Respiratory Volumes and Capacities

Respiratory volumes define the varying amounts of air that enter and exit the lungs, while respiratory capacities are combinations of respiratory volumes.

These can be measured using a spirometer and, shown here, is a spirographic record of a healthy adult.

The volume of air exchanged during normal breathing is called tidal volume or TV.

After normal breathing, the air that can be forcefully inhaled forms the inspiratory reserve volume or IRV.

In contrast, the air forcefully exhaled post-normal breathing is the Expiratory Reserve Volume or ERV, with the remaining air termed as Residual Volume or RV.

Inspiratory capacity, calculated by summing IRV  and TV, represents the maximum air intake possible after a normal exhale.

The functional residual capacity, the summing of ERV and RV, is the air remaining in the lungs after normal breathing.

Vital capacity, the sum of IRV, TV, and ERV is the total amount of exchangeable air in the lungs.

Total Lung Capacity, the sum of IRV, TV, ERV, and RV is the maximum volume of air the lungs can hold.