Under normal conditions, red blood cells in the human blood are continuously produced from the bone marrow. The average life span of red blood cells is 120 days. Hemoglobin is produced after the natural destruction of aging red blood cells. Approximately 250-300 mg of hemoglobin is converted into indirect bilirubin in the body daily. This indirect bilirubin reaches the liver with the blood circulation and is converted into direct bilirubin in the liver cells. After direct bilirubin reaches the capillary bile ducts, it becomes the main component of bile. Bile travels from the bile ducts to the large intestine via the small intestine. Bacteria in the lower small intestine and large intestine reduce the direct bilirubin and convert it into bilirubinogen, which excretes about 40-280 mg of fecal bile (made by oxidation of bilirubinogen) daily and stains the stool yellow. Another small portion of bilirubinogen is reabsorbed by the intestine into the bloodstream and then returned to the liver, where it is shed by the kidneys in the bloodstream (about 0.5-4.0 mg per day), i.e., urinary bilirubinogen. Cycle after cycle, the amount of production and excretion is in dynamic balance. Therefore, the amount of bilirubin in the normal human body is constant. The bilirubin level in the blood is 17.1 µM/L, urinary bilirubinogen is a small amount, and the stool remains normal yellow. When any one of the above processes becomes diseased or malfunctions, bilirubin will reflux or remain in the blood in large amounts and the amount of serum bilirubin can rise. When the serum bilirubin in the blood is >34.2 micromol/liter, the sclera and skin mucosa will become yellow, called jaundice.