Anatomy.—The interstitium consists of a continuum of connective tissue throughout the lung comprising three subdivisions: (a) the bronchovascular (axial) interstitium, surrounding and supporting the bronchi, arteries, and veins from the hilum to the level of the respiratory bronchiole; (b) the parenchymal (acinar) interstitium, situated between alveolar and capillary basement membranes; and (c) the subpleural connective tissue contiguous with the interlobular septa (73). [Fleischner Society]