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

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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{ 4 comments… read them below or add one }

avatar Ben June 10, 2008 at 10:45 pm

Blood does get a lot of its color from absorption. It’s not just scattering, though that is important. Oxyhemoglobin has a stark absorption peak at around 433 nm and that’s what makes oxygenated the blood distinctly red. Deoxygenated blood has a much flatter absorption spectrum (deoxyhemoglobin is more of a slight green than anything) so the apparent color is more dominated by scattering and contrast effects. The tissue transparency goes up even farther in the infrared, at least until about 950 nm, so that’s used a lot in optical tomography. If you have a digital camera without an IR filter (cell phone cameras often lack them) you can see that pretty strongly in a dark room with an NIR LED like they use in remote controls.

avatar sciencegeekgirl June 11, 2008 at 6:29 am

Hi Ben,
Thanks for your fascinating comments! I’ve got a few questions:
So you’re saying that deoxygenated blood *would* be *green* if its color were dependent just on the wavelength of light it absorbs. However, it too is red, and that is explained by scattering & contrast. Do I have that right? What do you mean by contrast? If the tissue transparency goes up in the IR, do you mean that I should be able to see a remote control LED through, say, my finger, in a dark room using a digital camera?

avatar Ben June 24, 2008 at 12:53 am

Well, the perceived color of anything depends on a lot of stuff. First, you’ve got the illumination itself (spectral content, directionality, even polarization a little bit) and then the absorption. The index of refraction can also make a big difference, especially if it’s either large enough to cause significant reflection or if it varies a lot with wavelength. Then scattering comes into play in two main ways. First, there can be directional scattering biases that depend on wavelength, usually with blue scattering more than red, but some highly structured media can do much stranger things. Second, a scattering object can allow some light to penetrate the interior and scatter back out even if you’re viewing it in a reflection geometry, so there’s a complex interplay of transmission from scattering in the interior and reflection from the surface. Plus, scattering from the surface itself can get into the act too, but that’s usually a minor thing. Finally, the way the eye perceives the color of an object depends some on the colors of things around it (a lot of optical illusions use this effect), and also the absolute illumination level, so with something as complex as a translucent blood vessel embedded in translucent skin, there are really 4 or 5 major effects contributing to the perceived color. Only in cases where one of these effects is very dominant, like absorption in oxygenated blood or scattering with the blue of a clear day sky, does it become fairly easy to understand the physical basis of the color. Deoxygenated blood in veins is one of those odd in-between things where none of the mechanisms clearly dominate.
See this nice picture, for example:

Yeah, if you have the right cell phone and the right remote control, if you go into a dark room, you can see shockingly good penetration through your hand. Scattered power roughly goes down with the 4th power of wavelength, so even the modest increase from 650 nm red light to 900 nm NIR light gives a pretty dramatic increase in light penetration. Remote control LEDs are usually 870-880 nm or 940-950 nm. The 950s are just starting to hit the water absorption peak at 970 nm so they’re not really ideal for penetrating tissue, but the 880s are in a good spectral spot to get low absorption and low scattering.

avatar Mike November 5, 2008 at 10:04 pm

I have had people argue this one with me in public. “I was a trained EMT!” they tell me as if that will make an untruth become true.
I have a picture on my website that goes a long way to helping convince people of this. Our skin is similar in content to milk. So dilute some milk with water and dip a red object in it. It will appear blue. I did it with a tube of red food coloring.
Something that many people get wrong even when explaining this misconception is that you can see blood at all through your skin and veins. You cannot. Blood veins are not transparent. The blue color is a function of the skin and the outside of the vein or artery. It has nothing to do with blood. They say that you see the blood in your veins and arteries and it appears blue through the skin. You do not see the blood, only the veins.
I really wish that they would stop coloring the veins and arteries in diagrams red and blue and that they would use different colors when they double-inject fetal pigs and such. Those really contribute to this misconception.

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