Bilirubin is a type of bile pigment. Bile pigments are one of the main basic components of animal bile, consisting of bilirubin and biliverdin, which give the bile its various colors due to the different proportions and concentrations of the two. Human bile contains almost exclusively the former. Bilirubin is a breakdown product of hemoglobin, which, under the action of heme oxygenase, produces biliverdin, which continues to be reduced to bilirubin. At this time the bilirubin is called free bilirubin or unconjugated bilirubin, unconjugated bilirubin because of its insoluble qualities, so it can not be filtered out of the glomerulus, the urine will not appear unconjugated bilirubin. Unconjugated bilirubin binds to serum albumin and is transported through the bloodstream to the liver, where it separates from serum albumin and is taken up by hepatocytes, where it binds to carrier proteins and is transported to the endoplasmic reticulum on the surface of the hepatocyte where it binds to glucuronic acid and forms bilirubin glucuronide, or conjugated bilirubin. Conjugated bilirubin has water-soluble properties and can be excreted in the urine through glomerular filtration. If jaundice occurs when bilirubin is elevated, it may be due to excessive destruction of hemoglobin, disease of the liver itself, bile duct related disease, or inborn inherited metabolic disease, and the patient needs to seek medical attention to identify the cause of the disease and to treat it.