# Gastric Secretion

> Gastric juice cells, hydrochloric acid mechanism, cephalic gastric and intestinal phases and MBBS Physiology exam points.

- Canonical URL: https://prepelephant.com/topics/mbbs/physiology/gastric-secretion
- Exam / course: MBBS · Subject: Physiology
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Gastric Secretion", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/gastric-secretion

## Direct answer

Gastric secretion is the daily production of about 2 litres of juice containing hydrochloric acid, pepsinogen, intrinsic factor and mucus by the gastric mucosa. The parietal cells secrete hydrochloric acid and intrinsic factor, the chief cells secrete pepsinogen, and the G, enterochromaffin-like and D cells release gastrin, histamine and somatostatin respectively; secretion is organised into cephalic, gastric and intestinal phases under neural and hormonal control.

## What you must remember

- **Cells and products:** parietal cells — hydrochloric acid and intrinsic factor; chief cells — pepsinogen; G cells — gastrin; enterochromaffin-like cells — histamine; D cells — somatostatin, the brake on everything else.
- **Acid secretion mechanism:** intracellular carbonic anhydrase supplies hydrogen ions for the H+-K+ ATPase (proton pump), chloride follows through apical channels, and bicarbonate exits basolaterally, producing the post-prandial alkaline tide.
- **Stimulatory trio:** acetylcholine from the vagus (muscarinic receptors), gastrin from G cells, and histamine from ECL cells act on the parietal cell, and histamine strongly potentiates the other two.
- **Cephalic phase:** roughly 20 to 30 per cent of secretion, driven by thought, sight, smell and taste of food through the vagus before food reaches the stomach.
- **Gastric phase:** the largest share, about half or more, triggered by distension, peptides and amino acids via vagovagal reflexes and gastrin release.
- **Intestinal phase:** the smallest fraction from duodenal gastrin, soon offset by inhibitory hormones.
- **Inhibition and clinical:** luminal pH below about 2 releases somatostatin; secretin, cholecystokinin and gastric inhibitory peptide inhibit acid; proton pump inhibitors block the H+-K+ ATPase and H2 blockers block histamine; absent intrinsic factor causes vitamin B12 deficiency and pernicious anaemia.

## Common confusion

Students often credit the vagus with acting only on the parietal cell. Postganglionic vagal fibres also release gastrin-releasing peptide onto G cells, so vagotomy removes both a direct and a gastrin-mediated drive. The phases are also mixed up by contribution: cephalic is vagal and occurs before food arrives, gastric is the largest phase, and intestinal is minor and largely inhibitory. Finally, pepsinogen is activated to pepsin by acid, and surface-cell mucus with bicarbonate forms the protective alkaline barrier.

## Exam-focused takeaway

In theory, tabulate — in words — the cell types with their secretions, explain the H+-K+ ATPase mechanism, and describe the three phases with their approximate contributions and controls, ending with the inhibitory hormones and one clinical corollary such as pernicious anaemia. In viva, expect how omeprazole and histamine blockers act, why histamine potentiates acetylcholine and gastrin, and the meaning of the alkaline tide. In practicals and OSCE, interpret acid output studies and the physiology of anti-ulcer drugs.

## Frequently asked questions

### Which cells secrete hydrochloric acid and intrinsic factor?

The parietal (oxyntic) cells of the gastric body secrete both. Loss of these cells, or of intrinsic factor, causes vitamin B12 malabsorption and pernicious anaemia.

### How is hydrochloric acid actually secreted?

Carbonic anhydrase generates hydrogen and bicarbonate inside the parietal cell; hydrogen is pumped into the lumen by the H+-K+ ATPase in exchange for potassium, and chloride follows. Bicarbonate enters the blood, creating the alkaline tide.

### What are the three phases of gastric secretion?

The cephalic phase before food enters, the gastric phase after food arrives, and the intestinal phase as chyme reaches the duodenum. Their contributions are roughly 30, 60 and 10 per cent respectively.

### Why does histamine matter so much in acid secretion?

Histamine from enterochromaffin-like cells acts on H2 receptors and strongly potentiates the effects of acetylcholine and gastrin on the parietal cell. Blocking H2 receptors therefore markedly cuts acid output.

### What inhibits gastric acid secretion?

Somatostatin from D cells when luminal pH falls below about 2, secretin, cholecystokinin and gastric inhibitory peptide from the intestine, and prostaglandins acting on the parietal cell.