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Digestive Secretion & Control Regulatory Mechanisms in General Glandular Secretion of the Stomach Neural Control of Secretion Endocrine Control of Secretion Control of Gastric Secretion Digestive Regulatory Mechanisms Allow communication between different parts of the digestive tract Ensure the presence of sufficient secretions when food present Help avoid overabundance of secretions in absence of food Two types of mechanisms: neural and endocrine Glandular Secretions of the Stomach Recall that the main secretory products of the stomach are: - mucus: protection from acidic chyme and pepsin - hydrochloric acid: acidic pH of stomach - intrinsic factor: absorption of vitamin B12 - pepsinogen: cleaved to pepsin - gastrin: from endocrine cells, stimulates parietal and chief cells How does a Cell Produce HCl? Parietal cells must maintain a normal pH to survive They don’t really produce HCl internally and excrete it, but instead release H+ ions and Cl- ions into the lumen of the stomach Here’s how: - CO2 is taken in from the cell - CO2 + H2O ==> carbonic acid (H2CO3) (reaction catalized by carbonic anhydrase) - H2CO3 dissociates into bicarbonate and H+ - H+ ions are actively pumped out of the cell, into the lumen of the stomach - Cl- ions diffuse with the charged H+ ions - bicarbonate enters bloodstream Production of H+ and Cl- Ions Neural Control of Secretion Stimulation by the parasympathetic nervous system - oral cavity: facial and glossopharyngeal nerves - other GI organs: vagus nerve, and local nerve reflexes (stretch receptors, chemo receptors) The neurotransmitter of the parasympathetic nervous system is acetylcholine (ACh) Endocrine Control of Digestive Secretions Feedback interactions between organs, and within organs Allow different parts of the digestive tract to influence each other Main hormones: - gastrin - secretin - cholecystokin (CCK) - gastric inhibitory peptide Source, Regulation, and Action of Hormones Gastrin: - produced from the stomach and duodenum - stimulated by stomach distension, peptides, amino acids, alcohol, caffeine - inhibited by acidic pH - functions: - increases stomach secretions - increases rate of gastric emptying Source, Regulation, and Action of Hormones Secretin: - produced by the duodenum - release stimulated by acidity of chyme - functions: - decreases gastric secretions - stimulates pancreatic secretion of bicarbonate - increases bile secretion by liver - increases intestinal mucus secretion - decreases gastric motility Source, Regulation, and Action of Hormones Gastric Inhibitory Peptide: - produced by small intestine - stimulated by lipids - functions: - inhibits gastric secretion - inhibits gastric motility Source, Regulation, and Action of Hormones Cholecystokinin (CCK): - produced by small intestine - stimulated by lipids - functions: - inhibits gastric secretion - stimulates pancreatic digestive secretions - causes contraction of gallbladder - decreases gastric motility The Control of Gastric Secretion The stomach produces 2 to 3 liters of secretions per day Secretion increases immediately after a meal The rate of gastric secretion is under neural and hormonal control There are three phases to gastric secretion: - cephalic phase - gastric phase - intestinal phase Cephalic Phase Stimulation: thought, chewing, taste of food Neural response: release of ACh from the vagus nerve Result: increased release of mucus, HCl, pepsinogen, intrinsic factor, gastrin (gastric secretions) in the stomach (about 500 ml/hour) Prepare stomach for oncoming food Gastric Phase Stimulus: distension of stomach, increase in pH of gastric contents, presence of proteins and amino acids Neural Response: Stretch receptors and chemoreceptors (detecting protein, alcohol, and caffeine) stimulate ACh release, activating parietal and chief cells Endocrine Response: Protein and distension also stimulate the release of gastrin Gastrin production declines at pH < 2.0 Duration of gastric phase: many hours The Intestinal Phase Purpose: Control the rate of gastric emptying, and ensure that the small intestine is ready to digest and absorb chyme Stimulus: Acidity of chyme entering the small intestine Neural Response: chyme in duodenum activates mechanoreceptors and chemoreceptors, resulting in the enterogastric reflex (slowing of gastric motility) The Intestinal Phase (cont.) Endocrine Responses: - low pH => increased secretin release - decreased gastric secretion - stimulation of pancreatic bicarbonate - increased bile secretion - increased intestinal mucus secretion - decreased gastric motility The Intestinal Phase (cont.) Endocrine Responses: - lipids increase CCK and gastric inhibitory peptide - both slow gastric secretion - CCK also causes gall bladder contraction and increased pancreatic digestive secretion The Intestinal Phase (cont.) Endocrine Responses: - partially digested proteins (or chyme of high pH) cause increased release of gastrin - result: increased acid and pepsin production Thus, gastric digestion can be regulated for specific components of food SUMMARY The secretion of digestive enzymes is tightly regulated, such that organs of the digestive tract signal each other to optimize effeciency of digestion and absorption. NEXT LECTURE... Absorption