One part of bile is a yellow (or brownish) substance called bilirubin. Bilirubin starts out as a product of the breakdown of red blood cells. When red blood cells break down, the heme molecule is converted into bilirubin.
Bilirubin starts out as "unconjugated" bilirubin and, after biochemical alteration in the liver, becomes "conjugated" (or direct) bilirubin. Conjugated bilirubin is a major part of bile. Bile is a substance that breaks down fats, and is often yellow or brown in color. It is released into the small intestine by the gallbladder when it's needed to break down fats that are being digested.
When an abnormally high number of red blood cells are broken down, bilirubin can build up quickly in the body. Liver disease can also cause bilirubin levels to be abnormally high. Too much bilirubin is one cause of a condition called jaundice, which causes the skin and sclera of the eyes to become yellow. Once the cause of excess bilirubin is taken care of, yellowing of the eyes and skin disappears. Bilirubin is then removed from the body through the stool (feces) and gives stool its normal brown color.
Bilirubin circulates in the bloodstream in two forms:
* Indirect (or unconjugated) bilirubin. This form of bilirubin does not dissolve in water (it is insoluble). Indirect bilirubin travels through the bloodstream to the liver, where it is changed into a soluble form (direct or conjugated).
* Direct (or conjugated) bilirubin. Direct bilirubin dissolves in water (it is soluble) and is made by the liver from indirect bilirubin.