Friday, 30 May 2008

WRONG: Veins are blue because of deoxygenated blood

An amazing number of people seem to believe that deoxygenated blood is blue. In a way, it makes sense – veins (in pale skin) look blue, after all, and we were all taught in school that veins carry blood back to the heart and lungs to be refreshed with oxygen. But ask yourself this: have you ever seen blue blood? Anywhere? (Other than in a mollusc or a horseshoe crab, smartarse.)

It’s true that deoxygenated blood – blood returning from the organs and extremities – is a darker shade than “fresh” blood. The stuff you see when you cut your finger or donate blood may appear a deep, burgundy-red, compared to the bright letterbox-red familiar to surgeons and axe-murderers. (Though this is largely down to light conditions, too.)

Blood vessels near the surface are slightly translucent, as is the skin. Light from outside penetrates the skin and is reflected back out, but shorter [EDIT:] longer wavelengths (ie colours from the red end of the spectrum) are more likely to be blocked by the skin on the journey out again. As a result, what you see appears bluer than it otherwise would.

Optical physicists determined that the blueness of a vein depends on the depth of the blood vessel, its width and the blood content in the tissue around it. Others have speculated that the blue effect is also the product of a contrast with yellowy-pink skin, an optical illusion called the retinex effect.

But even the Queen bleeds red. The expression “blue blooded” in the sense of aristocratic is a direct translation of the Spanish term sangre azul, which referred to the nobility’s descent from “pure” Northern European stock rather than from Moorish ancestors, like the proles. Pale-skinned Northerners simply had more visible blue veins.

Having said all that, not everyone’s blood is red. In 2007 a 42-year-old Canadian man underwent surgery for tissue damage to his legs and surprised the surgeons by bleeding green. The rare condition, called sulfhaemoglobinaemia, was caused by sulphur in his migraine medication binding with his blood cells.


Rhonelle said...

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Anonymous said...

red wavelengths are the longer on the visible light spectrum, shifting up through the o-y-g-b-i-v spectrum