The greatest challenge that humans face when exposed to high altitude is the reduction in the partial pressure of O2, which ultimately also leads to a reduction in blood oxygen content (called hypoxia). To compensate for this lack the human organism induces several defense mechanisms. While these are all important, it seems safe to state that those occurring within the blood are key to successful acclimatisation.
For many years, athletes have experimented with altitude exposure as a means to boost their athletic performance. The overall idea being if red blood cell volume becomes increased through altitude acclimatization so will exercise performance. Today there are numerous altitude training strategies such as live high – train high, where athletes live high (at altitude) and train high (at altitude) or live high – train low, where athletes reside at altitude but conduct their training at or close to sea level.
Read more about altitude training in the link.