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". Eur Respir J 1999; 13: 468±472. This article reviews definitions of human lung volumes and the mechanisms that set them, in the context of pulmonary function testing. It uses and reconciles existing conventions and terminology [1, 2] and notes some common semantic and conceptual difficulties."Lung volume" generally means the volume of gas or of gas-containing spaces in the lung, or one or more of the "lung volume compartments" which they comprise. Gas in the lung is considered to be at body temperature, saturated with water vapour, and (for many purposes) at ambient pressure.To start, three primary lung volumes, set independently by different mechanisms will be considered: total lung capacity (TLC), residual volume (RV), and functional residual capacity (FRC). Total lung capacityTLC in cooperating humans is the greatest lung volume achieved by maximum voluntary inspiration. It is set by a static balance between inspiratory muscle forces and elastic recoil forces arising in the respiratory system. At TLC, these two sets of forces are equal and opposite in sign. Thus, TLC can be thought of as lying at the intersection of the static volume±pressure curves of the relaxed respiratory system and the maximally active inspiratory muscles ( fig. 1) [3]. It is the lung, rather than the chest wall,

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