Beyond a certain effort, higher pleural pressure compresses airways and flow cannot increase
Expansion
During forced expiration, pleural pressure becomes positive and is applied to both the alveoli and the airways. As gas flows towards the mouth, airway pressure falls progressively.
At some point along the airway, the pressure inside equals the pleural pressure outside: the equal pressure point. Downstream of this, the airway is compressed.
Consequences:
- Beyond a modest effort, further effort raises both the driving pressure and the compressing pressure equally, so flow does not increase. Expiratory flow at low lung volumes is effort independent
- Flow is then determined by elastic recoil and upstream airway resistance
In emphysema, loss of recoil and loss of the tethering that holds small airways open move the equal pressure point towards the alveoli, so airways collapse early and gas is trapped. Pursed lip breathing creates back pressure that keeps airway pressure above pleural pressure for longer, splinting them open.