Bilirubin, a bile pigment, is the main pigment in human bile and is orange in color. It is the main metabolite of iron porphyrin compounds in the body and is toxic, causing irreversible damage to the brain and nervous system, but it also functions as an antioxidant and can inhibit the oxidation of linoleic acid and phospholipids. Bilirubin is an important clinical basis for determining jaundice and is an important indicator of liver function. Normal serum total bilirubin concentration is 1.7 to 17.1 μmol/L. Jaundice is clinically detected when the blood-stained total bilirubin concentration exceeds 34.2 μmol/L. So where does the bilirubin in the body come from, and where does it go? 1, the source of bilirubin The human body produces 250-350mg of bilirubin every day, of which 80%-85% comes from the disintegration of aging red blood cells, about 15% is formed in the process of hematopoiesis immature red blood cells in the bone marrow (ineffective red blood cell formation in the bone marrow) is destroyed, a small amount from the destruction of heme-containing proteins, such as myoglobin, peroxidase, cytochrome, etc. 2, types of bilirubin (1) indirect bilirubin / unconjugated bilirubin: normal bilirubin is mainly derived from hemoglobin in senescent red blood cells, not transformed by liver uptake is, is fat-soluble, insoluble in water, can not be filtered from the glomerulus. (2) Direct bilirubin/conjugated bilirubin: indirect bilirubin/unconjugated bilirubin is transported to the liver and is combined with glucuronic acid after being taken up by hepatocytes to form direct bilirubin/conjugated bilirubin, which is water-soluble and can pass through the glomerulus, but is mainly secreted with bile for enterohepatic circulation. 3.Transport of bilirubin The erythrocytes in the body are constantly renewed, and the aging erythrocytes are recognized and engulfed by reticuloendothelial cells due to changes in the cell membrane. In the reticuloendothelial cells of the liver, spleen and bone marrow, hemoglobin is broken down into pepsin and hemoglobin. The heme is catalyzed by heme oxygenase in microsomes, and the carbon atoms on both sides of the α-submethyl bridge (=CH-) on the protoporphyrin IX ring of heme break, opening the protoporphyrin IX ring and releasing CO and Fe3+ and biliverdin IX. Fe3+ can be reused and CO can be excreted from the body. The biliverdin of linear tetrapyrrole is further catalyzed by biliverdin reductase (coenzyme is NADPH) in the cytosol and is rapidly reduced to bilirubin. 4, conversion of bilirubin Conjugated bilirubin is excreted into the intestine with bile through the biliary tract, and is reduced to urinary (fecal) bilirubinogen by the cells. Most of the urobilinogen is excreted in the feces, while a small portion (about 1/10) is absorbed by the intestinal mucosa and reaches the hepatic sinus through the portal vein. Most of the urobilinogen that reaches the hepatic sinusoids is excreted from the bile duct through the liver (hepatic-intestinal circulation), and only a small portion is excreted through the kidneys via the body circulation.