Final answer:
The tunica media of a blood vessel wall is primarily made up of smooth muscle, essential for regulating blood flow and pressure through vasoconstriction and vasodilation.
Step-by-step explanation:
The tunica media is a key component of the walls of blood vessels, particularly in the arteries. This middle layer is composed predominantly of smooth muscle, bolstered by connective tissue filled with elastic fibers. In arteries, the tunica media is much thicker compared to veins and plays a crucial role in regulating blood pressure and flow via vasoconstriction and vasodilation. Contractile smooth muscle fibers allow for adjustments to the diameter of the vessel lumen, hence controlling blood flow.
Within the tunica media, the smooth muscle is interspersed with circular sheets of elastic fibers, and toward the outer boundary, it is supported by longitudinal muscle layers. When these smooth muscles contract, they can make the lumen narrower, leading to increased blood pressure and decreased blood flow—a process known as vasoconstriction. Conversely, during relaxation of these muscles, the lumen widens, reducing blood pressure and increasing blood flow—a process termed vasodilation.
The nervi vasorum, small nerve fibers within the vessel walls, largely consist of sympathetic fibers that can induce both vasoconstriction and vasodilation, depending on the receptors and neurotransmitters involved. The autonomic nervous system, including both sympathetic and parasympathetic divisions, helps to regulate these dynamic processes.