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Dear Petra Johnson

Because blood isn't always red.

The colour of your blood comes from the colour of the big haemoglobin molecules that transport oxygen to your cells and take away carbon dioxide.

When the haemoglobin molecule has oxygen attached to it it has a red colour. When it has carbon dioxide attached its colour is blue.

Blood in your veins (like the veins you can see in your arm) is bringing carbon dioxide back from the cells to the lungs (via your heart) to exchange the carbon dioxide for oxygen.

So the blood there is blue.

Yours sincerely 

Virginia R. Claire

 

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