Morphometric Basis of the Sheet-Flow Concept of the Pulmonary Alveolar Microcirculation in the Cat

The purpose of this study was to describe quantitatively the geometric organization of the pulmonary alveolar capillary bed. Conventional representation of these vessels as a tight network of circular cylindrical tubes (tube-flow model) was contrasted with a suggested alternative of a continuous sheet bounded on two sides by endothelium held apart by connective tissue and cellular posts (sheet-flow model). A vascular space-tissue ratio, VSTR, is defined as the ratio of vascular lumen volume to the circumscribing tissue volume. The VSTR was obtained by planimetric measurements. Formulas were developed to relate VSTR to the geometric models. Cat lungs were perfused in situ with liquid silicone elastomer at 25 mm Hg, and the liquid elastomer was allowed to catalyze at 15 or 25 mm Hg static pressure while the airway pressure was maintained at 10 cm H2O. After the silicone became solid, the lungs were fixed with formalin-steam, and morphometric analysis was carried out on photographs of frozen sections. The mean values of VSTR of each cat (six lobes) were 0.91 to 0.92, irrespective of intracapillary pressure. These data are consistent with a sheet-flow model. Sheet thickness then becomes a significant variable.