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What makes blood turn red when it is exposed to air?

The protein hemoglobin, which carries oxygen in the blood, changes shape when it binds oxygen. When it changes shape, it absorbs different wavelengths of light, making it change color. When blood is exposed to air, much more of the hemoglobin absorbs oxygen than had in the vein the blood came from (in the veins, the hemoglobin has already given up most of its oxygen to the body). Therefore, the blood turns red.


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