The intima is a single layer of endothelial cells on the innermost section of the vessel wall. Media refers to the middle section of the vessel wall and consists of smooth muscle cells surrounded by collagen and elastic tissue. Adventitia, the outermost covering of the vessel wall, consists of a mixture of collagen, elastic tissue, smooth muscle, nerve fibers, vaso vasorum, and lymphatic vessels which accommodate lymphatic flow to nourish and remove metabolic waste products from the vessel wall. The endothelium comprises a flat layer of endothelial cells lining the entire vascular system. Below the endothelium is the basement membrane, composed of various proteins and polysaccharides which serve as a support structure and transport medium for various materials.
Elastic tissue encompasses the endothelium and basement membrane.
Collagen, a major protein of the white fibers of connective tissue, cartilage, and bone, resists stretching and thereby prevents overdistension of the vasculature. Smooth muscle provides the contracting component of the vascular system that regulates vasoconstriction and dilation. Elastic arteries contain large amounts of elastic tissue, which enables them to distend and recoil during systole and diastole to help propel blood. The muscular arteries mainly comprise smooth muscle cells and control blood flow to the periphery and major organs. These arteries are capable of constricting and dilating to allot varying degrees of blood flow to certain tissues according to their needs.