Everyone knows that blood is red, but when we see the veins under our skin, they are blue instead of red. What is the cause of this phenomenon?
Human blood is always red because of its iron content, the iron is dissolved with oxygen in the lungs, making the blood be in dark red or magenta depending on the amount of oxygen in the blood. The vein itself is not blue, this blue color appears when we look through the skin and by the four following factors:
The first factor is the interaction of light with the skin:
Sunlight composes of many colors and it is equivalent with many wavelengths, it will have different colors.
Human skin always absorbs light and reflects back to the environment with thousands of times in an eye blink. Scientists have found that veins reflect more blue light than red, so we see that veins are so blue.
The second factor is the amount of oxygen in the blood:
A red blood cell may carry up to 4 molecules of oxygen. If the blood transport is affected by an external environment such as high temperature or acid, one or more oxygen molecules will separate from the red blood cells. That’s reason the veins look easier to be blue.
The third factor is the diameter and location of the vein:
If the vein is just below the skin, it will be red like blood; but as it goes deeper, it will gradually turn into blue. In fact, the vast majority of veins are more than half a millimeter deep under the skin. This optical phenomenon is related to complex blood transport equations.
The scientists observe the blue color in the arteries, but it is very little because blood in the arteries is bright red. When light passes through the skin, the difference in color between the veins and the arteries is very clear, so the veins are greener. In addition, the arteries are usually small and deep under the skin, so we often do not see the artery.
The last factor is the brain:
The color contrast of the skin is a cause for the brain to be “fooled” and tend to see the veins are blue.
Information collected from the retina to the brain is processed many times. For example, when we put purple next to red, the brain will not completely reflect the purple color, but rather the purple color with blue. In the case of veins, the contrast of the surrounding skin also tends to make veins to be blue.