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Blood is never blue - it's always some shade of red. However, veins carrying red blood may appear bluish because of how human tissue and blood reflect and absorb light.
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When light enters human tissue, it is scattered in all directions (or bounced around) by cells and other structures. That light is either absorbed into the tissue, or it escapes from the skin's surface. It is the escaped, or reflected, light that we see with our eyes.
Blood usually looks red because hemoglobin, an oxygen-carrying protein in red blood cells, absorbs lots of blue light and reflects lots of red light. (Think of someone turning "blue in the face" - it means they're not getting enough oxygen.)
But veins - unlike arteries - carry deoxygenated blood, which absorbs more red light than the oxygen-rich blood in the tissue around the vein. The more red light that's absorbed, the less of it we see.
When red light is absorbed into veins but reflected off the tissue around veins, what we see on the surface of the skin is that it's less reddish around the veins than directly above them.
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"When we compare the light reaching the surface above the vessel to the light reaching the surface nearby, the amount of blue light is the same, but the amount of red light is less," explains Michael Patterson, a professor of medical physics at McMaster University in Ontario and co-author of a 1996 paper that investigated why veins appear blue.
Dina Spector/Business InsiderDeep veins will appear bluer because the deoxgenated blood they contain reflects less red red light than the surrounding tissues.
This creates somewhat of an optical illusion. A vein looks blue not because more blue light is being reflected, but because less red light is being reflected from the vein than from the tissue around it.
Because blue light waves are shorter than red light waves, veins that are deep below the surface of the skin will look the bluest. This seems counterintuitive, but it means they'll absorb the (long) red light, but the (short) blue light will hit the space above the vessel and bounce back to our eyes before it's ever absorbed. [See diagram at right.]
In short, veins look blue because they carry deoxygenated blood, which means they reflect less red light than the surrounding tissues. And the deeper the vein (as the bluer it will seem.
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This post is part of a continuing series that answers all of your "why" questions related to science. Have your own question? Email dspector@buisnessinsider with the subject line "Q&A"; tweet your question to @BI_Science; or post to our Facebook page.
