Thursday July 24 2014
Science Photo Answer #845

Science photo 845

If you look at the back of your hand, you may be able to see some of your veins through your skin. Is the blood in your veins ever blue?

No, human blood is never blue. Even when it is totally lacking in oxygen, it is a dark red color. With oxygen, it is a bright red. OK, then why do your veins look blue? First, your veins are not transparent, so you cannot see the red blood through them. Your veins are not blue either, but scattering of light and partial absorption of the red end of the spectrum causes them to look blue when seen through your skin. Some creatures, such as horseshoe crabs, have blue blood, but mammalian blood is red all the time.

One of the most convincing

One of the most convincing pieces of evidence that veins and arteries are not blue is physical examination during surgery. When seen directly, veins have no blue color.

But wait...

So I've been looking into this a bit and I'm not convinced of the scattering of light idea. I can't imagine that 2 mm of skin can diffuse that much color so dramatically. Would the slightly lighter red in the arteries diffuse a bit as well? One of the best places to see the blue of veins is the bottom of your tongue and they are practically right at the surface. Me thinks there's more to this....

How do you know arteries are not blue...

...since they are all too deep to be seen through the skin. Even the large and superficial carotid is too deep to show any surface color. Also, veins are larger and have thinner walls compared to arteries, so perhaps some of the deeper red color of venous blood is contributing to the surface appearance.

So, why *are* the veins blue?

What is the purpose of blue veins? Why aren't they red like arteries?

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