Which process reduces the oxygen content in the atmosphere?

Why is there less oxygen in the air in the mountains?

The maybe 100 kilometers thick "lower" atmosphere of our earth, the homosphere, mainly contains a mixture of water vapor and so-called "dry air", which is made up of 78 percent nitrogen, about 0.03 percent carbon dioxide and traces of noble gases - and around 21 percent oxygen, an unusually high proportion compared to other planets. It is likely that it gradually increased over a long period of time as a by-product of photosynthesis. The ratio of the "dry" gases to one another remains relatively constant in our atmosphere and does not differ significantly in layers close to the ground or very high layers.

However, as the distance from sea level increases, the air becomes thinner and thinner - and thus all molecules become rarer, including oxygen. The reason for the decreasing density towards the top is the air pressure gradient that arises in the atmosphere because the upper air layer masses load on the lower. Now gases are compressible, and so not only does the pressure increase, but also the molecular density down to sea level (where an average of 1013.25 hectopascals prevail). Conversely, the higher you go, the less and less air mass presses from above. Therefore, the air density is reduced by half about every 5.5 kilometers upwards. Or, in other numbers: one cubic centimeter of air on the earth's surface contains around 1019 Molecules, at an altitude of 250 kilometers there are around a million and at 800 kilometers only a single one.

There, in the exosphere, gas molecules can also completely escape the earth: gravity pulls them towards the earth's surface, but this effect decreases quadratically with increasing distance from the earth. At the same time, temperature-dependent diffusion processes constantly distribute the gas molecules in the atmosphere in all directions - and molecules accelerated outwards by high thermal energy beyond the escape speed can accidentally escape the gravitational influence of the earth into space.

Humans hundreds of kilometers further down the lack of air becomes a problem - or more precisely, the lack of oxygen, the falling partial pressure of which can cause us problems from an altitude of around 2000 to 2500 meters. The oxygen partial pressure determines how much oxygen our blood can absorb with each breath: At higher pressure, a larger amount of gas can be dissolved in liquid. Does our body feel a lack of O2, so he increases the breathing rate: We gasp for air. The lack of oxygen can cause headaches, drowsiness and nausea - and in the worst case, it can lead to fatal edema in the brain or lungs.

Normally, however, our body is able to adapt to life at higher altitudes within a few days: We produce more red blood cells for the transport of oxygen after the hormone erythropoietin (EPO), which is increased in the kidneys, has given the order to do so. EPO is also known as a doping agent - and the reason why many top athletes do altitude training before competitions. People whose ancestors lived in the mountains for hundreds of generations, such as the Tibetans, on the other hand, have gene variants that cause even better altitude adaptation: They ensure, for example, more dilated vessels, which reduces the risk of clumping of blood cells and enables faster blood flow. In this way, the cells are supplied with sufficient oxygen even if the oxygen content in the blood is low.