Physiology, Biliary

The biliary system refers to bile production, storage, and secretion via the liver, gallbladder, and bile ducts. Bile ducts are categorized into intrahepatic and extrahepatic bile ducts (see Diagram of the Biliary System). Intrahepatic bile ducts include the left and right hepatic ducts, which join to form the common hepatic duct (CHD), while extrahepatic bile ducts include the common bile duct (CBD), which is formed from the CHD and cystic duct. The CBD and pancreatic duct converge to form the ampulla of Vater, which bile travels through before passing through the sphincter of Oddi and into the second portion of the duodenum. Initially, bile is a unique alkaline (7.5 to 8.1 pH) fluid secreted by hepatocytes (600-1000 mL/day), further altered and refined by the epithelial cells lining the biliary tract, and becoming acidic in the gallbladder (5.2 to 6.0 pH). The gallbladder stores this fluid, where it gets concentrated and subsequently released into the digestive tract via the CBD. After receiving stimulation via the hormone cholecystokinin (CCK) from the intestinal tract due to food in the intestinal lumen, the gallbladder contracts and secretes bile into the duodenum. Bile is predominantly water with multiple dissolved substances, including cholesterol, amino acids, enzymes, vitamins, heavy metals, bile salts, bilirubin, and phospholipids.