Chapter 16 The Digestive System PDF

Chapter 16 The Digestive System Duodenum, Pancreas, Bile Sherwood Modified, Edited and Presented by Marios Z Panos Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  DIGESTIVE SYSTEM OUTLINE - Duodenum - Pancreas - Secr...

Chapter 16 The Digestive System Duodenum, Pancreas, Bile Sherwood Modified, Edited and Presented by Marios Z Panos Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  DIGESTIVE SYSTEM OUTLINE - Duodenum - Pancreas - Secretions, Digestion - Control of secretions - Bile, bile salts, - Secretion - Control of secretion - Role in fat digestion Pancreas-Duodenum-Biliary Tree Pancreas - Contains both exocrine and endocrine tissue - Elongated gland located behind and below the stomach Pancreas  Pancreas Endocrine function – Islets of Langerhans Found throughout pancreas Secrete insulin and glucagon Exocrine function – Secretes pancreatic juice consisting of Pancreatic enzymes actively secreted by exocytosis by the acinar cells that line the acini Aqueous alkaline solution actively secreted by the duct cells that line pancreatic ducts Pancreatic Acinus Enzymes stored in zymogen granules - released on stimulus by exocytosis  Pancreatic Enzymes Proteolytic enzymes Trypsinogen - converted to active form trypsin Chymotrypsinogen – converted to active form chymotrysin Procarboxypeptidase – converted to active form carboxypeptidase Pancreatic amylase – Converts polysaccharides into the disaccharide maltose Pancreatic lipase – Only enzyme secreted throughout entire digestive system that can digest fat All 3 enzymes packaged together in zymogen granules therefore proportion of enzymes released is always the same irrespective of food content  Pancreatic Enzymes – Activation Proteolytic enzymes Trypsinogen - > secreted into duodenal lumen - > active form trypsin by enteropeptidase* Trypsin auto-catalytically activates more trypsinogen Trypsin also converts (within duodenal lumen) Chymotrypsinogen - active form chymotrypsin Procarboxypeptidase - > active form carboxypeptidase Enteropeptidase (enterokinase) = enzyme embeded in the luminal membrane of duodenal mucosa  Pancreatic Enzymes Proteolytic enzymes (contd.) Conversion to active enzymes and enzyme activity require alkali environment (contrast with pepsinogen in stomach which requires acid environment) Duodenal activation ensures protection of pancreas from autodigestion Trypsin inhibitor affords further protection - produced by the pancreas – in case of inadvertent activation of trypsin within pancreas  Pancreatic Enzymes (contd.) Pancreatic amylase – Converts polysaccharides into the disaccharide maltose and α-limit dextrins – Secreted in the active form – safe because - > no polysaccharides in pancreatic secretory cells Pancreatic lipase – Only enzyme secreted throughout entire digestive system that can digest fat * – Dietary triglycerides - > monoglycerides + FFAs – Secreted in the active form - > triglycerides not a structural component of pancreatic cells * In humans also lingual + gastric lipase in negligible amounts  Clinical Note Pancreatic insufficiency Diminished secretion of pancreatic enzymes - > Incomplete digestion of fats - > STEATORRHOEA = malabsorption of fat (mainly) – up to 60-70% not absorbed - > fatty stool = pale, fatty, offensive + floats in pan Calorie + fat soluble Vitamin malabsorption Note: Protein and CHO digestion affected less - salivary, gastric, small intestinal enzymes mitigate CHO = carbohydrate  Pancreas – Acqueous Alkaline Secretion Bicarbonate secretion Pancreatic enzymes - > function best at neutral or slightly alkaline pH Acidic chyme must be neutralised quickly - > enzyme function - > prevent acid damage to duodenal mucosa Sodium bicarbonate (NaHCO3- ) rich fluid secreted by pancreatic duct cells Secretion of water and electrolytes Na, K – the same as in plasma Bicarbonate concentration – up to 5 times higher than in plasma  Pancreas – Bicarbonate secretion HCO3- CFTR Capillary Pancreatic venous blood slightly acid Bicarbonate exits through a) Cl- / HCO3- antiporter b) CFTR instead of Cl- CFTR = Cystic Fibrosis Transmembrane conductance Regulator  pH change D1-D4 D1 pH 1-2 D2 pH 6-7 D4 D3 Pancreatic Secretion - Enzymes and Bicarbonate Regulated by Secretin and CCK - both released from duod. mucosa CCK= cholecystokinin Other action of CCK? Other action of Secretin?  PANCREAS Note: Release of the enterogastrones Secretin and CCK in response to chyme in the duodenum regulates pancreatic exocrine secretion Secretin and CCK - - > trophic to pancreas (as is gastrin trophic to the stomach) In contrast to fat and protein, presence of CHO in duodenum does not directly influence digestive enzyme secretions by the pancreas NB. Presence of protein in duodenum - > CCK - > pancr. enzyme secretion The Liver, Gallbladder, Bile and Bile Salts The Liver Gallbladder Bile Bile Salts Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  Liver Largest and most important metabolic organ in the body Body’s major biochemical factory Importance to digestive system - > secretion of bile and bile salts Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  Liver Functions not related to digestion – Metabolic processing of the major categories of nutrients – Detoxifying or degrading body wastes and hormones, drugs, and other foreign compounds – Synthesizes plasma proteins – Stores glycogen, fats, iron, copper, and many vitamins – Activates vitamin D – Removes bacteria and worn-out red blood cells – Excretes cholesterol and bilirubin  Liver Bile – Actively secreted by liver and actively diverted to gallbladder between meals – Stored and concentrated in gallbladder – Consists of Bile salts Cholesterol Lecithin Bilirubin – After meal, bile enters duodenum Total volume bile secreted by liver 250ml - 1L /day Bile - Continuously secreted by liver - Sphincter of Oddi closed between meals - > bile diverted to gallbladder - > stored Sphincter of Oddi Effect of CCK on bile release into duodenum  Liver Bile – Continuously secreted by liver – Sphincter of Oddi closed between meals – Bile up against closed SOD - > diverted back to gallbladder - > stored – Meal - > chyme in duodenum - > CCK release - > SOD relaxes, GB contracts - > bile enters duodenum SOD = sphincter of Oddi  Gallbladder Sac like structure beneath the liver Storage and concentration of bile Bile concentrated by Outward active transport of salt (and water) - > X 5-10 Î conc. bile constituents - > predisposes to stone formation Cholecystectomy - > continuous secretion bile into Common Bile Duct - > SOD closed between meals - > CBD dilated (post cholecystectomy CBD diameter > 6 mm on U/S examination)  Liver Bile salts Cholic Acid Chenodeoxycholic acid – Derivatives of cholesterol – Convert large fat globules into a liquid emulsion = small lipid droplets - > Î surface area - > lipase attack Bile salts = emulsifiers (similar action to detergents)  Bile salts – Enterohepatic Circulation Bile Salts - > aid fat digestion and absorption Reabsorbed via the terminal ileum - > portal system (Portal Vein) - > liver - > re-secreted into bile On average bile salts cycle twice between liver and small intestine during digestion of a meal Bile Salts  Bile Salts (contd.) Negatively charged coat - repel each other preventing fat droplets from coalescing  Formation of micelles With bile salts, cholesterol and lecithin contribute to formation of micelles  Bile Salts (contd.) Cholesterol, lecithin contribute to formation of micelles Lecithin = phospholipid - also constituent of cell membranes Colipase (amphiphilic peptide) - > anchors lipase to surface amid bile salt coating  Stimulation of Bile Secretion CCK release in response to presence of fat in the duodenum Bile Salts during enterohepatic circulation - > stimulate Liver to secrete more bile salts Vagus n. - only minor stimul. during cephalic phase - Substances that cause secretion of bile = choleretic ENTERO-HEPATIC CIRCULATION Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  BILIRUBIN Bilirubin – main bile pigment Plays no part in digestion A waste product of red cell degradation Gives bile yellow colour & skin colour of jaundice Gut bacteria degrade - > brown colour of stool Some reabsorbed - > modified by liver - > excreted by kidneys - > yellow colour of urine Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  DIGESTIVE SYSTEM In this lecture we discussed - Duodenum - Pancreas - Secretions, Digestion - Control of secretions - Bile, bile salts, - Secretion - Control of secretion - Role in fat digestion  Chapter 16 The Digestive System Duodenum, Pancreas, Bile Thank you for your attention !