pressurevolume

Pressure-volume (PV) relationships describe how the pressure in a system varies with its volume. PV data are commonly shown on a pressure-volume diagram, a plot with pressure on the vertical axis and volume on the horizontal axis. The area under a process on the diagram represents the mechanical work exchanged with the surroundings during that volume change; the area enclosed by a closed loop corresponds to the net work over a cycle.

In thermodynamics, the state of a simple compressible system is described by an equation of state, such as PV = nRT for an ideal gas. Real substances require corrections. For any quasi-static process, the work done by the system as it changes volume is W = ∫ P dV. Common processes include isothermal (constant temperature), isobaric (constant pressure), isochoric (constant volume; no work), and adiabatic (no heat exchange; for an ideal gas, PV^γ = constant).

PV diagrams are used to analyze cycles in engines, compressors, and refrigerators. Cycles such as Carnot, Otto, and Diesel are represented as closed loops on the diagram; the path taken corresponds to successive compression and expansion stages, with the loop area giving the total work and, in many cases, informing efficiency and power considerations.

In physiology, a left ventricular pressure-volume loop plots heart pressure against volume during a cardiac cycle. The loop’s area equals stroke work, and the positions of end-diastolic and end-systolic points reflect preload, contractility, and afterload, providing a useful tool for assessing cardiac function and disease.