The hardy people of the Andes live an isolated existence. As with the Himalayan mountains, the Andes impose severe conditions and many limitations on life. Consequently, Andean people are physically adapted to the heights in special ways. In contrast to persons born and raised at sea level, those living at Andean altitudes 2,500 meters or more above sea level have as much as 25 percent more blood that is more viscous and richer in red cells, a heart that is proportionately larger, and specially adapted, larger lungs, with an enhanced capacity to take in oxygen from the rare atmosphere. Biological adaptations have permitted the native highlanders to work efficiently and survive successfully in the Andean altitudes for 20,000 years.