Human lung volumes and the mechanisms that set them.

Definitions of human lung volumes and the mechanisms that set them are reviewed in the context of pulmonary function testing, with attention to the distinction between functional residual capacity (FRC) and the static relaxation volume of the respiratory system, and to the circumstances in which FRC and residual volume are set by dynamic rather than by static mechanisms. Related terms, conventions, and issues are addressed, including some common semantic and conceptual difficulties, with attention to "gas trapping", "hyperinflation", and "restriction".