Combined pressure-volume curves for the lung and chest wall

Pressure-volume curves for the lungs are obtained when a patient breathes in and out of a spirometer. When the spirometer valve is closed and the patient relaxes the respiratory muscles, this pressure is recorded as relaxation pressure. The slope of each curve within the pressure-volume curve is compliance (ΔVolume/ΔPressure). Functional residual capacity is the resting equilibrium volume of the combined lung and chest-wall system. When the volume in the system is less than functional residual capacity, there is less volume in the lungs and the collapsing force of the lungs is smaller. When the volume in the system is greater than the functional residual capacity, there is greater volume in the lungs and the collapsing force is greater.