Hi Yousaf, nice to have you back asking questions again!
From what I can find (I will be honest here and say that I don’t know – this is not my area of expertise), bilirubin is what is left over when heme breaks down to produce energy. Heme is found in haemoglobin, one of the main parts of red blood cells.
It is what gives lots of things the yellow colour they have – including bruises and urine 🙂
Most of our blood is made up of red cells (erythrocytes). These only live in the circulation for about 120 days – then they are broken in the liver or spleen. There, the haemoglobin (which makes the red colour, and carries oxygen in the body) is broken down, and one of the main things left over is bilirubin. This gives the plasma (the watery bit of the blood – without the cells) a slightly yellow colour and also makes urine slightly yellow.
While some bilirubin is lost in the urine, most of it is removed in the liver (where it is then dumped into the gut, which contributes to the colour of stools). But, when your liver doesn’t work, the bilirubin builds up in the blood and turns your skin and the whites of your eyes yellow. This is called jaundice, and usually means either liver problems or too many red cells being broken down too quickly.