There is an updated version of this post here: "Is Blood Ever Blue, Science Teachers Want To Know!"
According to one of the leading experts on the human circulatory system, blood flowing through veins is blue.
I'm not going to mention any names. All I'll say is this: A person I know visited a major research center last year and saw a demonstration of organ removal and some other experimental stuff. A person also visiting asked the famous high-level researcher doing this work if blood was ever blue. What he said was not recorded in detail, but it was very much like this statement I found on the Internet:
blood is red as soon as it is oxygenated. Blue blood flows through veins back to the heart and lungs.....
[source: Some Guy on Yahoo Answers]
My friend was disturbed by this, as s/he had been teaching high school students for years that blood is not blue. Her understanding of the situation was that people thought blood was blue because standard anatomical drawings and models depict arteries as red and veins as blue, and because if you look at your veins they are blue. Obviously veins are not clear, but if you don't think that out you might assume that you were seeing blue blood.
So another year goes by and the same thing happens again. Another visit to the operating theatre, another person asks about blue blood, another confirmation that blood is blue.
Now, I've seen both veins and arterial blood either seeping or gushing (respectively) out of various organisms, including humans and various other mammals, on a number of occasions. My grandmother used to spurt out blood now and then because of a condition she had. As I study hunting, I've observed lots of thrashing around blood spurting and seeping mammals. I've cut myself and I've donated blood. And so on.
I've never seen blue blood. I've seen darker red and lighter red blood. But never blue.
Now, going back to Yahoo Answers, which I am NOT recommending as a source for actual information, but which is a good source for what regular people sometimes think, we have the following three quotes:
Melissa says: When blood gets oxygen it turns red but in your veins it is blue just look at them.
Avondro says: Myth, it's always red. It goes a darker red, purple-like (Some call it blue) when starved of Oxygen.
SS Agent Dick Wakka says: Somewhat true. Blood is very bright red when it is in the pulmonary vein in the lungs, when it is highly oxygenated. During it's journey back to the heart after circulating through the body, it is a little blue when it is deoxygenated, but more of a maroon-blue mix. ... This is the truth.
Agent Dick gives as a citation a "medical student." Well, I've got a citation of a leading blood researcher at a major research institution that says blood is blue.
I think there are two things going on here, one having to do with physics and the other with culture.
The physical issue is about color. Is "purple" a kind of red, or is it a kind of blue? Beyond that, is blood that is "dark red" or "purple" really purple? Or is it dark red. See my point?
The cultural issue is that more surgeons and folks like that, for much of recent history, are males, and males are bad at color, on average. I'm not taking about color blindness, but rather, color indifference. See my point?
So here is what I think: If a person who says to themselves "Blood is blue in our veins" thinks either of the following:
... That blood is blue, like this:
... Or, that blood is "blue" in that you look at your veins and see blue, thus you are seeing your blue blood....
... Or, that you look at an anatomical chart and see the veins drawn in as blue, therefore the blood inside them is blue...
... then that person is laboring under a misconception.
If a person thinks that this "blue blood" is purple, then they may also be laboring under a misconception. The HTML Internet Purple looks like this:
(I know, it looks dark blue to me as well.)
And the Pantone purple looks like this:
(I've never seen blood that looks like this)
Pantone Dark Red looks like this:
... very close to my blog's colors, but not very much like the darker shades of blood that I've seen.
I think dark blood looks a little like this:
This color is 24% red, 2% green, 2% blue, but at a saturation of 92 with a color value of 24 and a hue of 0 degrees. Whatever that means.
(By the way if your computer's video display is not set to a high value for number of colors shown, all of the above may look like only one or two colors. And, since all video screens are different, I might be seeing something different than you are...)
Anyway, the color that I personally think resembles blood in its darker state is not purple. It is red with a lot of darkness added to it. Or a lack of lightness, or whatever. But it is red.
Blood is red. But finding out if this is "true" is like squeezing blood from a stone.
Here's my take on the colour of blood:
Arterial (oxygenated) blood - bright red
Venous (oxygen depleted) blood - dark red
Carbon monoxide poisoning - bright, bright red (cherry red)
Methemoglobinemia - blue blood
Next time you donate blood or have blood drawn for lab tests, look at the colour, because that is venous blood (so called blue blood).
You can cite me. I have been a pathologist for 24 years, and in that time I have worked in Hematology (study of blood), surgical pathology (lots of blood) and autopsies (lots more blood). Blood is red.
My two cents worth: I personally bleed red.
After years of experience, I agree that blood is always various shades of red. However, there's no doubt that veins *look* blue, and that people/tissues with anoxia/hypoxia "turn blue."
Maybe it's time to get a biophysicist to actually research what's going on.
All I can say is that, when I donate blood, the stuff in the tubes and in the bag looks an awful lot like your "dark red."
I'm guessing that the bluish color of my veins is an artifact of the dark red passing through skin.
The term blue blood came from the fact that old royalty ate off off silver containers and the silver gradually does indeed turn the color of peoples blood bluish.
Anyone who has suffered large amounts of silver colloid ingestion actually has blue skin.
click to see here blue bloods
Blood is always a shade of red. As you said, it is a brighter red when oxygenated, but it is a darker red when deoxygenated, similar to the color of rust. The reason blood is red is because of the iron in hemoglobin and the oxygen reacting with it. This is the same reason rust is red; it's basically the same thing (it's more complicated than that, but that's the general idea).
As for why blood looks blue, it's caused by the diffraction of light (I believe that's the correct physical term for it). Since your veins and skin are not perfectly transparent, the light it changed a little as it passes through so that it looks blue. It's similar to looking at the world through a color filter.
...so what colour are veins?
There must be a way to test this. I'm thinking of clear tubing with some blood, and some sort of semipermeable membrane with a low O2 high CO2 environment on the other side. I'm not sure how to do it, but it sounds like something we should be able to set up.
George: I don't believe that yet.
KL: Thanks' for that paper!
Like Achuah, I use the example of venous blood in a vacuum collection tube as an example. This paper from Applied Optics (pdf) takes an objective, experimental approach to the question. Their conclusion is that it is variable transmittance of different wavelengths of light through the various tissues that produces the blue vein effect.
I think they refer to methemoglobinemia as "blue baby" syndrome. That's what I heard learning about ground water (the condition was aid to be caused by high concentrations of dissolved nitrate).
Actually, George is right - at least on the second half of his statement - there is a man who has ingested silver colloid for years and he has blue skin. Search for "blue man -group".
People eat off of silver all the time and their blood does not turn blue. Are silver workers blue?
I've heard that Paul Revere was blue-ish.
...so what colour are veins?
Posted by: meiamzz
Veins are a pale pink-tan and are translucent. The blue colour as seen through pale skin is explained in the article that Dave S. posted.
Veins are blue in the same way the the sky is blue. If you look at them, they look blue, so they are blue . Unless you dissect them out and clean them up.
Jeez, from the link, I thought this was going to be about horseshoe crabs...