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The effects of high altitude on humans are considerable. The percentage oxygen saturation of haemoglobin determines the content of oxygen in blood. After the human body reaches around 2,100 m (7,000 feet) above sea level, the saturation of oxyhemoglobin begins to plummet. 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. Athletes use these adaptations to help their performance. There is a limit to the level of adaptation; mountaineers refer to the altitudes above as the "death zone", where no human body can acclimatize, although the subject is debated.〔 ==Effects as a function of altitude== 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 21.136 kPa. In healthy individuals, this saturates hemoglobin, the oxygen-binding red pigment in red blood cells. Atmospheric pressure decreases exponentially with altitude while the O2 fraction remains constant to about 100 km, so pO2 decreases exponentially 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.〔(【引用サイトリンク】title=Introduction to the Atmosphere )〕 When pO2 drops, the body responds with altitude acclimatization. Mountain medicine recognizes three altitude regions that 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). The higher the altitude, the greater the risk. Research also indicates elevated risk of permanent brain damage in people climbing to extreme altitudes.〔 〕 Expedition doctors commonly stock a supply of dexamethasone, or "dex," to treat these conditions on site. Humans have survived for two years at (millibars of atmospheric pressure ), which is the highest recorded permanently tolerable altitude; the highest permanent settlement known, La Rinconada, is at . At extreme altitudes, above (millibars of atmospheric pressure ), sleeping becomes very difficult, digesting food is near-impossible, and the risk of HAPE or HACE increases greatly.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Effects of high altitude on humans」の詳細全文を読む スポンサード リンク
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