vital capacity
Vital capacity is the biggest single breath you can take in and let out: fill your lungs completely, then blow out everything you can. It is the full usable swing of your breathing, from brimful to as empty as muscles can make it.
In numbers, it is the sum of three volumes — the inspiratory reserve, the tidal volume, and the expiratory reserve. It does not include the residual volume, the air that always stays behind because the lungs and chest can never be fully emptied.
Vital capacity depends on body size, age, sex, and posture; it is largest in tall young adults and declines slowly with age. Clinicians compare a person's measured value to predicted values for someone of the same age, height, sex, and ethnicity.
A falling vital capacity is a useful warning sign across very different problems: scarred or stiff lungs that cannot expand, fluid or air pressing on the lung from outside, or weak respiratory muscles (as in some neuromuscular diseases) all reduce it, which is why it is tracked over time in those conditions.
A healthy young man might have a vital capacity around 4.8 litres, while a much smaller measured value in someone with pulmonary fibrosis reflects lungs too stiff to expand fully.
Vital capacity falls when lungs cannot expand.
When measured during a forced, maximal blow it is called forced vital capacity (FVC), the value spirometry reports. A slow, relaxed exhalation can give a slightly larger slow vital capacity, especially in obstructive disease where forced effort prematurely collapses airways.