22.3: Lung Capacity
Lung size can vary, depending on experience and body size. The amount of air that can enter or leave the lungs is measured as four volumes—single functions like inhalation—and four capacities—comparative amounts of multiple volumes.
The amount of air normally inhaled and exhaled is called the tidal volume, whereas the air that can be further breathed in or out after normal inhalation or exhalation is referred to as the inspiratory and expiratory reserve volumes.
The fourth volume is the amount of air that remains in the lungs after the expiratory reserve volume—known as the residual reserve volume—the small amount of air that doesn’t leave the lungs, keeping the lungs inflated.
This amount combined with the other volumes is the total lung capacity, the entire amount of air that the lungs can hold, approximately six liters!
Within a single respiratory cycle, the maximal amount of air that can be moved in or out is the vital capacity, essentially a summation of the tidal and both reserve volumes.
The remaining capacities are the inspiratory capacity—how much can be inhaled after normal exhalation—and the functional residual capacity—how much air remains after breathing out. Ultimately, these measurements are used to diagnose lung diseases, like asthma—where more air remains in the lungs, taking longer for exhalation—or pulmonary fibrosis, where volume is rapidly emptied, resulting in being short of breath.