Bilirubin Metabolism and Pathogenesis of Jaundice

Introduction

Bilirubin is a yellow pigment formed as a byproduct of hemoglobin degradation in the body. It is primarily excreted through bile and feces. Understanding its metabolism and excretion is crucial in diagnosing jaundice, a condition characterized by excessive bilirubin levels in the bloodstream.

Formation and Transport of Bilirubin

Hemoglobin Breakdown

Bilirubin is derived from the breakdown of heme in red blood cells. The process occurs in macrophages of the reticuloendothelial system, leading to the formation of unconjugated bilirubin.

Transport to the Liver

Unconjugated bilirubin is transported to the liver via albumin, where it undergoes conjugation for better solubility.

Excretion of Bilirubin in Bile and Feces

Hepatic Conjugation

In hepatocytes, bilirubin is conjugated with glucuronic acid by the enzyme UDP-glucuronosyltransferase, making it water-soluble.

Biliary Excretion

Conjugated bilirubin is secreted into bile and then into the intestines, where bacterial enzymes convert it into urobilinogen.

Fecal and Urinary Excretion

Urobilinogen is further oxidized into stercobilin and excreted in feces, giving stool its characteristic brown color. A small portion is reabsorbed and excreted in urine as urobilin.

Pathogenesis of Jaundice

Types of Jaundice

• Prehepatic Jaundice: Excessive hemolysis leads to increased bilirubin production.
• Hepatic Jaundice: Liver dysfunction causes impaired bilirubin processing.
• Posthepatic Jaundice: Obstruction in bile ducts prevents bilirubin excretion.

Clinical Presentation

Patients exhibit yellowing of the skin and sclera, dark urine, and pale stools, indicating disrupted bilirubin metabolism.

Conclusion

Understanding bilirubin metabolism and its excretion pathways helps in diagnosing and managing jaundice effectively. Early intervention can prevent complications associated with hyperbilirubinemia.