Bilirubin is converted to a di- or monoglucuronide before active secretion by hepatocytes into the biliary canaliculi. It descends the biliary tree to the duodenum as a constituent of bile. On route, it may be degraded to urobilinogen or partially reabsorbed into plasma.
Urobilinogen may be reabsorbed by the gut and returned to the liver, converted to the derivative urobilin that colours faeces, or reabsorbed into plasma for excretion by the kidneys.
Alternatively, conjugated bilirubin can be acted upon by bacterial enzymes within the gut to form the bile pigment stercobilinogen. Stercobilinogen may be reabsorbed into plasma for recycling to the liver or for excretion by the kidney, or, it may be oxidized to stercobilin. Stercobilin is bile pigment which appears brown in faeces.
Hence, failure of bilirubin to reach the gut e.g. due to obstructive jaundice, results in a reduction in pigment within the stool; pale stools are witnessed clinically.
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