Final answer:
Arterioles, which lead into capillary beds, are very small arteries that regulate blood flow to the capillaries and have only a few layers of smooth muscle in their walls.
Step-by-step explanation:
Leading into capillary beds, arterioles are extremely thin and contain only a few layers of smooth muscle. Arterioles are very small arteries that connect to capillaries and are crucial for regulating blood flow into the capillary beds. The walls of arterioles are composed of three tunics.
The tunica intima is the innermost layer and includes the endothelial lining, the tunica media consists mainly of one or two layers of smooth muscle cells, and the tunica externa, which is the outermost layer, is very thin. This structural organization allows arterioles to adjust their diameter, thereby controlling the blood pressure and flow reaching the capillaries. In contrast, capillaries themselves are composed of a single layer of epithelial cells and are the primary sites for the exchange of nutrients, waste, and oxygen with tissues at the cellular level.