Pulmonary function tests that measure lung volumes and capacities are performed to evaluate the mechanical function of the lungs. Lung volumes and capacities are related to a person’s age, weight, sex, and body position and are altered by disease. Two or more lung volumes, when combined, are described as a capacity. A basic understanding of these measurements and what the values reflect is useful for a therapist treating patients with pulmonary dysfunction.

Total Lung Capacity
Total lung capacity (TLC) is the total amount of air contained in the lungs after a maximum inspiration. TLC can be subdivided into four volumes: tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume. The vital capacity plus the residual volume equal the TLC, which is approximately 6000 mL in a healthy, young adult.

Tidal Volume
The amount of air exchanged during a relaxed inspiration followed by a relaxed expiration is called the tidal volume (TV). In a healthy, young adult, TV is approximately 500 mL per inspiration. Approximately 350 mL of the tidal volume reaches the alveoli and participates in gas exchange (respiration).

Inspiratory Reserve Volume
Inspiratory reserve volume (IRV) is the amount of air a person can breathe in after a resting inspiration (approximately 3000 mL).

Expiratory Reserve Volume
Expiratory reserve volume (ERV) is the amount of air a person can exhale after a normal resting expiration (approximately 1000 mL).

Residual Volume
Residual volume (RV) is the amount of air left in the lungs after a maximum expiration (approximately 1500 mL). RV increases with age and with restrictive and obstructive pulmonary diseases.

Inspiratory Capacity
Inspiratory capacity (IC) is the maximum amount of air a person can breathe in after a resting expiration (approximately 3500 mL).

Functional Residual Capacity
Functional residual capacity (FRC) is the amount of air remaining in the lungs after a resting (tidal) expiration (approximately 2500 mL). It is the sum of the ERV and RV. FRC represents the point during ventilation at which the forces that expand the thoracic wall are in balance with the forces that tend to collapse the lungs.

Vital Capacity
Vital capacity (VC) is the sum of the TV, IRV, and ERV. It is measured by a maximum inspiration followed by a maximum expiration (approximately 4500 mL). Vital capacity decreases with age and is less in the supine position than in an erect posture (sitting or standing). VC decreases in the presence of restrictive and obstructive diseases.

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