If veins carry dark red blood, and the vessels themselves are transparent, why is it that in the arms, hands, and legs of fair-skinned people they nonetheless look blue? After carefully studying how light travels through skin, some Canadian researchers have come up with a surprisingly simple answer to this commonly asked question.
Lothar Lilge, a physicist at the Ontario Laser and Light Wave Research Center in Toronto, and his colleagues approached the problem by setting up a simulated version of blood vessels near the skin’s surface. They filled glass tubes of varying sizes with blood and immersed them in a milk-like fatty fluid that was chemically very similar to light-colored skin in the way it transmits and reflects light. As they lowered the tubes into the fluid, the tubes gradually changed color. Right at the surface, they look red, says Lilge. But when you start lowering them down, the tubes look really nicely blue.
Why the color change? Lilge’s group used a camera and various light filters to measure how the milky liquid and the glass-enclosed blood absorbed light of different wavelengths. When not immersed in the fluid, the blood in the glass tubes absorbed almost all light, although it did reflect some red light, which, of course, is why blood outside our bodies appears red.
But when surrounded by fluid--or, presumably, skin tissue--it becomes blood of a different color. Fair skin normally reflects most of the light that hits it. Longer, redder wavelengths, though, can penetrate more deeply into the skin than shorter, bluer wavelengths before reflecting out. A vein looks blue because red light travels far enough into the skin to be absorbed by the blood in the vein. If the blood vessel is far enough below the skin, however, blue light--which would normally also be absorbed by the vein--reflects out of the skin before reaching the vein. So the light reflecting from tissue over the vein contains less red light than blue, giving the vein a bluish cast.
Lilge found that for a vein to look blue, it has to be at least .02 inch below the surface, which explains why fair-skinned people look pink and not blue when they blush. The small surface capillaries that become engorged during blushing lie just a thousandth of an inch below the skin’s surface. Past about .08 inch, light can’t penetrate, and blood courses unseen.