Lung volumes and capacities - definitions and age-related changes
Core concept
- Four volumes; capacities are sums of them
| Volume | Definition | Typical |
|---|---|---|
| Tidal volume (VT) | Quiet breath | ~500 mL |
| IRV | Extra in above tidal | ~3000 mL |
| ERV | Extra out below tidal | ~1100 mL |
| RV | Left after maximal expiration | ~1200 mL |
| Capacity | = | Typical |
|---|---|---|
| IC | VT + IRV | ~3500 |
| FRC | ERV + RV | ~2400 |
| VC | IRV + VT + ERV | ~4600 |
| TLC | VC + RV | ~5800 |
- Spirometry cannot measure anything containing RV - so RV, FRC and TLC require body plethysmography, helium dilution or nitrogen washout
- Plethysmography measures all intrathoracic gas including trapped air; dilution/washout measure only communicating gas -> plethysmographic TLC > dilutional TLC in bullous emphysema and air trapping (the difference is the trapped volume)
- FRC = the equilibrium point where inward lung elastic recoil exactly balances outward chest wall recoil
3 more sections, plus exam facts
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