digestion in stomach 2

• Motor function


  • Acts as a temporary reservoir for the food
  • Mixing food with gastric secretions
  • Propels digested food to the duodenum
• Fundus


  The upper portion of the stomach that is located above the lower esophageal sphincter

  
• The proximal part of the stomach (fundus and proximal part of the body) is thin-walled

  
• Parasympathetic stimulation and the hormone gastrin


  Increase the force of gastric smooth muscle contractions
• Tonic contractions


  Sustained increase in muscle tension, compressing the food and reducing the intragastric volume
• Propulsion and retropulsion


  Peristaltic waves initiated by the gastric pacemaker, intensifying towards the antrum, moving gastric content from the body to the antrum (propulsion), and forcing it back into the body (retropulsion)
• Gastric emptying


  1. Phasic contraction of the smooth muscle in the stomach antrum (antral pump)

  2. Relaxation of the pyloric sphincter
• Gastric emptying occurs within 2-4 hours after eating a meal
• Presence of fat and presence of gastric acid (low pH) in the duodenum

  Inhibit gastric emptying
• Liquids


  Leave the stomach more rapidly than solids
• Gastric juice


  Contains hydrochloric acid, pepsin, and gastric lipase
• Foods remain in the fundus for about an hour without becoming mixed with gastric juice, allowing digestion by salivary amylase to continue
• Mucous cells


  Secrete mucus
• G cells


  Endocrine cells in the gastric glands, secrete the hormone gastrin
• Oxyntic glands


  In the gastric body
• Chief cells


  • Secrete pepsinogen and gastric lipase
• Gastric mucosa with numerous gastric pits (electron micrograph)
• Gastrin


  Hormone secreted by G cells in the gastric antrum that stimulates parietal cells to secrete hydrochloric acid
• G cells secrete gastrin into the bloodstream
• Gastrin secretion is increased by

  Vagal stimulation, presence of amino acids in the gastric content
• Gastrin secretion is reduced when

  High amount of hydrochloric acid is present in the stomach (negative feedback mechanism)
• Types of glands in the stomach

  • Oxyntic glands (gastric body)
  • Pyloric glands (gastric antrum)
• Components of gastric juice

  • Gastric acid
  • Pepsin
  • Gastric lipase
• Parietal cells


  Secrete hydrochloric acid
• Secretion of hydrochloric acid

  1. Carbonic anhydrase catalyzes formation of carbonic acid from H2O and CO2

  2. Carbonic acid dissociates to H+ and HCO3- ions

  3. Proton pump (H+/K+ ATPase) actively transports H+ into the stomach lumen, bringing K+ into the cell

  4. HCO3- ions exit the parietal cell in exchange for Cl- ions through the Cl-/HCO3- antiporter at the basolateral membrane

  5. Cl- diffuses from the cell to the gastric lumen via the Cl- channel in the luminal membrane
• K+, Cl-, and H+ concentrations in gastric juice are higher than in plasma, Na+ concentration in gastric juice is lower than in plasma, gastric juice is isotonic to plasma
• The rate of gastric acid secretion varies with the time of the day, being lower in the morning and higher in the afternoon and evening
• Agents that regulate gastric acid secretion

  • Gastrin, histamine, acetylcholine, somatostatin, prostaglandin E2 (produced in the stomach)
  • Secretin, cholecystokinin (produced in the duodenum)
• Regulation of secretion of hydrochloric acid

  1. HCl secretion is stimulated by ACh, gastrin, histamine

  2. HCl secretion is inhibited by somatostatin and prostaglandin E2

  3. Receptors for these agents are present at the basolateral membrane of parietal cells

  4. These agents stimulate or inhibit translocation of vesicles containing proton pumps to the luminal membrane
• Secretion of gastric acid by parietal cells is inhibited by

  Hormones secretin and cholecystokinin produced by endocrine cells in the duodenum
• Gastric acid secretion is stimulated by

  Gastrin and histamine
• Gastric acid secretion is inhibited by

  Somatostatin
• Hydrochloric acid (HCl)


  Kills microbes, denatures proteins, converts pepsinogen to pepsin, creates optimum pH for pepsin, stimulates release of enteric hormones in the duodenum
• Protein denaturation


  Process by which proteins lose their native conformation and structure
• Pepsin


  • Proteolytic enzyme acting as endopeptidase, splits long polypeptides into smaller fragments, active in acidic pH
• Activation of pepsinogen


  By hydrochloric acid and pepsin
• Gastric lipase


  Enzyme secreted by chief cells that splits triglycerides into fatty acids and glycerol, operates best at pH 5-6
• In neonates, gastric lipase plays an important role in fat digestion as pancreatic lipase is not fully active yet
• In patients with pancreatic insufficiency, gastric lipase may partly compensate for impaired fat digestion
• Differences in secretory and motor function between proximal and distal stomach

  • Secretion: Oxyntic glands (body) vs Pyloric glands (antrum)
  • Motility: Tonic contractions (reservoir) vs Phasic contractions (emptying)