Respiratory Volumes And Capacities
The Big Idea
During quiet breathing you use only a small part of your lungs' total range — like sipping air through a straw. The lungs can expand much further and can also be squeezed out much further than a normal breath uses. And even after you force out every possible bit of air, some always remains, keeping the alveoli from collapsing. Volumes are the individual amounts of air involved in specific breathing manoeuvres; capacities are sums of two or more volumes, describing the total air the lungs can hold under different conditions.
The Four Basic Volumes
Approximate values for a healthy young adult (actual numbers vary with age, sex, body size, and fitness):
- Tidal Volume (TV) — about 500 mL: the air moved in a single normal, quiet breath.
- Inspiratory Reserve Volume (IRV) — about 3000 mL: the extra air you can forcefully inhale after a normal tidal inhalation.
- Expiratory Reserve Volume (ERV) — about 1200 mL: the extra air you can forcefully exhale after a normal tidal exhalation.
- Residual Volume (RV) — about 1200 mL: the air that remains in the lungs even after the most forceful exhalation. It cannot be measured with a simple spirometer because it can never be exhaled; it needs an indirect technique such as gas dilution.
A common mistake is thinking the lungs are empty after a maximal exhalation. They are not — Residual Volume always remains. Without it, alveoli would collapse and be very hard to re-inflate.
The Four Capacities
Capacities describe the total air the lungs can hold in different circumstances, by adding volumes together:
- Vital Capacity (VC) = IRV + TV + ERV, about 4700 mL — the maximum air you can exhale after a maximum inhalation. It is the most clinically useful measure of how much air the lungs can actually move; a low VC suggests a restrictive disease such as fibrosis, where the lungs are stiff.
- Total Lung Capacity (TLC) = VC + RV, about 5900 mL — the absolute maximum the lungs can hold.
- Inspiratory Capacity (IC) = TV + IRV, about 3500 mL — how much the lungs can expand from their resting position at the end of a normal breath. …