Oxygen Requirements of Exercise

The most fundamental problem confronting any organism, from ameba to man, is that of securing an adequate supply of oxygen. The very existence of living organisms depends on the constant availability of free energy, derived ultimately from oxidations. When any organ or system of the body becomes "active" (as when a resting muscle contracts, or when a gland begins to secrete), its oxygen requirement is proportionately increased. The oxygen consumption of the human body under basal conditions (12 to 14 hours after eating and following a 30 minute period of resting in bed, at a comfortable room temperature) is approximately 200 to 250 ml. per minute. During maximal physical exertion this may rise to 4000 ml. per minute, or 15 to 20 times the basal consumption. This remarkable factor of safety involves the coordinated adjustment of the heart and circulation, the blood and the respiratory system.

While all the tissues of the body require oxygen for their continued activity, some are able to function for short periods at a level of intensity which far exceeds their capacity for obtaining oxygen. They do this by calling on anaerobic sources of energy such as the breakdown of glycogen to lactic acid. This is a wasteful procedure, as compared with oxidation, since only a portion of the energy of the fuel is liberated. Nevertheless, it serves an extremely useful function in enabling the organism to meet emergency situations requiring an energy expenditure in excess of that which can be supplied by oxidations alone. It must be emphasized, however, that anaerobic metabolism is not a substitute for oxidation--it merely postpones it. Following a period of activity powered by anaerobic energy, the anaerobic reactions must be reversed in order to restore the fuel supply and to remove acid metabolites, and this is accomplished by oxidative energy. During the period of exercise the subject has gone into debt for oxygen and this debt must be repaid during recovery.

While some of the organs of the body arc able to meet emergencies by incurring an oxygen debt, others, notably the heart and brain, cannot. These vital organs depend entirely on oxidative energy and their function begins to suffer as soon as their oxygen supply falls short of their requirement. The oxygen requirement of the brain is not particularly affected by exercise, lint that of the heart may be tremendously increased, and it will be demonstrated in a later chaplet that the inability of the heart to obtain its full oxygen requirement is one of the most important factors which limits the intensity and duration of exercise.