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Your blood is red, so why are your veins blue?

Business Insider Australia logo Business Insider Australia 15/05/2017 Fernando Marinho, David Anderson

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If our blood is red, then why do our veins look blue? Don't be fooled, your blood is always red whether it's pulsing in your veins or gushing from a wound.

Skin scatters more blue light than red, making the skin around the vein look redder than the vein itself. So the colour contrast enhances the veins' bluish hue. Human blood is always red because of the iron in it. The iron mixes with oxygen in your lungs, turning blood red. 

However, the blood of other animals is different because not all blood has the same chemical makeup. Crustacean and spider blood has more copper than iron. When the copper mixes with oxygen it turns blood blue. By contrast, leech blood contains iron but it's less concentrated than human blood, so it looks green.

Turns out, blood is more colourful than you might think!

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