Basic Mechanism of Secretion

Secretory Functions of the Alimentary Tract Gastric Secretion  Stomach mucosa has two important types of tubular glands: • Oxyntic glands (gastric glands) • Secrete hydrochloric acid, pepsinogen, intrinsic factor, and mucus. • Pyloric glands. • Secrete mucus for protection of the pyloric mucosa from the stomach acid. • Secrete the hormone gastrin. Secretory Functions of the Alimentary Tract Secretions from the Oxyntic (Gastric) Glands • Oxyntic gland is composed of three types of cells: • Mucous neck cells (secrete mucus) • Peptic (chief) cells, (secrete pepsinogen) • Parietal (oxyntic) cells, (HCl and intrinsic factor) Secretory Functions of the Alimentary Tract Basic Mechanism of Hydrochloric Acid Secretion • • • The pH is about 0.8. Concentration of the hydrogen ions requires more than 1500 calories of energy per liter of gastric juice. At the same time that hydrogen ions are secreted, bicarbonate ions diffuse into the blood (gastric venous blood has a higher pH than arterial blood). Secretory Functions of the Alimentary Tract Basic Mechanism of Hydrochloric Acid Secretion • • The main driving force for HCl secretion is a hydrogen-potassium pump (H+-K+ ATPase). A major part of the stomach’s ability to prevent backleak of acid can be attributed to the gastric barrier due to the formation of alkaline mucus and to tight junctions between epithelia cells. Secretory Functions of the Alimentary Tract Basic Mechanism of Hydrochloric Acid Secretion Blood flow +H2O HCO3- HCl Increase blood pH Secretory Functions of the Alimentary Tract Changes in the concentration of electrolytes Secretory Functions of the Alimentary Tract Basic Factors That Stimulate Gastric Secretion Indirect Direct Secretory Functions of the Alimentary Tract Basic Factors That Stimulate Gastric Secretion Direct • Muscarinic acetylcholine receptor (M3, IP3 and Ca2+) Secretory Functions of the Alimentary Tract Basic Factors That Stimulate Gastric Secretion Indirect • • Histamine receptor (H2, Gs and cAMP) Cimetidine, Ranitidine, famotidine Secretory Functions of the Alimentary Tract Basic Factors That Stimulate Gastric Secretion Indirect • CCK-B receptor, IP3/Ca2+ Secretory Functions of the Alimentary Tract Gastric Secretion      Acetylcholine excites secretion of pepsinogen by peptic cells, hydrochloric acid by parietal cells, and mucus by mucous cells. Gastrin and histamine strongly stimulate secretion of acid by parietal cells but have little effect on the other cells. Secretion and Activation of Pepsinogen. When pepsinogen is first secreted, it has no digestive activity. However, as soon as it comes in contact with HCl, it is activated to form active pepsin. Pepsin functions as an active proteolytic enzyme in a highly acidic medium (optimum pH 1.8 to 3.5), but above a pH of about 5 it has almost no proteolytic activity and becomes completely inactivated. Secretory Functions of the Alimentary Tract Gastric Secretion    Secretion of Intrinsic Factor by Parietal Cells. Intrinsic factor is essential for absorption of vitamin B12 in the ileum. When the acid-producing parietal cells of the stomach are destroyed, which frequently occurs in persons with chronic gastritis, not only does achlorhydria (lack of stomach acid secretion) develop, but pernicious anemia also often develops because of failure of maturation of the red blood cells.   Pyloric Glands—secretion Of Mucus And Gastrin They contain mostly mucous cells. These cells secrete a small amount of pepsinogen, and large amount of thin mucus (lubricate food movement, protect the stomach wall from digestion by the gastric enzymes). The pyloric glands also secrete gastrin, which plays a key role in controlling gastric secretion.  Secretory Functions of the Alimentary Tract Phases of Gastric Secretion • • Cephalic Phase (30 percent of the gastric secretion) Gastric Phase (60 percent of the total gastric secretion) • • • • Long vagovagal reflexes Local enteric reflexes Gastrin mechanism Intestinal Phase (10 percent of the gastric secretion)  Because of small amounts of gastrin released by the duodenal mucosa Secretory Functions of the Alimentary Tract Inhibition of Gastric Secretion by Other Intestinal Factors • • 1. The presence of food in the small intestine initiates a reverse enterogastric reflex, transmitted through the myenteric nervous system and extrinsic sympathetic and vagus nerves, that inhibits stomach secretion. This reflex can be initiated by (a) distending the small bowel, (b) the presence of acid in the upper intestine, (c) the presence of protein breakdown products, or (d) irritation of the mucosa. 2. The presence of acid, fat, protein breakdown products, hyperosmotic or hypo-osmotic fluids, or any irritating factor in the upper small intestine causes release of several intestinal hormones. • Secretin, GIP, VIP, and somatostatin.