Vasa vasorum

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The vasa vasorum is a network of small arterioles, capillaries and venules that supply the outer tissues of large blood vessels like the aorta. 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 vasa vasorum penetrate the tunica adventitia, as well as the deeper tunica media of larger vessels such as the aorta.

The exact makeup of the vasa vasorum depends 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 considered to be present more frequently in arteries than in veins. However, several books hypothesize that vasa vasorum would be more abundant in large veins, as partial oxygen pressure is lower than in arteries as well as osmotic pressure.^{[citation needed]} This would lead to more vasa vasorum needed to supply the vessels sufficiently.