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Why does blood turn red when it is exposed to oxygen?

Oxygen in the blood of vertebrates is carried by a special protein called hemoglobin. This allows the blood to carry about 50 times more oxygen than if the oxygen were simply dissolved in the fluid of the blood. When hemoglobin binds to oxygen (each hemoglobin molecule binds four oxygen molecules), it changes shape. In its new shape, it absorbs light differently, and for this reason appears red.


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