Recall that spirometry measures the amount of usable air in the lungs. This is the vital capacity. And because the subject is asked to blow out as hard and fast as possible, the value obtained is the forced vital capacity.
Note that spirometry cannot measure residual volume, functional residual capacity, or total lung capacity since residual volume cannot be exhaled. These values need to be obtained by other means in a pulmonary function lab.
Recall that part of spirometry is determining how fast the subject is able to exhale (or blow out) their vital capacity. The primary reason this is measured is because in certain diseases such as COPD and asthma, the patient’s ability to exhale is compromised.
There are generally three or four measures of a subject’s ability to exhale hard and fast.