• Question: Why is our blood red? I know about red blood cells, but why is our blood red though?

    Asked by sofiegrey to Ed, Katie, Sam, Steve, Vera on 22 Jun 2011.
    • Photo: Edward Morrison

      Edward Morrison answered on 21 Jun 2011:


      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.

    • Photo: Steven Daly

      Steven Daly answered on 21 Jun 2011:


      Blood is red because it contains iron, specifically iron(II), which is iron with two electrons removed. This is what gives blood its red colour.

      If we had copper instead, we would have green blood.

    • Photo: Vera Weisbecker

      Vera Weisbecker answered on 21 Jun 2011:


      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!

    • Photo: Sam Tazzyman

      Sam Tazzyman answered on 22 Jun 2011:


      Of course our blood is also blue if it’s got no oxygen in it – is this because haeomoglobin is blue when it doesn’t have oxygen attached?

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