G.I.T+secretion - Semi-quantitative analysis of amounts(g...

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Unformatted text preview: Semi-quantitative analysis of amounts (g or ml) of substances ingested, secreted, absorbed and excreted per day in human adult Note that the total volume of secretory products of the G.I.T. is several times greater each 24 hours than the volume (mass) of solids and liquids ingested. Fig 15.5 Vander's Human Physiology, McGrawHill, 10th ed. Physiology of the stomach's various functions Note: multiple cell types in gastric pits. Mucous layer at surface of pits not shown. Fig 41.1, Boron & Boulpaep, Medical Physiology, Updated ed., 2005 Elsevier Saunders Figure 21.4, etal Selected secretory products of gastric pits of stomach Pepsinogen secreted by chief cells Precursor for pepsin, enzyme that digests proteins Hydrogen ions secreted by parietal cells Maintain acidic environment of stomach Intrinsic factor secreted by parietal cells Necessary for absorption of vitamin B12 Gastrin secreted from G cells (hormone) Mucus secreted from neck cells From G&S 2nd ed., 2005 Diffusion barrier in gastric mucosa The surface mucus barrier serves two purposes, 1) to prevent diffusion of H+, and 2) to trap HCO3. These actions prevent erosion of the mucosal layer of the stomach/intestine. If the barrier is damaged, H+ damages mast cells, which release histamine and other proinflammatory mediators which can lead to tissue ischemia (reduced mucosal blood flow) and necrosis. Fig 41.13 Boron & Boulpaep, Medical Physiology, Saunders, 2003. Breakdown of gastric mucosal barrier Agents that alter diffusion barrier: aspirin, alcohol, bile acids Allow acid to damage mast cells which release: histamine,PAF (plateletactivating factor), leukotrienes, endothelins, thromboxanes, oxidants (collectively cause inflammation, ischemia, tissue damage) Acid reflux and diseases of the esophageal mucosa Helicobacter pylori (H. pylori): gram negative bacillus colonizing antral mucosa causes gastritis and peptic ulcer (Nobel Prize in Physiology or Medicine, 2005, Barry Marshall and Robin Warren) (Barrett's Disease/Barrett's esophagus, on the rise, treatment requires antibiotic therapy) Control of the secretion of gastric acid Fig 41.10 Boron & Boulpaep, Medical Physiology, Updated Edition, 2005, Elsevier Saunders Movement of newly-synthesized proteins through the secretory pathway To identify newlysynthesized proteins, the investigators radiolabelled (3H amino acids) pancreatic acinar cells. The cell at the top of the picture illustrates the structures involved, and the curves at the bottom indicate the time and per cent of initial label found in them. Fig 42.2 Boron & Boulpaep, Medical Physiology, Saunders, 2003. Summary secretions All G.I.T tissues are secretory Secretions moisten, lubricate, digest, release, and transport Excess secretions (H+ and ulcers) H. pylori, stomach, esophageal erosion and Barrett's syndrome Summary Gastric acid important to protein digestion Several mechanisms involved in acid secretion Pernicious anemia (lack parietal cells, high plasma gastrin but no hydrogen ion secretion) For the pancreas and salivary glands, the thought, sight, smell, and taste of food are all important determinants of secretion For guidance on what and how to eat, see for example, Report of the Dietary Guidelines Advisory Committee on the Dietary Guidelines for Americans, 2005 USDA, August, 2004 Structural and functional units of exocrine salivary and pancreatic glands Figs 6.10, 21.8 and 21.9 G&S, 2nd ed. Acinar cells are characteristic of the salivary glands and exocrine pancreas. Main secretory products are proteins, aqueous medium, bicar bonate, and electrolytes (what is primary vs secondary secretion?) Summary Phases = basal, cephalic, gastric, intestinal Exocrine salivary secretions = aqueous, amylases, electrolytes Exocrine pancreatic secretions = aqueous, enzymes, bicarbonate, electrolytes Secretory products begin and end with circulation Q/A Pancreatic secretagogues Pancreatic secretagogues such as ACh, CCK, substance P, secretin, and VIP stimulate the exocrine pancreas to release enzymes that are needed for digestion of lipids, carbohydrates, and proteins. Fig 33.12 Berne & Levy, Principles of Physiology, Mosby, 3rd ed., 2000. ...
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