Why Do Blood Vessels Look Blue Under the Skin?
- Blood vessels that look blue are the veins, which carry deoxygenated blood toward the heart and lungs, where they will become oxygenated. Blood can appear blue when seen through the skin, although it is not blue in the body. Blood in the arteries is bright red, whereas blood in the veins is dark red.
- Besides erroneously thinking that the blood in the veins is really blue, some people also think that the reason we don't ever see blue blood is because blood oxygenates on contact with air. This is not true. However, textbooks commonly use blue to illustrate veins and red to illustrate arteries, so this may be the source of the misconception.
- If veins do not appear blue because of blue blood, why do veins look blue? Alwin Kienle and other researchers tackled this problem from an optical perspective in 1995. They found that blood vessel thickness and depth beneath the skin are important factors. Their report also cites a common error in perception: relativism. Even though a shallow blood vessel reflects slightly more red light than blue light, because the human brain is designed to make comparisons, the difference between red-reflecting skin and the vein, which is less red, creates the illusion of blue. This is the same as putting something purple near something red; the purple item will appear bluish.