The tunica media is the middle layer of blood vessel walls, primarily composed of smooth muscle and elastic fibers. This layer plays a crucial role in regulating blood flow and pressure by contracting and relaxing, which helps control the diameter of the vessel. In arteries, the tunica media is thicker than in veins, allowing for greater strength and elasticity to withstand high pressure from the heart's pumping action. In contrast, veins have a thinner tunica media, reflecting their role in returning blood to the heart at lower pressure.