The air in the lungs is measured in volumes and capacities. Lung volume measures reflect the amount of air taken in, released, or left over after a lung function, like a single inhalation. Lung capacity measures are sums of two or more lung volume measures.

There are four defined lung volume measures: tidal volume, inspiratory reserve volume, expiratory reserve volume, and residual volume.

Tidal volume is the amount of air inhaled and exhaled in a normal breath. Inspiratory (inhalation-related) reserve volume is the additional amount of air that can be inhaled after a regular inspiration. Expiratory (exhalation-related) reserve volume is the amount of air that can be exhaled after normal exhalation. Residual volume is the amount of air remaining in the lungs after forced exhalation (i.e., blowing).

There are four defined lung capacity measures: inspiratory capacity, functional residual capacity, vital capacity, and total lung capacity.

Inspiratory capacity is the volume of air that can be inhaled after normal exhalation. It is the sum of the two volume measures involving inhalation: tidal volume and inspiratory reserve volume. Functional residual capacity is the volume of air remaining in the lungs after normal exhalation and is equal to the sum of the expiratory reserve and residual volumes. Vital capacity is the maximum volume of air that can be exhaled after a maximal inhalation. It is the sum of the tidal, expiratory reserve, and inspiratory reserve volumes.

Total lung capacity is the total volume of air the lungs can hold. It is the sum of all four lung volume measures. Total lung capacity in humans is 6 liters in an average adult male and 4.2 liters in an average adult female! However, lungs do generally not operate at this maximal capacity.

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