Expansion
A capacity is two or more volumes added together
Expansion
Volumes (cannot be subdivided)
- Tidal volume: about 500 ml
- Inspiratory reserve volume: about 3000 ml
- Expiratory reserve volume: about 1100 ml
- Residual volume: about 1200 ml
Capacities (two or more volumes)
- Inspiratory capacity = tidal + inspiratory reserve
- Functional residual capacity = expiratory reserve + residual
- Vital capacity = tidal + inspiratory reserve + expiratory reserve
- Total lung capacity = vital capacity + residual volume, about 5800 ml
The single most important rule is that residual volume cannot be exhaled, so spirometry cannot measure it. Any capacity containing it, namely functional residual capacity and total lung capacity, therefore requires helium dilution, nitrogen washout or body plethysmography.
Plethysmography measures all gas in the chest including trapped gas, so it gives a larger value than dilution methods in emphysema; the difference is itself a measure of gas trapping.