Step by Step
Ceph
Cephalic phase (30%)
Triggered by the sight, smell, taste, or even thought of food — a Pavlovian response occurring before food even reaches the stomach. Signals travel via the vagus nerve, releasing acetylcholine that stimulates parietal cells (HCl) and chief cells (pepsinogen).
Gast
Gastric phase (60% — the largest phase)
Triggered once food is actually in the stomach — distension and the presence of protein stimulate gastrin release from G cells in the antrum, which then stimulates parietal cells to secrete HCl.
Int
Intestinal phase (10%)
Triggered once chyme reaches the duodenum. Initially, a small amount of gastrin is released, but this is quickly followed by secretin and CCK, which actually INHIBIT further gastric secretion — helping to wind down stomach activity once digestion has moved onward.
Cells
The key cells involved
Parietal cells secrete HCl, stimulated by gastrin, acetylcholine, and histamine (via H2 receptors) — and are blocked by PPIs like omeprazole (which block the H+/K+ ATPase pump) and H2 blockers like ranitidine. Chief cells secrete pepsinogen, which is activated into pepsin by the acidic environment (pH below 2). Parietal cells also secrete intrinsic factor, essential for vitamin B12 absorption.
Applied Walkthrough
1
Just smelling a favorite meal cooking triggers the cephalic phase — vagal stimulation causes parietal and chief cells to begin secreting HCl and pepsinogen, even before any food has been eaten.
2
Once food actually enters the stomach, distension and protein content trigger the gastric phase — by far the largest contributor, with gastrin from G cells driving substantial additional HCl secretion.
3
As partially digested chyme moves into the duodenum, the intestinal phase begins: a brief small burst of gastrin, quickly followed by secretin and CCK, which now work to inhibit further gastric acid secretion — appropriately winding down stomach activity as digestion moves further along.
4
A patient taking a PPI like omeprazole has their parietal cells' H+/K+ ATPase pump blocked directly, reducing HCl secretion regardless of which phase (cephalic, gastric, or intestinal) is triggering the signal.
Exam Application
Exams test whether you can correctly order and assign approximate percentages to the three phases of gastric secretion, and whether you know which cells and stimulating molecules are involved in HCl and pepsinogen secretion.
⚠ Common Trap
The most common trap is assuming the intestinal phase stimulates more gastric secretion like the other two phases — in fact, after an initial small burst of gastrin, the intestinal phase is primarily inhibitory, via secretin and CCK.
✓ Quick Self-Check
1. What triggers the cephalic phase of gastric secretion?
The sight, smell, taste, or thought of food, via vagal stimulation — before food reaches the stomach.
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2. What triggers the gastric phase, and what percentage of secretion does it represent?
Distension and protein in the stomach; about 60%, the largest phase.
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3. What happens during the intestinal phase?
An initial small burst of gastrin, followed by secretin and CCK, which inhibit further gastric secretion.
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4. What do parietal cells secrete?
HCl and intrinsic factor (essential for vitamin B12 absorption).
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5. How do PPIs like omeprazole reduce stomach acid?
By blocking the H+/K+ ATPase pump in parietal cells.
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