“Blood is blue inside your body.”

Many people have looked at the prominent lines beneath their skin, especially on their wrists, and concluded that blood must be blue inside the body, only turning red when exposed to air. This common visual observation is the root of a widespread misconception about the true color of our vital fluid.

However, the truth is that blood is always red. The exact shade of red depends entirely on its oxygenation level. Blood that has just been oxygenated in the lungs and is being pumped to the rest of the body through arteries is rich in oxygen and appears a bright, vivid red. As this blood travels through the body, delivering oxygen to tissues and picking up carbon dioxide, it becomes deoxygenated. This oxygen-poor blood, found in veins, is a darker, deeper red, often described as maroon, but it never turns blue. The red color comes from hemoglobin, an iron-containing protein in red blood cells that binds with oxygen.

The reason veins often appear bluish or greenish through the skin is a fascinating trick of light and human perception, not an indication of blue blood. Our skin and the layers of tissue beneath it absorb certain wavelengths of light while reflecting others. Red light, for example, is absorbed more efficiently by the blood and surrounding tissues, and thus doesn't penetrate as deeply. Blue light, on the other hand, is scattered and reflected back to our eyes more effectively from the deeper veins. This optical illusion makes the veins seem blue, much like how deep water can appear blue even though water itself is colorless. So, while your veins might look blue, the blood flowing through them remains steadfastly red.