The systemic, or peripheral, circulation comprises a circuit of blood vessels that transport substances needed for cellular metabolism to body systems and remove the waste products of metabolism from those same tissues. With the exception of the interconnecting capillaries, the anatomy of all blood vessel walls is a similar three-layer design:

• Tunica intima-The smooth single-cell layer that lines the inside of all blood vessel walls.
• Tunica media-The middle layer of elastic fibers and muscle that gives strength and recoil. Contraction or relaxation of this muscle layer varies the diameter of the blood vessel lumen, the cavity through which blood flows.
• Tunica externa-A tough outer layer of fibrous tissue that protects the blood vessel from damage.