The energy used in overcoming compliance during lung expansion is stored as potential energy in the elastic tissues. This energy is used to generate the pressure gradient needed for expiration, which is a passive process under normal circumstances.

In contrast, when gas flows through the conducting airways, the forces involved in overcoming resistance are frictional, both for movement of air molecules and lung tissues sliding on each other. This energy is dissipated as heat and is therefore expended by the body to achieve ventilation – the work of breathing.

On a pressure-volume curve, this is seen as a different pathway for inspiration and expiration, known as hysteresis. The work of breathing is represented by the area contained within the ellipse.