In short, to acclimatize to the altitudes of Kilimanjaro means to develop certain physiological adaptations of your body to survive in the decompressed environment with abnormally low levels of oxygen. Such adaptation is the most important issue in mountaineering, and it is directly impacted by your fitness, good equipment, adequate supplies and professional crew. Essentially, all these arrangements are made to simplify the process of adaptation to the abnormal altitudes. We will discuss it further in greater detail.  

Before we start explaining how the process of altitude adaptation (acclimatization) works, you are probably interested in what is really abnormal at the slopes of Kilimanjaro, and what impacts do this abnormality exercise on the climbers.

As we know from elementary physics, the higher we go, the lower barometric pressure becomes. For example, at sea level it is approximately 760 mmHg, while at Uhuru Peak (5,895 m) a barometer will show something between 350-352 mmHg, meaning that the density of air shall be reduced twofold. As a result, there will be substantially less pressure that keeps the molecules of oxygen together. To put it simply, the distance between them increases, and therefore, a fewer number of oxygen molecules is inhaled with each breath, what leads to well-known discomfort symptoms.