MBBS Year 1 Gastrointestinal Lectures PDF

Prof GE Mann, Waterloo Campus (Email: [email protected]) MBBS Year 1 Gastrointestinal Lectures L1: Overview of the digestive system L2: Salivary, gastric and pancreatic secretions L3: Digestion and absorption of nutrients L4: Motility of the gut L5: Bile and biliary system Physiology, 5...

Prof GE Mann, Waterloo Campus (Email: [email protected]) MBBS Year 1 Gastrointestinal Lectures L1: Overview of the digestive system L2: Salivary, gastric and pancreatic secretions L3: Digestion and absorption of nutrients L4: Motility of the gut L5: Bile and biliary system Physiology, 5th Edition, eds. RM Berne, MN Levy, BM Koeppen, BA Stanton (2004) (see Chapters 31-33) [see also recent Editions] Medical Physiology, eds. WF Boron & EL Boulpaep (2003) Color Atlas of Physiology, 4th Edition, eds. A. Despopoulos & S. Silbernagl, (1991) 1 L1: Overview of the digestive system Learning Objectives Describe functions of the layers of the gastrointestinal (GI) tract: serosa, muscularis externa, submucosa, and mucosa Explain the physiological functions of salivary, gastric and pancreatic secretions Describe the mechanisms regulating digestion and absorption of nutrients Describe the endocrine control of the gastrointestinal tract Describe the physiological mechanisms that regulate the secretion of bile and role of the enterohepatic circulation Note this introductory lecture aims to provide an overview for the subsequent lectures 2 3 4 Subdivision of GI tract and its common features After mouth, digestive tract differentiated into 4 major organs: esophagus, stomach, small intestine and large intestine. Organs separated by muscular valves or sphincters, with mucosal layer lining the inner surface of the digestive tract. Digestion of carbohydrates initiated in oral cavity through amylase, and majority of digestion and absorption of carbohydrates and amino acids occurs across the mucosae of the stomach and small intestine. Secretions from the exocrine pancreas (e.g. HCO3) and liver (bile salts) pass directly into the lumen of the small intestine. Lymphatic vessels are distributed throughout the tract and involved in intestinal absorption of dietary fat. 5 Salivary glands are compound organs that secrete electrolytes and proteins (e.g. amylase) as a fluid into the oral cavity. Saliva lubricates food for swallowing and glycoproteins secreted from submandibular, 6 sublingual glands, buccal glands aid in starch digestion. 1. Histological organisation of the digestive tract: Mucosa, submucosa, muscularis externa, adventitia/serosa 2. Circulatory and lymphatic vessels for nutrient supply/removal: Vascular and lymphatic vessels aid nutrient absorption. Moreover, epithelial tissue requires a continual supply of nutrients for cell maintenance and repair. 3. Innervation of the GI tract: Autonomic motor and sensory fibres found in GI tract. Motor fibres are both parasympathetic and sympathetic, ramifying throughout GI tract forming a plexus in each layer. 4. Endocrine control of the GI tract: Influence of gut hormones, e.g. neurocrine, paracrine and endocrine control. Endocrine cells widely distributed in epithelia of stomach, small and large intestine, appendix, distal esophageal glands and even ducts of pancreas and liver. 7  Phases of Gastric Acid Secretion Acid secretion divided into basal (fasting) and stimulated (post- prandial) phases, cephalic, gastric and intestinal phases 8 9 Resting parietal cell Activated parietal cell Tubulovesicles Intracellular canaliculus 10 with permission Prof Rod Dimaline, Physiology, Univ. Liverpool Parietal Cell Receptors and Regulation of Acid Secretion Parietal cell acid secretion is regulated by chemical messengers: acetylcholine, histamine & gastrin (Soll's three receptor hypothesis). ACh released at or near basolateral surface of cells from postganglionic neurons (neurocrine mechanism). Gastrin released by G cells of antral mucosa and first part of duodenum into bloodstream, which carries this hormone to the parietal cells (endocrine mechanism). Histamine released from mast-like cells of lamina propria of oxyntic (acid secreting) mucosa into extracellular fluid and subsequently diffuses to the parietal cells (paracrine mechanism). 11  Acid Proton pump inhibitor Food (eg omeprazole) x G-cell D-cell histamine ECL-cell x H2 antagonist (eg famotidine) Gastrin Gastrin 12 with permission Prof Rod Dimaline, Physiology, Univ. Liverpool 13 Secretion of pancreatic juice Highest rate of protein synthesis of any secretory tissue with exception of lactating mammary gland Aqueous component rich in HCO3 which neutralises duodenal content and enzyme component Proteolytic enzymes – trypsinogen, chymotrypsinogen, procarboxypolypeptidase, ribonuclease, deoxyribonuclease Amylase - starch & glycogen Lipase - hydrolyses neutral fat into glycerol & fatty acids. 14 Digestion and absorption of carbohydrates 15 Digestion and absorption of amino acids and oligopeptides 16 Regulation of bile secretion and role of endocrine hormones 17 15 Role of bile salts in the absorption of dietary fats 18 Stimulatory and inhibitory mechanisms regulating digestion and absorption of dietary nutrients 19 20