We hear a lot of supposed facts in grade school that are completely false. For example, did you ever hear that blood is actually blue inside your body, but turns red instantly on contact with air?
That’s incorrect, but it’s not a bad guess. The confusion comes whenever someone notices that a lighter-skinned person’s veins around the surface of the wrist or inner arm appear bluish, whereas blood always comes out red. What’s really happening?
The answer is that blood appears a different color when viewed through skin. That’s because some wavelengths of light have a harder time traveling through skin than others.
Red light penetrates down far enough to reach those slightly-under-the-skin veins, where much of it is absorbed by the blood. Blue light would normally also be absorbed, but it doesn’t make it through skin as easily as red, and thus more bounces back.
The result is that veins look blue, not because they are full of blue blood, but because blue light isn’t being swallowed up by them and more comes back to our eyes.
If the skin isn’t thick enough to cause this difference in light absorption, veins will still look red—such as the flush seen on your face after exercise. On the other hand, if the vein is too deep for light to reach, then we don’t see the vein at all.
“The Skinny on Blue Blood” – Discover Magazine, December 1996