“Blood in veins is blue.”

The idea that blood running through our veins is blue is a widespread misconception, often fueled by simplified anatomical diagrams that use blue to represent deoxygenated blood and red for oxygenated blood. Historically, the term "blue blood" also emerged among European nobility with very pale skin, whose visible veins appeared bluish, leading to the belief that their blood was distinct. However, the true color of human blood is never blue.

All human blood is red, varying in shade depending on its oxygen content. Oxygenated blood, found in arteries, is a bright, vibrant red. Deoxygenated blood, which circulates in veins, is a darker, deeper red, sometimes described as reddish-purple or crimson. This color comes from hemoglobin, an iron-rich protein in red blood cells that binds with oxygen. If you've ever had blood drawn or witnessed a cut, you've seen firsthand that blood, regardless of its oxygen level, is always red.

The bluish appearance of veins through the skin is an optical illusion, a fascinating trick of light and human perception. White light, which contains all colors, penetrates the skin. Red light, with its longer wavelength, travels deeper into the tissue and is absorbed by the blood's hemoglobin. Blue light, having a shorter wavelength, does not penetrate as deeply and tends to scatter and reflect closer to the skin's surface. When less red light is reflected back from the area directly above a vein (because the deoxygenated blood absorbs more of it), and the blue light from the surrounding superficial skin layers is more readily scattered and reflected, our brains interpret this contrast as a bluish hue. The depth of the vein and individual skin tone can also influence how pronounced this optical effect appears.