The Digestive System

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Chapter 23

• The Digestive System

G.R. Pitts, Ph.D., J.R. Schiller, Ph.D. and
James F. Thompson, Ph.D.
2
Digestive Processes

• Ingestion
• Movement of food
• Digestion
• Mechanical digestion
• Chemical digestion
• Absorption
• Defecation

3
General Structure

• Digestive organs divided into 2 main groups
• GI (alimentary) tract
• Accessory structures
• cheeks, teeth, tongue, salivary glands
• liver, gallbladder, pancreas

4
Salivary Glands

• 3 pairs salivary glands
• Parotid glands
• Submandibular glands
• Sublingual glands

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Salivary Glands

• Composition of Saliva
• 99.5 water, 0.5 solutes
• Na, K, Cl-, HCO3-, and PO4-, proteins,
• waste products
• lysozyme
• salivary amylase digests carbohydrates
• Saliva composition differs among the 3 glands
• parotid - watery saliva, amylase
• submandibular - thicker mucous, amylase
• sublingual - mostly mucous, a little amylase

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Salivary Glands

• Functions of Saliva
• Water dissolves food for taste and digestion
• Mucous moistens and lubricates food
• Mucous lubricates oral surfaces for smooth
  actions in swallowing and speech
• Cl- ions activate amylase
• HCO3- and PO4- ions buffer bacterial acids
• IgA, lysozymes, cyanide, defensins protect
  against microorganisms

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Salivary Glands

• Secretion of Saliva - 1-1.5 L l day
• Primarily under nervous control
• Parasympathetic (ANS) ? normal salivary
  secretions
• saliva swallowed
• most reabsorbed
• Sympathetic (ANS) ? reduced flow (dry mouth)
• Food (mechanically, chemically) stimulates
  salivation
• behavioral memories from cortex
• starts digestion
• continues after ingestion is complete
• irritating foods or nausea

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Physiology of Digestion in Mouth

• Mechanical digestion
• Chewing mastication
• Food mixed with saliva
• Shaped into a bolus
• Chemical digestion salivary amylase breaks down
  and converts polysaccharides (starches) to
  disaccharides (maltose) and monosaccharides
  (glucose) no enzymatic action with cellulose
  which is also a polymer of glucose

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Physiology of Deglutition (Swallowing)

• Moving bolus from mouth to stomach
• Three phases
• Facilitated by saliva, mucous secretions
• Involves mouth, pharynx, esophagus
• Buccal phase
• Voluntary
• Moves bolus to oropharynx

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Physiology of Deglutition

• Pharyngeal phase
• Involuntary
• Receptors in oropharynx stimulate medulla and
  pons to
• Block mouth with tongue
• Block nasopharynx with soft palate
• Raise larynx to seal epiglottis, blocking airways
• Relax upper esophageal sphincter
• Bolus is moved through pharynx into esophagus

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Physiology of Deglutition

• Esophageal stage
• Upper esophageal sphincter closes
• Gastroesopaheal sphincter opens
• Esophagus controls involuntary peristaltic
  movement
• Epiglottis reopens
• Bolus moves from esophagus to stomach

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Esophagus

• Peristalsis
• Involuntary, rhythmic contraction of muscularis
• Controlled by medullary centers
• A movement activity inner circular layer of
  smooth muscle contracts behind bolus to push it
  forward outer longitudinal muscle contracts to
  pull esophagus wall up

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Esophagus

• Physiology
• Upper esophageal sphincter
• Peristalsis
• Lower esophageal (cardiac) sphincter
• Sharp transition from nonkeratinized stratified
  squamous epithelium to simple columnar epithilium
• Esophageal epithelium resistant to abrasion but
  not to acid and proteolytic enzyme attack acid
  reflux disease

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Stomach

• Physiology of digestion - Mechanical digestion
• peristaltic movement (mixing waves) back and
  forth between body and pylorus
• 3 muscle layers longitudinal, circular, and
  oblique
• chyme

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Stomach

• Physiology of digestion - Chemical digestion
• parietal cells secrete intrinsic factor for B12
  absorption
• parietal cells secrete HCl by active transport
• kills microbes, denatures proteins
• causes some acid hydrolysis of food molecules
• stimulates secretion of hormones for bile
  pancreatic juice flow
• chief cells secrete pepsinogen (inactive
  precursor)
• activated to pepsin by HCl acid and by other
  pepsins
• only an effective protease at acid pH
• cleaves proteins into smaller peptides

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Stomach Mucosa

• Gastric gland chief cells
• Secrete pepsinogen (inactive precursor)
• activated to pepsin by HCl acid and by other
  activated pepsin enzymes
• only an effective protease at acid pH
• cleaves proteins into smaller peptides
• Secrete rennin in neonates
• curdles milk to increase time for gastric
  processing
• Secrete gastric lipase in neonates
• splits short chain triglycerides common in milk
• limited role in digestion since it works best at
  pH 5-6

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Stomach Mucosa

• Simple columnar epithelium with goblet cells and
  gastric pits
• Secretes 2-3 L l day
• Gastric gland parietal cells
• Secrete intrinsic factor for B12 absorption
• Secrete HCl by active transport
• kills microbes, denatures proteins
• causes some acid hydrolysis of food molecules
• stimulates secretion of hormones for bile
  pancreatic juice flow
• Goblet gastric pit mucous cells secrete mucin
• 1-3 mm mucus layer in the stomach prevents
  self-digestion

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Stomach Mucosa

• Gastric gland G cells (enteroendocrine)
• Secrete gastrin, histamine, serotonin,
  somatostatin
• Absorption
• Impermeable to diffusion of most molecules into
  the bloodstream
• Absorbs a few lipid soluble compounds
• certain drugs (e,g., aspirin)
• alcohol

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Stomach Regulation of Secretion and Motility

• Regulated by combination of neuronal and hormonal
  factors
• 3 phases
• Cephalic
• Gastric
• Intestinal

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Stomach Regulation of Secretion and Motility

• Cephalic phase
• Stimuli
• sight
• smell
• taste
• thoughts/memories
• Effect
• Parasympathetic impulses increase gastric
  secretion

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Stomach Regulation of Secretion and Motility

• Gastric phase
• Neural negative feedback mechanisms
• Distension activates stretch receptors causing
  myenteric and vagovagal reflexes to release Ach
• Ach stimulates gastric juice secretion
• Chemoreceptors respond to partially digested
  proteins, caffeine and rising pH
• Stimulate gastrin secretion from G cells

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Stomach Regulation of Secretion and Motility

• Gastric phase (cont.)
• Gastrin
• Inhibited at pH lt 2
• Gastrin transported in the blood to the gastric
  glands
• Greatly stimulates HCl secretion
• Stimulates histamine secretion
• Slightly stimulates pepsinogen secretion
• Contracts lower esophageal sphincter
• Increases gastric motility
• Relaxes pyloric sphincter

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Stomach
KHCO3

• Gastric phase (continued)
• Control of HCl secreting parietal cells
• stimulation by three signal chemicals
• gastrin
• acetylcholine
• histamine
• All three needed for strong H secretion
• H pumps work in conjunction with carbonic
  anhydrase
• blockage of the histamine H2 receptor decreases
  HCl secretion
• Tagamet
• Zantac

HCl
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Stomach Regulation of Secretion and Motility

• Intestinal phase has excitatory and inhibitory
  components
• Excitatory
• Very short phase
• Initiated by chyme entry into duodenum
• Stretch receptors stimulate release of intestinal
  (enteric) gastrin
• Chemoreceptors detect fatty acids, glucose in
  the duodenum
• Stimulate enteric gastrin release

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Stomach Regulation of Secretion and Motility

• Intestinal phase (cont.)
• Inhibitory
• Enterogastric reflex stretch receptors,
  chemoreceptors trigger 3 reflexes that
• Inhibit vagoval reflex
• Inhibit myenteric reflex
• Activate sympathetic nervous system to close
  pyloric sphincter
• Inhibit gastric secretion
• Enterogastrone secretion
• Enteroendocrine cells in the small intestine
  release
• Cholecystokinin (CCK)
• Gastric inhibitory peptide (GIP)
• Secretin
• Vasoactive intestinal peptide (VIP)
• Hormones inhibit gastric secretion

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Stomach Regulation of Gastric Emptying

• Food normally passes through stomach in 4 hours
• Hormonal/neuronal reflexes regulate gastric
  emptying
• Large meals and large amounts of liquid increase
  stomach distension ? increasing rate of emptying
• Stomach emptying inhibited by the enterogastric
  reflex, enterogastrones, and fat in the duodenum

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Stomach
Summary
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Pancreas

• Pancreatic juice
• 1.2-1.5 L/day
• Mostly water some salts, bicarbonate, enzymes
• alkaline, pH 7.1-8.2
• buffers acidic gastric juice, stops pepsin
  activity, creates proper alkaline pH for enzymes
  acting in the intestine
• Enzymes include
• pancreatic amylase
• trypsinogen, chymotrypsinogen, procarboxypeptidase
  (inactive zymogens)
• pancreatic lipase
• ribonuclease and deoxyribonuclease

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Regulation of Pancreatic Secretion

• Neural control from parasympathetic division of
  ANS via vagus nerve
• Autoregulation by sensing the presence of fatty
  acids and amino acids in the acidic chyme
• Hormonal control by the secretion of
  enteroendocrines from duodenum
• Secretin stimulates secretion of water, HCO3-
• CCK stimulates secretion of enzymes

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Liver Blood Supply

• Two sources
• Hepatic artery - oxygenated blood from aorta
• Hepatic portal vein -deoxygenated blood
• absorbed nutrients and toxins from the stomach
  and intestines
• hormones from the pancreas
• breakdown products of RBCs from the spleen
• Blood mixes in the sinusoids
• Hepatocytes (liver cells) modify and exchange
  molecules with the blood

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Liver
Central veins return blood to the systemic
circulation via the hepatic vein and inferior
vena cava
Note the portal triads and fenestrated capillary
sinusoids
hepatic portal venous blood and arterial blood
mix in the sinusoids
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Liver

• Sinusoids are patrolled by monocyte-derived
  stellate reticuloendothelial (Kupffer's) cells
• Phagocytize damaged blood cells, bacteria, and
  other microbes
• Phagocytize some foreign molecules such as toxins
• Hepatocytes can also store or breakdown toxic
  molecules

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Liver

• Hepatocytes
• Receive, process and store nutrients from meals
• Process heme and cholesterol breakdown products
  to make bile components

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Liver Bile Secretion

• Bile from the hepatocytes enters bile capillaries
  (canaliculi)
• Canaliculi empty into small bile ducts
• Hepatic ducts join the cystic duct from the
  gallbladder to form the common bile duct
• Gallbladder stores bile
• Common bile duct meets pancreatic duct at the
  hepatopancreatic ampulla (of Vater)

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Liver

• Bile
• 800-1000 ml/day
• Yellow, brownish, or olive-green liquid
• pH 7.6-8.6, mostly water, bile salts, bile acids,
  cholesterol, lecithin (phospholipid), bile
  pigments, ions
• Part digestive secretion, part excretory product
• bile salts help in emulsification of ingested
  fats
• bilirubin and other bile pigments are wastes from
  lipid catabolism

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Liver Bile Synthesis

• Regulation of bile production/secretion
• nervous control from parasympathetic division of
  ANS via vagus nerve
• autoregulation by sensing the resence of fatty
  acids and amino acids in the acidic chyme
• hormonal control by the secretion of the
  enteroendocrines, CCK and secretin, from the
  duodenum

Cholecystokinin gall bladder moves
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Liver

• Physiology of the liver processes vital to life
• Carbohydrate metabolism regulates blood glucose
  levels
• glycogenesis (insulin)
• glycogenolysis (glucagon)
• gluconeogenesis (glucagon)
• Lipid metabolism -
• stores, metabolizes some triglycerides
• synthesizes new cholesterol
• degrades excess cholesterol for bile salt
  production
• Protein metabolism -
• deaminates AAs by removing amino groups (-NH2)
  from AAs
• deaminated AA's used for ATP production or
  changed to carbohydrates or fats as needed
• detoxifies ammonia (NH3) by synthesizing urea (1
  CO2 2 NH3 urea)
• can convert AA's from one to another
  (transamination)
• synthesizes and secretes most plasma proteins

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Liver

• Physiology of the liver - processes vital to life
  
• Storage oil-soluble vitamins, iron, other
  nutrients and minerals
• Phagocytosis
• Removal of dietary toxins, hormones, drugs
• detoxify or store or secrete compounds into bile
• metabolize thyroid, steroid hormones
• Synthesis of bile salts
• Excretion of bile - bilirubin
• Activation of Vitamin D (?)
• Pathologies of the liver hepatitis (viral,
  toxic), cirrhosis, cancer

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Gall Bladder

• Pear-shaped sac, 7-10 cm long
• Physiology
• stores and concentrates bile between meals
• CCK stimulates bile release for fatty meals
• when the small intestine is empty, the
  hepatopancreatic sphincter closes, forcing bile
  into the gallbladder for storage

Pathology gallstones
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Summary Digestive Hormonesenteroendocrines

• Gastrin
• Gastric Inhibitory Peptide
• Secretin
• Cholecystokinin
• (There are others.)

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Small Intestine Segmentation

• primary action of small intestine when food is
  present
• a form of mechanical digestion
• a mixing activity
• alternate contraction, relaxation of antagonistic
  smooth (circular and longitudinal) muscle
  segments in the intestine
• controlled by the autonomic nervous system

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Small Intestine Peristalsis

• as absorption continues, distension decreases and
  true peristalsis starts
• a movement which propels chyme onward
• these weak movements which occur only after most
  nutrients have been absorbed

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Small Intestine Motility and Secretion

• Intestinal secretions
• 1-2 L/day, pH 7.6
• mostly water and mucus
• bicarbonate buffer neutralizes gastric acid
• provide enzymes for final chemical digestion
• Regulation of intestinal secretion and motility
• stimulated by distension and acidic chyme
• local reflexes increase Ach release
• VIP stimulates production of intestinal
  secretions
• basal motility is controlled by autorhythmic
  pacemakers
• local hormones and parasympathetic ANS reflexes
  increase motility

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Small Intestine Chemical Digestion

• Intestinal secretions
• 1-2 L/day, pH 7.6
• mostly water and mucus
• along with pancreatic secretions provide acid
  neutralization, final chemical digestion, and
  more water for absorption
• Brush border enzymes
• brush border enzymes complete digestion of
  protein and carbohydrate molecules

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Small Intestine Chemical Digestion

• Brush border enzymes
• Enteropeptidase (enterokinase) converts
  trypsinogen to trypsin
• Trypsin activates other zymogens
• Various other brush border enzymes complete
  digestion of protein and carbohydrate molecules

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Small Intestine Chemical Digestion

• Complete digestion is a function of bile,
  pancreatic secretions and intestinal secretions
• Although produced by different organs, they all
  function in the small intestine
• Prior to small intestine, only limited activity
• mouth salivary amylase
• stomach
• pepsin
• lingual lipase

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Small Intestine Chemical Digestion

• Chemical digestion in the small intestine
• Carbohydrate digestion
• pancreatic amylase digests starches
• disaccharidases liberate monosaccharides
• Protein digestion
• pancreatic proteases (trypsin, chymotrypsin,
  carboxypeptidase)
• finished by brush border proteases in the lining
  epithelium
• Lipid digestion
• bile salts for emulsification
• pancreatic lipase
• Nucleic acid digestion
• pancreatic ribonuclease and deoxyribonuclease
• brush border enzymes digest nucleotides

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Small Intestine Absorption

• About 90 of all absorption takes place in small
  intestine
• Nutrient absorption
• Monosaccharides facilitated diffusion and
  Na-driven secondary active transport
• Amino acids primary and Na-driven secondary
  active transport
• Di- and tripeptides H-driven secondary active
  transport

• Nutrients enter capillaries via diffusion,
  facilitated diffusion, or active transport
• Nutrients are transported in the blood to the
  liver via the hepatic portal circulation

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Small Intestine Nutrient Absorption

• Electrolytes (minerals)
• Na Primary active transport
• K facilitated diffusion
• Fe Active transport
• Ca2 Active transport, vitamin D is a cofactor
• Vitamins
• Water-soluble vitamins (B complex C) absorbed
  by diffusion - B12 absorbed with intrinsic
  factor
• Fat-soluble vitamins (A, D, E, K) included with
  other lipids in micelles/chylomicrons

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Small Intestine Nutrient Absorption

• Lipids (neutral fats, cholesterol, phospholipids,
  etc.) are emulsified by bile salts, forming
  micelles
• Pancreatic Lipase breaks triglycerides into 2
  fatty acids and 1 monoglyceride
• monoglycerides, fatty acids and other lipids
  diffuse into cells
• SER re-synthesizes triglycerides
• all lipids packaged into chylomicrons by Golgi
  apparatus
• chylomicrons leave the cell by exocytosis and
  enter lacteals of the lymphatic system

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Small Intestine Water Absorption

• Total volume added to the small intestine/day -
  9.3 L
• 2.3 L from ingestion
• 7.0 L from secretions
• Small intestine absorbs 8.3 L /day
• passive absorption following nutrient molecules
• osmosis
• The rest of the water (1.0L/day) passes to large
  intestine where most is reabsorbed (0.9 L/day)

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Large Intestine

• Functions
• Completion of absorption, especially final
  absorption of H2O
• Normal flora manufacture certain vitamins (B
  complex, K)
• Formation and expulsion of feces
• Anatomy
• 1.5 m L, 6.5 cm W
• Divided into 4 general areas
• cecum
• colon
• rectum
• anal canal

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Large Intestine Digestion

• Mechanical digestion
• Chyme passage regulated by ileocecal sphincter
• valve generally closed - slow passage
• following a meal ? gastroileal reflex ileal
  motility increases, sphincter relaxes, chyme
  moves to the cecum
• when the cecum is full, the sphincter contracts
• Colon movements start when chyme passes sphincter
• haustral churning
• haustra relaxed, distended until full
• then contract, squeeze contents into next
  haustrum
• peristalsis is slow
• mass peristalsis (gastrocolic reflex)
• during or immediately following a meal, 3-4 times
  day
• strong peristaltic waves from middle of
  transverse colon
• push contents into the rectum

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Large Intestine Digestion

• Chemical digestion
• much mucus but no enzymes are secreted
• some digestion of chyme by bacteria in colon
• final breakdown of substances, mostly
  carbohydrates
• bacteria produce some vitamins, B complex and K
• some bacterial metabolites are toxic, but the
  liver normally deals successfully with them

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Large Intestine

• Absorption and feces formation
• Chyme
• after 3-10 hours in the large intestine, chyme
  becomes solidified (due to water reabsorption)
  into feces
• large intestine absorbs water, electrolytes, some
  vitamins and any toxins
• Feces
• water, inorganic salts, sloughed off intestinal
  epithelial cells, bacteria, products of bacterial
  decomposition, undigested parts of food
• most water is reabsorbed in small intestine, but
  the large intestine is also important in water
  reabsorption

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Large Intestine

• Physiology of defecation
• Mass peristalsis
• pushes fecal matter into rectum
• distension stimulates stretch
• receptors initiating reflex for
• defecation
• Parasympathetic ANS
• stimulated by stretch receptors
• stimulates contraction of rectum
• shortens and increases pressure in
• rectum
• parasympathetic stimulation relaxes internal
  sphincter
• Conscious stimulation relaxes external sphincter
• feces expelled

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Large Intestine

• Physiology of defecation
• Mass peristalsis
• pushes fecal matter into rectum
• distension stimulates stretch receptors
  initiating reflex for defecation
• Parasympathetic ANS stimulated by stretch
  receptors and voluntary commands contribute
• stimulates contraction of rectum
• shortens and increases pressure in rectum
• parasypathetic stimulation relaxes internal
  sphincter
• conscious stimulation relaxes external sphincter
• feces expelled

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Large Intestine

• Pathology of the intestines
• diarrhea/dysentery
• enteritis
• appendicitis
• constipation/fecal impaction
• diverticulosis
• Crohns disease/ulcerative colitis
• polyps/colon cancer

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End Chapter 23