The common bile duct is a small, tube-like structure formed where the common hepatic duct and the cystic duct join.Its physiological role is to carry bile from the gallbladder and empty it into the upper part of the small intestine (the duodenum). The common bile duct is part of the biliary system.he common bile duct is a small, tube-like structure formed where the common hepatic duct and the cystic duct join.