<P> The human body can adapt to high altitude through both immediate and long - term acclimatization . At high altitude, in the short term, the lack of oxygen is sensed by the carotid bodies, which causes an increase in the breathing depth and rate (hyperpnea). However, hyperpnea also causes the adverse effect of respiratory alkalosis, inhibiting the respiratory center from enhancing the respiratory rate as much as would be required . Inability to increase the breathing rate can be caused by inadequate carotid body response or pulmonary or renal disease . </P> <P> In addition, at high altitude, the heart beats faster; the stroke volume is slightly decreased; and non-essential bodily functions are suppressed, resulting in a decline in food digestion efficiency (as the body suppresses the digestive system in favor of increasing its cardiopulmonary reserves). </P> <P> Full acclimatization, however, requires days or even weeks . Gradually, the body compensates for the respiratory alkalosis by renal excretion of bicarbonate, allowing adequate respiration to provide oxygen without risking alkalosis . It takes about four days at any given altitude and can be enhanced by drugs such as acetazolamide . Eventually, the body undergoes physiological changes such as lower lactate production (because reduced glucose breakdown decreases the amount of lactate formed), decreased plasma volume, increased hematocrit (polycythemia), increased RBC mass, a higher concentration of capillaries in skeletal muscle tissue, increased myoglobin, increased mitochondria, increased aerobic enzyme concentration, increase in 2, 3 - BPG, hypoxic pulmonary vasoconstriction, and right ventricular hypertrophy . Pulmonary artery pressure increases in an effort to oxygenate more blood . </P> <P> Full hematological adaptation to high altitude is achieved when the increase of red blood cells reaches a plateau and stops . The length of full hematological adaptation can be approximated by multiplying the altitude in kilometres by 11.4 days . For example, to adapt to 4,000 metres (13,000 ft) of altitude would require 45.6 days . The upper altitude limit of this linear relationship has not been fully established . </P>

How does changes in altitude affect breathing system