“Blood in your veins is blue”

It is a common sight to observe the veins just beneath our skin as distinctly blue or sometimes even greenish, leading many to believe that the blood flowing within them must also be blue, only turning red when exposed to air. This widespread misconception, however, is incorrect. The truth is that human blood is always red, a consistent color determined by its fundamental composition, regardless of whether it is rich in oxygen or not.

The vibrant color of blood stems from hemoglobin, an iron-rich protein found in our red blood cells. When hemoglobin is saturated with oxygen, as it is in the arteries carrying blood from the lungs, it takes on a bright, scarlet red hue. This oxygenated blood is essential for nourishing our body's tissues. As the blood circulates and delivers oxygen to the various cells, the oxygen detaches from the hemoglobin. This deoxygenated blood, which then returns to the heart through the veins, is still red, but it is a darker, deeper shade of red, often described as maroon or a purplish-red, never blue.

The visual illusion of blue veins is a fascinating phenomenon explained by the way light interacts with our skin and blood. Our skin and the tissues beneath it absorb red light wavelengths more effectively than blue light wavelengths. When ambient light strikes the skin, the red light penetrates and is absorbed, particularly by the hemoglobin in the blood. The blue light, however, is scattered and reflected back towards our eyes. Because the deoxygenated blood in our veins is closer to the surface and its darker red color allows for more red light absorption, the blue light reflecting back makes the veins appear blue or greenish to our perception, despite the blood itself remaining a shade of red.