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PowerPoint® Lecture Slide Presentation by Vince Austin Human Anatomy & Physiology FIFTH EDITION Elaine N. Marieb Chapter 24 The Digestive System Part D Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Digestion in the Stomach • The stomach: • Holds ingested food • Degrades it both physically and chemically • Delivers chyme to the small intestine • Enzymatically digests proteins with pepsin • Secretes intrinsic factor required for absorption of vitamin B12 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Regulation of Gastric Secretion • Neural and hormonal mechanisms regulate the release of gastric juice • Stimulatory and inhibitory events occur in three phases • Cephalic (reflex) phase: prior to food entry • Gastric phase: once food enters the stomach • Intestinal phase: as partially digested food enters the duodenum Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Cephalic Phase • Excitatory events include: • Sight or thought of food • Stimulation of taste or smell receptors • Inhibitory events include: • Loss of appetite or depression • Decrease in stimulation of the parasympathetic division Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gastric Phase • Excitatory events include: • Stomach distension • Activation of stretch receptors (neural activation) • Activation of chemoreceptors by peptides, caffeine, and rising pH • Release of gastrin to the blood • Inhibitory events include: • A pH lower than 2 • Emotional upset which overrides the parasympathetic division Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Intestinal Phase • Excitatory phase – low pH and partially digested food enters the duodenum • Inhibitory phase – distension of duodenum, presence of fatty, acidic, or hypertonic chyme, and/or irritants in the duodenum • Initiate inhibition of local reflexes and vagal nuclei • Closes the pyloric sphincter • Releases enterogastrones that inhibit gastric secretion Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Release of Gastric Juice Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Figure 24.16 Regulation and Mechanism of HCl Secretion • HCl secretion is stimulated by ACh, histamine, and gastrin • All work through second messenger systems • Release of hydrochloric acid: • Is low if only one ligand binds to parietal cells • Is prolific if all three ligands bind to parietal cells • Antihistamines and cimetidine block H2 receptors and decrease HCl release Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Regulation and Mechanism of HCl Secretion Figure 24.17 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Response of the Stomach to Filling • Stomach pressure remains constant until about 1L of food is ingested • Relative unchanging pressure results from reflexmediated relaxation and plasticity • Reflex-mediated events include: • Receptive relaxation – as food travels in the esophagus, stomach muscles relax • Adaptive relaxation – the stomach dilates in response to gastric filling • Plasticity – intrinsic ability of smooth muscle to exhibit the stress-relaxation response Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gastric Contractile Activity • Peristaltic waves move toward the pylorus at the rate of 3 per minute • This basic electrical rhythm (BER) is initiated by pacemaker cells (cells of Cajal) • Most vigorous peristalsis and mixing occurs near the pylorus • Chyme is either: • Delivered in small amounts to the duodenum or • Forced backward into the stomach for further mixing Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Gastric Contractile Activity Figure 24.18 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Regulation of Gastric Emptying • Gastric emptying is regulated by: • The neural enterogastric reflex • Hormonal (enterogastrone) mechanisms • These mechanisms inhibit gastric secretion and duodenal filling • Carbohydrate-rich chyme moves through the duodenum quickly • Fat-laden chyme is digested more slowly causing food to remain in the stomach longer Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Regulation of Gastric Emptying Figure 24.19 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Homeostatic Imbalance • Vomiting (emesis) – the stomach empties via a different route (oral) • Causes include extreme stretching, irritants such as bacterial toxins, excessive alcohol, spicy foods, and certain drugs • The emetic center of the medulla initiates a number of motor responses • Diaphragm and abdominal wall muscle contract • Cardiac sphincter relaxes and soft palate closes off the nasal passages • Excessive vomiting can cause dehydration and upset electrolyte and pH balance Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Small Intestine: Gross Anatomy • Runs from pyloric sphincter to the ileocecal valve • Has three subdivisions: duodenum, jejunum, and ileum • The bile duct and main pancreatic duct: • Join the duodenum at the hepatopancreatic ampulla • Are controlled by the sphincter of Oddi • The jejunum extends from the duodenum to the ileum • The ileum joins the large intestine at the ileocecal valve Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Microscopic Anatomy of the Small Intestine • Structural modifications of the small intestine wall increase surface area • Plicae circulares: deep circular folds of the mucosa and submucosa • Villi: fingerlike extensions of the mucosa • Microvilli: tiny projections of absorptive mucosal cells’ plasma membranes Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Microscopic Anatomy of the Small Intestine Figure 24.21a-c Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Small Intestine: Histology of the Wall • The epithelium of the mucosa is made up of: • Absorptive cells and goblet cells • Interspersed T cells (intraepithelial lymphocytes), and • Enteroendocrine cells • Intestinal crypts cells secrete intestinal juice • Peyer’s patches are found in the submucosa • Brunner’s glands in the duodenum secrete alkaline mucus Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Intestinal Juice • Secreted by intestine glands in response to distension or irritation of the mucosa • It is slightly alkaline and isotonic with blood plasma • Is largely water, enzyme-poor, but contains mucus Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Liver • The largest gland in the body • Superficially has four lobes – right, left, caudate, and quadrate • The falciform ligament: • Separates the right and left lobes anteriorly • Suspends the liver from the diaphragm and anterior abdominal wall • The ligamentum teres: • Is a remnant of the fetal umbilical vein • Runs along the free edge of the falciform ligament Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Liver: Associated Structures • The lesser omentum anchors the liver to the stomach • The hepatic blood vessels enter the liver at the porta hepatis • The gallbladder rests in a recess on the inferior surface of the right lobe • Bile leaves the liver via • Bile ducts which fuse into the common hepatic duct • The common hepatic duct fuses with the cystic duct • These two ducts form the bile duct Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings Liver: Associated Structures Figure 24.20 Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings