Change in volume per unit change in pressure, determined by elastic tissue and surface tension
Expansion
Compliance = change in volume / change in pressure, normally about 200 ml per cm of water for the lung alone, and about 100 for lung and chest wall together.
Increased (floppier) in
- Emphysema, where elastin is destroyed
- Normal ageing
Reduced (stiffer) in
- Pulmonary fibrosis
- Pulmonary oedema and acute respiratory distress syndrome
- Atelectasis and surfactant deficiency
- Consolidation, and reduced chest wall compliance from obesity or kyphoscoliosis
Two determinants underlie it: the elastic tissue of the lung, and surface tension at the air-liquid interface, which contributes about two-thirds of total recoil and is reduced by surfactant.
Compliance is not constant: it is greatest in the mid-range of the pressure-volume curve and falls at both extremes, which is why lungs are hard to inflate when collapsed and hard to inflate further when already full. Tidal breathing normally sits on the steepest, most efficient part of the curve.