If you've ever glanced at the thin veins on your wrist, you could be forgiven for thinking that the blood within them is blue. We are taught this from an early age: deoxygenated blood is blue, and once the lungs have furnished it with oxygen, it is red.
However, when we cut ourselves, the blood is always red. This, we have been told, is because the blood is oxygenated as soon as it touches the air. Despite the way things appear, none of the above is true. Blood is never blue. When it is deoxygenated, it is a deep shade of red, and, once oxygenated, it is cherry red.
So why do the veins look blue? It's actually a rather complex answer that involves at least four factors:
The way in which the skin scatters and absorbs light is complicated. Because the skin is made of numerous compounds with a variety of optical properties, the way that light travels through it, or bounces off it, is difficult to predict.
Blood's oxygenation state affects the way that light is absorbed. When it is deoxygenated, its absorption coefficient is altered.
The depth and diameter of the blood vessels has an effect. For instance, smaller vessels near to the surface appear red, whereas a larger vessel, at the same depth, will look bluer.
So, why veins look blue is a very simple question with a very complicated answer.
Another blood-based misconception is that the iron within the hemoglobin gives blood its red color. In fact, it is hemoglobin's interaction with other molecules, such as porphyrin, that produces the redness.