Formation, Secretion, and Composition of Bile

Bile is formed and secreted continuously by polygonally shaped liver parenchymal cells called hepatocytes. An aqueous buffer component (e.g., HCOj ) is added to the bile by the hepatic bile duct cells that carry the secretion toward the common bile duct. The membrane of the hepatocytes in contact with the blood has microvilli that facilitate the exchange of substances between plasma and the cells. Hepatocytes are rich in mitochondria and endoplasmic reticulum. Hepatic bile flows into the gallbladder, where it is concentrated, stored, and emptied into the duodenum when the partially digested contents of the stomach enter 

Schematic representation of the radial architecture of the plates formed by the liver cells. Blood from the portal vein and the hepatic artery flows into sinusoids and eventually enters the central vein. The bile canaliculi are located between the liver cells. Bile flows in the opposite direction and empties into the bile duct in portal triads. [Reproduced with permission from A. W. Ham Histology, 8th ed., J. B. Lippincott, Philadelphia, 1979.] 

Schematic representation of structures within the hepatic lobule. H = hepatocyte, BC = bile canaliculus, KC = Kupffer cell, EC = endothelial cell, N = nerve fiber, F = reticulin fibers, S = sinusoid, D = space of Disse, X = gap between sinusoid lining cells, and RBC = red blood cell. 

Bile produced by hepatocytes is secreted into the bile canaliculi between adjacent hepatic cells. The wall of the canaliculus is formed by the plasma membrane of the hepatocytes, which are held together by tight junctions. Canaliculi arise near central veins and extend to the periphery of the lobules. The direction of bile flow in the canaliculi is centrifugal, whereas that of the blood flow is centripetal. Canaliculi coalesce to form ducts, which are lined by epithelium, and the ducts coalesce to form the right and left hepatic ducts. Outside the liver these ducts form the common hepatic duct. 

The bile acids are 24-carbon steroid derivatives. The two primary bile acids, cholic acid and chenodeoxycholic acid, are synthesized in the hepatocytes from cholesterol by hy-droxylation, reduction, and side chain oxidation. They are conjugated by amide linkage to glycine or taurine before they are secreted into the bile (see cholesterol metabolism. Chapter 19). The mechanism of secretion of bile acids across the canalicular membrane is poorly understood. Bile acids are present as anions at the pH of the bile, and above a certain concentration (critical micellar concentration) they form polyanionic molecular aggregates, or micelles (Chapter 11). The critical micellar concentration for each bile acid and the size of the aggregates are affected by the concentration of Na+ and other electrolytes and of cholesterol and lecithin. Thus, bile consists of mixed micelles of conjugated bile acids, cholesterol, and lecithin. While the excretion of osmotically active bile acids is a primary determinant of water and solute transport across the canalicular membrane, in the canaliculi they contribute relatively little to osmotic activity because their anions aggregate to form micelles. 

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