Is Deoxygenated Blood Blue?

Many people believe that that deoxygenated blood is blue, but this is not the case; human blood is red both inside and outside the body. There are several reasons why people believe that blood is blue, including the fact that veins appear blue under the skin and that some biology books illustrate the circulatory system with blue veins and red arteries. All human blood, however, is red, because it gets its color from red blood cells, which contain iron.

More about the color of blood:

  • Both venous and arterial blood is red, but arterial blood is brighter in color, and venous blood is darker. This can have some effect on the appearance of veins, making them appear to be blue.

  • Male cockroaches have colorless blood, and some adult female roaches have orange-tinged blood.

  • The term "blue blood" is sometimes used to describe people of aristocratic birth. One explanation for the term is that highborn people would not be tanned as a consequence of outdoor labor. As a result, their blue veins would show more prominently under their fair skin.

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Post 4

The blood appears blue because it is deoxygenated and darker than arterial blood which is oxygenated. Deoxygenated blood only runs through the veins (venous blood). Venous blood could be compared to a bruise. A bruise is simply ruptured capillaries (tiny blood vessels).

Unlike an open sore, the blood is contained under the skin. As the bruise ages, the contained blood gets darker because it is depleted of oxygen (deoxygenated) giving the bruise a blue or black appearance. Veins have a continuous supply of deoxygenated blood therefore they always appear blue. For your information, the veins that can be seen are superficial and very close to the outer layer of skin (epidermis) as is a bruise.

Post 3

@Gurenda: Your answer makes sense, much more than any other answer I have seen. Thank you.

Post 2

Never thought about the subject before, but something springs to my mind here spontaneously: blood does not course "freely" under our skin and body, but is channeled in veins, arteries and whatever other blood vessels. These could be called sheaths with a color appearance in situ of, say, grey and not the red color of blood itself.

Any surgeon should be able to shed light on this phenomenon, well, actually any doctor or nurse will be seeing exposed blood channels rather frequently. However, I haven't a clue as to why the color of blood vessels - as filtered through our skin - has to be blue.

Post 1

What makes the veins appear blue through the skin?

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