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Here is another question that students ask me all the time: ”Is it true that your blood is blue in your veins when it doesn’t have any oxygen?” The short answer is ”No, your blood is always red”; however, if you’re like my students, it’s going to take a lot of proof to convince you. So, here we go!

The Misconception

Many people, including teachers, college professors, and even some doctors and vets seem to think that deoxygenated venous blood is blue while it is in the vein. It doesn’t turn red until it becomes oxygenated, either in the lungs or when it comes into contact with oxygen in the atmosphere. It is a misconception that just won’t die. The idea has become so ingrained that people refuse to change their mind, even when they are given hard evidence!

Where Does The Idea Come From?

When pressed for proof, most people who believe the “blue blood” misconception will point to the visible bluish colored blood vessels in their wrists. I imagine this is where the myth originated. Afterall, it is thought that royalty picked up the nickname “blue bloods” hundreds of years ago due to the way their blood vessels showed through their pasty pale skin. To the casual observer, it seems quite obvious that they must contain a blue substance.

The myth was probably reinforced when medical diagrams began color coding the circulatory system with red and blue: red for oxygenated blood and blue for deoxygenated blood. The rest was probably a bit of creative, on-the-spot scientific conjecture where people who thought they knew that blood was blue started trying to find the chemistry that would cause the phenomenon.

Debunking The Myth

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Point/Counterpoint: The Blood Turns Blue Because It Doesn’t Have Any Oxygen

Those who have bought into the blue blood myth try to support their argument by invoking chemistry. We know why oxygenated blood is red. The part of a red blood cell that attracts oxygen is called hemoglobin. Hemoglobin is a protein that contains four iron atoms, each bonded to a heme group. If you’ve ever seen a rusty nail, then you know that oxidized iron takes on a reddish color. The same is true in hemoglobin. So, it makes sense that deoxygenated blood would be not-red, right? So, that explains why it would be blue?

Except, even the deoxygenated blood in your body isn’t really deoxygenated. Normal blood oxygen levels for oxygenated arterial blood usually range anywhere from 95% - 100%. Normal levels for deoxygenated venous blood range from 60% - 80%. Even deoxygenated blood is really quite oxygenated! So, it stands to reason that it, too, would be red in color. Indeed, it is just a duller, darker red. However, under extreme cases of hypoxia, it may take on a purple hue.

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Point/Counterpoint: The Blue Blood Turns Red As Soon As It Touches The Air

The first few times I was asked this question, my reply went something like ”Ummmm, have you ever cut yourself? Did you see any blue blood?” Their retort was ”No, because it turns red as soon as it hits the oxygen in the air.” Isn’t that convenient? You’ve reverse engineered an explanation for why we never see this mysterious blue blood by coming up with something we can’t test, because any time blood leaves the body, it has to touch air.

Or, does it? Whenever I teach pressure or states of matter, I use a demonstration where the students get a large syringe (sans needle!) with some water in it. They are instructed to cover the opening of the syringe and pull the plunger back. You see, as the plunger is pulled back, it does not allow any new air to enter the syringe. As a result, the water begins to boil, even at room temperature.

Whenever blood is drawn for testing or for donation, it is usually taken from a vein, because they are closer to the skin surface, and there is less blood pressure in veins. So, the blood that goes into that syringe is deoxygenated. If a syringe is air tight, then having blood drawn puts the blood in a vacuum environment, where there is no oxygen. If you’ve ever had blood taken for any reason, I’m sure you noticed that it was, indeed, dull red in color. So spilled blood isn’t just red because of the oxygen in the environment. It came out of the body that color.

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Point/Counterpoint: But, The Veins In My Wrists Are Blue, So The Blood Has To Be Blue!

True, the veins in your wrist do appear to be blue, but that is just because you’re looking at them through your skin. They actually get their color from the blood that they carry, so they are red inside the body. But, what is it about the skin that makes veins look blue? It has to do with the properties of light.

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The white light that comes from the sun or that we use to brighten a room is actually made up of 7 different colors. I’m sure you all know ROY G BIV. And, I’m sure you’ve seen how a prism can separate white light into those 7 colors based on their wavelength. The long wavelength of red light allows it to more easily penetrate through the skin, and once it hits a vein, a lot of it gets absorbed by the hemoglobin in your blood. You can test this by shining a red light onto your wrist. You should notice that the red light reflects back really well in most places; however, you will see dark lines. That is where the red light is being absorbed by your veins. On the contrary, if you shine a blue light onto your wrist, it will be reflected back evenly without any dark lines. In addition to the fact that blood doesn’t absorb as much blue light, the smaller wavelength of the blue light is more easily deflected by your body tissue, so it bounces back to your eye. When regular white light hits your wrist, those two phenomena overlap. Both red and blue bounce back to your eye in most places, but wherever there is a vein, you will have mostly the blue end of the spectrum coming back to your eye.

Are You Convinced, Yet?

So, can we just let this myth die, already? Blood is absolutely NOT blue when it is in your body. It is always some shade of red. The idea that it is blue is a throwback to 1800’s misconceptions that have inexplicably survived into modern times.

I hope you’ve enjoyed this article, and if you know of anyone (especially teachers) who still believe that blood is blue, please send them this way.