Is Blood Ever Blue? Science Teachers Want to Know!

There is an updated version of this post here: "Is Blood Ever Blue, Science Teachers Want To Know!"

Why ask the question, "Is blood ever blue?"

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 "Is blood 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 she 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 as they are visible through your skin they are blue. Obviously veins are not clear so you are not seeing the blood inside them, 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.

So, is blood ever blue, or what?

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 old blog's official 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...)

So, what color is blood?>

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.

See: Human Anatomy for Kids.

Having said all that, please do note that here we are speaking only of mammalian blood. Other blood is red, and other things that we may equate to blood is a different color, but often, the term "bloood" is not applied. For example, insect "blood" is called hemolymph. What color is insect hemolymph (blood)? Well, kinda blue, actually....


Other posts of interest:

Also of interest: In Search of Sungudogo: A novel of adventure and mystery, which is also an alternative history of the Skeptics Movement.

Photo Credit: postbear via Compfight cc


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THANK YOU! I have been so frustrated by this issue. My students will be better informed now!

wanna see blue-green blood? Go scuba diving below about 30 feet, then let an eel bite you. Weirdest color blood you'll ever see.

Next time you give blood, either for testing or for altruism, just look at it. That's venous blood (bad, bad idea to draw from an artery!), it has not been in contact with air (those little glass tubes are evacuated), and it ain't blue. I think your color swatch is indeed pretty close.

By Sven DiMilo (not verified) on 11 Feb 2008 #permalink

If you are science teachers, for goodness sake see if there is an experiment where you can deoxygenate blood to see what color it is. If so, that would be much better than all this speculative blather. Show rather than tell.

A girl I knew painted her walls in that last brownish-red colour. I asked her whether she'd smeared her walls with cow's blood or something. She wasn't amused.

Is blood ever blue? Doesn't it depend on the species?

By bob koepp (not verified) on 11 Feb 2008 #permalink

I agree with stan. This is the kind of thing that can easily be proved. If you don't want to or can't think it out then just do a test to prove it.

The blood of the Horseshoe Crab is actually a fairly light blue in color. The blood has coagulant properties that are useful in medicine and Pharmaceutical companies are draining some blood from about 500,000 Horseshoe Crabs each year before returning them to the ocean.

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.

Are you joking, or did you really need this explained?

R/G/B and Hue/Saturation/Value are just two ways of expressing a colour. RGB is basically a rectangular coordinate system, whereas HSV is polar (cylindrical, actually).

James, did you just watch which actually showed scenes of the crabs being "bloodletted"? I think they said pharma pays $15,000/quart and the use of the blood is a critical part of the industry. Turns out a dieoff of the crabs was noticed only after a dieoff in a small bird, the Red Knot, dependented on them experienced a 70% dieoff.

It was a great sustainability story. Humans depending on an ancient species to keep Big Pharma going, yet only noticing a serious supply problem when a bird that only bird-lovers would know about began to decline.

About 10% of the horseshoe crabs die due to the blood harvesting (they take 1/3 of the crabs blood). Copper makes the blood blue, I guess it plays a similar role to the iron in human blood. It's those copper compounds that pharma uses.

Eamon, I wasn't joking, but I do know what it means. As a scientist, an archaeologist, and a couple of other things I've had the opportunity to learn about the physics of color, as well as systems of color mapping both cross culturally and in western professional contexts. I was hoping to elicit a discussion on the technical side of color. Thanks for starting that, I hope it develops further.

I should have said in my post that this is all about mammal blood (minimally).

Yes, this could be a good experiment, but in K-12 classrooms these days experiments that use bodily fluids are not being developed too often, and the previously extant experiments are often phasing out. But I think it could be done.

How do you get the blood without bleeding students? It is possible that there are supplies available. Would pasteurizing the blood a) ruin the experiment and/or b) really make the blood "safer"?

Very entertaining. I enjoyed this post.

Deoxygenated blood does look blue, when seen through skin. But that is only an illusion.

By Dunkleosteus (not verified) on 11 Feb 2008 #permalink

I can see the problem. I'm not a blood expert like you, but I've seen enough blood to know that it never comes close to anything that could be considered "blue" -- even by someone from a culture that has fewer distinctions in color terms.

Only time I would lend any real credence to blood being blue would be in cephalopods or another creature that has copper-based blood. Is it, in fact, true that cephalopods have blue-green blood?

My favorite demonstration on color perception: Start with a dimly lit room. Using a standard computer projector and projection screen, project an image of a black square on a white background. Ask the audience, "What color is the square?" Typically they will respond "Black." "Really, that's black?" "Sure, it's black." Cover the projector lens. "What color are you seeing there now?" "White." "White? Really? That is exactly the same color that ten seconds ago you just called black! If anything, there is probably less light falling on that area now than when the lens was uncovered. But now it's white?"

Only time I would lend any real credence to blood being blue would be in cephalopods or another creature that has copper-based blood. Is it, in fact, true that cephalopods have blue-green blood?

Hemocyanin is found in most molluscs (including cephalopods) as well as some arthopods; besides horseshoe crabs, stoneflies have it as well, if I remember correctly.

Fun fact: deoxygenated hemocyanin blood is actually clear.

This won't help!
You need to explain why blood in your veins *does* look blue.


When you look at your veins, your veins look blue, but they are not clear. Empty veins do not look like clear plastic tubing.

Is that what you mean?

Just wanted to mention that horseshoe crabs aren't crabs. They aren't crustaceans at all they are most closely related to the spiders & scorpions.

Copper makes the blood blue, I guess it plays a similar role to the iron in human blood.


However, it isn't the iron that makes our blood red, it's the carrier molecule (heme). Even the last precursor without iron, protoporphyrin IX, is already red. Just like how chlorophyll is green it's not because of the magnesium.

I should have said in my post that this is all about mammal blood (minimally).

Vertebrate blood minimally.

By David Marjanović (not verified) on 12 Feb 2008 #permalink

Blood in your veins looks blue because the light reflecting off of your veins has to pass through tissue (including fatty tissue) before it reaches your eye. The tissue bends the reflected light a bit, making it appear blue.

Try this as an experiment, and you'll see something similar: Take a dark red marker (or some object that is dark read), and plunge it into a bowl of milk (similar lipid content, as I understand). Slowly raise it toward the surface, and watch the color change. When it is just under the surface, it will look blue.

I think I learned that from Mr. Wizard when I was a little kid...

LM, that is an interesting experiment. But again, veins are not clear, so any reddishness one is seeing as bluishness is not blood looking blue. It is a vein looking blue.

For instance, why, when I look under my sink I see copper colored pipes even though water is "colorless"???

But, that is a cool sounding experiment, I'll have to try it.

Lab Cat, well, yes, there are some humans with blue blood, including but not limited to the British...

I would agree with you on this and say that I've always been taught that blood is red to dark red because oxygenated iron is red (like rust). Even deoxygenated blood isn't really all that deoxygenated anyway. It only loses, like what, 5% of its oxygen? Further, as others have pointed out, some marine organisms really do have blue blood. That's because their oxygen carrying element is copper and not iron, like ours. And oxygenated copper is blue to blue-green.

Anyway, iron, that which gives blood cells their color, is never, at any point, blue, if I remember correctly. So the only way to really solve this debate would be to look at blood cells that have no heme group. I would imagine that they would be essentially colorless.

Furthermore, as my understanding goes, blue light can travel farther than red light and that is why aquatic plants are all tuned to be most receptive to blue light, as all the red light is filtered out by the water. So it could be much the same way with how we see our veins. It is possible that only blue light can penetrate our veins, reach our blood cells and be reflected back to our eyes. But don't hold me to this, it's just a guess and I don't know anything about light.

Greg, if it is the veins themselves that are blue, what is in them that makes them so?

And what causes the colors of a bruise?

LM- the experiment you mentioned sounds cool. But I tried it and it failed for me- but I only have a 5% nonfat milk in PBS handy (presumably, no major lipid component).

I don't know about Sena's reflection hypothesis, but here is a simple experiment on transmission of light through tissue.

Put a small bright white light source up against a finger tip and observe it through the fingernail (clear nail polish only). I used one of those 'white' LEDs on a key chain.

You will see a predominantly red colour. In melanin deficient individuals, the ear can be used. Blood vessels can be detected in the thin part of the upper ear.

I verified the spectrum of the LED by shining it on a diffraction grating in the form of a CD. It was almost continuous from red to violet, with an intriguing small gap between green and blue.

Pulse oximetry monitors use both a red and an infrared source/detector pair in this manner to measure arterial oxygen saturation (SaO2), which is typically near 95%.

A venous saturation (SvO2) below 60% is considered medically significant. I don't know what a normal range would be.

As mentioned above, venous blood is what you see in the blood test vials and pint bags. Definitely not blue, and not properly described as purple. Like Greg says, brownish dark red.

By JohnnieCanuck, FCD (not verified) on 12 Feb 2008 #permalink

Right .... as a matter of fact, my wife and I did this experiment weeks ago, when I was thinking about writing this post to begin with. Get a really really strong light and shine it through a human body, and you see red stuff but no blue stuff.

Veins and stuff are greayish-blue / bluish-grey when you dissect them out of an unprepared mammal, and the "blue" color that you see is the veins as they appear through pinkish skin. I would have generally called veins "grey" in my own personal simplified color scheme. At least, that goes for the ones I have personally cut out of a body.

They really, honestly, are not clear. Really. Honest.

Why is a bruise blue? Good question!

Note that classroom dissections are often of material that is dyed and otherwise prepared.

Can you eat a horshoe crab? Would it taste like a crab or a spider ( what ever that tastes like?)

Just in the hope that others here care about science stuff other than in living things, I found out what that gap is in a white LED spectrum.

The LED I have is described as an "InGaN-GaN structure is covered with a yellowish phosphor coating usually made of cerium-doped yttrium aluminum garnet (Ce3+:YAG)" on the Wiki page for Light Emitting Diode.


By JohnnieCanuck, FCD (not verified) on 15 Feb 2008 #permalink

Phil -- Sortof. Horseshoe crabs are not generally considered edible, and the bulk of their body is not only unpalatable, they also tend to harbor flatworms and other parasites that you really wouldn't want to ingest. Having said that, the Chinese "harvest" gravid females for their eggs, and there is some (not definitive) evidence that Native Americans used to eat the tail and leg muscles. Some people may still do.

Their main use to humans, though: their blood is drained to obtain Limulus Amoebocyte Lysate, which is in high demand around the world for use in tests for bacterial contamination and for the diagnosis of some diseases. Although this blood harvesting isn't supposed to kill the crab -- practice is to drain about 1/3 of the crab's blood and then release it -- evidence has come to light recently that this might be contributing to high mortality after all. But the major threat to horseshoe crabs is their use as bait in eel fisheries; millions of them are taken for this purpose every year, and numbers of them are dropping.

By Luna_the_cat (not verified) on 16 Feb 2008 #permalink

I just put up a post about this on my blog. What it comes down to is that there are three reasons why veins in skin appear blue: the absorptive properties of blood, the reflective properties of skin, and relative color perception. As pointed out here, blood is never blue, and it actually takes a bit of complex optics to comprehensively explain the blue appearance of veins in skin.

well think of it this way When a person donates blood, it comes from a vein, not an artery. It is also not exposed to air, or it would be contaminated and need to be disposed of. It also happens to be dark red, NOT blue.

This is an interesting topic. Simple question but need high explanations. I prefer to your explanation that it was veins substances interacts with red blood made veins looks blue to human eyes. But I don't know what substances they are. That's why bruises are bluish. They are related. Human blood is always red, even The Royal Highness Prince Charles.

blood appears blue because blue is the only light that isn't filtered by the skin. It's an optical illusion much like the illusion of a blue sky or blue oceans.

Search âScuba Diving Green Bloodâ on You Tube for a demonstration of blood colour while diving.

By Tim Powell (not verified) on 08 Dec 2010 #permalink

So is there blue blood in your body.

By Elijah Croft (not verified) on 13 Oct 2013 #permalink

When very ill I lacked oxygen in my blood and when they drew blood it was a a dark purple blue red color.

my teacher said the blood in our body is white,blue, and green i want to know wherther is true thank you.

By charlotte (not verified) on 21 Feb 2014 #permalink

this has been such a sigh of relief thank you greg it has been such a huge misconception about red blood turning blue in my class and my teacher is teaching the students that human blood is blue and its been so frustrating beacuse i was the only one trying to explain to them that its a misconception but no one believed me so thank you for this closure

For those who wish to try this, go for it: Prick a finger to make it bleed inside a VACUUM chamber that"s fitted with a glove so the gloved-hand can be stuck inside the chamber, as they do it when working in labs and such, so whatever they are working with or manipulating inside the vacuum chamber is not contaminated with oxygen and other stuff flying around in the air? Maybe that way it can be proven whether blood is really blue or not before it bleeds out and becomes oxygenated in the regular environment? Try it, I'll wait for your posting with your results. Could be interesting.

numbnutz think again about "deoxygenated blood" now remember people breathe out carbon dioxide. so blood on the way out should have twice the oxygen as CO2. get on my level. its common sense

By chris maser (not verified) on 01 Jul 2015 #permalink