Bilirubin is a yellow compound that occurs in the normal catabolic pathway of breaking down red blood cells. It is primarily known as the substance responsible for the yellow color of bruises, urine, and jaundice. Bilirubin is produced when heme, a component of hemoglobin in red blood cells, is broken down. This process takes place in the spleen and liver.
There are two forms of bilirubin: unconjugated (indirect) and conjugated (direct). Unconjugated bilirubin is not water-soluble and travels to the liver bound to albumin. In the liver, it is conjugated with glucuronic acid, making it water-soluble and allowing it to be excreted in bile. This conjugated bilirubin is then transported to the intestines and further metabolized by gut bacteria into urobilinogen. Some urobilinogen is reabsorbed and excreted in urine, while the rest is converted into stercobilin, which gives feces their brown color.
Clinically, bilirubin levels are an important diagnostic marker. Elevated bilirubin levels can indicate liver dysfunction, bile duct obstruction, or excessive breakdown of red blood cells, leading to conditions such as jaundice. Jaundice is characterized by a yellowing of the skin and eyes and can be observed in newborns due to immature liver function or in adults with liver diseases such as hepatitis or cirrhosis.
Measuring bilirubin levels involves blood tests, where total, direct, and indirect bilirubin are quantified. The balance between direct and indirect bilirubin levels helps in diagnosing the underlying cause of bilirubin elevation. For instance, high levels of indirect bilirubin typically suggest hemolysis or a problem with bilirubin processing in the liver, while high levels of direct bilirubin usually point to obstructive liver diseases or bile duct issues.