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Digestive Physiology

Digestive Physiology. By: M.H.Dashti. Lecture 4. Physiological functions of the stomach. Anatomy of Stomach. Size when empty? large sausage stretches due to rugae Parts of stomach cardia fundus---air in x-ray body pylorus---starts to narrow as approaches pyloric sphincter.

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Digestive Physiology

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  1. Digestive Physiology By: M.H.Dashti Lecture 4 Physiological functions of the stomach

  2. Anatomy of Stomach • Size when empty? • large sausage • stretches due to rugae • Parts of stomach • cardia • fundus---air in x-ray • body • pylorus---starts to narrow as approaches pyloric sphincter

  3. Physiological functions of the Stomach - secretions • By Oxyntic and Pyloric glandular cells • exocrine • Gastric juice-2-3 L/day • Water & electrolytes • enzymes • Mucus • Endocrine: Gastrin • Paracrine • Histamine • Somatostatin

  4. ,rennin Gastric glandular cells & their secretions • surface mucous cells & mucous neck cells Alkaline Mucin • Parietal cell • Hydrochloric acid and Intrinsic factor • Chief ,peptic or zymogene cell • Gastric lipase, pepsinogen and rennin • G cell • Gastrin hormone Entrochromafin cells produce paracrine secretions of Histamine & D cells Somatostatin

  5. Mechanisms of acid secretion K+ ATP Na+

  6. Histamine Ach M3 IP3 Gastrin CckB IP3 Somatostatin - cAMP Prostaglandins PGE2 EGF control of acid secretion

  7. Ionic concentrations of gastric juice as a function of secretion rate • .At low flow rates the juice is hypotonic • At high flow rates it approaches isotonicity and contains predominantly H+ and Cl-

  8. Gastric alkaline mucosal barrier

  9. pH~7 at Cell surface Factors affecting Gastric alkaline mucosal barrier • stimulation • Prostaglandins • parasympathetic • inhibition • Sympathetic • Bile salts • Alcohol • Citric acid

  10. cytokines Helicobacter pylori & Gastritis

  11. Pepsinogen & Gastric lipase Control of the chief cells • Secretin , VIP & β-adrenergic agonists act via CAMP • Ach Gastrin & cck act via IP3 & Ca2+ • Mechanism for Potentiation effect of H+ is not known

  12. type I type II typeIII Gastric motility : • Gentle mixing waves (type I) • every 20 seconds (BER) • mixes bolus with 2 .5 L/day of gastric juice to turn it into chyme • More vigorous waves (type II) • travel from body of stomach to pyloric region Intense waves (type III )open thepylorus and squirt out 3ml of chyme with each wave type II & III are stimulated by Ach & Gastrin and inhibited by NE

  13. Gastric motility: Mechanical Digestion

  14. Pyloric sphincter Pyloric sphincter & Pyloric pump Myenteric plex Sub mucosal plexus Circular muscle layer Longitudinal muscle layer duodenum antrum

  15. Control of Gastric functions Three Phases

  16. Cephalic Phase = “Stomach Getting Ready” • Sight, smell, taste & thought • stimulate parasympathetic nervous system • Vagus nerve • increases stomach muscle and glandular activity • Receptive relaxation

  17. Gastric Phase = “Stomach Working” • Nervous control via stretch receptors & chemoreceptor keeps stomach active • vigorous peristalsis and glandular secretions continue • chyme is released into the duodenum • Endocrine gastrinhormone increases stomach churning and pyloric sphincter relaxation

  18. Hormones ,short & Inhibit gastric function Intestinal Phase = “Stomach Emptying” • Stretch receptors in duodenum slow stomach activity & increase intestinal activity • Distension, fatty acids or sugar signals medulla • sympathetic nerves slow stomach activity • Hormonal influences (from SI) • secretin hormone decreases stomach secretions • cholecystokinin(CCK) decreases stomach emptying Takes 2-4 hrs to completely empty the stomach

  19. Regulation of Gastric Emptying

  20. Chemical Digestion in the Stomach • Protein digestion begins • HCl denatures (unfolds) protein molecules • HCl transforms pepsinogen into pepsin that breaks peptides bonds between certain amino acids (aromatics) • Rennin in infants • Fat digestion continues • gastric lipase splits the triglycerides in milk fat • most effective at pH 5 to 6 (infant stomach)

  21. Absorption of Nutrients by the Stomach • Water especially if it is cold • Electrolytes • Some drugs (especially aspirin) & alcohol

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