text: In its journey from the heart to the tissues, the blood passes through channels of six principal types: elastic arteries, muscular arteries, arterioles, capillaries, venules and veins. In this system, the arteries show a progressive diminution in diameter as they recede from the heart, from about 25 mm in the aorta to 0.3 mm in some arterioles. The reverse is true for the veins; the diameter is small in the venules and progressively increases as the veins approach the heart. All arteries are comprised of three distinct layers, intima, media and adventitia, but the proportion and structure of each varies with the size and function of the particular artery. A large artery, like the aorta is comprised of the following layers, going from the lumen to the most external layers:
  1. the intima, or intermost layer, consists of a layer of endothelial cells separated from the inner layer by a narrow layer of connective tissues which anchors the cells to the arterial wall.
  2. A large layer of elastic fibers forming the elastica interna layer.
  3. Below this layer are concentric waves of smooth muscle cells intermixed with elastic fibers. Elastic lamellae and smooth muscle cells are imbedded in a ground substance rich in proteoglycans. Proteoglycans are formed of disaccharides bound to protein and serve as binding or "cement" material in the interstitial spaces. The outer layer of the media is penetrated by branches of the vasa vasorum.
  4. Between the smooth muscle layer and the adventitia, there is again another layer of elastic fibers, the elastica externa. Layers 2, 3 and 4 form the media.
  5. The outer layer or adventitia is formed of irregularly arranged collagen bundles, scattered fibroblasts, a few elastic fibers and blood vessels which, because of their location, are called vasa vasorum or vessels of the vessels.
  This structure of the aorta and large arteries corresponds well to their function which serves as a blood reservoir and to stretch or recoil with the pumping action of the heart. The wall of the arterioles, contains less elastic fibers but more smooth muscle cells than that of the aorta. The arterioles represent the major site of the resistance to blood flow and small changes in their caliber cause large changes in total peripheral resistance. Muscle cells are innervated by noradrenergic nerve fibers which are constrictor in function, and in some cases, by cholinergic nerve fibers which dilate the vessels.
  The capillary structure shows a diameter just large enough to permit the red blood cells to squeeze through in single file. In the same manner as the intima of the arteries, the capillary wall is formed of a layer of endothelial cells resting on a basement membrane. The major function of the capillaries is to promote exchange of nutrients and metabolic end-products between the blood and the interstitial tissues. Such exchanges are facilitated by the presence of specialized junctions, gaps or fenestrations.