Gastric Juice: Composition, Function, and Clinical Significance

Composition of Gastric Juice

Secreted by gastric mucosa (~2–3 L/day). pH 1.5–3.5 (fasting). Key components:

Component	Source	Function
Hydrochloric Acid (HCl)	Parietal cells	Denatures proteins, activates pepsinogen → pepsin, kills microbes, intrinsic factor release, stimulates pancreatic/biliary secretion
Pepsinogen → Pepsin	Chief cells	Proteolytic enzyme (optimal pH 1.5–2.5); cleaves peptide bonds
Intrinsic Factor (IF)	Parietal cells	Essential for vitamin B12 absorption in terminal ileum
Mucus (MUC5AC, MUC6)	Surface mucous cells, neck cells	Barrier: HCO₃⁻ gradient (pH 7 at epithelium), physical protection, lubrication
Bicarbonate (HCO₃⁻)	Surface mucous cells	Trapped in mucus gel; neutralizes back-diffusing H⁺
Water & Electrolytes	All secretory cells	Vehicle, osmotic balance
Gastrin	G cells (antrum)	Stimulates HCl, pepsinogen, growth; feedback by pH
Histamine, Somatostatin, Ghrelin	ECL cells, D cells, P/D1 cells	Local regulation of secretion, appetite

Phases of Secretion

• Cephalic (30%): Vagus (sight/smell/taste) → ACh → parietal/chief/G cells.
• Gastric (60%): Distension + peptides/amino acids → local reflexes + gastrin → robust secretion.
• Intestinal (10%): Duodenal distension, acid, fat, hyperosmolarity → enterogastric reflex + secretin/CCK/GIP → inhibit gastric secretion.

Clinical Correlates

• Achlorhydria: Pernicious anemia (anti-IF/parietal cell Ab), atrophic gastritis, PPIs, H. pylori (late). → B12 malabsorption, bacterial overgrowth, iron malabsorption.
• Zollinger-Ellison: Gastrinoma → massive acid → refractory ulcers, diarrhea.
• Stress Ulcers: Mucosal ischemia → barrier breakdown.
• Gastric Analysis: Rarely done; basal/peak acid output (BAO/PAO) via nasogastric aspiration.

Color Note

Normally clear/colorless. Yellow/green = bile reflux (duodenogastric reflux). Blood = red/coffee-ground (digested).