The importance of the common bile duct is best appreciated by understanding the biliary system and the functions of the organs involved, which includes the liver, gallbladder, hepatic duct, cystic duct, and common bile duct.

The liver is the organ that produces bile, yellow-colored fluid used to emulsify or digest fat in the duodenum. The liver is prompted to store the bile in the gallbladder, a small pouch-like structure inferior to the liver, when the Sphincter of Oddi is closed that happens when there is no fatty food in the duodenum. The bile then travels through the hepatic duct and goes to the gallbladder by passing through the cystic duct.

In the presence of fatty food in the duodenum, the hormone cholecystokinin is released. This hormone relaxes the Sphincter of Oddi, triggers contraction of the gallbladder and increases bile production in the liver. The gallbladder releases its stored bile as it contracts. Since the Sphincter of Oddi  is relaxed,  it is open and allows flow of bile from the gallbladder, going through the cystic duct, into the common bile duct and finally, into the duodenum. When the Sphincter of Oddi closes back, the bile produced in the liver will replace the bile lost by the gallbladder.