Figure 4

A Schematic Showing the Relationship between Lung SA/V and Density under two scenarios. The top panel represents normal lung architecture with the dimensions of each "alveolus" being 100 × 100 μm yielding a total volume of the "lung" = 16,000 μm³ with a surface area of 6,400 μm² and a SA/V of 0.4. If we assign a mass of 10 units to each 100 μm length of "alveolar wall" this "lung" has a mass of 400 units and a density of 0.025 units/μm³ (= 400 units/16,000 μm³). In A, the volume and thickness of the "alveolar walls" remains the same as those in "normal lung architecture" but the surface area is decreased due to destruction of "alveolar walls". In this scenario, the reduction in SA/V and density are proportional. However in scenario B, the thickness of the "alveolar walls" is doubled therefore increasing the mass. The resultant SA/V is the same as in A whereas the density is higher than in A and even higher than the Normal. Thus if there is addition of tissue, the relationship between SA/V and density is disrupted.