When someone asks the question "why are veins blue?" a likely response is that they're blue because the blood in veins is deoxygenated. While it's true that venous blood vessels carry a lower concentration of oxygen than their arterial counterparts, this isn't the reason for their blue appearance in your skin. Still, when someone invariably responds to the veins-are-blue-because-they're-deoxygenated argument with the observation that "I've never seen blue blood before" one might then hear the slightly more sophisticated-sounding but increasingly far-fetched claim that we don't ever observe blue blood because it is immediately oxidized upon contact with air. (Check out these beauties from Yahoo Answers to witness these arguments yourself.)

Wrong. Blood is never blue. Ever. Period. (And, this is an excellent example of why you should never ingest information from Yahoo Answers uncritically.)

The bright red color of arterial blood stems from a complex that's formed between hemoglobin, iron, and molecular oxygen. This complex mainly absorbs higher energy (shorter wavelength) blue and green light, leaving behind primarily just red wavelengths for our eyes to detect. However, even when blood is largely depleted of oxygen, it is never blue: it's more of a deep maroon color. And, although I have not participated in a surgery myself, those who have (including my girlfriend, a veterinary student) assure me that in surgery, veins within the body do not appear blue either. So, not only is blood not blue, veins on their own aren't either. "Blue veins", then, are a phenomenon unique to the skin.

So, what's going on here?

The most comprehensive answer that I've been able to find comes from a paper by Kienle et al. published over twelve years ago in the journal Applied Optics and entitled "Why do veins appear blue? A new look at an old question". Based on the findings of the authors, three reasons emerge for the blue appearance of veins in skin. The first two are physical and stem directly from the way in which light interacts with blood (how it is absorbed) and with skin (in this case, how light is reflected). The final reason is psychological, dealing with the way in which our brain processes information relatively to generate color perception. (Greg Laden sort of hinted at some of this recently on his blog).

Since the paper in question is a study in optics, it is way outside of my area of expertise. So, just in case you wanted an in depth technical critique of its methodology, you're not going to find it here. However, I can give you a basic summary of the paper's major findings. And then, if you still have the desire (and the ability) to explore the subject in greater depth, you can parse through the paper yourself.

To tackle this problem, the authors measured how much light of various wavelengths was reflected from both real blood vessels in skin and imitation vessels in a skin-like environment, using a sophisticated technique that gave them spatially-resolved measurements. The synthetic vessel (which was a capillary tube filled with blood and placed in a milky substance with optical properties similar to skin) allowed the authors to experiment with a variety of parameters (particularly vessel depth and diameter), and they were then able to validate their results by taking measurements on actual vessels in skin. Using this set-up, they were able to demonstrate that the optical properties of skin and blood (combined with the influence of relative color perception) explain why veins in skin appear blue, despite not actually being blue.

Skin does not absorb much light at any wavelength, making it look white (depending on how much melanin is present, of course--making this discussion only really relevant to people with lighter skin). Blood, on the other hand, absorbs light of all wavelengths (but less in the red part of the spectrum). However, blue light does not penetrate the skin as well as red light. If a vessel is near the surface of the skin, almost all blue light is absorbed by the vessel, so even though only about 1/4 of the red light is reflected, the ratio of red light reflected to blue light reflected is about 10:1. This vessel appears red.

If the vessel is deeper (about 0.5 mm or more), not as much blue or red light will be absorbed. Importantly, this effect will be more pronounced on blue light than on red light since blue light doesn't penetrate skin very well (the ratio of red light reflected to blue light reflected is about 3:2 or less). This is the case for the "blue veins" observed in skin. Once the vessel is deep enough, though, it won't be seen at all, as light of all wavelengths will be reflected before it can interact with the blood.

Perplexingly, this 0.5-mm-deep vessel appears blue despite reflecting slightly more red light than blue light. This is where relative color perception comes into play. The surrounding skin reflects more red light than blue light (by a ratio of about 5:3), and it does not absorb as much of either type of light as a blood vessel does. Since vision is influenced in part by relative perception, if something purple is placed next to something red, the purple object will appear 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 protected from injury.

The take-home message here is that the bluish appearance of veins in the skin has everything to do with where they are located, and nothing to do with the concentration of oxygen within them. In fact, if we could see them through the skin as well, even arteries would look blue.

Kienle, A., Lilge, L., Vitkin, I.A., Patterson, M.S., Wilson, B.C., Hibst, R., Steiner, R. (1996). Why do veins appear blue? A new look at an old question. Applied Optics, 35(7), 1151-1160. (link)