<P> Studies have shown that the approximately 140 million people who live at elevations above 2,500 metres (8,200 ft) have adapted to the lower oxygen levels . These adaptations are especially pronounced in people living in the Andes and the Himalayas . Compared with acclimatized newcomers, native Andean and Himalayan populations have better oxygenation at birth, enlarged lung volumes throughout life, and a higher capacity for exercise . Tibetans demonstrate a sustained increase in cerebral blood flow, lower hemoglobin concentration, and less susceptibility to chronic mountain sickness (CMS). These adaptations may reflect the longer history of high altitude habitation in these regions . </P> <P> A significantly lower mortality rate from cardiovascular disease is observed for residents at higher altitudes . Similarly, a dose response relationship exists between increasing elevation and decreasing obesity prevalence in the United States . This is not explained by migration alone . On the other hand, people living at higher elevations also have a higher rate of suicide in the United States . The correlation between elevation and suicide risk was present even when the researchers control for known suicide risk factors, including age, gender, race, and income . Research has also indicated that oxygen levels are unlikely to be a factor, considering that there is no indication of increased mood disturbances at high altitude in those with sleep apnea or in heavy smokers at high altitude . The cause for the increased suicide risk is as yet unknown . </P> <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>

How are red blood cells affected at altitude