For a long time, people of noble descent were called blue-blooded. Some veins in the body have a bluish shimmer through the skin, even though the blood that flows through them is red. But how come veins look blue when they are not?

Hand Apearing Blue Veins
Source: Wikimedia Commons

Please make a fist says the nurse, who is about to pierce the protruding vein in the crook of the arm. Veins shimmer blue through the skin, although the blood that flows out of it is red. Let’s explain how this different color perception comes about. It is a matter of physics and not, as some might think, the color of the vein wall.

The blue color registered in anatomy books, which stands for oxygen-poor blood, has nothing to do with the actual color of the blood in the veins. Blue was already chosen in old anatomy books because oxygen-poor blood is much darker than oxygen-rich blood.

Why Veins look Blue
©YouTube / Australian Museum

This phenomenon is not about the color of the blood, but about the optical properties of the skin and the layers beneath it. The daylight that hits the skin has different wavelengths. These are reflected in different ways by the layers of the skin. The short-wave (blue) portion is reflected in blue by the very superficial veins, i.e. between half a millimeter and two millimeters deep in the skin. The long-wave portion of the light, on the other hand, penetrates deeper into the skin but is absorbed by the blood, so that deeper veins can no longer be seen with the naked eye. In people with dark skin or very overweight, however, even the short-wave light cannot penetrate the blood vessels.

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Blue Blood In Nobles? (Why Do Veins Look Blue)

The lighter the skin, the bluer, sometimes even blue-green, the veins appear. On this basis, it can also be explained why people with the past ideal of beauty of light skin were called blue-blooded by the nobility. The attribution, which is said to come from the medieval ages, is an indication of the very light skin of the People of Royal descent. The Arab Moors who invaded Europe, are said to have given the nobles that name. Over time, the term became a synonym for aristocrats throughout Europe and is still used today.

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Even if everyone has red blood, there are living things in nature whose blood is blue. Squids as well as some snail and crustaceans are among them. With them, the oxygen is not bound to iron, but copper in the blood. In so-called hemocyanin, the blood pigment in blue blood, oxygen is bound to two copper ions and turns blue as a result. In contrast, hemocyanin without oxygen is colorless.

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