Why is blood red?

Blood is red because of haemoglobin, the protein in red blood cells that carries oxygen. Inside haemoglobin sits a group called haem with a single iron atom at its centre, and it is that iron-bearing structure which absorbs certain wavelengths of light and reflects red.

Blood is never blue inside the body. Oxygen-carrying arterial blood is bright scarlet and oxygen-depleted venous blood is a dark brownish red — but both are red.

So why do the veins in your arm look blue? That is your skin, not your blood. Skin and fat absorb red light more than blue and send more blue back, so dark vessels beneath look bluish. It is entirely an optical effect.

Not every animal has red blood, though. Crabs, octopuses and some snails use haemocyanin instead of haemoglobin, with copper in place of iron — and their blood really is bluish.

The colour carries some medical information as well. In anaemia there is less haemoglobin, so the blood looks paler — which is why a doctor glances at the inside of your eyelid or the beds of your nails for a first impression.