GI Secretions PDF - Student Version

GI SECRETIONS Dra. Carla Romo Jaramillo Anesthesiology / Critical Care Medicine OBJECTIVES • State the components of saliva & the substrates and digestion products of salivary amylase (ptyalin). • Contrast the plasma and saliva concentrations at low and high secretion rates. • Identify the stimuli...

GI SECRETIONS Dra. Carla Romo Jaramillo Anesthesiology / Critical Care Medicine OBJECTIVES • State the components of saliva & the substrates and digestion products of salivary amylase (ptyalin). • Contrast the plasma and saliva concentrations at low and high secretion rates. • Identify the stimuli and cell types involved in GI secretion & the cell type and anatomical location of the endocrine cells secreting main hormones. • Describe function of somatostatin and histamine as paracrine regulators of acid secretion in the stomach. • State the effects of acid, fat, and solutions of high osmolarity in the duodenum on gastric secretion. • Describe the regulation of H+-K+ ATPase & the mechanism of gastric H+ generation and secretion. • Describe the modulation of gastric acid secretion by the enterochromaffin-like cell. • Describe the generation of an “alkaline tide”. • Describe the bile secretion with its characteristics. Secretions? Addition of fluids, enzymes, and mucus to the lumen of the GI Salivary secretions FUNCTIONS • • • • • • • • • Disruption of food resulting in smaller particles. Aid in the formation of a bolus for swallowing. Initiation of starch and lipid digestion. Facilitation of taste. Cleansing of mouth and selective antibacterial action. Clearance and neutralization of refluxed gastric material in the esophagus. Production of intraluminal gastric and duodenal stimuli. Regulation of food intake and eating behavior. Aids in speech. Production and secretion • Acinar Cells: Water – Ions – Enzymes (Ptyalin) – Mucus • Ductal Cells: Modify Electrolites • Na+ and Cl- absorption • Na+: apical epithelial Na+ channels; basolateral Na-K pumps • Cl-: apical membrane by ClHCO3 exchanger; basolateral Cl channels • Myoepithelial Cells: Contract and eject Relationship between Salivary production and Flow rate Dependent on how many time is saliva in contact with the ductal cells. Constituents Function Mucins Amylase Lipase Lysozyme IgA Nerve growth Factor Lubrication Digestion of starches Digestion of Fat Immune Protection Protective? Protective? Epidermal growth factor Regulation of Salivary Secretion Parasympathetic system • Main regulatory system • Disruption of PS causes glandular atrophy Sympathetic system • Disruption does not cause major change Gastric Secretion Gastric Juice = Hydrochloric acid (HCl) + Pepsinogen + Intrinsic factor + Mucus THE STOMACH SECRETES SIX PRODUCTS WITH SPECIFIC FUNCTION • HYDROGEN ION • Converts pepsinogen into pepsin. • Kills microbes. • Denatures proteins. • PEPSINOGENS • Pepsin partly digests proteins. • MUCUS • Lubricates and protects mucosa. • BICARBONATE • Protects mucosa. • INTRINSIC FACTOR • Necessary for normal absorption of vit B12. • WATER • Dissolves & dilutes ingested material. Cells – Anatomy – Secretion Mucin and HCO3Mucus intrinsic factor Only gastric function essential for human life Enterochromaffin-like (ECL) cells, which release Histamine. Ach, Gastrin, and Histamine Directly and Indirectly Induce HCl secretion by Parietal Cells QUESTION During vacations you go home and your aunt comes to visit your mom, she was recently diagnosed with a gastric ulcer, and she is using omeprazole 20 mg every morning, she asks you about the mechanism of action of this treatment. Omeprazole inhibits: A. H+-K+ ATPase B. Cl- channel C. Na+-K+ ATPase D. Cl—-HCO3- channel Mechanism of HCl secretion –parietal cell- Vagal activation stimulates multiple cell responses via neurotransmitters Secretion during Cephalic and Gastric Phases Cephalic phase • • • • 30% HCL. Smelling, tasting, conditioned reflexes. Direct stimulation by Vagus – Ach. Indirect stimulation by Vagus – G cell – GRP. Gastric phase • 60% HCl. • Distention + presence of protein, amino acids and small peptides. • 4 mechanisms: • Distention – Vagus – Parietal cells. • Indirect stimulation of the parietal cells via gastrin release. • Distention – local reflexes stimulate gastrin release. • Amino acids and small peptides on the G cells stimulate gastrin release. Regulation of HCl secretion during cephalic and gastric phases Inhibition of secretion Vit B12 absorption and storage depends on several organs Haptocorrin = TCI or Cobalophilin BILE SECRETION • Is necessary for the digestion and absorption of lipids in the small intestine, solving the problem of insolubility of the fat. • Bile is produced and secreted by the liver, stored in the gallbladder, and ejected into the lumen of the small intestine. • Composition of the bile: The organic constituents of bile are bile salts (50%), bile pigments such as bilirubin (2%), cholesterol (4%), and phospholipids (40%). BILE SALTS (50%) • The function of the bile salts depends on their amphipathic properties: hydrophobic and hydrophilic portions. • The first role of bile salts is to emulsify dietary lipids. • The negatively charged bile salts surround the lipids, creating small lipid droplets in the intestinal lumen, they repel each other, so the droplets disperse. • The second role of bile salts is to form micelles with the products of lipid digestion including monoglycerides, lysolecithin, and fatty acids. PHOSPHOLIPIDS (40%) AND CHOLESTEROL (4%) • Both are secreted into bile by the hepatocytes and are included in the micelles with the products of lipid digestion. • Phospholipids are amphipathic and aid the bile salts in forming micelles. BILIRRUBIN (2%) • Yellow-colored byproduct of hemoglobin metabolism. • The liver extracts bilirubin from blood -> conjugates it with glucuronic acid -> bilirubin glucuronide (secreted into bile and accounts for bile’s yellow color.) • In the intestinal lumen is converted back to bilirubin -> then converted to urobilinogen (intestinal bacteria). • A portion of the urobilinogen is recirculated to the liver. • A portion is excreted in the urine,. • A portion is oxidized to urobilin and stercobilin, the compounds that give stool its dark color. FUNCTION OF THE GALLBLADDER • Stores the bile: During the interdigestive periods, the gallbladder can fill because it is relaxed and the sphincter of Oddi is closed. • Concentration of bile: The epithelial cells of the gallbladder absorb ions and water but not the organic components -> concentration of bile. • Ejection of bile: begins within 30 minutes after a meal is ingested. • The major stimulus for ejection of bile is CCK: causing contraction of the gallbladder and relaxation of the sphincter of Oddi. SECRETION AND ENTEROHEPATIC CIRCULATION OF BILE SALTS The rate-limiting enzyme in the biosynthetic pathway, cholesterol 7αhydroxylase, is inhibited by bile salts. Recirculation of bile salts to the liver also stimulates biliary secretion, which is called a choleretic effect. Na+-bile salt cotransporters The fecal loss is about 600 mg/day (out of the total bile salt pool of 2.5 g) BIBLIOGRAPHY • Linda S. Costanzo (2018), 6th edition. Physiology, chapter 8, pages 356-374.