Bilirubin
Bilirubin is a bile pigment that is a degradation product of heme. In particular, bilirubin is a yellow breakdown product of normal heme catabolism. Its levels are elevated in certain diseases and it is responsible for the yellow color of bruises. Bilirubin is an excretion product, and the body does not control levels. Bilirubin levels reflect the balance between production and excretion. Thus, there is no normal level of bilirubin. Bilirubin consists of an open chain of four pyrroles (tetrapyrrole); by contrast, the heme molecule is a closed ring of four pyrroles, called porphyrin.