Expansion
Unconjugated bilirubin is conjugated in the liver, excreted in bile, and converted to urobilinogen
Expansion
- Haem from senescent red cells is broken down by macrophages to biliverdin then unconjugated bilirubin
- Transported in blood bound to albumin; water-insoluble, so not filtered by the kidney
- Liver: taken up, then conjugated with glucuronic acid by UDP-glucuronosyl transferase, becoming water-soluble
- Excreted in bile into the gut
- Colonic bacteria convert it to urobilinogen; most becomes stercobilin, colouring the stool, while some is reabsorbed and either re-excreted or passed in urine as urobilin
Jaundice by site:
- Prehepatic (haemolysis): unconjugated, normal urine colour, dark stool
- Hepatic: mixed. Gilbert syndrome is a reduced conjugating enzyme causing isolated unconjugated hyperbilirubinaemia on fasting or illness
- Posthepatic (obstruction): conjugated, dark urine and pale stools, since no bilirubin reaches the gut, plus pruritus from bile salt retention
Neonatal jaundice reflects immature conjugation, and kernicterus occurs because unconjugated bilirubin is lipid soluble and crosses the immature blood-brain barrier.