Well, it has to be some colour. If it were green we’d be asking “why is blood green”? There’s no reason for it, it’s just a by-product of how chemicals reflect light.
Steven and Ed are right. There is a lot of a particular protein in our red blood cells that is called hemoglobin, and each of these hemoglobin molecules contain 4 sub-proteins. Each of these sub-proteins has one iron II atom sitting in its center. The way that this iron is arranged it absorbs other colours and only leaves red behind, which we then see. The default colour of blood, which is yellow, simply gets “swamped out” with all the hemoglobin-containing blood cells!
Comments
Katie commented on :
A good answer from Steven there!
Steve commented on :
Sam, blood is just a darker shade of red when it is deoxygenated. Veins look blue because of skin pigmentation.
Sam commented on :
Wow – I never knew that. Thanks!