Science Questions

Why is blood red?

Why is blood red? Fiona in Oulten Broad

The reason it's red is because it's got iron in it. If you look down a microscope at blood, what you'll see are thousands of tiny little red cells that are referred to as looking like concave discs. If you look at them from the side, they look like a number eight and that's because they've got a thick ring round the edge and a flattened centre, a bit like a doughnut. They're crammed with a substance called haemoglobin, and haemoglobin is that stuff that carries oxygen around the body. It's a protein - in fact it's four proteins stuck together. In the centre of those four proteins there is an iron ion. Because it's got iron in there it's the colour red. Now lots of animals have a different version of that protein. For instance, the horseshoe crab has blue blood because they use copper instead of iron. The real hippies of the haemoglobin world are a kind of annelid worm that lives in the sea. It actually has blood that's purple when it's oxygenated.

September 2006