Figure 6.

Model of trans-septal forces. (A) Model of septum between two alveoli in normal, air-filled lung. In each alveolus, airway pressure is indicated as P_alv. A thin liquid layer with liquid pressure (P_liq) lines each alveolus. With all forces balanced across the septum, the septum is planar. Radius (R) of air–liquid interface in each alveolus is thus infinite. (B) Model of septum between an air-filled alveolus (1) and an edematous alveolus (2). In each alveolus, airway pressure is indicated as P_alv. Because of the presence of a meniscus, however, R₂ in the edematous alveolus is less than R₁ in the air-filled alveolus. According to the Laplace relationship, a greater pressure drop across the air–liquid interface occurs in the edematous than in the air-filled alveolus. Thus, liquid phase pressure in the edematous alveolus (P_liq2) is lower than P_liq1 in the air-filled alveolus. The difference in liquid phase pressures acting across the septum displaces the septum toward the edematous alveolus. Thus, liquid-filling diminishes the edematous alveolus, and expands the adjacent air-filled alveolus.