Lung volumes are measured by spirometry. This instrument measures the volume taken during a breath. Tidal volume is the amount of air moving into and out of lungs in each breath. It is about 500 mL during quite breathing and can increase significantly with exercise.
Inspiratory reserve (IRV) is the additional volume that can be inspired above that of quiet breathing. It is the result of forced inspiration. Expiratory reserve (ERV) is the additional volume of air that can be forced out the lungs (forced expiration). Residual volume (RV) is the volume of air that remains in the lungs and cannot be expelled.
Lung capacities are sums of the individual volumes. Vital capacity, which is the sum of IRV + TV + ERV is a constant value that is commonly calculated.
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