The source of bilirubin in the blood is hemoglobin. Under normal circumstances, aging red blood cells are destroyed by mononuclear macrophages, and the released hemoglobin is broken down into bilirubin, iron and pepsin. At this time, bilirubin is insoluble in water and is called unconjugated bilirubin or free bilirubin because it is in a non-conjugated state. The unconjugated bilirubin then travels with the blood to the liver. In the liver, free bilirubin, also known as unconjugated bilirubin, combines with glucuronic acid in the liver cells to form glucuronide bilirubin, which is then called conjugated bilirubin. Conjugated bilirubin is water-soluble and can be excreted from urine through glomerular filtration, while conjugated bilirubin that enters the capillary bile ducts enters the intestine with bile and is reduced to colorless urobilinogen by the action of bacteria in the intestine. Most of the urobilinogen is excreted from the feces, and a small portion is reabsorbed into the blood in the intestine and then returned to the liver. It becomes conjugated bilirubin in the hepatocytes and then enters the intestine with bile. This becomes the enterohepatic circulation of bilirubin that is often spoken of in clinical practice.