The mechanics of a forced expiration

By taking a deep breath and then exhaling with maximal force, we apply force to the thoracic cavity and its contents: pleural and alveolar pressure increase to well above barometric pressure, and gas is expelled from the lung. The energy required to overcome resistance to flow translates into a pressure drop. In the simplest case (that of laminar flow) the pressure drop (dP) is proportional to airways resistance (Raw) and flow (V'):

dP = V'·Raw

Going from the alveoli to the mouth, the pressure falls steadily; it is high in the alveoli (Palv), falls steadily within bronchi (Pbr) to the level of mouth pressure and subsequently barometric pressure (Pbar). The pressure outside intrathoracic airways is pleural pressure, the pressure outside extrathoracic airways approximates barometric pressure.

It follows that during a forced expiration all intrathoracic airways are exposed to a high outside (pleural) pressure, whilst pressure within airways drops from a high level near the alveoli to a low level near the thoracic outlet. The intrathoracic airways can be divided into three sections:

1. A section nearest the alveoli where bronchial pressure exceeds outside pressure: the pressure difference expands the airways.
2. A point where intra- and extrabronchial pressures are equal (‘equal pressure point’); the pressure drop from the alveoli at this point equals Palv – Pbr = Palv – Ppl = PL,el.
3. A section nearest the mouth where bronchial pressure is lower than outside pressure: the airway will be compressed.

Airway compression always occurs first in the trachea. The horseshoe shaped cartilaginous rings have a dorsal membranous part, which easily gives way to pressure. The cartilaginous rings bend in the process, so that a slit-like airway cross-section remains. This narrow airway segment functions as a check valve where the speed of gas molecules is limited to the speed at which a wave can propagate in the airway wall, and no longer governed by the pressure difference from alveoli to mouth. This resembles a waterfall, where the amount of water falling down is similarly independent of height of the waterfall. The creation of an intrathoracic check valve (see literature) is demonstrated in the accompanying animation.
 

Back Next