Notice the layers of smooth muscle in the tunica media of the artery (HA24, upper left). The associated vein is pictured in the next image. There are several examples of arterioles, venules, and blood and lymphatic capillaries. Use the RBCs at lower left in HA24 to help you pick out the vessels that have the diameter of capillaries. Slightly larger vessels that have one or two smooth muscle layers are arterioles. An internal elastic lamina may or may not be present. The venules have a very thin wall consisting of a few fibroblasts in loose CT and an endothelial lining. The lumen is two or more erythrocytes in diameter. The lymphatics have widely spread endothelial cell nuclei (middle) and valves.