“Blood in your veins is blue.”

It's a common misconception that the blood flowing through our veins is blue until it picks up oxygen. This idea, often seen in simplified diagrams where deoxygenated vessels are colored blue, is actually a myth. The truth is, human blood is always red, though its exact shade can vary.

Oxygenated blood, which is rich in oxygen and typically found in arteries, appears a vibrant, bright red. As blood circulates through the body and delivers oxygen to tissues, it becomes deoxygenated. This deoxygenated blood, carried by veins back to the heart and lungs, is a darker, duller red, sometimes described as a deep maroon or reddish-purple. It never turns blue.

So, if blood is always red, why do our veins often appear blue or greenish through the skin? This is an optical illusion. When light hits the skin, different wavelengths are absorbed and reflected. Red light, with its longer wavelength, penetrates deeper into the skin and is absorbed by the hemoglobin in the blood. Blue light, however, has a shorter wavelength and doesn't penetrate as deeply; instead, it is scattered and reflected back to our eyes. This scattering of blue light, combined with the way our brains perceive color against the warmer tones of our skin, makes the underlying veins appear blue.