Why Is Blood Red?
Blood gets its red color from a substance called “hemoglobin.” Hemoglobin has a molecule called a “heme” which has the metal iron in it. When the iron is oxygenated, it becomes red. When the iron is deoxygenated, it becomes darker red.
Iron has the property of reflecting red light and because there is so much iron in our blood, blood looks red. One of the most important functions of your blood is to carry oxygen throughout your body.
“Red cells” in the blood accomplish this task. They contain the pigment hemoglobin, which binds with oxygen and delivers it to the body’s tissues. When the hemoglobin is carrying oxygen, it makes the blood a bright red.
As the oxygen is distributed and your blood circulates back toward the heart, it turns dark and bluish. If you cut yourself, the exposed blood turns red again as it is exposed to the oxygen in the air.
Therefore, contrary to popular belief, blood is never blue. Veins under light colored skin only look blue because the skin changes the optical properties of the light that passes through the skin.
In other words, skin changes the way light is reflected from the blood and we perceive that as being a different color.