- Compliance is measured as the change in volume for a given change in pressure. A structure that is very stiff (low elasticity) would show a very high resistance to deformity and is considered to have a low compliance. In a lung with a high compliance, a small pressure change would result in a large volume change and thus the work performed by the respiratory muscles would be less than in a lung with a low compliance. However, the expiratory force would be less in a lung with high compliance than in a lung with a low compliance.
- The lungs alone, when removed from the chest, are almost twice as distensible as the lungs and thorax together. This is because the chest has to be expanded together with the lungs and muscle have to expend energy to expand the lungs and the chest wall.
Any condition that destroys lung tissue, causes it to become fibrotic or edematous, blocks the bronchioles or in any way impeded lung expansion and contraction, causes decreased lung compliance. Obese people often have reduced chest cage compliance because abdominal fat impedes normal descent of the diaphragm and upward movement of the rib cage during inspiration.
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Compliance work is the work required to expand the lungs against the elastic forces.
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