Why does clotted blood change colour over time?
My question is about blood. In my medical terminology class, we were told that you can distinguish an old internal bleed from a relatively fresh one by the colour of the blood: a fresh bleed produces bright red blood but an old bleed produces brown blood. I was curious as to what makes the blood turn brown.
Initially, the blood is red due to the oxygen in the haemoglobin. As it sits around, it loses its oxygen and turns a purple colour. After that, the body starts to metabolise the blood products into more red and green pigments. You can tell the approximate timing of bleeding by the combination of those pigments. Haemoglobin is made of four rings joined together, a bit like a clover leaf. When you first bleed, you get that stuff in your bleed. Very quickly after that, cells move in and start attacking that molecule and break it open into a long chain of these rings. That's a molecule called bilirubin, and another related molecule called biliverdin. Biliverdin is a green colour and bilirubin is a browny-yellow colour. Too much bilirubin is what makes your skin go yellow when you get jaundice. All these things happen just in the site of your bruise or bleed, and that's why your bruise changes colour from a red to a browny-green to yellow.