Blood is red
I was recently surprised to find out a number of my students thought deoxygenated blood (the blood in your veins that has already delivered oxygen to your body cells) is blue.
Wait, what? People still think this???
To be fair it used to be a fairly common misconception - what surprised me is that the misconception is still hanging around.
Here’s the thing - blood is red. All the time. Oxygenated blood is a nice bright red, whilst deoxygenated blood is a darker red. But still red.
The erythrocytes (red blood cells) in your blood contain haemoglobin. This is composed of four haem subunits, and there is an iron atom in the middle. It is the iron to which the oxygen is attracted, and the interaction between the iron and the oxygen is what causes your blood to be red.
Granted, your veins look bluish when looking at them through your skin, and this is due to a number of factors.
Firstly, let’s look at the physics of light and colour. Different colours of light are due to different wavelengths of the light. Blue light has a certain wavelength (450–495 nm), green has a different wavelength (495–570 nm), and red light has yet a different wavelength (620–750 nm). Whatever wavelength of light makes it to our eyes determines the colour we see. For example, incident light (light from the Sun that hits an object) is white, and is composed of all colours. Leaves look green because the blue and red light get absorbed, so they can’t make it to our eyes. The green light, however, gets reflected and can make it to our eyes, so we see the green-ness of the leaf.
When considering the blue look of veins, you have to realise that blue and red light penetrate our body tissue (skin) with different degrees of success. Blue light does not penetrate human tissue as deeply as red light does. As a result, veins that are close to the surface of the skin will be more likely to reflect blue light back to the eye. So we see blue veins.
Also, deoxygenated blood tends to absorb red light more than blue light, so it is blue light that is more likely to be reflected back to our eyes.
It is also believed that blue light tends to scatter more easily when it passes through human tissue. This contributes to veins appearing blue despite the dark red blood within them.
Another factor to consider is that of optical illusions. Basically, our brains are pretty easy to deceive. When you look at something, your brain also takes in information from around the thing you are looking at. If the surroundings are pale (e.g. the skin of your arm), the object your are looking at will appear darker (e.g. a dark blue vein). In fact, nobility used to be called “bluebloods”, as they weren’t out working in the fields, so didn’t get tanned skin, and so their veins looked more blue compared to the working people out in the fields with darker, tanned skin. Looking at veins of a worker against darker surrounding skin, the vein did not look as blue…
Anyhoo, as you can see, there are a number of factors that come into play to make your veins look blue. And I suppose biology textbooks don’t help, in that they tend to have diagrams of the circulatory system using red for oxygenated vessels and blue for deoxygenated vessels. Not just textbooks, but all over the internet too...
When you think about it, though, have you ever seen blood in a movie look blue??? Now I know movies are often far from realistic, but I for one have never seen blue blood portrayed in a movie, so at least that’s one thing they get right!!!
So, blood is red. Assuming we’re talking about human blood. Because the horseshoe crab, e.g. Limulus polyphemus (there are actually four species of horseshoe crab), has... Wait for it... Blue blood. That’s because the horseshoe crab doesn’t use haemoglobin to bind oxygen, but haemocyanin. This has copper instead of iron, giving a bluish tinge to the blood.