  bilirubin (formerly referred to as haematoidin) is the yellow breakdown product of normal heme catabolism. heme 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 color 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. it has also been found in plants. == chemistry == bilirubin consists of an open chain of four pyrrole-like rings (tetrapyrrole). in heme, these four rings are connected into a larger ring, called a porphyrin ring. bilirubin can be "conjugated" with a molecule of glucuronic acid whic
Biological_pigments Hepatology Liver_function_tests Metabolism Pediatrics Tetrapyrroles