As soon as a person begins to exercise, breathing becomes deeper and more rapid to produce a total minute volume that is many times the resting value. This increased ventilation, particularly in well-trained athletes, is exquisitely matched to the simultaneous increase in oxygen consumption and carbon dioxide production by the exercising muscles. The arterial blood PO2, Pœ2, and pH thus remain surprisingly constant during exercise (fig. 16.41).
It is tempting to suppose that ventilation increases during exercise as a result of the increased CO2 production by the exer cising muscles. Ventilation and CO2 production increase simultaneously, however, so that blood measurements of PCO2 during exercise are not significantly higher than at rest. The mechanisms responsible for the increased ventilation during exercise must therefore be more complex.
Two kinds of mechanisms—neurogenic and humoral— have been proposed to explain the increased ventilation that occurs during exercise. Possible neurogenic mechanisms include the following: (1) sensory nerve activity from the exercising limbs may stimulate the respiratory muscles, either through spinal reflexes or via the brain stem respiratory centers, and/or (2) input from the cerebral cortex may stimulate the brain stem centers to modify ventilation. These neurogenic theories help to explain the immediate increase in ventilation that occurs as exercise begins.
Rapid and deep ventilation continues after exercise has stopped, suggesting that humoral (chemical) factors in the blood may also stimulate ventilation during exercise. Since the PO2, PCO2, and pH of the blood samples from exercising subjects are within the resting range, these humoral theories propose that (1) the PCO2 and pH in the region of the chemoreceptors may be different from these values "downstream," where blood samples are taken, and/or (2) cyclic variations in these values that cannot be detected by blood samples may stimulate the chemoreceptors. The evidence suggests that both neurogenic and humoral mechanisms are involved in the hyperpnea, or increased total minute volume, of exercise. (Note that hyperpnea differs from hyperventilation in that the blood PCO2 is decreased in hyperventilation.)
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.