Shephard R J
Department of Physiological Hygiene, School of Hygiene University of Toronto, Toronto, Canada.
J Sports Med Phys Fitness. 1970 Jun;10(2):72-83.
Human endurance, both at altitude and at sea level, depends upon the ability of the subject to transport oxygen from the atmosphere to the working tissues. Many physiological processes were involved in oxygen transport, including the oxygen cost of breathing, the ventilation of alveolar gas, diffusion in the lungs, blood transport, and diffusion within the tissues. At sea level, the blood transport term (effectively the product of haemoglobin level and maximum cardiac out) offers the major resistance to oxygen uptake. At moderate altitudes, such as Denver (5000 ft) and Mexico City (7350 ft), alveolar ventilation (measured under STPD conditions) is well maintained, and the normal shape of the oxygen dissociation curve increases the effective solubility of oxygen in the blood stream, partially compensating for the increasing resistance offered by the pulmonary diffusion/blood transport interaction. However, if unacclimatized men are exposed acutely to an altitude of 7350 ft, their maximum oxygen intake is decreased by at least 8-10%, and there is a corresponding loss of performance in endurance events. The average sedentary indi vidual notices nothing more than a little breathlessness at this altitude, probably because in sub-maximum exercise, compensation is readily achieved by an increase of ventilation (measured at body temperature and pressure) and pulse rate. With more prolonged residence at altitude, compensation is also affected by adjustment of buffering systems and an increase of haemoglobin level. The altitude of Mexico City is rather critical from the physiological standpoint; at higher altitudes, compensation is no longer possible by increase of ventilation and use of the normal oxygen dissociation characteristics of haemoglobin. The pulmonary diffusion/blood transport resistance becomes increasingly significant, arterial saturation falls, and there is a progressive loss of maximum oxygen intake. Above 10,000 ft, the problem is compounded by the danger that unaccustomed severe exertion may induce pulmonary oedema.
人类在高原和海平面的耐力取决于个体将氧气从大气输送到工作组织的能力。氧气运输涉及许多生理过程，包括呼吸的氧气消耗、肺泡气体的通气、肺部扩散、血液运输以及组织内的扩散。在海平面，血液运输这一因素（实际上是血红蛋白水平与最大心输出量的乘积）对氧气摄取构成主要阻力。在中等海拔地区，如丹佛（5000英尺）和墨西哥城（7350英尺），肺泡通气（在标准温度、压力和干燥条件下测量）能得到良好维持，并且氧解离曲线的正常形状增加了氧气在血流中的有效溶解度，部分抵消了肺扩散/血液运输相互作用带来的阻力增加。然而，如果未适应环境的人突然暴露在7350英尺的海拔高度，他们的最大摄氧量至少会降低8 - 10%，并且在耐力项目中的表现会相应下降。一般久坐不动的个体在这个海拔高度只会感觉到有点气喘，这可能是因为在次最大运动量的运动中，通过增加通气量（在体温和压力下测量）和脉搏率很容易实现代偿。在高原停留时间更长时，缓冲系统的调整和血红蛋白水平的增加也会影响代偿。从生理学角度来看，墨西哥城的海拔相当关键；在更高海拔地区，通过增加通气量和利用血红蛋白的正常氧解离特性来进行代偿已不再可能。肺扩散/血液运输阻力变得越来越显著，动脉血氧饱和度下降，最大摄氧量逐渐降低。在10000英尺以上，不习惯的剧烈运动可能诱发肺水肿这一危险使问题更加复杂。
