Which layer houses smooth muscle responsible for constriction and dilation?

• A. Endothelium
• B. Tunica adventitia
• C. Tunica media
• D. Internal elastic lamina

Answer: (C)

The smooth muscle that controls vessel diameter is in the middle layer of the vessel wall. This tunica media contains circularly arranged smooth muscle cells that can contract to narrow the lumen (vasoconstriction) or relax to widen it (vasodilation), regulating blood flow and pressure. The inner lining (endothelium) handles the barrier and signals but doesn’t contain smooth muscle. The outer layer (tunica adventitia) is mainly connective tissue that provides support and houses nerves and blood vessels, not muscle for constriction. The internal elastic lamina sits between the inner lining and the muscle layer, contributing to elasticity, not muscle tissue.