expiratory reserve volume

Expiratory reserve volume is the extra air you can still squeeze out after a normal breath out — the difference between a relaxed exhale and forcing every last bit you can. Breathe out gently, then keep pushing: that final stretch of air is the reserve.

It sits between two landmarks: the resting end-of-quiet-breath point (functional residual capacity) and the truly emptied point (residual volume). Subtract one from the other and you have the ERV, typically around 1 to 1.5 litres in an adult.

Forcing out this reserve is an active effort, driven by the abdominal and internal intercostal muscles squeezing the chest down. It is recruited during exercise, forced coughing, and singing or playing wind instruments.

The expiratory reserve volume is reduced whenever the resting lung volume is pushed up or down. Obesity and lying flat raise the diaphragm and shrink it; air-trapping in obstructive disease raises functional residual capacity toward total lung capacity, also leaving little room to exhale further.