During Expiration:
1. The respiratory muscles relax, causing an abrupt increase in pleural pressure to a less negative value.
2. Because lung volume has not yet changed, the recoil pressure of the lung must remain the same, so the rise in pleural pressure causes the alveolar pressure to rise by the same amount.
3. This establishes a pressure gradient from the alveoli to the mouth, down which air flows.
4. Lung and chest volume decrease as air flows out, causing lung recoil pressure to fall as well, until a new equilibrium is reached at FRC, the equilibrium volume.
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