The gallbladder is a pear-shaped sac in a depression on the liver’s under surface. The gallbladder is lined with epithelial cells and has a strong layer of smooth muscle in its wall. The gallbladder stores bile between meals, reabsorbs water to concentrate bile, and contracts to release bile into the small intestine. It connects to the cystic duct, which in turn joins the common hepatic duct (Figure 2).
The common hepatic duct and cystic duct join to form the bile duct (common bile duct). It leads to the duodenum where the hepatopancreatic sphincter guards its exit (Figure 3). Because this sphincter normally remains contracted, bile collects in the bile duct. It backs up into the cystic duct and flows into the gallbladder, where it is stored.
Figure 1. Gallbladder location
Figure 2. Gallbladder anatomy
Figure 3. The common bile duct is closely associated with the pancreatic duct and the duodenum
Following a meal, the mixing movements of the stomach wall aid in producing a semifluid paste of food particles and gastric juice called chyme.
Bile is a yellowish-green liquid continuously secreted from hepatic (liver) cells. In addition to water, bile contains bile salts, bile pigments (bilirubin and biliverdin), cholesterol, and electrolytes. Of these, bile salts are the most abundant and are the only bile components that have a digestive function.
Bile pigments are breakdown products of hemoglobin from red blood cells and are normally secreted in the bile.
Normally bile does not enter the duodenum until cholecystokinin stimulates the gallbladder to contract. Proteins and fats in chyme in the duodenum stimulate
the intestinal wall to release cholecystokinin. Cholecystokinin travels via the bloodstream to the pancreas also, where it stimulate the pancreas to release its pancreatic juice that has a high concentration of digestive enzymes.
The hepatopancreatic sphincter usually remains contracted until a peristaltic wave in the duodenal wall approaches it. Then the sphincter relaxes, and bile is squirted into the duodenum (see Figure 4).
Note: Cholecystokinin produced by the intestinal wall cells, in response to proteins and fats in the small intestine, decreases secretory activity of gastric glands and inhibits gastric motility; stimulates pancreas to secrete fluid with a high digestive enzyme concentration and stimulates gallbladder to contract and release bile.
Figure 4. Fatty chyme entering the duodenum stimulates the gallbladder to release bile