- Bilirubin is a yellow-orange compound that comes from the breakdown of hemoglobin in red blood cells by phagocytic cells (cells that consume and digest other cells). Hemoglobin is broken down into 2 parts, heme and globin. The heme is converted into bilirubin while the globin is split off to amino acids used for protein synthesis.
- Bilirubin is secreted into the blood and carried to the liver where it is conjugated with glucuronic acid. Conjugated bilirubin (also called direct bilirubin) is excreted in the bile as bile pigments into the small intestine.
Bile is an emulsifying agent produced in the liver and secreted into the duodenum (the part of the small intestine between the stomach and the jejunum). Bile helps to digest fats in the duodenum. Its composition includes bile acids, bile salts, cholesterol and electrolytes.
- Bilirubin is further metabolized by bacteria in the intestine to form 3 colorless pigments known collectively as urobilinogens upon the removal of the conjugated glucuronic acid : stercobilinogen (6 hydrogen atoms), mesobilirubinogen (8 hydrogen atoms) and urobilinogen (12 hydrogen atoms).
- These 3 compounds in the intestine are oxidized to become urobilin, mesobilin and stercobilin, which turn feces somewhat brownish.
- A small amount of urobilinogens can get reabsorbed into the blood from the intestines and 90% of these urobilinogens will eventually land up in the liver through the portal veins (then re-excreted into bile) while the remaining 10% will be excreted into urine. Urobilinogen in urine is converted to yellow urobilin.
- Abnormal levels of urobilinogens in the urine can be caused by liver disorders, hepatitis and hemolytic anemias (a destruction of red blood cells throughout the bloodstream).
Chapter: Urine :: 25 April 2007
