Myth 7: Blood is blue

by Stephanie Chasteen on June 7, 2008

I saw this on a teachers’ listserv, and realized that I had been told the same myth as a child, and it was one of those many things that worms its way into your knowledge base and then you never question it again. It’s funny how this happens, because with any thought, you often realize that these don’t make sense (like the idea that polar bear fur is fiber optic, which just doesn’t stand up to scrutiny). There is a delightful episode of This American Life called “A little bit of knowledge” (and when that can be a dangerous thing). One of the stories is about a woman who believed until very late in life that unicorns actually exist. She wasn’t naive or stupid, it was just one of those beliefs that never got questioned, but when confronted with defending the idea, she realized that it was ludicrous. Anyone else got any weird things like this that have cropped up?

So, here’s the myth that was stuck in my brain since I was a kid: Blood in our veins is blue! Here’s the question the teacher posed.

Can someone please clarify for me why blood is blue–or red? What
are the current misconceptions as well as understanding? I would appreciate any
insights or updates. Thanks so much!

Another teacher agrees:

Many of my students–amd adults–categorically state that blood is
really blue. After my years of training as a health care
professional and many biology and physiology courses, the only
‘blue blood’ I’d heard about, were tales recounted by long-
deceased family relatives, waxing nostagically about their royal,
geneological, family tree.

I know that for me, the explanation was that after blood left the heart, it was deoxygenated, and thus blue. That’s why the veins in my wrists look blue. But the arteries were red. Here’s the real story from the Exploratorium’s physicist:

Flesh proteins scatter light, like the sky they scatter slightly more
blue than red light.

The slight blue is lightly pastel when seen against white flesh,
however against a dark background it is a clearer blue. Thus against
the dark background of an artery or vein the skin between the artery
and the surface will be seen to be blue.

This is similar to a blue feather. A feather is blue not because of
pigment but because the physical structure of the feather scatters
blue light. The back of the feather is black with melanin so the blue
scattered light stands out against the dark background. Bleach will
destroy the melanin but not the blue scattering structure. So by
bleaching the feather it still scatters blue but without the black
background the feather looks white. Paint the back of the feather
black again and once again it appears blue.

This same effect gives some pasty faced men with black beard hair the
look of blue skin. The dark hair follicles beneath the skin provide a
dark background against which the blue scattering of the skin can be
seen

So once again understanding perception is important to
science…someone has got to point this out to the California State
Science Standards people.

Paul D

Of course, the text books are helping this misconception stay alive by continuing to illustrate veins
as blue and arteries as red!

So, one enterprising teacher tried a neat experiment:

Hi Paul,

Based on your answer here I tried something really cool. You said red light penetrates skin the deepest, so that is the white light color we see best through the finger tip. That lead me to try the following: I replaced the white light from the mini-mag light with a red laser pointer and then a green laser pointer. The red shone through, but not the green. Very cool!! The green laser is even slightly higher wattage, so it would have the advantage that way. I have no blue laser, but am assuming it wouldn’t make it through the finger tip either.

One final question based on your response:

Bones of the finger are translucent?? I thought they were opaque. Are finger bones much different from other bones (thinner or less dense??). X-rays penetrate tissue effectively, but mostly bounce off bones, right?? It seems strange that finger bones would be better transmitters of visible light than x-rays.

Thank you!

And the physicists’s reply:

Bone absorbs x-rays more or less depending on the density of the bone, so we can see bone structure in x-rays.

Bones are white in the visible. They scatter white light.
But they don’t absorb it much. So the light comes into the bone and goes out in a random direction. So while you cannot see through a bone, it does not block the passage of all white light. Bones are translucent not opaque. (Small thin bones allow more light through than larger bones.

Neat stuff!

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