The role of haemoglobin is to transport oxygen.In human lungs haemoglobin binds or associates with oxygen readily when there is a high partial pressure of oxygen in the environment.The loading of haemoglobin with oxygen in the lungs is due to its high affinity for oxygen at high partial pressures of oxygen.The amount of oxygen bound is measured by the percentage saturation of haemoglobin.Haemoglobin dissociates or binds less readily with oxygen when there is a low partial pressure of oxygen in the environment.The unloading of haemoglobin in the respiring tissue is due to its low affinity for oxygen at low partial pressures of oxygen.At low partial pressures of oxygen, the percentage saturation with oxygen is low.