Development and Anatomy
Vessels--The pulmonary arteries accompany the airways, but the veins run in the interlobular septa . There are three types of pulmonary arteries: elastic; muscular, including axial and supernumerary; and partially muscular or non-muscular arterioles. Elastic arteries (>1 mm in diameter) (left) have a mixture of smooth muscle and elastic tissue in their medias. Muscular arteries (1 mm to 0.1 mm in diameter) (middle) have medial muscle delimited by internal and external elastic membranes. Arterioles (<0.1 mm in diameter) (right) normally are partially muscular or lack muscle completely. They supply the capillary bed.
The above photo shows vessels stained with the elastic van Gieson stain, which stains elastic tissue black. They have been distended by a barium-gelatin mixture (red).
The dichotomously branching axial arteries have gradually decreasing diameters. The supernumerary arteries (below), which supply the alveolar capillary bed directly, branch at right angles from the muscular parent artery and are more common peripherally. There, they are 4 times as numerous as axial branches and have 1/4 the diameter of the parent artery .
In the above photo, the endothelial cells are stained black immunohistochemically for von Willebrand factor (factor VIII). A small supernumerary artery branches at right angles (arrow) from the larger muscular artery. Note the relative thicknesses of the vessel walls.
Veins (below)--On the venous side, venules cannot be distinguished from arterioles morphologically. They lead to veins (>0.1 mm diameter) with a thin muscular media and single elastic lamina. Also, veins have supernumerary branches that arise at right angles. Valves are absent in the venous system of the lungs.
The photo shows a vein in an interlobular septum. It is surrounded by a loose adventitia.