“Blood in your veins is blue.”

It's a common sight: the network (Review) of vessels visible beneath our skin, often appearing distinctly blue or greenish. This visual has led many to believe that the blood flowing through our veins is, in fact, blue, only turning red upon exposure to air. This widespread misconception likely stems directly from this visual observation, reinforced by diagrams in older textbooks or medical illustrations that sometimes depicted oxygenated blood as red and deoxygenated blood as blue to differentiate their functions.

However, the truth is that all human blood is red. The vibrant crimson color we associate with blood comes from hemoglobin, a protein in red blood cells that contains iron and binds to oxygen. When hemoglobin is carrying a full load of oxygen, as it does in the arteries, the blood appears bright scarlet. As oxygen is delivered to the body's tissues and picked up carbon dioxide, the hemoglobin changes slightly, and the blood in the veins becomes a darker, duller red, often described as maroon or purplish-red. It never, at any point, turns blue.

The bluish tint we perceive when looking at our veins is an optical illusion, not a reflection of the blood's actual color. Light must penetrate the skin to reach the blood vessels. Red light wavelengths are absorbed more readily by the blood and surrounding tissues, while blue light wavelengths are reflected more efficiently back to our eyes. Additionally, the depth of the veins beneath the skin and the way our eyes and brains interpret these light interactions contribute to this visual trick, making the darker red blood appear blue through the layers of skin.

This fascinating interplay of light, skin, and blood vessel depth explains why the myth persists. Despite the scientific reality, the visual evidence is so compelling that it's easy to assume the blood itself is blue. Understanding the science behind light absorption and reflection helps to bust this common myth, revealing that our blood's true color is always within the red spectrum, regardless of its oxygenation level.