“Blood is blue inside your body”

It's a common sight to glance at your wrist and notice the bluish lines beneath your skin, leading many to believe that blood circulating within our bodies is actually blue. However, this is a widespread misconception. Human blood is always red, regardless of its oxygen content. Blood rich in oxygen, typically found in arteries, is a bright, vibrant red. As oxygen is delivered to the body's tissues, the blood returning through veins becomes deoxygenated, appearing a darker, duller red, but never blue.

The blue appearance of veins is an optical illusion, a trick of light and human perception. When white light, which contains all colors, hits our skin, different wavelengths are absorbed and reflected differently. Red light, with its longer wavelength, penetrates deeper into the skin and is absorbed by the hemoglobin in our blood. Blue light, having a shorter wavelength, doesn't penetrate as deeply and is reflected back to our eyes more readily. This means that the light returning from the relatively shallow veins contains more blue wavelengths, making the veins appear blue against the surrounding skin. The depth of the veins and the thickness and tone of a person's skin can also influence how pronounced this blue appearance is.

This enduring myth is likely fueled by the visual evidence of blue-looking veins and has been reinforced in some educational diagrams that color veins blue to distinguish them from red arteries. Historically, the term "blue blood" even emerged in some cultures to describe nobility with pale skin, through which their veins were more visible. However, despite these visual cues and linguistic associations, the iron-rich hemoglobin in our red blood cells consistently ensures that blood remains within the red spectrum, from a bright cherry red to a deep maroon, throughout its journey in the body.