Many people believe that oxygen-rich blood is red, while oxygen-poor blood is blue, which is why veins often appear bluish. But is this belief correct?
The answer is: No. Blood is always red.
Each hemoglobin molecule – a type of protein in red blood cells responsible for transporting oxygen – contains four iron atoms, which reflect red light. This is what gives our blood its red color.
This red color changes depending on the oxygen level in the blood. When hemoglobin picks up oxygen from the lungs, the blood appears bright red as it travels through the arteries and into the surrounding tissues of the body.
However, during its return to the lungs, after delivering oxygen to tissues throughout the body, “oxygen-poor blood” flows through the veins and appears dark red, according to Dr. Kleber Fertrin, an assistant professor of hematology at Washington University School of Medicine.
But why do veins appear blue when viewed from the skin’s surface?
According to Dr. Fertrin, this is similar to an illusion caused by the phenomenon of wavelength scattering. “The colors we see are based on the wavelengths that the retina can perceive, and different layers of skin scatter wavelengths in various ways,” explains Dr. Fertrin.
Due to this scattering, veins often appear greenish or bluish under darker skin, while those with lighter skin tones may see their veins appear purplish.
“This is because the wavelengths of green and blue light are shorter than those of red light,” says Dr. Fertrin. “Red light penetrates human tissue better than blue light. Therefore, while the red wavelengths are absorbed by our skin, green and blue light are reflected and scattered back to us.”
Veins are blue not because of the color of the blood.
It can also be observed that some other blood vessels, such as small capillaries located closer to the skin’s surface, are less affected by this “illusion.” “Fingertips are pink because the capillaries are much closer to the surface than the veins,” Dr. Fertrin explains.
A similar case occurs when we have a bruise from an impact. If the bruise is near the surface, it will appear red or purplish-red. In contrast, if the bruise is deeper, it will appear bluish-purple.
In simple terms, the color of blood does not change. It is merely how the human eye perceives it through the skin.
