All blood vessels have an outer adventitia, a medial layer of smooth muscle cells, and an intima lined by endothelial cells. Contraction of the vascular smooth muscle causes changes in the diameter and wall tension of blood vessels. In the aorta and large arteries vascular smooth muscle contraction affects mainly the compliance (the reciprocal of stiffness) of the vessel. At the precapillary level, contraction of vascular smooth muscle will regulate blood flow to different organs, and contribute to the peripheral resistance. Compliance of large vessels and resistance of arterioles both contribute most of the impedance of the vascular circuit and therefore the afterload of the heart. The capacity of the circulation is determined by the degree of contraction of the veins (capacitance vessels) especially in the splanchnic area; this will affect the venous filling pressure, or preload, of the heart.