The bile duct forms part of the biliary system, responsible for the transportation of bile, a thick brownish-yellow or greenish-yellow digestive fluid necessary for proper digestion of food. The biliary system consists of other organs such as the gallbladder, the hepatic ducts, the common bile duct, the cystic duct, and the bile ducts inside and outside the liver. In short, the biliary system is a whole network of various sized bile ducts that run through a series of organs that include the liver, gallbladder, pancreas, and the small intestine.
The term bile duct generally refers to any of the biliary passages in the biliary system. Specifically, it is the terminal segment of the biliary system extending from the union of the common hepatic duct and cystic duct to the major duodenal papilla. From the liver the network of various sized ducts extends down to the pancreas through which it passes on its way to the small intestine. The bile secreted by the liver cells flows through this system of ducts until it reaches the part of the small intestine called duodenum where it helps convert fats in food into fatty acids. Unless converted in to fatty acids, fats cannot be absorbed by the digestive tract.
Not all bile directly runs from the liver to the small intestine. The gallbladder, located below the liver, stores about 50 percent of the bile produced by the liver. When food is eaten the gallbladder releases stored bile into the small intestine to break down the fats. Aside from carrying bile from the liver, the bile duct also drains waste products from the liver into the small intestine. Bile secreted by the liver consists of bile salt, bilirubin, cholesterol, and other waste products. It is the bile salt component that helps break down fats into fatty acids.
Health problems occur when the bile duct is blocked by gallstones, pancreatic cancer, and scarring from injury. Blockage prevents the bile from being transported to the small intestines causing bilirubin, a reddish-yellow pigment, to accumulate in the blood which leads to the condition called jaundice. The presence of bilirubin in the blood causes the skin and eyes to turn yellow. Other symptoms of jaundice include noticeably darker urine and paler stools due to bilirubin being filtered into the urine by the kidneys instead of being lost in the stools. Cancer of the bile duct is called cholangiocarcinoma.