Final answer:
Bilirubin is the yellowish waste product generated by the liver from the degradation of heme in worn-out red blood cells, secreted into bile, and contributing to the green color of bile. It is excreted in stool after being converted to stercobilin in the intestine.
Step-by-step explanation:
Bilirubin is a yellowish waste product of the liver that is formed when worn-out red blood cells are destroyed, and it is secreted into bile. Direct answer: Bilirubin is a yellowish waste product formed by the liver and secreted into bile.
Explanation: When red blood cells reach the end of their life cycle, they are broken down by the spleen and other parts of the reticuloendothelial system. The non-iron portion of the heme from the red blood cells is degraded into a waste product known as biliverdin, which is then converted into bilirubin in the liver. This breakdown product is then transported to the liver via the splenic vein of the hepatic portal system. In the liver, bilirubin is conjugated and secreted into bile, which is a fluid produced by the liver and stored in the gall bladder.
Bilirubin is responsible for the characteristic green color of bile. Once excreted into the small intestine, bilirubin is eventually transformed by intestinal bacteria into stercobilin, a brown pigment that gives stool its normal color. In cases where bilirubin cannot effectively be excreted, for example, due to liver failure, it leads to jaundice, giving the skin and the whites of the eyes a yellowish tinge. Furthermore, it's the breakdown pigments like bilirubin and stercobilinogen that color bruises and feces, indicating the vital role of bilirubin not just as a waste product, but also as a pigment contributing to several color-associated health indicators.