compliance
Compliance describes how easily the lungs stretch — how much they expand for a given push of pressure. A highly compliant lung is like a soft, well-worn balloon that puffs up with the gentlest blow; a low-compliance lung is like a thick, stiff new balloon that fights every breath.
Defined precisely, it is the change in lung volume divided by the change in distending (transpulmonary) pressure. High compliance means a large volume gain for little pressure; low compliance means it takes a lot of pressure to gain a little volume.
Compliance is not always good in large amounts. The healthy lung balances compliance against elastic recoil: enough stretchiness to fill easily, but enough recoil to empty and to hold the airways open. Surfactant raises compliance by lowering alveolar surface tension.
Disease pushes compliance both ways. It falls in restrictive disease — fibrosis, pulmonary oedema, ARDS — where stiff or wet lungs resist expansion, so breathing demands more pressure and effort. It rises in emphysema, where destruction of elastic tissue makes the lung floppy and easy to inflate but very hard to empty.
In ARDS the lungs become so stiff that a ventilator must use higher pressures to deliver even small tidal volumes; low compliance is a hallmark of the syndrome.
Low compliance defines stiff-lung syndromes.
Compliance and elastic recoil are two sides of one coin: a stiff lung has low compliance and strong recoil; a floppy emphysematous lung has high compliance and weak recoil. Each describes the same elastic property from a different angle.