Δ Volume / Δ pressure of alveoli **factors that ↑ compliance** - age - emphysema **decrease** - alveolar oedema - pulm fibrosis - unventilated lung (de-recruitment) - atelectasis - loss of surfactant (eg [[ARDS]]) From a ventilator perspective: Compliance of resp system = Tidal Volume/ Driving pressure ∴ , in a patient with more severe ARDS with ↓ compliance, will take more pressure to get a given tidal volume