Why Are Veins Blue?

ResearchBlogging.orgWhen someone asks the question "why are veins blue?" a likely response is that they're blue because the blood in veins is deoxygenated. While it's true that venous blood vessels carry a lower concentration of oxygen than their arterial counterparts, this isn't the reason for their blue appearance in your skin. Still, when someone invariably responds to the veins-are-blue-because-they're-deoxygenated argument with the observation that "I've never seen blue blood before" one might then hear the slightly more sophisticated-sounding but increasingly far-fetched claim that we don't ever observe blue blood because it is immediately oxidized upon contact with air. (Check out these beauties from Yahoo Answers to witness these arguments yourself.)

Wrong. Blood is never blue. Ever. Period. (And, this is an excellent example of why you should never ingest information from Yahoo Answers uncritically.)

The bright red color of arterial blood stems from a complex that's formed between hemoglobin, iron, and molecular oxygen. This complex mainly absorbs higher energy (shorter wavelength) blue and green light, leaving behind primarily just red wavelengths for our eyes to detect. However, even when blood is largely depleted of oxygen, it is never blue: it's more of a deep maroon color. And, although I have not participated in a surgery myself, those who have (including my girlfriend, a veterinary student) assure me that in surgery, veins within the body do not appear blue either. So, not only is blood not blue, veins on their own aren't either. "Blue veins", then, are a phenomenon unique to the skin.

So, what's going on here?

The most comprehensive answer that I've been able to find comes from a paper by Kienle et al. published over twelve years ago in the journal Applied Optics and entitled "Why do veins appear blue? A new look at an old question". Based on the findings of the authors, three reasons emerge for the blue appearance of veins in skin. The first two are physical and stem directly from the way in which light interacts with blood (how it is absorbed) and with skin (in this case, how light is reflected). The final reason is psychological, dealing with the way in which our brain processes information relatively to generate color perception. (Greg Laden sort of hinted at some of this recently on his blog).

Since the paper in question is a study in optics, it is way outside of my area of expertise. So, just in case you wanted an in depth technical critique of its methodology, you're not going to find it here. However, I can give you a basic summary of the paper's major findings. And then, if you still have the desire (and the ability) to explore the subject in greater depth, you can parse through the paper yourself.

To tackle this problem, the authors measured how much light of various wavelengths was reflected from both real blood vessels in skin and imitation vessels in a skin-like environment, using a sophisticated technique that gave them spatially-resolved measurements. The synthetic vessel (which was a capillary tube filled with blood and placed in a milky substance with optical properties similar to skin) allowed the authors to experiment with a variety of parameters (particularly vessel depth and diameter), and they were then able to validate their results by taking measurements on actual vessels in skin. Using this set-up, they were able to demonstrate that the optical properties of skin and blood (combined with the influence of relative color perception) explain why veins in skin appear blue, despite not actually being blue.

Skin does not absorb much light at any wavelength, making it look white (depending on how much melanin is present, of course--making this discussion only really relevant to people with lighter skin). Blood, on the other hand, absorbs light of all wavelengths (but less in the red part of the spectrum). However, blue light does not penetrate the skin as well as red light. If a vessel is near the surface of the skin, almost all blue light is absorbed by the vessel, so even though only about 1/4 of the red light is reflected, the ratio of red light reflected to blue light reflected is about 10:1. This vessel appears red.

If the vessel is deeper (about 0.5 mm or more), not as much blue or red light will be absorbed. Importantly, this effect will be more pronounced on blue light than on red light since blue light doesn't penetrate skin very well (the ratio of red light reflected to blue light reflected is about 3:2 or less). This is the case for the "blue veins" observed in skin. Once the vessel is deep enough, though, it won't be seen at all, as light of all wavelengths will be reflected before it can interact with the blood.

Perplexingly, this 0.5-mm-deep vessel appears blue despite reflecting slightly more red light than blue light. This is where relative color perception comes into play. The surrounding skin reflects more red light than blue light (by a ratio of about 5:3), and it does not absorb as much of either type of light as a blood vessel does. Since vision is influenced in part by relative perception, if something purple is placed next to something red, the purple object will appear blue.

None of this, however, addresses the question of why veins specifically appear blue. To answer this one, I can fortunately once again rely on the expertise of Meredith, my vet student girlfriend. The reason why only veins appear blue is that veins are the only vessels we actually observe through the skin. This is due to the fact that veins are larger, have thinner walls, and are more superficial than arteries (and, no, I don't meant that veins prefer People or Us Weekly over The New Yorker--"superficial" is just medical speak for closer to the surface). All of these aspects of veins have clear biological rationales. Beyond just carrying blood back to the heart, the primary function of the venous system is as a blood reservoir. In fact, about two-thirds of your blood volume is held in your veins at any given time, hence their larger size. Because the heart has to push blood directly through arteries, their walls are subject to higher pressures than the walls of veins, so they need to be thicker. Finally, veins are located closer to the surface of the skin, because they also play an important role in heat exchange with the outside environment (to help cool the body). Arteries could perform this function just as well, but it's much more advantageous to keep those higher pressure blood conduits deeper in the body and protected from injury.

The take-home message here is that the bluish appearance of veins in the skin has everything to do with where they are located, and nothing to do with the concentration of oxygen within them. In fact, if we could see them through the skin as well, even arteries would look blue.

Kienle, A., Lilge, L., Vitkin, I.A., Patterson, M.S., Wilson, B.C., Hibst, R., Steiner, R. (1996). Why do veins appear blue? A new look at an old question. Applied Optics, 35(7), 1151-1160. (link)

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Excellent summary. Informative, but concise post.

By Victoria Carrell (not verified) on 11 Oct 2008 #permalink

sorry to say but i still say that blood is not blue in the body because why is ur face red when u blush or why do they show red blood cells in blood video's in health. don't mention the veins i htink that the veins are blue not the blood

wrong, wrong, wrong, your blood is a light colored blue before hitting oxygen, when you look down at your veins, thats the original color of blood, blood turns a dark red when hitting oxygen.

I don't think your veins are blue....mine is greenish even what angle i look at it at the floures lamp. Medically, Oxygen content is really different between the two..I said Oxygen content, and this is the basis for such terms as red and blue to medically remember that its the vein or the arteries...It is also a question why Pulmonary artery is called an artery when it carries less oxygen? But they answered that already in the medical profession. This research is just a redundant expenditure of funds when the medical profession agreed already on the terms of their anatomy even since Curie discovered the piezoelectic crystals to proclaim "superman"

By Sunooogna (not verified) on 03 Mar 2012 #permalink

I agree. Blood is never blue. It is always red. Simple. That's it.

Interesting... I was under the 'deoxygenated' delusion too.

So how does that explain cyanosis when someone is severely hypoxic? I always thought cyanosis was because of blood in vessels very near the skins surface (in the lips for example)... So shouldn't that still be red under this analysis?

Maybe I should just read the paper :D

Nick- nice post. I understood what was going on much more quickly than I did when Laden covered the topic. Granted, I was originally under 'the deoxygenated delusion' and this may just reflect me having time to absorb that that idea is wrong and let go of it- but I like to think your answer was better.

Also, Ian, I'm not sure this helps, but acording to the wikipedia article (http://en.wikipedia.org/wiki/Methemoglobinemia) sufferers of methemoglobinemia, a rather extreme disorder including cyanosis actually have *brown* (rather than red) arterial blood because so much of it is deoxygenated. Brown arterial blood apparently can produce a striking blue hue in the skin- from what I've heard about the blue Fugates of Kentucky. This is also most predominant in infants- though I haven't yet worked out why that should be.
Anyway, the deoxygenated blood being brown appearing blue could explain cyanosis... now I just don't quite understand why being *cold* makes your lips blue (unless being cold interfers with oxygenation of blood).

I haven't been able to find as thorough a physical/optical explanation of cyanosis, but I can certainly speculate. Firstly, I should point out that while I did emphasize that if we observed arteries in the skin they too would be blue, this isn't to say that the oxygen concentration in blood has no effect. Clearly it does, since arterial blood is normally bright red, and venous blood is normally deep maroon. This fact is irrelevant in regards to the "blue veins" we see in skin. However, skin is also full of much smaller and more superficial microvessels. Due to their smaller size and closer proximity to the surface, the optical situation is different here, and these vessels give the skin a slightly reddish tint. However, one could imagine that if their optical properties were significantly perturbed (by severe deoxygenation), they could give the skin a slightly bluish tint due to the same optical processes explained in the post. Of course, this is just speculation, and if anyone knows of a better explanation, I would be interested to hear it.

Great post. Follow up question: I have very pale, relatively transparent skin. The veins on my hands and elsewhere actually look purple. On my feet / ankles where veins are most prominent, half look purple and half look green. I've never seen 'blue' veins on myself. Does anyone else see green?

This one, kinda like the myth of the "giant moon on the horizon" phenomena is exactly the sort of thing I love to hear about. I was told the deoxygenation story when I was a kid, but it never came up much for me as I got older.

I do a little painting, and learned a while ago that painting veins on skin isn't done with blue paint - it doesn't look realistic (Also, steel isn't gray either). It was a very interesting lesson to learn.

By Dread Polack (not verified) on 17 Apr 2008 #permalink

Becca: Cold makes the veins go deeper into the body. The veins that are blue normally probably disappear or get hard to see if it gets cold enough. In the lips the veins are normally very close to the surface because the skin is really thin. That's why they are red. When the veins in the lips go deeper because it is cold they appear blue for the same reason that Nick described.

Wait, blue light does not penetrate the skin as well as red light? Do they suggest a reason for this oddity? As the energy of light increases (i.e. frequency increases) the penetrating power of light increases. Note that UV penetrates the skin fairly well, X-Rays go right through it, and gamma radiation/cosmic rays will zip right though your entire body. Blue light is (albeit only slightly) higher energy than red light, so it ought to penetrate better, not worse, than red light. Perhaps I'm missing something? Or else there's a mechanism in place to prevent blue light from getting through?

Actually, within the UV-visible range, longer wavelengths penetrate the skin more effectively than shorter wavelengths. So, visible light penetrates more than UV, red light more than blue light, and UVA more than UVB. Even though it does not penetrate the skin well, enough UV radiation penetrates sufficiently far into the skin to cause the damage that leads to skin cancer.

Ah, curious. My understanding of physics once again outstrips my knowledge of biology. Why the inverse relationship in skin? Do the pigments just have an absorption curve with peaks closer to the UV?

I'm not sure if this is the cause of this phenomenon, but once again I can speculate. In our body, the only types of compounds that interact with UV-visible radiation to a significant degree are organic molecules and transition metal complexes (with the latter only being relevant in special cases, such as iron in the blood). Organic molecules only absorb significant amounts of UV-visible radiation when they contain double bonds, but these are pretty common (throughout the peptide backbone of a protein, for example). Double bonds only absorb higher energy (shorter wavelength) UV (220 nm in the case of peptide bonds), unless they're found in conjugated double bond systems. Aromatic groups in DNA and proteins, for example, absorb at 260 nm and 280 nm, respectively. Since aromatic groups are less common than single double bonds, though, more radiation will be absorbed at shorter UV wavelengths than at longer UV wavelengths. Compounds with longer conjugated double bond systems (or transition metal complexes) will absorb longer wavelengths (in the visible region of the spectrum), but those will be rarer still. I'm not sure if this is the reason for the increased penetrance of longer wavelengths, but it's a possibility.

A logical hypothesis. Appreciate your explanation.

LOL - yeah sheeple think veins are blue cause no O2 like they think the sky is blue cuse it reflects the ocean.

Amazing how close that ocean thing is, while being the exact wrong concept.

Funny thing is, if blood is blue due to lack of oxygen, and we all know that low oxygen in the blood causes brain damage, then what were they thinking in the middle ages when nly so-called blue-bloods ruled? Oh, wait - those were the dark ages.

Amazing that you could get through your very interesting post without using the word scattering. Check the ever authoritative wikipedia. This is the last (and therefore correct) explanation for blue appearing veins I have heard:)Before open-heart surgery, there were unfortunate kids with heart defects that allowed (I think) oxygenated and unoxygenated blood to mix in their hearts. They were quite "blue" in appearance. Unfortunately, they did not live much beyond their 20's.

By BlindSquirrel (not verified) on 18 Apr 2008 #permalink

A diagnostic check for hypoxia is to look at the finger nails for a colour change. Decades ago an altitude record-holding glider pilot told me of her training session at a US military hypobaric chamber. They went to 25,000 feet and took off their masks. The task was to identify the point where the colour change occurred.

The point of her anecdote was that she stared intently at her fingers and became aware that the colour change had happened only after her observer had restored the mask.

I keep saying colour change because my memory is not completely certain that it was black. Maybe it was blue.

By JohnnieCanuck, FCD (not verified) on 19 Apr 2008 #permalink

A good discussion. Did not see this point in the comments but maybe I missed it -- If you donate blood, they take it out of veins (see Wikipedia) and you can verify yourself that it looks red or maroon...

By John Abbott (not verified) on 19 Apr 2008 #permalink

As a surgeon I can tell you veins are blue to purple, even deep in the body with no skin covering. Blood itself ranges from bright red to deep purplish maroon depending on how oxygenagted it is. Arteries appear white because they have a thick muscular wall made of three layers and do not transmit the color of the blood. Veins, conversely, have a thin translucent wall that transmits the color of the blood inside, distorting the color slightly and making it bluish. If you look at a vein in the body you can see the blood flowing inside of it, and if air bubbles get in (from an IV or from an intentional opening of the vein) you can also see the bubbles through the wall of the vein.

I'm not sure I understand � if veins appear blue because _that's what color they are_, to the naked eye, as reported by a surgeon, then why the need for a discussion of optics, or for relative color? If I isolate a vein in my vision from the surrounding "red" skin by surrounding it with say, a green cloth, so that the only skin visible is the vein, it still appears blue. Why can't we say that veins appear blue because that's what color they are, and that arteries, even if they were visible, would appear white, because that's what color they are?

By Ciro Faienza (not verified) on 20 Apr 2008 #permalink

The most important point here is that blood is never blue. In addition to that, there's the point that the oxygenation state of blood isn't the primary determinant of what color a vessel appears in skin. To explain these points, a discussion of optics is necessary. Beyond that, I'm still under the impression that exposed veins within the body do not appear blue, as this is what I'm told by reliable sources, but I cannot vouch for that personally. Regardless, that point is secondary to the rest.

Blood is never blue. Ever. Period.

Well, not hemoglobin-based blood, at any rate...

This is what happens when I read this blog during class (this time, small animal oncology):

In veterinary medicine we have a pretty (unfortunately) common tumor of the dog arising from the vascular endothelium called a hemangioma or hemangiosarcoma, depending on malignancy. These tumors, located in the subcutaneous layer or in the dermis, can appear to have a bluish tinge when filled with blood, due to the same phenomena discussed here.

And, for the record, every vessel I've seen in surgery is a nice maroon, maybe purple at best--definitely no blue veins. In fact, many a surgical student has mistaken a large red vein for an artery (overlooking the smaller more muscular and therefore less red artery) and almost caused a patient to bleed out.
It's almost a trick of the mind that the veins seem more blue in your mind's eye because you want them to be, because you've always been taught they were, but alas, even the venae cavae are a deep dark red.

If you don't believe me--and aren't too squeamish--you can check out some intraoperative pictures online. As a caveat, I'm told that veins in the brain can look bluer than those elsewhere in the body, but I can't confirm... or explain.

By Meredith M. Clancy (not verified) on 23 Apr 2008 #permalink

hej is not quite correct on what happens in cold. The surface veins do not move, rather blood flow is redirected to deeper veins which, of course, flow toward the heart, alongside arteries, thus creating a counter current situation which helps retain body heat. This means the surface is not kept as warm and frostbite can happen.

By Jim Thomerson (not verified) on 30 Apr 2008 #permalink

Isn't there ANY article or experiment done on whether blood is actually dark red in veins? I managed convinced my classmates that veins look blood, but they still say blood is blue. When blood is drawn, is there vacumm inside the vial? This way, I could show them. Seriously, everyone managed to contradict me, even the freaking teacher with the whole blood is blue, oxygenated is red. I tell them hemoglobin is red, but nooo, they respond with contempt and stupid logical leap. A class mate even mocked me, saying to put a person in a vacumm and make them bleed, as if what I was suggesting is stupid. "It is all true because everyone believes it," that is how the argument ran. Anyway, sorry for the rant, but I am still pissed by what happened.

whoops, I meant blue, not blood in the second sentence.

Venous blood needs to be drawn out using a vacuum because it lacks the self-propelled force that muscular arteries give the arterial blood; in fact, one way that venous blood is drawn back to the heart using a kind of vacuum created by the heart's pump (the other way involves skeletal muscle compression, thus explaining why people who are laid up in hospital beds have blood pooling and must be turned, etc.) Therefore, most blood vials that are used have a vacuum present (although not too much of a vacuum or your veins will collapse). When you get your blood drawn at the doctor's, they are most likely using a nifty little invention called a vacutainer to place blood in multiple vials (each vial according to the contens and tests to be run).

It's really hard to dissuade people from this "deoxygenation delusion" but kudos for trying. Like I said earlier, the best way to prove it to yourself is to either see a surgery in real-time or find pictures of one and realize that everything there looks a beautiful shade of maroon. Even in this picture (apologies for quality--my school doesn't permit the publication of our own intraoperative pictures or those from lectures due to obvious legal constraints) you can see the portal vein (PV) where blood has essentially gone through circulation twice and so should be quite deoxygenated, and even it looks purplish at best.

By Meredith M. Clancy (not verified) on 01 May 2008 #permalink

If you examine an area of the body with really thin skin, you'll notice the veins near the surface are indeed red. And, you definitely don't see red & blue veins of the same size.

Examples of these areas are: the eyes, eyelids, underside of the tongue, and the scrotum.

By Larry Walker (not verified) on 19 May 2008 #permalink

Wait a minute isn't blood purple when it contains the CO2 and such?

By Student of a S… (not verified) on 29 Jun 2008 #permalink

One diagnostic marker for Ehlers Danlos Syndrome (type IV) is veins that are very easily seen through the skin. Other EDS types have this as well, though.

My skin is like a roadmap haha.

Excellent summary of a long article.... and in English!!

By Shinga Bappa (not verified) on 28 Sep 2008 #permalink


Excellent summary. Informative, but concise post.

By Victoria Carrell (not verified) on 11 Oct 2008 #permalink

"Blood is never blue. Ever. Period."

I draw your attention to the horseshoe crab. Because its blood is bound to copper, its blood is indeed blue. The quick clotting blue blood of this ancient non-crab has been used since the 1950s to test vaccines.
Nature nearly always can provide an exception to any rule.

I am glad stumbled upon this article. After reading the aforementioned Applied Optics article entitled "Why do veins appear blue? A new look at an old question," I found Nick's summation of points very helpful. The research paper is very dense and hard to digest, if you are not used to reading scientific peer reviewed papers (The last one I read was over two years ago during my senior year as a biology major).

After reading the responses, I feel like there could be a potential follow up article on this subject. People are still concentrating on the properties of gaseuos molecules bound to blood, when the extensive research from 1996 concluded that the amount oxygen saturation, or even the CO2 saturation, is only a small piece of the puzzle. The proerties of light, which Nick wrote about, is the important issue. An understanding of the refraction of light coupled with the remittance is the key to understanding the answer here, in addition to the Retinex effect. By no means am I an expert on the matter, but that's what I reasoned from Kienle's studies on this matter.

I think if you conjoined this topic with the color of the sky it might offer this post even more clarity. Either way, it was a delightful read.

By Patrick Wright (not verified) on 12 Jan 2009 #permalink

If I may chime in- In my Bio Class last semester, we discussed gas exchange in blood and the Bohr Shift. The blood does not release 100% of the O2 it is carrying, but instead a lower percentage, then goes back through the systemic circuit. If the blood in your vein is blue because it has given off all it's 02, how do they expect us to hold our breath for more than a few seconds?

By Dr. D-Rag (not verified) on 26 Jan 2009 #permalink

I watch way too much television which is why I write at a 5th grade level at best. But I will make my grammatically imperfect point. I watch a lot of shows where surgeries (cosmetic, c-sections, gastric bypass, etc.) are performed and I never see anything blue inside anyone's body, so I'll buy this explanation.

By TV Zombie (not verified) on 18 Feb 2009 #permalink

I remember learning this back when I was at shcool. I remeber predicting why it was beforehand too. We have an uncanny knowing within us that Science then exaborates.

Excellent article, thank you. This is how scientists should always talk. Rephrase: This is how science should perceived. I think it's important to always keep in mind, and mention(!), that everything is only our perception. I hate all those arrogant science blogs saying 'this IS that' like if it was an absolute truth and pushing non-believers in to the opposite extreme. - BTW, I'm subscribing =)

By Daniel K. (not verified) on 27 Feb 2010 #permalink

I've been teaching 9th grade physical education for three years now. I try and try and try to prove to my 15 year old know-it-all geniuses that blood isn't ever blue no mather how little oxygen is in it. I tell them that I am by no means a doctor and that I only took BIO 131 to get my degree so my medical classes are limited to that. I was however informed in my Exercise Physiology class that blood was never blue. That there was a chemical in the skin called callogen that cause the blood to tint to a blue shade. Can someone speak to the validity of this statement please?

By P.E. Teacher (not verified) on 30 Apr 2010 #permalink

The effect does not have to involve collagen specifically; in the study described above, no collagen (as far as I know) was present in the vessel-mimicking experimental system. However, collagen is a major component of the skin and connective tissue that is involved in scattering and light, leading to the appearance of blue vessels. But, this effect is not specific to collagen (and may not involve it at all).

Thank you for your blog! It was very helpful and answered my questions! My husband and I had the EXACT conversation in your first paragraph and I was not satisfied with that answer at all! So, I asked him to grab my laptop and I found your entry - thanks so much! Makes more sense than deoxygenated blood. :) 'Preciate this!

By Tif Burns (not verified) on 10 Feb 2011 #permalink

Almost without exception in the nursing home I used to work in, the elderly folks in their 70's and 80's have large, protruding, very blue veins in their hands and arms. I always wondered why the veins enlarge with age. The advantage is this - you cannot miss in starting an I.V., or drawing blood.

Fantastic article! I've recently encountered several junior and senior high students who've heard this "blood is blue until it the air" baloney and I cannot convince them otherwise....now, if I can just figure out how to get it to them without friending them on fb!!

Blood is never blue. Ever. Period.

So is it Urban Legend that people with Rh(Neg) blood have a tinge of blue to their blood when it's exposed to the air?

Supposedly, this is the reason why the upper 2% in our society are called "Blue Bloods". Apparently, this is due to an inordinate amount of silver in their blood, as opposed to iron.

Forgive my Junior HS question, but I ran across this on that venue of academic excellence, YouTube.