The partial oxygen pressure (pO2) should be high for the maximum loading of hemoglobin with the oxygen.
Hemoglobin is a quaternary protein that transports oxygen in the blood. Hemoglobin is made up of four heme subunits and an Iron atom. Blood oxygen is bound to the hemoglobin in red blood cells. One molecule of hemoglobin binds the four oxygen molecules and forms the oxyhemoglobin.
The partial pressure of the oxygen is an important factor that determines the binding affinity of oxygen to the hemoglobin. The higher the partial pressure of oxygen the more oxygen binds with the hemoglobin. The hemoglobin binds oxygen rapidly in the pulmonary capillaries of the lungs due to higher partial pressure of oxygen.
