total lung capacity
Total lung capacity is the most air your lungs can possibly hold — a full tank, reached at the very top of the deepest breath you can take. It is the absolute ceiling of lung volume.
It is the sum of every subdivision of lung volume: the vital capacity (everything you can move) plus the residual volume (the air that can never leave). Equivalently, it is what you can breathe out maximally added to what always stays behind.
How big TLC can become is set by a tug-of-war: the inspiratory muscles pulling the chest open versus the rising elastic recoil of lung and chest wall resisting further stretch. Maximal inspiration stops where these balance. A typical adult value is about 6 litres.
TLC sorts lung diseases into two broad families. A reduced TLC defines a restrictive pattern — stiff, scarred, or compressed lungs that cannot expand. An increased TLC marks hyperinflation in obstructive disease, where air-trapping over-stretches the lungs. Because it includes residual volume, TLC requires full lung-volume measurement, not spirometry alone.
In advanced COPD, hyperinflation can push TLC above 7 or 8 litres, yet the patient is breathless because so much of that volume is stale trapped air, not fresh ventilation.
TLC rises in hyperinflation but does not help.
Spirometry can suggest restriction (a low vital capacity with a preserved ratio), but only a measured TLC confirms it. A genuinely reduced TLC is the defining feature of a true restrictive defect.