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15 Cards in this Set
- Front
- Back
pancreatic secretion: exocrine (2)
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1) enzymes (released from acinar cells, contains enzymes for protein / fat / carbohydrate digestion but proteases released as pro-enzymes, stored as zymogen granules and released via exocytosis);
2) aqueous (released from epithelial cells of pancreatic ducts, contains HCO3- & water; raises pH to neutralize chyme and stop pepsin action + optimal pH for pancreatic enzymes) |
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pancreatic secretion: endocrine
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insulin & glucagon secreted by islet of langerhans cells
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pancreatic enzymes: activation
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trypsinogen --> [enterokinase] --> trypsin; trypsin converts: chymotrypsinogen --> chymotrypsin; proelastase --> elastase; procarboxypeptidase --> carboxypeptidase
(anti-trypsin by pancreas prevents autodigestion) |
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pancreatic enzymes: functions
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protein digestion: trypsin, chymotrypsin, elastase (breaks peptide bonds) + carboxypeptidase (breaks off terminal AA at carboxyl end of peptide);
carbohydrate digestion: amylase (polysaccharides --> glucose + maltose); lipid digestion: lipase (TG --> 2 FAs + MG) nucleic acid digestion: ribonuclease, deoxyribonuclease |
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control of pancreatic exocrine secretions
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cephalic & gastric phases negligible, mainly by intestinal phase:
1) hormones (secretin from S cells, stimulated by acidic chyme, cause release of HCO3- from ductal cells & CCK from I cells, stimulated by partially digested fat & protein, cause release of pancreatic enzymes and release of bile from gall bladder); 2) nerves: vagus nerve |
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composition of bile (6)
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water, electrolytes, bile salt, bilirubin, cholesterol, phospholipids
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stages of biliary secretion
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1) hepatocytes secrete fluid with bile salt and cholesterol into canaliculi --> bile ducts;
2) as bile flows through ducts, ductal epithelium secretes HCO3- in watery fluid; 3) between meals, bile stored & concentrated in gall bladder (via absorption of water and NaCl); 4) during meals, gall bladder contracts & secretes bile into duodenum |
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bile salt formation
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synthesized from cholesterol by hepatocytes; primary bile acids --> hydroxylation by intestinal bacteria --> secondary bile acids --> conjugation by glycine & taurine --> bile salts;
(cholic acid --> deoxycholic acid; chenodeoxycholic acid --> lithocholic acid) |
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bile salt property: amphipathic
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hydrophobic domain of sterol nucleus + hydrophilic domain of OH- / COO- / CONH
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bile acid / salt function: fat digestion (emulsification)
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emulsification: non-polar portion of bile salt attaches to non-polar lipid droplet, polar side faces out and repels other droplets to prevent aggregation, lipid droplets broken off fat globules by mechanical disruption of intestinal contraction, water-soluble pancreatic lipase can digest at surface of lipid droplet
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bile acid / salt function: fat digestion (micelle formation)
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bile salts, phospholipids, FAs, monoglycerides assemble with polar ends towards water and non-polar towards core; micelles remain in equilibrium to free fat digestion products
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fat absorption
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1) FAs & MGs diffuse into intestinal epithelial cells;
2) reassembled into TGs in ER 3) released as chylomicrons into lacteals --> lymph --> thoracic duct --> systemic veins |
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enterohepatic circulation of bile salts
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~95% bile salts reabsorbed from ileum via hepatic vein to liver where it is absorbed by hepatocytes and re-secreted into bile; 5% lost in feces replenished by synthesis in liver; several cycles in one meal
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functions of biliary secretions (2 +3)
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digestive: 1) bile salts --> fat digestion & absorption, emulsification & micelle formation, absorption of fat-soluble vitamins; 2) HCO3- --> neutralize acidic chyme, optimal pH for pancreatic enzymes;
excretory: 1) bile pigment (bilirubin); 2) excess cholesterol; 3) drugs |
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control of biliary secretion (3)
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1) bile acids (most potent, presence in portal vein stimulates uptake and re-secretion & inhibits synthesis);
2) hormones: secretin (acidic chyme --> secretin --> HCO3-) & CCK (fat & peptides --> CCK --> release of enzymes and bile via contraction of gall bladder + relaxation of sphincter of Oddi); 3) neural: vagus nerve (contraction of gall bladder |