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16.1 I. digestive processes A. secretion 1. controlled by nervous and endocrine systems 2. general components: water, ions, mucus, enzymes, special materials 3. most water and electrolytes later reabsorbed B. motility smooth muscle cells are connected to each other by gap junctions when stimulated they contract as a unit contraction is caused by intrinsic and extrinsic (ANS, hormones) factors 1. tone = constant partial contraction 2. mixing – increases rate of digestion and absorption 3. propulsion – moves contents through gut peristalsis = wavelike muscular contraction bolus of food initiates localized muscle responses contraction occurs behind bolus relaxation occurs ahead of bolus C. digestion = breaking ingested food into molecules small enough to be absorbed 1. mechanical – breaks food into smaller pieces increases surface area for enzymes 2. chemical – uses enzymes to break chemical bonds in nutrient molecules BIOL 2305/Strong/Spring 2008 16.2 (hydrolysis) D. absorption – molecules are moved from the gut lumen, through the intestinal epithelium and into vascular or lymphatic capillaries. II. regulation local vs systemic neural vs endocrine feed-forward vs feed-back A. GI hormones gastrin secretin cholecystokinin (CCK) glucose-dependent insulinotropic peptide (GIP) 1. release stimulated by: local chemical signals local mechanical signals ANS 2. targets are smooth muscle and exocrine glands B. digestive reflexes BIOL 2305/Strong/Spring 2008 16.3 receptors: chemo-, mechano-, or osmoeffectors: smooth muscle, exocrine glands, endocrine glands 1. short reflexes - entire reflex arc is located in gut: integrating center is enteric nervous system 2. long reflexes - autonomic reflexes with integrating centers in the CNS C. feedback vs feedforward feedback is a response in reaction to a change feedforward is a response in anticipation of a change III. Mouth A. saliva 1. volume = 1 to 2 liters/day 2. composition a. enzymes: salivary amylase substrate = polysaccharides (starch) end products = disaccharides (maltose) optimal pH range = 6.6 to 6.8 b. mucus - moistens and lubricates food c. lysozyme - antibacterial B. mechanical digestion = chewing/mastication C. propulsion = swallowing BIOL 2305/Strong/Spring 2008 16.4 reflex initiated by voluntarily moving a bolus of food into the pharynx with the tongue 1. receptors = pressure receptors in pharynx 2. integrating center = medulla oblongata a. oropharyngeal phase soft palate elevated larynx elevated; epiglottis covers glottis respiration inhibited pharyngeal muscles contract b. esophageal phase pharyngoesophageal sphincter relaxes peristalsis in esophagus gastroesophageal (cardiac) sphincter relaxes IV. Stomach A. stomach function = storage up to 1 liter B. gastric juice 1. volume = 2 liters / day 2. composition a. mucus - protects mucosa from enzymes and acid b. pepsinogen = inactive form of protease pepsin activated by HCl or pepsin substrate = proteins end product = peptides optimal pH range = c. HCl: denatures proteins, facilitates hydrolysis, activates pepsin d. intrinsic factor - binds to Vit. B12 and causes it to be absorbed in the ileum e. gastrin - increases gastric activity 3. control of gastric secretion a. cephalic phase - before food reaches stomach stimuli: food in mouth, chewing, thought of food (act via ANS/vagus nerve) BIOL 2305/Strong/Spring 2008 16.5 effectors: gastric mucosa response: increased secretion of gastric juice b. gastric phase - caused by food in stomach stimuli: peptides, distention (act via: short and long reflexes, gastrin) effectors: gastric mucosa response: increased secretion of gastric juice c. intestinal phase - caused by food in intestine stimuli: distention, fats, acid, hypertonicity (act via: short & long reflexes, hormones) effectors: gastric mucosa response: decreased secretion of gastric juice enterogastrones = hormones produced by the intestine that inhibit stomach activity enterogastric reflex = reflex inhibition of stomach activity by small intestine C. mechanical digestion peristaltic contractions in antrum mix food with gastric juice to make chyme D. propulsion and emptying 1. peristaltic contractions in antrum propel chyme into duodenum when pyloric sphincter is open 2. factors that affect rate of emptying: a. gastric distention - increases emptying by increasing gastric muscle excitability, stimulating short and long reflexes and increasing gastric secretion b. fluidity of chyme - increases emptying c. intestinal distention - decreases emptying by initiating enterogastric reflex and enterogastrone secretion d. fat or acid in intestine - decreases emptying by initiating enterogastric reflex and enterogastrone secretion V. Pancreas A. acini (exocrine tissue) produce pancreatic juice, volume = 1 to 2 liters / day B. pancreatic juice composition BIOL 2305/Strong/Spring 2008 16.6 1. HCO3 - neutralizes stomach acid 2. enzymes: optimal pH range = a. proteases: inactive form trypsinogen chymotrypsinogen procarboxypeptidase substrate = peptides end products = smaller peptides active form trypsin chymotrypsin carboxypeptidase b. amylase substrate = polysaccharides end products = disaccharides c. lipase substrate = triglycerides end products = fatty acids, monoglycerides C. control 1. acidic chyme in duodenum release of secretin from intestinal mucosa increase HCO3 content of pancreatic juice 2. fat and peptides in chyme in duodenum release of CCK from intestinal mucosa increase enzyme content of pancreatic juice VI. Liver and Gall Bladder A. functions of liver 1. metabolic processing of nutrients 2. detoxification of wastes and hormones 3. synthesis of plasma proteins BIOL 2305/Strong/Spring 2008 16.7 4. 5. 6. 7. 8. storage of glycogen, fats, Fe, Cu, vitamins activation of Vit. D phagocytosis of bacteria and RBCs excretion of cholesterol and bilirubin bile synthesis B. bile - produced by hepatocytes; 250 ml to 1 liter / day 1. composition water HCO3 bile salts cholesterol bilirubin lecithin 2. bile salts polar and nonpolar ends derived from cholesterol recycled via enterohepatic circulation emulsify fats in chyme to increase surface area for lipase 3. secretion and storage of bile a. bile is secreted at constant rate into bile ducts if hepatopancreatic sphincter is open, bile enters duodenum if hepatopancreatic sphincter is closed, bile backs up through cystic duct into gall bladder b. gall bladder removes water; concentrates bile 4. control of secretion and release of bile a. choleretic: bile salts from the enterohepatic circulation increase bile synthesis and release b. secretin: increases bile and HCO3 secretion c. parasympathetic: increases synthesis and release d. CCK: causes gall bladder contraction and sphincter of Oddi relaxation VII. Small Intestine A. function = digestion and absorption B. intestinal juice - 1.5 liters / day; water and electrolytes, no enzymes C. mechanical digestion BIOL 2305/Strong/Spring 2008 16.8 segmentation = alternating contraction and relaxation in a segment of the gut occurs when stimuli bring smooth muscle cells to threshold at the peaks of their rhythmic membrane potential fluctuations D. chemical digestion - intestinal enzymes are components of the luminal membrane of the intestinal cells 1. disaccharidases (maltase, sucrase, lactase) substrates = disaccharides end products = monosaccharides 2. aminopeptidases substrates = peptides end products = amino acids E. absorption 1. Na a. passive - down conc. gradient b. active - pumped out of cell at basal surface 2. Cl - follows Na passively 3. water - follows Na by osmosis 4. amino acids, glucose, galactose - secondary active transport with Na 5. fructose - facilitated diffusion 6. fats a. bile salts and lecithin form a shell around fat digestion products, cholesterol, and fat-soluble vitamins in the intestinal lumen (micelles) b. when micelles approach mucosal cells, fatty acids, monoglycerides and cholesterol diffuse into cell BIOL 2305/Strong/Spring 2008 16.9 c. in sER, fatty acids and monoglycerides form triglycerides d. triglycerides coated with lipoproteins form chylomicrons e. chylomicrons leave mucosal cell via basal membrane and enter lymphatic capillaries 7. Fe a. Fe pumped into mucosal cells from lumen b. if Fe is needed by the body, transferrin is present in plasma and can pick up Fe from mucosal cell c. unneeded Fe sequestered in non-absorbable form called ferritin d. ferritin lost when mucosal cell sloughed 8. Ca - vitamin D increases uptake 9. vitamin B12 F. propulsion 1. segmentation - propels food during active absorption following a meal 2. migrating motility complex - moves unabsorbed remnants out of small intestine between meals VIII. Large Intestine A. function: converts remaining chyme into feces by removing water and electrolytes B. mixing - haustral contractions increase absorption controlled by intrinsic plexuses (30 min intervals) C. propulsion BIOL 2305/Strong/Spring 2008 16.10 1. mass movements strong peristaltic waves propel feces towards rectum (3-4 times/day) 2. gastrocolic reflex - mass movements are initiated by gastric distention D. defecation reflex 1. stretch receptors in rectum 2. integrating center in sacral spinal cord 3. effectors: a. rectum and sigmoid colon contract (parasympathetic) b. internal anal sphincter relaxes (parasympathetic) c. external sphincter voluntarily relaxes (somatic) BIOL 2305/Strong/Spring 2008