The function of the common bile duct is to carry bile from the liver and the gallbladder into the duodenum, the top of the small intestine directly after the stomach. The bile it carries interacts with ingested fats and fat-soluble vitamins to enable them to be absorbed by the intestine.
Bile is produced in the liver, with bile salts as its major component. Bile has a yellow-green color that comes from a different substance, bilirubin, that comes from broken down red blood cells. During fasting, about 75 percent of the bile produced in the liver is released into the gallbladder, while the rest travels on through the common bile duct into the intestines. The gallbladder absorbs water from the bile stored in it, and concentrates it to a large degree.
Once food is ingested, hormonal signals cause the gallbladder to release bile. The gallbladder releases between 50 and 75 percent of its contents at any one time. The concentrated bile is released through the common bile duct to meet the food as it passes into the intestine. The bile salts are only poorly absorbed in the upper intestine, but further down, they are absorbed into the bloodstream, where they are carried back to the liver to be reused.