No one, no matter how snooty, has actual blue blood. So why do veins look blue under the skin? Turns out, the answer lies in physics, not biology.
Although de-oxygenated blood is coded blue in medical diagrams and anatomical models, we all began suspecting that this was not an actual representation around the third time we got an owwie. No matter where blood is drawn from, it's always red, not blue. Nor do blue veins tend to emerge like frozen worms from the body. But look through skin (at least light skin), and the veins show clearly blue. Why?
This explanation hinges on something that sounded trippy to me in high school. We only see light that's reflected back, that's rejected by the thing we're looking at. So the light absorbed by the thing is exactly the opposite of what we see. So, like, the light 'inside' of blood is like, the complete opposite of red, man. Does that mess with your head? The vessels seem red to us because they throw one particular wavelength of red light back in our face. Everything else, including blue light, they absorb.
Of course, they can only do that if that's what they have to work with. Red light penetrates the skin farther than blue light does. Blue light gets flung back sooner by the skin than red light does. If a blood vessel is on the surface of the skin, it absorbs all the blue light it gets (as well as most of the shades of red light and other colors) and coughs back up that one shade of red. In the absence of other colors, only that red shows up. So when we look at a blood vessel within half a millimeter under the skin, it looks red.
However, when it's farther under the skin, the blue light can't get to it to be absorbed. It's already thrown back at us by the layer of skin. All that struggles through to the blood vessel is the red light. Since the blood vessel can absorb most shades of red, it sucks those up and only throws that one shade back at us. We still see that one shade of red, but we also see all those shades of blue that our skin reflected, and so the blood vessels appear blue.
Image: Colin Davis
Via Science Blogs.