Normally, the bile, a substance used to emulsify fat, is produced by the liver and is stored in the gallbladder. During the digestive process, the gallbladder is triggered to release bile in the presence of fat in the duodenum. To reach the duodenum, the bile has to be excreted from the gallbladder through the spiral valve of the cystic duct and then it moves into the  common bile duct.

The common bile duct joins the terminal end of the pancreatic duct forming the biliopancreatic ampulla. Together with the pancreatic juice/enzymes, the bile reaches the duodenum by passing through the ampulla of vater.

Overall, the cystic duct is like a bridge between the gallbladder and the common bile duct. If it gets obstructed, the flow of bile towards the duodenum will be interrupted as well. In most cases that the cystic duct is blocked by gallstones, it often results to the inflammation/swelling of the gallbladder or worst of all is the possibility for a ruptured gallbladder.