How brain, lungs, blood cells and muscles struggle to survive
A fact of life is that of all the elements essential to human life, oxygen alone cannot be stored by the body. Moreover, the supply of vital oxygen must be uninterruptedly replenished almost as rapidly as it is used. If it is not, the body fails and collapses into unconsciousness. In medical terminology, this is anoxia, insufficient oxygen. At an altitude of 7,600 feet, or even 76,000 feet, the components of the atmosphere are precisely the same as at sea level. The problem is that the air is much less dense. Consequently, breath for breath, a runner in Mexico City can take in, no matter how hard he gasps for air, only 80% of the oxygen that he might be accustomed to at sea level. What this does to the runner is explained in detail in these drawings.
The brain (left), the most oxygen-sensitive organ of the body, is the first to react to lack of oxygen. When insufficient amounts reach it, the runner lapses into unconsciousness. Blacking out, in fact, is actually a defense mechanism of the body to enable it to remain alive. When unconscious, the body requires the least possible amount of oxygen and, sprawled motionless on the ground, permits a maximum supply of oxygen-carrying blood to replenish the brain.
The lungs (right) consist about 400 million alveoli, minute air sacs resembling clusters of grapes, each surrounded by countless capillaries. Here oxygen and carbon dioxide move into and out of the blood stream. Since the weight of air in Mexico City is one-quarter less than at sea level, less oxygen presses its way from the lungs into the blood stream. Thus, the entire anatomy of the runner who is built for lower altitudes is constantly hungry for oxygen.
Red corpuscles (right) in the blood pick up oxygen from the alveoli, carry it throughout the body and bring back carbon dioxide to be exhaled. Runners who live at sea level have about 25 trillion red corpuscles, not enough to fulfill their needs at 7,600 feet. Natives of higher altitudes have as many as one-third more and thus can exercise strenuously without ill effects. After a few weeks, however, a body accustomed to low altitudes increases its production of red cells and it becomes acclimatized.
Muscles (left) need oxygen to combine with sugar and produce energy, and the greater the muscle exertion, the more oxygen is required. Since the number of red blood cells remains constant, the runner gasps for breath and his heart beat increases to force as much oxygen-carrying blood as possible even faster to oxygen-starved muscles. When unacclimatized to the rarefied atmosphere of higher altitudes he therefore tires more quickly and may collapse after the race.