<P> The human body can perform best at sea level, where the atmospheric pressure is 101,325 Pa or 1013.25 millibars (or 1 atm, by definition). The concentration of oxygen (O) in sea - level air is 20.9%, so the partial pressure of O (pO) is 21.136 kPa . In healthy individuals, this saturates hemoglobin, the oxygen - binding red pigment in red blood cells . </P> <P> Atmospheric pressure decreases exponentially with altitude while the O fraction remains constant to about 100 km, so pO decreases exponentially with altitude as well . It is about half of its sea - level value at 5,000 m (16,000 ft), the altitude of the Everest Base Camp, and only a third at 8,848 m (29,029 ft), the summit of Mount Everest . When pO drops, the body responds with altitude acclimatization . </P> <P> Mountain medicine recognizes three altitude regions that reflect the lowered amount of oxygen in the atmosphere: </P> <Ul> <Li> High altitude = 1,500--3,500 metres (4,900--11,500 ft) </Li> <Li> Very high altitude = 3,500--5,500 metres (11,500--18,000 ft) </Li> <Li> Extreme altitude = above 5,500 metres (18,000 ft) </Li> </Ul>

How does high altitude affect oxygen dissociation curve