Lung compliance is defined as the ratio of change in volume to change in pressure, written as which expression?

• A. ∆V/∆P
• B. ∆P/∆V
• C. V/∆P
• D. ∆V×∆P

Answer: (A)

Lung compliance describes how easily the lungs expand when pressure changes. It is defined as the change in volume per unit change in pressure, which is written as ΔV/ΔP. This ratio is the slope of the pressure–volume relationship: a larger ΔV for a given ΔP means higher compliance (easier to inflate), while a smaller ΔV means lower compliance (stiffer lungs).

If you used ΔP/ΔV, you’d be looking at elastance (stiffness), the inverse concept of compliance. Using V/ΔP uses an absolute volume rather than how much the volume actually changes, so it doesn’t quantify distensibility. ΔV×ΔP would mix units and isn’t a ratio, so it doesn’t measure compliance either.