📡 Digestive System Lesson

GSC: the three digestive hormones

Digestion isn't just mechanical and enzymatic — it's coordinated by hormones that sense what's in the gut and signal other organs to respond.

Gastrin

Secretin

CCK

📖 Full Breakdown

Three hormones, three triggers, three distinct jobs

Each hormone is released by a different cell type in response to a different stimulus.

Gastrin

Stomach acid stimulator

Released by G cells in the stomach when protein enters. Stimulates HCl secretion and gastric motility — more protein means more acid needed to digest it.

Secretin

The neutralizer

Released by S cells in the duodenum when acidic chyme arrives from the stomach. Signals the pancreas to release bicarbonate, which neutralizes the acid before it can damage the intestinal lining.

CCK (Cholecystokinin)

The fat responder

Released by I cells in the duodenum when fat and protein enter. Triggers gallbladder contraction (releasing stored bile) and pancreatic enzyme release — both needed to handle incoming fat.

🩺 Clinical / Exam Application

A patient eats a high-fat meal and soon after develops right upper quadrant pain from gallbladder disease. The pain occurs specifically after fatty meals because CCK is released in response to fat entering the duodenum, triggering the gallbladder to contract — if the gallbladder is inflamed or contains stones, this contraction is what produces the pain. Understanding CCK's specific trigger (fat) explains why this pain pattern is so consistently tied to meal timing and fat content.

⚠️ Exam Alert

A frequently tested contrast: secretin responds to ACID and triggers bicarbonate (a neutralizing response), while CCK responds to FAT/PROTEIN and triggers enzymes and bile (a digestive response) — different triggers, different downstream effects, easy to swap under time pressure.

🚧 Common Trap

Don't assume all three hormones come from the same location. Gastrin comes from the stomach; secretin and CCK both come from the duodenum but from different cell types (S cells vs. I cells) responding to different stimuli.

✅ Quick Check

A patient just ate a fatty meal. Which hormone is released, from where, and what two things does it cause to happen?

📝 Exam Prep

Common Exam Questions

❓ What triggers secretin release, and what does it do?

✅ Secretin is released by S cells in the duodenum when acidic chyme arrives from the stomach. It stimulates the pancreas to release bicarbonate, which neutralizes the acid.

❓ What triggers CCK release, and what two effects does it have?

✅ CCK is released by I cells in the duodenum when fat and protein enter. It triggers gallbladder contraction (releasing bile) and pancreatic enzyme release — both needed to digest the incoming fat and protein.