GI 3

Secretion In typical day, 9 L fluid pass through GI, 7 L come from body water The digestive system secretes ions and water Particularly Na+, K+, Cl-, HCO3-, and H+ Ion channels include cystic fibrosis transmembrane conductance regulator (CFTR chloride channel) Acid secretion Parietal cells ? secret HCl from gastric glands Acid secretion by parietal cells HCl is secreted into lumen of stomach while bicarbonate is absorbed into gastric blood supply H+ from water pumped into stomach lumen by H+K+ATPase in exchange for K+ entering parietal cell Cl- follows H+ into lumen via open Cl- channel net secretion HCl Buffering action of HCO3- makes stomach less acidic ?alkaline tide? as meal is digested Bicarbonate secretion Most bicarbonate comes from pancreas (small amount from duodenal cells) Acini ? lobules of pancreas that open into ducts; Acinar cells secrete digestive enzymes; Duct cells secret NaHCO3 Pancreatic islet cells secrete hormones Cells that produce bicarbonate have high concentrations of carbonic anhydrase (CA) HCO3- is exchanged for Cl- at apical membrane, and H+ is exchanged for Na+ at basolateral membrane. Bicarbonate secretion occurs in exocrine pancreas and duodenum NaCl secretion [by intestinal and colonic crypt cells] Cl- enters cells by indirect active transport and leaves apical side through CFTR channel. Na+ and H2O follow passively by paracellular pathway Na+ , K+ , and Cl- enter by cotransport Cl- enters lumen through CFTR channel Na+ is reabsorbed Negative Cl- in lumen attracts Na+ by paracellular pathway. Water follows Digestive enzymes are secreted into the mouth, stomach, and intestine Some enzymes made and stored in vesicles, released on demand by exocytosis; some when released are not free but are anchored by transmembrane proteins or lipids Some secreted in inactive proenzyme form known as zymogens Parasympathetic stimulation enhances enzyme secretion Specialized cells secrete mucus Mucins ? glycoproteins found in mucus Signals for mucus release: parasympathetic innervations, infections & inflammatory responses (to fortify protective barrier) Saliva is an exocrine secretion Made of water, ions, mucus, proteins Slightly acidit pH 6-7 Stress decreases salivary secretion Liver secretes bile Bile ? nonenzymatic solution secreted from hepatocytes (liver cells) Bile salts ? at as detergents to facilitate enzymatic fat digestion; made from steroid bile acids combined w/ amino acids Bile pigments ? i.e. bilirubin, the waste products of Hb degradation Cholesterol ? excreted in feces (along w/ drugs & other zenobiotics that are cleared from blood by hepatic processing) Gallbladder ?stores and concentrates bile solution; during meal, contraction of gallbladder sends bile + solution of bicab & digestive enzymes from pancreas via common bile duct into duodenum