Final answer:
The layer of the blood vessel that is composed of smooth muscle, collagen and sometimes elastic tissue is called the tunica media.
Step-by-step explanation:
The tunica media is the middle layer of arteries and veins. It consists mainly of elastic fibers and connective tissues. In arteries, this is the thickest layer because it also contains smooth muscle tissues, which control the diameter of the vessels.
In addition to smooth muscle, the tunica media also contains collagenous fibers and elastic fibers. Collagen provides strength and elastic fibers allow the blood vessel to stretch and recoil, maintaining the vessel's shape and allowing it to respond to changes in blood pressure.