Lecture 21: Digestive System PDF

About This Chapter 21.1 Anatomy of the Digestive System 21.2 Digestive Function and Processes Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Digestive System is a Tube Digestion carried out by intestinal enzymes aided by 2 accessory glandular organs: pancreas and...

About This Chapter 21.1 Anatomy of the Digestive System 21.2 Digestive Function and Processes Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Digestive System is a Tube Digestion carried out by intestinal enzymes aided by 2 accessory glandular organs: pancreas and liver —> secretions from these 2 enter the initial section of the duodenum through ducts and a tonically contracted sphincter (of Oddi) keeps pancreatic fluid and bile from entering small intestine except during a meal • Accessory glandular organs secrete materials that aid in digestion Salivary glands: secretion of saliva by sublingual glands under the tongue, glands under the mandible (jawbone) and parotid glands – Salivary glands, pancreas, liver submandibular lying near the hinge of the jaw • Chyme is the mixture of food and secretions mixing food with acid and enzymes • Products of digestion are absorbed – Across epithelium into interstitial fluid – From interstitial fluid to blood or lymph to be distributed • Waste is excreted from the GI tract by way of the anus • Digestive system is exposed to external environment – Home to commensal microorganisms Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Digestive System is a Tube • • Digestion begins in the oral cavity – Chewing – Salivary glands Esophagus swallowed food passes through it narrow tube that travels through thorax to abdomen Wall: skeletal muscle initially but transition to smooth muscle about 2/3 of the way down the length upper fundus central lower • Stomach: fundus → body → antrum • Pylorus with pyloric valve • Small intestine: duodenum → jejunum → ileum with help of pancreas and liver • creation of chyme opening between stomach and small intestine guarded by pyloric valve terminal section skeletal muscle, voluntary control Large intestine: colon → recturm → anus with external anal Colon = proximal section of large intestine : watery chyme becomes semisolid feces as sphincter water and electrolytes are absorbed out of the chyme and into the ECF • Feces Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.1(d-e) The Digestive System Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The GI Tract Wall Has Four Layers (1 of 2) • Modifications increase surface area – – – folds in stomach folds in small intestine Rugae and plicae fingerlike extension Villi small projected by intestinal mucosa into lumen Gastric glands, crypts, and submucosal glands intestine stomach • tubular invaginations of the surface that extend down into supporting connective tissue (gastric glands, crypts, and submucosal glands) lining of Mucosa inner GI tract with 3 deepest, form secretory submucosal glands that open into lumen through ducts facing lumen cells: enterocytes in small intestine (transporting epithelial cells), endocrine and exocrine secretory cells, and stem cells Secrete ions, enzymes, mucus and paracrine molecules into lumen sub epithelial connective tissue that holds the epithelium in place contain nerve fibres and small blood and lymph vessels absorbed nutrient pass into blood and lymph Peyer’s patches of the gut-associated lymphoid tissues (GALT) intestine lymphoid aggregations of small nodules and larger Peyer’s thin layer of smooth muscle patches create visible bumps in separates lamina propria from submucosa contraction of muscles alters effective surface area for absorption by mucosa moving villi back and forth middle layer of gut wall layers 1. Mucosal epithelium 2. Lamina propria (connective tissue) ▪ 3. Muscularis mucosae • Submucosa – Connective tissue with larger blood and lymph vessels running through it – Contains submucosal plexus of the enteric nervous system Meissner’s plexus innervates cells in epithelial layer as well as smooth muscle of the muscularis mucosae Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The GI Tract Wall Has Four Layers (2 of 2) • Submucosa – middle layer – Connective tissue – • circular layer and outer Contains submucosa plexus of the enteric nervous systeminner longitudinal layer, contraction of Muscularis externa – outer wall of GI tract circular layer decreases diameter of lumen vs longitudinal shortens tube incomplete third layer of oblique muscle between circular muscles and submucosa – Consists of two layers of smooth muscle in small intestine – longitudinal and circular Contains myenteric plexus of the enteric nervous systembetween muscle layers Auerbach’s plexus controls and coordinates motor activity of muscularis externa • Serosa – outer covering of digestive tract – connective tissue membrane lining abdominal cavity Continuation of the peritoneal membrane, which forms sheets of mesentery that holds intestines in place so that they don’t tangled as they move Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.1(f) The Digestive System Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.2 Four processes of the digestive system Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved GI Smooth Muscle Exhibits Different Patterns of Contraction Perstalsis: progressive waves of contraction that move from 1 section of GI to the next Circular muscles contract just behind bolus of food, which pushes it forward into a receiving segment where the circular muscles are relaxed Receiving segment then contracts, continuing the forward movement In esophagus propels material from pharynx to stomach contributes to food mixing in stomach, but in normal digestion, intestinal peristaltic waves are limited to short distances • Muscle contractions during and following a meal fall into one of the 2 patterns: Contractions occur in 3 patterns Between meals when tract is largely empty, series of contractions begins in stomach passes slowly from section to section, each series taking 90 minutes to reach – Migrating motor complex and the large intestine —> housekeeping function that sweeps food remnants and bacteria out of the upper GI tract and into large intestine – Peristalsis moves bolus forward short segments of intestine alternately contract and relax segments: circular muscles contract while longitudinal muscles relax – Segmental contractions Contracting May be random along the intestine or regular interval Alternating segmental contractions churn the intestinal contents, mixing them and keeping them in contact with absorptive epithelium When segments contract sequentially, in an oral-to-aboral direction, intestinal contents are propelled short distance • Slow-wave potentials originate in interstitial cells of Cajal cells of Cajal: pacemakers and link between GI smooth muscle and enteric and autonomic NS Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.4(a-b) Gastrointestinal Motility Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.4(c-d) Gastrointestinal Motility Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 21.4 Integrated Function: The Cephalic Phase • • Smelling, seeing, or thinking about food – Reflex begins in brain – Anticipation of or presence of food in oral cavity activates neurons in medulla – Initiates cephalic phase Chemical and mechanical digestion begins in the mouth. – Salivary secretions have 4 functions 1. Soften and lubricate food 2. Digestion of starch (chemical) 3. Taste 4. Defense – Mechanical digestion begins with chewing ▪ Mastication Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 21.5 Integrated Function: The Gastric Phase • 3 functions of the stomach 1. Storage ▪ Upper stomach 2. Digestion ▪ Lower stomach ▪ Lipids and proteins ▪ Acid, enzymes, paracrine signal molecules, and hormones 3. Defense ▪ Against swallowed pathogens • Digestive activity in stomach begins with long vagal reflex of cephalic phase then food in stomach initiates short reflexes of gastric phase • The stomach stores food Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.8 Cephalic and gastric phase reflexes Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Gastric Secretions Protect and Digest deep within mucosal layer • Gastric glands found in the gastric pits • Gastrin secretion – found deep in gastric glands G cells secrete gastrin ▪ • hormone neurotransmitter that mediates short reflexes Gastrin-releasing peptide (GRP) ENS gastrin release by neural reflexes Acid secretion deep in gastric glands – • openings in mucus-producing epithelium Parietal cells secrete gastric acid (HCl) into lumen Enzyme secretion – pepsin carries out initial digestions of protein, particularly effective on collagen in gastric cells Chief cells secrete pepsinogen (activated to pepsin) and gastric lipase Acid stimulates pepsinogen release from chief cells through short reflex mediated in ENS • break down TG but less than 1/3 of fat digestion in stomach Paracrine secretions from gastric mucosa paracrine signal in response to gastrin or acetylcholine stimulation stimulates acid secretion by combining with H2 receptors on parietal cells – Enterochromaffin-like (ECL) cells secrete histamine – the lumen, form complexes with vitamin B12, a step needed for Parietal cells secrete intrinsic factor Invitamin’s absorption in intestine – growth hormone-inhibiting hormone D cells secrete somatostatin (SS) hypothalamic secreted by D cells in stomach protein secreted by same gastric parietal cells that secrete acid Primary - feedback for gastric phase secretion Shuts down acid secretion directly and indirectly by decreasing gastrin and histamine section + pepsinogen secretion Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Stomach Balances Digestion and Defense • Stomach protects itself with a mucus-bicarbonate barrier – Prevents autodigestion • Mucous cells – Located ▪ On luminal surface ▪ Neck of gastric glands – Secrete mucus and bicarbonate chemical buffer barrier underlying mucus Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.9(a) Gastric Secretions Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.9(b-c) Gastric Secretions Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.10 Integration of cephalic and gastric phase secretion Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 21.6 Integrated Function: The Intestinal Phase • Motility in the small intestine is controlled – Segmental and peristaltic contractions – Ensure that small intestine is not overwhelmed by chyme – Need to complete digestive process and allow for absorption • Maximization of surface area – Villi and crypts of intestinal epithelium – Brush border surface bristle-like appearance of microvilli • Absorbed nutrients from intestinal epithelium – Most go to circulatory system for distribution ▪ Hepatic portal system – Fats use lymphatic circulation to enter circulatory system Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.11 The villus and a crypt in the small intestine Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.12 The hepatic portal system Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Intestinal Secretions Promote Digestion 1. Digestive enzymes 2. Bile produced by intestinal epithelium and exocrine pancreas made in liver and released from gall bladder nonenzymatic solution facilitating digestion of fat 3. Bicarbonate secretion into small intestine neutralizes highly acidic chyme that enters from stomach most comes from pancreas and is released in response to neural stimuli and secretin 4. Mucus from intestinal goblet cells protects epithelium and lubricates the gut’s contents 5. Isotonic NaCl secretion mixes with mucus to help lubricate contents of gut – Cystic fibrosis transmembrane conductance regulator, or CFTR from ECF enters cell via NKCC transporters, then exits into lumen via apical gated Cl- channel = channel Chloride CFTR which draws Na+ down the electrical gradient through leaky cell functions Water follows Na+ along osmotic gradient created by redistribution of NaCl = secretion of isotonic saline solution Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.13 Isotonic NaCl secretion Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Pancreas Secretes Enzymes and Bicarbonate • Endocrine portion (islets) – Secretes insulin and glucagon • Exocrine portion – Secretes digestive enzymes (acini) and sodium bicarbonate (duct cells) – Enzyme secretion ▪ Brush border enteropeptidase converts trypsinogen to trypsin then converts other pancreatic zymogens to their active fort – Bicarbonate secretion ▪ Neutralizes gastric acid Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.14 The Pancreas (1 of 2) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.14 The Pancreas (2 of 2) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Liver Secretes Bile • Bile is secreted by hepatocytes • Composition 1) Bile salts 2) Bile pigments 3) Cholesterol • Aids in fat digestion • Stored and concentrated in gall bladder • Gall bladder sends bile to duodenum through common bile duct Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.15(a-b) The Liver Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.15(c) The Liver Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Figure 21.15(d) The Liver Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Key words: Chyme, salivary glands, esophagus, stomach, fundus, body, antrum, pylorus, pyloric valve, small intestine, duodenum, jejunum, ileum, pancreas, liver, large intestine, colon, feces, rectum, anus, external anal sphincter, villi, gastric glands, crypts, submucosal glands, mucosa, submucosa, muscularis externa, serosa, mucosa, mucosal epithelium, lamina propria, enterocytes, mucosal surface, submucosal plexus (Meissner’s plexus), enteric nervous system, muscularis externa, myenteric plexus, digestion, absorption, secretion, motility, proenzyme, zymogen, mucus, mucins, mucous cells, goblet cells. tonic contractions, phasic contractions, slow wave potentials, interstitial cells of Cajal, migrating motor complex, peristalsis, bolus, segmental contractions. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved