Atmospheric pressure decreases exponentially with altitude while the O2 fraction remains constant to about , so PO2 decreases exponentially with altitude as well. It is about half of its sea level value at , the altitude of the Mount Everest base camp, and only a third at , the summit of Mount Everest. When PO2 drops, the body responds with altitude acclimatization. Additional red blood cells are manufactured; the heart beats faster; non-essential body functions are suppressed, food digestion efficiency declines (as the body suppresses the digestive system in favor of increasing its cardiopulmonary reserves); and one breathes more deeply and more frequently. But acclimatization requires days or even weeks. Failure to acclimatize may result in altitude sickness, including high altitude pulmonary edema (HAPE) or cerebral edema (HACE).