Membranous bronchioles and connective tissue network of normal and emphysematous lungs.

Three-dimensional reconstructions of the septal system of normal human lungs revealed that airways course within the interlobular septa, i.e., between the two blades formed by the peripheral boundaries of adjacent lobuli of whatever order, and enter the supplied pulmonary unit at its side. This is not in keeping with the classic view of a peripheral airway in the center of a lung unit and submitted to radial traction by attached alveolar septa. The basic design of the lung fibrous scaffold appears to be in conformity with the laws of fractal geometry. Similar reconstructions in centrilobular emphysema disclosed tortuosities of both intra-acinar and interlobular septa, with consequent distortions of the corresponding intraseptal bronchioles and collapse of lung units of different sizes. It is suggested that in centrilobular emphysema competition for space, besides intrinsic airways narrowing because of inflammation and loss of elastic recoil, is a cause of flow limitation.