Fig. 2.

Enterohepatic circulation of bile acids. In humans, about 0.2–0.6 g (averaging 0.5 g) bile acids are synthesized daily in human liver. Conjugated bile acids are secreted into bile and stored in the gallbladder. Some bile acids are spilled over into sinusoid blood and reabsorbed when passing through the renal tubules in the kidney and circulated back to the liver through mensenteric and arterial blood flow. Some bile acids secreted in the bile duct are reabsorbed in the cholangiocytes and recycled back to hepatocytes (cholangiohepatic shunt). After each meal, gallbladder contraction empties bile acids into the intestinal tract. When passing through the intestinal tract, some bile acids are reabsorbed in the upper intestine by passive diffusion, but most bile acids (95%) are reabsorbed in the ileum. Bile acids are transdiffused across the enterocyte to the basolateral membrane and excreted into portal blood circulation back to the sinusoid of hepatocytes. In the colon, DCA is reabsorbed by passive transport and recycled with CA and CDCA to the liver. A bile acid pool of about 3 g is recycled 4–12 times a day. Bile acids lost in the feces (0.2–0.6 g/day) are replenished by de novo synthesis in the liver to maintain a constant bile acid pool.