“Deoxygenated blood is blue”

The idea that human blood is blue until it hits the air is a widespread misconception, but the truth is, human blood is always red. This common belief likely stems from the visual appearance of veins under the skin, which often look distinctly blue or greenish. However, this observation is a trick of light and human perception, not an accurate representation of the blood's true color.

Scientifically, the color of blood is determined by hemoglobin, a protein in red blood cells that contains iron and is responsible for carrying oxygen. When hemoglobin is saturated with oxygen, such as in arterial blood, it takes on a bright, vivid red hue. As the oxygen is delivered to the body's tissues, the hemoglobin releases it, causing a slight change in its molecular structure. This deoxygenated blood, found in veins, becomes a darker, duller shade of red, sometimes described as maroon, but never blue.

The reason veins appear blue through the skin is purely an optical illusion. Light must penetrate the skin to reach the veins and then reflect back to our eyes. Red light, with its longer wavelength, is absorbed more readily by the blood and surrounding tissues. Blue light, with its shorter wavelength, does not penetrate as deeply and is scattered more effectively by the skin and blood vessels. This scattering effect, combined with the way our eyes perceive color at different depths, makes the deoxygenated blood in our veins appear blue or sometimes even green to us. When blood is drawn from a vein, it is immediately apparent that it is a deep red color.