inspiratory reserve volume

Inspiratory reserve volume is the extra air you can still pull in after a normal breath in — the gap between an ordinary inhale and a maximal one. Take a quiet breath, then keep going as deep as you can: that additional gulp is the reserve.

It is the largest of the breathing subdivisions, roughly 2.5 to 3 litres in a healthy adult — far bigger than the tidal volume it sits on top of. This generous headroom is what lets you ramp up breathing dramatically during exertion.

During exercise the body dips into this reserve, deepening each breath so that tidal volume expands toward total lung capacity while the reserve shrinks. The inspiratory reserve is therefore a measure of how much spare capacity you can recruit on demand.

It is calculated, not directly measured: it equals the inspiratory capacity minus the tidal volume. Conditions that limit how far the lungs can expand — stiff fibrotic lungs, or hyperinflated lungs already sitting near their ceiling — eat into this reserve and limit exercise tolerance.