Gi L18 (Biliary & Pancreatic Secretion)

Front 1

pancreatic secretion: exocrine (2)

Back 1

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)

Front 2

pancreatic secretion: endocrine

Back 2

insulin & glucagon secreted by islet of langerhans cells

Front 3

pancreatic enzymes: activation

Back 3

trypsinogen --> [enterokinase] --> trypsin; trypsin converts: chymotrypsinogen --> chymotrypsin; proelastase --> elastase; procarboxypeptidase --> carboxypeptidase
(anti-trypsin by pancreas prevents autodigestion)

Front 4

pancreatic enzymes: functions

Back 4

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

Front 5

control of pancreatic exocrine secretions

Back 5

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

Front 6

composition of bile (6)

Back 6

water, electrolytes, bile salt, bilirubin, cholesterol, phospholipids

Front 7

stages of biliary secretion

Back 7

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

Front 8

bile salt formation

Back 8

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)

Front 9

bile salt property: amphipathic

Back 9

hydrophobic domain of sterol nucleus + hydrophilic domain of OH- / COO- / CONH

Front 10

bile acid / salt function: fat digestion (emulsification)

Back 10

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

Front 11

bile acid / salt function: fat digestion (micelle formation)

Back 11

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

Front 12

fat absorption

Back 12

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

Front 13

enterohepatic circulation of bile salts

Back 13

~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

Front 14

functions of biliary secretions (2 +3)

Back 14

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

Front 15

control of biliary secretion (3)

Back 15

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