Figure 17-11 Relationship between surface tension (T), alveolar radius (R), and alveolar transmural pressure (P). The left side of the diagrams shows the starting condition. The right side of the diagrams shows the expected result in alveolar size (using the Laplace equation to calculate the starting pressure). In the upper example (A), the surface tension in the fluid lining both the large and the small alveolus is the same (no surfactant). Accordingly, the direction of gas flow is from the higher-pressure small alveolus to the lower-pressure large alveolus, which results in one large alveolus (R_final = ΣR_initial ). In the bottom example (B), the expected changes in surface tension when surfactant lines the alveolus are shown (less tension in the smaller alveolus). The direction of gas flow is from the larg er alveolus to the smaller alveolus until the two alveoli are of equal size and are volume stable (R_K ). K, constant; ΣR, sum of all individual radii.