Effect of Exercise on Respiration (Guyton & Hall, 4th SA Edition)

1. Increase in oxygen consumption and ventilation At rest, O2 consumption in an average young man is about 250 mL/min. During maximal exercise this can rise to about 3600 mL/min in an untrained man, 4000 mL/min in an athletically trained man, and up to 5100 mL/min in a marathon runner. Alveolar (pulmonary) ventilation increases in an almost exact linear relationship with oxygen consumption - both can increase about 20-fold from rest to maximal exercise in a well-trained athlete (Fig. 85.7 / Fig. 42.9).

2. Arterial blood gases remain normal Despite this huge rise in ventilation, arterial PO2, PCO2, and pH remain almost exactly normal during exercise. This shows that chemical stimuli (CO2, O2, H+) are not the main drivers of the increased ventilation seen in exercise.

3. Neurogenic (anticipatory) mechanism The principal mechanism is believed to be neural: when the motor cortex sends impulses to the exercising muscles, it simultaneously sends collateral impulses to the brainstem respiratory center, stimulating ventilation even before any change in blood chemistry occurs - this is analogous to the simultaneous excitation of the vasomotor center during exercise. This is why ventilation rises almost instantaneously at the onset of exercise, sometimes overshooting and transiently lowering arterial PCO2 below normal.

4. Chemical fine-tuning After about 30-40 seconds, CO2 released from the active muscles matches the elevated ventilation, and arterial PCO2 returns to normal. Chemical factors (CO2, H+, O2) act as a secondary "fine-adjustment" mechanism if the neurogenic drive is too strong or too weak.

5. Ventilatory reserve and limiting factor Maximal pulmonary ventilation during exercise reaches only 100-110 L/min, while maximal breathing capacity is 150-170 L/min - about 50% greater. This reserve protects against high altitude, heat, or respiratory disease. Hence the respiratory system is normally not the limiting factor for O2 delivery to muscles during maximal aerobic work; the pumping capacity of the heart (cardiac output) is usually the greater limiting factor.

Definition: VO2 max is the maximum rate of oxygen utilization (in L/min) by the body during maximal aerobic metabolism/exercise.

Values: It averages higher in trained individuals - a marathon runner's VO2 max is about 45% greater than that of an untrained person.

Effect of training: Structured aerobic training programs (7-13 weeks) raise VO2 max by only about 10% on average, and training frequency (2 vs 5 times/week) makes little difference to this rise. The much larger VO2 max seen in elite endurance athletes is therefore attributed partly to genetic factors, such as a larger chest size relative to body size and greater intrinsic cardiac/muscle oxidative capacity, rather than training alone.

Significance: VO2 max is used clinically and in sports physiology as the best single index of cardiorespiratory fitness and the overall capacity of the cardiopulmonary system to deliver and utilize oxygen during maximal exertion.