During inhalation, air is warmed and saturated with water vapor as it passes through the nose and pharynx before it enters the alveoli. The saturated vapor pressure of water is dependent only on temperature; at a body core temperature of 37 °C it is 6.3 kPa (47.0 mmHg), regardless of any other influences, including altitude. Consequently, at sea level, the tracheal air (immediately before the inhaled air enters the alveoli) consists of: water vapor (PH2O = 6.3 kPa), nitrogen (PN2 = 74.0 kPa), oxygen (PO2 = 19.7 kPa) and trace amounts of carbon dioxide and other gases, a total of 100 kPa. In dry air, the PO2 at sea level is 21.0 kPa, compared to a PO2 of 19.7 kPa in the tracheal air (21% of [100 - 6.3] = 19.7 kPa). At the summit of Mount Everest tracheal air has a total pressure of 33.7 kPa, of which 6.3 kPa is water vapor, reducing the PO2 in the tracheal air to 5.8 kPa (21% of [33.7 - 6.3] = 5.8 kPa), beyond what is accounted for by a reduction of atmospheric pressure alone (7.1 kPa).