Microscopic anatomy of the pulmonary vascular bed in the cat lung

Abstract

The pulmonary microcirculation of the cat was analyzed by light and electron microscopy. In order to establish the precise structure and ultrastructure of the components of the vascular wall of each segment, special efforts were made to identify positively the pulmonary arterioles and venules by tracing their connection to small pulmonary arteries and veins, respectively. Also, the pulmonary arterioles and venules were studied with respect to their relationship to the alveolar capillary network via the precapillary sphincter areas and the postcapillary venules. It was confirmed that the small pulmonary arteries, arterioles, and precapillary sphincter areas are provided with smooth muscle cells which are present up to the point where the pulmonary capillaries branch out, although the number of smooth muscle cells decreases gradually toward the capillary bed. Cholinergic and noradrenergic nerves accompany all arterial segments. The capillary network described by many investigators in several mammalian species was studied only to the extent that a three-dimensional conceptualization could be obtained. With respect to the postcapillary venules and pulmonary venules, it was discovered that, in the cat lung, true smooth muscle cells, albeit widely scattered, are present in these segments of the pulmonary microcirculation. These smooth muscle cells display extensive areas of myoendothelial junctions. That is, the cell membranes of the endothelial cells and the smooth muscle cells make contact without an intervening basal lamina. Some myoendothelial junctions were identified also in the arterioles and precapillary sphincter areas. However, they were few in number and had points of only limited membrane contact. The functional implications of these findings are discussed in terms of possible regulatory influence on the pulmonary microcirculation and hypothetical role in the development of pulmonary hypertension.