Chapter 19
The Digestive System
Copyright 2010, John Wiley & Sons, Inc.
Functions of the Digestive System
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Ingestion: eating
Secretion: release of water, enzymes, buffers
Mixing and propulsion: movement along GI
tract
Digestion: breakdown of foods
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Mechanically: by movements of digestive organs
Chemically: by enzymes
Absorption: moving products of digestion into
the body
Defecation: dumping waste products
Copyright 2010, John Wiley & Sons, Inc.
Organs of the Digestive System
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Gastrointestinal (GI) tract
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A tube through which foods pass and where
digestion and absorption occur.
Includes: mouth, pharynx, esophagus, stomach,
small intestine, large intestine
Accessory organs:
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Organs that help in digestion but through which
food never passes.
Includes: teeth, tongue, salivary glands, liver,
gallbladder, and pancreas
Copyright 2010, John Wiley & Sons, Inc.
Organs of the Digestive System
Copyright 2010, John Wiley & Sons, Inc.
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)
Copyright 2010, John Wiley & Sons, Inc.
Layers of the Gastrointestinal (GI) Wall
3. Muscularis: inner circular layer, outer longitudinal
layer
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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
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Also forms extensions: greater omentum and mesentery
Copyright 2010, John Wiley & Sons, Inc.
Layers of the Gastrointestinal (GI) Wall
Copyright 2010, John Wiley & Sons, Inc.
Layers of the Gastrointestinal (GI) Wall
Copyright 2010, John Wiley & Sons, Inc.
Layers of the Gastrointestinal (GI) Wall
Copyright 2010, John Wiley & Sons, Inc.
Mouth (Oral Cavity)
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Formed by
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Uvula
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U-shaped extension of soft palate posteriorly
During swallowing, uvula blocks entry of food or
drink into nasal cavity
Tongue: muscular accessory organ
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Cheeks and tongue
Hard palate anteriorly, soft palate posteriorly
Maneuvers food for chewing
Adjusts shape for speech and swallowing
Lingual tonsils at base of tongue
Copyright 2010, John Wiley & Sons, Inc.
Salivary Glands
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Exocrine glands with ducts that empty into
oral cavity
Three pairs of salivary glands
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Parotid
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Submandibular
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In floor of mouth; medial and inferior to mandible
Sublingual
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Largest; inferior and anterior to ears
Inferior to tongue and superior to submandibular
Saliva: 99.5% water, salivary amylase,
mucus and other solutes
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Dissolves food and starts digestion of starches
Copyright 2010, John Wiley & Sons, Inc.
Salivary Glands
Copyright 2010, John Wiley & Sons, Inc.
Teeth
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Accessory organs in bony sockets of
mandible and maxilla
Three external regions
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Crown: above gums
Root: part(s) embedded in socket
Neck: between crown and root near gum line
Three layers of material
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Enamel: hardest substance in body; over crown
Dentin: majority of interior of tooth
Pulp cavity: nerve, blood vessel, and lymphatics
Copyright 2010, John Wiley & Sons, Inc.
Teeth
Copyright 2010, John Wiley & Sons, Inc.
Teeth
Humans have two sets of teeth
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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
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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
Copyright 2010, John Wiley & Sons, Inc.
Digestion in the Mouth
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Mechanical digestion
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Chewing mixes food with saliva
Rounds up food into a soft bolus for swallowing
Chemical digestion
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Salivary amylase (enzyme) breaks down
polysaccharides (starch)  maltose and larger
fragments
Continues in the stomach for about an hour until
acid inactivates amylase
Copyright 2010, John Wiley & Sons, Inc.
Pharynx and Esophagus
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Food passages from mouth  stomach
Swallowing: 3 stages
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Voluntary stage: bolus of food  oropharynx
Pharyngeal stage: oropharynx  esophagus
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Soft palate moves up and epiglottis moves down;
prevent food from entering nasopharynx and larynx
Esophageal: food  stomach by peristalsis
Esophageal sphincters:
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Upper: controls entry  esophagus
Lower: controls entry  stomach; GERD affects
Copyright 2010, John Wiley & Sons, Inc.
Pharynx and Esophagus
Copyright 2010, John Wiley & Sons, Inc.
Pharynx and Esophagus
Copyright 2010, John Wiley & Sons, Inc.
Stomach
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J- shaped enlargement of GI tract
Mixing chamber and holding reservoir
Very elastic/expandable and muscular
Four regions
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Cardia: surrounds upper opening
Fundus: superior and to left of cardia
Body: large central portion
Pylorus: lower part leading to pyloric sphincter
and duodenum
Copyright 2010, John Wiley & Sons, Inc.
Stomach
Copyright 2010, John Wiley & Sons, Inc.
Stomach
Copyright 2010, John Wiley & Sons, Inc.
Stomach Wall: Four Layers
1. Mucosa
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Empty stomach lies in folds called rugae
Epithelium: simple columnar; glands secrete
mucus
Gastric glands line gastric pits
2. Secretory cells
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Mucous cells  mucus
Parietal cells  HCl and intrinsic factor
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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
Copyright 2010, John Wiley & Sons, Inc.
Stomach Wall: Four Layers
3. Muscularis: Three layers
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Outer: longitudinal
Middle: circular
Inner: oblique (extra layer not in other organs)
provides for efficient gastric contractions
4. Serous membrane (serosa)
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Visceral peritoneum: covers organs
Extensions of serosa
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Greater omentum: hangs from curve of stomach
Mesentery: attaches small intestine to posterior wall of
abdomen and provides route for vessels
Copyright 2010, John Wiley & Sons, Inc.
Stomach Wall: Four Layers
Copyright 2010, John Wiley & Sons, Inc.
Digestion and Absorption
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Digestion
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Mechanical digestion
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Chemical digestion
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Pepsin (pepsinogen + HCl) digests protein  peptides
(small chains of amino acids)
Gastric emptying through pyloric sphincter
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Stretching of stomach wall  nerve impulses 
Secretion + mixing waves 
Food mixed with juice  now called chyme
Carbohydrates fastest, proteins next, fats last
Once in duodenum  feedback inhibition of stomach
Little absorption: water, ions, some drugs
Copyright 2010, John Wiley & Sons, Inc.
Pancreas
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Location: behind stomach
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Produces pancreatic juice in acinar cells
Passes into duodenum via pancreatic duct
Secretions that help digestion
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Sodium bicarbonate (NaHCO3): pH 7.1-8.2)
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Digestive enzymes: many
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Pancreatic lipase: fat-digesting
Pancreatic amylase: starch-digesting
Proteases: made in inactivated form
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Activated by enterokinase from small intestine
Chymotrypsinogen, trypsinogen, carboxypeptidase
RNAase and DNAase
Copyright 2010, John Wiley & Sons, Inc.
Liver and Gallbladder
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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
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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
Copyright 2010, John Wiley & Sons, Inc.
Liver and Gall Bladder
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Functional unit is lobule
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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
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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
Copyright 2010, John Wiley & Sons, Inc.
Bile
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Functions of bile
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Formation and recycling of bile
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Emulsification: breaking apart clusters of fats so
they are more digestible
Absorption of fats
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
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Obstruct bile ducts from gallbladder  pain
Copyright 2010, John Wiley & Sons, Inc.
Liver, Gallbladder, Duodenum
Copyright 2010, John Wiley & Sons, Inc.
Liver
Copyright 2010, John Wiley & Sons, Inc.
Liver
Copyright 2010, John Wiley & Sons, Inc.
Liver Functions
1. Carbohydrate metabolism
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Polysaccharide stored in liver as glycogen
Converts glycogen, fructose, galactose, lactic
acid, amino acids  glucose to  blood glucose
2. Lipid metabolism
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Produces cholesterol, triglycerides; makes bile
Makes lipoproteins for lipid transport
3. Protein metabolism
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Remove NH2 from amino acids  ammonia
(NH3)  urea  to kidneys (urine)
Synthesize most plasma proteins: albumin
Copyright 2010, John Wiley & Sons, Inc.
Liver Functions
4. Removes many harmful substances from
blood
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Detoxifies alcohol
Inactivates steroid and thyroid hormones
Eliminates some drugs (like penicillin) into bile
5. Excretion of bilirubin
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From heme (in RBCs) to bile  feces
6. Stores fat-soluble vitamins (ADEK) and
minerals (Fe, Cu)
7. Activates vitamin D
Copyright 2010, John Wiley & Sons, Inc.
Small Intestine
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Length
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Three major regions: duodenum, jejunum,
ileum
Functions
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10 feet long in living person
Extends from pylorus of stomach to cecum of
large intestine
Site of most of digestion
Essentially all nutrient absorption occurs here
Ends in ileocecal sphincter (in RLQ)
Copyright 2010, John Wiley & Sons, Inc.
Small Intestine
Copyright 2010, John Wiley & Sons, Inc.
Small Intestine
Copyright 2010, John Wiley & Sons, Inc.
Intestinal Wall Structure
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Same 4 layers but with modifications
Epithelium in mucosa: simple columnar
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Intestinal glands secrete
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Absorptive cells with microvilli
Goblet cells: secrete mucus
Enzymes that complete digestion
Secretin, cholecystokinin (CCK), glucosedependent insulinotropic peptide (GIP)
Lymphatic tissue within wall: defense
Copyright 2010, John Wiley & Sons, Inc.
Intestinal Wall Structure
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Submucosa has duodenal glands 
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Circular folds
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Alkaline mucus  helps neutralize stomach acid
In mucosa and submucosa; increase surface area
Villi: fingerlike projections of mucosa
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Increase absorptive surface area
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Microvilli on absorptive cells further enhance absorption
Contain vessels that absorb nutrients:
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Arteriole, capillary, venule
Lacteal (lymph capillary) for lipid absorption
Copyright 2010, John Wiley & Sons, Inc.
Intestinal Wall Structure
Copyright 2010, John Wiley & Sons, Inc.
Digestion in Small Intestine
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Mechanical digestion
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Segmentation activity: for mixing
Peristalsis for movement of intestinal contents
after most absorption completed: slow waves
Chemical digestion: 2 L/d of secretions
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Alkaline chyme due to bicarbonate
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From pancreas and alkaline mucus from small intestine
Enzymes produced by cells on villi
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Peptidases: breaks small peptides
Disaccharidases: sucrase, lactase, and galactase
Copyright 2010, John Wiley & Sons, Inc.
Absorption in the Small Intestine
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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
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Monosaccharides; amino acids
Fatty acids and monoglycerides
Phosphate sugar, and bases of DNA, RNA
Copyright 2010, John Wiley & Sons, Inc.
Summary: Carbohydrate Digestion
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Amylases (salivary and pancreatic):
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Starch and dextrin  maltose
Disaccharidases (from small intestine):
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Maltose: maltose  glucose + glucose
Lactase: lactose  glucose + galactose
Sucrase: sucrose  glucose + fructose
Copyright 2010, John Wiley & Sons, Inc.
Protein and Fat Digestion
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Pepsin, trypsin, chymotrypsin, and
carboxypeptidase
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Peptidases at surface:
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Proteins small peptides
Peptides  amino acids, dipeptides, and
tri-peptides
Lipase (pancreatic)
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Triglyceridesfatty acids + monoglycerides
Copyright 2010, John Wiley & Sons, Inc.
Absorption of Products of Digestion
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By diffusion, facilitated diffusion, osmosis and
active transport
Carbohydrates monosaccharides
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Proteins (jejunum + ileum)  amino acids
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Via portal system (blood) to liver
Via portal system (blood) to liver
Lipids 
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Short-chained fatty acids or monoglycerides or 
blood in villi
Larger lipids coated by proteins in chlyomicrons
 lacteals  lymphatics (lymph)  then blood
Copyright 2010, John Wiley & Sons, Inc.
Absorption of Products of Digestion
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Water and salt
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Primarily osmotic movement that accompanies
other nutrients
Vitamins
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Fat-soluble (A, D, E, K) absorbed with fat
Water-soluble (B’s, C) with simple diffusion
B12
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Combines with intrinsic factor for transport through
duodenum and jejunum
Finally can be absorbed by active transport in ileum
Copyright 2010, John Wiley & Sons, Inc.
Absorption of Products of Digestion
Copyright 2010, John Wiley & Sons, Inc.
Absorption of Products of Digestion
Copyright 2010, John Wiley & Sons, Inc.
Large Intestine
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Structure: 4 regions
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Cecum
 Ileocecal sphincter
 Appendix attached
Colon: ascending, transverse, descending and
sigmoid
Rectum
Anal canal with sphincters
Wall: standard 4 layers
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Mucosa: goblet cells secrete mucus
Muscularis: incomplete longitudinal layer
Copyright 2010, John Wiley & Sons, Inc.
Large Intestine
Copyright 2010, John Wiley & Sons, Inc.
Large Intestine
Copyright 2010, John Wiley & Sons, Inc.
Large Intestine
Copyright 2010, John Wiley & Sons, Inc.
Digestion and Absorption
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Ileocecal sphincter limits rate of emptying of
ileum
Slow peristalsis
Mass peristalsis
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Triggered by presence of food in stomach
Wastes move from mid-colon  rectum
Bacterial digestion
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Produce some B-vitamins + vitamin K
Produce gases: flatus
Colon absorbs salt + water
Copyright 2010, John Wiley & Sons, Inc.
Defecation Reflex
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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
Copyright 2010, John Wiley & Sons, Inc.
Control: Phases of Digestion
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Rule: activate forward and inhibit behind
Three phases: cephalic, gastric, intestinal
1. Cephalic: smell, sight, thought of food 
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Cranial nerves VII + IX stimulate salivary glands
Cranial nerve X (vagus) stimulates gastric glands
2. Gastric: stretching, pH of stomach 
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Gastrin activates stomach and relaxes pyloric sphincter
3. Intestinal phase: intestinal hormones play key
roles
Copyright 2010, John Wiley & Sons, Inc.
Control: Phases of Digestion
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Secretin
Released when acidic chyme enters intestine
 Stimulates release of pancreatic juice high in
bicarbonate to buffer acidic chyme from stomach
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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

Copyright 2010, John Wiley & Sons, Inc.
Aging
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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
Copyright 2010, John Wiley & Sons, Inc.
End of Chapter 19

Copyright 2010 John Wiley & Sons, Inc.
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Copyright 2010, John Wiley & Sons, Inc.