Why are veins blue?

Category: Biology
Published: December 18, 2012

By: Christopher S. Baird, author of The Top 50 Science Questions with Surprising Answers and physics professor at West Texas A&M University

The veins themselves are not blue, but are mostly colorless. It is the blood in the veins that gives them color. Furthermore, the blood in human veins is also not blue. Blood is always red. Blood that has been oxygenated (mostly flowing through the arteries) is bright red and blood that has lost its oxygen (mostly flowing through the veins) is dark red. Anyone who has donated blood or had their blood drawn by a nurse can attest that deoxygenated blood is dark red and not blue. The blood in your veins appears blue because you are looking at your veins through layers of skin and fat according to Alwin Kienle in his paper "Why do veins appear blue? A new look at an old question" published in the Journal of Applied Optics. Skin scatters a lot of the red portion of white light before it can reflect off the blood, leaving the blue light to reflect off the blood and back to our eyes. It is a similar effect to how the white sun appears red at sunset due to the blue colors being scattered away by the atmosphere.

Venous blood is dark red and not blue. Public Domain Image, source: FDA.

Topics: blood, blood color, blue, blue blood, blue veins, color, light, scattering, veins