Let us briefly review what happens in airways during a forced expiration, making use of a simplified model.
- The alveolar pressure rises rapidly to a high level, often to 10 kPa, and alveolar gas flows towards the mouth.
- From the alveoli towards the mouth the total cross-sectional area of airways diminishes rapidly (see the animation), so that gas molecules accelerate. Acceleration implies greater kinetic energy, and is associated with a pressure drop.
- When the pressure drop along the airways equals the elastic recoil pressure of the lung, the pressure inside the airway at that point equals Palv - Pel, i.e. pleural pressure. Hence pressure inside and outside the airway are the same.
- Further downstream from this equal pressure point pressure outside the airway exceeds inside pressure, so that the airway is compressed.
- Airway compression leads to a check-valve within the airway, where extra expiratory effort will not lead to larger flows.
We can use this mechanism to study growth of lungs and intrathoracic airways, as we will now demonstrate.