Digestive System
Your digestive system isn’t just mechanical — it’s beautifully hormonal. Three key hormones act as the “chemical messengers” that coordinate digestion, ensuring enzymes, acid, and bile are released right on time.
🍽️ 1️⃣ Gastrin — The Acid Booster
Source: G cells of the stomach (mainly antrum)
Stimuli: Peptides, amino acids, stomach distension, vagal stimulation (ACh)
Main Actions:
- ↑ Gastric acid (HCl) secretion from parietal cells
- ↑ Pepsinogen release from chief cells
- ↑ Gastric motility and mucosal growth
Clinical note: Zollinger–Ellison syndrome (gastrinoma) → excess gastrin → hyperacidity, peptic ulcers.
💧 2️⃣ Secretin — The Acid Neutralizer
Source: S cells of duodenum
Stimuli: Acidic chyme (low pH) entering duodenum
Main Actions:
- ↑ Bicarbonate secretion from pancreas and bile ducts → neutralizes acid
- ↓ Gastric acid secretion
- ↑ Bile flow
Mnemonic: Secretin “secretes” base to fix acid.
Clinical tip: Secretin protects duodenal mucosa from acid injury and creates the right pH for enzyme activity.
🧈 3️⃣ Cholecystokinin (CCK) — The Fat Digester
Source: I cells of duodenum and jejunum
Stimuli: Fatty acids and amino acids in chyme
Main Actions:
- ↑ Pancreatic enzyme secretion (lipase, amylase, protease)
- ↑ Gallbladder contraction → bile release
- ↓ Gastric emptying → gives more time for digestion
- Relaxes sphincter of Oddi
Mnemonic: “CCK Contracts the gallbladder, Controls gastric emptying, and Kicks pancreatic enzymes.”
🧩 Summary Table
| Hormone | Source | Stimulus | Main Actions |
|---|---|---|---|
| Gastrin | G cells (stomach) | Peptides, distension | ↑ HCl, ↑ motility |
| Secretin | S cells (duodenum) | Acidic chyme | ↑ HCO₃⁻, ↓ HCl |
| CCK | I cells (duodenum, jejunum) | Fats, amino acids | ↑ Enzymes, ↑ bile, ↓ emptying |