A common misconception about blood is that it looks blue when depleted of oxygen, explaining why lighter-skinned people see blue veins in their arms. Here I’ll talk about how deoxygenated blood isn’t blue at all, and that the real reason for the color shows off some interesting aspects of how light interacts with matter.
Special thanks to everyone who let me bother them about the color of their veins over the past few weeks!
An object’s color comes from how its atoms and molecules interact with light
Unlike water waves, light waves are able to travel in the absence of a medium—they come from stars to our eyes through empty space just fine. That’s not to say light isn’t affected by what it’s traveling through. The atomic and molecular structure of most materials gives them certain natural frequencies of vibration, and atoms will tend to absorb incoming energy if it causes them to wiggle at just that right frequency.
Imagine pushing a child on a swing: you probably know that the best time to push is at the back end of the arc, just as they stop before moving forward again. If instead you tried to run up and push them partway down the arc, or worse try to push them forward when they were still headed towards you, you’ll end up with a poor swing motion and possibly a dirty, upset child.
Light behaves similarly as it wiggles electrons in an atom—if the light frequency matches a natural atomic frequency, that electron absorbs the energy of the light and continues to wiggle about, probably transferring the energy away as heat a bit later. Since some of this frequency (or color) light got absorbed not much gets reflected from the object to your eyes. In this way a banana looks yellow because its atoms are absorbing all frequencies of visible light except for yellow.
Because of the absorption and scattering of light, more red light than blue penetrates deeper into skin
The more a frequency of light is absorbed by a material, the less penetrates to lower depths. Another effect that prevents light from going further into a material is scattering, where light gets deviated from its original path because of something in the way. This is close to but not quite the same as reflection, since here the new direction won’t necessarily have the same angle as the incoming light.
The details of scattering lie at the heart of a lot of everyday physics—things like why the sky is blue, or why beer foam is white—but that’s another post. It turns out that the size of the scattering particle relative to the wavelength of the incoming light determines the nature of the scattering, and the little density changes in skin and fat tissue are such that blue light tends to be scattered more than red. This means blue light won’t reach as far down before it gets knocked into a new direction. You can test this by holding a bright flashlight on one side of the thin skin between your thumb and fingers—it’s red-tinted because those frequencies are making it through your skin.
The ratio of red to blue light is smaller over a vein than just beside a vein, so your brain interprets the vein as more blue
Let’s focus now on two areas of your arm: one area where there is a big vein underneath skin and an area right beside that with no vein. Skin will be lighter if it has less melanin, which means most incoming light gets reflected—so it appears white. Now, the blood inside the (mostly translucent) vein instead mostly absorbs light of all frequencies, but tends to reflect a little bit in the red area of the spectrum—that’s why blood looks red. By the way, it really is red regardless of oxygen content: oxygenated blood in arteries is cherry red, and de-oxygenated blood in veins is dark red, but neither is blue!
Both red and blue light get absorbed some on the way to the vein, more blue light gets scattered on its way down to the vein, and blue light gets absorbed more than red once it reaches the vein—so then why do the veins look blue? In fact, there is more red light than blue light coming from the region of skin above the vein and from the region of skin right beside a vein.
Your brain can be fooled sometimes when it observes colors and patterns—this is the basis for most optical illusions—and there is a similar thing going on here. Even though both regions of skin reflect more red light than blue light, the no-vein region reflects a greater ratio of red to blue light than the vein region. This is because less blue light made it down to the vein to be absorbed strongly there. Your brain interprets the vein area as a little bit bluer by comparison to the surrounding skin. The same thing happens when you look at the image below: the blue squares are the same color everywhere, but look a bit darker when surrounded by a darker color.
It turns out this blue vein effect only happens for bigger blood vessels that are a certain depth under your skin. It’s easier to see on veins because they are typically larger when they are near your skin surface, like in your wrists and forearms. If you happen to have a vessel with less skin covering it will appear its natural red—take for example the vessels in your eyes, or on your retina.
Bonus physics—Non-red blood
Human blood (and that of many other animals) uses the molecule hemoglobin, where oxygen is bound to iron atoms. It’s this iron content that gives blood its red color, which the presence of oxygen makes a bit brighter. Creatures that don’t use iron to bind oxygen can have different color blood. For example, copper is the main oxygen receptor in hemocyanin, which is present in the blood of lobsters, snails, and spiders—as a result, their blood looks blue. It’s copper carbonate that makes copper statues turn green (like the Statue of Liberty), so perhaps this is involved in the famously green blood of Vulcans.