Veins visible beneath your skin often appear blue, despite blood itself being red. The perceived blue color of veins is not due to the blood within them, but rather an optical phenomenon involving light and human tissue.
The Actual Color of Your Blood
The blood flowing inside your veins is always red. The color of blood is determined by hemoglobin, a protein found in red blood cells that contains iron.
When hemoglobin binds with oxygen, such as in the arteries carrying blood from the lungs, it forms oxyhemoglobin and appears a bright, red. Conversely, when hemoglobin releases oxygen to the body’s tissues, it becomes deoxygenated hemoglobin, or deoxyhemoglobin. This deoxygenated blood, which is primarily found in veins returning to the heart, is a darker, maroon-red, not blue. Anyone who has had blood drawn can confirm that venous blood, even though deoxygenated, is a deep red color when it leaves the body.
How Light Makes Veins Appear Blue
The blue appearance of veins is an optical illusion. White light penetrates the skin. Different wavelengths of light penetrate human tissue to varying depths.
Red light, with its longer wavelength, travels deeper into the skin and is absorbed by the hemoglobin in the blood. Blue light, with a shorter wavelength, does not penetrate as deeply into the skin. Instead, it is scattered and reflected by the superficial layers of the skin.
When white light hits the skin above a vein, much of the red light is absorbed by the blood within the vein, while the blue light is scattered back to our eyes. This combination creates the perception of a blue or greenish tint. The depth of the veins beneath the skin and the thickness of the skin also influence this effect, as deeper veins and thicker skin can enhance the scattering of blue light.
Oxygen and Blood’s Changing Hues
The internal color of blood changes depending on its oxygen saturation, but it always remains within the red spectrum. When hemoglobin picks up oxygen in the lungs, it undergoes a slight structural change that makes the blood bright cherry red. As this oxygenated blood circulates through the arteries and capillaries, it delivers oxygen to various cells and tissues throughout the body.
Once oxygen is released, the hemoglobin molecule changes again, resulting in the darker, maroon-red color characteristic of deoxygenated blood. This darker shade of red is what flows through your veins as it returns to the heart and lungs to be re-oxygenated.
While this internal color shift from bright red to dark red is a biological reality, it is distinct from the external blue appearance of veins. The blue color seen through the skin is purely an optical effect, not an indication that the blood itself has turned blue.