Gastric acid secretion: the parietal cell, step by step
Three signals converge on one proton pump, and acid also brakes its own release. Follow the parietal cell, then see where PPIs, H₂ blockers and somatostatin act.
Chapters (10)
Explain how gastrin, histamine and acetylcholine stimulate the parietal-cell H⁺/K⁺ ATPase and where PPIs, H₂ blockers and somatostatin act.
The short version
- Parietal cells in the body of the stomach make hydrochloric acid and intrinsic factor.
- Acetylcholine (M₃), histamine (H₂) and gastrin (CCK₂) all end at the H⁺/K⁺ ATPase, and histamine is the hub because gastrin works largely through it.
- Acid brakes itself: a falling pH makes D cells release somatostatin, which cuts gastrin (antrum) and histamine (body).
- PPIs bind the H⁺/K⁺ ATPase covalently and irreversibly; H₂ blockers block only the histamine receptor.
The puzzle: gastric acid secretion against a gradient
Gastric juice has a pH between about 1 and 3.5. The cells that make it sit in tissue at blood pH, around 7.4.[1][5][6]
So how does one cell pump out acid against that gradient, and where can a drug step in? This lesson follows gastric acid secretion from the parietal cell to the drugs that shut it down.
The parietal cell and how it makes HCl
Parietal cells sit in the body of the stomach. They make hydrochloric acid and intrinsic factor.[1][2]
- Make the protonInside the cell, carbonic anhydrase turns carbon dioxide and water into carbonic acid, which splits into a proton and bicarbonate.[5]
- Pump it outThe proton leaves into the lumen through the H⁺/K⁺ ATPase, which swaps it for potassium.[3][5]
- Let chloride followChloride follows the proton into the lumen through a channel.[5]
- Send bicarbonate to the bloodBicarbonate leaves the other side of the cell in exchange for chloride.[5]
Carbonic anhydrase makes the H₂CO₃; H⁺ exits to the lumen via the H⁺/K⁺ ATPase; HCO₃⁻ exits to the blood in exchange for Cl⁻.
Because bicarbonate enters the blood, blood leaving the stomach carries extra bicarbonate after a meal. This is the alkaline tide.[12]
Three signals, one pump: acetylcholine, histamine and gastrin
| Signal | Receptor | Second messenger |
|---|---|---|
| Acetylcholine (from the vagus) | M₃ | Raises calcium |
| Histamine | H₂ | Raises cyclic AMP |
| Gastrin | CCK₂ | Raises calcium (main route is indirect, via histamine) |
Gastrin acts on CCK₂ receptors, but its main route is indirect: it makes ECL cells release histamine. So histamine is the hub, because gastrin works largely through it.[2][5][7][8]
Acid brakes itself: somatostatin from D cells
When the pH falls, D cells release somatostatin. In the antrum it acts on G cells to cut gastrin; in the body of the stomach it acts on ECL cells to cut histamine. Either way, the parietal cell gets less drive.[2][9]
The vagus suppresses somatostatin. Eating therefore releases the brake as well as pressing the accelerator.[9]
Following a meal: the cephalic and gastric phases
- Cephalic phase: the sight and smell of food act through the vagus.[1][2]
- Gastric phase: food reaching the stomach starts a phase that lasts for hours.[1]
- During the gastric phase, stretch and partly digested protein release acetylcholine and gastrin, and the falling pH calls in somatostatin to taper the response.[1][2][9]
Where PPIs and H₂ blockers act
PPIs, such as omeprazole, bind the H⁺/K⁺ ATPase covalently and irreversibly. That shuts the last common step, so blocking the pump blocks all three signals at once.[3][10][11]
H₂ blockers, such as famotidine, compete with histamine at the H₂ receptor. That also dampens gastrin-driven acid. PPIs have replaced H₂ blockers in most settings because they work better.[3][11]
| Drug | Target | Effect |
|---|---|---|
| PPI (omeprazole) | H⁺/K⁺ ATPase | Covalent, irreversible; blocks the last common step |
| H₂ blocker (famotidine) | H₂ receptor | Competes with histamine; also dampens gastrin-driven acid |
Practice question
Original · USMLE styleA 52-year-old woman has gastroesophageal reflux disease. Her symptoms resolved after 8 weeks of omeprazole, 20 mg daily, and upper endoscopy is normal. Her fasting serum gastrin concentration is elevated.
Common questions
What are the three stimulants of gastric acid secretion?
Acetylcholine from the vagus (M₃ receptors, raises calcium), histamine (H₂ receptors, raises cyclic AMP) and gastrin (CCK₂ receptors). All three end at the H⁺/K⁺ ATPase.
Why is histamine called the hub?
Gastrin's main route is indirect: it makes ECL cells release histamine, so gastrin works largely through histamine.
How does acid inhibit its own secretion?
When the pH falls, D cells release somatostatin. In the antrum it cuts gastrin from G cells; in the body of the stomach it cuts histamine from ECL cells, so the parietal cell gets less drive.
How do PPIs differ from H₂ blockers?
PPIs bind the H⁺/K⁺ ATPase covalently and irreversibly, which blocks the last common step for all three signals. H₂ blockers compete with histamine at the H₂ receptor only.
What is the alkaline tide?
Bicarbonate leaves the parietal cell into the blood in exchange for chloride, so blood leaving the stomach carries extra bicarbonate after a meal.
Transcript
Read the full narration, by chapter622 words
0:00The puzzle: acid at pH 1 to 3.5, tissue at 7.4
Gastric juice has a pH between about one and three and a half. The cells that make it sit in tissue at blood pH, around 7.4. So how does one cell pump out acid against that gradient, and where can a drug step in? Today: show how gastrin, histamine and acetylcholine drive the parietal cell, and where PPIs, H2 blockers and somatostatin act. Parietal cells sit in the body of the stomach,
0:25The parietal cell: HCl and intrinsic factor
and they make hydrochloric acid and intrinsic factor. Inside the cell, carbonic anhydrase turns carbon dioxide and water into carbonic acid, which splits into a proton and bicarbonate. The proton leaves into the lumen through the
0:42H+/K+ ATPase, Cl- channel, bicarbonate and the alkaline tide
H plus K plus ATPase, which swaps it for potassium, and chloride follows through a channel. Bicarbonate leaves the other side in exchange for chloride, so blood leaving the stomach carries extra bicarbonate after a meal: the alkaline tide.
1:01Three signals: acetylcholine, histamine, gastrin
Three signals tell that pump to run. Acetylcholine, from the vagus, acts on M3 receptors and raises calcium. Histamine acts on H2 receptors and raises cyclic AMP. Gastrin acts on CCK2 receptors, but its main route is indirect: it makes ECL cells release histamine. So histamine is the hub, because gastrin works largely through it.
1:28Acid brakes itself: somatostatin from D cells
Acid also brakes itself. When the pH falls, D cells in the antrum release somatostatin. Somatostatin acts on G cells to cut gastrin, and on ECL cells to cut histamine, so the parietal cell gets less drive. The vagus suppresses somatostatin, so eating releases the brake as well as pressing the accelerator.
1:52The cephalic and gastric phases
Now follow a meal. The sight and smell of food start the cephalic phase, through the vagus. Food reaching the stomach starts the gastric phase, which lasts for hours. Stretch and partly digested protein release acetylcholine and gastrin, and the falling pH calls in somatostatin to taper the response.
2:13The drugs: PPIs and H2 blockers
Now the drugs. PPIs, such as omeprazole, bind the H plus K plus ATPase covalently and irreversibly, which shuts the last common step. H2 blockers, such as famotidine, compete with histamine at the H2 receptor, and that also dampens gastrin-driven acid. PPIs have replaced H2 blockers in most settings, because they work better. Block the pump and you block all three signals at once.
2:44Your turn: a practice question
Your turn. A 52-year-old woman has gastro-oesophageal reflux disease. Her symptoms resolved after eight weeks of omeprazole, 20 milligrams daily, and upper endoscopy is normal. Her fasting serum gastrin concentration is elevated. Which of the following best explains this finding? A, direct stimulation of antral G cells by the drug. B, less somatostatin release from antral D cells. C, increased vagal acetylcholine release to the antrum. D, increased histamine release from ECL cells. E, autonomous gastrin secretion by a tumour. Pause here and decide.
3:30The answer, one option at a time
A is out: omeprazole works on the parietal-cell pump, and it is not a stimulant of G cells. C is out: the vagus drives acid, but a PPI does not increase vagal output. D is out: ECL cells release histamine in response to gastrin, so they follow the rise rather than cause it, and ECL hyperplasia is a result of high gastrin, not its cause. E is out: a gastrinoma would be autonomous and often shows up as ulcers or diarrhoea, and here the rise is an expected drug effect. So the answer is B. The PPI raises the pH, the D cells release less somatostatin, and the brake on gastrin and histamine is lost.
4:19Recap pocket card and the next lesson
Three things to keep. One: three drivers, one pump. Acetylcholine, gastrin and histamine all end at the H plus K plus ATPase, and histamine is the hub. Two: acid brakes itself, through somatostatin from D cells. Three: PPIs block the pump irreversibly; H2 blockers block only the histamine receptor. Next, Helicobacter pylori and peptic ulcer disease.
Sources
Numbers in the text, e.g. [1], point here- OpenStax. Anatomy and Physiology 2e, 23.4 The Stomach: openstax.org/books/anatomy-and-physiology-2e/pages/23-4-the-stomach
- MSD Manual Professional Version. Overview of Acid Secretion: msdmanuals.com/professional/gastrointestinal-disorders/gastritis-and-peptic-ulcer-disease/overview-of-acid-secretion
- MSD Manual Professional Version. Medications for the Treatment of Gastric Acidity: msdmanuals.com/professional/gastrointestinal-disorders/gastritis-and-peptic-ulcer-disease/medications-for-the-treatment-of-gastric-acidity
- MSD Manual Professional Version. Peptic Ulcer Disease: msdmanuals.com/professional/gastrointestinal-disorders/gastritis-and-peptic-ulcer-disease/peptic-ulcer-disease
- In Pursuit of the Parietal Cell, 2019 review: europepmc.org/article/PMC/PMC6920182
- Helicobacter pylori infection process review, 2025 (intragastric pH): europepmc.org/article/PMC/PMC11903457
- Pathways regulating acid secretion, 1994 (gastrin and cholinergic receptors, CCK-B type): europepmc.org/article/PMC/PMC2588918
- Gastrin and H. pylori carcinogenesis, 2026 (ECL cell, hypergastrinaemia with PPIs): europepmc.org/article/PMC/PMC13457304
- The role of somatostatin in the GI tract, 2025 (SSTR2, G and ECL cells): europepmc.org/article/PMC/PMC12109247
- Rebound acid hypersecretion after PPI withdrawal, 2024 (covalent PPI binding, somatostatin feedback): europepmc.org/article/PMC/PMC11122117
- Potassium-competitive acid blockers, 2025 (PPIs and H2 receptor antagonists): europepmc.org/article/PMC/PMC12115254
- Postprandial alkaline tide (apical H+ secretion, basolateral HCO3- into blood): europepmc.org/article/PMC/PMC13200723
The schematics are teaching models; cell sketches are labelled schematic and the acid-output curve carries no values.
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined, resized): parietal (border) cell, stomach, fundic gland, bread. Capsule and tablet icons ("drug-capsule-1", "drug-tablet-1") by Servier via Bioicons (bioicons.com), CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/), adapted. The pH ruler, pumps, channels, cell sketches, curves, cards and all handwriting are drawn in code (original). No AI-generated teaching images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-GI-002.
For exam study only, not medical advice. Read the medical disclaimer.