The cystic duct is the short duct that joins the gallbladder to the common bile duct. It usually lies next to the cystic artery. It is of variable length. It contains 'spiral valves of Heister,' which do not provide much resistance to the flow of bile. Bile can flow in both directions between the gallbladder and the common hepatic duct and the (common) bile duct. In this way, bile is stored in the gallbladder in between meal times. The hormone cholecystokinin, when stimulated by a fatty meal, promotes bile secretion by increased production of hepatic bile, contraction of the gallbladder, and relaxation of the Sphincter of Oddi. Gallstones can enter and obstruct the cystic duct, preventing the flow of bile. The increased pressure in the gallbladder leads to swelling and pain. This pain is sometimes referred to as a gallbladder "attack" because of its sudden onset. During a cholecystectomy, the cystic duct is clipped two or three times and a cut is made between the clips, freeing the gallbladder to be taken out. To carry out its function to aid digestion of fats, the gallbladder receives bile secreted by the liver through the cystic duct. The liver secretes bile through the left and right hepatic ducts, which come together to form the common hepatic duct. The cystic duct from the gallbladder joins the common hepatic duct to form the common bile duct, which carries bile to the small intestines. The sphincter of Oddi regulates the flow of bile into the small intestines, and when it contracts, the bile flows through the cystic duct into the gallbladder.