Lung Volumes vs. Lung Capacities: Fundamental Distinction
Lung volumes are individual, discrete compartments of air in the lungs that cannot be subdivided, while lung capacities are the sum of two or more lung volumes combined together. This is the fundamental physiological distinction that underpins all pulmonary function testing 1.
The Four Standard Lung Volumes
The respiratory system is divided into four standard static lung volumes that represent distinct, non-overlapping compartments 1:
- Tidal Volume (TV): The amount of air moved in or out during normal breathing
- Inspiratory Reserve Volume (IRV): The additional air that can be inhaled beyond a normal breath
- Expiratory Reserve Volume (ERV): The additional air that can be exhaled beyond a normal breath
- Residual Volume (RV): The air remaining in the lungs after maximal exhalation
These volumes are measured at different degrees of inspiration or expiration and are static (not dependent on airflow rate) 1.
The Four Standard Lung Capacities
Lung capacities are calculated by adding together two or more of the basic lung volumes 1, 2:
- Inspiratory Capacity (IC) = TV + IRV
- Functional Residual Capacity (FRC) = ERV + RV (the volume remaining in lungs at end of normal expiration)
- Vital Capacity (VC) = TV + IRV + ERV (the maximum amount of air that can be exhaled after maximal inhalation)
- Total Lung Capacity (TLC) = TV + IRV + ERV + RV (all four volumes combined; the total volume of air in the lungs after maximal inhalation)
Clinical Significance of the Distinction
Understanding this distinction is critical because:
- Volumes cannot be further subdivided - they represent the fundamental building blocks
- Capacities provide integrated information about multiple compartments simultaneously
- Different measurement techniques are required: Some volumes (like RV) and capacities (like FRC and TLC) require special techniques such as body plethysmography, inert gas dilution, or nitrogen washout, whereas others can be measured during standard spirometry 3
Key Clinical Applications
The distinction matters clinically because 2, 4:
- TLC measurement is essential for confirming true restriction when VC or FVC is reduced
- FRC changes are particularly important in obesity (where it decreases exponentially) and in monitoring response to bronchodilators
- RV and TLC increases are characteristic of hyperinflation in obstructive diseases like COPD
- Body size, sex, and age are the most important determinants of both volumes and capacities, with height being proportional to TLC, VC, RV, FVC, and FEV₁ 2, 5
Common pitfall: Clinicians sometimes use VC or FVC as if they represent total lung volume, but these are capacities that comprise only part of TLC - they exclude the residual volume that remains after maximal exhalation 4.