Normally, it is supposed that oxygen concentration in air decreases with altitude. However, air is composed of 78% nitrogen, 21% oxygen and 1% carbon dioxide in constant measures at any altitude. As altitude increases, the only thing that decreases is atmospheric pressure, and consequently, the oxygen pressure diminishes (partial oxygen pressure pO2), which act so as to make the vital gas enter the lungs. At 5000 m the oxygen pressure has already reduced to half (see diagram).
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