Arteries and veins of the normal dog lung: qualitative and quantitative structural differences

As a necessary preliminary step to the study of pulmonary hypertension and edema, the structure of the pulmonary vasculature of seven normal dogs was examined in detail to distinguish arteries and veins. For light microscopy and morphometry, the left lung was injected from the arterial and venous sides with pigmented gelatin masses of different colors. The right lung was fixed for electron microscopy. The percentage of medial muscle thickness of arteries was greater (P less than 0.05) than that of veins, for vessels over 200 micrometer diameter. Smooth muscle cells extended more peripherally into arteries (including in vessels less than 50 micrometer) than into veins. The larger arteries were elastic or transitional in type, whereas larger veins were muscular. The arteries branched with the airways. Fifty percent of arteries under 50 micrometer and more than 50% of veins under 200 micrometer were surrounded by alveoli. Muscular arteries had a thick media between distinct internal and external elastic laminae, whereas veins had no internal lamina but had a thin media separated from a thick adventitia by an external elastic lamina. By electron microscopy, the muscular arteries had tightly packed smooth muscle cells with few myoendothelial junctions; the venous smooth muscle cells were arranged loosely, and more numerous myoendothelial junctions were seen. no definite differences were noted between nonmuscular arteries and veins. The functional implications of these morphological findings (differential reactions to pharmacological agents, distensibility of pulmonary arteries and veins, and responses of small vessels to alveolar pressure) are discussed.