Normal Pulmonary Vessels
These vessels were all photographed at the same magnification and stained with the elastic van Gieson stain that colors elastic tissue black, collagen red, and smooth muscle yellow. Arteries are divided into 3 types: elastic (>1 mm in diameter) (left), muscular (1 to 0.1 mm in diameter) (middle), and arterioles (<0.1 mm to about 0.01 mm in diameter) (right).
Elastic arteries have a mixture of smooth muscle and elastic tissue throughout the media. Muscular arteries have a muscular media limited by internal and external elastic laminas. Arterioles normally are muscular, partially muscular, or lack smooth muscle.
Capillaries, about 10 µm in diameter, run in the alveolar walls, and connect arterioles and venules. The capillary endothelial basement membrane is fused with that of the alveolar epithelium on the gas-exchanging side. Venules are 0.1 mm to about 0.01 mm in diameter and cannot be distinguished from arterioles except by their connections. Veins >0.1 mm in diameter have a muscular media and a single elastic lamina.
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