Start with one quiet breath
Imagine the lungs as a bucket of air that never fully empties. The amount of air you move in and out during one relaxed breath is the tidal volume — about half a litre, or roughly 500 mL, in a typical adult at rest. It is a small fraction of what the lungs can hold, which is exactly why you have so much breath in reserve when you need it.
The four basic volumes
- Tidal volume: the air in one quiet breath, ~500 mL.
- Inspiratory reserve volume: the extra you can still pull in after a normal breath in, ~3000 mL.
- Expiratory reserve volume: the extra you can still push out after a normal breath out, ~1100 mL.
- Residual volume: the air that always stays behind, even after the hardest breath out, ~1200 mL.
The residual volume is worth a pause. You can never voluntarily empty your lungs completely; if you could, they might collapse and stick shut. That leftover air keeps the air sacs propped open and lets gas exchange continue even between breaths. It is a feature, not a flaw.
Stacking volumes into capacities
Now add them up. Vital capacity is the biggest breath you can manage: all the way in, then all the way out. It is tidal volume plus both reserves — everything except the air you can never expel. Total lung capacity is vital capacity plus the residual volume, the absolute most the lungs can hold. And functional residual capacity, or FRC, is the comfortable resting point: the air left in the chest after a normal, relaxed breath out, where the lung's tendency to deflate is exactly balanced by the chest wall's tendency to spring open.
Approximate adult values (litres)
Volumes Capacities
TV (tidal) = 0.5 VC = TV + IRV + ERV = 4.6
IRV (insp. reserve) = 3.0 TLC = VC + RV = 5.8
ERV (exp. reserve) = 1.1 FRC = ERV + RV = 2.3
RV (residual) = 1.2 IC = TV + IRV = 3.5
Sanity check: TLC = TV + IRV + ERV + RV
5.8 = 0.5 + 3.0 + 1.1 + 1.2 ✓