The network of veins beneath the skin often appears to have a distinct blue or greenish tint. This leads many to believe that deoxygenated human blood must be blue, only turning red when exposed to air. This idea is a pervasive misconception, contradicted the moment one experiences a cut. The true color of human blood, and the reason for the bluish appearance of veins, involves biochemistry and the physics of light.
The Chemistry of Red Blood
Human blood is unequivocally red, a color determined by the respiratory pigment hemoglobin found within red blood cells. Hemoglobin is a complex protein containing four heme groups, each centered around an iron atom. This iron content is primarily responsible for binding oxygen and giving the blood its characteristic color, as the iron changes its light absorption properties depending on oxygen presence.
When hemoglobin is fully saturated with oxygen, such as in the arteries carrying blood from the lungs, it creates a bright, scarlet-red hue. Once oxygen is delivered to the body’s tissues, the hemoglobin changes structure slightly. This deoxygenated blood, found in the veins returning to the heart, is a darker, more maroon-red color, not blue. The difference is simply a shade of red.
Why Veins Appear Blue
The bluish appearance of veins is an optical illusion created by the way light interacts with your skin and the underlying blood vessels. Light must first pass through several layers of skin and tissue before it reaches the vein and is then reflected back to your eye.
Red light, which has a longer wavelength, penetrates deeper into the tissue than blue light, which has a shorter wavelength. The dark, deoxygenated blood in the vein absorbs most of the red light that manages to reach it. Consequently, less red light is scattered back to the surface.
Blue light does not penetrate as deeply and is more likely to be scattered back by the skin’s surface and surrounding tissues before reaching the deeper vein. When the eye receives the reflected light, the relative absence of red light combined with the presence of scattered blue light causes the vein to appear blue or greenish. This effect is most noticeable in veins approximately half a millimeter beneath the skin’s surface.
When Blood Is Actually Blue or Green
While human blood is always red, the animal kingdom offers true examples of blue and green blood, which rely on different respiratory pigments. Several species of mollusks (like octopuses and squids) and some arthropods (such as horseshoe crabs) possess blue blood. Their blood uses hemocyanin, a copper-containing protein, to transport oxygen instead of iron-based hemoglobin. Hemocyanin is colorless when deoxygenated but turns blue when oxygen binds to the copper atom.
Green blood is a reality for some creatures, including certain marine worms that use chlorocruorin, an iron-containing pigment producing a light green hue when deoxygenated. In very rare human cases, a condition called sulfhemoglobinemia can cause the blood to appear greenish. This occurs when a sulfur atom binds to the hemoglobin molecule, creating a non-functional pigment that results in a dark, greenish tint.