Fig. 8.

A simple model of alveolar collapse and recruitment showing residual V_i⁰ and fractional δV_i⁰ volumes for the ith representative alveolar unit. Postatelectasis states are shown as V_i¹ and δV_i¹, respectively. A collapsed airway unit is described by both V_i¹ → 0 and δV_i¹ → 0, with nominal boundary conditions: FRC^s ≈ Σ V_i^s and V_T^s ≈ Σδ V_i^s, where FRC is functional residual capacity, V_T is tidal volume, and s is the state of interest. The underlying assumptions are V_T⁰ = V_T¹ and V_i⁰ = V_i¹ for aerated alveoli. When a fraction of alveoli is collapsed, for a fixed V_T the patent airspaces undergo a proportional increase in size due to the redistribution of inspired gas. Traction forces due to alveolar interdependence may also cause airspace enlargement in the presence of atelectasis.