The oxygen dissociation curve is a graph showing the percentage saturation of oxyhemoglobin at various partial pressures of oxygen. The curve shows the equilibrium of oxyhemoglobin and hemoglobin at various partial pressures.

In the lungs, the partial pressure of oxygen is high. Hence, hemoglobin binds to oxygen
and forms oxyhemoglobin. Tissues have a low oxygen concentration. Therefore, at the tissues, oxyhemoglobin releases oxygen to form hemoglobin.

The sigmoid shape of the dissociation curve is because of the binding of oxygen to hemoglobin. As the first oxygen molecule binds to hemoglobin, it increases the affinity for the second molecule of oxygen to bind. Subsequently, hemoglobin attracts more oxygen.