Most cells need to be within a few cell-widths of a capillary to stay alive, and the cells that make up the outer walls of a blood vessel are no exception. The network of smaller vessels that supply these cells is known as the vasa vasorum. The vasa vasorum penetrate the tunica adventitia, as well as the deeper tunica media of larger vessels such as the aorta.
The vasa vasorum consist of a network of arterioles, capillaries and venules, depending on the vessel. They provide the vessel wall that they penetrate with metabolites which they would otherwise not receive due to the thickness of the layers making up the wall. The vasa vasorum are present more frequently in arteries than in veins.
In several books vasa vasorum are more abundant in large veins, as patial oxygen pressure is lower than in arteries as well as osmotic pressure. This would lead to to more vasa vasorum needed to supply the vessels sufficiently.
- Histology image: 05702loa – Histology Learning System at Boston University