Many people believe blood in veins is blue until exposed to air, a common misconception fueled by the blue-green lines visible beneath the skin. This “blue blood” idea persists, but scientific understanding reveals the true color of blood and the optical principles behind the bluish appearance of veins.
Why Blood is Red
Human blood is always red, though its shade can vary depending on its oxygen content. This color is primarily due to hemoglobin, an iron-rich protein found within red blood cells. Hemoglobin is responsible for transporting oxygen from the lungs to the body’s tissues and carbon dioxide back to the lungs.
When hemoglobin binds with oxygen, it forms oxyhemoglobin, which gives arterial blood a bright, cherry-red color. This is the blood that has just left the lungs and is circulating throughout the body. As oxygen is delivered to the tissues, hemoglobin releases it and becomes deoxygenated hemoglobin. This deoxygenated blood, which flows through the veins, appears a darker, purplish-red or burgundy, but it is never blue. You can observe this darker red color when blood is drawn.
The Illusion of Blue Veins
The appearance of veins as blue or sometimes green is an optical illusion, not an indication of blood color. This phenomenon occurs due to the way light interacts with your skin, tissues, and the blood within your veins. Skin and tissues absorb and scatter different wavelengths of light selectively.
Red light, which has longer wavelengths, penetrates human tissue more deeply than blue or green light. As light hits the skin, much of the red light is absorbed by hemoglobin in the blood, even deoxygenated blood in veins. In contrast, blue and green light, with shorter wavelengths, do not penetrate as far and are scattered and reflected more efficiently by skin and surrounding tissues. This preferential scattering and reflection of blue light back to our eyes makes veins appear blue or greenish. The depth of the vein beneath the skin also influences its perceived color, as deeper veins may appear bluer.
Settling the Blue Blood Myth
The persistent notion that blood is blue inside the body is a myth. The bluish appearance of veins is solely a trick of light and visual perception. This optical phenomenon is similar to how the sky appears blue because blue light is scattered more by the atmosphere than other colors. Ultimately, while veins may appear blue, the blood flowing through them is always a shade of red.