GI secretions

Vivian Imbriotis | Jan. 18, 2026

The GI tract secretes 9L/day of fluid = 1.5+2+2.5+3.

Saliva - water, calcium and phosphate (for teeth), amylase, lipase, IgA - 1.5L

Gastric acid - pH 2, pepsin - 2L

Pancreatic juice - many enzymes, pH 8, Cl 30, HCO3 130 - 2.5L

Small bowel - bicarb-rich mucus - 3L

Cells

  • Goblet \(\to\) mucus
  • Chief \(\to\) pepsinogen \(\to\) pepsin
  • Parietal \(\to\) HCl
  • Enterochromaffin \(\to\) histamine
  • G-cells \(\to\) gastric
  • D-cells \(\to\) somatostatin

Composition

  • 95% water
  • HCl - pH ~2.5
  • HCO3- rich mucus forms unstirred layer
  • Enzymes (pepsin, lipase, intrinsic factor)
  • Electrolytes (low Na, high K, high Cl)

Cephalic phase (increased secretion)

  • Central control \(\to\) vagus \(\to\) M3 receptor on parietal cell
  • Central control \(\to\) vagus \(\to\) M3 receptor on enterochromaffin cell \(\to\) histamine \(\to\) H2 receptor on parietal cell

Gastric phase (increased secretion)

  • Mechanical stretch, peptides, alcohol \(\to\) gastrin from G-cells
  • Gastrin \(\to\) CCKB receptor on parietal cell
  • Gastrin \(\to\) CCKB receptor on enterochromaffin cell \(\to\) H2 receptor on parietal cell

Intestinal phase (decreased secretion)

  • CCK and VIP from duodenum

Negative feedback

  • Somatostatin from D-cell (if pH < 1.5)
  • Prostaglandin E2 promotes bicarb + mucus release

In the parietal cell, basolaterally:

  • CO2 diffuses into the cell
  • Carbonic anhydrase converts it to H+ and HCO3-
  • The HCO3- is antiported with Cl-

Apically,

  • The H+ is primary active antiported with K+
  • The K+ diffuses back out of the cell via passive channels to be recycled
  • The Cl- is passively excreted

Overall,

  • The gastric pH is reduced to ~2
  • The body becomes more alkalotic

90% of pancreas is exocrine; 2.5L is secreted per day

Composed of acini \(\to\) intercalated ducts \(\to\) main pancreatic duct

Acini secrete enzymes and fluid, intercalated ducts alkalinize it.

Enzymes include typsin, pancreatic lipase, RNAase, amylase, and exopeptidases.

Stimulated by

  • Vagal stimulus (cephalic phase) 20%
  • CCK (intestinal phase) 80%

Inhibited by

  • Somatostatin
  • Catecholamines

Overall,

  • The intestinal pH is reduced
  • This activates pancreatic enzymes (trypsin, pancreatic lipase)
  • The body becomes more acidotic