During ExerciserVentiIation Must Increase Because the Tissues Demand More Oxygen and Produce More Carbon Dioxide
The increase in ventilation after the onset of exercise is initially rapid, then progresses more slowly, and, provided the workload remains constant, reaches a steady state after about 4 minutes.
Although the ventilatory response to exercise has been well described, the reasons for the increase in ventilation are still not well understood.The primary chemical stimuli for ventilation—PaO2, Paco2, and pH—do not change in most animals during moderate aerobic exercise. This observation shows that the increase in ventilation is well matched to the needs of the tissues, and that other factors besides chemical drive increase ventilation during exercise. These proposed other factors are (1) reflexes originating from motion of the exercising limbs, (2) factors related to the increase in cardiac output, (3) thermoregulatory factors, and (4) psychogenic factors that anticipate the onset of exercise. Current exercise control theory favors “central command neurons" that may be coupled to the neurons that regulate breathing. I hese command neurons control the responses of the respiratory and cardiovascular systems that are necessary to maintain oxygen delivery and carbon dioxide removal during exercise.
Once the anaerobic threshold is exceeded, it is easier to explain the increased ventilation. The production of Iacticacid decreases blood pH. The latter stimulates an increase in ventilation, which leads to a decrease in Paco2. In the horse, the increase in ventilation that can occur during exercise may be limited by respiratory rate being linked, one breath per stride, with stride frequency. During strenuous anaerobic exercise, the horses arterial pH decreases progressively, although ventilation remains constant. When exercise ceases, ventilation increases further, presumably because the restrictions imposed by locomotion are removed.