The four volumes (RV, ERV, TV, IRV) are the building blocks; the four capacities (TLC, VC, IC, FRC) are their sums. Pick a disease pattern below and watch the spirogram, capacity bars, and arrows update together — 14 patterns from normal to upper airway obstruction.
Four volumes (RV, ERV, TV, IRV) are the non-overlapping building blocks. Four capacities (TLC, VC, IC, FRC) are combinations: TLC = all four · VC = IRV + TV + ERV · IC = IRV + TV · FRC = ERV + RV. Only volumes that move with breathing can be measured by spirometry — RV and FRC require plethysmography or gas dilution.
A lung volume is a single, non-overlapping compartment (RV, ERV, TV, IRV). A capacity is a sum of two or more volumes — TLC = all four, VC = IRV + TV + ERV, IC = IRV + TV, FRC = ERV + RV.
Spirometry only measures air that moves in and out. RV is the gas remaining after a maximal exhale — you can't blow it out, so a spirometer can't count it. Measuring it requires body plethysmography or gas dilution (helium or nitrogen washout).
Air trapping means RV rises — gas is left behind after expiration because small airways collapse before the lungs fully empty. An RV/TLC ratio above 35% suggests trapping; above 50% is severe. COPD and acute asthma both show this.
FRC is the passive equilibrium point where inward lung recoil equals outward chest-wall recoil — no muscle effort is required to sit at FRC. Inspiratory weakness lowers TLC and expiratory weakness raises RV, but the resting position stays roughly normal.
Both show a reduced TLC and a preserved FEV1/FVC ratio. DLCO is the key: ILD destroys the alveolar membrane, so DLCO falls; chest-wall and neuromuscular restriction leave the membrane intact, so DLCO is normal or near-normal.