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. The mean value remains constant, Fig C . Chemoreceptors are sensitive to change in stimulus. Further drive to ventilation comes from lactic acidosis. This has been shown to stimulate chemoreceptors.