Respiratory Volumes and Capacities

A person's breathing can be measured and broken down into a standard set of respiratory volumes and capacities, most directly recorded using an instrument called a spirometer, which traces the volume of air moved with each breath over time. These measurements give a precise, quantitative picture of how much air the lungs move under normal, quiet breathing, how much further air remains available as a reserve for forced breathing, and how much air the lungs can never fully expel even after the most forceful possible expiration.

Four basic volumes, none of which overlaps with any other, together account for the full range of lung filling. The tidal volume (TV) is simply the volume of air inspired or expired during one normal, quiet breath at rest, and is the smallest and most frequently repeated of the four, at approximately 500 mL in a healthy adult. Beyond this normal tidal breath, a person can, by inspiring forcibly, draw in a further substantial volume of air -- the inspiratory reserve volume (IRV), approximately 2500 to 3000 mL -- considerably larger than the tidal volume itself, and reflecting how much further the lungs can still be filled beyond an ordinary quiet breath. Symmetrically, after a normal, quiet expiration, a person can forcibly expire a further volume of air still remaining in the lungs -- the expiratory reserve volume (ERV), approximately 1000 to 1100 mL. Even after the most forceful possible expiration, however, the lungs are never completely emptied: a certain volume of air, the residual volume (RV), approximately 1100 to 1200 mL, always remains trapped within the alveoli and airways, chiefly because the alveoli do not fully collapse and because the airways themselves retain some air within their own volume; this residual air is essential, since it keeps the alveoli permanently expanded and available for immediate gas exchange even between breaths, rather than repeatedly collapsing and having to be forcibly reopened. …