Final answer:
The vasa vasorum are small blood vessels that supply blood to the outer wall of larger arteries and veins, playing a crucial role in nutrient and waste exchange due to the thick walls that prevent direct diffusion.
Step-by-step explanation:
Small vessels that supply blood to the outer half of larger vessel walls are the vasa vasorum. These are small blood vessels located within the wall or tunics of larger vessels that supply nourishment to and remove wastes from the cells of the vessels. The vasa vasorum are critical for the exchange of nutrients and waste because the larger arteries and veins have thick walls that inhibit direct diffusion of such substances. The high pressure in arteries necessitates that the vasa vasorum be situated in the outer layers of the vessel wall (tunica externa) to function properly without collapsing. In contrast, the lower pressure within veins permits the vasa vasorum to be located closer to the lumen, facilitating nutrient exchange.