Regulation of the ventilation at the beginning of muscular exercise

Summary

The ventilation, breathing frequency andPa _{co 2} have been studied in two subjects at the beginning of an exercise, consisting in walking at a speed of 2,7 km/h, resp. 4 km/h, on a treadmill at a 10% inclination.

The ventilation increases before the exercise begins, leading to reduction of the alveolar CO₂ tension. The increase in the ventilation remains high at an almost constant, value, for about twenty seconds.

Such an early increase in the ventilation cannot be due to a direct chemical stimulation of the respiratory centre, because the CO₂ or the H⁺ion concentration of the blood do not increase but rather decrease. Furthermore, the chemical factor cannot enter into play until after a time greater than the circulation time.

It seems therefore to take place through a nervous mechanism of which two components have been shown: an earlier being a conditioned reflex, the stimulus being the warning signal; it initiates six seconds before and it dies down after the start of work. The second component is also of nervous nature: it initiates with the beginning of the exercise and it remains almost constant for about 16–20 seconds, that is, until the respiratory activity is established at a level dictated by the chemical variations in the blood induced by the muscular activity.

The two nervous components also differ, besides from their timing, because the increase in ventilation due to the conditioned reflex is mainly sustained by an increased frequency, while that due to the second component is essentially sustained by an increased depth.