Further work relies on anaerobic mechanism. Fig A For light or moderate exercise, increase in ventilation is proportional to O 2 consumption and CO 2 production. Mean arterial PCO 2 DOES NOT RISE in exercise. In severe exercise ventilation can become inappropriately high and alveolar and arterial PCO 2 will then fall. This is when lactic acid appears in significant amounts.
Ventilation is unlikely to be a limiting factor in exercise in a healthy subject.
The prompt rise in ventilation at onset of exercise suggests neural drive to respiratory centres coincident with voluntary drive to contracting muscles. Sensory receptors in muscles, tendons and joints may reflexly stimulate ventilation but these effects are weak. Close matching of ventilation and CO 2 production suggests strongly that chemoreceptor mechanism is important. Patients lacking functional chemoreceptors have poor ventilatory response to exercise. During exercise in normal subjects the alveolar and arterial pCO 2 oscillates more than at rest, rising in expiration and falling in inspiration.