Exercise Physiology A Tale of Two Pumps

Exercise Physiology: The Concept of Limits

We have been discussing the interaction of two organ systems during exercise as they strive to deliver oxygen to metabolically active tissue, remove carbon dioxide, and maintain an acceptable acid–base balance. These organ systems do an extraordinary job of allowing humans to perform feats of strength and endurance. However, there are limits to what the human body can do. What physiologically prevents us from exercising to even greater levels, the respiratory system or the cardiovascular system?

Limits Of The Respiratory System ↑

The ability to deliver oxygen to and remove carbon dioxide from the blood depends on the successful function of all three components of the respiratory system. Adequate ventilation is needed (requiring an adequate controller and pump), and there must not be a significant problem with the gas exchanger in the lungs. Based on exercise studies, we approximate the maximal ventilation that can be sustained in healthy individuals during exercise by multiplying the forced expired volume in 1 second (the FEV₁) by 40. The typical young person has an FEV₁ of roughly 4L. The maximal sustainable ventilation is, therefore, approximately 160 L/min. The highest ventilation reached during exercise rarely exceeds 80 L/min. Ample reserve capacity is available with respect to the functioning of the controller and the ventilatory pump; this is not the limit to exercise.

What about the respiratory gas exchanger? As noted earlier in this chapter, P_aO₂ and P_aCO₂ are normal through most of exercise. Carbon dioxide typically decreases with intense exercise as the respiratory system attempts to compensate for the development of metabolic acidosis. As cardiac output increases and the blood races more quickly through the alveolar capillaries, diffusion of oxygen into the RBCs may not be completed by the time the RBC exits the alveolus. In healthy persons, however, this process rarely causes a significant decrease in P_aO₂ during exercise. Again, ample reserve capacity is available; the gas exchanger is not a limit to exercise.

Limits Of The Cardiovascular System ↑

The first sign that there is a problem supporting the needs of the body during exercise is the development of metabolic acidosis. At the AT, the muscle is unable to meet its metabolic needs by aerobic metabolism. What does this tell us physiologically? AT means that there is insufficient delivery of oxygen, or a limited ability of the muscle to use oxygen for metabolism. If the P_aO₂ is normal and the oxygen saturation of the blood is normal, as it should be based on the reserve capacity of the respiratory system, then a limitation on oxygen delivery must be attributable to the cardiovascular system. Once SV has been optimized, cardiac output increases, largely as a result of increasing HR. At the point that the average individual cannot exercise any further, she has reached a limit in cardiac output, typically evidenced by the fact that the HR is greater than 85% of the maximal predicted level (a rough estimate of the maximal predicted HR for a healthy individual can be made by subtracting the person’s age from 220). Healthy individuals are limited in their ability to exercise because of the limits of the cardiovascular system in delivering oxygen and the ability of the muscles to use oxygen. Well-trained athletes have more cardiovascular reserve than sedentary individuals, but both groups are limited ultimately by the cardiovascular system (Table 9-1).