enterohepatische

Entrohepatic circulation is the recycling system by which bile acids and related compounds cycle between the liver and the intestine. Bile acids are synthesized in liver cells from cholesterol and secreted into bile, then stored in the gallbladder and released into the small intestine after meals to aid fat digestion. Most bile acids are reabsorbed in the terminal ileum and return to the liver via the portal vein, where they are taken up by hepatocytes and secreted again into bile, continuing the cycle.

Key steps involve specific transporters and binding proteins. In the ileum, bile acids are reabsorbed by the apical sodium-dependent bile acid transporter (ASBT) and then transferred within enterocytes by ileal bile acid-binding protein (IBABP) before being exported to the portal blood by the OST alpha-OST beta transporter. The liver takes up bile acids again through NTCP and OATPs and secretes them into bile via the bile salt export pump (BSEP).

Regulation of the cycle helps maintain bile acid levels and cholesterol homeostasis. The nuclear receptor FXR senses bile acids and induces production of FGF19, which travels to the liver and suppresses bile acid synthesis by downregulating CYP7A1. This feedback helps balance production and reabsorption of bile acids.

Clinical relevance includes conditions where the cycle is disrupted, such as ileal disease or cholestasis, which can cause fat malabsorption, diarrhea, or pruritus. Treatments may include bile acid sequestrants to bind intestinal bile acids or ursodeoxycholic acid to modify bile acid composition.