If we bleed in a vacuum would it bleed blue?

01 June 2008



Do we bleed blue blood out of our veins? Presumably if you bleed blood it’s the oxygen that makes it turn red. If we bleed in a vacuum would it bleed blue?


The answer is definitely no. In hospital they use things called vacutainers which is a vacuum in a tube. You put a needle into a patient's vein, put the tube onto the needle and this exposes the inside of the vein to the vacuum and draws the blood into the tube. Momentarily the blood is exposed to a vacuum and the blood comes out very red. There's still too much oxygen in venous blood to make sure it stays a red colour. So why do veins look blue?

This is because when you have a bit of the tissue which doesn't have enough bloodflow going through it tissue will remove more oxygen from the blood. It does mean haemoglobin can get to be a blue colour. Normally that doesn't happen but if you have an area of the body that doesn't have enough bloodflow through it then the tissue gets oxygen hungry and removes enough oxygen to make it go blue. Veins look blue as an optical illusion. They're not really blue and if you cut through the skin you'll see they're a whitish pink colour.


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