How is bilirubin formed and excreted in the biliary system?

• A. Bilirubin is formed from heme breakdown; it is conjugated in the liver and excreted into bile, eventually eliminated in stool (and some in urine if conjugation is defective).
• B. Bilirubin is formed in the intestine by bacterial metabolism; excreted in urine only.
• C. Bilirubin is formed in the spleen and excreted directly in bile without conjugation.
• D. Bilirubin is a protein derived from iron.

Answer: (A)

Bilirubin comes from the breakdown of heme in macrophages of the reticuloendothelial system, mainly from aging red blood cells. This yields unconjugated bilirubin, which travels in the blood bound to albumin because it is not water-soluble. The liver takes up this bilirubin and conjugates it by attaching glucuronic acid through the action of UDP-glucuronyl transferase, forming water-soluble conjugated bilirubin. This conjugated form is then secreted into bile and delivered to the intestine.

In the gut, bacterial enzymes convert the conjugated bilirubin to urobilinogen. Most of this is further processed to stercobilin and excreted in feces, giving stool its characteristic color; a portion is reabsorbed and may be excreted in urine as urobilinogen. Thus, bilirubin is formed from heme breakdown, conjugated in the liver, excreted into bile, and ultimately eliminated in stool, with urinary excretion depending on the conjugation and biliary integrity.