The bilirubin in the body is broken down into bilirubin, iron and pearl protein after the hemoglobin has been phagocytosed. The bilirubin at this time is called non-conjugated bilirubin, also known as free bilirubin. In the liver, it is combined with glucuronic acid by hepatocytes to produce conjugated bilirubin. After conjugated bilirubin is excreted into the intestinal cavity through the bile ducts, it is broken down into colorless urobilinogen and fecal bilinogen by the action of intestinal bacteria. Most of the urobilinogen is excreted in the feces, while a small portion of urobilinogen is reabsorbed in the intestine and then secreted back into the intestine via the portal vein into the liver, which is called the enterohepatic circulation of bilirubin. In addition, for a small portion of urobilinogen, it is excreted out of the body by the kidneys after being metabolized by the kidneys through the body circulation, which is called urobilirubin. The excretion of bilirubin is normal for the normal condition of human body, and the bilirubin is in the range of 1.7-17.2umol/L.