Why does hemoglobin bind oxygen in the lungs and release it in the tissues?

In the lungs, the partial pressure of oxygen is high. Since oxygen appears on the left side of the reaction, the equilibrium will shift towards the right, product side of the reaction, increasing the concentration of oxygen-bound hemoglobin. In the tissues, the partial pressure of oxygen is quite low. The equilibrium will shift towards reactants and the bound oxygens will be given up in the tissues.
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