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What's the reason the skin looks blue when having cyanosis? What's the relationship between lack of oxygen of the bluish appearance?

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  • Have you thought about the coloring of arterial and venous blood? Jul 5 '20 at 1:09
  • Yes, and it still doesn't explain to me the bluish colour. (lack of red doesn't mean blue) Jul 5 '20 at 1:35
  • What color is venous blood? Jul 5 '20 at 2:16
  • Darker red than the arterial one. Jul 5 '20 at 2:21
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    @UbiquitousStudent No, it's not simply darker red. It has a purplish hue, and to make purple you mix which two colors?
    – Carey Gregory
    Jul 5 '20 at 4:06
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The skin of a caucasian-skinned person with cyanosis is described as bluish, but it is still more reddish than blue. The bluish jumps out at us in comparison with healthy well-oxygenated blood.

The colors we see are the product of the incoming light, and the proportion of the wavelengths that are absorbed, scattered, or reflected. Because skin and arteries/capillaries/veins are translucent, there's normally quite a bit of red light scattering and reflection going on. However when looking at veins or cyanotic skin, there's less reflection of the red wavelengths, and the skin looks darker and bluer than we're used to.

If you look at a photograph of someone with Raynaud's or cyanotic hands, you'll see that there's still quite a bit of red!

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  • This may partly explain why we see different colors, but it doesn't answer the question of why cyanosis causes those changes to occur.
    – Carey Gregory
    Sep 24 '20 at 23:41

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