Bile flow is generated by the active transport of bile salts and electrolytes and the accompanying obligate passive movement of water. After virtually complete conjugation in the hepatocyte with both glycine and taurine, bile acids are excreted in bile, which flows from the intrahepatic collecting system into the proximal or common hepatic duct. Bile remaining in the gallbladder is thus a concentrated solution consisting primarily of bile acids and sodium. During fasting, bile acids are concentrated in the gallbladder, and little bile acid-dependent bile flows from the liver. Food entering the duodenum initiates an exquisite hormonal and neural sequence.
Bile acids induce water secretion by the colon as they enter that organ, thus promoting catharsis.
Bilirubin is excreted in bile as degradation products of heme compounds from worn-out RBCs. Various proteins important in GI function are secreted in bile. Bile salts are efficiently extracted by the liver, promptly modified, and secreted back into bile. Bile acids undergo enterohepatic circulation 10 to 12 times per day. During each pass, a small amount of primary bile acids reaches the colon, where anaerobic bacteria containing 7 -hydroxylase form secondary bile acids.