Question
Define the four respiratory volumes and four respiratory capacities. How are capacities calculated from volumes? What is the significance of residual volume — why can we never completely empty our lungs?
(NEET + CBSE Board — definitions + calculations)
Solution — Step by Step
| Volume | Full Name | Definition | Typical Value |
|---|---|---|---|
| TV | Tidal Volume | Air breathed in or out during normal breathing | ~500 mL |
| IRV | Inspiratory Reserve Volume | Extra air that can be forcibly inhaled after normal inhalation | ~2500-3000 mL |
| ERV | Expiratory Reserve Volume | Extra air that can be forcibly exhaled after normal exhalation | ~1000-1100 mL |
| RV | Residual Volume | Air remaining in lungs after maximum exhalation — cannot be expelled | ~1100-1200 mL |
| Capacity | Formula | Typical Value |
|---|---|---|
| IC (Inspiratory Capacity) | TV + IRV | ~3000-3500 mL |
| EC (Expiratory Capacity) | TV + ERV | ~1500-1600 mL |
| FRC (Functional Residual Capacity) | ERV + RV | ~2100-2300 mL |
| VC (Vital Capacity) | TV + IRV + ERV | ~4000-4600 mL |
| TLC (Total Lung Capacity) | TV + IRV + ERV + RV = VC + RV | ~5100-5800 mL |
After the most forceful exhalation, about 1100-1200 mL of air remains trapped in the lungs. This is the residual volume. It exists because:
- The lungs have structural rigidity — the alveoli do not collapse completely
- The negative intrapleural pressure keeps some alveoli partially inflated
- Biologically, this is critical — RV prevents the alveoli from collapsing (atelectasis) and maintains continuous gas exchange even between breaths
Without residual volume, the alveoli would collapse with every exhalation and need to be re-inflated with every inhalation — enormously increasing the work of breathing.
- Vital Capacity is used to assess lung health — reduced VC indicates restrictive lung disease
- FRC increases in emphysema (trapped air) and decreases in fibrosis
- Spirometry measures all volumes EXCEPT residual volume (RV cannot be measured by spirometry — requires body plethysmography or helium dilution)
graph TD
A["Total Lung Capacity"] --> B["Vital Capacity"]
A --> C["Residual Volume"]
B --> D["IRV"]
B --> E["TV"]
B --> F["ERV"]
D --> G["IC = TV + IRV"]
F --> H["FRC = ERV + RV"]
style A fill:#fbbf24,stroke:#000,stroke-width:2px
style B fill:#86efac,stroke:#000
style C fill:#fca5a5,stroke:#000Why This Works
Lung volumes and capacities describe the limits of our breathing ability. Tidal volume is our “default” breathing. The reserves (IRV, ERV) represent our “extra” capacity for exercise or forceful breathing. Residual volume is the permanent minimum.
Capacities are simply meaningful combinations of these volumes. Vital capacity (the maximum air you can move in one breath) is the most clinically important — it measures total functional lung capacity.
Common Mistake
The most common error: students include residual volume in the calculation of vital capacity. VC = IRV + TV + ERV (NOT including RV). Only TLC includes RV. Remember: Vital Capacity is the air you CAN voluntarily move. Residual volume is the air you CANNOT move — so it is excluded from VC. NEET tests this formula every 2-3 years.
Quick formula check: TLC = VC + RV. If you know any two of TLC, VC, and RV, you can find the third. Also remember: RV cannot be measured by spirometry — this is a favourite NEET distractor option. Spirometry measures TV, IRV, ERV, and derived capacities, but NOT RV.