- The alveolar pressure becomes greater (i.e., becomes positive) because alveolar gas is compressed by the elastic forces of the lung.
- Thus, alveolar pressure is now higher than atmospheric pressure, the pressure gradient is reversed, and air flows out of the lungs.
b. Intrapleural pressure returns to its resting value during a normal (passive) expiration.
- However, during a forced expiration, intrapleural pressure actually becomes positive. This positive intrapleural pressure compresses the airways and makes expiration more difficult.
- In chronic obstructive pulmonary disease (COPD), in which airway resistance is increased, patients learn to expire slowly with "pursed lips" to prevent the airway collapse that may occur with a forced expiration.
c. Lung volume returns to FRC.
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