O1i: Describe the Functions of Gastric Secretions

Composition

Slightly hyperosmotic 325mOsml/L

Higher K (10mmol/L), H+ (150mmol/L), Cl (180mmol/L)

Lower Na (2mmol/L)

Function

Digestive

  • H2O
    • Aid in the breakdown of food products and formation of chyme
  • Pepsin (present as pepsinogen)
    • Pepsinogen (proenzyme) produced by chief cells o Cleaved in gastric lumen
    • Breaks down proteins (10% ingested protein)
  • Gastric lipase
    • Minimal activity
    • Breaks down triglycerides and short chain FA
  • Intrinsic Factor
    • Produced by parietal cell
    • Cofactor required for the absorption of Vit B12 in the terminal ileum
      • Forms complex

Hormonal

    • Gastrin
      • Released from G cells in antrum
      • Stimulates parietal cell HCl release (directly and via ↑histamine release from paracrine cells) → 1500 x more potent than histamine
      • ↑GI/SI motility
      • ↑pancreatic secretions
      • ↑GB contraction

Mucosal Protection

  • Mucus
    • Part of gastric protection
    • Lines luminal surface of stomach
      • Protects from autodigestion/ulceration (HCl, pepsin)
      • Retains Na+, repels H+ o Lubrication of food
    • Traps bacteria
  • Bicarbonate
    • Within mucus layer
    • Aids in protection of luminal surface and buffers against autodigestion
      • ↑pH close to surface (↓pepsin activity)
      • Buffers H+ ions

Immunity

    • HCl (pH 1.6)
      • Produced by parietal cell
      • Part of innate immunity → bacteriocidal
      • Stimulates pancreatic and biliary secretions
      • Provides optimal pH for pepsin activity
      • aids ferric iron conversion to the more soluble ferrous ion

Volume

2L/day

Regulation

3 phases: Gastric, Cephalic, Intestinal

Cephalic 30% gastric juice secretion

  • Thought, sight, taste food
  • Vagal stimulation
  • Enteric nerves release
    • Gastrin-Releasing Peptide → stimulates Gastrin secretion
    • ACh → stimulates secretion of mucus, pepsinogen, HCl, HCO3-, H2O

Gastric 50% gastric juice secretion

    • Entry of food into stomach
    • Antral distention -> increases Gastrin secretion from G cells
    • Gastrin/Histamine/ACh -> increase HCl secretion
    • Acidic pH -> increase pepsinogen secretion

Intestinal 10% gastric juice secretion

  • Acidic contents enter duodenum
  • Somatostatin -> supresses gastrin release
  • CCK -> decreases gastric emptying