Respiratory Volumes and Capacities

When we breathe in and out, the amount of air moved is not always the same — a quiet breath moves only a little air, while a forced breath moves a great deal more. Measuring these amounts (using an instrument called a spirometer) tells doctors how healthy the lungs are, so respiratory physiology defines a set of standard respiratory volumes and, by combining them, a set of respiratory capacities.

The four respiratory volumes

• Tidal Volume (TV): the volume of air breathed in or out during a single normal, resting breath — about 500 mL. A healthy adult breathes roughly this much 12 to 16 times a minute, moving 6 to 8 litres of air per minute.
• Inspiratory Reserve Volume (IRV): the extra air that can be forcibly breathed IN over and above the tidal inspiration — about 2500 to 3000 mL.
• Expiratory Reserve Volume (ERV): the extra air that can be forcibly breathed OUT after a normal expiration — about 1000 to 1100 mL.
• Residual Volume (RV): the air that always remains in the lungs even after the most forceful expiration, so the lungs never fully empty — about 1100 to 1200 mL.

The respiratory capacities are useful sums of two or more of these volumes:

• Inspiratory Capacity (IC): the total air a person can breathe in after a normal expiration — the tidal volume plus the inspiratory reserve volume.
• Expiratory Capacity (EC): the total air a person can breathe out after a normal inspiration — the tidal volume plus the expiratory reserve volume.
• Functional Residual Capacity (FRC): the air that remains in the lungs after a normal expiration — the expiratory reserve volume plus the residual volume. …