Which vessel layer is primarily composed of elastic tissue and smooth muscle cells?

The tunica media is the layer of a blood vessel that is mainly made up of elastic tissue and smooth muscle cells. This structure allows the tunica media to play a crucial role in regulating blood pressure and flow. The presence of smooth muscle enables the vessel to constrict and dilate, a process known as vasoconstriction and vasodilation, respectively. This dynamic change in diameter helps maintain adequate blood flow according to the body's changing demands, such as during exercise or rest. Additionally, the elastic tissue provides the necessary resilience for the vessel to withstand and absorb the pressure fluctuations that occur with each heartbeat.

In contrast, the tunica externa, which is the outer layer of the vessel, primarily consists of connective tissue, providing structural support and protection. The tunica intima is the innermost layer that features endothelial cells, contributing to a smooth lining that reduces friction as blood flows through the vessel. The endothelium, while crucial for maintaining vascular health, does not have the elastic and muscular properties found in the tunica media. These differences highlight why the tunica media is vital for the vessel's function in the circulatory system.