GIT – Secretions

Master answer

Functions of Gastric Secretions: Gastric Juice

1. Gastric Acid

• Activation of pepsin (from pepsinogen I) → protein digestion
• Facilitates protein digestion by ↓ pH alone
• Defence against micro-organisms
• Facilitates iron absorption in duodenum
• Stimulates biliary/pancreatic juice secretion (via duodenal CCK/secretin)
• –ve feedback on further HCl secretion

2. Pepsinogen

• Pepsinogen undergoes autocatalytic cleavage by acidic pH of stomach → forms pepsin → proteolytic enzyme that aids protein digestion

3. Intrinsic Factor

• glycoprotein that facilitates Vitamin B12 (cobalamin) absorption

4. Mucous in alkaline-rich fluid

• Forms “gastric mucosal barrier” → keeps H+ out of mucosa (prevents autodigestion by HCl → ulceration) and Na+ in it (maintains potential difference across mucosal surface)
• Lubricates food
• Traps bacteria

5. Gastric Lipase and Amylase

• Aid digestion of fats and CHO (minor role only)

Functions of Gastric Secretions: Non-Gastric Juice

1. Gastrin

• ↑ gastric HCl secretion (1500x more potent vs. histamine) → stimulates parietal cells directly and indirectly (via histamine from ECL cells)
• ↑ pepsinogen secretion from chief cells
• +ve trophic effect on small intestinal/colonic mucosa and parietal cell mass
• ↑ gastric and intestinal motility
• ↑ LOS contraction (preventing reflux)
• ↑ GB contractions and pancreatic secretions

2. Histamine

• MAJOR stimulus for gastric HCl secretion → acts via H2R (Gs) on parietal cells

3. Somatostain

• Potent inhibitor of gastric acid secretion (acts directly on parietal cells and indirectly by inhibiting gastrin release from G-cells)
• Inhibiting secretion of most GI hormones (Eg. gastrin, VIP, GIP, secretin)
• Inhibits pancreatic exocrine secretion
• Inhibits gastric motility (including ↓ gastric emptying rate)
• Inhibits gallbladder contraction
• Inhibits intestinal absorption of nutrients

JC 2019

Answer by JC