Fig. 18:  In normal circumstances, air escapes from the lung when the pleural pressure increases.  The pressure in the alveoli is still higher than the pleural pressure due to summation of pleural pressure and elastic retraction pressure from the lung tissue.  Along the bronchi, the pressure of alveoli decreases due to the airway resistance.  At one specific point, the equal pressure point, the pressure inside the bronchi equals the pleural pressure, and more central the pressure inside the bronchi is lower than pleural pressure leading to compression of airways.  Further increase in pleural pressure by ongoing contraction of expiratory muscles has no effect on expiratory flow. 

In patients with COPD, the equal pressure point due to instability of airways is shifted to the lung periphery.  If a positive expiratory pressure (PEP) by CPAP or PEP interventions is applied, the equal pressure point can be shifted back to the more central airways where the bronchial walls contain more cartilage that itself stabilizes the airway and thereby supports coughing.