“The blood in your veins is blue”

It's a common visual trick of the human body: the blood inside your veins appears blue through your skin. This observation has led to a widespread misconception that deoxygenated blood itself is blue. However, human blood is always red, regardless of its oxygen content. The origin of this myth lies in how our eyes perceive light interacting with our skin and the blood vessels beneath.

Scientifically, blood gets its red color from hemoglobin, an iron-rich protein in red blood cells. When hemoglobin carries oxygen, as it does in arteries, the blood is a bright, cherry red. When it has delivered oxygen to the body's tissues and is returning to the lungs through veins, the blood is a darker, duller red, often described as maroon or reddish-purple, but never blue.

The illusion of blue veins is due to the way light penetrates and is absorbed by your skin. White light, which contains all colors, hits your arm. Red light, with its longer wavelength, travels deeper into the skin and is absorbed by the hemoglobin in your blood. Blue light, having a shorter wavelength, doesn't penetrate as deeply and is mostly reflected back to your eyes. This means that the light returning from the area around your veins contains more blue wavelengths than red, making the veins appear blue against the surrounding skin. Factors like skin tone, thickness, and vein depth can influence how prominent this blue appearance is.