Digestive System-Stomach Part 1: Stomach Physiology PDF

Digestive System-Stomach Part 1: Stomach Physiology Dr Seley Gharanei, Assistant Professor Cell and Tissue Biomedicine [email protected] MB ChB Phase I Block 1 Health, Metabolism and  Digestive System  Part 1: Stomach Physiology...

Digestive System-Stomach Part 1: Stomach Physiology Dr Seley Gharanei, Assistant Professor Cell and Tissue Biomedicine [email protected] MB ChB Phase I Block 1 Health, Metabolism and  Digestive System  Part 1: Stomach Physiology  Part 2: Gastric Secretion  Part 3: Gastric Motility PPT - Pharmacology of Gastric Acid Secretion Signposts to other Themes Questions and Block teaching  Learning outcomes:  Describe the physiology of the stomach  Identify the different layers of the stomach wall and their functions  Identify the cells of the gastric gland and describe their function and secretions  Stomach function  The site of chemical and mechanical digestion  Store food  Chyme: ingested food plus stomach secretions  Acidic digestive fluid produced in the Structure and function of stomach anatomy system vector image stomach Gastric Juice  pH 1-3 - HCl - Electrolytes Q1: What is the main component  Per day secretion 2-3 litres - Mucus of the gastric - Water juice?  Oesophageal sphincter - Intrinsic factors  - Bicarbonate pyloric sphincter Divisions of Stomach 1. The Cardia: surrounds the opening of the esophagus into the stomach. 2. The fundus of stomach: the area above the level of the cardial orifice. 3. The body of stomach: the largest region of the stomach. 4. The pyloric part: divided into the pyloric antrum and pyloric canal and is the distal end of the stomach. Orad region  The Orad regions is also known as the oxyntic gland area  The Caudad region is known is the pyloric Caudad region gland area Microscopic histology of the stomach Gastrointestinal Tract Histology  Innervation of the GI tract Autonomic Nervous system The main innervations are Left and Right Vagus Nerves. Extrinsic component and an intrinsic component. The extrinsic: sympathetic and parasympathetic innervation The intrinsic component is called the enteric Berne and Levy, Principles of Physiology (Mosby) Autonomic Nervous nervous system. System “Catch Up Biology” Ch.  The Enteric Plexuses  Submucosal plexus: in the deeper submucosal layer. This positions it closer to the gastric glands where it controls gastric secretion, blood flow and detects the nutritional composition of the bolus.  Myenteric plexus: in the more superficial muscularis externa layer allowing it to modulate tone and velocity of smooth muscle contractions. Q2: How is the enteric nervous systems regulated?  Gastric Gland  Gastric pits extend into the mucosa as straight and branched tubules, forming gastric glands.  They are lined with 4 major types of secretory epithelial cells: 1.Parietal cells: hydrochloric acid (HCl) and intrinsic factors. 2.Neck cell: secrete bicarbonate to buffer the pH, mucus and water 3.Chief (Zymogenic) cells: secrete digestive enzymes 4.Endocrine cells (D cells, G cells, enterochromaffinlike (ECL) cells, A-like cells and enterochromaffin (EC) cells) Q3: Which nutrients are secrete hormones e.g gastrin, ghrelin, mainly digested in the somatostatin Gastric secretion  Functional distribution of secretory cells in the gastric mucosa  Parietal (HCl-secreting) cells are located in the proximal 80 % of the stomach (“oxyntic gland area”).  Gastrin-producing cells are found typically in the antrum (“pyloric gland area”).  Parietal cell HCL and intrinsic factors  Secretes  HCL: converts pepsinogen to pepsin, provides optimal pH environment destroys bacteria  Intrinsic factor is a glycoprotein essential for the absorption of vitamin B12 in the terminal ileum of the small intestine.  Intrinsic factor production is the only indispensable Davies et al, Human Physiology (Churchill Livingstone) function of the stomach.  Specialised structure: Truncated pyramidal shape, with apex oriented towards gastric lumen.  Highly folded luminal membrane forming canaliculi, whose membranes are lined with “proton pumps” that secrete H+.  High mitochondrial content. Q4: Why do parietal cells Q5: What is the have a high consequence of prolonged mitochondrial content? vitamin B12 deficiency?  Chief cell  Chief cells are at the lower part of the gastric gland  Secret the digestive enzyme Pepsin in the form of the inactive precursor, pepsinogen.  Exposure to low pH in the gastric lumen causes the conversion of pepsinogen to pepsin. Q6: Why is pepsin synthesised by and  Pepsinogen secretion parallels acid secreted from chief cells secretion. in the form of the inactive precursor  Pepsin is active only at low pH. pepsinogen?   Endocrine cells G cells D cells ECL located in the antrum Located in the cells Enterochromaffin-like ECL (“pyloric gland antrum and body of cell scattered throughout area“). the stomach the gastric mucosa, Gastrin Somatost close to parietal cells. principal hormone atin polypeptide hormone secreted from the Inhibition of gastrin gastric epithelium. Histamine stimulates the secretion of release. Inhibition of acid is a paracrine acid, pepsinogens, mucus, HCO3-. secretion. agent stimulation of acid secretion. stimulation of gastric increased local blood flow, motility. which supports the increase in inhibition of gastric metabolism associated with Q7: What is a emptying. paracrine agent? acid secretion.  Mucus secreting cells  Mucus Neck cells and surface mucus cells  To protect the stomach lining from the acid and the enzymes  Mixture of glycoproteins and lipopolysaccharides.  Mucus forms a gel (up to 200 mm thick) on the luminal surface of the gastric mucosa.  Secret Bicarbonate HCO3- to buffer the pH  Mucus and the alkaline secretions entrapped within it constitute a “gastric mucosal barrier” that protects the stomach against acid, proteolytic enzymes and mechanical damage.  The Mucosal Barrier  Bicarbonate rich mucus  The epithelial cells meet at tight junctions, block gastric juice from penetrating the underlying tissue layers  Local irritation stimulates the production of prostaglandins (PGs), increase production of mucus and HCO3- and inhibit acid secretion.  Cells of the stomach lining have a high rate of cell division.  Renewal of the Gastric Mucosal Barrier protects the stomach against autodigestion Q8: What are prostaglandins and how/ where are they produced? Consequences of failure of this protection  Breaching the gastric mucosal barrier leads to gastritis (inflammation of underlying tissue).  Persistent erosion of the gastric mucosa can lead to formation of gastric ulcers. Benign gastric ulcer Deep gastric ulcer A gastric ulcer is a lesion in the gastric mucosa. Resources Clinical Key Student (https://www.clinicalkey.com/student): Physiology. Seventh Edition. Costanzo, L. S. 2022. Elsevier. Costanzo, Linda S., PhD, Physiology, Chapter 8, 339-393. Engevik AC, Kaji I, Goldenring JR. 2020. The Physiology of the Gastric Parietal Cell. Physiol Rev. Apr 1;100(2):573-602. PMID: 31670611; Medical Sciences, Naish and Syndercombe Court, Chapter 15: The Alimentary System https://0-www-clinicalkey-com.pugwash.lib.warwick.ac.uk/student/content/book/3-s2.0-B 9780702073373000158  Answers to the questions Q1. What is the main component of the gastric juice? HCL is the main component of the gastric juice, converts pepsinogen to pepsin, Provides optimal pH environment destroys bacteria, promotes the absorption of electrolytes. Q2: What is the main function of the enteric nervous systems? It connects the brain and the digestive system through its afferent and efferent nerves, which pass messages back and forth between the CNS and the ENS. The ENS controls secretions, blood flow, hormone release, and motility (the movement of food through the digestive tract), all of which are part of the digestive process. Q3: Which nutrients are mainly digested in the stomach? Proteins are mainly digested in the stomach through the actions of pepsin and hydrochloric acid. Q4: Why do parietal cells have a high mitochondrial content? Parietal cells have high content mitochondria to synthesise ATP to support the function of proton pumps to secret hydrogen ions and provide CO2. Q5:What is the consequence of prolonged vitamin B12 deficiency? Vitamin B12 deficiency can lead to pernicious anaemia – where the immune system attacks healthy cells in the stomach, preventing your body absorbing vitamin B12 from the food. Q6: Why is pepsin synthesised by and secreted from chief cells in the form of the inactive precursor pepsinogen? Pepsin is an active enzyme; it could digest the proteins in the cells that produce it. Q7: What is a paracrine agent? A paracrine agent is a substance secreted by a cell and acts on adjacent cells. Q8: What are prostaglandins and how/ where are they produced? Prostaglandins are a group of lipid