fig8

Figure 8. The cholehepatic shunt. Illustrated is the hepatic secretion process of PC through the ABC transporter MDR3 (ABCB4) of the canalicular plasma membrane into the biliary lumen. Additionally, conjugated (fat symbols) and unconjugated (lean symbols) bile acids are secreted through the ABC transporter BSEP (ABCB11) into bile. These substances combine to form micelles that travel down the biliary system to the intestinal lumen for fat absorption. Unconjugated bile acids, which are more lipophilic, are reabsorbed by cholangiocytes and recirculated back to hepatocytes via the periductular plexus. Once taken up by hepatocytes, they are excreted again in bile, enhancing bile flow through this mechanism (cholehepatic shunt). The accompanying phosphatidylcholine remains in the biliary lumen and is available to occupy empty binding sites for phosphatidylcholine on mucin 2, compensating for its absence in mucus and reestablishing a protective hydrophobic barrier. PC: Phosphatidylcholine.