Lung volumes and capacities
The amounts of air the lungs move and hold, measured by spirometry and used to separate obstructive from restrictive disease.
A quiet breath moves about half a litre (tidal volume). Above it lies the inspiratory reserve and below it the expiratory reserve; together with tidal volume these make the vital capacity, roughly 4 to 5 litres in an adult man. Even after the deepest expiration about 1.2 litres remains as residual volume, keeping the alveoli open.
Spirometry records the forced vital capacity (FVC) and the volume blown out in the first second (FEV1). Obstructive disease (asthma, COPD) narrows the airways so FEV1 falls out of proportion to FVC and the FEV1/FVC ratio drops below about 0.7. Restrictive disease (fibrosis, chest wall problems) shrinks all volumes but keeps the ratio normal.
Volumes fall with age, height and smoking and rise with training; reference values are matched to age, sex, height and ethnicity.
Key facts
| Fact | Value |
|---|---|
| Tidal volume | ≈500 ml |
| Vital capacity (adult male) | ≈4.5 L |
| Obstruction threshold | FEV1/FVC < 0.7 |