Bile acid metabolism General		Hepatocytes have a unique structure that allows for simultaneous uptake and excretion. These 2 processes occur at defined portions of the cell membrane. 80% of the liver's blood supply comes via the portal vein, which drains the stomach, intestines, spleen and pancreas. This brings nutrients and other substances to the hepatic sinusoids. The basolateral surfaces of the hepatocyte membrane are in contact with sinusoidal blood and it is here that uptake occurs. The basolateral surfaces end at tight junctions which serve as the delimiters of the apical surface of the membrane, where excretion takes place. Bile flows countercurrently to sinusoidal blood within the hepatic lobule. It is primarily an aqueous solution with a specific composition. There are 3 determinants of bile flow: Bile acid dependent flow (BADF) - the amount of bile acids in bile determines the osmotic diffusion of water and electrolytes into the bile, i.e. increased bile acid concentration results in increased bile flow Bile acid independent flow (BAIF) - this is relatively constant and is primarily related to liver cell mass Cholangiocytes, which line the entire biliary tree, contribute a fixed amount by secreting water and bicarbonate Although there can be significant variation in the amounts contributed to bile flow by these 3 mechanisms, under normal circumstances BADF and BAIF are predominant and almost equal. Return to top of page