“Blood is blue in your veins.”

It is a common sight to look at the veins beneath your skin and observe a distinct bluish tint, leading many to believe that blood inside the body must be blue. This widely held belief, however, is a fascinating misconception. The truth is, human blood is always red, and its shade varies depending on the amount of oxygen it carries.

The vibrant color of blood comes from hemoglobin, an iron-rich protein found in red blood cells. When hemoglobin is saturated with oxygen, such as in the arteries carrying blood from the heart to the body, it appears a bright, cherry red. As blood travels through the body and delivers oxygen to tissues, the hemoglobin releases oxygen, becoming deoxygenated. This deoxygenated blood, which flows through your veins back to the heart, turns a darker, maroon-red, but it never becomes blue. Anyone who has had blood drawn can attest that venous blood, even when deoxygenated, is distinctly red.

So, why do our veins appear blue through the skin? This is an optical illusion caused by how light interacts with your skin and blood vessels. White light, which contains all colors, hits the skin. Red light, with its longer wavelength, penetrates deeper into the skin and is absorbed by the red hemoglobin in your blood. Blue light, however, has a shorter wavelength and does not penetrate as deeply. Instead, it is scattered and reflected back to your eyes more readily. This phenomenon, combined with the depth of the veins and the thickness and tone of your skin, makes the veins appear blue or greenish to our perception. Simplified diagrams in textbooks that color deoxygenated veins blue may also inadvertently reinforce the myth.