Ventilation rises linearly with work until the anaerobic threshold, then disproportionately
Expansion
Moderate exercise
- Minute ventilation rises linearly with oxygen uptake and carbon dioxide production, from about 6 to over 100 litres per minute
- Achieved by increasing tidal volume first, then rate
- Arterial oxygen, carbon dioxide and pH remain nearly unchanged
Above the anaerobic threshold (about 50 to 60 per cent of maximum uptake in the untrained)
- Lactate accumulates and is buffered by bicarbonate, generating extra carbon dioxide
- Ventilation rises disproportionately, and carbon dioxide begins to fall
The control mechanisms are still debated, but include central command from the motor cortex, afferents from muscle and joint receptors, rising body temperature, increased sensitivity of chemoreceptors to oscillating carbon dioxide, and potassium released from exercising muscle.
Note that ventilation is not normally the limiting factor in healthy exercise; cardiac output is. Even at maximum effort a healthy person uses only about 60 to 70 per cent of maximum voluntary ventilation, which is why breathing reserve is preserved and its loss points to lung disease.