Why Are Veins Blue?

Have you ever wondered whenever you see blood outside your body that blood is red, but veins are blue?

Let us explain to you the reason behind this, but keep in mind the answer depends on a number of things, including how your eyes perceive color, how light behaves when it contacts your body, and the special properties of blood. Blood that has been oxygenated (mostly flowing through the arteries) is bright red and blood that has lost its oxygen (mostly flowing through the veins) is dark red. Anyone who has donated blood or had their blood drawn by a nurse can attest that deoxygenated blood is dark red and not blue.

 

The Color of Human Blood

Human blood is red because of the protein hemoglobin, which contains a red-colored compound called heme that’s crucial for carrying oxygen through your bloodstream. Heme contains an iron atom that binds to oxygen; it’s this molecule that transports oxygen from your lungs to other parts of the body.

 

Chemicals appear particular colors to our eyes based on the wavelengths of light they reflect. Hemoglobin bound to oxygen absorbs blue-green light, which means that it reflects red-orange light into our eyes, appearing red. That’s why blood turns bright cherry red when oxygen binds to its iron. Without oxygen connected, blood is a darker red color.

Wavelengths of Light

To understand what color our veins appear, we need to think about what happens to different wavelengths of light when they hit our skin, how far they can travel through our skin, and what happens when they get to our veins.

As we know that the light that hits our skin during the day is basically white, this is a mixture of all the visible wavelengths. But to explain why our veins look blue, we will look at just the red and blue ends of the spectrum.

Red light has a long wavelength – and this means it is less likely to be deflected by materials and can more easily travel through. Red light can travel pretty well through the skin and body tissues, reaching up to 5-10mm below the skin, which is where many veins are. When it gets into the veins, the red light is absorbed by the hemoglobin (the protein that makes our blood red). You can demonstrate this to yourself. If you shine a red light on your arm, you will see some red light reflected back and dark lines where the veins are, as the red light is absorbed by the hemoglobin. Thus, the bluish color of veins is only an optical illusion. Blue light does not penetrate as far into the tissue as red light. If the blood vessel is sufficiently deep, your eyes see more blue than red reflected light due to the blood’s partial absorption of red wavelengths.

True-Blue Blooded

But the blue blood does exist elsewhere in the animal world. It’s common in animals such as squid and horseshoe crabs, whose blood relies on a chemical called hemocyanin, which contains a copper atom, to carry the oxygen. Despite exceptions, the majority of blood from animals is red. But that doesn’t mean it’s exactly the same as what courses through our veins.