Hormonal Control of Digestion
Think about what happens when you even see or smell food. Your mouth waters. That's a nervous reflex — fast, but short-lived. Now imagine the real work begins once food actually hits your stomach and small intestine. The body needs a different kind of signal: one that lasts longer, coordinates multiple organs, and adjusts the amount of juice based on what's actually in the food. That's where hormones come in.
The gut is not just a passive tube. It's an endocrine organ — it releases hormones into the blood that travel to distant targets (stomach, pancreas, gallbladder) and tell them exactly what to secrete and when.
The Key Players and Their Jobs
There are three major gut hormones you need to know. Each is released by a specific part of the gut in response to a specific stimulus, and each has a precise effect.
Gastrin — Released by G-cells in the stomach antrum when food (especially proteins) enters the stomach.
Effect: Stimulates gastric acid (HCl) and pepsinogen secretion from the stomach wall. Also promotes gastric motility.
Secretin — Released by S-cells in the duodenum when acidic chyme (pH < 4.5) enters from the stomach.
Effect: Stimulates the pancreas to secrete bicarbonate-rich juice (neutralises stomach acid). Also inhibits gastric acid secretion and gastric emptying.
Cholecystokinin (CCK) — Released by I-cells in the duodenum and jejunum when fats and proteins (digestion products) arrive.
Effect: Stimulates the gallbladder to contract and release bile; stimulates the pancreas to secrete digestive enzymes (lipase, amylase, proteases). Also slows gastric emptying.
How They Work Together — The Sequence
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Stomach phase: Food stretches the stomach and proteins trigger gastrin release. Gastrin tells the stomach to pump out acid and pepsinogen. Digestion begins.
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Duodenal arrival: The acidic, partially digested chyme hits the duodenum. This is a critical checkpoint. The duodenum now releases secretin (to neutralise the acid) and CCK (to handle fats and proteins).
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Pancreas and gallbladder respond: Secretin makes the pancreas pour out bicarbonate (which raises pH). CCK makes the gallbladder squirt bile (to emulsify fats) and the pancreas release its enzyme cocktail.
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Negative feedback: Once the duodenal pH rises and fats/proteins are being digested, secretin and CCK release stop. Gastrin secretion is also inhibited by low pH in the stomach (a local feedback) and by secretin itself. The system shuts itself off when the job is done.
Think of gastrin as the "starter" — it gets the stomach going. Secretin is the "neutraliser" — it protects the small intestine from acid damage. CCK is the "digester" — it brings in bile and pancreatic enzymes to break down fats and proteins.
Other Gut Hormones (Mention-Worthy)
- Gastric inhibitory peptide (GIP) — Released by the small intestine in response to glucose and fats. It inhibits gastric acid secretion and motility (slows down stomach emptying) and stimulates insulin release from the pancreas. This is why your body starts releasing insulin even before blood sugar rises — it's a "preparation" signal.
- Motilin — Released during fasting. It stimulates migrating motor complexes — waves of contraction that sweep the gut clean between meals. This is why you feel hungry and your stomach growls.
Why This Matters for Exams …