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 acclimatization and will be shortly outlined here.
First response to Altitude Training
The most important acute response from altitude training is a reduction in plasma volume (and hence total blood volume also becomes reduced). The decrease in plasma volume is typically around 15% and will cause the hematocrit and [Hb] to increase and this largely normalizes the arterial oxygen content of blood within the first days of exposure. Plasma volume remains reduced throughout the altitude exposure period.
Benefits of the Hypoxic Exposure from Altitude
Altitude training is aimed at improving performance by the way of adaptation to reduced oxygen. With the acute hypoxic altitude exposure, the Hypoxic Inducible Factor-2a (HIF-2a) becomes stabilized (this discovery lead to the Nobel Prize in Physiology or Medicine in 2019) which initiates the release of erythropoietin (epo) from the kidney which subsequently binds to its receptor on bone leading to an enhanced synthesis of red blood cells. While the HIF-2a pathway is instantaneously activated with hypoxia, the production of red blood cells is a lengthy process and will take weeks to manifest.