Lecture 9 the digestive system

Copyright 2010, John Wiley & Sons, Inc.
Chapter 19
The Digestive System

Functions of the Digestive System
 Ingestion: eating
 Secretion: release of water, enzymes, buffers
 Mixing and propulsion: movement along GI
tract
 Digestion: breakdown of foods
 Mechanically: by movements of digestive organs
 Chemically: by enzymes
 Absorption: moving products of digestion into
the body
 Defecation: dumping waste products

Organs of the Digestive System
 Gastrointestinal (GI) tract
 A tube through which foods pass and where
digestion and absorption occur.
 Includes:
mouth, pharynx, esophagus, stomach, small
intestine, large intestine
 Accessory organs:
 Organs that help in digestion but through which
food never passes.
 Includes: teeth, tongue, salivary
glands, liver, gallbladder, and pancreas

Organs of the Digestive System

Layers of the Gastrointestinal (GI) Wall
 Four layers from lower esophagus to anus
1. Mucosa: epithelium in direct content with food;
made of connective tissue, glands, and thin
muscularis mucosae
2. Submucosa: connective tissue, blood
vessels, lymphatic vessels, and enteric nervous
system (ENS)

3. Muscularis: inner circular layer, outer longitudinal
layer
 Smooth muscle in most of GI tract
 Except skeletal (voluntary muscle) in
mouth, pharynx, upper esophagus, and external anal
sphincter
4. Serosa: visceral layer of peritoneum
 Also forms extensions: greater omentum and mesentery

Mouth (Oral Cavity)
 Formed by
 Cheeks and tongue
 Hard palate anteriorly, soft palate posteriorly
 Uvula
 U-shaped extension of soft palate posteriorly
 During swallowing, uvula blocks entry of food or
drink into nasal cavity
 Tongue: muscular accessory organ
 Maneuvers food for chewing
 Adjusts shape for speech and swallowing
 Lingual tonsils at base of tongue

Salivary Glands
 Exocrine glands with ducts that empty into
oral cavity
 Three pairs of salivary glands
 Parotid
 Largest; inferior and anterior to ears
 Submandibular
 In floor of mouth; medial and inferior to mandible
 Sublingual
 Inferior to tongue and superior to submandibular
 Saliva: 99.5% water, salivary
amylase, mucus and other solutes
 Dissolves food and starts digestion of starches

Salivary Glands

Teeth
 Accessory organs in bony sockets of
mandible and maxilla
 Three external regions
 Crown: above gums
 Root: part(s) embedded in socket
 Neck: between crown and root near gum line
 Three layers of material
 Enamel: hardest substance in body; over crown
 Dentin: majority of interior of tooth
 Pulp cavity: nerve, blood vessel, and lymphatics

Teeth

Teeth
 Humans have two sets of teeth
 The 20 deciduous teeth are replaced by the
permanent teeth between ages 6 and 12 years.
 The 32 permanent teeth appear between 6 years
and adulthood.
 Four types of teeth
 Incisors (8): used to cut food
 Cuspids (canines) (4): used to tear food
 Premolars (8): for crushing and grinding food
 Molars (12): used for crushing and grinding food

Digestion in the Mouth
 Mechanical digestion
 Chewing mixes food with saliva
 Rounds up food into a soft bolus for swallowing
 Chemical digestion
 Salivary amylase (enzyme) breaks down
polysaccharides (starch)  maltose and larger
fragments
 Continues in the stomach for about an hour until
acid inactivates amylase

Pharynx and Esophagus
 Food passages from mouth  stomach
 Swallowing: 3 stages
 Voluntary stage: bolus of food  oropharynx
 Pharyngeal stage: oropharynx  esophagus
 Soft palate moves up and epiglottis moves down;
prevent food from entering nasopharynx and larynx
 Esophageal: food  stomach by peristalsis
 Esophageal sphincters:
 Upper: controls entry  esophagus
 Lower: controls entry  stomach; GERD affects

Pharynx and Esophagus

Stomach
 J- shaped enlargement of GI tract
 Mixing chamber and holding reservoir
 Very elastic/expandable and muscular
 Four regions
 Cardia: surrounds upper opening
 Fundus: superior and to left of cardia
 Body: large central portion
 Pylorus: lower part leading to pyloric sphincter
and duodenum

Stomach

Stomach Wall: Four Layers
1. Mucosa
 Empty stomach lies in folds called rugae
 Epithelium: simple columnar; glands secrete
mucus
 Gastric glands line gastric pits
2. Secretory cells
 Mucous cells  mucus
 Parietal cells  HCl and intrinsic factor
 These secretions collectively called gastric juice
 Intrinsic factor helps with vitamin B12 absorption needed
for RBC formation. If missing  anemia
 Chief cells  inactive enzyme pepsinogen
 G cells secrete gastrin (hormone) into blood

3. Muscularis: Three layers
 Outer: longitudinal
 Middle: circular
 Inner: oblique (extra layer not in other organs)
provides for efficient gastric contractions
4. Serous membrane (serosa)
 Visceral peritoneum: covers organs
 Extensions of serosa
 Greater omentum: hangs from curve of stomach
 Mesentery: attaches small intestine to posterior wall of
abdomen and provides route for vessels

Digestion and Absorption
 Digestion
 Mechanical digestion
 Stretching of stomach wall  nerve impulses 
 Secretion + mixing waves 
 Food mixed with juice  now called chyme
 Chemical digestion
 Pepsin (pepsinogen + HCl) digests protein  peptides
(small chains of amino acids)
 Gastric emptying through pyloric sphincter
 Carbohydrates fastest, proteins next, fats last
 Once in duodenum  feedback inhibition of stomach
 Little absorption: water, ions, some drugs

Pancreas
 Location: behind stomach
 Produces pancreatic juice in acinar cells
 Passes into duodenum via pancreatic duct
 Secretions that help digestion
 Sodium bicarbonate (NaHCO3): pH 7.1-8.2)
 Digestive enzymes: many
 Pancreatic lipase: fat-digesting
 Pancreatic amylase: starch-digesting
 Proteases: made in inactivated form
 Activated by enterokinase from small intestine
 Chymotrypsinogen, trypsinogen, carboxypeptidase
 RNAase and DNAase

Liver and Gallbladder
 Weighs 1.4 kg (3 lb): 2nd largest organ in the
body; large right lobe + 3 smaller parts
 In right upper quadrant, below diaphragm
 Bile production and pathway
 Hepatocytes (liver cells) make bile 
 Bile canaliculi  bile ducts  hepatic duct 
 Gallbladder (green, pear-shaped organ that stores
bile) 
 Cystic duct  common bile duct  duodenum

Liver and Gall Bladder
 Functional unit is lobule
 Consists of hepatocytes in rows that radiate
around central vein
 Sinusoids (permeable capillaries with phagocytic
[Kuppfer] cells) are between cells
 Blood reaches liver lobules from
 Hepatic artery (branch of celiac): blood high in O2
 Hepatic portal vein (formed by veins from digestive
organs and spleen): blood low in O2 but rich in
nutrients from digestive organs

Bile
 Functions of bile
 Emulsification: breaking apart clusters of fats so
they are more digestible
 Absorption of fats
 Formation and recycling of bile
 Bilirubin from heme when RBCs broken down
 Bile is digested  stercobilin: gives feces brown
color
 Bile salts reabsorbed into blood in small intestine
(ileum)  portal vein  liver
 Gallstones may form from bile
 Obstruct bile ducts from gallbladder  pain

Liver, Gallbladder, Duodenum

Liver

Liver Functions
1. Carbohydrate metabolism
 Polysaccharide stored in liver as glycogen
 Converts glycogen, fructose, galactose, lactic
acid, amino acids  glucose to blood glucose
2. Lipid metabolism
 Produces cholesterol, triglycerides; makes bile
 Makes lipoproteins for lipid transport
3. Protein metabolism
 Remove NH2 from amino acids  ammonia
(NH3)  urea  to kidneys (urine)
 Synthesize most plasma proteins: albumin

Liver Functions
4. Removes many harmful substances from
blood
 Detoxifies alcohol
 Inactivates steroid and thyroid hormones
 Eliminates some drugs (like penicillin) into bile
5. Excretion of bilirubin
 From heme (in RBCs) to bile  feces
6. Stores fat-soluble vitamins (ADEK) and
minerals (Fe, Cu)
7. Activates vitamin D

Small Intestine
 Length
 10 feet long in living person
 Extends from pylorus of stomach to cecum of
large intestine
 Three major regions:
duodenum, jejunum, ileum
 Functions
 Site of most of digestion
 Essentially all nutrient absorption occurs here
 Ends in ileocecal sphincter (in RLQ)

Small Intestine

Intestinal Wall Structure
 Same 4 layers but with modifications
 Epithelium in mucosa: simple columnar
 Absorptive cells with microvilli
 Goblet cells: secrete mucus
 Intestinal glands secrete
 Enzymes that complete digestion
 Secretin, cholecystokinin (CCK), glucose-
dependent insulinotropic peptide (GIP)
 Lymphatic tissue within wall: defense

 Submucosa has duodenal glands 
 Alkaline mucus  helps neutralize stomach acid
 Circular folds
 In mucosa and submucosa; increase surface area
 Villi: fingerlike projections of mucosa
 Increase absorptive surface area
 Microvilli on absorptive cells further enhance absorption
 Contain vessels that absorb nutrients:
 Arteriole, capillary, venule
 Lacteal (lymph capillary) for lipid absorption

Digestion in Small Intestine
 Mechanical digestion
 Segmentation activity: for mixing
 Peristalsis for movement of intestinal contents
after most absorption completed: slow waves
 Chemical digestion: 2 L/d of secretions
 Alkaline chyme due to bicarbonate
 From pancreas and alkaline mucus from small intestine
 Enzymes produced by cells on villi
 Peptidases: breaks small peptides
 Disaccharidases: sucrase, lactase, and galactase

Absorption in the Small Intestine
 Chyme enters small intestine carrying
partially digested carbohydrates and proteins
 Intestinal juice (composed of bile, pancreatic
juice, intestinal juice) completes digestion
 90% of absorption of products of digestion
occurs in the small intestine
 Monosaccharides; amino acids
 Fatty acids and monoglycerides
 Phosphate sugar, and bases of DNA, RNA

Summary: Carbohydrate Digestion
 Amylases (salivary and pancreatic):
 Starch and dextrin  maltose
 Disaccharidases (from small intestine):
 Maltose: maltose  glucose + glucose
 Lactase: lactose  glucose + galactose
 Sucrase: sucrose  glucose + fructose

Protein and Fat Digestion
 Pepsin, trypsin, chymotrypsin, and
carboxypeptidase
 Proteins small peptides
 Peptidases at surface:
 Peptides  amino acids, dipeptides, and
tri-peptides
 Lipase (pancreatic)
 Triglyceridesfatty acids + monoglycerides

Absorption of Products of Digestion
 By diffusion, facilitated diffusion, osmosis and
active transport
 Carbohydrates monosaccharides
 Via portal system (blood) to liver
 Proteins (jejunum + ileum)  amino acids
 Via portal system (blood) to liver
 Lipids 
 Short-chained fatty acids or monoglycerides or 
blood in villi
 Larger lipids coated by proteins in chlyomicrons
 lacteals  lymphatics (lymph)  then blood

 Water and salt
 Primarily osmotic movement that accompanies
other nutrients
 Vitamins
 Fat-soluble (A, D, E, K) absorbed with fat
 Water-soluble (B’s, C) with simple diffusion
 B12
 Combines with intrinsic factor for transport through
duodenum and jejunum
 Finally can be absorbed by active transport in ileum

Large Intestine
 Structure: 4 regions
 Cecum
 Ileocecal sphincter
 Appendix attached
 Colon: ascending, transverse, descending and
sigmoid
 Rectum
 Anal canal with sphincters
 Wall: standard 4 layers
 Mucosa: goblet cells secrete mucus
 Muscularis: incomplete longitudinal layer

Large Intestine

Digestion and Absorption
 Ileocecal sphincter limits rate of emptying of
ileum
 Slow peristalsis
 Mass peristalsis
 Triggered by presence of food in stomach
 Wastes move from mid-colon  rectum
 Bacterial digestion
 Produce some B-vitamins + vitamin K
 Produce gases: flatus
 Colon absorbs salt + water

Defecation Reflex
 Stretch of rectum wall  neural reflex
 contraction of longitudinal muscle
 Combined pressure + parasympathetic
activity relaxes internal anal sphincter
 External anal sphincter is voluntary
 Contraction of diaphragm and abdominal
muscles aid defecation

Control: Phases of Digestion
 Rule: activate forward and inhibit behind
 Three phases: cephalic, gastric, intestinal
1. Cephalic: smell, sight, thought of food 
 Cranial nerves VII + IX stimulate salivary glands
 Cranial nerve X (vagus) stimulates gastric glands
2. Gastric: stretching, pH of stomach 
 Gastrin activates stomach and relaxes pyloric sphincter
3. Intestinal phase: intestinal hormones play key
roles

Control: Phases of Digestion
 Secretin
 Released when acidic chyme enters intestine
 Stimulates release of pancreatic juice high in
bicarbonate to buffer acidic chyme from stomach
 Cholecystokinin (CCK)
 Released when chyme rich in amino acids and
fatty acids enters intestine
 Stimulates release of pancreatic juice high in
digestive enzymes
 Decreases gastric motility and secretion
 Causes gallbladder to contract and eject bile

Aging
 Decreased GI secretion, motility, strength of
responses
 Loss of taste, increased risk for periodontal
disease, difficulty swallowing, hiatal
hernia, gastritis, peptic ulcer disease
 Increased risk for gallbladder
problems, cirrhosis of
liver, pancreatitis, constipation, hemorrhoids,
diverticulitis

End of Chapter 19
 Copyright 2010 John Wiley & Sons, Inc.
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Lecture 9 the digestive system

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Lecture 9 the digestive system