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. Heart rate continues to rise with increasing severity of exercise, but SV levels off when exercise reaches half maximum. SV is usually less than doubled. Together the increase in HR and SV gives an increase in Cardiac Output of 5 fold .