Final answer:
Larger venules and small veins have a tunica media with a continuous layer of smooth muscle cells that help regulate blood flow. They are part of the low-pressure venous system and have larger lumens and valves to prevent the backflow of blood.
Step-by-step explanation:
Venules that are above 1 mm in diameter with scattered smooth muscle cells that become a continuous layer as the vessels get larger are the larger venules and small veins. When studying the structure of these blood vessels, we can see that they are composed of three layers known as the tunica intima, tunica media, and tunica externa. The tunica media in larger venules and veins is the middle layer that predominantly contains smooth muscle fibers and elastic fibers which help regulate blood flow through vasoconstriction and vasodilation. Veins, including these larger venules, are part of the low-pressure venous system, have larger lumens, and are equipped with valves to prevent the backflow of blood, a structural difference from arteries. The skeletal muscle also assists in the flow of blood through veins back to the heart.