ABSTRACT
Both the lungs and the surrounding chest wall are elastic structures, i.e. they tend to return to their previous configuration when a distending force is removed.At the end of a quiet expiration in a normal subject, the volume of the lungs (functional residual capacity, FRC) represents the mechanically ‘neutral’ position of the respiratory system as a whole. The respiratory muscles are relaxed and there is no net force acting across the combined lungs and chest well, i.e. the pressure within the alveoli is the same as the external atmospheric pressure. There is, however, a balance of passive forces acting across the lungs and chest wall individually. Functional residual capacity corresponds to the neutral (relaxed) position of the overall respiratory system provided that the lungs and chest wall are coupled together, but neither component is at its own unstressed neutral volume. When a surgeon opens the chest or if a pneumothorax occurs, the lungs collapse due to their inward (positive) recoil and the ribs spring apart because at FRC the recoil of the chest wall is outwards (negative). The introduction of air into the pleural space thus uncouples the lungs from the chest wall and allows each to approach its own neutral volume. Functional residual capacity is the volume adopted by the relaxed intact respiratory system
because inward recoil of the lungs is exactly balanced by outward recoil of the chest wall.
