The oxygen dissociation curve is a sigmoidal graph showing the percentage saturation of haemoglobin at various partial pressures of oxygen. The curve shows the equilibrium of oxyhaemoglobin with haemoglobin at various partial pressures. In the lungs, the partial pressure of oxygen is high. So, haemoglobin binds to oxygen and forms oxyhaemoglobin. On the other hand, in the tissues, the oxygen concentration is low. So, oxyhaemoglobin releases oxygen to form haemoglobin.

The sigmoid shape of the dissociation curve is due to the nature of the binding of oxygen to haemoglobin. As the first oxygen molecule binds to haemoglobin, it increases the affinity for the second molecule of oxygen to bind. Subsequently, haemoglobin attracts more oxygen. At one point, hemoglobin gets saturated with oxygen, and at this point the curve plateus off.