Dihedral angles of septal "bend" structures in lung parenchyma

Alveolar parenchyma comprises two interacting tensile systems: the cable system (a network of linear condensations of connective tissue) and the membrane system (a network of quasiplanar alveolar septa). Inferences can be drawn about the mechanics of this structure from it configuration. We reported earlier (E.H. Oldmixon, J.P. Butler, and F.G. Hoppin, Jr. J. Appl. Physiol. 64: 299-307, 1988) that the angles between alveolar septa at the common three-way junctions (J) are nearly uniform, indicating that septal tensions are also nearly uniform. We now report on the interseptal angles at the next most common class of septal junction (B), a structure where two septa meet along a segment of the cable system. We find, first, that the distributions of interseptal angles at B junctions have means > 120 degrees, are narrow, and have few, if any, angles < 120 degrees. The findings of uniform 120 degrees angles at J junctions and a cutoff below 120 degrees at B junctions are also characteristic of soap films supported on a frame, which follows the physical principle of surface area minimization. We suggest that this principle may be operative in parenchymal development and remodeling.