There are 4 distinct respiratory volumes: - Tidal volume (TV): volume of air that enters or leaves the lungs during one respiratory cycle; the average is ~500 mL
- inspiratory reserve volume (IRV): volume of air that can be inhaled in addition to the tidal volume, during forced inspiration; the average is ~3,000 mL
- Expiratory reserve volume (ERV): volume of air that can be exhaled during a maximal forced expiration, beyond the tidal volume; the average is ~1,200 mL
- Residual volume (RV): volume of air that remains in the lungs after a maximal expiration; the average is ~1,200 mL; cannot be measured with a spirometer