Figure 37.9 Sensitivity of the Respiratory Control System Changes with Exercise

C. R. Bainton tested the hypothesis that a rising level of CO₂ during exercise is the feedback signal that stimulates an increase in respiratory rate. The researcher equipped dogs for measuring their respiratory rate and trained them to run on a treadmill. During exercise, blood samples were taken to determine CO₂ levels. In the first series of experiments, the dogs ran on the treadmill set at various speeds. The respiratory rate data were then plotted as a function of arterial CO₂ concentration. Analysis revealed that while the respiratory rate changed with speed, the arterial levels of CO₂ remained constant; these results did not support Bainton’s hypothesis. In the second set of experiments, the dogs again ran on the treadmill, but at a constant speed. However, the slope of the treadmill was elevated so as to increase the workload of the dog. As above, the respiratory rate data were plotted as a function of arterial CO₂ concentration. Analysis revealed that both the respiratory rate and arterial levels of CO₂ rose as the workload increased, thus supporting the original hypothesis. Bainton concluded that the arterial level of CO₂ is the metabolic feedback signal that controls respiratory rate in response to workload. Importantly, the results suggested that information from sensors in joints and muscles that signal the rate of limb movement can also change the body’s sensitivity to CO₂ levels. This experiment addressed the sensitivity of the respiratory control system in response to exercise. In addition to exercise, however, other factors such as cold exposure are also known to increase respiration. To test whether cold-induced respiration is also driven by increased arterial blood CO₂ levels, Bainton induced shivering in the dogs running treadmills at a steady rate. As with the increased treadmill slope, respiratory rate and arterial levels of CO₂ increased with shivering. To confirm the importance of arterial CO₂ levels on respiratory rate, dogs inhaled CO₂ while on a flat treadmill. In the absence of increased CO₂ due to shivering or workload (due to treadmill slope), respiratory rate increased, providing additional support for a role of CO₂ in metabolic rate signaling.

Original Paper

Bainton, C. R. 1972. Effect of speed vs. grade and shivering on ventilation in dogs during active exercise. Journal of Applied Physiology 33: 778–787.

Links

Eastern Kentucky University: Gary Ritchison: Human Physiology: Respiration
http://people.eku.edu/ritchisong/RITCHISO//301notes6.htm

Kimball’s Biology Pages: The Human Respiratory System
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/Pulmonary.html