Gastrointestinal Physiology Chelsea Dawn Unruh The Basics Functions of the GI System Ingestion, digestion, absorption, & defecation Principle Processes Motility, secretion, & membrane transport Stages of Digestion Mechanical & Chemical Mechanical: Increasing surface area Chemical reaction of digestion: hydrolysis Anatomical Tube 2 The Basics The gastrointestinal tract is A tube that propels a substance from one end to the other while attempting to it break down with various digestive juices, absorb as much of its digestive products as possible, & excret what is left Associated with accessory glands that produce the majority of digestive juices Controlled by both nervous and hormonal systems 3 Anatomy of GI System The Gut Tube Accessory Glands Oral cavity Salivary glands Esophagus Pancreas Stomach Liver Duodenum Small Intestine Large Intestine Rectum Anus Which structures of the GI system are retroperitoneal? • Most of the pancreas (except tail) • Most of the duodenum (except first segment) • Ascending & Descending Colon 4 Despopoulos, Color Atlas of Physiology © 2003 Thieme GI Bloodflow Supply Celiac a. Superior Mesenteric a. Inferior Mesenteric a. Drainage Portal Vein Portal vein carries absorbed substances to liver 5 General GI Wall Structure 6 http://www.enel.ucalgary.ca/People/Mintchev/stomach.htm Nervous System of the GI Tract Enteric Nervous System (ENS) Consists of about 10 −100 million nerve cells located in the ganglia of the intestinal wall. Consists of sensory neurons, interneurons, and motor neurons Some sensory signals are transmitted to the CNS ENS is modulated by the ANS Two internal nerve plexuses Auerbach’s & Meissner’s 7 Nervous System of the GI Tract Myenteric or Auerbach’s plexus Between the longitudinal and circular smooth muscle layers Coordinates the movements of muscularis externa Peristalsis, Segmentation, Propulsion, Grinding, etc. Excitatory motoneurons contract muscle by ACh to muscarinic receptors; Inhibitory use VIP and NO Sensory neurons respond to chemical or mechanical stimuli – receptors in all layers of GI tract Interneurons are involved in the many reflex arcs through GI 8 Motility 9 Nervous System of the GI Tract Submucosal or Meissner’s plexus Located in the submucosa, between the muscularis mucosae and the circular layer of muscularis externa involved in internal processing – modulates secretion and blood flow stimulatory secretomotoneurons stimulate glands and epithelium (via Ach, et al.) sensory neurons respond to chemical or mechanical stimuli to produce secretomotor reflexes submucosal vasodilator neurons dilate blood vessels in the submucosa 10 Nervous System of the GI Tract Autonomic Control of GI Both ANS systems modulate the ENS Parasympathetic: Vagus nerve stimulates secretions and motility through distal colon Pelvic splanchnic nerves innervate distal colon & rectum Sympathetic: Innervation through splanchnic & prevertebral ganglia Reduces motility, increases sphincter tone 11 Nervous System of the GI Tract Neurotransmitters Norepinephrine – adrenergic post-gang neurons Ach – pre- and postgang enteric fibers & post gang parasymp VIP – mediates relaxation of circular & vascular muscles Enkaphalin – intensifies contraction of sphincters GRP – mediates release of gastrin CGRP (calcitonin gene-related peptide) – stimulates release of somatostatin 12 Nervous System & GI Wall 13 Segments 14 Segments & Structure Mouth Pharynx Esophagus Stomach Small Intestine Large Intestine Rectum Anus 15 Mouth Ingestion & Digestion Mechanical Mastication (chewing, cutting & grinding with teeth), mixing with tongue Chemical Salivary glands: Parotid, Sublingual, Submandibular Secretions (1.5L/day) Mostly salt & water but also includes amylase, lingual lipase, lysozyme, peroxidase, R-binder (important protein for Vit B12 metabolism), IgA, glycoproteins 16 http://medical-dictionary.thefreedictionary.com/ Functions of Saliva Moistens, cleans, & protects enamel Lubricant aiding in speech, chewing, & swallowing (mucins) Dissolves food – allows for taste Acts as a bonding agent to form bolus Alkaline secretions neutralize regurgitated gastric acid Enzymes (amylase & lipase) begin digestion Immunological: Lysozyme, IgA, & lactoferrin 17 Salivary secretion 18 Salivary Reflexes Conditioned Unconditioned 19 Pharynx Circular muscles Superior, middle ,and inferior pharyngeal constrictors force food down esophagus while swallowing Other muscles Close off oral cavity, nasopharynx, and trachea during swallowing 20 http://www.home-speech-home.com/dysphagia.html Esophagus Straight muscular tube Propels food bolus from pharynx to stomach 1/3 composed of striated muscle, 2/3 smooth muscle LES – physiological sphincter at cardiac orifice Clinical: Correlation Achalasia – inability of the LES to relax causing dysphagia, regurgitation, and chest 21 pain. Causes: cancer, Chagas disease Deglutition Coordinated by swallowing center in medulla oblongata & pons ≥ 22 muscles involved 2 stages Buccal - voluntary Pharyngeal/esophageal - involuntary 22 Deglutition 23 Stomach 24 Stomach J-shaped muscular sack for food storage & digestion located in upper abdomen, directly below diaphragm Empty volume 50 mL, may hold up to 4 L Structures 4 regions: Cardia, fundus, body, and pylorus/antrum 2 sphincters: Lower esophageal/cardiac, pyloric Rugae Innervation ENS, PNS (vagal), SNS Circulation Supplied by celiac artery Drains to hepatic portal circulation 25 Stomach Wall Mucus Only 26 Stomach Secretory Cells Secretory Products - Surface mucous & mucous Mucus, HCO 3 neck cells Oxyntic (Parietal) cells HCl, intrinsic factor Chief (Peptic) cells Pepsinogen, gastric lipase, rennin/chymosin (infancy?) Neurendocrine cells Peptide Hormones G cells Gastrin D cells Somatostatin 27 Gastric Protection H. pylori • Gram negative microaerophilic rod • Flagella to bury itself in the protective mucus NSAIDs • • Urease breaks down urea toCO and ammonia COX inhibitors 2 • (neutralizes) ↓ PGs ↑ acid •Treatment: Triple Therapy 28 Regulation of Gastric Function Cephalic Phase Stimulated by sight, smell, or thought of food (1/3 of secretion) Medicated by vagus nerve, secretions begin before food arrives Gastric Phase Stimulated by food in the stomach (2/3 of gastric secretion) Ingested food stretches and raises the pH activating mechanical and chemical receptors Gastric secretion stimulated by ACh, histamine, and gastrin Intestinal Phase Stimulated by chyme entering the duodenum Enterogastric reflexes reduce gastric motility and secretions 29 Gastric Acid Basal Acid Output: <10 mmol/hr Maximum Acid Output: <50 mmol/hr ↑ w/ pepc ulcer, duodenal ulcer, Zollinger-Ellison Syn. ↓ w/ vagotomy, pernicious anemia Gastric HCl has pH ~1.0, Activates pepsin, dissolves food, disinfects, & stimulates duodenum to secrete secretin Gastric Enzymes Pepsin – cleaves protein into smaller peptides Lipase – digests fats, of little importance except with pancreatic insufficiency Gastric juices (pH 1.5 −2, about 2 −3 liters/day) 30 31 Mechanism of HCl secretion by gastric parietal cells. 32 Agents that stimulate and inhibit H + secretion by gastric parietal cells. 33 Gastric Motility Migrating Motor Complexes (MMC) – 90 min cycle Pacemaker cells (Interstitial cells of Cajal) create 3 phases Phase I – no contractions Phase II – 6 min, irregular contractions Phase III – 3 min, regular contractions (gastric 3 per min, duodenal 11 per min) Receptive reflex: Relaxes stomach to allow entry of food, modulated by swallowing center (vagus) Enterogastric reflexes: (act on motility & secretion) Hormonal signaling from intestines slows gastric emptying and reduces motility Neural reflex: hyperosmolar chyme & mechanical distention 34 Gastric Motility When receptive reflex is activated, MMC is replaced with peristaltic actions Mechanical churning for digestion Emptying time usually ~4hrs Sympathetic System can prolong emptying Antrum holds ~30mL of chyme and releases ~3mL into duodenum with each peristaltic wave 35 36 Small & Large Intestines 37 Small Intestine In the small intestine (duodenum, jejunum, ileum), contractile activity serves to mix the chyme with the alkaline exocrine secretions of the intestine, pancreas, and liver. These secretions contain a number of digestive enzymes, including proteases, amylases, and lipases. Protein, lipids, carbohydrate, vitamins, water, and electrolytes are absorbed primarily in the small intestine 3 parts Duodenum (25cm) Jejunum (2.5 m) Ileum (3.5m) and ends at ileocecal junction 38 Small Intestine 39 Small Intestine: Duodenum Segment between pylorus of stomach and jejunum 4 segments: only 1 st segment (duodenal cap) is intraperitoneal Submucosal Brunner’s glands Produce alkaline mucus to neutralize stomach acid (pepsin is inactivated by the elevated pH and pancreatic enzymes take over the job of chemical digestion) Receives pancreatic digestive juices and bile juices from liver Ampulla of Vater (major duodenal papilla), sphincter of Oddi Primary site for Fe 2+ absorption 40 Small Intestine: Jejunum & Ileum Major site of digestion: brush border contains numerous digestive enzymes 90% absorption – specialized transporters, simple diffusion, facilitated diffusion, active/passive transport Protection against infection – Peyer’s patches, M cells Hormone secretion – CCK, Secretin, Somatostatin, etc Intestinal juice secretion – isotonic fluid (w/ enzymes) secreted by crypts of Lieberkuhn,