Final answer:
The common bile duct is formed by the union of the common hepatic duct and the gallbladder's cystic duct, which allows bile to flow into the small intestine.
Step-by-step explanation:
The common bile duct is the structure formed by the union of the common hepatic duct and the gallbladder's cystic duct. This duct is an essential component of the biliary system, allowing bile to flow from the liver and gallbladder into the small intestine, particularly into the duodenum. The common bile duct, alongside the main pancreatic duct, also meets at the hepatopancreatic ampulla before bile and pancreatic juice are released into the duodenum, which is controlled by the hepatopancreatic sphincter (also known as the sphincter of Oddi).