When the chest wall is relaxed (and only when it is relaxed), it behaves as an elastic container, similar to the lung. The pressure difference across the chest wall, Pw (pleural pressure minus the pressure at the body surface), determines its size. In the absence of any pressure difference (Ppl=0 and Pw=0), the chest wall would come to itsunstressed volume, which is roughly 75% of Total Lung Capacity. When the pleural pressure is below atmospheric (Pw is negative), the chest wall is pulled inward.
When the pleural pressure rises aboveatmospheric pressure (Pw is positive), the chest wall bows out. Again, however, it is important to understand that these conditions only hold when the chest wall muscles are relaxed (during quiet expiration, certain states of paralysis or anesthesia, and whenthe respiratory muscles are consciously relaxed). The pressure difference across the chest wall will have no relationship to its size if the respiratory muscles are being used either to move the chest or to keep it at a particular volume.
In certain disease states the chest wall may stiffen. In such cases, a restrictive ventilatory defect may result.