Different degrees of effort in breathing result in different volumes of air being moved in or out of the lungs. The measurement of such air volumes is called spirometry, and these measurements reveal the presence of four distinct respiratory volumes. For example, the amount of air that enters the lungs during a normal, quiet inspiration is about 500 cubic centimeters (cc). Approximately the same amount leaves during a normal expiration. This volume is termed the tidal volume.
During forced inspiration, a quantity of air in addition to the tidal volume enters the lungs. This additonal volume is called the inspiratory reserve volume (complemental air), and it equals about 3,000 cc.
During forced expiration, about 1,100 cc of air in addition to the tidal volume can be expelled from the lungs. This quantity is called the expiratory reserve volume (supplemental air). Even after the most forceful expiration, however, about 1,200 cc of air remains in the lungs. This volume is called the residual volume.
Residual air remains in the lungs at all times, and consequently, newly inhaled air is always mixed with the air that is already in the lungs. This prevents the oxygen and carbon dioxide concentrations in the lungs from fluctuating excessively with each breath.
Once the respiratory volumes are known, four repiratory capacites can be calculated by combining two or more of the volumes. Thus, if the inspiratory reserve volume (3,000 cc) is combined with the tidal volume (500 cc) and the expiratory reserve volume (1,100 cc), the total is termed the vital capacity (4,600 cc). This capacity is the maximum amount of air a person can exhale after taking the deepest breath possible.
The tidal volume (500 cc) plus the inspiratory reserve volume (3,000 cc) gives the inspiratory capacity (3,500 cc), which is the maximum volume of air a person can inhale following the exhalation of the tital volume. Similarly, the expiratory reserve volume (1,100 cc) plus the residual volume (1,200 cc) equals the functional residual capacity (2,300 cc), which is the volume of air that remains in the lungs following the exhalation of the tital volume.
The vital capacity plus the residiual volume equals the total lung capacity (about 5,800 cc). This total varies with age, sex, and body size.
Some of the air that enters the respitory tract during breathing fails to reach the alveoli. This volume (about 150 milliliters) remains in the passageways of the trachea, bronchi, and bronchioles. Since gas exchanges do not occour through the walls of these passages, this air is said to occupy anatomic dead space.