FactSentinel

Blood is never blue, whether it is inside the body or outside it. Human blood is always some shade of red, and its color depends on how much oxygen it carries. Blood leaving the lungs is bright, cherry red because its hemoglobin is loaded with oxygen. Blood returning to the heart through the veins has given up much of that oxygen and appears a darker, deeper red, sometimes described as maroon or burgundy. Its color does not change from blue to red when exposed to air. It simply becomes slightly brighter as it picks up oxygen.

The evidence is easy to observe. When blood is drawn into a sealed vacuum tube for a lab test, it is dark red even though it has never touched air. Medical staff, surgeons, and anyone who has had a cut or a blood draw have seen venous blood directly, and it is never blue. The iron-containing protein hemoglobin gives blood its color, and both its oxygenated and deoxygenated forms absorb and reflect light in the red part of the spectrum.

The myth likely persists because veins often look blue or green through the skin. This is an optical effect. Skin and tissue scatter and absorb different wavelengths of light, and red light penetrates deeper than blue, so the light that returns from a vein reaches the eye with a bluish appearance. Diagrams that color veins blue and arteries red in textbooks have also reinforced the misconception.