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Lung tissue has a natural elasticity. Left to its own devices, a lung would tend to shrink to little more than the size of a tennis ball. This can sometimes be observed radiographically in the context of a complete pneumothorax (see Chapter 16). The lung’s tendency to contract is counteracted by the semi‐rigid chest wall, which itself has a tendency to spring outward from its usual position. At the end of a normal tidal expiration, the two opposing forces are nicely balanced and no muscular effort is required to hold this ‘neutral’ position. Breathing at close to this lung volume (normal tidal breathing) is therefore relatively efficient and minimises work. It is rather like gently stretching and relaxing a spring from its neutral, tension‐free position. In some diseases (asthma or COPD), tidal ventilation is obliged to occur at higher lung volumes (see Chapter 3). Breathing then is rather like stretching and relaxing a spring that is already under a considerable degree of tension. The work of breathing is therefore increased, a factor that contributes to the sensation of breathlessness. Test this yourself: take a good breath in and try to breathe normally at this high lung volume for a minute.

The natural tendencies for the chest wall to spring outwards and the lung to contract down present opposing forces, which generate a negative pressure within the pleural space. This negative pressure (‘vacuum’) maintains the lung in its stretched state. Clearly, at higher lung volumes, the lung is at greater stretch and a more negative pleural pressure is required to hold it in position. The relationship between pleural pressure (the force on the lung) and lung volume can be plotted graphically (Fig. 1.5). The lung does not behave as a perfect spring, however. You may recall that the length of a spring is proportional to the force applied to it (Hooke’s law). In the case of the lung, as its volume increases, greater and greater force is needed to achieve the same additional increase in volume; that is, the lung becomes less ‘compliant’ as its volume increases. Lung compliance is defined as ‘the change in lung volume brought about by a unit change in transpulmonary (intrapleural) pressure’.

Figure 1.5 Graph of (static) lung volume against oesophageal pressure (a surrogate for intrapleural pressure). In both subjects A and B, we see that lung compliance – the change in lung volume per unit change in intrapleural pressure (or slope of the curve) – is reduced at higher lung volumes. A: normal individual. B: individual with reduced lung compliance, such as lung fibrosis.