Chapter 16 The Digestive System Stomach PDF

Chapter 16 The Digestive System The Stomach Sherwood Modified, Edited and Presented by Marios Z Panos Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  DIGESTIVE SYSTEM OUTLINE - The Stomach - Func...

Chapter 16 The Digestive System The Stomach Sherwood Modified, Edited and Presented by Marios Z Panos Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  DIGESTIVE SYSTEM OUTLINE - The Stomach - Functions - Storage, Secretions, - Mixing, Motility - Control of HCL Secretion Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Stomach  Stomach J-shaped sac-like chamber lying between esophagus and small intestine Divided into three sections – Fundus, Body, Antrum Three main functions – Store ingested food until it can be emptied into small intestine – Secretes hydrochloric acid (HCl) and enzymes that begin protein digestion – Mixing movements convert pulverized food to chyme Pyloric sphincter – Serves as barrier between stomach and upper part of small intestine Gastric pit and Gland Stomach  Stomach Types of Gland located in gastric mucosa: ❖ Cardiac Glands ❖ Pyloric glands (many G cells) ❖ Oxyntic glands (most abundant, found in fundus and corpus) Cardia = just below gastroesophageal junction G cells = gastrin secreting cells Oxyntic glands = secrete HCl  Types of Cell ❖ Parietal cells HCl & intrinsic factor ❖ Chief cells Pepsinogen ❖ Mucus neck cells - HCO3- - Mucus Parietal cells = oxyntic cells  Stomach – types of cells (contd.) Gastric hormone-secreting cells: ❖ G Cells: Gastrin (hormone) ---> HCl secretion ❖ D Cells: Somatostatin (antrum) ❖ Enterochromaffin-like cell: Histamine Location and function of gastric cell types  Gastric juice Contents - HCL Pepsinogen Electrolytes Intrinsic factor Mucus (mucus gel layer)  Gastric Mucosal Barrier Enables stomach to contain acid without injuring itself Gastric motility NB. An extra layer of smooth muscle Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  Gastric motility Functions 1. Allows the stomach to serve as reservoir 2. Breaks food to small particles and mix it with gastric juice 3. Empties gastric contents at a controlled rate Allows contents to become iso-osmolar V. IMPT - before entering small bowel  Gastric Motility Four aspects – Filling Involves receptive relaxation - fundus – Enhances stomach’s ability to accommodate the extra volume of food with little rise in stomach pressure – Triggered by act of eating – Mediated by vagus nerve – Storage Takes place in body of stomach – Mixing Takes place in antrum of stomach – Emptying Largely controlled by factors in duodenum  Gastric motility Reservoir part fundus + 1/3 corpus (tonic contraction) Antral pump 2/3 corpus + antrum & pylorus (phasic contraction)  Mixing & emptying of gastric contents Gastric contents may remain unmixed (1h) Fat takes longer time to empty* than other Liquids are emptied easier and first Major mixing activities in the antrum Retropulsion * Fat / oil causes delay in gastric emptying Gastric Emptying and Mixing as a Result of Antral Peristaltic Contractions Constriction of the pyloric sphincter Hormones promote constriction 1. CCK 2. Secretin 3. Gastrin 4. GIP Sympathetic innervation CCK = cholecystokinin GIP = gastric Inhibitory peptide = “gastrointestinal peptide” Relaxation of the gastric reservoir (Fundus and upper body) Receptive relaxation - triggered by swallowing reflex Adaptive relaxation - triggered by stretch receptors (vago-vagal reflex) - lost in vagotomy  threshold of fullness and pain Feedback relaxation - triggered by chyme in small intestine  Gastric Emptying Factors in stomach – Amount of chyme in stomach is main factor that influences strength of contraction Factors in duodenum – delay emptying – Fat Fat digestion and absorption takes place only within lumen of small intestine When fat is already in duodenum, further gastric emptying of additional fatty stomach contents is prevented – Acid Unneutralized acid in duodenum inhibits further emptying of acidic gastric contents until neutralization can be accomplished – Hypertonicity Gastric emptying is reflexly inhibited when osmolarity of duodenal contents starts to rise – Distension Too much chyme in duodenum inhibits emptying of even more gastric contents  Gastric Emptying-motility Factors trigger either – Neural response Mediated through both intrinsic nerve plexuses (short reflex) and autonomic nerves (long reflex) Collectively called enterogastric reflex – Hormonal response Involves release of hormones from duodenal mucosa collectively known as enterogastrones – Secretin – Cholecystokinin (CCK) – Vasoactive Intestinal Peptide (VIP) Additional factors that that influence gastric motility – Emotions Sadness and fear – tend to decrease motility Anger and aggression – tend to increase motility – Intense pain – tends to inhibit motility NB. Presence of food or acid in the duodenum delays gastric emptying Scintigraphy - Normal Gastric emptying Scintigraphy – Gastroparesis Disorders of Gastric emptying GASTROPARESIS - SCINTIGRAPHY Delayed gastric emptying  GASTRIC SECRETIONS Chapter 16 The Digestive System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning  Gastric Secretions Two distinct areas of gastric mucosa that secrete gastric juice – Oxyntic mucosa Lines body and fundus – Pyloric gland area (PGA) Lines the antrum Gastric pits at base of gastric glands Three types of gastric exocrine secretory cells – Mucous cells Line gastric pits and entrance of glands Secrete thin, watery mucus – Chief cells Secrete enzyme precursor, pepsinogen – Parietal (oxyntic) cells Secrete HCl and intrinsic factor Gastric Secretions  HCl Secretion Functions of HCl – Activates pepsinogen to active enzyme pepsin and provides acid medium for optimal pepsin activity – Aids in breakdown of connective tissue and muscle fibers – Denatures protein – Along with salivary lysozyme, kills most of the microorganisms ingested with food  Gastric juice HCL Secretion Secreted by parietal cells Fundus Body HCl Secretion  HCL Secretion (cont) Mechanism of HCl production: H/K ATPase ❖ Inhibited by: omeprazole (PPI = Proton Pump Inhibitor) H/K pump depends on [K]out [HCl] drives water into gastric content to achieve/maintain iso-osmolality During gastric acid secretion: amount of HCO3-  in blood = amount of HCl being secreted Alkaline tide ( gastric venous blood pH Î )  Neural & Hormonal Control of Gastric Secretion ❖ Vagus nerve (neural effector) ❖ Gastrin (hormonal effector) ❖ Enterochromaffin-like cells - > Histamine - > H2 receptor (parietal cells) - > acid secretion Cimetidine (H2 receptor blocker) - > acid secretion - > Rx peptic ulcer and gastro-esophageal reflux e.g.s of H2 receptor blockers: Cimetidine, ranitidine, famotidine for the treatment of peptic ulcer and gastro-esophageal reflux PPIs have supplanted H2 antagonists Proton pump inhibitors (PPIs) block the gastric H,K-ATPase  Phases of Gastric Secretion Cephalic phase – Refers to increased secretion of HCl and pepsinogen that occurs in response to stimuli acting in the head before food reaches stomach Gastric phase – Begins when food actually reaches the stomach – Presence of protein increases gastric secretions Intestinal phase – Inhibitory phase – Helps shut off flow of gastric juices as chyme begins to empty into small intestine  Inhibition of Acid Secretion ❖ Inhibitory hormones (Enterogastrones) Somatostatin (D-cells) in antrum Secretin (S-cells) in duodenum Glucose-dependent insulinotropic peptide (GIP) in duodenum GIP = also known as gastric inhibitory peptide or gastrointestinal peptide  DIGESTIVE SYSTEM In this lecture we covered The Stomach - Functions - Storage, Secretions, - Mixing, Motility - Control of HCL Secretion Next Lecture - Duodenum - Exocrine Pancreas  Chapter 16 The Digestive System Mouth, Oesophagus and Stomach Sherwood Modified, Edited and Presented by Marios Z Panos Thank you for your attention ! Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning