Why is my blood blue

Aha : Why do veins shimmer blue?

Soak a frog in alcohol. Such thoughts come to researchers when they discover a coral finger in Australian trees. The green tree frog is preserved in alcohol - and immediately changes its color to blue. The alcohol dissolves pigments from the skin, which act like a filter, absorb some of the light and produce the typical green tree frog. If you remove it, the color changes.

The color change reveals something about the nature of light. This is usually a mixture of different colors. Sunlight, although white, is made up of a wide range of colors. But complementary colors such as yellow and blue or red and blue-green also add up to white.

When sunlight hits our skin, the color components become noticeable. "Light is constantly scattered on its way through the tissue," says Jürgen Helfmann from Laser- und Medizin-Technologie GmbH Berlin. Blue light is more energetic. It shakes the electrons in the skin molecules more vigorously and is itself scattered back and forth more than red light. The reflected blue light that reaches our eyes therefore comes from shallow skin depths. "Red light penetrates deeper."

As long as the sunlight does not hit brown pigments or blood vessels, the reflected light components mix to form a whitish color impression. For example, the skin on the insides of the arms stays white even in summer. On the other hand, there are red areas on the palms of the hands. This is where the finest blood vessels run near the surface. We see the color of the blood. Likewise on the lips. Or with shame.

Larger, slightly deeper veins shimmer bluish through the skin. But not because it was venous blood of a different color. The lower oxygen content of the blood flowing back changes the color only a little. The blue cast is caused by the different depths of penetration of blue and red light.

Only red light penetrates to the thick veins. “The blood absorbs part of this light,” says the physicist. “That is why less red light comes back from skin regions with larger blood vessels.” This red deficit catches our eye as a complementary color: white minus red results in blue-green.

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