From the atmospheric level, the partial pressure of oxygen falls in 3 stages before the arterial blood is reached. Firstly the inspired air is humidified by the upper respiratory tract, the saturated vapor pressure of water (6.2kPa or 47mmHg) reducing the PO2 to around 19.7kPa (148mmHg). As air enters the alveoli it mixes with gas remaining in the alveoli at the end of expiration. Less than 10% of the total lung volume is replaced by each respiration. In the alveoli the partial pressure of oxygen is reduced to about 14.4kPa (108mmHg) and finally the small physiological shunt normally present reduces it to approximately 13.3kPa (100mmHg).