Observations on some ultrastructural features of normal pulmonary blood vessels in collapsed and distended lungs.

In this paper we describe certain special features of the pulmonary vasculature of rats from both distended and collapsed lungs. Evaginations of medial smooth muscle cells are a generalised response to contraction and are absent from distended lungs. They form at surfaces of low tissue pressure, such as the intima of veins and the adventitia of arteries. Contacts between endothelium and smooth muscle are common, particularly in veins, and often terminate in tight junctions. We also describe a variety of cystic spaces. Those between the endothelium and elastic lamina of pulmonary arteries are probably a fixation artefact. Those within the endothelium, except discoid bodies, are all extracellular spaces, and are not vacuoles as they may seem to be at first sight.