Vital capacity is defined as the maximum volume of air that can be exhaled following a maximum inhalation. It is one of several lung volumes that physiologists measure to understand how the respiratory system works.

The four primary lung volumes are: tidal volume — the air moved in a normal, relaxed breath (approximately 0.5 litres); inspiratory reserve volume — the extra air you can inhale beyond a normal breath (approximately 3 litres); expiratory reserve volume — the extra air you can push out beyond a normal exhale (approximately 1.1 litres); and residual volume — the air that permanently remains in the lungs after maximum exhalation (approximately 1.2 litres).

Vital capacity = tidal volume + inspiratory reserve + expiratory reserve. It is the total range of air you can voluntarily move. It does not include residual volume, because residual volume cannot be controlled — it is set by the elastic properties of the lung tissue itself. If the lungs fully collapsed with every exhale, reinflating them would require enormous muscular effort and would damage the delicate alveoli. The residual volume keeps them slightly inflated at all times.

Your lungs contain approximately 600 million alveoli — tiny air sacs with walls only one cell thick. The total surface area of all these alveoli is approximately 70 square metres — the size of a singles tennis court. This enormous surface area allows rapid gas exchange: oxygen diffuses from the air in the alveoli into the blood, and carbon dioxide diffuses in the opposite direction, in the time it takes for a red blood cell to pass through a capillary.

Vital capacity is influenced by height (taller chest cavity), age (peaks around age 20–25), biological sex (males average slightly more due to larger chest size), aerobic fitness (respiratory muscles strengthen with training), and specific training such as swimming or wind instrument playing. Doctors use spirometry — a precise clinical measurement of lung volumes — to diagnose conditions including asthma, COPD, and pulmonary fibrosis, all of which reduce vital capacity in characteristic ways.