The adaptation of man to life at high altitude
Bragg, Ernest Atherton
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When a resident from sea level ascends to high altitude, several important adaptive responses must be made if the body is to maintain its efficiency in the new environment. At altitudes above 12,000 or 13,000 feet, the necessary responses are of such magnitude as to require several days for completion, and the efficiency of the body is likely to be seriously impaired for a time. Mountain sickness, characterized by dyspnoea, severe headache, nausea, muscular weakness, loss of appetite, and various other symptoms, is the most common high altitude ailment. Glacier lassitude, Cheyne-Stokes respiration, and Monge's disease are other conditions resulting from faulty acclimatization. The chief factor by which the high altitude environment differs from that at sea level are: reduced oxygen tension in the atmosphere; more extreme conditions of air temperature; dryness of the air; and increased solar radiation. Of these, the low oxygen is the most important, and the difficulties at high altitude are chiefly a matter of anoxemia. Respiration responds immediately to high altitude by an increase in depth. At very high altitude and during prolonged stays at moderately high elevations, there is also an increase the respiratory rate. When the proper change has been made in the carbon dioxide level of the blood, the volume of respiration is sufficient to cause a rise of several millimeters of Hg in the alveolar oxygen tension, thus adding materially to the ability of the body to obtain oxygen from the reduced atmosphere. The muscles controlling respiration become much stronger during the long residence at high altitude. The vital capacity decreases somewhat, probably due to congestion of the lungs with blood. Circulation shows response in all its phases. The pulse increases markedly, reaching its maximum in about two days and returning later toward normal. Cardiac output reaches its maximum in a few days, then drops off as does the pulse rate. Systolic, diastolic, and pulse pressure all rise after a few hours at high altitude. Pulse presure returns to normal, but the other pressures remain slightly higher except at very high altitudes, where the pulse pressure also drops. Venous pressure drops off proportionately as the altitude increases. The condition of the heart remains normal up to the limit of acclimitization, but at extreme altitudes the heart rhythm becomes irregular, and the heart is dilated. All of these changes in circulation work together to increase the rate of circulation and help to lessen the degree of oxygen lack. The blood makes two very important adaptive responses. The first of these is a change in the number and condition of the red blood corpuscules. At very high altitude, the red cell counts may increase as much as 50 percent above the sea level value. The total hemoglob1n content of th blood is thus increases, but not to such a degree, since the quantity of hemeglobin in each cell is somewhat reduced. The cell volume and diameter are increased. This change provides a much larger surface area, per unit volume of hemoglobin, so that the absorption of oxygen as the blood passes through the lungs is greatly speeded up. The second blood change is a reduction in the alkaline reserve to compensate for excessive loss or carbon dioxide through the increased pulmonary ventilation. The changes in the carbon dioxide level have several effects. The initial loss interferes with the response or the respiratory center to the oxygen shortage, but does belp the situation by changing the oxygen dissociation curve of the blood so that the hemoglobin combines more readily with oxygen. After a few days, the blood regains its normal pH, or may become a little more acid than normal. By this time, the chnages in the red cells have improved the ability of the blood to absorb oxygen from the lungs, and the acidity of the blood aids in the unloading or oxygen from the blood to the bodily tissues. The acidity also stimulates the respiratory center, and produces a further rise in respiration. When the changes of the blood have reached completion, circulation returns almost to normal. The rate of oxygen consumption seems to be slightly increased during rest at high altitude, probably because of the greater activity of the respiratory and circulatory systems. During exercise, the oxygen consumption is markedly lover at high altitude than at sea level, because the body can not obtain oxygen as rapidly. Since oxygen consumption is the ultimate control for physical work, the ability of the body to perform work is reduced at high altitudes. During work at high altitude, respiration and circulation are speeded up more than for the same load of work at sea level, and the efficiency of these functions drops considerably. In healthy men it can not be demonstrated that any one organ or function fails during work; the body as a whole merely reaches the maximum of its ability to obtain oxygen from the air and distribute it to the tissues. The digestive processes are subject to important alterations at high altitudes. The emptying time of the stomach is increased proportionately as the oxygen tension falls. At very high altitudes, the diet must be changed, as most proteins can no longer be easily digested. Even with the best possible diet, digestion is impaired, and tbe appetite is poor. Malnutrition and loss of weight result. Altitudes of 5,000 to 10,000 feet seem to have a bennificial influence upon health, especially in people who suffer from any disease of the respiratory tract. This is probably to the clean dry air and the strong sunlight. Sufferers from any serious cardiac disorder are endangered by ascent to high altitude, even by train or plane, because of the strain of the increased circulation. Adaptation to life at high altitude seems to be most rapid and most complete in healthy young adults who are in good physical condition. True adaptation seems to be limited to about 18,000 feet. Healthy men can live for indefinite periods at 17,000 to 18,000 with no apparent ill eftects. Mountaineers have been as high as 28,000 feet for short periods, and have lived for weeks at 20,000 feet and above, but these exploits are always accompanied by a steady decline in health and vigor. At 20,000 feet or above, there is loss of weight, loss of physical strength, impairment of mental acuity, and a general souring of the disposition. Certain organs, including heart, lungs, and liver, may undergo actual histological deterioration.
Thesis (M.A.)--Boston University, 1940