At the cellular level, the aorta and the aortic arch are composed of three layers: The tunica intima, which surrounds the lumen and is composed of simple squamal epithelial cells; the tunica media, composed of smooth cell muscles and elastic fibers; and, the tunica adventitia, composed of loose collagen fibers. Innervated by barometric nerve terminals, the aortic arch is responsible for sensing changes in the dilation of the vascular walls, inducing changes in heart rate to compensate for changes in blood pressure.