Vasa vasorum

The vasa vasorum (Latin, "vessels of the vessels") is a network of small blood vessels that supply large blood vessels.

The vasa vasorum are found in large arteries and veins such as the aorta and its branches. Studies conducted with 3-dimensional microcomputed tomography (3D Micro-CT) on porcine and human arteries from different vascular beds have shown that there are three different types of vasa vasorum: 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).
 * 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.

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.

Most authorities say that the vasa vasorum is present more frequently in arteries than veins, because arteries are thicker. But 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.

An interesting point of fact is that, in the human descending aorta, vasa vasorum cease to supply the arterial walls with oxygenated blood at the level of the renal arteries. Thus, below this point, the aorta is dependent on diffusion for its metabolic needs, and is necessarily markedly thinner. This leads to an increased likelihood of aneurysm at this location, especially in the presence of atherosclerotic plaques. Other species, such as dogs, do have vasa vasorum below their renal vasculature, and aneurysms at this site are substantially less likely.