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True Blue Inside of You?

Did you ever hear that blood is actually blue inside your body, but turns red instantly on contact with air? Learn more on this Moment of Science.

Blue vein in wrist

Photo: Hana's Attic (flickr)

Although this vein appears blue through the skin, it is actually red due to different wavelengths of light traveling through skin easier than others

We hear a lot of supposed facts in grade school that are completely false.

For example, did you ever hear that blood is actually blue inside your body, but turns red instantly on contact with air?

That’s incorrect, but it’s not a bad guess. The confusion comes whenever someone notices that veins around the surface of the wrist or inner arm appear bluish, whereas blood always comes out red. What’s really happening?

The answer is that blood appears a different color when viewed through skin. That’s because some wavelengths of light have a harder time traveling through skin than others. Red light penetrates down far enough to reach those slightly-under-the-skin veins, where much of it is absorbed by the blood. Blue light would normally also be absorbed, but it doesn’t make it through skin as easily as red, and thus more bounces back.

The result is that veins look blue, not because they are full of blue stuff, but because blue light isn’t being swallowed up by them and more comes back to our eyes.

If the skin isn’t thick enough to cause this difference in light absorption, veins will still look red–such as the flush seen on your face after exercise. On the other hand, if the vein is too deep for light to reach, then we don’t see the vein at all.

  • Petunia Evans

    I’ve always wondered about the color changes from blood in and out of your veins! Of coarse as a kid, I heard about the theory of the blood turning red with the air, but that always sounded a bit odd to me. I’m glad I know now that it’s because of the way our eyes see the light coming back to us, and the thickness of the skin!

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