Vasa vasorum are small blood vessels that comprise a vascular network supplying the walls of large blood vessels, such as elastic arteries (e.g., the aorta) and large veins (e.g., the venae cavae). The name derives . Occasionally two different singular forms are seen: vasa vasis () and vas vasis (). Structure Studies conducted with 3D micro-CT on pig and human arteries from different vascular beds have shown that there are three different types of vasa vasorum: Vasa vasorum internae, that originate directly from the main lumen of the artery and then branch into the vessel wall. Vasa vasorum externae, that originate from branches of the main artery and then dive back into the vessel wall of the main artery. Venous vasa vasorae, that originate within the vessel wall of the artery but then drain into the main lumen or branches of concomitant vein. Depending on the type of vasa vasorum, it penetrates the vessel wall starting at the intimal layer (vasa vasorum interna) or the adventitial layer (vasa vasorum externa). Due to higher radial and circumferential pressures within the vessel wall layers closer to the main lumen of the artery, vasa vasorum externa cannot perfuse these regions of the vessel wall (occlusive pressure). The structure of the vasa vasorum varies with the size, function and location of the vessels. Cells need to be within a few cell-widths of a capillary to stay alive. In the largest vessels, the vasa vasorum penetrates the outer (tunica adventitia) layer and middle (tunica media) layer almost to the inner (tunica intima) layer. In smaller vessels it penetrates only the outer layer. In the smallest vessels, the vessels' own circulation nourishes the walls directly and they have no vasa vasorum at all. Vasa vasorum are more frequent in veins than arteries. Some authorities hypothesize that the vasa vasorum would be more abundant in large veins, as partial oxygen pressure and osmotic pressure is lower in veins. This would lead to more vasa vasorum needed to supply the vessels sufficiently. The converse argument is that generally artery walls are thicker and more muscular than veins as the blood passing through is of a higher pressure. This means that it would take longer for any oxygen to diffuse through to the cells in the tunica adventitia and the tunica media, causing them to need a more extensive vasa vasorum. A later method of scanning is optical coherence