Why Are Veins Blue?

The Scientific Activist: Why Are Veins Blue?: "None of this, however, addresses the question of why veins specifically appear blue. To answer this one, I can fortunately once again rely on the expertise of Meredith, my vet student girlfriend. The reason why only veins appear blue is that veins are the only vessels we actually observe through the skin. This is due to the fact that veins are larger, have thinner walls, and are more superficial than arteries (and, no, I don't meant that veins prefer People or Us Weekly over The New Yorker--'superficial' is just medical speak for closer to the surface). All of these aspects of veins have clear biological rationales. Beyond just carrying blood back to the heart, the primary function of the venous system is as a blood reservoir. In fact, about two-thirds of your blood volume is held in your veins at any given time, hence their larger size. Because the heart has to push blood directly through arteries, their walls are subject to higher pressures than the walls of veins, so they need to be thicker. Finally, veins are located closer to the surface of the skin, because they also play an important role in heat exchange with the outside environment (to help cool the body). Arteries could perform this function just as well, but it's much more advantageous to keep those higher pressure blood conduits deeper in the body and"

Here is the beginning of my post. And here is the rest of it.