During acclimatization over a few days to weeks, the body produces more red blood cells to counteract the lower oxygen saturation in blood in high altitudes. Full adaptation to high altitude is achieved when the increase of red blood cells reaches a plateau and stops.Heart rate is controlled by the two branches of the autonomic (involuntary) nervous system. The sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). ... At the beginning of exercise, your body removes the parasympathetic stimulation, which enables the heart rate to gradually increase.
