Abstract: Altitude sickness (mountain sickness) is a syndrome caused by decreased oxygen at high altitudes. It is known that when we are at an altitude of more than 1 500 m, the process of breathing and gas exchange (oxygen and carbon dioxide) inside the lungs does not take place in the same way as at sea level. The fraction of atmospheric oxygen remains constant at all altitudes (21%), but as the altitude increases, the partial pressure of oxygen (PiO2) decreases with atmospheric pressure.