Altitude sickness is caused by a oxygen deficiency In the body. The percentage of oxygen in the air surrounding us is 21%; that percentage is virtually independent of the height at which we are. Air also has a weight: the air pressure, usually indicated in millimeters of mercury barometric pressure (mm Hg) or in Pascal (Nm-2), where 1 mm Hg = 133.3226 Pascal. At sea level, the air is compressed by the air column above it, causing it comfortable 'thick' is.
weight of the air column
However, as the altitude increases, the air pressure drops due to thedecrease in weight from the air column above: the air becomes 'thinner' and contains fewer molecules of oxygen. The airtightness is greatest around the equator, so the farther away the mountain to climb is from the equator, the lower the air pressure is and the less oxygen is available at the same height. The climate and the seasons also influence the air pressure, but the influence of the altitude dominates.
On average, barometric pressure is halved every 5500 meters: from sea level to 5500 meters it drops from 760 to around 380 mm Hg. Each liter of air then contains much less of all types of normal molecules: oxygen, nitrogen oxide, hydrogen, carbon dioxide, and a few minor molecules. Because fewer oxygen molecules are available per liter of air, the amount of oxygen that can be absorbed by the body per breath decreases.
Because less oxygen is absorbed, the amount of oxygen in (the arterial oxygen pressure: PaO2) decreases over that height difference (0-5500m) from 94 to about 40 mm Hg. The body 'cleverly' deals with the lack of oxygen, among other things by ensuring that relatively more oxygen is bound to the hemoglobin in the red blood cells than under normal circumstances. As a result, this works out less badly than you would expect. The percentage of red cells bound oxygen (% SaO2) in the arteries nevertheless falls between sea level and 5500 meters from 97 to about 75% on average.
Not everyone succeeds equally well and this causes altitude sickness
However, the body continues to have the same need for oxygen to be able to function and perform normally. To maintain the oxygen supply, the body adapts itself to the changed conditions. This does not work equally well for everyone and this is why altitude sickness occurs.
Author: Han Willems - More info about altitude sickness