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

Anatomy of Stomach cardia fundus---air in x-ray body 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

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

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 ,rennin Entrochromafin cells produce paracrine secretions of Histamine & D cells Somatostatin

Mechanisms of acid secretion K+ ATP Na+

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

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-

Gastric alkaline mucosal barrier

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

Helicobacter pylori & Gastritis cytokines

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 Pepsinogen & Gastric lipase

type II & III are stimulated by Ach & Gastrin and inhibited by NE Gastric motility : type I type II typeIII 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 the pylorus and squirt out 3ml of chyme with each wave type II & III are stimulated by Ach & Gastrin and inhibited by NE

Gastric motility: Mechanical Digestion

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

Control of Gastric functions Three Phases

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

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 gastrin hormone increases stomach churning and pyloric sphincter relaxation

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 Hormones ,short & Inhibit gastric function

Regulation of Gastric Emptying

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)

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