Why Are Veins Blue? | Does Blood Change Color Like That?

(Last Updated On: August 21, 2022)

If you’ve ever opened a biology textbook or looked at some diagrams of the human circulatory system, you’ve probably seen the red and blue veins running along the human body. Even way back you might have figured the red and blue was just to represent oxygenated or deoxygenated blood respectively–but then you may have looked at the veins in your arms or hands and saw they were actually kind of blue or green. So what’s up with that? Why are veins blue?

Does Blood Change Color At All?

The answer, for our blood, is “kind of.” But your blood isn’t going to go from red to blue or purple. Our blood is red, and stays red. You can thank the hemoglobin in our blood for that. Specifically it’s all the iron that’s in hemoglobin that makes everything all red. 

Further Reading: Why Is Blood Red? Does It Have to Be Red?

As far as our blood goes, it’s really just going to bounce between a very bright red and a much darker, but still red, red. When your blood is leaving your lungs and making its way to all the other parts of your body, it’s full of oxygen. At this time it’s the bright candy red. Once your blood cells are making their way back to your lungs, it’s deoxygenated–it’s called venous blood at this time. 

Venous blood is still red, but it would be significantly darker in color if you were to go take a look. Oxygen binds to hemoglobin in your blood, and when there isn’t any oxygen bound to the hemoglobin your red blood cells change shape. If you’ve ever had a blood test done before and seen the blood the phlebotomist took out of you, you probably saw the darker venous blood. 

Your little sort-of-donut-shaped red blood cells are also capable of changing shape to get into capillaries in your body–which are thinner than your cells normally are. That’s part of what makes diseases like malaria or sickle cell anemia so dangerous; they restrict the mobility of your red blood cells. 

Okay, but why do our veins look blue?

Alright so we’ve established that your blood is always red. But you might look at your arm and point to your blue/green looking veins as a counterpoint. 

When it comes to the blue-ish appearance of your veins, you can thank your skin. If you were to rip out all your arteries, veins, and capillaries; drain out all your blood; and splay it on a table (we do not recommend this) you would probably find difficulty in telling everything apart.

This is less a matter of your blood than it is how light works. Most of the light you encounter is white light, which contains all light in the visible spectrum. When you have a red apple, the apple absorbs all colors of light except red. It reflects the red light off of it and into your eyes, so you see the apple as red. 

Now the thing about veins; they’re under your skin. Normally they’re about 0.5 millimeters under your skin. Before light from the outside gets to your veins, it has to go through your skin first. When light goes through your skin, your skin will absorb and scatter the warmer tones (red, oranges, yellows), and really just leave the cooler ones (green, blue, purple) behind. That’s because red light penetrates through our tissues better than blue. Since our skin is scattering all the warm tones, that just leaves the blue behind when you’re looking at your veins.

Speaking of blood, here’s a blood type logic puzzle.

About the Author:

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Kyler is a content writer at Sporcle living in Seattle, and is currently studying at the University of Washington School of Law. He's been writing for Sporcle since 2019; sometimes the blog is an excellent platform to answer random personal questions he has about the world. Most of his free time is spent drinking black coffee like water.