• Question: Why are red blood cells red?

    Asked by Ayrton to Ed, Kerrianne, Nina, Oli, yoyehudi on 6 Nov 2017.
    • Photo: Ed Bracey

      Ed Bracey answered on 6 Nov 2017:

      Good question. Red blood cells carry oxygen. They have special proteins inside them that have iron atoms in them. They use the iron atoms to grab a hold of the oxygen.
      The iron reflects red light back into our eyes, but absorbs the other colours. That means it seems red to us. If you’ve seen rust, you’ll notice it’s red too – it’s also reflecting red light.
      Some creatures, for example some octopuses use copper to hold the oxygen instead of iron, so their their blood is green!

    • Photo: Oli Wilson

      Oli Wilson answered on 6 Nov 2017:

      Edward’s absolutely right, though another aspect to this is that, unlike some other types of octopus, our blood is never actually as blue as some diagrams (or your own wrist) make it look. The blood in veins is a darker red because it is carrying carbon dioxide rather than oxygen, but the blue colour we see is just because that’s the colour of light that can make it through the skin best. We don’t see blue arteries because they have thicker walls and tend to be deeper below the skin, where we can’t see them. You’d never be able to see the veins – or arteries – of the ocellated icefish: its blood doesn’t have any colour at all..!