Content on this page requires a newer version of Adobe Flash Player. In this chapter we will model how changes in airway structure affect the resistance to flow. For the purpose of modelling the intrathoracic or lower airways can be schematically depicted as structures with the following components:
Consult the animation on the left for further information. Shortly we will go into the role of lung elastic recoil.
To better understand the mechanical behavior of airways it helps to model the various constituents within the airway wall. We can model the airway as a circular structure with an inner and an outer diameter. The airway wall thickness can be increased internal to the smooth muscle layer due to inflammatory changes: cellular infiltration, hypertrophy, hyperplasia, edema, vascular engorgement, and by the accumulation of mucus at the inner surface of the airway. Increased thickness of the airway wall is usually due to smaller internal diameter and hence associated with increased airways resistance. However, outer airway diameter may also increase due to swelling of the adventitia external to the smooth muscle layer. The alveolar attachments confer stability to the airway wall as they pull the airway wall outwards. When the airway muscle contracts, the muscles are loaded by the elastic recoil of the surrounding tissues, or the subatmospheric pleural pressure, which exert a tethering force on intrapulmonary airways. Thus the surrounding structures can be depicted as springs inserting on the outer airway wall.