12 Myths About The Human Body Debunked
11. Blood Isn't Blue In Your Veins
It is commonly believed that the reason your veins are blue is that the blood flowing inside them is blue. This is also often combined with scientific facts, namely that veins carry deoxygenated blood, with people saying it only appears red in a cut because it is reacting with the oxygen in the air. Once again this is not true, unless you suffer a horrible accident with blue dyes.
The blood in your body is almost certainly red (though there are some incredibly rare instances of it being green due to medication).
The blue colour of your veins actually comes from the way your eyes perceive the light that passes through and is reflected off of your blood vessels and skin. It just happens that the wavelengths of light that pass through our veins and reach our eyes the best are those that correspond to blue light. Meaning that our veins get a blue hue.
The red colour comes from the pigment haemoglobin, an iron-containing molecule that binds oxygen, present in your red blood cells.
This is not to say that there is no difference between oxygenated and deoxygenated blood. Once the blood cells pass this oxygen into different tissues, it can become much darker - though it is never blue.