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Chapter The Digestive System
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The Gall bladder Hollow, pear-shaped organ
Stores, modifies and concentrates bile Contraction of the gall bladder and release of bile is controlled by CCK and vagal stimulation Empties into the Cystic duct Gall stones
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The Gallbladder
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Bile Made of water, billirubin, ions, cholesterol, other lipids
Acts as an emulsifier to aid lipid digestion 90% is reabsorbed in the ileum - enterohepatic circulation of bile
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Functions of the large intestine
Reabsorb water and compact material into feces 1500 mL chyme enter the cecum, 90% of volume reabsorbed yielding mL of feces Absorb vitamins produced by bacteria Store fecal matter prior to defecation
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General anatomy of the large intestine
Lies inferior to the stomach Frames the small intestine Parts of the large intestine Cecum Colon Ascending Transverse Descending Sigmoid Rectum Anal canal
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The Large Intestine
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Cecum and Rectum Cecum opening protected by ileocecal valve
veriform appendix Rectum Last portion of the digestive tract Terminates at the anal canal Internal anal sphincter - involuntary smooth muscle External anal sphincter - voluntary muscle Hemorrhoids -inflamed veins
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Rectum
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Histology of the large intestine
Columnar cells except the rectum and anal canal (stratified squamous) Muscularis externa circular muscle only 3 bands longitudinal smooth muscle - taeniae coli Absence of villi Presence of goblet cells Deep intestinal glands
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Physiology of the large intestine
Absorption in the large intestine includes: Water Vitamins – K, biotin, and B5 Organic wastes – urobilinogens and sterobilinogens Bile salts Some ammonia and other toxins Mass movements of material through colon and rectum Defecation reflex triggered by distention of rectal walls
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Movement in Large Intestine
Mass movements Common after meals Integrated by the enteric plexus Local reflexes instigated by the presence of food in the stomach and duodenum Gastrocolic reflex: initiated by stomach Duodenocolic reflex: initiated by duodenum
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Long reflexes of defecation
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Coordination secretion and absorption
Neural and hormonal mechanisms coordinate glands GI activity stimulated by parasympathetic innervation Inhibited by sympathetic innervation Enterogastric, gastroenteric and gastroileal reflexes coordinate stomach and intestines Several hormones alter motility and secretion
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Summary Hormone Table GIP duodenum fats, carbohydrates stim pancreas insulin secretion, inhibits stomach, stim adipose glucose and FA uptake & lipogenesis Marieb Table
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Summary Hormone Table (continued)
Marieb Table
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Activities of Major Digestive Tract Hormones
From Martini Figure 24.22
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Digestion, Absorption, Transport
Disassembles organic food into smaller fragments Mechanical: breaks large food particles to small Chemical: breaking of covalent bonds by digestive enzymes Hydrolyzes carbohydrates, proteins, lipids and nucleic acids for absorption Absorption and transport Molecules are moved out of digestive tract and into circulation for distribution throughout body
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Carbohydrate digestion and absorption
Begins in the mouth (salivary amylase) completed in small intestine Salivary and pancreatic amylase Convert starches to disaccharides, trisaccharides, monosaccharides Brush border enzymes Make monosaccharides from disaccharides maltase (glucose / glucose) sucrase (glucose / fructose) lactase (glucose / galactose) Absorption of monosaccharides occurs across the intestinal epithelia by facilitaed diffusion or Na+ linked co-transporters
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Lipid digestion and absorption
Begins in stomach (minor) completed in small intestine Lipid digestion utilizes lingual and pancreatic lipases converts triglycerides to monoglycerides Bile salts improve chemical digestion by emulsifying lipid drops Lipid-bile salt complexes called micelles are formed Micelles diffuse into intestinal epithelia which re-synthesis triglycerides and then release lipids into the circulation as chylomicrons - mixtures of phopholipids and proteins carried by lymphatic system
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Lipid Absorption also (see Saladin fig 25.30)
from Seeley, Stephens and Tate
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Protein digestion and absorption
(See Saladin Fig 25.29) In the stomach, the low pH destroys tertiary and quaternary structure allows pepsin to digest specific peptide bonds In the small intestine, pancreatic enzymes and intestinal brush border peptidases work at pH 7-8 Trypsin - breaks bonds next to arginine or lysine Chymotrypsin- breaks bonds next to phenylalanine or tyrosine Liberated amino acids, and some dipeptides are absorbed through by several different carrier proteins via by facilitaed diffusion or Na+ linked co-transporters
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Absorption of other molecules
Water Nearly all that is ingested is reabsorbed via osmosis Ions Absorbed via diffusion, cotransport, and active transport Vitamins Water soluble vitamins (C and B vitamins) are absorbed by diffusion Vitamin B12 requires intrinsic factor Fat soluble vitamins (A,D, E, K) are absorbed as part of micelles
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Water Intake, Secretion and Absorption
Diarrhea Constipation from Seeley, Stephens and Tate
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Great Summary slide (a)
Marieb Figure 23.33a
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Great Summary slide (b)
Marieb Figure 23.33b
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