Bilirubin (formerly referred to as haematoidin) is the yellow breakdown product of normal haeme catabolism. Haeme is found in hemoglobin, a principal component of red blood cells. Bilirubin is excreted in bile and urine, and elevated levels may indicate certain diseases. It is responsible for the yellow color of bruises, the background straw-yellow color of urine (via its reduced breakdown product,urobilin the more obvious but variable bright yellow colour of urine is due to thiochrome, a breakdown product of thiamine), the brown color of feces (via its conversion to stercobilin), and the yellow discoloration in jaundice.
Jaundice (also known as icterus, from the Greek word, adjectival form, icteric) is a yellowish pigmentation of the skin, the conjunctival membranes over the sclera (whites of the eyes), and other mucous membranes caused by hyperbilirubinemia (increased levels of bilirubin in the blood). This hyperbilirubinemia subsequently causes increased levels of bilirubin in the extracellular fluid. Concentration of bilirubin in blood plasma is normally below 1.2 mg/dL (<25mol/L). A concentration higher than 2.5 mg/dL (>50mol/L) leads to jaundice. The term jaundice comes from the French word jaune, meaning yellow.