Looking down at your hand often reveals a network of blue lines beneath the skin, suggesting the blood inside veins is blue. This visual observation leads to the persistent misconception that the circulatory system contains two different colors of blood: bright red in arteries and blue in veins. The reality is that the blood coursing through both arteries and veins is always a shade of red, and the perceived blue color is a trick of light and human vision.
The Role of Oxygen in Blood Color
The color of blood is determined by a protein called hemoglobin, which is packed inside red blood cells. Hemoglobin contains four subunits, each with an iron-containing heme group that reversibly binds to oxygen. When oxygen attaches to the iron atom within the heme group, it changes the protein’s molecular structure and its light absorption properties.
Oxygenated hemoglobin, often called oxyhemoglobin, is responsible for the bright, vibrant color of blood carrying a full load of oxygen. This change in structure alters how light interacts with the molecule, causing it to strongly reflect red light. Conversely, when hemoglobin releases oxygen to the body’s tissues, it becomes deoxygenated hemoglobin, or deoxyhemoglobin. The deoxygenated form absorbs and reflects light differently, resulting in a darker, less brilliant color.
Actual Blood Colors in Arteries and Veins
Arterial blood, which has just passed through the lungs and is fully saturated with oxygen, is a striking, scarlet red. This oxygen-rich blood travels away from the heart to supply the body’s tissues with necessary oxygen and nutrients. In this state, over 95% of the hemoglobin is bound to oxygen.
Venous blood, returning to the heart after delivering its oxygen payload, is a much darker, maroon or reddish-purple color. Although labeled “deoxygenated,” venous blood still contains a significant amount of oxygen, with saturation levels generally ranging between 50% and 80%. No human blood ever turns blue; the color difference is purely a shift between two shades of red. If a person were to experience a cut on an artery or a vein, the blood that flows out would always appear red, confirming its true color. This distinction between bright and dark red is often mistakenly interpreted as the dramatic difference between red and blue.
The Optical Illusion of Blue Veins
The blue appearance of veins seen through the skin is an optical illusion created by how light interacts with human tissue and the blood vessels beneath. Veins are typically situated close to the skin’s surface, usually at a depth of 0.5 millimeters or more, which is crucial for the illusion to occur.
When white light hits the skin, it must penetrate the tissue layers to reach the vein and reflect back. The skin and underlying tissue preferentially absorb longer wavelengths, such as red light, while allowing shorter wavelengths, like blue light, to scatter more effectively. As a result, much of the red light is absorbed before it can reach the vein and reflect back.
The dark-red venous blood absorbs the light that manages to penetrate, including the remaining red light. However, the blue light, which has a shorter wavelength, is more likely to be scattered by the tissue above the vein. The light that eventually returns to the viewer’s eye is dominated by these scattered blue wavelengths. The perceived color is not the color of the blood itself, but rather the color of the light scattered back to the observer after filtering through the skin. This effect is compounded because the human eye perceives colors relatively, making the darker vein area appear bluer in contrast to the surrounding, red-reflecting skin.