GI tract secretion

Secretory function of the GI tract Digestion –Movement of food materials Linear movement Mixing –Digestion Mechanical Chemical –Absorption

Secretion by the GI tract –Aids movement Mucus –Provides protection –Digestion Enzymes Bile acids –In response to the presence of foods in the tract

Anatomical structures Single cell mucous gland –Mucous cells/goblet cells Pits –Submucosal glands Crypts of Lieberkun Tubular glands –Oxyntic glands in stomach Complex gland –Salivary –Liver –Pancreas

GI tract secretion –Epithelial stimulation Direct contact with food –Enteric nerve stimulation Tactile Chemical Distension –Parasympathetic stimulation Increased secretion in the upper GI tract –Sympathetic stimulation Increase in some area Reduced parasympathetic-induced secretion

Mechanism of secretion Organic materials –Exocytosis Water and electrolytes –Increased ion uptake Inward movement of chlorine –Movement of sodium ion Osmosis Release of fluids by opening of channels –Hydrostatic pressure

Mucus Chemical composition –Different among different regions within the tract Adhesiveness Ability to coat the tract Low resistance against movement High resistance to digestion Buffering

Saliva

Composition –Serous Amylase –Mucous Mucin –Ions Bicarbonate Potassium Low sodium and chloride ion concentrations

Salivary Glands Type of % of total Gland saliva Secreted Parotid Serous Submandibular Mucous / Serous Sublingual Mucous 10% / Serous Buccal Mucous <1% 90%

Saliva and oral hygiene –Removal of pathogens and food particles –Bactericide activity –Immunity

Nervous regulation of secretion –Parasympathetic nervous system Tactile and taste stimulation of salivatory nuclei –Higher CNS Regulation of salivatory nuclei –Reflexes Lower GI tract

Esophageal secretion Mucus –Lubrication for swallowing Simple mucous cells –Protection of the lower esophagus Compound glands located in the lower portion

Gastric secretion Types of secretion –Mucous cells Mucus –Oxyntic glands Hydrochloric acid Pepsinogen Intrinsic factor –Pyloric glands Mucus gastrin

Phase of secretion 20 % 70 %

Oxyntic glands –Cells Mucous neck cells Parietal cells –HCL –Intrinsic factor Peptic/chief cells –Pepsinigen

HCL –Formed at the villus- like structures inside of canaliculi

K+K+ K+K+ K+K+ K+K+ Na + H+H+ Cl - H2OH2O H2OH2O osmosis H2OH2O HO - + H + HCO 3 CO 2 Carbonic Anhydrase Final Results HCl mEq/L KCl - 15 mEq/L NaCl - 3 mEq.L pH = 0.8 P P BLOOD LUMEN P P

Pepsinogen –Activated by HCL –Secretion Acetylcholine Stomach acid Inhibition of gastric secretion –Reverse enterogastric reflex –Presence of chemicals within the intestine

Pancreatic secretion Digestive enzymes –Pancreatic acini Sodium bicarbonate –Ducts

Secretion –Presence of chyme Determines the composition of pancreatic secretion –Stimulated by Acetylcholine –Acinar secretion Cholecystokinin –Acinar secretion Secretin –Ductal secretion

Pancreatic enzymes –Carbohydrate digestion Amylase –Protein digestion Trypsin Chymotrypsin Carboxylpolypeptidase –Lipid digestion Lipase Cholesterol esterase Phospholipase

Phase of secretion 20 % pancreatic secretion 5-10 % Pancreatic secretion Majority of secretion (secretin)

Bile secretion Secretion –Hepatocytes Initial secretion –Drained into ducts Secretion to the SI Accumulation in the gallbladder Gallbladder –Increased concentrations of bile salts Removal of sodium ions –Absorption of chloride ions –Osmosis of water

–Emptying In response to fats in the chyme –Secretion of cholecystokinin Contraction of gallbladder wall and relaxation of the sphincter of Oddi

Function –Fat digestion and absorption Emulsification –Detergent function Formation of micelles –Bile is amphipathic in nature (cholesterol plus salt) –Hydrophobic core/pocket for lipids Bile salts –Recycled Enterohepatic circulation

Secretion of the small intestine Compound mucous cells –Brunner’s gland Site where pacreatic secretion and bile enter the intestine Alkaline mucus for protection –Added effects with pacreatic bicarbonate ions and bile Secretion –Tactile stimulation –Irritation –Vagus stimulation –Secretin

Crypts of Liererkuhn –Goblet cells Mucus –Enterocytes Water and electrolytes Secretion –Rapidly reasborbed by the villi

Secretion of the watery mucus –Active secretion of chloride ions –Active secretion of bicarbonate ions –Passive/drag movement of sodium ions –Results of these: Osmosis

Digestive enzymes –Peptidases –Enzymes for disaccharide digestion Sucrase Maltase Lactase Isomaltase –Lipase Secretion –Regulated by the local stimuli Enteric nervous reflex

Secretion by the large intestine No enzyme secretion –Only mucus Bicarbonate ions –Non-mucous cells Secretion –Tactile stimulation –Local nervous reflex to the cryts of Leiberkuhn Protection of the large intestine –Acids from bacterial metabolism Adhesive for feces formation