Step by Step
G
Gastrin
Released from G cells in the stomach antrum, triggered by protein, distension, and vagal stimulation. Action: stimulates HCl secretion, gastric motility, and mucosal growth.
S
Secretin
Released from S cells in the duodenum, triggered by acidic (low pH) chyme entering the duodenum. Action: stimulates pancreatic bicarbonate secretion (to neutralize the acid) and inhibits gastric acid secretion and motility.
C
CCK (Cholecystokinin)
Released from I cells in the duodenum, triggered by fat and protein in the duodenum. Action: stimulates gallbladder contraction (releasing bile) and pancreatic enzyme secretion, while inhibiting gastric emptying.
Other
Two additional GI hormones worth knowing
GIP (glucose-dependent insulinotropic peptide), released from K cells in response to glucose and fat, stimulates insulin release — this "incretin effect" is the basis for GLP-1 agonist drugs. VIP (vasoactive intestinal peptide) promotes intestinal secretion and relaxes smooth muscle.
Applied Walkthrough
1
A meal containing protein enters the stomach, triggering G cells in the antrum to release gastrin — which stimulates HCl secretion, increased motility, and mucosal growth.
2
As acidic chyme passes into the duodenum, S cells release secretin, which stimulates the pancreas to secrete bicarbonate — neutralizing the acid — while simultaneously telling the stomach to slow down its own acid production and motility.
3
If that same meal contains significant fat, I cells in the duodenum release CCK, causing the gallbladder to contract and release bile, and the pancreas to secrete digestive enzymes — while also signaling the stomach to slow gastric emptying, giving the intestine more time to process the fat.
4
Together, these three hormones coordinate a smooth handoff: gastrin gets digestion started in the stomach, then secretin and CCK take over to manage the next phase in the duodenum, each responding to a specific trigger (acid for secretin, fat/protein for CCK).
Exam Application
Exams test whether you can match each of the three major GI hormones (gastrin, secretin, CCK) to its source cell, trigger, and specific physiological action — and whether you know the additional roles of GIP and VIP.
⚠ Common Trap
The most common trap is confusing secretin and CCK's triggers — secretin responds specifically to acid in the duodenum, while CCK responds specifically to fat and protein; mixing these up leads to assigning the wrong downstream effects.
✓ Quick Self-Check
1. What triggers gastrin release, and what does it do?
Protein, distension, and vagal stimulation trigger it; it stimulates HCl secretion, gastric motility, and mucosal growth.
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2. What triggers secretin release, and what does it do?
Acidic chyme entering the duodenum; it stimulates pancreatic bicarbonate secretion and inhibits gastric acid/motility.
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3. What triggers CCK release, and what does it do?
Fat and protein in the duodenum; it stimulates gallbladder contraction and pancreatic enzyme secretion, while inhibiting gastric emptying.
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4. What is the "incretin effect," and which hormone is responsible?
GIP's stimulation of insulin release in response to glucose and fat; this is the basis for GLP-1 agonist drugs.
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5. Where is CCK released from?
I cells in the duodenum.
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