Final answer:
Bilirubin is a yellow pigment produced from the breakdown of red blood cells, transported to the liver, converted into a water-soluble form, and excreted with bile into the intestine where it is transformed into the brown pigment stercobilin, giving stool its color.
Step-by-step explanation:
Bilirubin is a byproduct of the breakdown of red blood cells in the body. It is initially formed as unconjugated bilirubin, which is not water-soluble. To become excretable, it must be made water-soluble, a process that occurs in the liver. Here, bilirubin binds to albumin in the bloodstream and is transported to the liver, where it is conjugated with glucuronic acid by the enzyme UDP glucuronyl transferase, forming bilirubin diglucuronide, which is water-soluble. This process allows bilirubin to be carried via the bile into the small intestine. Most of bilirubin is ultimately converted into stercobilinogen and then into stercobilin, which gives feces its characteristic brown color. The rest may end up in urine as urobilin, adding a yellow color to it. Conditions such as obstructive jaundice or hepatic diseases can disrupt this excretion process, leading to the characteristic yellowing of the skin and eyes known as jaundice.