“Blood is blue in your veins until it hits the air.”

It’s a common visual trick of the human body: the sight of bluish lines beneath our skin, leading many to assume that the blood flowing through our veins is actually blue until exposed to air. This widespread misconception likely stems directly from this visual observation, combined with the often-depicted blue color of veins in anatomical diagrams or medical illustrations, which are used to differentiate deoxygenated from oxygenated blood.

The truth is, your blood is never blue. It's always red, a color owed to the iron-rich protein hemoglobin within red blood cells. When hemoglobin is saturated with oxygen, such as in arterial blood leaving the lungs, it takes on a bright, vibrant red hue. As oxygen is delivered to the body's tissues and picked up by cells, the hemoglobin loses its oxygen, becoming deoxygenated. This deoxygenated blood, found in your veins, is a darker, more maroon-red, not blue.

So why do veins look blue through the skin? It's an optical illusion caused by a combination of factors, including the way light penetrates and reflects off your skin, the depth of the veins, and the way your eyes perceive color. Blue and violet light wavelengths are scattered more by the skin and reflected back to our eyes, while red light penetrates deeper and is absorbed by the blood. This interaction of light with our tissues makes the dark red venous blood appear bluish beneath the surface, much like a deep body of water can appear blue even though the water itself is clear.