Cholecystokinin and gallbladder contraction after a fatty meal
Cholecystokinin (CCK) is a hormone (a chemical messenger carried in the blood) that contracts the gallbladder when fat and amino acids (the building blocks of protein) reach the duodenum.
Remember
CCK, which the duodenum releases when fat and amino acids arrive, chiefly drives gallbladder emptying after a fatty meal: it contracts the gallbladder and relaxes the sphincter of Oddi (the muscular valve where bile enters the duodenum), so stored bile flows out.
The liver makes bile all the time. The gallbladder, a small sac under the liver, stores and concentrates bile between meals. A fatty meal triggers its release.
The key trigger is hormonal. When fat and amino acids reach the duodenum, cells in its lining release cholecystokinin (CCK) into the blood. CCK makes the muscular wall of the gallbladder contract. It also relaxes the sphincter of Oddi, the muscular valve where bile enters the duodenum, so stored bile is squeezed out into the gut.
Bile contains bile salts, which act like a detergent. They break fat into tiny droplets (emulsification), so digestive enzymes can act on it. CCK also stimulates the pancreas to secrete enzyme-rich fluid, so bile and enzymes arrive together when fat needs digesting.
The vagus nerve, the main nerve of the parasympathetic nervous system to the gut, can also stimulate contraction, but CCK drives most emptying after eating, so the process is mainly hormone-driven.
Biliary colic is right upper quadrant pain (pain in the upper right part of the abdomen) that comes and goes and is often triggered by fatty food. It happens when a stone, called a gallstone, temporarily blocks the cystic duct, the gallbladder's outlet. The contraction after fat then squeezes the gallbladder against that blocked outlet.
Key facts
- Trigger
- fat and amino acids in the duodenum make cells in its lining release CCK.
- Actions
- CCK contracts the gallbladder and relaxes the sphincter of Oddi, so bile enters the duodenum.
- Pancreas
- CCK also stimulates enzyme-rich pancreatic fluid.
- Production versus release
- the liver makes bile continuously, and CCK chiefly drives the release of the stored bile, not its production.
- Nerves
- the vagus nerve can also stimulate contraction, but CCK drives most emptying after eating.
- Clinical link
- gallstone pain (biliary colic) often follows a fatty meal.
Exam traps
- Secretin comes from the duodenum too, but it responds to acid and mainly stimulates bicarbonate-rich pancreatic fluid, so it is not the main trigger for gallbladder contraction after a fatty meal.
- Gastrin is released in the stomach and drives acid secretion there, so a link with protein digestion does not make it a trigger for bile release.
- Stimulation of the vagus nerve does make the gallbladder contract, but it is a supporting factor: CCK from the duodenum drives most emptying after eating.
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