This gradual adjustment to conditions is acclimatization.
When available, the incorporation of indigenous wild stock into domesticated herds doubtless aided animals' acclimatization, a practice that continued into historic times.
Respiratory acclimatization in humans is achieved through mechanisms that heighten the partial pressure of oxygen at all stages, from the alveolar spaces in the lung to the mitochondria in the cells, where oxygen is needed for the ultimate biochemical expression of respiration.
After living many years at high altitude, some highlanders lose this acclimatization and develop chronic mountain sickness, sometimes called Monge's disease, after the Peruvian physician who first described it.
The symptoms usually occur within six hours to four days after arrival at high altitude and disappear within two to five days as acclimatization occurs.
Humans and some mammalian species like cattle adjust to the fall in oxygen pressure through the reversible and non-inheritable process of acclimatization, which, whether undertaken deliberately or not, commences from the time of exposure to high altitudes.
In Tibet some infants of Han origin never achieve satisfactory acclimatization on ascent to high altitude.
At heights above 1,500 to 3,000 metres (5,000 to 10,000 feet), the pressure is low enough to produce mountain sickness and severe physiological problems unless careful acclimatization is undertaken.
Another surprising characteristic of acclimatization is its anticipatory nature it can develop before the change occurs.
Adaptations to strange or artificial conditions, however, are often difficult to describe, and only in a few cases can such adaptations be compared with acclimatization.
For example, in acclimatization to the low pressure of oxygen (hypoxia) in high mountains, animals, including man, improve the capacity of blood to transport oxygen by increasing the number of red blood cells (polycythemia); in the chronic disease emphysema, the inadequate supply of oxygen to the lungs is to some degree compensated for by a similar polycythemia.
