Enterohepatic circulation

Enterohepatic circulation is an especially important concept in the field of toxicology as many lipophilic xenobiotics undergo this process causing repeated liver damage.

Along the proximal and distal ileum, these conjugated primary bile salts are reabsorbed actively into hepatic portal circulation.

There, hepatocytes extract bile acids very efficiently, and little escapes the healthy liver into systemic circulation.

[1] Bilirubin is conjugated with glucuronic acid in the liver by the enzyme glucuronyltransferase, making it soluble in water.

All conjugated bilirubin in the large intestine is metabolised by colonic bacteria to urobilinogen, which is then further oxidized to urobilin and stercobilin.

Not only drugs but also endogenous substrates like bilirubin, steroidal hormones and thyroxine utilize this pathway.