Answered by Dr Mat Edwards
Blood (or rather Haemoglobin, the protein that carries oxygen in your red blood cells) changes colour depending on how much oxygen it has. Blood in your arteries, which comes directly from your heart and lungs is full of oxygen and is bright scarlet, almost orangey. As it goes around your body the oxygen leaves it (to feed your muscles and organs ) and it gets darker.Your veins carry this "used up" blood with little oxygen back to the heart and lungs and this blood is dark- maroon/red-wine coloured. (If people are really sick and not breathing properly than it can look almost purple. This may or may not be where the idea of royals having "blue blood" comes from). When you cut yourself it's a mixture of oxygen-rich scarlet blood and oxygen-less, venous blood that comes out so it looks midway between these colours. So veins look blue because the blood is already quite dark, and then you see in through a thick layer of skin, which makes it look darker.
Supplemental- The reason you can't see any of your bright arterial blood going around is because arteries are buried deep inside us, unlike veins. This is due to evolution. Cut a vein, you bleed (and can stop it by pressing on it). Cut an artery and you spray, squirt and hose. if you cut a main artery the blood will literally hit the ceiling.
There. I enjoyed that. Can't wait for someone to ask why poo is brown...
Blood (or rather Haemoglobin, the protein that carries oxygen in your red blood cells) changes colour depending on how much oxygen it has. Blood in your arteries, which comes directly from your heart and lungs is full of oxygen and is bright scarlet, almost orangey. As it goes around your body the oxygen leaves it (to feed your muscles and organs ) and it gets darker.Your veins carry this "used up" blood with little oxygen back to the heart and lungs and this blood is dark- maroon/red-wine coloured. (If people are really sick and not breathing properly than it can look almost purple. This may or may not be where the idea of royals having "blue blood" comes from). When you cut yourself it's a mixture of oxygen-rich scarlet blood and oxygen-less, venous blood that comes out so it looks midway between these colours. So veins look blue because the blood is already quite dark, and then you see in through a thick layer of skin, which makes it look darker.
Supplemental- The reason you can't see any of your bright arterial blood going around is because arteries are buried deep inside us, unlike veins. This is due to evolution. Cut a vein, you bleed (and can stop it by pressing on it). Cut an artery and you spray, squirt and hose. if you cut a main artery the blood will literally hit the ceiling.
There. I enjoyed that. Can't wait for someone to ask why poo is brown...
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