The Stomach

It is collapsible bag to left of midline, temporary storage of food, ~25 cm (10 in) long X 14 cm wide, highly variable in size & shape, inner surface constantly worn away and renewed

Structure:
(drawing in class)

 

 

 

 

 

 

 

 

 

 

 

  1. Mucosa - when empty lies in large folds or rugae, lined with simple columnar epithelium, contains many simple tubular gastric glands that empty into gastric pits, most acid & enzymes secreted in body and fundic regions
    Cell types:
    Mucous surface cells cover mucosal surface, secrete mucous & HCO3- to form barrier to protect epithelium

    Neck cells in glands, secrete mucous, divide to replace surface cells

    G cells in base of pyloric glands secrete gastrin (hormone)

    D cells release somatostatin, inhibits G cell release

    Chief cells in glands, secrete pepsinogens & gastric lipase

    Parietal cells in glands, secrete HCl and intrinsic factor (binds with Vit B12 & increases absorption in ileum)

  2. Muscularis externa - also contains innermost incomplete layer of oblique smooth muscle
Physical activity:
  1. Storage - esophagus (cardiac) sphincter normally prevents regurgitation
    Stomach stretches up to 1 liter, old food near walls & new in center
  2. Mixing - local mixing waves move outer food & gastric secretions toward antrum, antral peristaltic waves squeeze chyme toward pylorus, some passes thru but most squirts back toward body
  3. Slow emptying - pyloric sphincter allows fluids thru, strength of antral peristalsis & sphincter tonal contraction determine passage of chyme, regulated by both stomach & duodenum, chyme will pass if fluid, duodenum not too full, too low pH or too much fat
    • Cephalic phase - preganglionic parasympathetic n. to submucosal plexus activates gastric juice production, affected by stress, minutes long
    • Gastric phase - stomach regulation, 3-4 hr
      1. Stretching - stimulates submucosal plexus causing release of gastric juice & myenteric plexus for peristalsis
      2. Gastrin - release from G-cells stimulated by stretching or proteins acting as buffers & raising pH; causes gastric juice secretion, HCl secretion, strengthens stomach peristalsis, relaxes pyloric & ileocecal sphincters (gastroileal reflex); low pH inhibits gastrin release
    • Intestinal phase - duodenal regulation
      1. Enterogastric reflex - stimulated by duodenal stretching, irritation, low pH, too solid chyme; inhibits peristalsis & excites pyloric sphincter
      2. Enterocrinin - chyme stimulates mucin production
      3. CCK - (cholecystokinin) fats and partially digested proteins stimulate release; CCK stimulates pancreas to release high enzyme juice & gall bladder to release bile, inhibits peristalsis & excites pyloric sphincter
      4. Gastrin - intermediate protein products stimulate G cells in duodenal wall to secrete gastrin, increases stomach acid & enzyme release, positive feedback
      5. GIP (gastric inhibitory peptide) - glucose & fat stimulate release, inhibits gastric emptying & stimulates insulin secretion by pancreas, stimulates lipid storage in adipose & glucose use in skeletal muscle
      6. Secretin - pH <4.5 & peptides stimulate release; excites pyloric sphincter & stimulates pancreas to release alkaline juice low in enzymes, stimulates bile release rich in HCO3-
      7. Vasoactive intestinal peptide (VIP) - stimulates intestinal glands, dilates regional capillaries, inhibits stomach acid
Chemical digestion
Pepsinogen activated by HCl to form pepsin, optimal pH = 1.5-2.0, digests proteins to peptides

Gastric lipase found mostly in infants, digests milk fats, pH = 5-6

Rennin coagulates milk proteins

Absorption- ions, drugs (aspirin) & alcohol