The GI System 1-3 (2024) PDF

WEEK 15 MPharm Programme The Gastro-intestinal System 1 Dr G Boachie-Ansah [email protected] Dale 113 ext. 2617 MPharm PHA115 GI System WEEK 15 Outline of L...

WEEK 15 MPharm Programme The Gastro-intestinal System 1 Dr G Boachie-Ansah [email protected] Dale 113 ext. 2617 MPharm PHA115 GI System WEEK 15 Outline of Lectures The main functions of the GI or Digestive System Organisation & functional anatomy of the gastro- intestinal (digestive) system Digestive processes in the mouth & stomach Digestive processes in the small intestine Digestive processes in the large intestine Slide 2 of 83 MPharm PHA115 GI System WEEK 15 Learning Outcomes At the end of this lecture, you should be able to: Describe the basic anatomy & main functions of the gastro-intestinal (digestive) system Explain the digestive processes that take place in the major parts of the digestive tract Describe the neural & chemical control of the gastro-intestinal (digestive) system Discuss the major sites of absorption in the gastro- intestinal (digestive) system Slide 3 of 83 MPharm PHA115 GI System WEEK 15 The GI System A group of organs that work together to break down ingested food into smaller, absorbable nutrient molecules that can then be absorbed into blood for distribution to all the cells in the body Slide 4 of 83 MPharm PHA115 GI System WEEK 15 The GI System Main functions Digests or biochemically breaks down ingested food into small, simple nutrient molecules Absorbs & transfers the nutrient molecules, electrolytes & water derived from the ingested food into systemic circulation Makes the preformed organic nutrient molecules in ingested food available to body cells to be used as sources of fuel or energy building blocks Slide 5 of 83 MPharm PHA115 GI System WEEK 15 The GI System Performs 5 basic digestive processes Motility Secretion Digestion Absorption Storage & elimination of indigestible food Slide 6 of 83 MPharm PHA115 GI System WEEK 15 The Digestive System Motility Muscular contractions that mix & move the contents of the digestive tract forward Two types of digestive motility Propulsive movements propel or push contents forward through the digestive tract Mixing movements mix food with digestive juices & promote digestion facilitate absorption of digested food Slide 7 of 83 MPharm PHA115 GI System  The GI System WEEK 15 Secretion Several digestive juices secreted in GI tract in response to specific neural or hormonal stimulation Consist of water, electrolytes & specific organic constituents (enzymes, mucus, etc) Include both exocrine & endocrine secretions Exocrine – secreted into the lumen of GI tract H2O, HCl, HCO3-, bile, lipase, pepsin, amylase, trypsin & histamine Endocrine – secreted into the blood stream Gastrin, secretin, CCK, VIP & somatostatin Slide 8 of 83 MPharm PHA115 GI System WEEK 15 The GI System Digestion Biochemical breakdown of complex foodstuffs into smaller, absorbable units (chemical digestion) Accomplished by enzyme-mediated hydrolysis Complex foodstuffs & their absorbable units Carbohydrates → monosaccharides Proteins → amino acids Fats → glycerol & fatty acids Slide 9 of 83 MPharm PHA115 GI System WEEK 15 The GI System Absorption Transfer of small digested units, along with water, vitamins & electrolytes, from the GI tract into blood or lymph Occurs largely & most completely in the small intestine Slide 10 of 83 MPharm PHA115 GI System WEEK 15 Functional Anatomy of The Gastro-intestinal (Digestive) System Slide 11 of 83 MPharm PHA115 GI System WEEK 15 The GI System Slide 12 of 83 MPharm PHA115 GI System  The GI System WEEK 15 Gross Structure & Organisation Also called the Gastrointestinal (GI) Tract Comprises 2 main components The alimentary canal long, continuous hollow tube stretching from the mouth to the anus (~30 feet long) Accessory organs & glands Salivary glands Liver & Pancreas Slide 13 of 83 MPharm PHA115 GI System WEEK 15 Alimentary canal Accessory digestive – Continuous from mouth organs to anus – Salivary glands – Consists of – Exocrine pancreas Mouth – Biliary system Pharynx Liver Oesophagus Gallbladder Stomach Small intestine – Duodenum – Jejunum – Ileum Large intestine – Caecum – Appendix – Colon – Rectum Anus Slide 14 of 83 MPharm PHA115 GI System WEEK 15 The GI System The Alimentary Canal Gross Functional Organisation Divided into 3 main parts Upper part the mouth, oesophagus & stomach serves as food intake source & receptacle site of initial digestive processes Slide 15 of 83 MPharm PHA115 GI System WEEK 15 The GI System The Alimentary Canal Gross Functional Organisation Divided into 3 main parts (cont’d) Middle part the small intestine – duodenum, jejunum & ileum site of main digestive & absorptive processes Slide 16 of 83 MPharm PHA115 GI System WEEK 15 The GI System The Alimentary Canal Gross Functional Organisation Divided into 3 main parts (cont’d) Lower part the large intestine – caecum, colon & rectum serves as the storage channel for efficient elimination of waste Slide 17 of 83 MPharm PHA115 GI System WEEK 15 Wall Structure of Alimentary Canal Slide 18 of 83 MPharm PHA115 GI System WEEK 15 Wall Structure of Alimentary Canal Slide 19 of 83 MPharm PHA115 GI System WEEK 15 Wall Structure of Alimentary Canal Comprises 4 layers Mucosa innermost, moist, folded lining of epithelial cells, connective tissue & smooth muscle cells main functions protective barrier against pathogens, etc secretes mucus, digestive enzymes & hormones absorbs digested nutrients into the blood undergoes rapid cell division & regeneration Slide 20 of 83 MPharm PHA115 GI System WEEK 15 Wall Structure of Alimentary Canal Comprises 4 layers Submucosa dense connective tissue layer containing blood and lymphatic vessels, lymphoid follicles & nerve fibres contains the submucosal nerve plexus of Meissner main functions provides the digestive tract with distensibility and elasticity glands secrete digestive enzymes nerve plexus helps regulate gut motility Slide 21 of 83 MPharm PHA115 GI System WEEK 15 Wall Structure of Alimentary Canal Comprises 4 layers Muscularis externa consists of inner layer of circular smooth muscle & outer layer of longitudinal smooth muscle myenteric plexus of Auerbach lies in between main functions contractions facilitate mixing & movement of food contents (peristalsis & segmentation) intrinsic nerve plexus helps regulate gut motility Slide 22 of 83 MPharm PHA115 GI System WEEK 15 Wall Structure of Alimentary Canal Comprises 4 layers Serosa outer connective tissue & epithelial covering continuous with the mesentery main functions secretes a watery, slippery serous fluid  lubrication and  friction between digestive organs & surrounding viscera Slide 23 of 83 MPharm PHA115 GI System WEEK 15 Complex Extrinsic & Intrinsic Regulation of Digestive Function Mediated via Interstitial cells of Cajal (pacemaker cells)  slow-wave potentials (BER)  spontaneous rhythmic electrical activity Intrinsic nerve plexuses (ENS)  GI muscle contraction or relaxation Extrinsic nerves (ANS)  increase or decrease GI contraction & secretion Gastrointestinal hormones  increase or decrease GI contraction & secretion Local sensory receptors – chemoreceptors, mechanoreceptors & osmoreceptors Slide 24 of 83 MPharm PHA115 GI System WEEK 15 Complex Extrinsic & Intrinsic Regulation of Digestive Function Slide 25 of 83 MPharm PHA115 GI System WEEK 15 Complex Extrinsic & Intrinsic Regulation of Digestive Function Slide 26 of 83 MPharm PHA115 GI System WEEK 15 Digestive Processes Slide 27 of 83 MPharm PHA115 GI System WEEK 15 Digestive Processes From Mouth to Stomach Slide 28 of 83 MPharm PHA115 GI System  From Mouth to Stomach WEEK 15 Mastication (chewing) Aids in mechanical breakdown of larger food units Mixes food with saliva which contains salivary amylase  partial digestion of starch Saliva moistens ingested food, lubricates & exerts antibacterial effect Deglutition (swallowing) oropharyngeal stage oesophageal stage Slide 29 of 83 MPharm PHA115 GI System WEEK 15 Swallowing Oropharyngeal stage Movement of the bolus through the pharynx into the oesophagus Slide 30 of 83 MPharm PHA115 GI System  Swallowing WEEK 15 Oesophageal stage Slide 31 of 83 MPharm PHA115 GI System WEEK 15 MPharm Programme The Gastro-intestinal System 2 Dr G Boachie-Ansah [email protected] Dale 113 ext. 2617 MPharm PHA115 GI System  The Stomach WEEK 15 J-shaped sac-like chamber The most distensible part of GI tract Main functions Stores ingested food Secretes gastric juice (acid, mucus & intrinsic factor, digestive enzymes - pepsin, lipase) Mixes food with gastric juice initiates digestion of proteins & fat kills bacteria Moves food (chyme) at controlled & steady rate into the duodenum Slide 33 of 83 MPharm PHA115 GI System  The Stomach WEEK 15 Slide 34 of 83 MPharm PHA115 GI System WEEK 15 Gastric Mucosal Folds, Gastric Pits & Gastric Glands Slide 35 of 83 MPharm PHA115 GI System WEEK 15 Structure of Gastric Gland Slide 36 of 83 MPharm PHA115 GI System WEEK 15 Slide 37 of 83 MPharm PHA115 GI System WEEK 15 Gastric Glands Important Gastric Secretory Products Product Source Functions Hydrochloric acid Parietal cell Hydrolysis; sterilization of meal Intrinsic factor Parietal cell Vitamin B12 absorption Pepsinogen Chief cell Protein digestion Mucus, bicarbonate Surface mucous cells Gastroprotection Trefoil factors Surface mucous cells Gastroprotection Histamine ECL cells Regulation of gastric secretion Gastrin G cells Regulation of gastric secretion Gastrin releasing Nerves Regulation of gastric secretion peptide (GRP) Acetylcholine (ACh) Nerves Regulation of gastric secretion Somatostatin D cells Regulation of gastric secretion Slide 38 of 83 MPharm PHA115 GI System WEEK 15 Pepsinogen Activation in The stomach Slide 39 of 83 MPharm PHA115 GI System WEEK 15 Mechanism of Gastric HCl Secretion Slide 40 of 83 MPharm PHA115 GI System  Parietal Cell – Receptor Activation WEEK 15 of the Proton Pump Slide 41 of 83 MPharm PHA115 GI System WEEK 15 Parietal Cell – Receptor Activation of Proton Pump Slide 42 of 83 MPharm PHA115 GI System WEEK 15 Parietal Cell Receptor Activation of Proton Pump Slide 43 of 83 MPharm PHA115 GI System  Control of Gastric Secretion WEEK 15 The 3 Phases of Gastric Secretion Cephalic (“head”) phase Triggered by factors arising before food enters the stomach – thought, smell, sight or taste of food Gastric phase Triggered by factors resulting from the presence of food in the stomach Intestinal Phase Triggered by factors resulting from movement of food from stomach into duodenum Slide 44 of 83 MPharm PHA115 GI System  Control of Gastric Secretion WEEK 15 Excitatory Cephalic & Gastric Phases Slide 45 of 83 MPharm PHA115 GI System  Control of Gastric Secretion WEEK 15 Inhibitory Intestinal Phase Slide 46 of 83 MPharm PHA115 GI System WEEK 15 Control of Gastric Secretion Inhibitory Duodenal Mechanisms Slide 47 of 83 MPharm PHA115 GI System WEEK 15 Control of Gastric Secretion Slide 48 of 83 MPharm PHA115 GI System WEEK 15 Digestion & Absorption in the Stomach Carbohydrate digestion by salivary amylase continues partial inhibition of amylase by acidity Proteins are partially digested by pepsin in the antrum Only alcohol & aspirin are absorbed Slide 49 of 83 MPharm PHA115 GI System WEEK 15 The Gastric Mucosal Barrier Slide 50 of 83 MPharm PHA115 GI System  The Gastric Mucosal Barrier WEEK 15 Protects Stomach from Damage Cell membranes of gastric mucosal cells are impermeable to HCl on the luminal side Tight junctions between adjacent gastric mucosal (epithelial) cells are impermeable to HCl The HCO3–-rich mucus serves both as protective physical barrier & as chemical barrier that neutralizes acid close to gastric mucosa Rapid rate of cell division – entire stomach lining replaced in 3 days Prostaglandins inhibit acid secretion & promote alkaline mucus secretion Slide 51 of 83 MPharm PHA115 GI System WEEK 15 Digestive Processes From Small Intestine To Large Intestine Slide 52 of 83 MPharm PHA115 GI System WEEK 15 The Small Intestine Digestive & Absorptive Functions Main site for digestion & absorption of ingested food Secretes aqueous salt & mucus solution for lubrication & protection Utilizes segmentation & peristaltic contractions to mix & propel chyme, and facilitate absorption Digestion occurs via action of bile, pancreatic enzymes & intestinal (brush-border) enzymes Absorption occurs along the entire length – but largely in the duodenum & jejunum Slide 53 of 83 MPharm PHA115 GI System WEEK 15 The Small Intestine Structure-function Relationships Mucosal surface thrown into permanent circular folds  3-fold  in surface area Microscopic finger-like projections (villi) project from circular folds  10-fold  in surface area Villi are covered by epithelial cells Epithelial cells have numerous smaller, hair-like projections, called microvilli or brush border  20-fold  in surface area 3 types of digestive enzymes are expressed on the membranes of microvilli or brush border – enteropeptidase, disaccharidases & aminopeptidases Slide 54 of 83 MPharm PHA115 GI System WEEK 15 Intestinal Circular Folds, Villi & Microvilli Slide 55 of 83 MPharm PHA115 GI System WEEK 15 The Structure of Intestinal Villi Slide 56 of 83 MPharm PHA115 GI System WEEK 15 MPharm Programme The Gastro-intestinal System 3 Dr G Boachie-Ansah [email protected] Dale 113 ext. 2617 MPharm PHA115 GI System  The Small Intestine WEEK 15 Three Main Subdivisions Duodenum & Hepatopancreatic organs Jejunum Ileum Slide 58 of 83 MPharm PHA115 GI System WEEK 15 The Duodenum & Hepatopancreatic Region Slide 59 of 83 MPharm PHA115 GI System  The Duodenum WEEK 15 The first 10 inches of small intestine First site of intensive digestion Receives bile & pancreatic juice via the common bile duct & main pancreatic duct Arrival of chyme stimulates release of hepatic & pancreatic secretions fatty chyme in the duodenum  release of CCK  release of bile & pancreatic enzymes acid in chyme  release of secretin  release of pancreatic bicarbonate  neutralizes the acidity Slide 60 of 83 MPharm PHA115 GI System  Structure of the Pancreas WEEK 15 Slide 61 of 83 MPharm PHA115 GI System WEEK 15 Functional Anatomy of Pancreas Exocrine Duct cells secrete aqueous bicarbonate solution Acinar cells secrete pancreatic juice – proteases, amylase, lipase Endocrine Islets of Langerhans -cells – secrete insulin -cells – secrete glucagon Slide 62 of 83 MPharm PHA115 GI System  Secretions of the Pancreas WEEK 15 Slide 63 of 83 MPharm PHA115 GI System WEEK 15 Hormonal Control of Pancreatic Exocrine Secretion Slide 64 of 83 MPharm PHA115 GI System WEEK 15 Activation of Pancreatic Enzymes Slide 65 of 83 MPharm PHA115 GI System WEEK 15 The Liver & Hepatic Portal Circulation Slide 66 of 83 MPharm PHA115 GI System  The Liver WEEK 15 The largest gland in the body Composed of ‘plates’ of liver cells (hepatocytes) main digestive function is the production of bile Bile a fat emulsifier a yellow-green, alkaline solution contains bile salts, bile pigments, cholesterol, neutral fats, phospholipids & electrolytes stored & released from gallbladder Slide 67 of 83 MPharm PHA115 GI System WEEK 15 Fat Emulsification by Bile Salts Slide 68 of 83 MPharm PHA115 GI System WEEK 15 Control of Bile Release Slide 69 of 83 MPharm PHA115 GI System WEEK 15 Enterohepatic Circulation of Bile Salts Slide 70 of 83 MPharm PHA115 GI System WEEK 15 Digestion & Absorption in the Small Intestine Slide 71 of 83 MPharm PHA115 GI System WEEK 15 Absorption in Small Intestine Duodenum and jejunum main site of absorption freely absorb all products of carbohydrate, protein & fat digestion freely absorb most ingested electrolytes, vitamins & water absorption of calcium & iron is adjusted to the body’s needs Ileum absorbs bile salts, vitamin B12, electrolytes & water Slide 72 of 83 MPharm PHA115 GI System WEEK 15 Carbohydrate, Protein & fat Digestion Slide 73 of 83 MPharm PHA115 GI System WEEK 15 Carbohydrate Digestion Slide 74 of 83 MPharm PHA115 GI System  Carbohydrate Absorption WEEK 15 Slide 75 of 83 MPharm PHA115 GI System  Protein Digestion WEEK 15 Slide 76 of 83 MPharm PHA115 GI System  Protein Absorption WEEK 15 Slide 77 of 83 MPharm PHA115 GI System WEEK 15 Fat Digestion and Absorption Slide 78 of 83 MPharm PHA115 GI System WEEK 1. Dietary fat in the form of large fat globules composed of triglycerides is 15 emulsified by the detergent action of bile salts into a suspension of smaller fat droplets. This lipid emulsion prevents the fat droplets from coalescing and thereby increases the surface area available for attack by pancreatic lipase. 2. Lipase hydrolyzes the triglycerides into monoglycerides and free fatty acids. 3. These water-insoluble products are carried to the luminal surface of the small-intestine epithelial cells within water-soluble micelles, which are formed by bile salts and other bile constituents. 4. When a micelle approaches the absorptive epithelial surface, the monoglycerides and fatty acids leave the micelle and passively diffuse through the lipid bilayer of the luminal membranes. 5. The monoglycerides and free fatty acids are resynthesized into triglycerides inside the epithelial cells. 6. These triglycerides aggregate and are coated with a layer of lipoprotein from the endoplasmic reticulum to form water-soluble chylomicrons. 7. Chylomicrons are extruded through the basal membrane of the cells by exocytosis. 8. Chylomicrons are unable to cross the basement membrane of capillaries, so instead they enter the lymphatic vessels, the central lacteals. Slide 79 of 83 MPharm PHA115 GI System  Iron Absorption WEEK 15 Slide 80 of 83 MPharm PHA115 GI System  The Large Intestine WEEK 15 Consists of the colon, caecum, appendix & rectum Primarily a drying & storage organ Minor absorptive function absorbs water, electrolytes, several vitamin B complexes, vitamin K & folic acid Contains many beneficial bacteria (~500 -1000 sp) produce vitamin K & folic acid ferment indigestible molecules to produce short-chain fatty acids Slide 81 of 83 MPharm PHA115 GI System  The Large Intestine WEEK 15 Slide 82 of 83 MPharm PHA115 GI System WEEK 15 Recommended Reading Sherwood L. Human Physiology: From Cells to Systems, 9th ed. CENGAGE Learning (2015) ISBN: 978-1305273467 Sherwood L. Introduction to Human Physiology, 8th ed. Thomson Brooks/Cole/ CENGAGE Learning (2012) ISBN: 978-1133104544 VanPutte C et al. Seeley's Anatomy & Physiology, 13th ed. McGraw Hill (2022) ISBN: 978-1265129583 Slide 83 of 83 MPharm PHA115 GI System