Respiration in man at high altitudes
Lee, Robert Cleveland
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Respiration is the process of the exchange of gases between the body tissues and the environment. This is divided into external and internal respiration. External respiration involves the passage of air through the respiratory passages and into the alveoli of the lungs, together with its diffusion through the lung walls and into the blood. Internal respiration here includes the chemical and physical transport of the gases by the blood, the circulation of the blood, and the exchange of gases between the blood and the tissues. Respiration is affected by the low pressures encountered on high mountains and during flight. The regulation of respiration is controlled by the nervous system, regulated by impulses from the respiratory centers. The respiratory centers respond to changes in the tension of carbon dioxide and oxygen of the arterial blood. Blood tension affects respiration both by direct response of the center itself and by reflex impulses from the chemoreceptors of the carotid body and the aortic body. Respiration is also altered by the Hering-Breuer reflexes, impulses caused by pressure on the pressoreceptors in the carotid sinus and in the aortic arch, and by stretch receptors in the lungs. The gas tensions of the arterial blood are the chief regulators of respiration. An increase in the tension of carbon dioxide in the arterial blood is a stimulus to respiration. Appreciable lowering of the tension of oxygen in the arterial blood increases the excitability of the respiratory center. Blood gas tensions are altered by the level of the metabolism, the dead space in the respiratory system, the oxygen utilization, and changes in carbon dioxide and oxygen pressures in the air breathed. The internal respiration utilizes diffusion of gases to effect the transfer of gases into the blood and out, aided by an enzyme carbonic anhydrase. The gases are carried in the blood, largely in chemical combination. Carbon dioxide is carried as a bicarbonate in the blood plasma. Hemoglobin in the red cells carries the oxygen and much of the labile carbon dioxide. The quantity of gases carried depends upon the number of red cells and the amount of hemoglobin, the degree to which oxygen is utilized, and the rate of circulation. These factors are under nervous control, regulated in part by the factors altering the external respiration. Respiratory difficulties at high altitudes are due to the low barometric pressures encountered, which reduce the amount of oxygen available to the body. The constant water-vapor pressure in the lungs is also a factor limiting the amount of oxygen which can be supplied at high altitude. Respiration at high altitude is considered in two parts; first, respiration on high mountains, where altitude is maintained for days, weeks, or longer; and second, respiration during flight and conditions simulating flight, in which the altitude is maintained for a period of minutes or hours. Ascending high mountains causes the percentages of carbon dioxide and oxygen in the expired and in the alveolar air to change. The carbon dioxide percentage increases and the oxygen percentage decreases progressively with increase in altitude. These changes are more pronounced during muscular exertion. The tensions of carbon dioxide and of oxygen in both the alveolar air and the arterial blood are lowered, with increases in altitude, from 40 mm. and 102 mm., respectively, at sea level, to 21 mm. and 37 mm. at a 20,000-foot elevation. The respiration rate is not altered consistently, but the minute volume of respiration is increased with increase in altitude. During muscular work at high altitude the minute volume is higher than during the same amount of work at sea level. The minute volume increase is the result of an increase in the excitability of the respiratory centers due to lowered oxygen tension, sufficient to more than counteract the smaller stimulus incurred by a decreased carbon dioxide tension. Internal respiration is affected by high altitude. The red cell count and the hemoglobin content are increased, at first by temporary means; but later they are more permanently augmented. The hemoglobin content continues to increase for a number of weeks. Immediate increase in the rate of circulation aids in supplying the oxygen required until blood changes compensate for it. The oxygen requirement for rest is essentially the same as that at sea level, but a given amount of work requires more oxygen than is required by the same amount of work at sea level. Residents and natives at high altitude are acclimatized to altitude and perform muscular tasks with less respiratory effort than sea level residents at high altitude. The best explanation of adaptation appears to be an increase in the surface area of hemoglobin present in the blood of natives. Man, after becoming acclimatized, has climbed to 28,000 feet on Mount Everest, but respiration is labored and the amount of exertion is limited. Oxygen has been used in some mountain climbing but with no appreciable benefit. This is probably due to inadequate oxygen supply and to technical difficulties. When air is breathed during flight and in simulated flight in low pressure chambers, the same trends noted on high mountains are found. The alveolar partial pressures are 30 mm. for carbon dioxide and 35 mm. for oxygen at an altitude of 20,000 feet. The more permanent acclimatization factors are not found during flight because of the short duration, and even the immediate adjustments do not permit altitudes attainable on mountains to be reached without serious difficulties. The maximum altitude that can be attained when air is breathed is dependent on the rate of ascent, the length of time the altitude is maintained, and individual factors. The highest safe altitude for long exposures is probably about 15,000 feet, and for short exposures 25,000 feet is about the limit. By using oxygen, sufficient to maintain the partial pressure in the inspired air at 160 mm. up to 27,000 feet, or by using pure oxygen above 32,000 feet, the partial pressure of carbon dioxide in the alveolar air can be maintained at 36 to 40 mm. Under these conditions the volume of respiration is not increased above normal for sea level and internal respiration is probably normal. The altitude limit for flights using oxygen is probably below 50,000 feet, the present record being 47,358 feet. The highest recommended height is 40,000 feet. The breathing of pure oxygen or oxygen-rich air may be accomplished by the use of masks. Masks should be employed at 10,000 feet for long flights and at 15,000 feet for all flights. For flights above 30,000 feet it is desirable to breathe pure oxygen from the ground up. By using pressure enclosures, maintaining either the atmospheric pressure at sea level or the oxygen pressure at sea level, respiration is not restricted by altitudes. The highest altitude to which man has ascended was attained in a balloon with a pressurized gondola, in which the oxygen pressure was maintained above the sea level value of 160 mm. The altitude of 72,395 feet was reached in this flight without respiratory difficulty. The data presented prove that respiratory difficulties at high altitude are caused by insufficient oxygen, and that by maintaining an adequate oxygen pressure in the inspired air, normal respiration is possible at high altitudes.
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