“Blood in your veins is blue”

It's a common observation that the veins visible beneath our skin often have a distinctly bluish tint, leading many to assume that the blood flowing through them must also be blue. This visual phenomenon is a widespread source of misunderstanding about our circulatory system, perpetuated simply by what we see at a glance.

The truth is, human blood is never blue. Its characteristic red color comes from hemoglobin, an iron-rich protein in red blood cells responsible for transporting oxygen. When hemoglobin is carrying a full load of oxygen, such as in the arteries, the blood appears a bright, vibrant scarlet. Once the oxygen has been delivered to the body's tissues, the deoxygenated blood returning to the heart through the veins is still red, but a much darker, duller shade, sometimes described as maroon or purplish-red.

The reason our veins appear blue, despite containing red blood, is a fascinating trick of light and human biology. Our skin and tissues absorb certain wavelengths of light while reflecting others. Red light, which has a longer wavelength, penetrates deeper into the skin and is largely absorbed by the blood in the veins. Blue light, with its shorter wavelength, does not penetrate as deeply and is more readily reflected back to our eyes by the surrounding tissue before it can be absorbed. This selective absorption and reflection, combined with the way our eyes perceive color through layers of skin, creates the illusion of blue veins.