Bilirubin

Overview of Bilirubin and its Metabolism

• Bilirubin is derived from haemoglobin and is produced in the body at a rate of about 450 µmol per day.
• It is insoluble in water and is transported in the blood bound to albumin.
• Bilirubin is taken up by liver cells and conjugated to form mono- and diglucuronides, which are more soluble in water.
• The conjugated bilirubin is excreted into the bile, with bilirubin monoglucuronide accounting for 25% and diglucuronide accounting for 75% of the total.
• Bile salts, involved in fat digestion and absorption, are the main functional constituents of bile.
• Serum bile acid concentrations are a more sensitive indicator of hepatic transport function than total bilirubin measurements.
• In the terminal ileum and colon, bilirubin conjugates are attacked by bacteria to form stercobilinogen, which is mostly excreted in feces.
• Some stercobilinogen is reabsorbed and re-excreted through bile in a process called enterohepatic circulation.
• Small amounts of stercobilinogen, known as urobilinogen, can be found in urine.
• When the biliary tract is blocked, bilirubin is not excreted, leading to elevated serum concentrations and jaundice.
• Jaundice is a condition characterized by yellowing of the skin and eyes due to high levels of bilirubin in the blood.
• Further details about jaundice and its effects on patients are provided in the text.