Basic Science and Pathophysiology

In order to understand the physiology of jaundice, it is important to appreciate the normal physiology of haem metabolism.

(as shown in the image below)

Normal physiology:


As Red blood cells (RBC) reach the end of their lifespan (120 days) they pass through the reticulo-endothelial system in the spleen.

The RBCs are broken down by spleen macrophages into Haemoglobin, which is then further broken down into Haem and Globin.

The Globin portion is a protein which breaks down into amino acids.

The Haem portion is initially oxidised to form biliverdin, and then reduced to formĀ Unconjugated Bilirubin. Between 425 and 510 mmol of Unconjugated Bilirubin is produced from catabolism of haem every day.

Unconjugated bilirubin travels to the liver, bound to serum albumin.


In the liver an enzyme (UDP glucuronyl transferase) conjugates the bilirubin with glucuronic acid to formĀ Conjugated Bilirubin.

Post Hepatic

Conjugated bilirubin is excreted in the bile into the small intestine. Enzymes within the small intestine covert conjugated bilirubin to stercobilinogen and urobilinogen.

Stercolbilinogen converts to Stercobilin and is excreted through faeces. Stercobilin is responsible for colouration of faeces.

Urobilinogen is filtered by the blood and transported to the kidneys where it is oxidised to form urobilin. Urobilin is excreted through urine and is responsible for the colour of urine.

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