Bilirubin is the yellow breakdown product of normal heme catabolism, formed by failing red blood cells. Bilirubin is excreted in bile, and its levels are elevated in certain diseases. It is responsible for the yellow colour of bruises and the yellow discolouration in jaundice. Bilirubin reduction in the gut leads to a product called urobilinogen which is then oxidized to urobilin which is excreted in the urine.