RBC contains Hemoglobin having two parts, Heme- an iron containing component, and globin protein. Normally, RBCs undergo hemolysis (=breakdown) in Spleen once they reach the end of their life, roughly ~120 days. The are broken down by the White Blood Cells of Spleen to release the Hemoglobin in them. The globin protein is salvaged, the Heme part separates from the iron it contains. What remains of Heme is now known as Biliverdin. This is then converted to Bilirubin and released into blood where it binds with plasma protein and is called Unconjugated Bilirubin. Because protein molecules are larger than the pores of Kidney, they are not excreted by the Kidney. The Unconjugated Bilirubin, because it is bound to protein, is further bigger than the pores, so it isn’t excreted by Kidney either. This Unconjugated Bilirubin, roaming around in plasma is taken up by Liver where it is combined with Glucuronic acid to form Conjugated Bilirubin. The Liver, then releases this via Bile into Small Intestine where Conjugated Bilirubin undergoes modifications by your intestinal bacteria forming Urobilinogen and then Stercobilinogen, that gives yellow colour to feces. Music: ice flow by Kevin Macleod