1. Bilirubin Production: Bilirubin, a yellow pigment, is produced in the spleen, liver, and bone marrow as a byproduct of heme breakdown. Heme is the iron-containing molecule in red blood cells that carries oxygen.
2. Bilirubin Transport: Bilirubin is transported in the blood bound to albumin, a protein.
3. Liver Uptake and Conjugation: The liver takes up bilirubin and conjugates it with glucuronic acid. This conjugation makes bilirubin water-soluble, allowing it to be excreted in bile.
4. Bile Excretion: Conjugated bilirubin is secreted into the bile and travels through the bile ducts into the small intestine.
5. Bacterial Reduction: In the small intestine, bacteria reduce conjugated bilirubin into urobilinogen, a colorless compound.
6. Absorption and Excretion: Some urobilinogen is reabsorbed into the bloodstream and transported back to the liver. Most urobilinogen, however, enters the large intestine and is further oxidized by bacteria into stercobilin, a brown pigment that gives feces its color.
7. Urine Excretion: A small amount of urobilinogen is excreted in urine, contributing to its pale yellow color.
In summary: Urobilinogen is formed in the intestine by the bacterial reduction of bilirubin, a byproduct of heme breakdown.
