Compliance is defined as which of the following?

• A. Volume Change / Time Change
• B. Volume Change / Pressure Change
• C. Pressure Change / Time Change
• D. Flow Change / Volume Change

Answer: (B)

Compliance describes how much a vessel’s volume changes for a given change in pressure. In practice, it is defined as ΔV/ΔP, the change in volume per unit pressure change. That’s why Volume Change divided by Pressure Change is the best description. A vessel with high compliance expands readily with small increases in pressure, while a stiff vessel has low compliance and requires a larger pressure change for the same volume increase. The units are typically mL per mmHg. This is the inverse of elastance (ΔP/ΔV), which reflects stiffness rather than distensibility. The other options mix in time or flow, which are not part of the definition of compliance.