Vasa vasorum
Encyclopedia
The vasa vasorum is a network of small blood vessel
s 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).
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.
Blood vessel
The blood vessels are the part of the circulatory system that transports blood throughout the body. There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and...
s that supply large blood vessels.
The vasa vasorum are found in large arteries and veins such as the aorta
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...
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:
- 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.
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
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
and osmotic pressure
Osmotic pressure
Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane....
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
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...
, vasa vasorum cease to supply the arterial walls with oxygenated blood at the level of the renal arteries. Thus, below this point, the aorta
Aorta
The aorta is the largest artery in the body, originating from the left ventricle of the heart and extending down to the abdomen, where it branches off into two smaller arteries...
is dependent on diffusion for its metabolic needs, and is necessarily markedly thinner. This leads to an increased likelihood of aneurysm
Aneurysm
An aneurysm or aneurism is a localized, blood-filled balloon-like bulge in the wall of a blood vessel. Aneurysms can commonly occur in arteries at the base of the brain and an aortic aneurysm occurs in the main artery carrying blood from the left ventricle of the heart...
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.