Adaptation for Survival
from the book, "Petrified Lightning"
Adaptation for Survival
Many upland regions near the equator lack the change of seasons most people are accustomed to. In fact, in certain areas of the Peruvian and Bolivian Andes the climate is forever springtime. Temperatures are less extreme, rainfall is distributed more evenly throughout the year, and the soil is more productive than that of terrain near sea level. Here fruit trees bloom throughout the year, and many field crops may be harvested several times annually. Mountain habitats also are free from tropical diseases, since high altitudes are unsatisfactory to the needs of mosquitoes, tsetse flies, and other scourges of the tropics.
The environment in these areas was friendly enough to have fostered the great Inca civilization. In the course of its 4,000—year development, millions of people occupied the high valleys of the central Andes. Many established some of the highest permanent habitations on earth.
At extreme altitudes, however, the advantages of equatorial mountain living vanish. The air is too thin and cold to be considered friendly. For the ordinary person, the climate in the high Andes is so unfavorable that the problem of survival is almost unsolvable. The dilemma: how does one get enough air up there to breathe?
Most of the human population of the earth lives near the bottom of a 10—mile—deep ocean of air. This air has weight and is easily compressed; therefore it becomes denser as it gets deeper. At sea level humans are adapted to a density (atmospheric pressure) of 15 pounds per square inch. Sea—level humans' lungs are so conditioned that when one breathes in a gulp of air the 15—pound pressure forces a significant supply of oxygen through the thin linings of the lungs, giving the breather what is needed to function properly.
At higher elevations air pressure is reduced, and at 10,000 feet it is only 10 pounds per square inch. This is not quite enough to push an adequate supply of oxygen through the linings of the lungs. As a result blood carries up to 15 percent less than its normal load of oxygen, a short supply that may cause headaches, fatigue, and shortness of breath, commonly called "mountain sickness." At 18,000 feet air pressure is only half that at sea level, and few people escape more pronounced symptoms. If the human body is unable to overcome the shortage of oxygen, it will sicken and die.
Yet residents of the high Andes not only survive at such altitudes but spend their entire lives there, working and playing as normally as do most people at sea level. Clearly these people are benefiting from the adaptions of generations of their predecessors to an extreme environment. Some changes they have made to overcome the altitude problems are dramatic and permanent. They have become physically different from the rest of us, mainly as a result of changes in their respiratory and blood—circulating systems.
A visitor to the high Andes will immediately notice that the natives of these mountain slopes are short and stocky and have tremendous, barrel—shaped chests. They have lungs to match; the small pockets (alveoli) that line the lungs and give added capacity are always wide open in these high—altitude dwellers. This feature allows the greatest possible amount of blood to flow through the delicate lung tissues, thereby picking up all available oxygen that is breathed in.
The natives of the high Andes have about 20 percent more blood than a lowlander. An Andean mountaineer and a sea—level dweller, both weighing 130 pounds, would compare as follows: although the Andean is shorter in stature, his total blood content is about six quarts, more than half red blood cells. The lowlander has only five quarts, red cells making up about 40 percent. The Andean's extra red cells hold extra hemoglobin that can catch and absorb oxygen. People living at altitudes of 15,000 feet or higher have an almost 60 percent increase in this vital hemoglobin. Moreover, each of their red blood cells is larger than the cells of lowland people; this larger size gives a greater surface for the absorption of oxygen.
Since the mountaineer's blood is so rich in red cells, it is of course thicker than the lowlander's. The heart of the Andean is about a fifth larger because it must pump harder at these altitudes, but the beat is much slower. The blood doesn't have to be pumped very far as it circulates through the highlander's squat, compact body.
The ends of the Andean's broad, stubby fingers and toes contain an unusually large number of direct passages between arteries and small veins. This hastens circulation, enabling the mountain person to go about with hands and feet uncovered at times when lowlanders would suffer severe frostbite. As can ducks and penguins, these people walk barefoot in the snow without discomfort.
In 1981 a team of scientists from the United States was working on a newly discovered Inca ruin in the high Andes. The expedition cook, who had just gotten married, planned not to report for work the next day but promised to send his brother to fill in for him during his brief honeymoon. The scientists were disgruntled the next morning because breakfast time came and the replacement had not arrived. Since it had snowed during the night everyone was cold, hungry, and uncomfortable. A cheer went up as the new cook arrived on the run with an apologetic grin on his face; he had overslept. Then scientists realized that the man had plowed barefooted through the deep snow. Because he was late, he didn't take the time to put on shoes and carried them instead. Immediately the scientists, laced in warm boots, felt more comfortable.