The Pulmonary Circulation in Health and Disease

Abstract SEM analysis of pulmonary vascular casts shows basket-like alveolar capillaries that are about 6 μm in diameter. Generally pulmonary arteries are adjacent to airways and veins are not. The pleural surface and the airways capillaries have a planar meshwork with few large vessels. Uncommonly are lymphatics cast in normal lungs but they are regularly found in casts of edematous lungs. Casts of pulmonary veins in several species have smooth surfaces with a slight notch occurring at regular intervals; these notches will deepen after physiological interventions and are sharply constricted in spontaneously hypertensive rats (SHR) with pulmonary hypertension and cattle. The circular bands are caused by tufts of smooth muscle cells and have been called pulmonary venous sphincters (Fig. 1). The depth of the constrictions can be measured to show the effect of an experimental intervention on individual small veins, which is important in understanding the pulmonary microcirculation and its control.