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Effects of high altitude on humans
Résumé
The effects of high altitude on humans are mostly the consequences of reduced partial pressure of oxygen in the atmosphere. The oxygen saturation of hemoglobin determines the content of oxygen in blood. After the human body reaches around above sea level, the saturation of oxyhemoglobin begins to decrease rapidly. However, the human body has both short-term and long-term adaptations to altitude that allow it to partially compensate for the lack of oxygen. There is a limit to the level of adaptation; mountaineers refer to the altitudes above as the death zone, where it is generally believed that no human body can acclimatize. At extreme altitudes, the ambient pressure can drop below the vapor pressure of water at body temperature, but at such altitudes even pure oxygen at ambient pressure cannot support human life, and a pressure suit is necessary. A rapid depressurisation to the low pressures of high altitudes can trigger altitude decompression sickness.
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 (O2) in sea-level air is 20.9%, so the partial pressure of O2 (pO2) is . In healthy individuals, this saturates hemoglobin, the oxygen-binding red pigment in red blood cells.
Atmospheric pressure decreases following the Barometric formula with altitude while the O2 fraction remains constant to about , so pO2 decreases with altitude as well. It is about half of its sea-level value at , the altitude of the Everest Base Camp, and only a third at , the summit of Mount Everest. When pO2 drops, the body responds with altitude acclimatization.
Mountain medicine recognizes three altitude regions which reflect the lowered amount of oxygen in the atmosphere:
High altitude =
Very high altitude =
Extreme altitude = above
Travel to each of these altitude regions can lead to medical problems, from the mild symptoms of acute mountain sickness to the potentially fatal high-altitude pulmonary edema (HAPE) and high-altitude cerebral edema (HACE).
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