coloring matters that are among the components of bile and are present in small quantities in the blood and tissues.

The most important bile pigments, which are found in the bile of man and carnivorous animals, are the green pigment biliverdin and the reddish yellow pigment bilirubin. The structure of the bile pigments includes four pyrrhole rings interconnected by carbon atoms. Bile pigments are formed during the decomposition of hemoglobin, mainly in the spleen and liver. Under ordinary circumstances, about 280 mg of bile pigments are produced in the human body in 24 hours. In the liver, bilirubin is converted predominantly to diglucuronide. Human blood normally contains from 0.25 x 10^-3 percent to 1.2 x 10^-3 percent bilirubin, of which approximately three-quarters is free bilirubin. Normally, bile pigments entering the intestinal tract from the liver are discharged from the body with the stool in the form of stercobilinogen—bilirubin reduced by intestinal bacteria (40–280 mg per day). Stercobilinogen is oxidized in the presence of light to stercobilin. Part of the reduced bilirubin is absorbed from the intestine into the blood, enters the liver with the blood, and is again secreted with bile. A smaller amount of the bili-rubin bypasses the liver and is discharged through the kidneys in the form of urobilinogen with the urine (4 mg per day). Urobilinogen is converted to urobilin in the presence of light and air. The entire course of bile-pigment formation from hemoglobin is presented in the following flow-chart:

Bile pigments possess the properties of acids and yield salts (sometimes insoluble) with metals, which accounts for their part in the formation of gallstones. An increase in the bile-pigment content of cutaneous coverings, blood, and urine is of diagnostic significance in various forms of jaundice.

Phylloerythrin is a red pigment in the bile of herbivores; it is a derivative of chlorophyll and is not one of the bile pigments.