8th February 2024

Bilirubin metabolism

Bilirubin is produced from the degradation of haemoglobin by the reticuloendothelial system. Red blood cells are broken down within the spleen, and haemoglobin is broken down into haem and globin. Haem is catabolised into bilirubin, while the globin is broken down into amino acids and recycled. Iron released from haem is also recycled. 

Bilirubin is bound to albumin and is transported to the liver. The liver conjugates bilirubin with glucuronic acid, forming conjugated bilirubin. This is done by glucuronyl transferase. The urine will directly excrete a portion of this, while some will enter the intestinal system via bile and will be deconjugated by the intestinal bacteria to form urobilinogen. 

At the terminal ileum, the enterohepatic circulation reabsorbs a portion of urobilinogen and will return to the liver. Some reabsorbed urobilinogen will re-enter bile while the kidneys will excrete the remainder.

Intestinal bacteria will break down the remainder of the urobilinogen within the large intestine into urobilin and stercobilin before being excreted by faeces.

Image Source: Degruyter (2021). almed-2021-0047, Licensed under a creative commons licence. Available for free at: https://www.degruyter.com/document/doi/10.1515/almed-2021-0047/html#APA   (Accessed: 9 January 2024).