lung volume measurement

Lung volume measurement quantifies the static compartments of air in your chest — not just the air you can blow out, but the air that always stays behind. Picture the lungs as a set of nested containers: the breath you take, the reserve you can still squeeze out, and the residue you can never empty.

Spirometry alone cannot see the leftover air, because by definition it never reaches the mouth. To capture it, labs use either gas-dilution methods — breathing in a tracer gas like helium and measuring how much it gets diluted — or body plethysmography. From these, residual volume, functional residual capacity and total lung capacity are derived.

These static volumes are what confirm true restriction (a low total lung capacity) and quantify hyperinflation and air trapping in obstruction (a raised residual volume). They turn a suggestive spirogram into a definite physiological diagnosis.