De-oxygenated Blood Turns Blue Myth

The common misconception that blood which lacks oxygen turns blue probably comes from the fact that veins appear blue and blood in the veins is typically heading back to the lungs, hence depleted of oxygen.

People who perpetuate this myth often claim that the reason we never see blood in its blue form is that the instant we get cut, the blood is exposed to oxygen and instantly turns red. However, when you get blood drawn from your veins that isn't exposed to air, it is dark red.

When blood is deprived of oxygen it actually just turns dark red. When it’s oxygenated, it turns a brighter red. The red color primarily comes from the hemoglobin, which contains four heme groups. These heme group’s interactions with various molecules end up giving it the dark red or light red color we see. The hemoglobin itself is a protein that binds with oxygen to be distributed throughout the body in blood.

Veins are very close to the surface of skin. This location under the skin is largely why veins appear blue despite the fact that the blood is dark red. This is from the way light diffuses in the skin. Veins appear blue from the way subcutaneous fat absorbs low-frequency light. This permits only high frequency blue and violet wavelengths to penetrate through the skin to the vein, with the other wavelengths getting filtered off from the pigmentation of your skin.

If a person has darker or lighter skin the veins tend to appear green or brown. People with extremely light skin, such as an albino, will typically have veins that show up as dark purple or dark red, more closely resembling the actual color of the blood running through the veins.