body
(5000 mm Hg) compresses all the body tissues sufficiently to keep the excess nitrogen gas dissolved. But
when the diver suddenly rises to sea level (Figure
44–3B), the pressure on the outside of the body
becomes only 1 atmosphere (760 mm Hg), while the
gas pressure inside the body fluids is the sum of the
pressures of water vapor, carbon dioxide, oxygen, and
nitrogen, or a total of 4065 mm Hg, 97 per cent of
which is caused by the nitrogen. Obviously, this total
value of 4065 mm Hg is far greater than the 760 mm
Hg pressure on the outside of the body. Therefore, the
gases can escape from the dissolved state and form
actual bubbles, composed almost entirely of nitrogen,
both in the tissues and in the blood where they plug
many small blood vessels. The bubbles may not appear
for many minutes to hours, because sometimes the
gases can remain dissolved in the “supersaturated”
state for hours before bubbling.
Symptoms of Decompression Sickness (“Bends”).
The symptoms of decompression sickness are caused
by gas bubbles blocking many blood vessels in
different tissues. At first, only the smallest vessels
are blocked by minute bubbles, but as the bubbles
coalesce, progressively larger vessels are affected.
Chapter 44
Physiology of Deep-Sea Diving and Other Hyperbaric Conditions
Tissue ischemia and sometimes tissue death are the
result.
In most people with decompression sickness, the
symptoms are pain in the joints and muscles of the legs
and arms, affecting 85 to 90 per cent of those persons
who develop decompression sickness. The joint pain
accounts for the term “bends” that is often applied to
this condition.
In 5 to 10 per cent of people with decompression
sickness, nervous system symptoms occur, ranging
from dizziness in about 5 per cent to paralysis or collapse and unconsciousness in as many as 3 per cent.
The paralysis may be temporary, but in some instances,
damage is permanent.
Finally, about 2 per cent of people with decompression sickness develop “the chokes,” caused by massive
numbers of microbubbles plugging the capillaries of
the lungs; this is characterized by serious shortness of
breath, often followed by severe pulmonary edema
and, occasionally, death.
Nitrogen Elimination from the Body; Decompression Tables. If
a diver is brought to the surface slowly, enough of the
dissolved nitrogen can usually be eliminated by expiration through the lungs to prevent decompression
sickness. About two thirds of the total nitrogen is liberated in 1 hour and about 90 per cent in 6 hours.
Decompression tables have been prepared by the
U.S. Navy that detail procedures for safe decompression. To give the student an idea of the decompression
process, a diver who has been breathing air and has
been on the sea bottom for 60 minutes at a depth of
190 feet is decompressed according to the following
schedule:
10 minutes at 50 feet depth
17 minutes at 40 feet depth
19 minutes at 30 feet depth
50 minutes at 20 feet depth
84 minutes at 10 feet depth
Thus, for a work period on the bottom of only
1 hour, the total time for decompression is about
3 hours.
Tank Decompression and Treatment of Decompression Sickness. Another procedure widely used for decompres-
sion of professional divers is to put the diver into a
pressurized tank and then to lower the pressure gradually back to normal atmospheric pressure, using
essentially the same time schedule as noted above.
Tank decompression is even more important for
treating people in whom symptoms of decompression
sickness develop minutes or even hours after they
have returned to the surface. In this case, the diver
is recompressed immediately to a deep level. Then
decompression is carried out over a period several
times as long as the usual decompression period.
“Saturation Diving” and Use of Helium-Oxygen Mixtures in Deep
Dives. When divers must work at very deep levels—
between 250 feet and nearly 1000 feet—they frequently live in a large compression tank for days or
weeks at a time, remaining compressed at a pressure
549
level near that at which they will be working. This
keeps the tissues and fluids of the body saturated with
the gases to which they will be exposed while diving.
Then, when they return to the same tank after
working, there are no significant changes in pressure,
so that decompression bubbles do not occur.
In very deep dives, especially during saturation
diving, helium is usually used in the gas mixture
instead of nitrogen for three reasons: (1) it has only
about one fifth the narcotic effect of nitrogen; (2) only
about one half as much volume of helium dissolves in
the body tissues as nitrogen, and the volume that does
dissolve diffuses out of the tissues during decompression several times as rapidly as does nitrogen, thus
reducing the problem of decompression sickness; and
(3) the low density of helium (one seventh the density
of nitrogen) keeps the airway resistance for breathing
at a minimum, which is very important because highly
compressed nitrogen is so dense that airway resistance
can become extreme, sometimes making the work of
breathing beyond endurance.
Finally, in very deep dives it is important to reduce
the oxygen concentration in the gaseous mixture
because otherwise oxygen toxicity would result. For
instance, at a depth of 700 feet (22 atmospheres of
pressure), a 1 per cent oxygen mixture will provide all
the oxygen required by the diver, whereas a 21 per
cent mixture of oxygen (the percentage in air) delivers a Po2 to the lungs of more than 4 atmospheres,
a level very likely to cause seizures in as little as
30 minutes.
Scuba (Self-Contained
Underwater Breathing
Apparatus) Diving
Before the 1940s, almost all diving was done using a
diving helmet connected to a hose through which air
was pumped to the diver from the surface. Then, in
1943, Jacques Cousteau popularized a self-contained
underwater breathing apparatus, known as the SCUBA
apparatus. The type of SCUBA apparatus used in
more than 99 per cent of all sports and commercial
diving is the open-circuit demand system shown in
Figure 44–4. This system consists of the following components: (1) one or more tanks of compressed air or
some other breathing mixture, (2) a first-stage “reducing” valve for reducing the very high pressure from the
tanks to a low pressure level, (3) a combination inhalation “demand” valve and exhalation valve that allows
air to be pulled into the lungs with slight negative pressure of breathing and then to be exhaled into the sea
at a pressure level slightly positive to the surrounding
water pressure, and (4) a mask and tube system with
small “dead space.”
The demand system operates as follows: The firststage reducing valve reduces the pressure from the
tanks so that the air delivered to the mask has a pressure only a few mm Hg greater than the surrounding
water pressure. The breathing mixture does not
flow continually into the mask. Instead, with each
550
Unit VIII
Aviation, Space, and Deep-Sea Diving Physiology
Mask
Hose
Demand valve
First stage
valve
devices, especially when using helium, theoretically
can allow escape from as deep as 600 feet or perhaps
more.
One of the major problems of escape is prevention
of air embolism. As the person ascends, the gases in
the lungs expand and sometimes rupture a pulmonary
blood vessel, forcing the gases to enter the vessel and
cause air embolism of the circulation. Therefore, as the
person ascends, he or she must make a special effort
to exhale continually.
Health Problems in the Submarine Internal Environment.
Air cylinders
Except for escape, submarine medicine generally
centers around several engineering problems to keep
hazards out of the internal environment. First, in
atomic submarines, there exists the problem of radiation hazards, but with appropriate shielding, the
amount of radiation received by the crew submerged
beneath the sea has been less than normal radiation
received above the surface of the sea from cosmic rays.
Second, poisonous gases on occasion escape into the
atmosphere of the submarine and must be controlled
rapidly. For instance, during several weeks’ submergence, cigarette smoking by the crew can liberate
enough carbon monoxide, if not removed rapidly,
to cause carbon monoxide poisoning. And, on occasion, even freon gas has been found to diffuse out of
refrigeration systems in sufficient quantity to cause
toxicity.
Figure 44–4
Open-circuit demand type of SCUBA apparatus.
inspiration, slight extra negative pressure in the
demand valve of the mask pulls the diaphragm of the
valve open, and this automatically releases air from
the tank into the mask and lungs. In this way, only the
amount of air needed for inhalation enters the mask.
Then, on expiration, the air cannot go back into the
tank but instead is expired into the sea.
The most important problem in use of the selfcontained underwater breathing apparatus is the
limited amount of time one can remain beneath the
sea surface; for instance, only a few minutes are possible at a 200-foot depth. The reason for this is that
tremendous airflow from the tanks is required to wash
carbon dioxide out of the lungs—the greater the
depth, the greater the airflow in terms of quantity of
air per minute that is required, because the volumes
have been compressed to small sizes.
Special Physiologic Problems
in Submarines
Escape from Submarines. Essentially the same problems
encountered in deep-sea diving are often met in relation to submarines, especially when it is necessary
to escape from a submerged submarine. Escape is
possible from as deep as 300 feet without using any
apparatus. However, proper use of rebreathing
Hyperbaric Oxygen Therapy
The intense oxidizing properties of high-pressure
oxygen (hyperbaric oxygen) can have valuable
therapeutic effects in several important clinical
conditions. Therefore, large pressure tanks are now
available in many medical centers into which patients
can be placed and treated with hyperbaric oxygen.
The oxygen is usually administered at Po2s of 2 to
3 atmospheres of pressure through a mask or intratracheal tube, whereas the gas around the body is
normal air compressed to the same high-pressure
level.
It is believed that the same oxidizing free radicals
responsible for oxygen toxicity are also responsible for
at least some of the therapeutic benefits. Some of the
conditions in which hyperbaric oxygen therapy has
been especially beneficial follow.
Probably the most successful use of hyperbaric
oxygen has been for treatment of gas gangrene. The
bacteria that cause this condition, clostridial organisms, grow best under anaerobic conditions and
stop growing at oxygen pressures greater than about
70 mm Hg. Therefore, hyperbaric oxygenation of the
tissues can frequently stop the infectious process
entirely and thus convert a condition that formerly was
almost 100 per cent fatal into one that is cured in most
instances by early treatment with hyperbaric therapy.
Other conditions in which hyperbaric oxygen
therapy has been either valuable or possibly valuable
include decompression sickness, arterial gas embolism,
Chapter 44
Physiology of Deep-Sea Diving and Other Hyperbaric Conditions
carbon monoxide poisoning, osteomyelitis, and
myocardial infarction.
References
Butler PJ: Diving beyond the limits. News Physiol Sci 16:222,
2001.
Kooyman GL, Ponganis PJ: The physiological basis of diving
to depth: birds and mammals. Annu Rev Physiol 60:19,
1998.
Leach RM, Rees PJ, Wilmshurst P: Hyperbaric oxygen
therapy. BMJ 317:1140, 1998.
Neuman TS: Arterial gas embolism and decompression sickness. News Physiol Sci 17:77, 2002.
551
Nilsson GE: Surviving anoxia with the brain turned on. News
Physiol Sci 16:217, 2001.
Russi EW: Diving and the risk of barotrauma. Thorax
53(Suppl 2):S20, 1998.
Wang C, Schwaitzberg S, Berliner E, et al: Hyperbaric
oxygen for treating wounds: a systematic review of the
literature. Arch Surg 138:272, 2003.
Wang J, Li F, Calhoun JH, Mader JT: The role and effectiveness of adjunctive hyperbaric oxygen therapy in the management of musculoskeletal disorders. J Postgrad Med
48:226, 2002
West JB, Fu Z, Gaeth AP, Short RV: Fetal lung development
in the elephant reflects the adaptations required for snorkeling in adult life. Respir Physiol Neurobiol 138:325,
2003.
U
N
I
The Nervous
System: A. General
Principles and
Sensory Physiology
45. Organization of the Nervous System, Basic
Functions of Synapses, “Transmitter Substances”
46. Sensory Receptors, Neuronal Circuits for
Processing Information
47. Somatic Sensations: I. General Organization,
the Tactile and Position Senses
48. Somatic Sensations: II. Pain, Headache,
and Thermal Sensations
T
IX
C
H
A
P
T
E
R
4
5
Organization of the Nervous
System, Basic Functions of
Synapses, “Transmitter Substances”
The nervous system is unique in the vast complexity of thought processes and control actions it can
perform. It receives each minute literally millions of
bits of information from the different sensory
nerves and sensory organs and then integrates all
these to determine responses to be made by the
body.
However, before beginning this discussion of the
nervous system, the reader should review Chapters 5 and 7, which present the
principles of membrane potentials and transmission of signals in nerves and
through neuromuscular junctions.
General Design of the Nervous System
Central Nervous System Neuron: The Basic
Functional Unit
The central nervous system contains more than 100 billion neurons. Figure 45–1
shows a typical neuron of a type found in the brain motor cortex. Incoming
signals enter this neuron through synapses located mostly on the neuronal dendrites, but also on the cell body. For different types of neurons, there may be
only a few hundred or as many as 200,000 such synaptic connections from input
fibers. Conversely, the output signal travels by way of a single axon leaving the
neuron. Then, this axon has many separate branches to other parts of the
nervous system or peripheral body.
A special feature of most synapses is that the signal normally passes only in
the forward direction (from the axon of a preceding neuron to dendrites on cell
membranes of subsequent neurons). This forces the signal to travel in required
directions for performing specific nervous functions.
Sensory Part of the Nervous System—Sensory Receptors
Most activities of the nervous system are initiated by sensory experience
exciting sensory receptors, whether visual receptors in the eyes, auditory
receptors in the ears, tactile receptors on the surface of the body, or other
kinds of receptors. This sensory experience can either cause immediate reaction
from the brain, or memory of the experience can be stored in the brain
for minutes, weeks, or years and determine bodily reactions at some future
date.
Figure 45–2 shows the somatic portion of the sensory system, which transmits
sensory information from the receptors of the entire body surface and from
some deep structures. This information enters the central nervous system
through peripheral nerves and is conducted immediately to multiple sensory
areas in (1) the spinal cord at all levels; (2) the reticular substance of the
medulla, pons, and mesencephalon of the brain; (3) the cerebellum; (4) the thalamus; and (5) areas of the cerebral cortex.
555
556
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
Somesthetic areas
Motor cortex
Dendrites
Thalamus
Brain
Pons
Cell body
Medulla
Cerebellum
Spinal cord
Bulboreticular
formation
Skin
Pain, cold,
warmth (Free
nerve ending)
Pressure
(Pacinian corpuscle)
(Expanded tip
receptor)
Touch
(Meissner's corpuscle)
Axon
Muscle spindle
Golgi tendon
apparatus
Muscle
Kinesthetic receptor
Synapses
Joint
Spinal cord
Second-order
neurons
Figure 45–2
Somatosensory axis of the nervous system.
Figure 45–1
Structure of a large neuron in the brain, showing its important functional parts. (Redrawn from Guyton AC: Basic Neuroscience:
Anatomy and Physiology. Philadelphia: WB Saunders Co, 1987.)
Motor Part of the Nervous System—
Effectors
The most important eventual role of the nervous
system is to control the various bodily activities. This
is achieved by controlling (1) contraction of appropriate skeletal muscles throughout the body, (2) contraction of smooth muscle in the internal organs, and
(3) secretion of active chemical substances by both
exocrine and endocrine glands in many parts of the
body. These activities are collectively called motor
functions of the nervous system, and the muscles and
glands are called effectors because they are the actual
anatomical structures that perform the functions dictated by the nerve signals.
Figure 45–3 shows the “skeletal” motor nerve axis of
the nervous system for controlling skeletal muscle contraction. Operating parallel to this axis is another
system, called the autonomic nervous system, for con-
trolling smooth muscles, glands, and other internal
bodily systems; this is discussed in Chapter 60.
Note in Figure 45–3 that the skeletal muscles can be
controlled from many levels of the central nervous
system, including (1) the spinal cord; (2) the reticular
substance of the medulla, pons, and mesencephalon;
(3) the basal ganglia; (4) the cerebellum; and (5) the
motor cortex. Each of these areas plays its own specific role, the lower regions concerned primarily with
automatic, instantaneous muscle responses to sensory
stimuli, and the higher regions with deliberate
complex muscle movements controlled by the thought
processes of the brain.
Processing of Information—
“Integrative” Function of the
Nervous System
One of the most important functions of the nervous
system is to process incoming information in such a
way that appropriate mental and motor responses will
occur. More than 99 per cent of all sensory information is discarded by the brain as irrelevant and unimportant. For instance, one is ordinarily unaware of the
Chapter 45
Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances”
Motor nerve
to muscles
Motor
area
Caudate
nucleus
Thalamus
Putamen
Globus pallidus
Subthalamic nucleus
Cerebellum
Bulboreticular formation
Gamma motor fiber
Alpha motor fiber
Muscle spindle
Figure 45–3
Skeletal motor nerve axis of the nervous system.
parts of the body that are in contact with clothing, as
well as of the seat pressure when sitting. Likewise,
attention is drawn only to an occasional object in one’s
field of vision, and even the perpetual noise of our surroundings is usually relegated to the subconscious.
But, when important sensory information excites
the mind, it is immediately channeled into proper integrative and motor regions of the brain to cause desired
responses. This channeling and processing of information is called the integrative function of the nervous
system. Thus, if a person places a hand on a hot stove,
the desired instantaneous response is to lift the hand.
And other associated responses follow, such as moving
the entire body away from the stove, and perhaps even
shouting with pain.
Role of Synapses in Processing Information. The synapse
is the junction point from one neuron to the next.
Later in this chapter, we will discuss the details of
synaptic function. However, it is important to point out
here that synapses determine the directions that the
nervous signals will spread through the nervous
system. Some synapses transmit signals from one
neuron to the next with ease, whereas others transmit
signals only with difficulty. Also, facilitatory and
inhibitory signals from other areas in the nervous
557
system can control synaptic transmission, sometimes
opening the synapses for transmission and at other
times closing them. In addition, some postsynaptic
neurons respond with large numbers of output
impulses, and others respond with only a few. Thus, the
synapses perform a selective action, often blocking
weak signals while allowing strong signals to pass, but
at other times selecting and amplifying certain weak
signals, and often channeling these signals in many
directions rather than only one direction.
Storage of Information—Memory
Only a small fraction of even the most important
sensory information usually causes immediate motor
response. But much of the information is stored for
future control of motor activities and for use in
the thinking processes. Most storage occurs in the
cerebral cortex, but even the basal regions of the
brain and the spinal cord can store small amounts of
information.
The storage of information is the process we call
memory, and this, too, is a function of the synapses.
That is, each time certain types of sensory signals pass
through sequences of synapses, these synapses become
more capable of transmitting the same type of signal
the next time, a process called facilitation. After the
sensory signals have passed through the synapses a
large number of times, the synapses become so facilitated that signals generated within the brain itself can
also cause transmission of impulses through the same
sequences of synapses, even when the sensory input is
not excited. This gives the person a perception of experiencing the original sensations, although the perceptions are only memories of the sensations.
We know little about the precise mechanisms by
which long-term facilitation of synapses occurs in the
memory process, but what is known about this and
other details of the sensory memory process are discussed in Chapter 57.
Once memories have been stored in the nervous
system, they become part of the brain processing
mechanism for future “thinking.” That is, the thinking
processes of the brain compare new sensory experiences with stored memories; the memories then help
to select the important new sensory information and
to channel this into appropriate memory storage areas
for future use or into motor areas to cause immediate
bodily responses.
Major Levels of Central
Nervous System Function
The human nervous system has inherited special functional capabilities from each stage of human evolutionary development. From this heritage, three major
levels of the central nervous system have specific functional characteristics: (1) the spinal cord level, (2) the
lower brain or subcortical level, and (3) the higher
brain or cortical level.
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Unit IX
The Nervous System: A. General Principles and Sensory Physiology
Spinal Cord Level
We often think of the spinal cord as being only a
conduit for signals from the periphery of the body to
the brain, or in the opposite direction from the brain
back to the body. This is far from the truth. Even after
the spinal cord has been cut in the high neck region,
many highly organized spinal cord functions still
occur. For instance, neuronal circuits in the cord can
cause (1) walking movements, (2) reflexes that withdraw portions of the body from painful objects, (3)
reflexes that stiffen the legs to support the body
against gravity, and (4) reflexes that control local blood
vessels, gastrointestinal movements, or urinary excretion. In fact, the upper levels of the nervous system
often operate not by sending signals directly to the
periphery of the body but by sending signals to
the control centers of the cord, simply “commanding”
the cord centers to perform their functions.
Lower Brain or Subcortical Level
Many, if not most, of what we call subconscious activities of the body are controlled in the lower areas
of the brain—in the medulla, pons, mesencephalon,
hypothalamus, thalamus, cerebellum, and basal
ganglia. For instance, subconscious control of arterial
pressure and respiration is achieved mainly in the
medulla and pons. Control of equilibrium is a combined function of the older portions of the cerebellum
and the reticular substance of the medulla, pons, and
mesencephalon. Feeding reflexes, such as salivation
and licking of the lips in response to the taste of food,
are controlled by areas in the medulla, pons, mesencephalon, amygdala, and hypothalamus. And many
emotional patterns, such as anger, excitement, sexual
response, reaction to pain, and reaction to pleasure,
can still occur after destruction of much of the cerebral cortex.
system performs specific functions. But it is the cortex
that opens a world of stored information for use by the
mind.
Comparison of the
Nervous System with
a Computer
When computers were first developed, it soon became
apparent that these machines have many features in
common with the nervous system. First, all computers
have input circuits that are comparable to the sensory
portion of the nervous system, and output circuits that
are comparable to the motor portion of the nervous
system.
In simple computers, the output signals are controlled
directly by the input signals, operating in a manner
similar to that of simple reflexes of the spinal cord. In
more complex computers, the output is determined both
by input signals and by information that has already
been stored in memory in the computer, which is analogous to the more complex reflex and processing mechanisms of our higher nervous system. Furthermore,
as computers become even more complex, it is necessary to add still another unit, called the central processing unit, that determines the sequence of all operations.
This unit is analogous to the control mechanisms in our
brain that direct our attention first to one thought or
sensation or motor activity, then to another, and so
forth, until complex sequences of thought or action take
place.
Figure 45–4 is a simple block diagram of a computer.
Even a rapid study of this diagram demonstrates its similarity to the nervous system. The fact that the basic
components of the general-purpose computer are analogous to those of the human nervous system demonstrates that the brain is basically a computer that
continuously collects sensory information and uses this
along with stored information to compute the daily
course of bodily activity.
Higher Brain or Cortical Level
After the preceding account of the many nervous
system functions that occur at the cord and lower brain
levels, one may ask, what is left for the cerebral cortex
to do? The answer to this is complex, but it begins with
the fact that the cerebral cortex is an extremely large
memory storehouse. The cortex never functions alone
but always in association with lower centers of the
nervous system.
Without the cerebral cortex, the functions of the
lower brain centers are often imprecise. The vast storehouse of cortical information usually converts these
functions to determinative and precise operations.
Finally, the cerebral cortex is essential for most of
our thought processes, but it cannot function by itself.
In fact, it is the lower brain centers, not the cortex,
that initiate wakefulness in the cerebral cortex, thus
opening its bank of memories to the thinking machinery of the brain. Thus, each portion of the nervous
Problem
Input
Procedure
for solution
Central
processing unit
Output
Initial
data
Result of
operations
Answer
Information
storage
Computational
unit
Figure 45–4
Block diagram of a general-purpose computer, showing the basic
components and their interrelations.
Chapter 45
Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances”
Central Nervous System
Synapses
Every medical student is aware that information is
transmitted in the central nervous system mainly in the
form of nerve action potentials, called simply “nerve
impulses,” through a succession of neurons, one after
another. However, in addition, each impulse (1) may
be blocked in its transmission from one neuron to the
next, (2) may be changed from a single impulse into
repetitive impulses, or (3) may be integrated with
impulses from other neurons to cause highly intricate
patterns of impulses in successive neurons. All these
functions can be classified as synaptic functions of
neurons.
Types of Synapses—Chemical
and Electrical
There are two major types of synapses: (1) the chemical synapse and (2) the electrical synapse.
Almost all the synapses used for signal transmission
in the central nervous system of the human being are
chemical synapses. In these, the first neuron secretes at
its nerve ending synapse a chemical substance called a
neurotransmitter (or often called simply transmitter
substance), and this transmitter in turn acts on receptor proteins in the membrane of the next neuron to
excite the neuron, inhibit it, or modify its sensitivity in
some other way. More than 40 important transmitter
substances have been discovered thus far. Some of
the best known are acetylcholine, norepinephrine,
epinephrine, histamine, gamma-aminobutyric acid
(GABA), glycine, serotonin, and glutamate.
Electrical synapses, in contrast, are characterized by
direct open fluid channels that conduct electricity from
one cell to the next. Most of these consist of small
protein tubular structures called gap junctions that
allow free movement of ions from the interior of one
cell to the interior of the next. Such junctions were discussed in Chapter 4. Only a few examples of gap junctions have been found in the central nervous system.
However, it is by way of gap junctions and other
similar junctions that action potentials are transmitted
from one smooth muscle fiber to the next in visceral
smooth muscle (Chapter 8) and from one cardiac
muscle cell to the next in cardiac muscle (Chapter 10).
“One-Way” Conduction at Chemical Synapses. Chemical
synapses have one exceedingly important characteristic that makes them highly desirable for transmitting
most nervous system signals: they always transmit
the signals in one direction: that is, from the neuron
that secretes the transmitter substance, called the
presynaptic neuron, to the neuron on which the transmitter acts, called the postsynaptic neuron. This is the
principle of one-way conduction at chemical synapses,
and it is quite different from conduction through electrical synapses, which often transmit signals in either
direction.
559
Think for a moment about the extreme importance
of the one-way conduction mechanism. It allows
signals to be directed toward specific goals. Indeed, it
is this specific transmission of signals to discrete and
highly focused areas both within the nervous system
and at the terminals of the peripheral nerves that
allows the nervous system to perform its myriad functions of sensation, motor control, memory, and many
others.
Physiologic Anatomy of the Synapse
Figure 45–5 shows a typical anterior motor neuron in
the anterior horn of the spinal cord. It is composed of
three major parts: the soma, which is the main body of
the neuron; a single axon, which extends from the
soma into a peripheral nerve that leaves the spinal
cord; and the dendrites, which are great numbers of
branching projections of the soma that extend as
much as 1 millimeter into the surrounding areas of the
cord.
As many as 10,000 to 200,000 minute synaptic knobs
called presynaptic terminals lie on the surfaces of the
dendrites and soma of the motor neuron, about 80 to
95 per cent of them on the dendrites and only 5 to 20
per cent on the soma. These presynaptic terminals are
the ends of nerve fibrils that originate from many
other neurons. Later, it will become evident that many
Dendrites
Axon
Soma
Figure 45–5
Typical anterior motor neuron, showing presynaptic terminals on
the neuronal soma and dendrites. Note also the single axon.
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Unit IX
The Nervous System: A. General Principles and Sensory Physiology
of these presynaptic terminals are excitatory—that is,
they secrete a transmitter substance that excites the
postsynaptic neuron. But other presynaptic terminals
are inhibitory—they secrete a transmitter substance
that inhibits the postsynaptic neuron.
Neurons in other parts of the cord and brain differ
from the anterior motor neuron in (1) the size of the
cell body; (2) the length, size, and number of dendrites,
ranging in length from almost zero to many centimeters; (3) the length and size of the axon; and (4) the
number of presynaptic terminals, which may range
from only a few to as many as 200,000. These differences make neurons in different parts of the nervous
system react differently to incoming synaptic signals
and, therefore, perform many different functions.
Presynaptic Terminals. Electron microscopic studies of
the presynaptic terminals show that they have varied
anatomical forms, but most resemble small round or
oval knobs and, therefore, are sometimes called terminal knobs, boutons, end-feet, or synaptic knobs.
Figure 45–6 illustrates the basic structure of a
synapse, showing a single presynaptic terminal on the
membrane surface of a postsomatic neuron.The presynaptic terminal is separated from the postsynaptic
neuronal soma by a synaptic cleft having a width
usually of 200 to 300 angstroms. The terminal has
two internal structures important to the excitatory or
inhibitory function of the synapse: the transmitter vesicles and the mitochondria. The transmitter vesicles
contain the transmitter substance that, when released
into the synaptic cleft, either excites or inhibits the
postsynaptic neuron—excites if the neuronal membrane contains excitatory receptors, inhibits if the
membrane contains inhibitory receptors. The mitochondria provide adenosine triphosphate (ATP),
which in turn supplies the energy for synethesizing
new transmitter substance.
When an action potential spreads over a presynaptic terminal, depolarization of its membrane causes
a small number of vesicles to empty into the cleft.
The released transmitter in turn causes an immediate
change in permeability characteristics of the postsynaptic neuronal membrane, and this leads to excitation
or inhibition of the postsynaptic neuron, depending on
the neuronal receptor characteristics.
Mechanism by Which an Action Potential
Causes Transmitter Release from the
Presynaptic Terminals—Role of Calcium Ions
The membrane of the presynaptic terminal is called
the presynaptic membrane. It contains large numbers
of voltage-gated calcium channels. When an action
potential depolarizes the presynaptic membrane,
these calcium channels open and allow large numbers
of calcium ions to flow into the terminal. The quantity
of transmitter substance that is then released from
the terminal into the synaptic cleft is directly related
to the number of calcium ions that enter. The precise
mechanism by which the calcium ions cause this
release is not known, but it is believed to be the
following.
When the calcium ions enter the presynaptic terminal, it is believed that they bind with special protein
molecules on the inside surface of the presynaptic
membrane, called release sites. This binding in turn
causes the release sites to open through the membrane, allowing a few transmitter vesicles to release
their transmitter into the cleft after each single action
potential. For those vesicles that store the neurotransmitter acetylcholine, between 2000 and 10,000 molecules of acetylcholine are present in each vesicle, and
there are enough vesicles in the presynaptic terminal
to transmit from a few hundred to more than 10,000
action potentials.
Action of the Transmitter Substance
on the Postsynaptic Neuron—Function of
“Receptor Proteins”
Transmitter vesicles
Mitochondria
Presynaptic
terminal
Synaptic cleft
(200-300
angstroms)
Receptor
proteins
Soma of neuron
Figure 45–6
Physiologic anatomy of the synapse.
The membrane of the postsynaptic neuron contains
large numbers of receptor proteins, also shown in
Figure 45–6. The molecules of these receptors have
two important components: (1) a binding component
that protrudes outward from the membrane into the
synaptic cleft—here it binds the neurotransmitter
coming from the presynaptic terminal—and (2) an
ionophore component that passes all the way through
the postsynaptic membrane to the interior of the postsynaptic neuron. The ionophore in turn is one of two
types: (1) an ion channel that allows passage of specified types of ions through the membrane or (2) a
“second messenger” activator that is not an ion channel
but instead is a molecule that protrudes into the cell
cytoplasm and activates one or more substances inside
the postsynaptic neuron. These substances in turn
serve as “second messengers” to increase or decrease
specific cellular functions.
Chapter 45
561
Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances”
the process of memory—require prolonged changes in
neurons for seconds to months after the initial transmitter substance is gone. The ion channels are not suitable for causing prolonged postsynaptic neuronal
changes because these channels close within milliseconds after the transmitter substance is no longer
present. However, in many instances, prolonged postsynaptic neuronal excitation or inhibition is achieved
by activating a “second messenger” chemical system
inside the postsynaptic neuronal cell itself, and then it
is the second messenger that causes the prolonged
effect.
There are several types of second messenger
systems. One of the most common types uses a group
of proteins called G-proteins. Figure 45–7 shows in the
upper left corner a membrane receptor protein. A
G-protein is attached to the portion of the receptor
that protrudes into the interior of the cell. The
G-protein in turn consists of three components: an
alpha (a) component that is the activator portion of
the G-protein, and beta (b) and gamma (g) components that are attached to the alpha component and
also to the inside of the cell membrane adjacent to the
receptor protein. On activation by a nerve impulse, the
alpha portion of the G-protein separates from the beta
and gamma portions and then is free to move within
the cytoplasm of the cell.
Inside the cytoplasm, the separated alpha component performs one or more of multiple functions,
depending on the specific characteristic of each type
of neuron. Shown in Figure 45–7 are four changes that
can occur. They are as follows:
1. Opening specific ion channels through the
postsynaptic cell membrane. Shown in the upper
right of the figure is a potassium channel that is
opened in response to the G-protein; this channel
often stays open for a prolonged time, in contrast
to rapid closure of directly activated ion channels
that do not use the second messenger system.
Ion Channels. The ion channels in the postsynaptic neuronal membrane are usually of two types: (1) cation
channels that most often allow sodium ions to pass
when opened, but sometimes allow potassium and/or
calcium ions as well, and (2) anion channels that allow
mainly chloride ions to pass but also minute quantities
of other anions.
The cation channels that conduct sodium ions are
lined with negative charges. These charges attract the
positively charged sodium ions into the channel when
the channel diameter increases to a size larger than
that of the hydrated sodium ion. But those same negative charges repel chloride ions and other anions and
prevent their passage.
For the anion channels, when the channel diameters
become large enough, chloride ions pass into the
channels and on through to the opposite side, whereas
sodium, potassium, and calcium cations are blocked,
mainly because their hydrated ions are too large to
pass.
We will learn later that when cation channels open
and allow positively charged sodium ions to enter, the
positive electrical charges of the sodium ions will in
turn excite this neuron. Therefore, a transmitter substance that opens cation channels is called an excitatory transmitter. Conversely, opening anion channels
allows negative electrical charges to enter, which
inhibits the neuron. Therefore, transmitter substances
that open these channels are called inhibitory
transmitters.
When a transmitter substance activates an ion
channel, the channel usually opens within a fraction of
a millisecond; when the transmitter substance is no
longer present, the channel closes equally rapidly. The
opening and closing of ion channels provide a means
for very rapid control of postsynaptic neurons.
“Second Messenger” System in the Postsynaptic Neuron.
Many functions of the nervous system—for instance,
Transmitter substance
Receptor
protein
Figure 45–7
“Second messenger” system by
which a transmitter substance from
an initial neuron can activate a
second neuron by first releasing
a “G-protein” into the second
neuron’s cytoplasm. Four subsequent possible effects of the
G-protein are shown, including 1,
opening an ion channel in the
membrane of the second neuron;
2, activating an enzyme system in
the neuron’s membrane; 3, activating an intracellular enzyme system;
and/or 4, causing gene transcription in the second neuron.
g
Potassium
channel
b
a
G-protein
1
Opens
channel
K+
Membrane
enzyme
2
a
3
Activates one or
more intracellular
enzymes
Activates gene
transcription
4
Activates
enzymes
ATP
GTP
or
cAMP
Specific cellular
chemical activators
Proteins and
structural changes
cGMP
562
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
2. Activation of cyclic adenosine monophosphate
(cAMP) or cyclic guanosine monophosphate
(cGMP) in the neuronal cell. Recall that either
cyclic AMP or cyclic GMP can activate highly
specific metabolic machinery in the neuron and,
therefore, can initiate any one of many chemical
results, including long-term changes in cell
structure itself, which in turn alters long-term
excitability of the neuron.
3. Activation of one or more intracellular enzymes.
The G-protein can directly activate one or more
intracellular enzymes. In turn the enzymes can
cause any one of many specific chemical functions
in the cell.
4. Activation of gene transcription. This is one of the
most important effects of activation of the second
messenger systems because gene transcription
can cause formation of new proteins within the
neuron, thereby changing its metabolic machinery
or its structure. Indeed, it is well known that
structural changes of appropriately activated
neurons do occur, especially in long-term memory
processes.
It is clear that activation of second messenger
systems within the neuron, whether they be of the
G-protein type or of other types, is extremely important for changing the long-term response characteristics of different neuronal pathways. We will return to
this subject in more detail in Chapter 57 when we
discuss memory functions of the nervous system.
Excitatory or Inhibitory Receptors in the
Postsynaptic Membrane
Some postsynaptic receptors, when activated, cause
excitation of the postsynaptic neuron, and others cause
inhibition. The importance of having inhibitory as well
as excitatory types of receptors is that this gives an
additional dimension to nervous function, allowing
restraint of nervous action as well as excitation.
The different molecular and membrane mechanisms
used by the different receptors to cause excitation or
inhibition include the following.
excitatory membrane receptors or decrease the
number of inhibitory membrane receptors.
Inhibition
1. Opening of chloride ion channels through the
postsynaptic neuronal membrane. This allows
rapid diffusion of negatively charged chloride ions
from outside the postsynaptic neuron to the
inside, thereby carrying negative charges inward
and increasing the negativity inside, which is
inhibitory.
2. Increase in conductance of potassium ions out of
the neuron. This allows positive ions to diffuse to
the exterior, which causes increased negativity
inside the neuron; this is inhibitory.
3. Activation of receptor enzymes that inhibit cellular
metabolic functions that increase the number of
inhibitory synaptic receptors or decrease the
number of excitatory receptors.
Chemical Substances That Function
as Synaptic Transmitters
More than 50 chemical substances have been proved
or postulated to function as synaptic transmitters.
Many of them are listed in Tables 45–1 and 45–2, which
give two groups of synaptic transmitters. One group
comprises small-molecule, rapidly acting transmitters.
The other is made up of a large number of neuropeptides of much larger molecular size that are usually
much more slowly acting.
The small-molecule, rapidly acting transmitters are
the ones that cause most acute responses of the
nervous system, such as transmission of sensory signals
to the brain and of motor signals back to the muscles.
The neuropeptides, in contrast, usually cause more
prolonged actions, such as long-term changes in
numbers of neuronal receptors, long-term opening or
closure of certain ion channels, and possibly even longterm changes in numbers of synapses or sizes of
synapses.
Excitation
1. Opening of sodium channels to allow large
numbers of positive electrical charges to flow to the
interior of the postsynaptic cell. This raises the
intracellular membrane potential in the positive
direction up toward the threshold level for
excitation. It is by far the most widely used means
for causing excitation.
2. Depressed conduction through chloride or
potassium channels, or both. This decreases the
diffusion of negatively charged chloride ions to
the inside of the postsynaptic neuron or decreases
the diffusion of positively charged potassium ions
to the outside. In either instance, the effect is to
make the internal membrane potential more
positive than normal, which is excitatory.
3. Various changes in the internal metabolism of the
postsynaptic neuron to excite cell activity or, in
some instances, to increase the number of
Table 45–1
Small-Molecule, Rapidly Acting Transmitters
Class I
Acetylcholine
Class II: The Amines
Norepinephrine
Epinephrine
Dopamine
Serotonin
Histamine
Class III: Amino Acids
Gamma-aminobutyric acid (GABA)
Glycine
Glutamate
Aspartate
Class IV
Nitric oxide (NO)
Chapter 45
Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances”
Table 45–2
Neuropeptide, Slowly Acting Transmitters or Growth
Factors
Hypothalamic-releasing hormones
Thyrotropin-releasing hormone
Luteinizing hormone–releasing hormone
Somatostatin (growth hormone inhibitory factor)
Pituitary peptides
Adrenocorticotropic hormone (ACTH)
b-Endorphin
a-Melanocyte-stimulating hormone
Prolactin
Luteinizing hormone
Thyrotropin
Growth hormone
Vasopressin
Oxytocin
Peptides that act on gut and brain
Leucine enkephalin
Methionine enkephalin
Substance P
Gastrin
Cholecystokinin
Vasoactive intestinal polypeptide (VIP)
Nerve growth factor
Brain-derived neurotropic factor
Neurotensin
Insulin
Glucagon
From other tissues
Angiotensin II
Bradykinin
Carnosine
Sleep peptides
Calcitonin
Small-Molecule, Rapidly Acting Transmitters
In most cases, the small-molecule types of transmitters
are synthesized in the cytosol of the presynaptic terminal and are absorbed by means of active transport
into the many transmitter vesicles in the terminal.
Then, each time an action potential reaches the presynaptic terminal, a few vesicles at a time release their
transmitter into the synaptic cleft. This usually occurs
within a millisecond or less by the mechanism
described earlier. The subsequent action of the smallmolecule type of transmitter on the membrane receptors of the postsynaptic neuron usually also occurs
within another millisecond or less. Most often the
effect is to increase or decrease conductance through
ion channels; an example is to increase sodium conductance, which causes excitation, or to increase potassium or chloride conductance, which causes inhibition.
Recycling of the Small-Molecule Types of Vesicles. The vesicles that store and release small-molecule transmitters
are continually recycled and used over and over again.
After they fuse with the synaptic membrane and open
to release their transmitter substance, the vesicle membrane at first simply becomes part of the synaptic
membrane. However, within seconds to minutes, the
vesicle portion of the membrane invaginates back to
the inside of the presynaptic terminal and pinches
off to form a new vesicle. And the new vesicular
563
membrane still contains appropriate enzyme proteins
or transport proteins required for synthesizing and/or
concentrating new transmitter substance inside the
vesicle.
Acetylcholine is a typical small-molecule transmitter that obeys the principles of synthesis and release
stated earlier. This transmitter substance is synthesized
in the presynaptic terminal from acetyl coenzyme A
and choline in the presence of the enzyme choline
acetyltransferase. Then it is transported into its specific
vesicles. When the vesicles later release the acetylcholine into the synaptic cleft during synaptic neuronal signal transmission, the acetylcholine is rapidly
split again to acetate and choline by the enzyme
cholinesterase, which is present in the proteoglycan
reticulum that fills the space of the synaptic cleft. And
then again, inside the presynaptic terminal, the vesicles are recycled; choline is actively transported back
into the terminal to be used again for synthesis of new
acetylcholine.
Characteristics of Some of the More Important Small-Molecule
Transmitters. The most important of the small-molecule
transmitters are the following.
Acetylcholine is secreted by neurons in many areas
of the nervous system but specifically by (1) the terminals of the large pyramidal cells from the motor
cortex, (2) several different types of neurons in the
basal ganglia, (3) the motor neurons that innervate the
skeletal muscles, (4) the preganglionic neurons of
the autonomic nervous system, (5) the postganglionic
neurons of the parasympathetic nervous system, and
(6) some of the postganglionic neurons of the sympathetic nervous system. In most instances, acetylcholine
has an excitatory effect; however, it is known to have
inhibitory effects at some peripheral parasympathetic
nerve endings, such as inhibition of the heart by the
vagus nerves.
Norepinephrine is secreted by the terminals of many
neurons whose cell bodies are located in the brain
stem and hypothalamus. Specifically, norepinephrinesecreting neurons located in the locus ceruleus in the
pons send nerve fibers to widespread areas of the brain
to help control overall activity and mood of the mind,
such as increasing the level of wakefulness. In most of
these areas, norepinephrine probably activates excitatory receptors, but in a few areas, it activates inhibitory
receptors instead. Norepinephrine is also secreted by
most postganglionic neurons of the sympathetic
nervous system, where it excites some organs but
inhibits others.
Dopamine is secreted by neurons that originate in
the substantia nigra. The termination of these neurons
is mainly in the striatal region of the basal ganglia. The
effect of dopamine is usually inhibition.
Glycine is secreted mainly at synapses in the
spinal cord. It is believed to always act as an inhibitory
transmitter.
GABA (gamma-aminobutyric acid) is secreted by
nerve terminals in the spinal cord, cerebellum, basal
ganglia, and many areas of the cortex. It is believed
always to cause inhibition.
564
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
Glutamate is secreted by the presynaptic terminals
in many of the sensory pathways entering the central
nervous system, as well as in many areas of the cerebral cortex. It probably always causes excitation.
Serotonin is secreted by nuclei that originate in the
median raphe of the brain stem and project to many
brain and spinal cord areas, especially to the dorsal
horns of the spinal cord and to the hypothalamus.
Serotonin acts as an inhibitor of pain pathways in the
cord, and an inhibitor action in the higher regions of
the nervous system is believed to help control the
mood of the person, perhaps even to cause sleep.
Nitric oxide is especially secreted by nerve terminals
in areas of the brain responsible for long-term behavior and for memory. Therefore, this transmitter system
might in the future explain some behavior and
memory functions that thus far have defied understanding. Nitric oxide is different from other smallmolecule transmitters in its mechanism of formation
in the presynaptic terminal and in its actions on the
postsynaptic neuron. It is not preformed and stored in
vesicles in the presynaptic terminal as are other transmitters. Instead, it is synthesized almost instantly as
needed, and it then diffuses out of the presynaptic
terminals over a period of seconds rather than being
released in vesicular packets. Next, it diffuses into
postsynaptic neurons nearby. In the postsynaptic
neuron, it usually does not greatly alter the membrane
potential but instead changes intracellular metabolic
functions that modify neuronal excitability for
seconds, minutes, or perhaps even longer.
Neuropeptides
The neuropeptides are an entirely different class of
transmitters that are synthesized differently and
whose actions are usually slow and in other ways quite
different from those of the small-molecule transmitters. The neuropeptides are not synthesized in the
cytosol of the presynaptic terminals. Instead, they are
synthesized as integral parts of large-protein molecules by ribosomes in the neuronal cell body.
The protein molecules then enter the spaces inside
the endoplasmic reticulum of the cell body and subsequently inside the Golgi apparatus, where two changes
occur: First, the neuropeptide-forming protein is enzymatically split into smaller fragments, some of which
are either the neuropeptide itself or a precursor of it.
Second, the Golgi apparatus packages the neuropeptide into minute transmitter vesicles that are released
into the cytoplasm. Then the transmitter vesicles are
transported all the way to the tips of the nerve fibers
by axonal streaming of the axon cytoplasm, traveling
at the slow rate of only a few centimeters per day.
Finally, these vesicles release their transmitter at the
neuronal terminals in response to action potentials in
the same manner as for small-molecule transmitters.
However, the vesicle is autolyzed and is not reused.
Because of this laborious method of forming the
neuropeptides, much smaller quantities of them are
usually released than of the small-molecule transmitters. This is partly compensated for by the fact that the
neuropeptides are generally a thousand or more times
as potent as the small-molecule transmitters. Another
important characteristic of the neuropeptides is that
they often cause much more prolonged actions. Some
of these actions include prolonged closure of calcium
channels, prolonged changes in the metabolic machinery of cells, prolonged changes in activation or deactivation of specific genes in the cell nucleus, and/or
prolonged alterations in numbers of excitatory or
inhibitory receptors. Some of these effects last for
days, but others perhaps for months or years. Our
knowledge of the functions of the neuropeptides is
only beginning to develop.
Electrical Events During
Neuronal Excitation
The electrical events in neuronal excitation have been
studied especially in the large motor neurons of the
anterior horns of the spinal cord. Therefore, the events
described in the next few sections pertain essentially
to these neurons. Except for quantitative differences,
they apply to most other neurons of the nervous
system as well.
Resting Membrane Potential of the Neuronal Soma. Figure
45–8 shows the soma of a spinal motor neuron, indicating a resting membrane potential of about -65 millivolts. This is somewhat less negative than the -90
millivolts found in large peripheral nerve fibers and in
skeletal muscle fibers; the lower voltage is important
because it allows both positive and negative control
of the degree of excitability of the neuron. That is,
decreasing the voltage to a less negative value makes
the membrane of the neuron more excitable, whereas
increasing this voltage to a more negative value makes
the neuron less excitable. This is the basis for the two
Dendrite
Na+: 142 mEq/L
K+: 4.5 mEq/L
Cl-: 107 mEq/L
14 mEq/L
(Pumps)
120 mEq/L
8 mEq/L
?
Pump
-65
mV
Axon
Axon hillock
Figure 45–8
Distribution of sodium, potassium, and chloride ions across the
neuronal somal membrane; origin of the intrasomal membrane
potential.
Chapter 45
Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances”
modes of function of the neuron—either excitation or
inhibition—as explained in detail in the next sections.
Concentration Differences of Ions Across the Neuronal
Somal Membrane. Figure 45–8 also shows the concen-
tration differences across the neuronal somal membrane of the three ions that are most important for
neuronal function: sodium ions, potassium ions, and
chloride ions. At the top, the sodium ion concentration is shown to be high in the extracellular fluid
(142 mEq/L) but low inside the neuron (14 mEq/L).
This sodium concentration gradient is caused by a
strong somal membrane sodium pump that continually
pumps sodium out of the neuron.
The figure also shows that potassium ion concentration is high inside the neuronal soma (120 mEq/L) but
low in the extracellular fluid (4.5 mEq/L). It shows that
there is a potassium pump (the other half of the Na+K+ pump) that pumps potassium to the interior.
Figure 45–8 shows the chloride ion to be of high concentration in the extracellular fluid but low concentration inside the neuron. It also shows that the membrane
is quite permeable to chloride ions and that there may
be a weak chloride pump. Yet most of the reason
for the low concentration of chloride ions inside the
neuron is the -65 millivolts in the neuron. That is, this
negative voltage repels the negatively charged chloride ions, forcing them outward through the pores until
the concentration is much less inside the membrane
than outside.
Let us recall at this point what we learned in Chapters 4 and 5 about the relation of ionic concentration
differences to membrane potentials. It will be recalled
that an electrical potential across the cell membrane
can oppose movement of ions through a membrane if
the potential is of proper polarity and magnitude. A
potential that exactly opposes movement of an ion is
called the Nernst potential for that ion; the equation
for this is the following:
EMF (mV) = ± 61 ¥ log
Ê Concentration inside ˆ
Ë Concentration outside ¯
where EMF is the Nernst potential in millivolts on the
inside of the membrane. The potential will be negative
(-) for positive ions and positive (+) for negative ions.
Now, let us calculate the Nernst potential that will
exactly oppose the movement of each of the three separate ions: sodium, potassium, and chloride.
For the sodium concentration difference shown in
Figure 45–8, 142 mEq/L on the exterior and 14 mEq/L
on the interior, the membrane potential that will
exactly oppose sodium ion movement through the
sodium channels calculates to be +61 millivolts.
However, the actual membrane potential is -65 millivolts, not +61 millivolts. Therefore, those sodium ions
that leak to the interior are immediately pumped back
to the exterior by the sodium pump, thus maintaining
the -65 millivolt negative potential inside the neuron.
For potassium ions, the concentration gradient is
120 mEq/L inside the neuron and 4.5 mEq/L outside.
This calculates to a Nernst potential of -86 millivolts
inside the neuron, which is more negative than the
565
-65 that actually exists. Therefore, because of the high
intracellular potassium ion concentration, there is a
net tendency for potassium ions to diffuse to the
outside of the neuron, but this is opposed by continual
pumping of these potassium ions back to the
interior.
Finally, the chloride ion gradient, 107 mEq/L outside
and 8 mEq/L inside, yields a Nernst potential of -70
millivolts inside the neuron, which is only slightly more
negative than the actual measured value of -65 millivolts. Therefore, chloride ions tend to leak very slightly
to the interior of the neuron, but those few that do leak
are moved back to the exterior, perhaps by an active
chloride pump.
Keep these three Nernst potentials in mind and
remember the direction in which the different ions
tend to diffuse because this information is important
in understanding both excitation and inhibition of the
neuron by synapse activation or inactivation of ion
channels.
Uniform Distribution of Electrical Potential Inside the
Soma. The interior of the neuronal soma contains a
highly conductive electrolytic solution, the intracellular fluid of the neuron. Furthermore, the diameter of
the neuronal soma is large (from 10 to 80 micrometers), causing almost no resistance to conduction of
electric current from one part of the somal interior to
another part. Therefore, any change in potential in any
part of the intrasomal fluid causes an almost exactly
equal change in potential at all other points inside the
soma (that is, as long as the neuron is not transmitting
an action potential). This is an important principle,
because it plays a major role in “summation” of signals
entering the neuron from multiple sources, as we shall
see in subsequent sections of this chapter.
Effect of Synaptic Excitation on the Postsynaptic Membrane—
Excitatory Postsynaptic Potential. Figure 45–9A shows the
resting neuron with an unexcited presynaptic terminal
resting on its surface. The resting membrane potential
everywhere in the soma is -65 millivolts.
Figure 45–9B shows a presynaptic terminal that has
secreted an excitatory transmitter into the cleft
between the terminal and the neuronal somal membrane. This transmitter acts on the membrane excitatory receptor to increase the membrane’s permeability
to Na+. Because of the large sodium concentration
gradient and large electrical negativity inside the
neuron, sodium ions diffuse rapidly to the inside of the
membrane.
The rapid influx of positively charged sodium ions
to the interior neutralizes part of the negativity of the
resting membrane potential. Thus, in Figure 45–9B, the
resting membrane potential has increased in the positive direction from -65 to -45 millivolts. This positive
increase in voltage above the normal resting neuronal
potential—that is, to a less negative value—is called
the excitatory postsynaptic potential (or EPSP),
because if this potential rises high enough in the positive direction, it will elicit an action potential in the
postsynaptic neuron, thus exciting it. (In this case, the
566
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
A
-65 mV
Resting Neuron
Initial segment
of axon
B
Excitatory
-45 mV
Na+
influx
Excited Neuron
C
Spread of
action potential
Cl- influx
times as great a concentration of voltage-gated sodium
channels as does the soma and, therefore, can generate an action potential with much greater ease than
can the soma. The EPSP that will elicit an action
potential in the axon initial segment is between +10
and +20 millivolts. This is in contrast to the +30 or +40
millivolts or more required on the soma.
Once the action potential begins, it travels peripherally along the axon and usually also backward over
the soma. In some instances, it travels backward into
the dendrites, too, but not into all of them, because
they, like the neuronal soma, have very few voltagegated sodium channels and therefore frequently
cannot generate action potentials at all. Thus, in Figure
45–9B, the threshold for excitation of the neuron is
shown to be about -45 millivolts, which represents an
EPSP of +20 millivolts—that is, 20 millivolts more positive than the normal resting neuronal potential of
-65 millivolts.
Inhibitory
-70 mV
K+
efflux
Inhibited Neuron
Figure 45–9
Three states of a neuron. A, Resting neuron, with a normal intraneuronal potential of -65 millivolts. B, Neuron in an excited state,
with a less negative intraneuronal potential (-45 millivolts) caused
by sodium influx. C, Neuron in an inhibited state, with a more negative intraneuronal membrane potential (-70 millivolts) caused by
potassium ion efflux, chloride ion influx, or both.
EPSP is +20 millivolts—that is, 20 millivolts more positive than the resting value.)
However, we must issue a word of warning. Discharge of a single presynaptic terminal can never
increase the neuronal potential from -65 millivolts
all the way up to -45 millivolts. An increase of this
magnitude requires simultaneous discharge of many
terminals—about 40 to 80 for the usual anterior motor
neuron—at the same time or in rapid succession. This
occurs by a process called summation, which is discussed in detail in the next sections.
Generation of Action Potentials in the Initial Segment of the
Axon Leaving the Neuron—Threshold for Excitation. When
the EPSP rises high enough in the positive direction,
there comes a point at which this initiates an action
potential in the neuron. However, the action potential
does not begin adjacent to the excitatory synapses.
Instead, it begins in the initial segment of the axon
where the axon leaves the neuronal soma. The main
reason for this point of origin of the action potential
is that the soma has relatively few voltage-gated
sodium channels in its membrane, which makes it difficult for the EPSP to open the required number of
sodium channels to elicit an action potential. Conversely, the membrane of the initial segment has seven
Electrical Events During Neuronal
Inhibition
Effect of Inhibitory Synapses on the Postsynaptic Membrane—
Inhibitory Postsynaptic Potential. The inhibitory synapses
open mainly chloride channels, allowing easy passage
of chloride ions. Now, to understand how the
inhibitory synapses inhibit the postsynaptic neuron,
we must recall what we learned about the Nernst
potential for chloride ions. We calculated the Nernst
potential for chloride ions to be about -70 millivolts.
This potential is more negative than the -65 millivolts
normally present inside the resting neuronal membrane. Therefore, opening the chloride channels
will allow negatively charged chloride ions to move
from the extracellular fluid to the interior, which
will make the interior membrane potential more
negative than normal, approaching the -70 millivolt
level.
Opening potassium channels will allow positively
charged potassium ions to move to the exterior, and
this will also make the interior membrane potential
more negative than usual. Thus, both chloride influx
and potassium efflux increase the degree of intracellular negativity, which is called hyperpolarization. This
inhibits the neuron because the membrane potential is
even more negative than the normal intracellular
potential. Therefore, an increase in negativity beyond
the normal resting membrane potential level is called
an inhibitory postsynaptic potential (IPSP).
Figure 45–9C shows the effect on the membrane
potential caused by activation of inhibitory synapses,
allowing chloride influx into the cell and/or potassium
efflux out of the cell, with the membrane potential
decreasing from its normal value of -65 millivolts to
the more negative value of -70 millivolts. This membrane potential is 5 millivolts more negative than
normal and is therefore an IPSP of -5 millivolts, which
inhibits transmission of the nerve signal through the
synapse.
Chapter 45
567
Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances”
Presynaptic Inhibition
In addition to inhibition caused by inhibitory synapses
operating at the neuronal membrane, which is called
postsynaptic inhibition, another type of inhibition
often occurs at the presynaptic terminals before the
signal ever reaches the synapse. This type of inhibition,
called presynaptic inhibition, occurs in the following
way.
Presynaptic inhibition is caused by release of an
inhibitory substance onto the outsides of the presynaptic nerve fibrils before their own endings terminate
on the postsynaptic neuron. In most instances, the
inhibitory transmitter substance is GABA (gammaaminobutyric acid). This has a specific effect of
opening anion channels, allowing large numbers of
chloride ions to diffuse into the terminal fibril. The
negative charges of these ions inhibit synaptic transmission because they cancel much of the excitatory
effect of the positively charged sodium ions that also
enter the terminal fibrils when an action potential
arrives.
Presynaptic inhibition occurs in many of the sensory
pathways in the nervous system. In fact, adjacent
sensory nerve fibers often mutually inhibit one
another, which minimizes sideways spread and mixing
of signals in sensory tracts. We discuss the importance
of this phenomenon more fully in subsequent chapters.
Time Course of Postsynaptic Potentials
When an excitatory synapse excites the anterior motor
neuron, the neuronal membrane becomes highly permeable to sodium ions for 1 to 2 milliseconds. During
this very short time, enough sodium ions diffuse
rapidly to the interior of the postsynaptic motor
neuron to increase its intraneuronal potential by a few
millivolts, thus creating the excitatory postsynaptic
potential (EPSP) shown by the blue and green curves
of Figure 45–10. This potential then slowly declines
over the next 15 milliseconds because this is the time
required for the excess positive charges to leak out of
the excited neuron and to re-establish the normal
resting membrane potential.
Precisely the opposite effect occurs for an IPSP; that
is, the inhibitory synapse increases the permeability of
the membrane to potassium or chloride ions, or both,
for 1 to 2 milliseconds, and this decreases the intraneuronal potential to a more negative value than
normal, thereby creating the IPSP. This potential also
dies away in about 15 milliseconds.
Other types of transmitter substances can excite or
inhibit the postsynaptic neuron for much longer
periods—for hundreds of milliseconds or even for
seconds, minutes, or hours. This is especially true for
some of the neuropeptide types of transmitter
substances.
“Spatial Summation” in Neurons—
Threshold for Firing
Excitation of a single presynaptic terminal on the
surface of a neuron almost never excites the neuron.
The reason for this is that sufficient transmitter substance is released by a single terminal to cause an
EPSP usually no greater than 0.5 to 1 millivolt, instead
of the 10 to 20 millivolts normally required to reach
threshold for excitation. However, many presynaptic
terminals are usually stimulated at the same time.
Even though these terminals are spread over wide
areas of the neuron, their effects can still summate;
that is, they can add to one another until neuronal excitation does occur. The reason for this is the following:
It was pointed out earlier that a change in potential at
any single point within the soma will cause the potential to change everywhere inside the soma almost
exactly equally. This is true because of the very high
electrical conductivity inside the large neuronal cell
body. Therefore, for each excitatory synapse that discharges simultaneously, the total intrasomal potential
becomes more positive by 0.5 to 1.0 millivolt. When
+20
Action potential
Millivolts
0
–20
Excitatory postsynaptic potentials, showing that
simultaneous firing of only a few synapses will not
cause sufficient summated potential to elicit an
action potential, but that simultaneous firing of
many synapses will raise the summated potential
to threshold for excitation and cause a superimposed action potential.
16 synapses firing
8
8 synapses firing
4
4 synapses firing
16
–40
Excitatory postsynaptic
potential
8
Figure 45–10
16
4
–60
Resting membrane potential
– 80
0
2
4
6
8
Milliseconds
10
12
14
16
The Nervous System: A. General Principles and Sensory Physiology
the EPSP becomes great enough, the threshold for
firing will be reached and an action potential will
develop spontaneously in the initial segment of the
axon. This is demonstrated in Figure 45–10. The
bottom postsynaptic potential in the figure was caused
by simultaneous stimulation of 4 synapses; the next
higher potential was caused by stimulation of 8
synapses; finally, a still higher EPSP was caused by
stimulation of 16 synapses. In this last instance, the
firing threshold had been reached, and an action
potential was generated in the axon.
This effect of summing simultaneous postsynaptic
potentials by activating multiple terminals on widely
spaced areas of the neuronal membrane is called
spatial summation.
“Temporal Summation”
Each time a presynaptic terminal fires, the released
transmitter substance opens the membrane channels
for at most a millisecond or so. But the changed postsynaptic potential lasts up to 15 milliseconds after the
synaptic membrane channels have already closed.
Therefore, a second opening of the same channels can
increase the postsynaptic potential to a still greater
level, and the more rapid the rate of stimulation, the
greater the postsynaptic potential becomes. Thus, successive discharges from a single presynaptic terminal,
if they occur rapidly enough, can add to one another;
that is, they can “summate.” This type of summation is
called temporal summation.
Simultaneous Summation of Inhibitory and Excitatory Postsynaptic Potentials. If an IPSP is tending to decrease the
Most Dendrites Cannot Transmit Action Potentials—But They
Can Transmit Signals Within the Same Neuron by Electrotonic
Conduction. Most dendrites fail to transmit action
potentials because their membranes have relatively
few voltage-gated sodium channels, and their thresholds for excitation are too high for action potentials to
occur. Yet they do transmit electrotonic current down
the dendrites to the soma. Transmission of electrotonic
current means direct spread of electrical current by ion
conduction in the fluids of the dendrites but without
generation of action potentials. Stimulation (or inhibition) of the neuron by this current has special characteristics, as follows.
Decrement of Electrotonic Conduction in the Dendrites—Greater Excitatory (or Inhibitory) Effect by
Synapses Located Near the Soma. In Figure 45–11,
multiple excitatory and inhibitory synapses are shown
stimulating the dendrites of a neuron. On the two dendrites to the left, there are excitatory effects near the
tip ends; note the high levels of excitatory postsynaptic potentials at these ends—that is, note the less negative membrane potentials at these points. However, a
-2
E
E
0
Large Spatial Field of Excitation of the Dendrites. The dendrites of the anterior motor neurons often extend 500
to 1000 micrometers in all directions from the neu-
E
E
-1
Special Functions of Dendrites
for Exciting Neurons
E
E
“Facilitation” of Neurons
Often the summated postsynaptic potential is excitatory but has not risen high enough to reach the threshold for firing by the postsynaptic neuron. When this
happens, the neuron is said to be facilitated. That is, its
membrane potential is nearer the threshold for firing
than normal, but not yet at the firing level. Consequently, another excitatory signal entering the neuron
from some other source can then excite the neuron
very easily. Diffuse signals in the nervous system often
do facilitate large groups of neurons so that they can
respond quickly and easily to signals arriving from
other sources.
E
0
membrane potential to a more negative value while an
EPSP is tending to increase the potential at the same
time, these two effects can either completely or partially nullify each other. Thus, if a neuron is being
excited by an EPSP, an inhibitory signal from another
source can often reduce the postsynaptic potential to
less than threshold value for excitation, thus turning
off the activity of the neuron.
ronal soma. And these dendrites can receive signals
from a large spatial area around the motor neuron.
This provides a vast opportunity for summation of
signals from many separate presynaptic nerve fibers.
It is also important that between 80 and 95 per cent
of all the presynaptic terminals of the anterior motor
neuron terminate on dendrites, in contrast to only 5 to
20 per cent terminating on the neuronal soma. Therefore, the preponderant share of the excitation is provided by signals transmitted by way of the dendrites.
0
Unit IX
-2
568
E
-3
-2
E
0
I
-3
5
-5
0
-40
-50 -60
-30 -40
E
0-
-40
-50 I
-75
-70
-60
60
-60 mV
I
I
-70 -75
I
I
Figure 45–11
Stimulation of a neuron by presynaptic terminals located on dendrites, showing, especially, decremental conduction of excitatory
(E) electrotonic potentials in the two dendrites to the left and inhibition (I) of dendritic excitation in the dendrite that is uppermost.
A powerful effect of inhibitory synapses at the initial segment of
the axon is also shown.
Chapter 45
569
Organization of the Nervous System, Basic Functions of Synapses, “Transmitter Substances”
large share of the excitatory postsynaptic potential is
lost before it reaches the soma. The reason is that the
dendrites are long, and their membranes are thin and
at least partially permeable to potassium and chloride
ions, making them “leaky” to electric current. Therefore, before the excitatory potentials can reach the
soma, a large share of the potential is lost by leakage
through the membrane. This decrease in membrane
potential as it spreads electrotonically along dendrites
toward the soma is called decremental conduction.
The farther the excitatory synapse is from the soma
of the neuron, the greater will be the decrement, and
the less will be excitatory signal reaching the soma.
Therefore, those synapses that lie near the soma have
far more effect in causing neuron excitation or inhibition than those that lie far away from the soma.
Summation of Excitation and Inhibition in Dendrites. The
uppermost dendrite of Figure 45–11 is shown to be
stimulated by both excitatory and inhibitory synapses.
At the tip of the dendrite is a strong excitatory postsynaptic potential, but nearer the soma are two
inhibitory synapses acting on the same dendrite. These
inhibitory synapses provide a hyperpolarizing voltage
that completely nullifies the excitatory effect and
indeed transmits a small amount of inhibition by electrotonic conduction toward the soma. Thus, dendrites
can summate excitatory and inhibitory postsynaptic
potentials in the same way that the soma can. Also
shown in the figure are several inhibitory synapses
located directly on the axon hillock and initial axon
segment. This location provides especially powerful
inhibition because it has the direct effect of increasing
the threshold for excitation at the very point where the
action potential is normally generated.
Frequency of discharge per second
600
Relation of State of Excitation of the
Neuron to Rate of Firing
“Excitatory State.” The “excitatory state” of a neuron is
defined as the summated degree of excitatory drive to
the neuron. If there is a higher degree of excitation
than inhibition of the neuron at any given instant, then
it is said that there is an excitatory state. Conversely, if
there is more inhibition than excitation, then it is said
that there is an inhibitory state.
When the excitatory state of a neuron rises above
the threshold for excitation, the neuron will fire repetitively as long as the excitatory state remains at that
level. Figure 45–12 shows responses of three types of
neurons to varying levels of excitatory state. Note that
neuron 1 has a low threshold for excitation, whereas
neuron 3 has a high threshold. But note also that
neuron 2 has the lowest maximum frequency of discharge, whereas neuron 3 has the highest maximum
frequency.
Some neurons in the central nervous system fire
continuously because even the normal excitatory
state is above the threshold level. Their frequency of
firing can usually be increased still more by further
increasing their excitatory state. The frequency can be
decreased, or firing can even be stopped, by superimposing an inhibitory state on the neuron. Thus, different neurons respond differently, have different
thresholds for excitation, and have widely differing
maximum frequencies of discharge. With a little imagination, one can readily understand the importance of
having different neurons with these many types of
response characteristics to perform the widely varying
functions of the nervous system.
Neuron 1
Neuron 2
Neuron 3
500
400
300
200
Threshold
100
0
Figure 45–12
Response characteristics of different types of
neurons to different levels of excitatory state.
0
5
10
15
20
25
Excitatory state (arbitrary units)
30
35
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Unit IX
The Nervous System: A. General Principles and Sensory Physiology
Some Special Characteristics
of Synaptic Transmission
of Synaptic Transmission. When excitatory
synapses are repetitively stimulated at a rapid rate, the
number of discharges by the postsynaptic neuron is at
first very great, but the firing rate becomes progressively less in succeeding milliseconds or seconds. This
is called fatigue of synaptic transmission.
Fatigue is an exceedingly important characteristic of
synaptic function because when areas of the nervous
system become overexcited, fatigue causes them to
lose this excess excitability after awhile. For example,
fatigue is probably the most important means by which
the excess excitability of the brain during an epileptic
seizure is finally subdued so that the seizure ceases.
Thus, the development of fatigue is a protective mechanism against excess neuronal activity. This is discussed further in the description of reverberating
neuronal circuits in Chapter 46.
The mechanism of fatigue is mainly exhaustion or
partial exhaustion of the stores of transmitter substance in the presynaptic terminals. The excitatory terminals on many neurons can store enough excitatory
transmitter to cause only about 10,000 action potentials, and the transmitter can be exhausted in only a
few seconds to a few minutes of rapid stimulation. Part
of the fatigue process probably results from two other
factors as well: (1) progressive inactivation of many of
the postsynaptic membrane receptors and (2) slow
development of abnormal concentrations of ions
inside the postsynaptic neuronal cell.
Fatigue
Effect of Acidosis or Alkalosis on Synaptic Transmission. Most
neurons are highly responsive to changes in pH of
the surrounding interstitial fluids. Normally, alkalosis
greatly increases neuronal excitability. For instance, a
rise in arterial blood pH from the 7.4 norm to 7.8 to
8.0 often causes cerebral epileptic seizures because of
increased excitability of some or all of the cerebral
neurons. This can be demonstrated especially well by
asking a person who is predisposed to epileptic seizures
to overbreathe. The overbreathing blows off carbon
dioxide and therefore elevates the pH of the blood
momentarily, but even this short time can often precipitate an epileptic attack.
Conversely, acidosis greatly depresses neuronal activity; a fall in pH from 7.4 to below 7.0 usually causes a
comatose state. For instance, in very severe diabetic or
uremic acidosis, coma virtually always develops.
of Hypoxia on Synaptic Transmission. Neuronal
excitability is also highly dependent on an adequate
supply of oxygen. Cessation of oxygen for only a few
seconds can cause complete inexcitability of some
neurons. This is observed when the brain’s blood flow is
temporarily interrupted, because within 3 to 7 seconds,
the person becomes unconscious.
Effect
Effect of Drugs on Synaptic Transmission. Many drugs are
known to increase the excitability of neurons, and
others are known to decrease excitability. For instance,
caffeine, theophylline, and theobromine, which are
found in coffee, tea, and cocoa, respectively, all increase
neuronal excitability, presumably by reducing the
threshold for excitation of neurons.
Strychnine is one of the best known of all agents that
increase excitability of neurons. However, it does not
do this by reducing the threshold for excitation of
the neurons; instead, it inhibits the action of some normally inhibitory transmitter substances, especially the
inhibitory effect of glycine in the spinal cord. Therefore,
the effects of the excitatory transmitters become overwhelming, and the neurons become so excited that they
go into rapidly repetitive discharge, resulting in severe
tonic muscle spasms.
Most anesthetics increase the neuronal membrane
threshold for excitation and thereby decrease synaptic
transmission at many points in the nervous system.
Because many of the anesthetics are especially lipidsoluble, it has been reasoned that some of them might
change the physical characteristics of the neuronal
membranes, making them less responsive to excitatory
agents.
Synaptic Delay. During transmission of a neuronal signal
from a presynaptic neuron to a postsynaptic neuron, a
certain amount of time is consumed in the process of
(1) discharge of the transmitter substance by the presynaptic terminal, (2) diffusion of the transmitter to the
postsynaptic neuronal membrane, (3) action of the
transmitter on the membrane receptor, (4) action of
the receptor to increase the membrane permeability,
and (5) inward diffusion of sodium to raise the excitatory postsynaptic potential to a high enough level to
elicit an action potential. The minimal period of time
required for all these events to take place, even when
large numbers of excitatory synapses are stimulated
simultaneously, is about 0.5 millisecond. This is called
the synaptic delay. Neurophysiologists can measure the
minimal delay time between an input volley of impulses
into a pool of neurons and the consequent output volley.
From the measure of delay time, one can then estimate
the number of series neurons in the circuit.
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Cowley MA, Cone RD, Enriori P, et al: Electrophysiological
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6
Sensory Receptors, Neuronal
Circuits for Processing Information
Input to the nervous system is provided by sensory
receptors that detect such sensory stimuli as touch,
sound, light, pain, cold, and warmth. The purpose of
this chapter is to discuss the basic mechanisms by
which these receptors change sensory stimuli into
nerve signals that are then conveyed to and
processed in the central nervous system.
Types of Sensory Receptors and the Sensory
Stimuli They Detect
Table 46–1 lists and classifies most of the body’s sensory receptors. This table
shows that there are five basic types of sensory receptors: (1) mechanoreceptors, which detect mechanical compression or stretching of the receptor or of
tissues adjacent to the receptor; (2) thermoreceptors, which detect changes in
temperature, some receptors detecting cold and others warmth; (3) nociceptors
(pain receptors), which detect damage occurring in the tissues, whether physical damage or chemical damage; (4) electromagnetic receptors, which detect light
on the retina of the eye; and (5) chemoreceptors, which detect taste in the
mouth, smell in the nose, oxygen level in the arterial blood, osmolality of the
body fluids, carbon dioxide concentration, and perhaps other factors that make
up the chemistry of the body.
In this chapter, we discuss the function of a few specific types of receptors,
primarily peripheral mechanoreceptors, to illustrate some of the principles by
which receptors operate. Other receptors are discussed in other chapters in relation to the sensory systems that they subserve. Figure 46–1 shows some of the
types of mechanoreceptors found in the skin or in deep tissues of the body.
Differential Sensitivity of Receptors
The first question that must be answered is, how do two types of sensory receptors detect different types of sensory stimuli? The answer is, by “differential sensitivities.”That is, each type of receptor is highly sensitive to one type of stimulus
for which it is designed and yet is almost nonresponsive to other types of sensory
stimuli. Thus, the rods and cones of the eyes are highly responsive to light but
are almost completely nonresponsive to normal ranges of heat, cold, pressure
on the eyeballs, or chemical changes in the blood. The osmoreceptors of the
supraoptic nuclei in the hypothalamus detect minute changes in the osmolality
of the body fluids but have never been known to respond to sound. Finally, pain
receptors in the skin are almost never stimulated by usual touch or pressure
stimuli but do become highly active the moment tactile stimuli become severe
enough to damage the tissues.
Modality of Sensation—The “Labeled Line” Principle
Each of the principal types of sensation that we can experience—pain, touch,
sight, sound, and so forth—is called a modality of sensation. Yet despite the
fact that we experience these different modalities of sensation, nerve fibers
572
Chapter 46
Sensory Receptors, Neuronal Circuits for Processing Information
573
Table 46–1
Classification of Sensory Receptors
Free nerve
endings
Expanded tip
receptor
Tactile hair
Pacinian
corpuscle
Meissner’s
corpuscle
Krause’s
corpuscle
Ruffini’s
end-organ
Golgi tendon
apparatus
Muscle
spindle
Figure 46–1
Several types of somatic sensory nerve endings.
transmit only impulses. Therefore, how is it that
different nerve fibers transmit different modalities of
sensation?
The answer is that each nerve tract terminates at a
specific point in the central nervous system, and
the type of sensation felt when a nerve fiber is stimulated is determined by the point in the nervous system
to which the fiber leads. For instance, if a pain fiber
is stimulated, the person perceives pain regardless
of what type of stimulus excites the fiber. The stimulus
can be electricity, overheating of the fiber, crushing
of the fiber, or stimulation of the pain nerve ending
by damage to the tissue cells. In all these instances,
the person perceives pain. Likewise, if a touch fiber is
stimulated by electrical excitation of a touch receptor
or in any other way, the person perceives touch
because touch fibers lead to specific touch areas in
the brain. Similarly, fibers from the retina of the eye
terminate in the vision areas of the brain, fibers from
the ear terminate in the auditory areas of the brain,
and temperature fibers terminate in the temperature
areas.
This specificity of nerve fibers for transmitting only
one modality of sensation is called the labeled line
principle.
I. Mechanoreceptors
Skin tactile sensibilities (epidermis and dermis)
Free nerve endings
Expanded tip endings
Merkel’s discs
Plus several other variants
Spray endings
Ruffini’s endings
Encapsulated endings
Meissner’s corpuscles
Krause’s corpuscles
Hair end-organs
Deep tissue sensibilities
Free nerve endings
Expanded tip endings
Spray endings
Ruffini’s endings
Encapsulated endings
Pacinian corpuscles
Plus a few other variants
Muscle endings
Muscle spindles
Golgi tendon receptors
Hearing
Sound receptors of cochlea
Equilibrium
Vestibular receptors
Arterial pressure
Baroreceptors of carotid sinuses and aorta
II. Thermoreceptors
Cold
Cold receptors
Warmth
Warm receptors
III. Nociceptors
Pain
Free nerve endings
IV. Electromagnetic receptors
Vision
Rods
Cones
V. Chemoreceptors
Taste
Receptors of taste buds
Smell
Receptors of olfactory epithelium
Arterial oxygen
Receptors of aortic and carotid bodies
Osmolality
Neurons in or near supraoptic nuclei
Blood CO2
Receptors in or on surface of medulla and in aortic and
carotid bodies
Blood glucose, amino acids, fatty acids
Receptors in hypothalamus
Transduction of Sensory
Stimuli into Nerve Impulses
Local Electrical Currents at Nerve
Endings—Receptor Potentials
All sensory receptors have one feature in common.
Whatever the type of stimulus that excites the receptor, its immediate effect is to change the membrane
574
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
electrical potential of the receptor. This change in
potential is called a receptor potential.
receptor potential rises above the threshold level, the
greater becomes the action potential frequency.
Mechanisms of Receptor Potentials. Different receptors
can be excited in one of several ways to cause receptor potentials: (1) by mechanical deformation of the
receptor, which stretches the receptor membrane and
opens ion channels; (2) by application of a chemical to
the membrane, which also opens ion channels; (3) by
change of the temperature of the membrane, which
alters the permeability of the membrane; or (4) by the
effects of electromagnetic radiation, such as light on a
retinal visual receptor, which either directly or indirectly changes the receptor membrane characteristics
and allows ions to flow through membrane channels.
It will be recognized that these four means of exciting
receptors correspond in general with the different
types of known sensory receptors. In all instances, the
basic cause of the change in membrane potential is a
change in membrane permeability of the receptor,
which allows ions to diffuse more or less readily
through the membrane and thereby to change the
transmembrane potential.
Receptor Potential of the Pacinian Corpuscle—
An Example of Receptor Function
Maximum Receptor Potential Amplitude. The maximum
amplitude of most sensory receptor potentials is about
100 millivolts, but this level occurs only at an extremely
high intensity of sensory stimulus. This is about the
same maximum voltage recorded in action potentials
and is also the change in voltage when the membrane
becomes maximally permeable to sodium ions.
Relation of the Receptor Potential to Action Potentials. When
the receptor potential rises above the threshold for
eliciting action potentials in the nerve fiber attached
to the receptor, then action potentials occur, as illustrated in Figure 46–2. Note also that the more the
The student should at this point restudy the anatomical structure of the pacinian corpuscle shown in Figure
46–1. Note that the corpuscle has a central nerve fiber
extending through its core. Surrounding this are multiple concentric capsule layers, so that compression
anywhere on the outside of the corpuscle will elongate,
indent, or otherwise deform the central fiber.
Now study Figure 46–3, which shows only the
central fiber of the pacinian corpuscle after all capsule
layers but one have been removed. The tip of the
central fiber inside the capsule is unmyelinated, but
the fiber does become myelinated (the blue sheath
shown in the figure) shortly before leaving the corpuscle to enter a peripheral sensory nerve.
The figure also shows the mechanism by which a
receptor potential is produced in the pacinian corpuscle. Observe the small area of the terminal fiber that
has been deformed by compression of the corpuscle,
and note that ion channels have opened in the membrane, allowing positively charged sodium ions to
diffuse to the interior of the fiber. This creates
increased positivity inside the fiber, which is the
“receptor potential.” The receptor potential in turn
induces a local circuit of current flow, shown by the
arrows, that spreads along the nerve fiber. At the first
node of Ranvier, which itself lies inside the capsule of
the pacinian corpuscle, the local current flow depolarizes the fiber membrane at this node, which then sets
off typical action potentials that are transmitted along
the nerve fiber toward the central nervous system.
Relation Between Stimulus Intensity and the Receptor Potential. Figure 46–4 shows the changing amplitude of the
Membrane potential (millivolts)
receptor potential caused by progressively stronger
mechanical compression (increasing “stimulus
strength”) applied experimentally to the central core
of a pacinian corpuscle. Note that the amplitude
Action potentials
+30
0
Receptor potential
-30
Receptor potential
Threshold
-60
Deformed
area
+ + + +++
+
+
+
+
+ + + ++-
Resting membrane potential
-90
0
10
20
+
+
-
+
+
-
Action
potential
- + + + + ++ + + + + + + + + + + + + + +
+
+
- + + + + ++ + + + + + + + + + + + + + +
Node of
Ranvier
30 40 60 80 100 120 140
Milliseconds
Figure 46–3
Figure 46–2
Typical relation between receptor potential and action potentials
when the receptor potential rises above threshold level.
Excitation of a sensory nerve fiber by a receptor potential produced in a pacinian corpuscle. (Modified from Loëwenstein WR:
Excitation and inactivation in a receptor membrane. Ann N Y Acad
Sci 94:510, 1961.)
Chapter 46
Joint capsule receptors
Muscle spindle
Hair receptor
Pacinian corpuscle
100
250
Impulses per second
90
Amplitude of observed
receptor potential (per cent)
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Sensory Receptors, Neuronal Circuits for Processing Information
80
70
60
50
40
200
150
100
50
30
0
20
0
10
1
2
3
4
5
Seconds
6
7
8
0
0
20
40
60
80
Stimulus strength
(per cent)
100
Figure 46–4
Relation of amplitude of receptor potential to strength of a
mechanical stimulus applied to a pacinian corpuscle. (Data from
Loëwenstein WR: Excitation and inactivation in a receptor membrane. Ann N Y Acad Sci 94:510, 1961.)
increases rapidly at first but then progressively less
rapidly at high stimulus strength.
In turn, the frequency of repetitive action potentials
transmitted from sensory receptors increases approximately in proportion to the increase in receptor
potential. Putting this principle together with the data
in Figure 46–4, one can see that very intense stimulation of the receptor causes progressively less and less
additional increase in numbers of action potentials.
This is an exceedingly important principle that is applicable to almost all sensory receptors. It allows the
receptor to be sensitive to very weak sensory experience and yet not reach a maximum firing rate until the
sensory experience is extreme. This allows the receptor to have an extreme range of response, from very
weak to very intense.
Adaptation of Receptors
Another characteristic of all sensory receptors is that
they adapt either partially or completely to any constant stimulus after a period of time. That is, when a
continuous sensory stimulus is applied, the receptor
responds at a high impulse rate at first and then at a
progressively slower rate until finally the rate of action
potentials decreases to very few or often to none at all.
Figure 46–5 shows typical adaptation of certain
types of receptors. Note that the pacinian corpuscle
adapts extremely rapidly and hair receptors adapt
Figure 46–5
Adaptation of different types of receptors, showing rapid adaptation of some receptors and slow adaptation of others.
within a second or so, whereas some joint capsule and
muscle spindle receptors adapt slowly.
Furthermore, some sensory receptors adapt to a far
greater extent than others. For example, the pacinian
corpuscles adapt to “extinction” within a few hundredths of a second, and the receptors at the bases of
the hairs adapt to extinction within a second or more.
It is probable that all other mechanoreceptors eventually adapt almost completely, but some require hours
or days to do so, for which reason they are called “nonadapting” receptors. The longest measured time for
complete adaptation of a mechanoreceptor is about
2 days, which is the adaptation time for many carotid and aortic baroreceptors. Conversely, some of
the nonmechanoreceptors—the chemoreceptors and
pain receptors, for instance—probably never adapt
completely.
Mechanisms by Which Receptors Adapt. The mechanism of
receptor adaptation is different for each type of receptor, in much the same way that development of a
receptor potential is an individual property. For
instance, in the eye, the rods and cones adapt by changing the concentrations of their light-sensitive chemicals (which is discussed in Chapter 50).
In the case of the mechanoreceptors, the receptor
that has been studied in greatest detail is the pacinian
corpuscle. Adaptation occurs in this receptor in two
ways. First, the pacinian corpuscle is a viscoelastic
structure so that when a distorting force is suddenly
applied to one side of the corpuscle, this force is
instantly transmitted by the viscous component of the
corpuscle directly to the same side of the central nerve
fiber, thus eliciting a receptor potential. However,
within a few hundredths of a second, the fluid within
the corpuscle redistributes, so that the receptor potential is no longer elicited. Thus, the receptor potential
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Unit IX
The Nervous System: A. General Principles and Sensory Physiology
appears at the onset of compression but disappears
within a small fraction of a second even though the
compression continues.
The second mechanism of adaptation of the pacinian corpuscle, but a much slower one, results from a
process called accommodation, which occurs in the
nerve fiber itself. That is, even if by chance the central
core fiber should continue to be distorted, the tip of
the nerve fiber itself gradually becomes “accommodated” to the stimulus. This probably results from
progressive “inactivation” of the sodium channels in
the nerve fiber membrane, which means that sodium
current flow through the channels causes them gradually to close, an effect that seems to occur for all
or most cell membrane sodium channels, as was
explained in Chapter 5.
Presumably, these same two general mechanisms of
adaptation apply also to the other types of mechanoreceptors. That is, part of the adaptation results from
readjustments in the structure of the receptor itself,
and part from an electrical type of accommodation in
the terminal nerve fibril.
Slowly Adapting Receptors Detect Continuous Stimulus
Strength—The “Tonic” Receptors. Slowly adapting recep-
tors continue to transmit impulses to the brain as long
as the stimulus is present (or at least for many minutes
or hours). Therefore, they keep the brain constantly
apprised of the status of the body and its relation to
its surroundings. For instance, impulses from the
muscle spindles and Golgi tendon apparatuses allow
the nervous system to know the status of muscle contraction and load on the muscle tendon at each instant.
Other slowly adapting receptors include (1) receptors of the macula in the vestibular apparatus, (2) pain
receptors, (3) baroreceptors of the arterial tree, and (4)
chemoreceptors of the carotid and aortic bodies.
Because the slowly adapting receptors can continue
to transmit information for many hours, they are called
tonic receptors.
Rapidly Adapting Receptors Detect Change in Stimulus
Strength—The “Rate Receptors,” “Movement Receptors,”
or “Phasic Receptors.” Receptors that adapt rapidly
cannot be used to transmit a continuous signal because
these receptors are stimulated only when the stimulus
strength changes. Yet they react strongly while a
change is actually taking place. Therefore, these receptors are called rate receptors, movement receptors, or
phasic receptors. Thus, in the case of the pacinian corpuscle, sudden pressure applied to the tissue excites
this receptor for a few milliseconds, and then its excitation is over even though the pressure continues. But
later, it transmits a signal again when the pressure is
released. In other words, the pacinian corpuscle is
exceedingly important in apprising the nervous system
of rapid tissue deformations, but it is useless for transmitting information about constant conditions in the
body.
status is taking place, one can predict in one’s mind the
state of the body a few seconds or even a few minutes
later. For instance, the receptors of the semicircular
canals in the vestibular apparatus of the ear detect the
rate at which the head begins to turn when one runs
around a curve. Using this information, a person can
predict how much he or she will turn within the next
2 seconds and can adjust the motion of the legs ahead
of time to keep from losing balance. Likewise, receptors located in or near the joints help detect the rates
of movement of the different parts of the body. For
instance, when one is running, information from the
joint rate receptors allows the nervous system to
predict where the feet will be during any precise fraction of the next second. Therefore, appropriate motor
signals can be transmitted to the muscles of the legs to
make any necessary anticipatory corrections in position so that the person will not fall. Loss of this predictive function makes it impossible for the person to
run.
Nerve Fibers That Transmit
Different Types of Signals,
and Their Physiologic
Classification
Some signals need to be transmitted to or from the
central nervous system extremely rapidly; otherwise,
the information would be useless. An example of this is
the sensory signals that apprise the brain of the momentary positions of the legs at each fraction of a second
during running. At the other extreme, some types of
sensory information, such as that depicting prolonged,
aching pain, do not need to be transmitted rapidly, so
that slowly conducting fibers will suffice. As shown in
Figure 46–6, nerve fibers come in all sizes between 0.5
and 20 micrometers in diameter—the larger the diameter, the greater the conducting velocity. The range of
conducting velocities is between 0.5 and 120 m/sec.
General Classification of Nerve Fibers. Shown in Figure 46–6
is a “general classification” and a “sensory nerve classification” of the different types of nerve fibers. In the
general classification, the fibers are divided into types A
and C, and the type A fibers are further subdivided into
a, b, g, and d fibers.
Type A fibers are the typical large and medium-sized
myelinated fibers of spinal nerves. Type C fibers are the
small unmyelinated nerve fibers that conduct impulses
at low velocities. The C fibers constitute more than one
half of the sensory fibers in most peripheral nerves as
well as all the postganglionic autonomic fibers.
The sizes, velocities of conduction, and functions of
the different nerve fiber types are also given in Figure
46–6. Note that a few large myelinated fibers can transmit impulses at velocities as great as 120 m/sec, a
distance in 1 second that is longer than a football field.
Conversely, the smallest fibers transmit impulses as
slowly as 0.5 m/sec, requiring about 2 seconds to go from
the big toe to the spinal cord.
Alternative Classification Used by Sensory Physiologists.
Importance of the Rate Receptors—Their Predictive Function.
If one knows the rate at which some change in bodily
Certain recording techniques have made it possible to
separate the type Aa fibers into two subgroups; yet
Chapter 46
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Sensory Receptors, Neuronal Circuits for Processing Information
Myelinated
Unmyelinated
Diameter (micrometers)
20
15
120
80
1 2.0
0.5
Conduction velocity (m/sec.)
60
30
6 2.0
0.5
10
5
General classification
A
C
a
b
g
d
Sensory nerve classification
I
II
III
IV
IA
IB
Sensory functions
Muscle spindle
(primary ending)
Muscle spindle
(secondary ending)
Muscle tendon
(Golgi tendon organ)
Hair receptors
Vibration
(pacinian corpuscle)
High discrimination touch
(Meissner's expanded tips)
Crude touch
and pressure
Deep pressure
and touch
Tickle
Pricking pain
Aching pain
Cold
Warmth
Motor function
Skeletal muscle
(type Aa)
Figure 46–6
20
15
Muscle spindle
(type Ag)
Sympathetic
(type C)
10
5
1 2.0
Nerve fiber diameter (micrometers)
0.5
Physiologic classifications and functions of nerve
fibers.
these same recording techniques cannot distinguish
easily between Ab and Ag fibers. Therefore, the following classification is frequently used by sensory
physiologists:
Group Ia
Fibers from the annulospiral endings of muscle spindles
(average about 17 microns in diameter; these are a-type
A fibers in the general classification).
Group Ib
Group III
Fibers carrying temperature, crude touch, and pricking
pain sensations (average about 3 micrometers in diameter; they are d-type A fibers in the general classification).
Group IV
Unmyelinated fibers carrying pain, itch, temperature,
and crude touch sensations (0.5 to 2 micrometers in
diameter; they are type C fibers in the general classification).
Group II
Transmission of Signals
of Different Intensity in
Nerve Tracts—Spatial and
Temporal Summation
Fibers from most discrete cutaneous tactile receptors
and from the flower-spray endings of the muscle spindles (average about 8 micrometers in diameter; these
are b- and g-type A fibers in the general classification).
One of the characteristics of each signal that always
must be conveyed is signal intensity—for instance, the
intensity of pain. The different gradations of intensity
Fibers from the Golgi tendon organs (average about 16
micrometers in diameter; these also are a-type A
fibers).
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Unit IX
The Nervous System: A. General Principles and Sensory Physiology
can be transmitted either by using increasing numbers
of parallel fibers or by sending more action potentials
along a single fiber. These two mechanisms are called,
respectively, spatial summation and temporal
summation.
Spatial Summation. Figure 46–7 shows the phenomenon
of spatial summation, whereby increasing signal
strength is transmitted by using progressively greater
numbers of fibers. This figure shows a section of skin
innervated by a large number of parallel pain fibers.
Each of these arborizes into hundreds of minute free
nerve endings that serve as pain receptors. The entire
cluster of fibers from one pain fiber frequently covers
an area of skin as large as 5 centimeters in diameter.
This area is called the receptor field of that fiber. The
number of endings is large in the center of the field but
diminishes toward the periphery. One can also see
from the figure that the arborizing fibrils overlap those
from other pain fibers. Therefore, a pinprick of the skin
usually stimulates endings from many different pain
fibers simultaneously. When the pinprick is in the
center of the receptive field of a particular pain fiber,
the degree of stimulation of that fiber is far greater
than when it is in the periphery of the field, because
the number of free nerve endings in the middle of the
field is much greater than at the periphery.
Thus, the lower part of Figure 46–7 shows three
views of the cross section of the nerve bundle leading
from the skin area. To the left is the effect of a weak
stimulus, with only a single nerve fiber in the middle
of the bundle stimulated strongly (represented by the
red-colored fiber), whereas several adjacent fibers are
stimulated weakly (half-red fibers). The other two
views of the nerve cross section show the effect of a
moderate stimulus and a strong stimulus, with progressively more fibers being stimulated. Thus, the
stronger signals spread to more and more fibers. This
is the phenomenon of spatial summation.
Temporal Summation. A second means for transmitting
signals of increasing strength is by increasing the frequency of nerve impulses in each fiber, which is called
temporal summation. Figure 46–8 demonstrates this,
showing in the upper part a changing strength of signal
and in the lower part the actual impulses transmitted
by the nerve fiber.
Transmission and Processing
of Signals in Neuronal Pools
The central nervous system is composed of thousands
to millions of neuronal pools; some of these contain
few neurons, while others have vast numbers. For
instance, the entire cerebral cortex could be considered to be a single large neuronal pool. Other neuronal
pools include the different basal ganglia and the
specific nuclei in the thalamus, cerebellum, mesencephalon, pons, and medulla. Also, the entire dorsal
gray matter of the spinal cord could be considered one
long pool of neurons.
Each neuronal pool has its own special organization
that causes it to process signals in its own unique way,
Pin
Strength of signal
(impulses per second)
80
Nerve
Skin
60
40
20
Impulses
0
Weak
stimulus
Moderate
stimulus
Strong
stimulus
Figure 46–7
Pattern of stimulation of pain fibers in a nerve leading from an area
of skin pricked by a pin. This is an example of spatial summation.
Time
Figure 46–8
Translation of signal strength into a frequency-modulated series
of nerve impulses, showing the strength of signal (above) and the
separate nerve impulses (below). This is an example of temporal
summation.
Chapter 46
579
Sensory Receptors, Neuronal Circuits for Processing Information
thus allowing the total consortium of pools to achieve
the multitude of functions of the nervous system. Yet
despite their differences in function, the pools also
have many similar principles of function, described in
the following pages.
Relaying of Signals Through
Neuronal Pools
Organization of Neurons for Relaying Signals. Figure 46–9 is
a schematic diagram of several neurons in a neuronal
pool, showing “input” fibers to the left and “output”
fibers to the right. Each input fiber divides hundreds
to thousands of times, providing a thousand or more
terminal fibrils that spread into a large area in the pool
to synapse with dendrites or cell bodies of the neurons
in the pool. The dendrites usually also arborize and
spread hundreds to thousands of micrometers in the
pool.
The neuronal area stimulated by each incoming
nerve fiber is called its stimulatory field. Note in Figure
46–9 that large numbers of the terminals from each
input fiber lie on the nearest neuron in its “field,” but
progressively fewer terminals lie on the neurons
farther away.
Threshold and Subthreshold Stimuli—Excitation or Facilitation.
From the discussion of synaptic function in Chapter
45, it will be recalled that discharge of a single
1
a
excitatory presynaptic terminal almost never causes an
action potential in a postsynaptic neuron. Instead,
large numbers of input terminals must discharge on
the same neuron either simultaneously or in rapid succession to cause excitation. For instance, in Figure
46–9, let us assume that six terminals must discharge
almost simultaneously to excite any one of the
neurons. If the student counts the number of terminals
on each one of the neurons from each input fiber, he
or she will see that input fiber 1 has more than enough
terminals to cause neuron a to discharge. The stimulus
from input fiber 1 to this neuron is said to be an
excitatory stimulus; it is also called a suprathreshold
stimulus because it is above the threshold required for
excitation.
Input fiber 1 also contributes terminals to neurons
b and c, but not enough to cause excitation. Nevertheless, discharge of these terminals makes both these
neurons more likely to be excited by signals arriving
through other incoming nerve fibers. Therefore, the
stimuli to these neurons are said to be subthreshold,
and the neurons are said to be facilitated.
Similarly, for input fiber 2, the stimulus to neuron d
is a suprathreshold stimulus, and the stimuli to neurons
b and c are subthreshold, but facilitating, stimuli.
Figure 46–9 represents a highly condensed version
of a neuronal pool because each input nerve fiber
usually provides massive numbers of branching terminals to hundreds or thousands of neurons in its distribution “field,” as shown in Figure 46–10. In the central
portion of the field in this figure, designated by the
circled area, all the neurons are stimulated by the
incoming fiber. Therefore, this is said to be the discharge zone of the incoming fiber, also called the
excited zone or liminal zone. To each side, the neurons
are facilitated but not excited, and these areas are
called the facilitated zone, also called the subthreshold
zone or subliminal zone.
Inhibition of a Neuronal Pool. We must also remember
b
c
that some incoming fibers inhibit neurons, rather than
exciting them. This is the opposite of facilitation, and
the entire field of the inhibitory branches is called the
inhibitory zone. The degree of inhibition in the center
of this zone is great because of large numbers of
endings in the center; it becomes progressively less
toward its edges.
d
2
Facilitated zone
Input nerve
fiber
Discharge zone
Facilitated zone
Figure 46–9
Basic organization of a neuronal pool.
Figure 46–10
“Discharge” and “facilitated” zones of a neuronal pool.
580
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
Source
Source
#1
Source
#2
Convergence from
single source
Divergence in same tract
A
Divergence in multiple tracts
B
Figure 46–11
“Divergence” in neuronal pathways. A, Divergence within a
pathway to cause “amplification” of the signal. B, Divergence into
multiple tracts to transmit the signal to separate areas.
Divergence of Signals Passing Through
Neuronal Pools
Often it is important for weak signals entering a neuronal pool to excite far greater numbers of nerve fibers
leaving the pool. This phenomenon is called divergence. Two major types of divergence occur and have
entirely different purposes.
An amplifying type of divergence is shown in Figure
46–11A. This means simply that an input signal spreads
to an increasing number of neurons as it passes
through successive orders of neurons in its path. This
type of divergence is characteristic of the corticospinal
pathway in its control of skeletal muscles, with a single
large pyramidal cell in the motor cortex capable, under
highly facilitated conditions, of exciting as many as
10,000 muscle fibers.
The second type of divergence, shown in Figure
46–11B, is divergence into multiple tracts. In this case,
the signal is transmitted in two directions from the
pool. For instance, information transmitted up the
dorsal columns of the spinal cord takes two courses in
the lower part of the brain: (1) into the cerebellum and
(2) on through the lower regions of the brain to the
thalamus and cerebral cortex. Likewise, in the thalamus, almost all sensory information is relayed both
into still deeper structures of the thalamus and at the
same time to discrete regions of the cerebral cortex.
Convergence of Signals
Convergence means signals from multiple inputs
uniting to excite a single neuron. Figure 46–12A shows
convergence from a single source. That is, multiple terminals from a single incoming fiber tract terminate on
the same neuron. The importance of this is that
neurons are almost never excited by an action potential from a single input terminal. But action potentials
converging on the neuron from multiple terminals
provide enough spatial summation to bring the neuron
to the threshold required for discharge.
A
B
Source
#3
Convergence from
multiple sources
Figure 46–12
“Convergence” of multiple input fibers onto a single neuron.
A, Multiple input fibers from a single source. B, Input fibers from
multiple separate sources.
Convergence can also result from input signals (excitatory or inhibitory) from multiple sources, as shown
in Figure 46–12B. For instance, the interneurons of the
spinal cord receive converging signals from (1) peripheral nerve fibers entering the cord, (2) propriospinal
fibers passing from one segment of the cord to
another, (3) corticospinal fibers from the cerebral
cortex, and (4) several other long pathways descending from the brain into the spinal cord.Then the signals
from the interneurons converge on the anterior motor
neurons to control muscle function.
Such convergence allows summation of information
from different sources, and the resulting response is a
summated effect of all the different types of information. Convergence is one of the important means by
which the central nervous system correlates, summates, and sorts different types of information.
Neuronal Circuit with Both Excitatory and
Inhibitory Output Signals
Sometimes an incoming signal to a neuronal pool
causes an output excitatory signal going in one direction and at the same time an inhibitory signal going
elsewhere. For instance, at the same time that an excitatory signal is transmitted by one set of neurons in the
spinal cord to cause forward movement of a leg, an
inhibitory signal is transmitted through a separate set
of neurons to inhibit the muscles on the back of the
leg so that they will not oppose the forward movement. This type of circuit is characteristic for controlling all antagonistic pairs of muscles, and it is called the
reciprocal inhibition circuit.
Figure 46–13 shows the means by which the inhibition is achieved. The input fiber directly excites the
excitatory output pathway, but it stimulates an intermediate inhibitory neuron (neuron 2), which secretes
a different type of transmitter substance to inhibit the
second output pathway from the pool. This type of
circuit is also important in preventing overactivity in
many parts of the brain.
Chapter 46
Sensory Receptors, Neuronal Circuits for Processing Information
Input
Excitatory synapse
#1
Input fiber
#2
#3
Excitation
A
Inhibition
Input
581
Output
Output
Inhibitory synapse
Figure 46–13
B
Facilitation
Inhibitory circuit. Neuron 2 is an inhibitory neuron.
Input
Output
Prolongation of a Signal by a
Neuronal Pool—“Afterdischarge”
Thus far, we have considered signals that are merely
relayed through neuronal pools. However, in many
instances, a signal entering a pool causes a prolonged
output discharge, called afterdischarge, lasting a
few milliseconds to as long as many minutes after
the incoming signal is over. The most important
mechanisms by which afterdischarge occurs are the
following.
C
Inhibition
Input
Output
Synaptic Afterdischarge. When excitatory synapses dis-
charge on the surfaces of dendrites or soma of a
neuron, a postsynaptic electrical potential develops in
the neuron and lasts for many milliseconds, especially
when some of the long-acting synaptic transmitter substances are involved. As long as this potential lasts, it
can continue to excite the neuron, causing it to transmit a continuous train of output impulses, as was
explained in Chapter 45. Thus, as a result of this synaptic “afterdischarge” mechanism alone, it is possible for
a single instantaneous input signal to cause a sustained
signal output (a series of repetitive discharges) lasting
for many milliseconds.
Reverberatory (Oscillatory) Circuit as a Cause of Signal Prolongation. One of the most important of all circuits in the
entire nervous system is the reverberatory, or oscillatory, circuit. Such circuits are caused by positive feedback within the neuronal circuit that feeds back to
re-excite the input of the same circuit. Consequently,
once stimulated, the circuit may discharge repetitively
for a long time.
Several possible varieties of reverberatory circuits
are shown in Figure 46–14. The simplest, shown in
Figure 46–14A, involves only a single neuron. In this
case, the output neuron simply sends a collateral nerve
fiber back to its own dendrites or soma to restimulate
itself. Although this type of circuit probably is not an
important one, theoretically, once the neuron discharges, the feedback stimuli could keep the neuron
discharging for a protracted time thereafter.
Figure 46–14B shows a few additional neurons in the
feedback circuit, which causes a longer delay between
initial discharge and the feedback signal. Figure
46–14C shows a still more complex system in which
both facilitatory and inhibitory fibers impinge on the
reverberating circuit.A facilitatory signal enhances the
D
Figure 46–14
Reverberatory circuits of increasing complexity.
intensity and frequency of reverberation, whereas an
inhibitory signal depresses or stops the reverberation.
Figure 46–14D shows that most reverberating pathways are constituted of many parallel fibers. At each
cell station, the terminal fibrils spread widely. In such
a system, the total reverberating signal can be either
weak or strong, depending on how many parallel nerve
fibers are momentarily involved in the reverberation.
Characteristics of Signal Prolongation from a Reverberatory Circuit. Figure 46–15 shows output signals
from a typical reverberatory circuit. The input stimulus may last only 1 millisecond or so, and yet the output
can last for many milliseconds or even minutes. The
figure demonstrates that the intensity of the output
signal usually increases to a high value early in reverberation and then decreases to a critical point, at which
it suddenly ceases entirely. The cause of this sudden
cessation of reverberation is fatigue of synaptic junctions in the circuit. Fatigue beyond a certain critical
level lowers the stimulation of the next neuron in the
circuit below threshold level so that the circuit feedback is suddenly broken.
The Nervous System: A. General Principles and Sensory Physiology
Inhibited
Normal
Facilitated
Input stimulus
Output pulse rate
Unit IX
Output
Excitation
Inhibition
Impulses per second
582
Time
Time
Figure 46–15
Typical pattern of the output signal from a reverberatory circuit
after a single input stimulus, showing the effects of facilitation and
inhibition.
The duration of the total signal before cessation can
also be controlled by signals from other parts of the
brain that inhibit or facilitate the circuit. Almost these
exact patterns of output signals are recorded from the
motor nerves exciting a muscle involved in a flexor
reflex after pain stimulation of the foot (as shown later
in Figure 46–18).
Continuous Signal Output from Some
Neuronal Circuits
Some neuronal circuits emit output signals continuously, even without excitatory input signals. At least
two mechanisms can cause this effect: (1) continuous
intrinsic neuronal discharge and (2) continuous reverberatory signals.
Continuous Discharge Caused by Intrinsic Neuronal Excitability. Neurons, like other excitable tissues, discharge
repetitively if their level of excitatory membrane
potential rises above a certain threshold level. The
membrane potentials of many neurons even normally
are high enough to cause them to emit impulses continually. This occurs especially in many of the neurons
of the cerebellum, as well as in most of the interneurons of the spinal cord. The rates at which these cells
emit impulses can be increased by excitatory signals or
decreased by inhibitory signals; inhibitory signals often
can decrease the rate of firing to zero.
Continuous Signals Emitted from Reverberating Circuits as
a Means for Transmitting Information. A reverberating
circuit that does not fatigue enough to stop reverberation is a source of continuous impulses. And excitatory impulses entering the reverberating pool can
increase the output signal, whereas inhibition can
decrease or even extinguish the signal.
Figure 46–16 shows a continuous output signal from
a pool of neurons. The pool may be emitting impulses
because of intrinsic neuronal excitability or as a result
Figure 46–16
Continuous output from either a reverberating circuit or a pool of
intrinsically discharging neurons. This figure also shows the effect
of excitatory or inhibitory input signals.
of reverberation. Note that an excitatory input signal
greatly increases the output signal, whereas an
inhibitory input signal greatly decreases the output.
Those students who are familiar with radio transmitters will recognize this to be a carrier wave type of
information transmission. That is, the excitatory and
inhibitory control signals are not the cause of the
output signal, but they do control its changing level of
intensity. Note that this carrier wave system allows a
decrease in signal intensity as well as an increase,
whereas up to this point, the types of information
transmission we have discussed have been mainly positive information rather than negative information.
This type of information transmission is used by the
autonomic nervous system to control such functions as
vascular tone, gut tone, degree of constriction of the
iris in the eye, and heart rate. That is, the nerve excitatory signal to each of these can be either increased
or decreased by accessory input signals into the reverberating neuronal pathway.
Rhythmical Signal Output
Many neuronal circuits emit rhythmical output
signals—for instance, a rhythmical respiratory signal
originates in the respiratory centers of the medulla and
pons. This respiratory rhythmical signal continues
throughout life. Other rhythmical signals, such as those
that cause scratching movements by the hind leg of a
dog or the walking movements of any animal, require
input stimuli into the respective circuits to initiate the
rhythmical signals.
All or almost all rhythmical signals that have been
studied experimentally have been found to result from
reverberating circuits or a succession of sequential
reverberating circuits that feed excitatory or inhibitory
signals in a circular pathway from one neuronal pool
to the next.
583
Sensory Receptors, Neuronal Circuits for Processing Information
Phrenic nerve output
Flexor muscle contraction force (g)
Chapter 46
Flexor reflexes–decremental responses
Stimulus
50
40
30
20
10
0
0
Increasing carotid
body stimulation
15
30
Seconds
45
60
Figure 46–18
Figure 46–17
The rhythmical output of summated nerve impulses from the
respiratory center, showing that progressively increasing stimulation of the carotid body increases both the intensity and the frequency of the phrenic nerve signal to the diaphragm to increase
respiration.
Excitatory or inhibitory signals can also increase
or decrease the amplitude of the rhythmical signal
output. Figure 46–17, for instance, shows changes in
the respiratory signal output in the phrenic nerve.
When the carotid body is stimulated by arterial oxygen
deficiency, both the frequency and the amplitude of
the respiratory rhythmical output signal increase
progressively.
Instability and Stability
of Neuronal Circuits
Almost every part of the brain connects either directly
or indirectly with every other part, and this creates a
serious problem. If the first part excites the second, the
second the third, the third the fourth, and so on until
finally the signal re-excites the first part, it is clear that
an excitatory signal entering any part of the brain
would set off a continuous cycle of re-excitation of all
parts. If this should occur, the brain would be inundated by a mass of uncontrolled reverberating
signals—signals that would be transmitting no information but, nevertheless, would be consuming the circuits of the brain so that none of the informational
signals could be transmitted. Such an effect occurs in
widespread areas of the brain during epileptic seizures.
How does the central nervous system prevent this
from happening all the time? The answer lies mainly
in two basic mechanisms that function throughout the
central nervous system: (1) inhibitory circuits and (2)
fatigue of synapses.
Successive flexor reflexes showing fatigue of conduction through
the reflex pathway.
Inhibitory Circuits as a Mechanism for
Stabilizing Nervous System Function
Two types of inhibitory circuits in widespread areas
of the brain help prevent excessive spread of signals:
(1) inhibitory feedback circuits that return from the
termini of pathways back to the initial excitatory
neurons of the same pathways—these circuits occur in
virtually all sensory nervous pathways and inhibit
either the input neurons or the intermediate neurons
in the sensory pathway when the termini become
overly excited; and (2) some neuronal pools that exert
gross inhibitory control over widespread areas of the
brain—for instance, many of the basal ganglia exert
inhibitory influences throughout the muscle control
system.
Synaptic Fatigue as a Means for
Stabilizing the Nervous System
Synaptic fatigue means simply that synaptic transmission becomes progressively weaker the more prolonged and more intense the period of excitation.
Figure 46–18 shows three successive records of a flexor
reflex elicited in an animal caused by inflicting pain in
the footpad of the paw. Note in each record that the
strength of contraction progressively “decrements”—
that is, its strength diminishes; much of this effect is
caused by fatigue of synapses in the flexor reflex
circuit. Furthermore, the shorter the interval between
successive flexor reflexes, the less the intensity of the
subsequent reflex response.
Automatic Short-Term Adjustment of Pathway Sensitivity by the
Fatigue Mechanism. Now let us apply this phenomenon
of fatigue to other pathways in the brain. Those that
584
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
are overused usually become fatigued, so that their
sensitivities decrease. Conversely, those that are
underused become rested, and their sensitivities
increase. Thus, fatigue and recovery from fatigue constitute an important short-term means of moderating
the sensitivities of the different nervous system circuits. These help to keep the circuits operating in a
range of sensitivity that allows effective function.
Long-Term Changes in Synaptic Sensitivity Caused by Automatic
Downregulation or Upregulation of Synaptic Receptors. The
long-term sensitivities of synapses can be changed
tremendously by upregulating the number of receptor
proteins at the synaptic sites when there is underactivity, and downregulating the receptors when there is
overactivity. The mechanism for this is the following:
Receptor proteins are being formed constantly by the
endoplasmic reticular–Golgi apparatus system and are
constantly being inserted into the receptor neuron
synaptic membrane. However, when the synapses are
overused so that excesses of transmitter substance
combine with the receptor proteins, many of these
receptors are inactivated and removed from the synaptic membrane.
It is indeed fortunate that upregulation and downregulation of receptors, as well as other control mechanisms for adjusting synaptic sensitivity, continually
adjust the sensitivity in each circuit to almost the exact
level required for proper function. Think for a moment
how serious it would be if the sensitivities of only a few
of these circuits were abnormally high; one might then
expect almost continual muscle cramps, seizures, psychotic disturbances, hallucinations, mental tension,
or other nervous disorders. But fortunately, the automatic controls normally readjust the sensitivities of the
circuits back to controllable ranges of reactivity any
time the circuits begin to be too active or too
depressed.
References
Buzsaki G: Large-scale recording of neuronal ensembles.
Nat Neurosci 7:446, 2004.
Baev KV: Biological Neural Networks. Boston: Birkhauser,
1998.
Basar E: Brain Function and Oscillations. Berlin: Springer,
1998.
Fain GL, Matthews HR, Cornwall MC, Koutalos Y: Adaptation in vertebrate photoreceptors. Physiol Rev 81:117,
2001.
Gandevia SC: Spinal and supraspinal factors in human
muscle fatigue. Physiol Rev 81:1725, 2001.
Gebhart GF: Descending modulation of pain. Neurosci
Biobehav Rev 27:729, 2004.
Hamill OP, Martinac B: Molecular basis of mechanotransduction in living cells. Physiol Rev 81:685, 2001.
Ivry RB, Robertson LC: The Two Sides of Perception. Cambridge, MA: MIT Press, 1998.
Kandel ER, Schwartz JH, Jessell TM: Principles of Neural
Science, 4th ed. New York: McGraw-Hill, 2000.
Krupa B, Liu G: Does the fusion pore contribute to synaptic plasticity? Trends Neurosci 27:62, 2004.
McLachlan EM: Transmission of signals through sympathetic ganglia—modulation, integration or simply distribution? Acta Physiol Scand 177:227, 2003.
Mombaerts P: Genes and ligands for odorant, vomeronasal
and taste receptors. Nat Rev Neurosci 5:263, 2004.
Palmer MJ, von Gersdorff H: Phasic transmitter release from
tonic neurons. Neuron 35:600, 2002.
Pearson KG: Neural adaptation in the generation of rhythmic behavior. Annu Rev Physiol 62:723, 2000.
Renteria RC, Johnson J, Copenhagen DR: Need rods? Get
glycine receptors and taurine. Neuron 41:839, 2004.
Richerson GB, Wu Y: Dynamic equilibrium of neurotransmitter transporters: not just for reuptake anymore. J
Neurophysiol 90:1363, 2003.
Schwartz EA: Transport-mediated synapses in the retina.
Physiol Rev 82:875, 2002.
Shen K: Molecular mechanisms of target specificity during
synapse formation. Curr Opin Neurobiol 14:83, 2004.
Williams JT, Christie MJ, Manzoni O: Cellular and synaptic
adaptations mediating opioid dependence. Physiol Rev
81:299, 2001.
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4
7
Somatic Sensations: I. General
Organization, the Tactile and
Position Senses
The somatic senses are the nervous mechanisms that
collect sensory information from all over the body.
These senses are in contradistinction to the special
senses, which mean specifically vision, hearing,
smell, taste, and equilibrium.
CLASSIFICATION OF SOMATIC SENSES
The somatic senses can be classified into three physiologic types: (1) the
mechanoreceptive somatic senses, which include both tactile and position sensations that are stimulated by mechanical displacement of some tissue of the body;
(2) the thermoreceptive senses, which detect heat and cold; and (3) the pain
sense, which is activated by any factor that damages the tissues.
This chapter deals with the mechanoreceptive tactile and position senses.
Chapter 48 discusses the thermoreceptive and pain senses. The tactile senses
include touch, pressure, vibration, and tickle senses, and the position senses
include static position and rate of movement senses.
Other Classifications of Somatic Sensations. Somatic sensations are also often
grouped together in other classes, as follows.
Exteroreceptive sensations are those from the surface of the body. Proprioceptive sensations are those having to do with the physical state of the body,
including position sensations, tendon and muscle sensations, pressure sensations
from the bottom of the feet, and even the sensation of equilibrium (which is
often considered a “special” sensation rather than a somatic sensation).
Visceral sensations are those from the viscera of the body; in using this term,
one usually refers specifically to sensations from the internal organs.
Deep sensations are those that come from deep tissues, such as from fasciae,
muscles, and bone. These include mainly “deep” pressure, pain, and vibration.
Detection and Transmission of
Tactile Sensations
Interrelations Among the Tactile Sensations of Touch, Pressure, and Vibration. Although
touch, pressure, and vibration are frequently classified as separate sensations,
they are all detected by the same types of receptors. There are three
principal differences among them: (1) touch sensation generally results from
stimulation of tactile receptors in the skin or in tissues immediately beneath the
skin; (2) pressure sensation generally results from deformation of deeper
tissues; and (3) vibration sensation results from rapidly repetitive sensory
signals, but some of the same types of receptors as those for touch and pressure
are used.
Tactile Receptors. There are at least six entirely different types of tactile receptors, but many more similar to these also exist. Some were shown in Figure 46–1
of the previous chapter; their special characteristics are the following.
585
586
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
First, some free nerve endings, which are found
everywhere in the skin and in many other tissues, can
detect touch and pressure. For instance, even light
contact with the cornea of the eye, which contains no
other type of nerve ending besides free nerve endings,
can nevertheless elicit touch and pressure sensations.
Second, a touch receptor with great sensitivity is the
Meissner’s corpuscle (illustrated in Figure 46–1), an
elongated encapsulated nerve ending of a large (type
Ab) myelinated sensory nerve fiber. Inside the capsulation are many branching terminal nerve filaments.
These corpuscles are present in the nonhairy parts of
the skin and are particularly abundant in the fingertips, lips, and other areas of the skin where one’s ability
to discern spatial locations of touch sensations is
highly developed. Meissner’s corpuscles adapt in a
fraction of a second after they are stimulated, which
means that they are particularly sensitive to movement
of objects over the surface of the skin as well as to lowfrequency vibration.
Third, the fingertips and other areas that contain
large numbers of Meissner’s corpuscles usually also
contain large numbers of expanded tip tactile receptors,
one type of which is Merkel’s discs, shown in Figure
47–1. The hairy parts of the skin also contain moderate numbers of expanded tip receptors, even though
they have almost no Meissner’s corpuscles. These
receptors differ from Meissner’s corpuscles in that
they transmit an initially strong but partially adapting
signal and then a continuing weaker signal that adapts
only slowly. Therefore, they are responsible for giving
steady-state signals that allow one to determine continuous touch of objects against the skin.
Merkel’s discs are often grouped together in a
receptor organ called the Iggo dome receptor, which
projects upward against the underside of the epithelium of the skin, as also shown in Figure 47–1. This
causes the epithelium at this point to protrude
outward, thus creating a dome and constituting an
extremely sensitive receptor. Also note that the entire
group of Merkel’s discs is innervated by a single large
myelinated nerve fiber (type Ab). These receptors,
along with the Meissner’s corpuscles discussed earlier,
play extremely important roles in localizing touch
sensations to specific surface areas of the body and in
determining the texture of what is felt.
Fourth, slight movement of any hair on the body
stimulates a nerve fiber entwining its base. Thus, each
hair and its basal nerve fiber, called the hair end-organ,
are also a touch receptor. This receptor adapts readily
and, like Meissner’s corpuscles, detects mainly (a)
movement of objects on the surface of the body or (b)
initial contact with the body.
Fifth, located in the deeper layers of the skin and
also in still deeper internal tissues are many Ruffini’s
end-organs, which are multibranched, encapsulated
endings, as shown in Figure 46–1. These endings adapt
very slowly and, therefore, are important for signaling
continuous states of deformation of the tissues, such as
heavy prolonged touch and pressure signals. They are
also found in joint capsules and help to signal the
degree of joint rotation.
Sixth, pacinian corpuscles, which were discussed in
detail in Chapter 46, lie both immediately beneath the
skin and deep in the fascial tissues of the body. They
are stimulated only by rapid local compression of the
tissues because they adapt in a few hundredths of a
second. Therefore, they are particularly important for
detecting tissue vibration or other rapid changes in the
mechanical state of the tissues.
Transmission of Tactile Signals in Peripheral Nerve Fibers.
Almost all specialized sensory receptors, such as
Meissner’s corpuscles, Iggo dome receptors, hair
E
FF
C
Figure 47–1
CF
A
AA
10 mm
Iggo dome receptor. Note the
multiple numbers of Merkel’s
discs connecting to a single large
myelinated fiber and abutting
tightly the undersurface of the
epithelium. (From Iggo A, Muir
AR: The structure and function of
a slowly adapting touch corpuscle in hairy skin. J Physiol 200:
763, 1969.)
Chapter 47
Somatic Sensations: I. General Organization, the Tactile and Position Senses
receptors, pacinian corpuscles, and Ruffini’s endings,
transmit their signals in type Ab nerve fibers that have
transmission velocities ranging from 30 to 70 m/sec.
Conversely, free nerve ending tactile receptors transmit signals mainly by way of the small type Ad myelinated fibers that conduct at velocities of only 5 to
30 m/sec.
Some tactile free nerve endings transmit by way of
type C unmyelinated fibers at velocities from a fraction of a meter up to 2 m/sec; these send signals into
the spinal cord and lower brain stem, probably subserving mainly the sensation of tickle.
Thus, the more critical types of sensory signals—
those that help to determine precise localization on
the skin, minute gradations of intensity, or rapid
changes in sensory signal intensity—are all transmitted in more rapidly conducting types of sensory nerve
fibers. Conversely, the cruder types of signals, such as
crude pressure, poorly localized touch, and especially
tickle, are transmitted by way of much slower, very
small nerve fibers that require much less space in the
nerve bundle than the fast fibers.
Detection of Vibration
All tactile receptors are involved in detection of vibration, although different receptors detect different frequencies of vibration. Pacinian corpuscles can detect
signal vibrations from 30 to 800 cycles per second
because they respond extremely rapidly to minute and
rapid deformations of the tissues, and they also transmit their signals over type Ab nerve fibers, which can
transmit as many as 1000 impulses per second. Lowfrequency vibrations from 2 up to 80 cycles per second,
in contrast, stimulate other tactile receptors, especially
Meissner’s corpuscles, which are less rapidly adapting
than pacinian corpuscles.
TICKLE AND ITCH
Neurophysiologic studies have demonstrated the existence of very sensitive, rapidly adapting mechanoreceptive free nerve endings that elicit only the tickle
and itch sensations. Furthermore, these endings are
found almost exclusively in superficial layers of the
skin, which is also the only tissue from which the tickle
and itch sensations usually can be elicited. These sensations are transmitted by very small type C, unmyelinated fibers similar to those that transmit the aching,
slow type of pain.
The purpose of the itch sensation is presumably to
call attention to mild surface stimuli such as a flea
crawling on the skin or a fly about to bite, and the
elicited signals then activate the scratch reflex or other
maneuvers that rid the host of the irritant. Itch can
be relieved by scratching if this removes the irritant
or if the scratch is strong enough to elicit pain. The
pain signals are believed to suppress the itch signals
in the cord by lateral inhibition, as described in
Chapter 48.
587
Sensory Pathways for
Transmitting Somatic
Signals into the Central
Nervous System
Almost all sensory information from the somatic segments of the body enters the spinal cord through the
dorsal roots of the spinal nerves. However, from
the entry point into the cord and then to the brain, the
sensory signals are carried through one of two alternative sensory pathways: (1) the dorsal column–medial
lemniscal system or (2) the anterolateral system. These
two systems come back together partially at the level
of the thalamus.
The dorsal column–medial lemniscal system, as its
name implies, carries signals upward to the medulla of
the brain mainly in the dorsal columns of the cord.
Then, after the signals synapse and cross to the opposite side in the medulla, they continue upward through
the brain stem to the thalamus by way of the medial
lemniscus.
Conversely, signals in the anterolateral system,
immediately after entering the spinal cord from the
dorsal spinal nerve roots, synapse in the dorsal
horns of the spinal gray matter, then cross to the opposite side of the cord and ascend through the anterior
and lateral white columns of the cord. They terminate
at all levels of the lower brain stem and in the
thalamus.
The dorsal column–medial lemniscal system is composed of large, myelinated nerve fibers that transmit
signals to the brain at velocities of 30 to 110 m/sec,
whereas the anterolateral system is composed of
smaller myelinated fibers that transmit signals at
velocities ranging from a few meters per second up to
40 m/sec.
Another difference between the two systems is
that the dorsal column–medial lemniscal system has a
high degree of spatial orientation of the nerve fibers
with respect to their origin, while the anterolateral
system has much less spatial orientation. These
differences immediately characterize the types of
sensory information that can be transmitted by the
two systems. That is, sensory information that must
be transmitted rapidly and with temporal and
spatial fidelity is transmitted mainly in the dorsal
column–medial lemniscal system; that which does not
need to be transmitted rapidly or with great spatial
fidelity is transmitted mainly in the anterolateral
system.
The anterolateral system has a special capability
that the dorsal system does not have: the ability to
transmit a broad spectrum of sensory modalities—
pain, warmth, cold, and crude tactile sensations;
most of these are discussed in detail in Chapter 48.
The dorsal system is limited to discrete types of
mechanoreceptive sensations.
With this differentiation in mind, we can now
list the types of sensations transmitted in the two
systems.
588
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
Dorsal Column–Medial
Lemniscal System
1. Touch sensations requiring a high degree of
localization of the stimulus
2. Touch sensations requiring transmission of fine
gradations of intensity
3. Phasic sensations, such as vibratory sensations
4. Sensations that signal movement against the skin
5. Position sensations from the joints
6. Pressure sensations having to do with fine degrees
of judgment of pressure intensity
Anterolateral System
1. Pain
2. Thermal sensations, including both warmth and
cold sensations
3. Crude touch and pressure sensations capable only
of crude localizing ability on the surface of the
body
4. Tickle and itch sensations
5. Sexual sensations
then upward in the dorsal column, proceeding by way
of the dorsal column pathway all the way to the brain.
The lateral branch enters the dorsal horn of the cord
gray matter, then divides many times to provide terminals that synapse with local neurons in the intermediate
and anterior portions of the cord gray matter. These
local neurons in turn serve three functions: (1) A major
share of them give off fibers that enter the dorsal
columns of the cord and then travel upward to the brain.
(2) Many of the fibers are very short and terminate
locally in the spinal cord gray matter to elicit local spinal
cord reflexes, which are discussed in Chapter 54. (3)
Others give rise to the spinocerebellar tracts, which we
will discuss in Chapter 56 in relation to the function of
the cerebellum.
The Dorsal Column–Medial Lemniscal Pathway. Note in Figure
47–3 that nerve fibers entering the dorsal columns
pass uninterrupted up to the dorsal medulla, where
they synapse in the dorsal column nuclei (the cuneate
and gracile nuclei). From there, second-order neurons
Cortex
Transmission in the
Dorsal Column–Medial
Lemniscal System
Anatomy of the Dorsal
Column–Medial Lemniscal System
On entering the spinal cord through the spinal nerve
dorsal roots, the large myelinated fibers from the specialized mechanoreceptors divide almost immediately
to form a medial branch and a lateral branch, shown by
the right-hand fiber entering through the spinal root in
Figure 47–2. The medial branch turns medially first and
Internal
capsule
Ventrobasal
complex
of thalamus
Spinal nerve
Lamina marginalis
Substantia gelatinosa
Tract of
Lissauer
Spinocervical
tract
Dorsal
spinocerebellar
tract
Medulla oblongata
Dorsal
column
Medial lemniscus
Dorsal column nuclei
I
II
III
Ascending branches of
dorsal root fibers
IV
V
VI
Spinal cord
VII
Ventral
spinocerebellar
tract
Spinocervical tract
IX VIII
Dorsal root and spinal
ganglion
Anterolateral
spinothalamic
pathway
Figure 47–3
Figure 47–2
Cross section of the spinal cord, showing the anatomy of the cord
gray matter and of ascending sensory tracts in the white columns
of the spinal cord.
The dorsal column–medial lemniscal pathway for transmitting critical types of tactile signals. (Modified from Ranson SW, Clark SL:
Anatomy of the Nervous System. Philadelphia: WB Saunders Co,
1959.)
Chapter 47
589
Somatic Sensations: I. General Organization, the Tactile and Position Senses
decussate immediately to the opposite side of the brain
stem and continue upward through the medial lemnisci
to the thalamus. In this pathway through the brain stem,
each medial lemniscus is joined by additional fibers
from the sensory nuclei of the trigeminal nerve; these
fibers subserve the same sensory functions for the head
that the dorsal column fibers subserve for the body.
In the thalamus, the medial lemniscal fibers terminate
in the thalamic sensory relay area, called the ventrobasal
complex. From the ventrobasal complex, third-order
nerve fibers project, as shown in Figure 47–4, mainly to
the postcentral gyrus of the cerebral cortex, which is
called somatic sensory area I (as shown in Figure 47–6,
these fibers also project to a smaller area in the lateral
parietal cortex called somatic sensory area II).
Spatial Orientation of the Nerve Fibers in the
Dorsal Column-Medial Lemniscal System
One of the distinguishing features of the dorsal
column–medial lemniscal system is a distinct spatial
orientation of nerve fibers from the individual parts of
the body that is maintained throughout. For instance,
in the dorsal columns of the spinal cord, the fibers from
the lower parts of the body lie toward the center of the
cord, whereas those that enter the cord at progressively higher segmental levels form successive layers
laterally.
In the thalamus, distinct spatial orientation is still
maintained, with the tail end of the body represented
by the most lateral portions of the ventrobasal
complex and the head and face represented by the
POSTCENTRAL GYRUS
Lower extremity
Trunk
medial areas of the complex. Because of the crossing
of the medial lemnisci in the medulla, the left side of
the body is represented in the right side of the thalamus, and the right side of the body in the left side of
the thalamus.
Somatosensory Cortex
Before discussing the role of the cerebral cortex in
somatic sensation, we need to give an orientation to
the various areas of the cortex. Figure 47–5 is a map
of the human cerebral cortex, showing that it is divided
into about 50 distinct areas called Brodmann’s areas
based on histological structural differences. This map
is important because virtually all neurophysiologists
and neurologists use it to refer by number to many of
the different functional areas of the human cortex.
Note in the figure the large central fissure (also
called central sulcus) that extends horizontally across
the brain. In general, sensory signals from all modalities of sensation terminate in the cerebral cortex
immediately posterior to the central fissure. And,
generally, the anterior half of the parietal lobe is
concerned almost entirely with reception and interpretation of somatosensory signals. But the posterior
half of the parietal lobe provides still higher levels of
interpretation.
Visual signals terminate in the occipital lobe, and
auditory signals in the temporal lobe.
Conversely, that portion of the cerebral cortex anterior to the central fissure and constituting the posterior half of the frontal lobe is called the motor cortex
and is devoted almost entirely to control of muscle
contractions and body movements. A major share of
this motor control is in response to somatosensory
Upper
extremity
Central fissure
3 2
4
Face
7a
8
5
6
9
7a
1
Ventrobasal complex of thalamus
40
46
39
10
45
MESENCEPHALON
Spinothalamic tract
Medial lemniscus
47
11
44
43
41
19
42
22
37
38
21
18
17
Lateral fissure
20
Figure 47–4
Figure 47–5
Projection of the dorsal column–medial lemniscal system through
the thalamus to the somatosensory cortex. (Modified from Brodal
A: Neurological Anatomy in Relation to Clinical Medicine. New
York: Oxford University Press, 1969, by permission of Oxford
University Press.)
Structurally distinct areas, called Brodmann’s areas, of the human
cerebral cortex. Note specifically areas 1, 2, and 3, which constitute primary somatosensory area I, and areas 5 and 7, which constitute the somatosensory association area.
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
Trunk
Neck
Head
Shoulder
Arm
Elbow rm
a
Fore
st
W ri d g e r
n in er
Ha e f ng
ttl f i
Li ing
R
Somatosensory
area I
Thigh
Somatosensory
Thorax
area II
Neck
Shoulder
Hand
Fingers
Tongue
Abdomen Leg
Arm
Face
M
In id
Th dex dle
Ey umb fin fing
e
ge e
Nos
r r
e
Face
Upper lip
Hip
Leg
590
ot
Fo
s
Toe
l
a
t
i s
Gen
Lips
Lower lip
Teeth, gums, and jaw
Tongue
Figure 47–6
Pharynx
Intra-abdominal
Two somatosensory cortical areas, somatosensory areas I and II.
signals received from the sensory portions of the
cortex, which keep the motor cortex informed at each
instant about the positions and motions of the different body parts.
Figure 47–7
Representation of the different areas of the body in somatosensory area I of the cortex. (From Penfield W, Rasmussen T: Cerebral Cortex of Man: A Clinical Study of Localization of Function.
New York: Hafner, 1968.)
Somatosensory Areas I and II. Figure 47–6 shows two sep-
arate sensory areas in the anterior parietal lobe called
somatosensory area I and somatosensory area II. The
reason for this division into two areas is that a distinct
and separate spatial orientation of the different
parts of the body is found in each of these two
areas. However, somatosensory area I is so much
more extensive and so much more important than
somatosensory area II that in popular usage, the term
“somatosensory cortex” almost always means area I.
Somatosensory area I has a high degree of localization of the different parts of the body, as shown by the
names of virtually all parts of the body in Figure 47–6.
By contrast, localization is poor in somatosensory area
II, although roughly, the face is represented anteriorly,
the arms centrally, and the legs posteriorly.
Little is known about the function of somatosensory
area II. It is known that signals enter this area from
the brain stem, transmitted upward from both sides of
the body. In addition, many signals come secondarily
from somatosensory area I as well as from other
sensory areas of the brain, even from the visual and
auditory areas. Projections from somatosensory area I
are required for function of somatosensory area II.
However, removal of parts of somatosensory area II
has no apparent effect on the response of neurons in
somatosensory area I. Thus, much of what we know
about somatic sensation appears to be explained by
the functions of somatosensory area I.
Spatial Orientation of Signals from Different Parts of the Body
in Somatosensory Area I. Somatosensory area I lies
immediately behind the central fissure, located in the
postcentral gyrus of the human cerebral cortex (in
Brodmann’s areas 3, 1, and 2).
Figure 47–7 shows a cross section through the brain
at the level of the postcentral gyrus, demonstrating
representations of the different parts of the body in
separate regions of somatosensory area I. Note,
however, that each lateral side of the cortex receives
sensory information almost exclusively from the opposite side of the body.
Some areas of the body are represented by large
areas in the somatic cortex—the lips the greatest of all,
followed by the face and thumb—whereas the trunk
and lower part of the body are represented by relatively small areas. The sizes of these areas are directly
proportional to the number of specialized sensory
receptors in each respective peripheral area of the
body. For instance, a great number of specialized nerve
endings are found in the lips and thumb, whereas only
a few are present in the skin of the body trunk.
Note also that the head is represented in the most
lateral portion of somatosensory area I, and the lower
part of the body is represented medially.
Layers of the Somatosensory Cortex and
Their Function
The cerebral cortex contains six layers of neurons,
beginning with layer I next to the brain surface and
extending progressively deeper to layer VI, shown in
Figure 47–8. As would be expected, the neurons in
each layer perform functions different from those
in other layers. Some of these functions are:
Chapter 47
Somatic Sensations: I. General Organization, the Tactile and Position Senses
I
II
III
IV
V
VIa
VIb
Figure 47–8
Structure of the cerebral cortex, showing I, molecular layer; II,
external granular layer; III, layer of small pyramidal cells; IV, internal granular layer; V, large pyramidal cell layer; and VI, layer of
fusiform or polymorphic cells. (From Ranson SW, Clark SL [after
Brodmann]: Anatomy of the Nervous System. Philadelphia: WB
Saunders, 1959.)
1. The incoming sensory signal excites neuronal
layer IV first; then the signal spreads toward the
surface of the cortex and also toward deeper
layers.
2. Layers I and II receive diffuse, nonspecific input
signals from lower brain centers that facilitate
specific regions of the cortex; this system is
described in Chapter 57. This input mainly
controls the overall level of excitability of the
respective regions stimulated.
3. The neurons in layers II and III send axons to
related portions of the cerebral cortex on the
opposite side of the brain through the corpus
callosum.
4. The neurons in layers V and VI send axons to the
deeper parts of the nervous system. Those in layer
V are generally larger and project to more distant
areas, such as to the basal ganglia, brain stem,
and spinal cord where they control signal
transmission. From layer VI, especially large
numbers of axons extend to the thalamus,
providing signals from the cerebral cortex that
interact with and help to control the excitatory
levels of incoming sensory signals entering the
thalamus.
591
The Sensory Cortex Is Organized in Vertical
Columns of Neurons; Each Column Detects a
Different Sensory Spot on the Body with a
Specific Sensory Modality
Functionally, the neurons of the somatosensory cortex
are arranged in vertical columns extending all the way
through the six layers of the cortex, each column
having a diameter of 0.3 to 0.5 millimeter and containing perhaps 10,000 neuronal cell bodies. Each of
these columns serves a single specific sensory modality, some columns responding to stretch receptors
around joints, some to stimulation of tactile hairs,
others to discrete localized pressure points on the skin,
and so forth. At layer IV, where the input sensory
signals first enter the cortex, the columns of neurons
function almost entirely separately from one another.
At other levels of the columns, interactions occur that
initiate analysis of the meanings of the sensory signals.
In the most anterior 5 to 10 millimeters of the postcentral gyrus, located deep in the central fissure in
Brodmann’s area 3a, an especially large share of the
vertical columns respond to muscle, tendon, and joint
stretch receptors. Many of the signals from these
sensory columns then spread anteriorly, directly to
the motor cortex located immediately forward of the
central fissure. These signals play a major role in
controlling the effluent motor signals that activate
sequences of muscle contraction.
As one moves posteriorly in somatosensory area I,
more and more of the vertical columns respond to
slowly adapting cutaneous receptors, and then still
farther posteriorly, greater numbers of the columns are
sensitive to deep pressure.
In the most posterior portion of somatosensory area
I, about 6 per cent of the vertical columns respond only
when a stimulus moves across the skin in a particular
direction. Thus, this is a still higher order of interpretation of sensory signals; the process becomes even
more complex as the signals spread farther backward
from somatosensory area I into the parietal cortex, an
area called the somatosensory association area, as we
discuss subsequently.
Functions of Somatosensory Area I
Widespread bilateral excision of somatosensory area I
causes loss of the following types of sensory judgment:
1. The person is unable to localize discretely the
different sensations in the different parts of the
body. However, he or she can localize these
sensations crudely, such as to a particular hand, to
a major level of the body trunk, or to one of the
legs. Thus, it is clear that the brain stem, thalamus,
or parts of the cerebral cortex not normally
considered to be concerned with somatic
sensations can perform some degree of
localization.
2. The person is unable to judge critical degrees of
pressure against the body.
3. The person is unable to judge the weights of
objects.
4. The person is unable to judge shapes or forms of
objects. This is called astereognosis.
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
5. The person is unable to judge texture of materials
because this type of judgment depends on highly
critical sensations caused by movement of the
fingers over the surface to be judged.
Note that in the list nothing has been said about loss
of pain and temperature sense. In specific absence
of only somatosensory area I, appreciation of these
sensory modalities is still preserved both in quality and
intensity. But the sensations are poorly localized, indicating that pain and temperature localization depend
greatly on the topographical map of the body in
somatosensory area I to localize the source.
Strong stimulus
Discharges per second
592
Moderate
stimulus
Weak
stimulus
Somatosensory Association Areas
Brodmann’s areas 5 and 7 of the cerebral cortex,
located in the parietal cortex behind somatosensory
area I (see Figure 47–5), play important roles in deciphering deeper meanings of the sensory information
in the somatosensory areas. Therefore, these areas are
called somatosensory association areas.
Electrical stimulation in a somatosensory association area can occasionally cause an awake person to
experience a complex body sensation, sometimes even
the “feeling” of an object such as a knife or a ball.
Therefore, it seems clear that the somatosensory association area combines information arriving from multiple points in the primary somatosensory area to
decipher its meaning. This also fits with the anatomical arrangement of the neuronal tracts that enter the
somatosensory association area because it receives
signals from (1) somatosensory area I, (2) the ventrobasal nuclei of the thalamus, (3) other areas of the
thalamus, (4) the visual cortex, and (5) the auditory
cortex.
Effect of Removing the Somatosensory Association Area—
Amorphosynthesis. When the somatosensory association
area is removed on one side of the brain, the person
loses ability to recognize complex objects and complex
forms felt on the opposite side of the body. In addition, he or she loses most of the sense of form of his
or her own body or body parts on the opposite side.
In fact, the person is mainly oblivious to the opposite
side of the body—that is, forgets that it is there.
Therefore, he or she also often forgets to use the other
side for motor functions as well. Likewise, when
feeling objects, the person tends to recognize only
one side of the object and forgets that the other side
even exists. This complex sensory deficit is called
amorphosynthesis.
Overall Characteristics of Signal
Transmission and Analysis
in the Dorsal Column–Medial
Lemniscal System
Basic Neuronal Circuit in the Dorsal Column–Medial Lemniscal
System. The lower part of Figure 47–9 shows the basic
Cortex
Thalamus
Dorsal column nuclei
Single-point stimulus on skin
Figure 47–9
Transmission of a pinpoint stimulus signal to the cerebral cortex.
organization of the neuronal circuit of the spinal cord
dorsal column pathway, demonstrating that at each
synaptic stage, divergence occurs. The upper curves of
the figure show that the cortical neurons that discharge
to the greatest extent are those in a central part of the
cortical “field” for each respective receptor. Thus, a
weak stimulus causes only the centralmost neurons to
fire. A stronger stimulus causes still more neurons to
fire, but those in the center discharge at a considerably
more rapid rate than do those farther away from the
center.
Two-Point Discrimination. A method frequently used to
test tactile discrimination is to determine a person’s
so-called “two-point” discriminatory ability. In this
test, two needles are pressed lightly against the skin at
the same time, and the person determines whether two
points of stimulus are felt or one point. On the tips of
the fingers, a person can distinguish two separate
points even when the needles are as close together as
1 to 2 millimeters. However, on the person’s back,
the needles must usually be as far apart as 30 to 70
Somatic Sensations: I. General Organization, the Tactile and Position Senses
Discharges per second
Chapter 47
Cortex
Two adjacent points
strongly stimulated
Figure 47–10
Transmission of signals to the cortex from two adjacent pinpoint
stimuli. The blue curve represents the pattern of cortical stimulation without “surround” inhibition, and the two red curves represent the pattern when “surround” inhibition does occur.
millimeters before two separate points can be
detected. The reason for this difference is the different
numbers of specialized tactile receptors in the two
areas.
Figure 47–10 shows the mechanism by which the
dorsal column pathway (as well as all other sensory
pathways) transmits two-point discriminatory information. This figure shows two adjacent points on the
skin that are strongly stimulated as well as the areas
of the somatosensory cortex (greatly enlarged) that
are excited by signals from the two stimulated points.
The blue curve shows the spatial pattern of cortical
excitation when both skin points are stimulated simultaneously. Note that the resultant zone of excitation
has two separate peaks. These two peaks, separated by
a valley, allow the sensory cortex to detect the presence of two stimulatory points, rather than a single
point. The capability of the sensorium to distinguish
this presence of two points of stimulation is strongly
influenced by another mechanism, lateral inhibition, as
explained in the next section.
Effect of Lateral Inhibition (Also Called Surround Inhibition) to
Increase the Degree of Contrast in the Perceived Spatial
Pattern. As pointed out in Chapter 46, virtually every
sensory pathway, when excited, gives rise simultaneously to lateral inhibitory signals; these spread to the
sides of the excitatory signal and inhibit adjacent
neurons. For instance, consider an excited neuron
in a dorsal column nucleus. Aside from the central
593
excitatory signal, short lateral pathways transmit
inhibitory signals to the surrounding neurons. That is,
these signals pass through additional interneurons that
secrete an inhibitory transmitter.
The importance of lateral inhibition is that it blocks
lateral spread of the excitatory signals and, therefore,
increases the degree of contrast in the sensory pattern
perceived in the cerebral cortex.
In the case of the dorsal column system, lateral
inhibitory signals occur at each synaptic level—for
instance, in (1) the dorsal column nuclei of the
medulla, (2) the ventrobasal nuclei of the thalamus,
and (3) the cortex itself. At each of these levels, the
lateral inhibition helps to block lateral spread of the
excitatory signal. As a result, the peaks of excitation
stand out, and much of the surrounding diffuse stimulation is blocked. This effect is demonstrated by the
two red curves in Figure 47–10, showing complete
separation of the peaks when the intensity of lateral
inhibition is great.
Transmission of Rapidly Changing and Repetitive Sensations.
The dorsal column system also is of particular importance in apprising the sensorium of rapidly changing
peripheral conditions. Based on recorded action
potentials, this system can recognize changing stimuli
that occur in as little as 1/400 of a second.
Vibratory Sensation. Vibratory signals are rapidly
repetitive and can be detected as vibration up to 700
cycles per second. The higher-frequency vibratory
signals originate from the pacinian corpuscles in the
skin and deeper tissues, but lower-frequency signals
(below about 200 per second) can originate from
Meissner’s corpuscles as well. These signals are transmitted only in the dorsal column pathway. For this
reason, application of vibration (e.g., from a “tuning
fork”) to different peripheral parts of the body is an
important tool used by neurologists for testing functional integrity of the dorsal columns.
Interpretation of Sensory
Stimulus Intensity
The ultimate goal of most sensory stimulation is to
apprise the psyche of the state of the body and its surroundings. Therefore, it is important that we discuss
briefly some of the principles related to transmission of
sensory stimulus intensity to the higher levels of the
nervous system.
The first question that comes to mind is, how is it possible for the sensory system to transmit sensory experiences of tremendously varying intensities? For instance,
the auditory system can detect the weakest possible
whisper but can also discern the meanings of an explosive sound, even though the sound intensities of these
two experiences can vary more than 10 billion times; the
eyes can see visual images with light intensities that vary
as much as a half million times; and the skin can detect
pressure differences of 10,000 to 100,000 times.
As a partial explanation of these effects, Figure 46–4
in the previous chapter shows the relation of the receptor potential produced by the pacinian corpuscle to the
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
intensity of the sensory stimulus. At low stimulus intensity, slight changes in intensity increase the potential
markedly, whereas at high levels of stimulus intensity,
further increases in receptor potential are slight. Thus,
the pacinian corpuscle is capable of accurately measuring extremely minute changes in stimulus at lowintensity levels, but at high-intensity levels, the change
in stimulus must be much greater to cause the same
amount of change in receptor potential.
The transduction mechanism for detecting sound by
the cochlea of the ear demonstrates still another
method for separating gradations of stimulus intensity.
When sound stimulates a specific point on the basilar
membrane, weak sound stimulates only those hair cells
at the point of maximum sound vibration. But as the
sound intensity increases, many more hair cells in each
direction farther away from the maximum vibratory
point also become stimulated. Thus, signals are transmitted over progressively increasing numbers of nerve
fibers, which is another mechanism by which stimulus
intensity is transmitted to the central nervous system.
This mechanism, plus the direct effect of stimulus intensity on impulse rate in each nerve fiber, as well as several
other mechanisms, makes it possible for some sensory
systems to operate reasonably faithfully at stimulus
intensity levels changing as much as millions of times.
Importance of the Tremendous Intensity Range of Sensory
Reception. Were it not for the tremendous intensity
range of sensory reception that we can experience, the
various sensory systems would more often than not be
operating in the wrong range. This is demonstrated by
the attempts of most people, when taking photographs
with a camera, to adjust the light exposure without using
a light meter. Left to intuitive judgment of light intensity, a person almost always overexposes the film on
bright days and greatly underexposes the film at twilight. Yet, that person’s own eyes are capable of discriminating with great detail visual objects in bright
sunlight or at twilight; the camera cannot do this without
very special manipulation because of the narrow critical range of light intensity required for proper exposure
of film.
Judgment of Stimulus Intensity
Weber-Fechner Principle—Detection of “Ratio” of Stimulus
Strength. In the mid-1800s, Weber first and Fechner later
proposed the principle that gradations of stimulus
strength are discriminated approximately in proportion
to the logarithm of stimulus strength. That is, a person
already holding 30 grams weight in his or her hand can
barely detect an additional 1-gram increase in weight.
And, when already holding 300 grams, he or she can
barely detect a 10-gram increase in weight. Thus, in this
instance, the ratio of the change in stimulus strength
required for detection remains essentially constant,
about 1 to 30, which is what the logarithmic principle
means. To express this mathematically.
Fechner principle is still a good one to remember,
because it emphasizes that the greater the background
sensory intensity, the greater an additional change must
be for the psyche to detect the change.
Power Law. Another attempt by physiopsychologists to
find a good mathematical relation is the following
formula, known as the power law.
Interpreted signal strength = K • (Stimulus - k)y
In this formula, the exponent y and the constants K and
k are different for each type of sensation.
When this power law relation is plotted on a graph
using double logarithmic coordinates, as shown in
Figure 47–11, and when appropriate quantitative values
for the constants y, K, and k are found, a linear relation
can be attained between interpreted stimulus strength
and actual stimulus strength over a large range for
almost any type of sensory perception.
Position Senses
The position senses are frequently also called proprioceptive senses. They can be divided into two subtypes:
(1) static position sense, which means conscious perception of the orientation of the different parts of
the body with respect to one another, and (2) rate of
movement sense, also called kinesthesia or dynamic
proprioception.
Position Sensory Receptors. Knowledge of position, both
static and dynamic, depends on knowing the degrees
of angulation of all joints in all planes and their rates
of change. Therefore, multiple different types of receptors help to determine joint angulation and are used
500
Interpreted stimulus strength
(arbitrary units)
594
200
100
50
20
10
0
0
10
100
1000
10,000
Stimulus strength (arbitrary units)
Interpreted signal strength = Log (Stimulus)
+ Constant
More recently, it has become evident that the WeberFechner principle is quantitatively accurate only for
higher intensities of visual, auditory, and cutaneous
sensory experience and applies only poorly to most
other types of sensory experience. Yet the Weber-
Figure 47–11
Graphical demonstration of the “power law” relation between
actual stimulus strength and strength that the psyche interprets it
to be. Note that the power law does not hold at either very weak
or very strong stimulus strengths.
Chapter 47
Somatic Sensations: I. General Organization, the Tactile and Position Senses
together for position sense. Both skin tactile receptors
and deep receptors near the joints are used. In the
case of the fingers, where skin receptors are in great
abundance, as much as half of position recognition is
believed to be detected through the skin receptors.
Conversely, for most of the larger joints of the body,
deep receptors are more important.
For determining joint angulation in mid ranges of
motion, among the most important receptors are the
muscle spindles. They are also exceedingly important
in helping to control muscle movement, as we shall see
in Chapter 54. When the angle of a joint is changing,
some muscles are being stretched while others are
loosened, and the net stretch information from the
spindles is transmitted into the computational system
of the spinal cord and higher regions of the dorsal
column system for deciphering joint angulations.
At the extremes of joint angulation, stretch of the
ligaments and deep tissues around the joints is an additional important factor in determining position. Types
of sensory endings used for this are the pacinian corpuscles, Ruffini’s endings, and receptors similar to the
Golgi tendon receptors found in muscle tendons.
The pacinian corpuscles and muscle spindles are
especially adapted for detecting rapid rates of change.
It is likely that these are the receptors most responsible for detecting rate of movement.
Processing of Position Sense Information in the Dorsal
Column–Medial Lemniscal Pathway. Referring to Figure
47–12, one sees that thalamic neurons responding
to joint rotation are of two categories: (1) those
Impulses per second
100
80
60
#1
#4
#5
#2
40
#3
20
595
maximally stimulated when the joint is at full rotation
and (2) those maximally stimulated when the joint is
at minimal rotation. Thus, the signals from the individual joint receptors are used to tell the psyche how
much each joint is rotated.
Transmission of Less Critical
Sensory Signals in the
Anterolateral Pathway
The anterolateral pathway for transmitting sensory
signals up the spinal cord and into the brain, in contrast to the dorsal column pathway, transmits sensory
signals that do not require highly discrete localization
of the signal source and do not require discrimination
of fine gradations of intensity. These types of signals
include pain, heat, cold, crude tactile, tickle, itch, and
sexual sensations. In Chapter 48, pain and temperature
sensations will be discussed specifically.
Anatomy of the
Anterolateral Pathway
The spinal cord anterolateral fibers originate mainly in
dorsal horn laminae I, IV, V, and VI (see Figure 47–2).
These laminae are where many of the dorsal root
sensory nerve fibers terminate after entering the cord.
As shown in Figure 47–13, the anterolateral fibers
cross immediately in the anterior commissure of the
cord to the opposite anterior and lateral white columns,
where they turn upward toward the brain by way of the
anterior spinothalamic and lateral spinothalamic tracts.
The upper terminus of the two spinothalamic tracts is
mainly twofold: (1) throughout the reticular nuclei of the
brain stem and (2) in two different nuclear complexes
of the thalamus, the ventrobasal complex and the
intralaminar nuclei. In general, the tactile signals are
transmitted mainly into the ventrobasal complex, terminating in some of the same thalamic nuclei where the
dorsal column tactile signals terminate. From here, the
signals are transmitted to the somatosensory cortex
along with the signals from the dorsal columns.
Conversely, only a small fraction of the pain signals
project directly to the ventrobasal complex of the thalamus. Instead, most pain signals terminate in the reticular nuclei of the brain stem and from there are relayed
to the intralaminar nuclei of the thalamus where the
pain signals are further processed, as discussed in
greater detail in Chapter 48.
Characteristics of Transmission in the Anterolateral Pathway.
0
0
60
80
100 120 140
Degrees
160
180
Figure 47–12
Typical responses of five different thalamic neurons in the thalamic ventrobasal complex when the knee joint is moved through
its range of motion. (Data from Mountcastle VB, Poggie GF, Werner
G: The relation of thalamic cell response to peripheral stimuli
varied over an intensive continuum. J Neurophysiol 26:807, 1963.)
In general, the same principles apply to transmission
in the anterolateral pathway as in the dorsal
column–medial lemniscal system, except for the following differences: (1) the velocities of transmission
are only one third to one half those in the dorsal
column–medial lemniscal system, ranging between 8
and 40 m/sec; (2) the degree of spatial localization of
signals is poor; (3) the gradations of intensities are also
far less accurate, most of the sensations being recognized in 10 to 20 gradations of strength, rather than as
many as 100 gradations for the dorsal column system;
596
Unit IX
The Nervous System: A. General Principles and Sensory Physiology
Cortex
C2
C2
C3
C3
C5
C4
C4
C5
T3
T5
Internal
capsule
Ventrobasal
and intralaminar
nuclei of the
thalamus
T7
T2
T2
T4
T8
T10
T12
L3
L5
T11
T12
Medial lemniscus
L1
C6
Medulla oblongata
C7
L2
C8
T6
T7
T8
T9
T6
T9
T10
Mesencephalon
T2
T4
T5
T4
T5
T11
S2
L1
S4&5
L2
S3
L3
L3
Lateral
division of the
anterolateral
pathway
L4
L4
Spinal cord
Anterior
division of the
anterolateral
pathway
Dorsal root and spinal
ganglion
L5
S1
L5
S1
L5
L4
Figure 47–13
Anterior and lateral divisions of the anterolateral sensory pathway.
S2
Figure 47–14
Dermatomes. (Modified from Grinker RR, Sahs AL: Neurology, 6th
ed. Springfield, IL: Charles C Thomas, 1966. Courtesy of Charles
C Thomas, Publisher, Ltd., Springfield, Illinois.)
and (4) the ability to transmit rapidly changing or
rapidly repetitive signals is poor.
Thus, it is evident that the anterolateral system is a
cruder type of transmission system than the dorsal
column–medial lemniscal system. Even so, certain
modalities of sensation are transmitted only in this
system and not at all in the dorsal column–medial lemniscal system. They are pain, temperature, tickle, itch,
and sexual sensations, in addition to crude touch and
pressure.
Some Special Aspects of
Somatosensory Function
Function of the Thalamus
in Somatic Sensation
When the somatosensory cortex of a human being is
destroyed, that person loses most critical tactile sensibilities, but a slight degree of crude tactile sensibility
does return. Therefore, it must be assumed that the thalamus (as well as other lower centers) has a slight ability
to discriminate tactile sensation, even though the thalamus normally functions mainly to relay this type of
information to the cortex.
Conversely, loss of the somatosensory cortex has little
effect on one’s perception of pain sensation and only
a moderate effect on the perception of temperature.
Therefore, there is much reason to believe that the
lower brain stem, the thalamus, and other associated
basal regions of the brain play dominant roles in discrimination of these sensibilities. It is interesting that
these sensibilities appeared very early in the phylogenetic development of animals, whereas the critical
tactile sensibilities and the somatosensory cortex were
late developments.
Chapter 47
Somatic Sensations: I. General Organization, the Tactile and Position Senses
Cortical Control of Sensory
Sensitivity—“Corticofugal” Signals
In addition to somatosensory signals transmitted from
the periphery to the brain, corticofugal signals are transmitted in the backward direction from the cerebral
cortex to the lower sensory relay stations of the thalamus, medulla, and spinal cord; they control the intensity
of sensitivity of the sensory input.
Corticofugal signals are almost entirely inhibitory, so
that when sensory input intensity becomes too great, the
corticofugal signals automatically decrease transmission
in the relay nuclei. This does two things: First, it
decreases lateral spread of the sensory signals into adjacent neurons and, therefore, increases the degree of
sharpness in the signal pattern. Second, it keeps the
sensory system operating in a range of sensitivity that is
not so low that the signals are ineffectual nor so high
that the system is swamped beyond its capacity to differentiate sensory patterns. This principle of corticofugal sensory control is used by all sensory systems, not
only the somatic system, as explained in subsequent
chapters.
Segmental Fields of Sensation—
The Dermatomes
Each spinal nerve innervates a “segmental field” of the
skin called a dermatome. The different dermatomes are
shown in Figure 47–14. They are shown in the figure as
if there were distinct borders between the adjacent dermatomes, which is far from true because much overlap
exists from segment to segment.
The figure shows that the anal region of the body lies
in the dermatome of the most distal cord segment, dermatome S5. In the embryo, this is the tail region and the
most distal portion of the body. The legs originate
embryologically from the lumbar and upper sacral segments (L2 to S3), rather than from the distal sacral segments, which is evident from the dermatomal map. One
can use a dermatomal map as shown in Figure 47–14 to
determine the level in the spinal cord at which a cord
injury has occurred when the peripheral sensations are
disturbed by the injury.
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4
Somatic Sensations:
II. Pain, Headache, and
Thermal Sensations
Many, if not most, ailments of the body cause pain.
Furthermore, the ability to diagnose different diseases depends to a great extent on a physician’s
knowledge of the different qualities of pain. For
these reasons, the first part of this chapter is devoted
mainly to pain and to the physiologic bases of some
associated clinical phenomena.
Pain Is a Protective Mechanism. Pain occurs whenever any tissues are being
damaged, and it causes the individual to react to remove the pain stimulus. Even
such simple activities as sitting for a long time on the ischia can cause tissue
destruction because of lack of blood flow to the skin where it is compressed by
the weight of the body. When the skin becomes painful as a result of the
ischemia, the person normally shifts weight subconsciously. But a person who
has lost the pain sense, as after spinal cord injury, fails to feel the pain and, therefore, fails to shift. This soon results in total breakdown and desquamation of the
skin at the areas of pressure.
Types of Pain and Their Qualities—Fast Pain
and Slow Pain
Pain has been classified into two major types: fast pain and slow pain. Fast pain
is felt within about 0.1 second after a pain stimulus is applied, whereas slow pain
begins only after 1 second or more and then increases slowly over many seconds
and sometimes even minutes. During the course of this chapter, we shall see
that the conduction pathways for these two types of pain are different and that
each of them has specific qualities.
Fast pain is also described by many alternative names, such as sharp pain,
pricking pain, acute pain, and electric pain. This type of pain is felt when a needle
is stuck into the skin, when the skin is cut with a knife, or when the skin is acutely
burned. It is also felt when the skin is subjected to electric shock. Fast-sharp
pain is not felt in most deeper tissues of the body.
Slow pain also goes by many names, such as slow burning pain, aching pain,
throbbing pain, nauseous pain, and chronic pain. This type of pain is usually
associated with tissue destruction. It can lead to prolonged, unbearable suffering. It can occur both in the skin and in almost any deep tissue or organ.
Pain Receptors and Their Stimulation
Pain Receptors Are Free Nerve Endings. The pain receptors in the skin and other
tissues are all free nerve endings. They are widespread in the superficial layers
of the skin as well as in certain internal tissues, such as the periosteum, the arterial walls, the joint surfaces, and the falx and tentorium in the cranial vault. Most
other deep tissues are only sparsely supplied with pain endings; nevertheless,
any widespread tissue damage can summate to cause the slow-chronic-aching
type of pain in most of these areas.
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8
Chapter 48
599
Somatic Sensations: II. Pain, Headache, and Thermal Sensations
types of stimuli. They are classified as mechanical,
thermal, and chemical pain stimuli. In general, fast pain
is elicited by the mechanical and thermal types of
stimuli, whereas slow pain can be elicited by all three
types.
Some of the chemicals that excite the chemical type
of pain are bradykinin, serotonin, histamine, potassium
ions, acids, acetylcholine, and proteolytic enzymes. In
addition, prostaglandins and substance P enhance the
sensitivity of pain endings but do not directly excite
them. The chemical substances are especially important in stimulating the slow, suffering type of pain that
occurs after tissue injury.
Number of subjects
Three Types of Stimuli Excite Pain Receptors—Mechanical,
Thermal, and Chemical. Pain can be elicited by multiple
Nonadapting Nature of Pain Receptors. In contrast to most
other sensory receptors of the body, pain receptors
adapt very little and sometimes not at all. In fact,
under some conditions, excitation of pain fibers
becomes progressively greater, especially so for
slow-aching-nauseous pain, as the pain stimulus
continues. This increase in sensitivity of the pain receptors is called hyperalgesia. One can readily understand
the importance of this failure of pain receptors to
adapt, because it allows the pain to keep the person
apprised of a tissue-damaging stimulus as long as it
persists.
Rate of Tissue Damage as a Stimulus
for Pain
The average person begins to perceive pain when the
skin is heated above 45°C, as shown in Figure 48–1.
This is also the temperature at which the tissues begin
to be damaged by heat; indeed, the tissues are eventually destroyed if the temperature remains above this
level indefinitely. Therefore, it is immediately apparent
that pain resulting from heat is closely correlated
with the rate at which damage to the tissues is occurring and not with the total damage that has already
occurred.
The intensity of pain is also closely correlated with
the rate of tissue damage from causes other than heat,
such as bacterial infection, tissue ischemia, tissue contusion, and so forth.
Special Importance of Chemical Pain Stimuli During Tissue
Damage. Extracts from damaged tissue cause intense
pain when injected beneath the normal skin. Most of
the chemicals listed earlier that excite the chemical
pain receptors can be found in these extracts. One
chemical that seems to be more painful than others is
bradykinin. Many researchers have suggested that
bradykinin might be the agent most responsible for
causing pain following tissue damage. Also, the intensity of the pain felt correlates with the local increase
in potassium ion concentration or the increase in proteolytic enzymes that directly attack the nerve endings
and excite pain by making the nerve membranes more
permeable to ions.
43
44
45
46
Temperature (∞C)
47
Figure 48–1
Distribution curve obtained from a large number of persons
showing the minimal skin temperature that will cause pain. (Modified from Hardy DJ: Nature of pain. J Clin Epidemiol 4:22, 1956.)
Tissue Ischemia as a Cause of Pain. When blood flow to a
tissue is blocked, the tissue often becomes very painful
within a few minutes. The greater the rate of metabolism of the tissue, the more rapidly the pain appears.
For instance, if a blood pressure cuff is placed around
the upper arm and inflated until the arterial blood flow
ceases, exercise of the forearm muscles sometimes
can cause muscle pain within 15 to 20 seconds. In the
absence of muscle exercise, the pain may not appear
for 3 to 4 minutes even though the muscle blood flow
remains zero.
One of the suggested causes of pain during ischemia
is accumulation of large amounts of lactic acid in the
tissues, formed as a consequence of anaerobic metabolism (metabolism without oxygen). It is also probable that other chemical agents, such as bradykinin and
proteolytic enzymes, are formed in the tissues because
of cell damage and that these, in addition to lactic acid,
stimulate the pain nerve endings.
Muscle Spasm as a Cause of Pain. Muscle spasm is also
a common cause of pain, and it is the basis of many
clinical pain syndromes. This pain probably results
partially from the direct effect of muscle spasm in
stimulating mechanosensitive pain receptors, but it
might also result from the indirect effect of muscle
spasm to compress the blood vessels and cause
ischemia. Also, the spasm increases the rate of metabolism in the muscle tissue, thus making the relative
ischemia even greater, creating ideal conditions for the
release of chemical pain-inducing substances.
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Dual Pathways for
Transmission of Pain
Signals into the Central
Nervous System
Even though all pain receptors are free nerve endings,
these endings use two separate pathways for transmitting pain signals into the central nervous system. The
two pathways mainly correspond to the two types of
pain—a fast-sharp pain pathway and a slow-chronic
pain pathway.
Peripheral Pain Fibers—“Fast” and “Slow” Fibers. The fast-
sharp pain signals are elicited by either mechanical
or thermal pain stimuli; they are transmitted in the
peripheral nerves to the spinal cord by small type Ad
fibers at velocities between 6 and 30 m/sec. Conversely,
the slow-chronic type of pain is elicited mostly by
chemical types of pain stimuli but sometimes by
persisting mechanical or thermal stimuli. This slowchronic pain is transmitted to the spinal cord by type
C fibers at velocities between 0.5 and 2 m/sec.
Because of this double system of pain innervation,
a sudden painful stimulus often gives a “double” pain
sensation: a fast-sharp pain that is transmitted to the
brain by the Ad fiber pathway, followed a second or so
later by a slow pain that is transmitted by the C fiber
pathway. The sharp pain apprises the person rapidly of
a damaging influence and, therefore, plays an important role in making the person react immediately to
remove himself or herself from the stimulus. The slow
pain tends to become greater over time. This sensation
eventually produces the intolerable suffering of longcontinued pain and makes the person keep trying to
relieve the cause of the pain.
On entering the spinal cord from the dorsal spinal
roots, the pain fibers terminate on relay neurons in the
dorsal horns. Here again, there are two systems for
processing the pain signals on their way to the brain,
as shown in Figures 48–2 and 48–3.
Dual Pain Pathways in the
Cord and Brain Stem—The
Neospinothalamic Tract and the
Paleospinothalamic Tract
On entering the spinal cord, the pain signals take two
pathways to the brain, through (1) the neospinothalamic tract and (2) the paleospinothalamic tract.
Neospinothalamic Tract for Fast Pain. The fast type Ad
pain fibers transmit mainly mechanical and acute
thermal pain. They terminate mainly in lamina I
(lamina marginalis) of the dorsal horns, as shown in
Figure 48–2, and there excite second-order neurons of
the neospinothalamic tract. These give rise to long
fibers that cross immediately to the opposite side of
the cord through the anterior commissure and then
turn upward, passing to the brain in the anterolateral
columns.
C Ad
Fast-sharp
pain fibers
Spinal
nerve
Tract of
Lissauer
III
III
IV
V
VI
VII
IX VIII
Lamina
marginalis
Substantia
gelatinosa
Anterolateral
pathway
Slow-chronic
pain fibers
Figure 48–2
Transmission of both “fast-sharp” and “slow-chronic” pain signals
into and through the spinal cord on their way to the brain.
To: Somatosensory areas
Thalamus
Intralaminar
nuclei
Ventrobasal
complex and
posterior
nuclear
group
"Slow" Pain
Fibers
"Fast" Pain
Fibers
Reticular
formation
Pain tracts
Figure 48–3
Transmission of pain signals into the brain stem, thalamus, and
cerebral cortex by way of the fast pricking pain pathway and the
slow burning pain pathway.
Termination of the Neospinothalamic Tract in the Brain
Stem and Thalamus. A few fibers of the neospinothal-
amic tract terminate in the reticular areas of the brain
stem, but most pass all the way to the thalamus without
interruption, terminating in the ventrobasal complex
along with the dorsal column–medial lemniscal tract
for tactile sensations, as was discussed in Chapter 47.
A few fibers also terminate in the posterior nuclear
Chapter 48
Somatic Sensations: II. Pain, Headache, and Thermal Sensations
group of the thalamus. From these thalamic areas, the
signals are transmitted to other basal areas of the brain
as well as to the somatosensory cortex.
Capability of the Nervous System to Localize Fast Pain
in the Body. The fast-sharp type of pain can be local-
ized much more exactly in the different parts of the
body than can slow-chronic pain. However, when only
pain receptors are stimulated, without the simultaneous stimulation of tactile receptors, even fast pain may
be poorly localized, often only within 10 centimeters
or so of the stimulated area. Yet when tactile receptors
that excite the dorsal column–medial lemniscal system
are simultaneously stimulated, the localization can be
nearly exact.
Glutamate, the Probable Neurotransmitter of the Type
Ad Fast Pain Fibers. It is believed that glutamate is the
neurotransmitter substance secreted in the spinal cord
at the type Ad pain nerve fiber endings. This is one of
the most widely used excitatory transmitters in the
central nervous system, usually having a duration of
action lasting for only a few milliseconds.
Paleospinothalamic Pathway for Transmitting Slow-Chronic
Pain. The paleospinothalamic pathway is a much older
system and transmits pain mainly from the peripheral
slow-chronic type C pain fibers, although it does transmit some signals from type Ad fibers as well. In this
pathway, the peripheral fibers terminate in the spinal
cord almost entirely in laminae II and III of the dorsal
horns, which together are called the substantia gelatinosa, as shown by the lateral most dorsal root type C
fiber in Figure 48–2. Most of the signals then pass
through one or more additional short fiber neurons
within the dorsal horns themselves before entering
mainly lamina V, also in the dorsal horn. Here the last
neurons in the series give rise to long axons that mostly
join the fibers from the fast pain pathway, passing first
through the anterior commissure to the opposite side
of the cord, then upward to the brain in the anterolateral pathway.
Substance P, the Probable Slow-Chronic Neurotransmitter of Type C Nerve Endings. Research experi-
ments suggest that type C pain fiber terminals entering
the spinal cord secrete both glutamate transmitter and
substance P transmitter. The glutamate transmitter
acts instantaneously and lasts for only a few milliseconds. Substance P is released much more slowly, building up in concentration over a period of seconds or
even minutes. In fact, it has been suggested that the
“double” pain sensation one feels after a pinprick
might result partly from the fact that the glutamate
transmitter gives a faster pain sensation, whereas the
substance P transmitter gives a more lagging sensation. Regardless of the yet unknown details, it seems
clear that glutamate is the neurotransmitter most
involved in transmitting fast pain into the central
nervous system, and substance P is concerned with
slow-chronic pain.
601
Projection of the Paleospinothalamic Pathway (SlowChronic Pain Signals) into the Brain Stem and Thalamus. The slow-chronic paleospinothalamic pathway
terminates widely in the brain stem, in the large
shaded area shown in Figure 48–3. Only one tenth to
one fourth of the fibers pass all the way to the thalamus. Instead, most terminate in one of three areas: (1)
the reticular nuclei of the medulla, pons, and mesencephalon; (2) the tectal area of the mesencephalon
deep to the superior and inferior colliculi; or (3) the
periaqueductal gray region surrounding the aqueduct
of Sylvius. These lower regions of the brain appear to
be important for feeling the suffering types of pain,
because animals whose brains have been sectioned
above the mesencephalon to block pain signals from
reaching the cerebrum still evince undeniable evidence of suffering when any part of the body is traumatized. From the brain stem pain areas, multiple
short-fiber neurons relay the pain signals upward into
the intralaminar and ventrolateral nuclei of the thalamus and into certain portions of the hypothalamus and
other basal regions of the brain.
Very Poor Capability of the Nervous System to Localize Precisely the Source of Pain Transmitted in the
Slow-Chronic Pathway. Localization of pain transmit-
ted by way of the paleospinothalamic pathway is poor.
For instance, slow-chronic pain can usually be localized only to a major part of the body, such as to one
arm or leg but not to a specific point on the arm or leg.
This is in keeping with the multisynaptic, diffuse connectivity of this pathway. It explains why patients often
have serious difficulty in localizing the source of some
chronic types of pain.
Function of the Reticular Formation, Thalamus, and Cerebral
Cortex in the Appreciation of Pain. Complete removal of
the somatic sensory areas of the cerebral cortex does
not destroy an animal’s ability to perceive pain. Therefore, it is likely that pain impulses entering the brain
stem reticular formation, the thalamus, and other
lower brain centers cause conscious perception of
pain. This does not mean that the cerebral cortex has
nothing to do with normal pain appreciation; electrical stimulation of cortical somatosensory areas does
cause a human being to perceive mild pain from about
3 per cent of the points stimulated. However, it is
believed that the cortex plays an especially important
role in interpreting pain quality, even though pain perception might be principally the function of lower
centers.
Special Capability of Pain Signals to Arouse Overall Brain
Excitability. Electrical stimulation in the reticular areas
of the brain stem and in the intralaminar nuclei of the
thalamus, the areas where the slow-suffering type of
pain terminates, has a strong arousal effect on nervous
activity throughout the entire brain. In fact, these two
areas constitute part of the brain’s principal “arousal
system,” which is discussed in Chapter 59.This explains
why it is almost impossible for a person to sleep when
he or she is in severe pain.
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Unit IX
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Surgical Interruption of Pain Pathways. When a person has
severe and intractable pain (sometimes resulting from
rapidly spreading cancer), it is necessary to relieve the
pain. To do this, the pain nervous pathways can be cut
at any one of several points. If the pain is in the lower
part of the body, a cordotomy in the thoracic region of
the spinal cord often relieves the pain for a few weeks
to a few months. To do this, the spinal cord on the side
opposite to the pain is partially cut in its anterolateral
quadrant to interrupt the anterolateral sensory pathway.
A cordotomy, however, is not always successful in
relieving pain, for two reasons. First, many pain fibers
from the upper part of the body do not cross to the
opposite side of the spinal cord until they have reached
the brain, so that the cordotomy does not transect these
fibers. Second, pain frequently returns several months
later, partly as a result of sensitization of other pathways
that normally are too weak to be effectual (e.g., sparse
pathways in the dorsolateral cord). Another experimental operative procedure to relieve pain has been to
cauterize specific pain areas in the intralaminar nuclei
in the thalamus, which often relieves suffering types of
pain while leaving intact one’s appreciation of “acute”
pain, an important protective mechanism.
Pain Suppression
(“Analgesia”) System in the
Brain and Spinal Cord
The degree to which a person reacts to pain varies
tremendously. This results partly from a capability of
the brain itself to suppress input of pain signals to the
nervous system by activating a pain control system,
called an analgesia system.
The analgesia system is shown in Figure 48–4. It
consists of three major components: (1) The periaqueductal gray and periventricular areas of the mesencephalon and upper pons surround the aqueduct of
Sylvius and portions of the third and fourth ventricles.
Neurons from these areas send signals to (2) the raphe
magnus nucleus, a thin midline nucleus located in
the lower pons and upper medulla, and the nucleus
reticularis paragigantocellularis, located laterally in
the medulla. From these nuclei, second-order signals
are transmitted down the dorsolateral columns in the
spinal cord to (3) a pain inhibitory complex located in
the dorsal horns of the spinal cord. At this point, the
analgesia signals can block the pain before it is relayed
to the brain.
Electrical stimulation either in the periaqueductal
gray area or in the raphe magnus nucleus can suppress
many strong pain signals entering by way of the dorsal
spinal roots. Also, stimulation of areas at still higher
levels of the brain that excite the periaqueductal gray
area can also suppress pain. Some of these areas are
(1) the periventricular nuclei in the hypothalamus, lying
adjacent to the third ventricle, and (2) to a lesser
extent, the medial forebrain bundle, also in the hypothalamus.
Several transmitter substances are involved in the
analgesia system; especially involved are enkephalin
and serotonin. Many nerve fibers derived from the
Third
ventricle
Aqueduct
Mesencephalon
Periventricular
nuclei
Periaqueductal gray
Enkephalin neurons
Fourth
ventricle
Pons
Raphe magnus
nucleus
Medulla
Serotonergic
neurons
Pain fibers
Enkephalin
neurons
Presynaptic
pain inhibition
Anterolateral
somatosensory tract
Figure 48–4
Analgesia system of the brain and spinal cord, showing (1) inhibition of incoming pain signals at the cord level and (2) presence
of enkephalin-secreting neurons that suppress pain signals in
both the cord and the brain stem.
periventricular nuclei and from the periaqueductal
gray area secrete enkephalin at their endings. Thus, as
shown in Figure 48–4, the endings of many fibers in
the raphe magnus nucleus release enkephalin when
stimulated.
Fibers originating in this area send signals to the
dorsal horns of the spinal cord to secrete serotonin at
their endings. The serotonin causes local cord neurons
to secrete enkephalin as well. The enkephalin is
believed to cause both presynaptic and postsynaptic
inhibition of incoming type C and type Ad pain fibers
where they synapse in the dorsal horns.
Thus, the analgesia system can block pain signals at
the initial entry point to the spinal cord. In fact, it can
also block many local cord reflexes that result from
pain signals, especially withdrawal reflexes described
in Chapter 54.
Brain’s Opiate System—Endorphins
and Enkephalins
More than 35 years ago it was discovered that injection of minute quantities of morphine either into the
periventricular nucleus around the third ventricle or
Chapter 48
Somatic Sensations: II. Pain, Headache, and Thermal Sensations
into the periaqueductal gray area of the brain stem
causes an extreme degree of analgesia. In subsequent
studies, it has been found that morphine-like agents,
mainly the opiates, also act at many other points in the
analgesia system, including the dorsal horns of the
spinal cord. Because most drugs that alter excitability
of neurons do so by acting on synaptic receptors, it was
assumed that the “morphine receptors” of the analgesia system must be receptors for some morphine-like
neurotransmitter that is naturally secreted in the
brain. Therefore, an extensive search was undertaken
for the natural opiate of the brain. About a dozen such
opiate-like substances have now been found at different points of the nervous system; all are breakdown
products of three large protein molecules: proopiomelanocortin, proenkephalin, and prodynorphin.
Among the more important of these opiate-like
substances are b-endorphin, met-enkephalin, leuenkephalin, and dynorphin.
The two enkephalins are found in the brain stem
and spinal cord, in the portions of the analgesia system
described earlier, and b-endorphin is present in both
the hypothalamus and the pituitary gland. Dynorphin
is found mainly in the same areas as the enkephalins,
but in much lower quantities.
Thus, although the fine details of the brain’s opiate
system are not understood, activation of the analgesia
system by nervous signals entering the periaqueductal
gray and periventricular areas, or inactivation of pain
pathways by morphine-like drugs, can almost totally
suppress many pain signals entering through the
peripheral nerves.
Inhibition of Pain Transmission
by Simultaneous Tactile
Sensory Signals
Another important event in the saga of pain control was
the discovery that stimulation of large type Ab sensory
fibers from peripheral tactile receptors can depress
transmission of pain signals from the same body area.
This presumably results from local lateral inhibition in
the spinal cord. It explains why such simple maneuvers
as rubbing the skin near painful areas is often effective
in relieving pain. And it probably also explains why liniments are often useful for pain relief.
This mechanism and the simultaneous psychogenic
excitation of the central analgesia system are probably
also the basis of pain relief by acupuncture.
been reported in some instances. Also, pain relief has
been reported to last for as long as 24 hours after only
a few minutes of stimulation.
Referred Pain
Often a person feels pain in a part of the body that is
fairly remote from the tissue causing the pain. This is
called referred pain. For instance, pain in one of the
visceral organs often is referred to an area on the body
surface. Knowledge of the different types of referred
pain is important in clinical diagnosis because in many
visceral ailments the only clinical sign is referred pain.
Mechanism of Referred Pain. Figure 48–5 shows the prob-
able mechanism by which most pain is referred. In the
figure, branches of visceral pain fibers are shown to
synapse in the spinal cord on the same second-order
neurons (1 and 2) that receive pain signals from the
skin. When the visceral pain fibers are stimulated, pain
signals from the viscera are conducted through at least
some of the same neurons that conduct pain signals
from the skin, and the person has the feeling that the
sensations originate in the skin itself.
Visceral Pain
In clinical diagnosis, pain from the different viscera of
the abdomen and chest is one of the few criteria that
can be used for diagnosing visceral inflammation, visceral infectious disease, and other visceral ailments.
Often, the viscera have sensory receptors for no other
modalities of sensation besides pain. Also, visceral pain
differs from surface pain in several important aspects.
One of the most important differences between
surface pain and visceral pain is that highly localized
types of damage to the viscera seldom cause severe
pain. For instance, a surgeon can cut the gut entirely in
two in a patient who is awake without causing significant pain. Conversely, any stimulus that causes diffuse
stimulation of pain nerve endings throughout a viscus
causes pain that can be severe. For instance, ischemia
1
2
Treatment of Pain by Electrical
Stimulation
Several clinical procedures have been developed for
suppressing pain by electrical stimulation. Stimulating
electrodes are placed on selected areas of the skin or,
on occasion, implanted over the spinal cord, supposedly
to stimulate the dorsal sensory columns.
In some patients, electrodes have been placed stereotaxically in appropriate intralaminar nuclei of the thalamus or in the periventricular or periaqueductal area of
the diencephalon. The patient can then personally
control the degree of stimulation. Dramatic relief has
603
Visceral
nerve fibers
Skin nerve
fibers
Figure 48–5
Mechanism of referred pain and referred hyperalgesia.
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Unit IX
The Nervous System: A. General Principles and Sensory Physiology
caused by occluding the blood supply to a large area of
gut stimulates many diffuse pain fibers at the same time
and can result in extreme pain.
Causes of True Visceral Pain
Any stimulus that excites pain nerve endings in diffuse
areas of the viscera can cause visceral pain. Such stimuli
include ischemia of visceral tissue, chemical damage to
the surfaces of the viscera, spasm of the smooth muscle
of a hollow viscus, excess distention of a hollow viscus,
and stretching of the connective tissue surrounding or
within the viscus. Essentially all visceral pain that originates in the thoracic and abdominal cavities is transmitted through small type C pain fibers and, therefore,
can transmit only the chronic-aching-suffering type of
pain.
Ischemia. Ischemia causes visceral pain in the same way
that it does in other tissues, presumably because of the
formation of acidic metabolic end products or tissuedegenerative products such as bradykinin, proteolytic
enzymes, or others that stimulate pain nerve endings.
Chemical Stimuli. On occasion, damaging substances leak
from the gastrointestinal tract into the peritoneal cavity.
For instance, proteolytic acidic gastric juice often leaks
through a ruptured gastric or duodenal ulcer. This juice
causes widespread digestion of the visceral peritoneum,
thus stimulating broad areas of pain fibers. The pain is
usually excruciatingly severe.
Spasm of a Hollow Viscus. Spasm of a portion of the gut,
the gallbladder, a bile duct, a ureter, or any other hollow
viscus can cause pain, possibly by mechanical stimulation of the pain nerve endings. Or the spasm might cause
diminished blood flow to the muscle, combined with the
muscle’s increased metabolic need for nutrients, thus
causing severe pain.
Often pain from a spastic viscus occurs in the form of
cramps, with the pain increasing to a high degree of
severity and then subsiding. This process continues
intermittently, once every few minutes. The intermittent
cycles result from periods of contraction of smooth
muscle. For instance, each time a peristaltic wave travels
along an overly excitable spastic gut, a cramp occurs.
The cramping type of pain frequently occurs in appendicitis, gastroenteritis, constipation, menstruation, parturition, gallbladder disease, or ureteral obstruction.
Overdistention of a Hollow Viscus. Extreme overfilling of a
hollow viscus also can result in pain, presumably
because of overstretch of the tissues themselves.
Overdistention can also collapse the blood vessels that
encircle the viscus or that pass into its wall, thus perhaps
promoting ischemic pain.
Insensitive Viscera. A few visceral areas are almost completely insensitive to pain of any type. These include the
parenchyma of the liver and the alveoli of the lungs. Yet
the liver capsule is extremely sensitive to both direct
trauma and stretch, and the bile ducts are also sensitive
to pain. In the lungs, even though the alveoli are insensitive, both the bronchi and the parietal pleura are very
sensitive to pain.
“Parietal Pain” Caused by
Visceral Disease
When a disease affects a viscus, the disease process
often spreads to the parietal peritoneum, pleura, or
pericardium. These parietal surfaces, like the skin, are
supplied with extensive pain innervation from the
peripheral spinal nerves. Therefore, pain from the parietal wall overlying a viscus is frequently sharp. An
example can emphasize the difference between this
pain and true visceral pain: a knife incision through the
parietal peritoneum is very painful, whereas a similar
cut through the visceral peritoneum or through a gut
wall is not very painful, if painful at all.
Localization of Visceral Pain—
“Visceral” and the “Parietal” Pain
Transmission Pathways
Pain from the different viscera is frequently difficult to
localize, for a number of reasons. First, the patient’s
brain does not know from firsthand experience that the
different internal organs exist; therefore, any pain that
originates internally can be localized only generally.
Second, sensations from the abdomen and thorax are
transmitted through two pathways to the central
nervous system—the true visceral pathway and the parietal pathway. True visceral pain is transmitted via pain
sensory fibers within the autonomic nerve bundles, and
the sensations are referred to surface areas of the body
often far from the painful organ. Conversely, parietal
sensations are conducted directly into local spinal
nerves from the parietal peritoneum, pleura, or pericardium, and these sensations are usually localized
directly over the painful area.
Localization of Referred Pain Transmitted via Visceral Pathways.
When visceral pain is referred to the surface of the body,
the person generally localizes it in the dermatomal
segment from which the visceral organ originated in the
embryo, not necessarily where the visceral organ now
lies. For instance, the heart originated in the neck and
upper thorax, so that the heart’s visceral pain fibers pass
upward along the sympathetic sensory nerves and enter
the spinal cord between segments C-3 and T-5. Therefore, as shown in Figure 48–6, pain from the heart is
referred to the side of the neck, over the shoulder, over
the pectoral muscles, down the arm, and into the substernal area of the upper chest. These are the areas of
the body surface that send their own somatosensory
nerve fibers into the C-3 to T-5 cord segments. Most frequently, the pain is on the left side rather than on the
right because the left side of the heart is much more frequently involved in coronary disease than the right.
The stomach originated approximately from the
seventh to ninth thoracic segments of the embryo.
Therefore, stomach pain is referred to the anterior epigastrium above the umbilicus, which is the surface area
of the body subserved by the seventh through ninth thoracic segments. Figure 48–6 shows several other surface
areas to which visceral pain is referred from other
organs, representing in general the areas in the embryo
from which the respective organs originated.
Parietal Pathway for Transmission of Abdominal and Thoracic
Pain. Pain from the viscera is frequently localized to two
surface areas of the body at the same time because of
Chapter 48
605
Somatic Sensations: II. Pain, Headache, and Thermal Sensations
Hear t
T-10
Es ophagus
L-1
Stomach
Liver and
gallbladder
Pylorus
Umbilicus
Appendix and
small intestine
Right kidney
Visceral pain
Left kidney
Parietal pain
Colon
Ureter
Figure 48–6
Surface areas of referred pain from different visceral organs.
Figure 48–7
Visceral and parietal transmission of pain signals from the
appendix.
Herpes Zoster (Shingles)
the dual transmission of pain through the referred visceral pathway and the direct parietal pathway. Thus,
Figure 48–7 shows dual transmission from an inflamed
appendix. Pain impulses pass first from the appendix
through visceral pain fibers located within sympathetic
nerve bundles, and then into the spinal cord at about
T-10 or T-11; this pain is referred to an area around the
umbilicus and is of the aching, cramping type. Pain
impulses also often originate in the parietal peritoneum
where the inflamed appendix touches or is adherent to
the abdominal wall. These cause pain of the sharp type
directly over the irritated peritoneum in the right lower
quadrant of the abdomen.
Some Clinical Abnormalities
of Pain and Other
Somatic Sensations
Hyperalgesia
A pain nervous pathway sometimes becomes excessively excitable; this gives rise to hyperalgesia, which
means hypersensitivity to pain. Possible causes of hyperalgesia are (1) excessive sensitivity of the pain receptors
themselves, which is called primary hyperalgesia, and
(2) facilitation of sensory transmission, which is called
secondary hyperalgesia.
An example of primary hyperalgesia is the extreme
sensitivity of sunburned skin, which results from
sensitization of the skin pain endings by local tissue
products from the burn—perhaps histamine, perhaps
prostaglandins, perhaps others. Secondary hyperalgesia
frequently results from lesions in the spinal cord or the
thalamus. Several of these lesions are discussed in subsequent sections.
Occasionally herpesvirus infects a dorsal root ganglion.
This causes severe pain in the dermatomal segment subserved by the ganglion, thus eliciting a segmental type
of pain that circles halfway around the body.The disease
is called herpes zoster, or “shingles,” because of a skin
eruption that often ensues.
The cause of the pain is presumably infection of the
pain neuronal cells in the dorsal root ganglion by the
virus. In addition to causing pain, the virus is carried by
neuronal cytoplasmic flow outward through the neuronal peripheral axons to their cutaneous origins. Here
the virus causes a rash that vesiculates within a few days
and then crusts over within another few days, all of this
occurring within the dermatomal area served by the
infected dorsal root.
Tic Douloureux
Lancinating pain occasionally occurs in some people
over one side of the face in the sensory distribution area
(or part of the area) of the fifth or ninth nerves; this phenomenon is called tic douloureux (or trigeminal neuralgia or glossopharyngeal neuralgia). The pain feels like
sudden electrical shocks, and it may appear for only a
few seconds at a time or may be almost continuous.
Often it is set off by exceedingly sensitive trigger areas
on the surface of the face, in the mouth, or inside the
throat—almost always by a mechanoreceptive stimulus
rather than a pain stimulus. For instance, when the
patient swallows a bolus of food, as the food touches a
tonsil, it might set off a severe lancinating pain in the
mandibular portion of the fifth nerve.
The pain of tic douloureux can usually be blocked by
surgically cutting the peripheral nerve from the hypersensitive area. The sensory portion of the fifth nerve is
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Unit IX
The Nervous System: A. General Principles and Sensory Physiology
often sectioned immediately inside the cranium, where
the motor and sensory roots of the fifth nerve separate
from each other, so that the motor portions, which are
needed for many jaw movements, can be spared while
the sensory elements are destroyed. This operation
leaves the side of the face anesthetic, which in itself may
be annoying. Furthermore, sometimes the operation is
unsuccessful, indicating that the lesion that causes the
pain might be in the sensory nucleus in the brain stem
and not in the peripheral nerves.
Brown-Séquard Syndrome
If the spinal cord is transected entirely, all sensations
and motor functions distal to the segment of transection
are blocked, but if the spinal cord is transected on only
one side, the Brown-Séquard syndrome occurs. The
effects of such transection can be predicted from a
knowledge of the cord fiber tracts shown in Figure 48–8.
All motor functions are blocked on the side of the transection in all segments below the level of the transection. Yet only some of the modalities of sensation are
lost on the transected side, and others are lost on the
opposite side. The sensations of pain, heat, and cold—
sensations served by the spinothalamic pathway—are
lost on the opposite side of the body in all dermatomes
two to six segments below the level of the transection.
By contrast, the sensations that are transmitted only in
the dorsal and dorsolateral columns—kinesthetic and
position sensations, vibration sensation, discrete localization, and two-point discrimination—are lost on the
side of the transection in all dermatomes below the level
of the transection. Discrete “light touch” is impaired
on the side of the transection because the principal
pathway for the transmission of light touch, the dorsal
column, is transected. That is, the fibers in this column
do not cross to the opposite side until they reach the
medulla of the brain. “Crude touch,” which is poorly
localized, still persists because of partial transmission in
the opposite spinothalamic tract.
result from pain stimuli arising inside the cranium, but
others result from pain arising outside the cranium, such
as from the nasal sinuses.
Headache of Intracranial Origin
Pain-Sensitive Areas in Cranial Vault. The brain tissues
themselves are almost totally insensitive to pain. Even
cutting or electrically stimulating the sensory areas
of the cerebral cortex only occasionally causes pain;
instead, it causes prickly types of paresthesias on the
area of the body represented by the portion of the
sensory cortex stimulated. Therefore, it is likely that
much or most of the pain of headache is not caused by
damage within the brain itself.
Conversely, tugging on the venous sinuses around the
brain, damaging the tentorium, or stretching the dura at
the base of the brain can cause intense pain that is recognized as headache. Also, almost any type of traumatizing, crushing, or stretching stimulus to the blood
vessels of the meninges can cause headache. An especially sensitive structure is the middle meningeal artery,
and neurosurgeons are careful to anesthetize this artery
specifically when performing brain operations under
local anesthesia.
Areas of the Head to Which Intracranial Headache Is Referred.
Stimulation of pain receptors in the cerebral vault
above the tentorium, including the upper surface of the
tentorium itself, initiates pain impulses in the cerebral
portion of the fifth nerve and, therefore, causes referred
headache to the front half of the head in the surface
areas supplied by this somatosensory portion of the fifth
cranial nerve, as shown in Figure 48–9.
Conversely, pain impulses from beneath the
tentorium enter the central nervous system mainly
through the glossopharyngeal, vagal, and second cervical nerves, which also supply the scalp above, behind,
and slightly below the ear. Subtentorial pain stimuli
cause “occipital headache” referred to the posterior
part of the head.
Headache
Headaches are a type of pain referred to the surface of
the head from deep head structures. Some headaches
Cerebral vault
headaches
Brain stem and
cerebellar vault
headaches
Fasciculus gracilis
Fasciculus cuneatus
Lateral
corticospinal
Rubrospinal
Olivospinal
Tectospinal
Ventral
corticospinal
Vestibulospinal
Descending
Tracts
Dorsal
spinocerebellar
Lateral
spinothalamic
Nasal sinus
and eye
headaches
Ventral
spinocerebellar
Spinotectal
Ventral spinothalamic
Ascending
Tracts
Figure 48–8
Figure 48–9
Cross section of the spinal cord, showing principal ascending
tracts on the right and principal descending tracts on the left.
Areas of headache resulting from different causes.
Chapter 48
Somatic Sensations: II. Pain, Headache, and Thermal Sensations
Types of Intracranial Headache
Headache of Meningitis. One of the most severe
headaches of all is that resulting from meningitis, which
causes inflammation of all the meninges, including the
sensitive areas of the dura and the sensitive areas
around the venous sinuses. Such intense damage can
cause extreme headache pain referred over the entire
head.
Headache Caused by Low Cerebrospinal Fluid Pressure.
Removing as little as 20 milliliters of fluid from the
spinal canal, particularly if the person remains in an
upright position, often causes intense intracranial
headache. Removing this quantity of fluid removes part
of the flotation for the brain that is normally provided
by the cerebrospinal fluid. The weight of the brain
stretches and otherwise distorts the various dural
surfaces and thereby elicits the pain that causes the
headache.
Migraine Headache. Migraine headache is a special type
of headache that is thought to result from abnormal vascular phenomena, although the exact mechanism is
unknown. Migraine headaches often begin with various
prodromal sensations, such as nausea, loss of vision in
part of the field of vision, visual aura, and other types
of sensory hallucinations. Ordinarily, the prodromal
symptoms begin 30 minutes to 1 hour before the
beginning of the headache. Any theory that explains
migraine headache must also explain the prodromal
symptoms.
One of the theories of the cause of migraine
headaches is that prolonged emotion or tension causes
reflex vasospasm of some of the arteries of the head,
including arteries that supply the brain. The vasospasm
theoretically produces ischemia of portions of the brain,
and this is responsible for the prodromal symptoms.
Then, as a result of the intense ischemia, something
happens to the vascular walls, perhaps exhaustion of
smooth muscle contraction, to allow the blood vessels
to become flaccid and incapable of maintaining vascular tone for 24 to 48 hours. The blood pressure in the
vessels causes them to dilate and pulsate intensely, and
it is postulated that the excessive stretching of the walls
of the arteries—including some extracranial arteries,
such as the temporal artery—causes the actual pain
of migraine headaches. Other theories of the cause
of migraine headaches include spreading cortical depression, psychological abnormalities, and vasospasm
caused by excess local potassium in the cerebral extracellular fluid.
There may be a genetic predisposition to migraine
headaches, because a positive family history for
migraine has been reported in 65 to 90 per cent of cases.
Migraine headaches also occur about twice as frequently in women as in men.
Alcoholic Headache. As many people have experi-
enced, a headache usually follows an alcoholic binge. It
is most likely that alcohol, because it is toxic to tissues,
directly irritates the meninges and causes the intracranial pain.
Headache Caused by Constipation. Constipation causes
headache in many people. Because it has been shown
that constipation headache can occur in people whose
pain sensory tracts in the spinal cord have been cut, we
know that this headache is not caused by nervous
impulses from the colon. Therefore, it may result from
607
absorbed toxic products or from changes in the circulatory system resulting from loss of fluid into the gut.
Extracranial Types of Headache
Headache Resulting from Muscle Spasm. Emotional tension
often causes many of the muscles of the head, especially
those muscles attached to the scalp and the neck
muscles attached to the occiput, to become spastic, and
it is postulated that this is one of the common causes of
headache. The pain of the spastic head muscles supposedly is referred to the overlying areas of the head and
gives one the same type of headache as intracranial
lesions do.
Headache Caused by Irritation of Nasal and Accessory Nasal
Structures. The mucous membranes of the nose and
nasal sinuses are sensitive to pain, but not intensely so.
Nevertheless, infection or other irritative processes in
widespread areas of the nasal structures often summate
and cause headache that is referred behind the eyes or,
in the case of frontal sinus infection, to the frontal surfaces of the forehead and scalp, as shown in Figure 48–9.
Also, pain from the lower sinuses, such as from the maxillary sinuses, can be felt in the face.
Headache Caused by Eye Disorders. Difficulty in focusing
one’s eyes clearly may cause excessive contraction of
the eye ciliary muscles in an attempt to gain clear vision.
Even though these muscles are extremely small, it is
believed that tonic contraction of them can cause retroorbital headache. Also, excessive attempts to focus
the eyes can result in reflex spasm in various facial
and extraocular muscles, which is a possible cause of
headache.
A second type of headache that originates in the eyes
occurs when the eyes are exposed to excessive irradiation by light rays, especially ultraviolet light. Looking at
the sun or the arc of an arc-welder for even a few
seconds may result in headache that lasts from 24 to 48
hours. The headache sometimes results from “actinic”
irritation of the conjunctivae, and the pain is referred to
the surface of the head or retro-orbitally. However,
focusing intense light from an arc or the sun on the
retina can also burn the retina, and this could be the
cause of the headache.
Thermal Sensations
Thermal Receptors and
Their Excitation
The human being can perceive different gradations of
cold and heat, from freezing cold to cold to cool to
indifferent to warm to hot to burning hot.
Thermal gradations are discriminated by at least
three types of sensory receptors: cold receptors,
warmth receptors, and pain receptors. The pain receptors are stimulated only by extreme degrees of heat or
cold and, therefore, are responsible, along with the
cold and warmth receptors, for “freezing cold” and
“burning hot” sensations.
The cold and warmth receptors are located immediately under the skin at discrete separated spots. In
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Unit IX
The Nervous System: A. General Principles and Sensory Physiology
out slightly above 40°C. Above about 30°C, the
warmth receptors begin to be stimulated, but these
also fade out at about 49°C. Finally, at around 45°C,
the heat-pain fibers begin to be stimulated by heat and,
paradoxically, some of the cold fibers begin to be stimulated again, possibly because of damage to the cold
endings caused by the excessive heat.
One can understand from Figure 48–10 that a
person determines the different gradations of thermal
sensations by the relative degrees of stimulation of the
different types of endings. One can also understand
why extreme degrees of both cold and heat can be
painful and why both these sensations, when intense
enough, may give almost the same quality of sensation—that is, freezing cold and burning hot sensations
feel almost alike.
most areas of the body, there are 3 to 10 times as many
cold spots as warmth spots, and the number in different areas of the body varies from 15 to 25 cold spots
per square centimeter in the lips to 3 to 5 cold spots
per square centimeter in the finger to less than 1 cold
spot per square centimeter in some broad surface
areas of the trunk.
Although the existence of distinctive warmth nerve
endings is quite certain, based on psychological tests,
they have not been identified histologically. They are
presumed to be free nerve endings, because warmth
signals are transmitted mainly over type C nerve fibers
at transmission velocities of only 0.4 to 2 m/sec.
Conversely, a definitive cold receptor has been identified. It is a special, small type Ad myelinated nerve
ending that branches a number of times, the tips of
which protrude into the bottom surfaces of basal epidermal cells. Signals are transmitted from these receptors via type Ad nerve fibers at velocities of about
20 m/sec. Some cold sensations are believed to be
transmitted in type C nerve fibers as well, which suggests that some free nerve endings also might function
as cold receptors.
Stimulatory Effects of Rising and Falling Temperature—Adaptation of Thermal Receptors. When a cold receptor is sud-
denly subjected to an abrupt fall in temperature, it
becomes strongly stimulated at first, but this stimulation fades rapidly during the first few seconds and progressively more slowly during the next 30 minutes or
more. In other words, the receptor “adapts” to a great
extent, but never 100 per cent.
Thus, it is evident that the thermal senses respond
markedly to changes in temperature, in addition to
being able to respond to steady states of temperature.
This means that when the temperature of the skin is
actively falling, a person feels much colder than when
the temperature remains cold at the same level. Conversely, if the temperature is actively rising, the person
feels much warmer than he or she would at the same
temperature if it were constant. The response to
changes in temperature explains the extreme degree
of heat one feels on first entering a tub of hot water
and the extreme degree of cold felt on going from a
heated room to the out-of-doors on a cold day.
Stimulation of Thermal Receptors—Sensations of Cold, Cool,
Indifferent, Warm, and Hot. Figure 48–10 shows the
effects of different temperatures on the responses of
four types of nerve fibers: (1) a pain fiber stimulated
by cold, (2) a cold fiber, (3) a warmth fiber, and (4) a
pain fiber stimulated by heat. Note especially that
these fibers respond differently at different levels of
temperature. For instance, in the very cold region, only
the cold-pain fibers are stimulated (if the skin becomes
even colder, so that it nearly freezes or actually does
freeze, these fibers cannot be stimulated). As the temperature rises to +10° to 15°C, the cold-pain impulses
cease, but the cold receptors begin to be stimulated,
reaching peak stimulation at about 24°C and fading
Freezing
cold
Cool
IndifferWarm Hot
ent
Burning
hot
Cold-pain
Cold-receptors
Warmth receptors
Heat-pain
10
Impulses per second
Cold
8
6
4
2
5
10
15
20
25 30
35 40
Temperature (∞C)
45
50
55
60
Figure 48–10
Discharge frequencies at different skin temperatures of a cold-pain fiber, a cold fiber, a warmth
fiber, and a heat-pain fiber.
Chapter 48
Somatic Sensations: II. Pain, Headache, and Thermal Sensations
Mechanism of Stimulation of
Thermal Receptors
It is believed that the cold and warmth receptors are
stimulated by changes in their metabolic rates, and
that these changes result from the fact that temperature alters the rate of intracellular chemical reactions
more than twofold for each 10°C change. In other
words, thermal detection probably results not from
direct physical effects of heat or cold on the nerve
endings but from chemical stimulation of the endings
as modified by temperature.
Spatial Summation of Thermal Sensations. Because the
number of cold or warm endings in any one surface
area of the body is slight, it is difficult to judge gradations of temperature when small skin areas are stimulated. However, when a large skin area is stimulated
all at once, the thermal signals from the entire area
summate. For instance, rapid changes in temperature
as little as 0.01°C can be detected if this change affects
the entire surface of the body simultaneously. Conversely, temperature changes 100 times as great often
will not be detected when the affected skin area is only
1 square centimeter in size.
Transmission of Thermal Signals
in the Nervous System
In general, thermal signals are transmitted in pathways
parallel to those for pain signals. On entering the
spinal cord, the signals travel for a few segments
upward or downward in the tract of Lissauer and then
terminate mainly in laminae I, II, and III of the dorsal
horns—the same as for pain. After a small amount of
processing by one or more cord neurons, the signals
enter long, ascending thermal fibers that cross to the
opposite anterolateral sensory tract and terminate in
both (1) the reticular areas of the brain stem and (2)
the ventrobasal complex of the thalamus.
A few thermal signals are also relayed to the cerebral somatic sensory cortex from the ventrobasal
complex. Occasionally a neuron in cortical somatic
sensory area I has been found by microelectrode
studies to be directly responsive to either cold or warm
stimuli on a specific area of the skin. However, removal
of the entire cortical postcentral gyrus in the human
609
being reduces but does not abolish the ability to distinguish gradations of temperature.
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U
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The Nervous
System: B. The
Special Senses
49. The Eye: I. Optics of Vision
50. The Eye: II. Receptor and Neural
Function of the Retina
51. The Eye: III. Central Neurophysiology of Vision
52. The Sense of Hearing
53. The Chemical Senses—Taste and Smell
I
T
X
C
H
A
P
T
E
R
4
9
The Eye: I. Optics of Vision
Physical Principles
of Optics
Before it is possible to understand the optical
system of the eye, the student must first be thoroughly familiar with the basic principles of optics,
including the physics of light refraction, focusing,
depth of focus, and so forth. A brief review of these
physical principles is presented; then the optics of the eye is discussed.
Refraction of Light
Refractive Index of a Transparent Substance. Light rays travel through air at a velocity of
about 300,000 km/sec, but they travel much slower through transparent solids
and liquids. The refractive index of a transparent substance is the ratio of the
velocity of light in air to the velocity in the substance. The refractive index of air
itself is 1.00. Thus, if light travels through a particular type of glass at a velocity of
200,000 km/sec, the refractive index of this glass is 300,000 divided by 200,000, or
1.50.
Refraction of Light Rays at an Interface Between Two Media with Different Refractive Indices.
When light rays traveling forward in a beam (as shown in Figure 49–1A) strike an
interface that is perpendicular to the beam, the rays enter the second medium
without deviating from their course. The only effect that occurs is decreased velocity of transmission and shorter wavelength, as shown in the figure by the shorter
distances between wave fronts.
If the light rays pass through an angulated interface as shown in Figure 49–1B,
the rays bend if the refractive indices of the two media are different from each other.
In this particular figure, the light rays are leaving air, which has a refractive index
of 1.00, and are entering a block of glass having a refractive index of 1.50. When the
beam first strikes the angulated interface, the lower edge of the beam enters the
glass ahead of the upper edge. The wave front in the upper portion of the beam continues to travel at a velocity of 300,000 km/sec, while that which entered the glass
travels at a velocity of 200,000 km/sec. This causes the upper portion of the wave
front to move ahead of the lower portion, so that the wave front is no longer vertical but angulated to the right. Because the direction in which light travels is always
perpendicular to the plane of the wave front, the direction of travel of the light beam
bends downward.
This bending of light rays at an angulated interface is known as refraction. Note
particularly that the degree of refraction increases as a function of (1) the ratio of
the two refractive indices of the two transparent media and (2) the degree of angulation between the interface and the entering wave front.
Application of Refractive Principles to Lenses
Convex Lens Focuses Light Rays. Figure 49–2 shows parallel light rays entering a convex
lens. The light rays passing through the center of the lens strike the lens exactly perpendicular to the lens surface and, therefore, pass through the lens without being
refracted. Toward either edge of the lens, however, the light rays strike a progressively more angulated interface. The outer rays bend more and more toward the
center, which is called convergence of the rays. Half the bending occurs when
the rays enter the lens, and half as they exit from the opposite side. (At this time,
the student should pause and analyze why the rays bend toward the center on
leaving the lens.) Finally, if the lens has exactly the proper curvature, parallel light
613
614
A
Unit X
Wave fronts
The Nervous System: B. The Special Senses
Glass
Light from
distant source
B
Figure 49–3
Figure 49–1
Light rays entering a glass surface perpendicular to the light rays
(A) and a glass surface angulated to the light rays (B). This figure
demonstrates that the distance between waves after they enter
the glass is shortened to about two thirds that in air. It also shows
that light rays striking an angulated glass surface are bent.
Light from
distant source
Bending of light rays at each surface of a concave spherical lens,
showing that parallel light rays are diverged.
Focal length
Figure 49–2
Bending of light rays at each surface of a convex spherical lens,
showing that parallel light rays are focused to a focal point.
A
rays passing through each part of the lens will be bent
exactly enough so that all the rays will pass through a
single point, which is called the focal point.
Concave Lens Diverges Light Rays. Figure 49–3 shows the
effect of a concave lens on parallel light rays. The rays
that enter the center of the lens strike an interface that
is perpendicular to the beam and, therefore, do not
refract. The rays at the edge of the lens enter the lens
ahead of the rays in the center. This is opposite to the
effect in the convex lens, and it causes the peripheral
light rays to diverge from the light rays that pass through
the center of the lens. Thus, the concave lens diverges
light rays, but the convex lens converges light rays.
Cylindrical Lens Bends Light Rays in Only One Plane—Comparison
with Spherical Lenses. Figure 49–4 shows both a convex
spherical lens and a convex cylindrical lens. Note that
the cylindrical lens bends light rays from the two sides
of the lens but not from the top or the bottom. That is,
bending occurs in one plane but not the other. Thus, parallel light rays are bent to a focal line. Conversely, light
rays that pass through the spherical lens are refracted
B
Figure 49–4
A, Point focus of parallel light rays by a spherical convex lens.
B, Line focus of parallel light rays by a cylindrical convex lens.
Chapter 49
615
The Eye: I. Optics of Vision
at all edges of the lens (in both planes) toward the
central ray, and all the rays come to a focal point.
The cylindrical lens is well demonstrated by a test
tube full of water. If the test tube is placed in a beam of
sunlight and a piece of paper is brought progressively
closer to the opposite side of the tube, a certain distance
will be found at which the light rays come to a focal line.
The spherical lens is demonstrated by an ordinary magnifying glass. If such a lens is placed in a beam of sunlight and a piece of paper is brought progressively closer
to the lens, the light rays will impinge on a common
focal point at an appropriate distance.
Concave cylindrical lenses diverge light rays in only
one plane in the same manner that convex cylindrical
lenses converge light rays in one plane.
Combination of Two Cylindrical Lenses at Right Angles
Equals a Spherical Lens. Figure 49–5B shows two
convex cylindrical lenses at right angles to each other.
The vertical cylindrical lens converges the light rays that
pass through the two sides of the lens, and the horizontal lens converges the top and bottom rays. Thus, all the
light rays come to a single-point focus. In other words,
two cylindrical lenses crossed at right angles to each other
perform the same function as one spherical lens of the
same refractive power.
Focal Length of a Lens
The distance beyond a convex lens at which parallel rays
converge to a common focal point is called the focal
length of the lens. The diagram at the top of Figure 49–6
demonstrates this focusing of parallel light rays.
In the middle diagram, the light rays that enter the
convex lens are not parallel but are diverging because
the origin of the light is a point source not far away from
the lens itself. Because these rays are diverging outward
from the point source, it can be seen from the diagram
that they do not focus at the same distance away from
the lens as do parallel rays. In other words, when rays
of light that are already diverging enter a convex lens,
the distance of focus on the other side of the lens is
farther from the lens than is the focal length of the lens
for parallel rays.
The bottom diagram of Figure 49–6 shows light rays
that are diverging toward a convex lens that has far
greater curvature than that of the other two lenses in
the figure. In this diagram, the distance from the lens at
which the light rays come to focus is exactly the same
as that from the lens in the first diagram, in which the
lens is less convex but the rays entering it are parallel.
This demonstrates that both para