Compliance of the combined lungchest wall system Figure

a. At rest (identified by the filled circle in the center of Figure 4-3), lung volume is at FRC and the pressure in the lungs is equal to atmospheric pressure. Under these equilibrium conditions, there is a collapsing force on the lungs (positive transmural pressure) and an expanding force on the chest wall (negative transmural pressure). At FRC, these two forces are equal and opposite.

Combined lung and chest wall

Transmural pressure

Figure 4-3. Compliance of the lung and chest wall separately and together. FRC = functional residual capacity.

b. As a result of these two opposing forces, intrapleural pressure is subatmospheric, or negative.

c. If air is introduced into the intrapleural space (pneumothorax), the intrapleural pressure becomes equal to atmospheric pressure. The lung will collapse (its natural tendency) and the chest wall will spring outward (its natural tendency).

d. Figure 4-3 shows the pressure-volume relationships for the lung alone (hysteresis has been eliminated for simplicity), the chest wall alone, and the lung and chest wall together.

- Compliance of the lung-chest wall system is less than that of the lungs alone or the chest wall alone (the slope is flatter).

e. Changes in lung compliance (Table 4-1)

- In a patient with emphysema, lung compliance is increased and the tendency of the lungs to collapse is decreased. Therefore, at the original FRC, the tendency of the lungs to collapse is much less than the tendency of the chest wall to expand. The lung-chest wall system will seek a new, higher FRC so that the two opposing forces can be balanced; the patient's chest becomes barrel-shaped to provide this new volume.

-In a patient with fibrosis, lung compliance is decreased and the tendency of the lungs to collapse is increased. Therefore, at the original FRC, the tendency of the lungs to collapse is greater than the tendency of the chest wall to expand. The lung-chest wall system will seek a new, lower FRC so that the two opposing forces can be balanced.

Table 4-1. Causes of Increased and Decreased Lung Compliance

Causes of I Lung Compliance

Causes of T Lung Compliance

High expanding pressures T pulmonary venous pressure Fibrosis (deposition of collagen) Lack of surfactant

Emphysema (4- elastic fibers) Age

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