“Deoxygenated blood is blue.”

Many people believe that deoxygenated blood, the blood returning to our hearts and lungs, is blue. This widespread misconception likely stems from the common sight of our veins, which often appear blue or greenish beneath the skin. Another contributing factor to this myth's endurance is the historical idiom "blue blood," a term used to describe aristocracy, whose pale skin often made their veins more visible. However, this belief is a classic example of how visual cues can mislead us.

The truth is, human blood is always red. The color of our blood is determined by hemoglobin, a protein in red blood cells that contains iron. When hemoglobin is carrying a full supply of oxygen, as it does in arteries, the blood appears a bright, vibrant red. As oxygen is delivered to the body's tissues, the hemoglobin changes its structure slightly, causing the blood to become a darker, duller red, but it never turns blue. If you've ever had blood drawn from a vein, you'll notice it's a dark red, not blue.

So, why do our veins appear blue if blood is always red? It's primarily an optical illusion, a trick of light and human perception. Our skin absorbs certain wavelengths of light and reflects others. Red light, with its longer wavelength, penetrates deeper into the skin and is absorbed by the hemoglobin in our blood. Blue light, having a shorter wavelength, does not penetrate as deeply and is reflected back to our eyes. This phenomenon, combined with the scattering properties of our skin and the depth of the veins, makes the deoxygenated, dark red blood appear blue or greenish from the surface.