The peripheral vascular system (PVS) includes all the blood vessels that exist outside the heart. The peripheral vascular system is classified as follows: The aorta and its branches:
- The arterioles
- The capillaries
- The venules and veins returning blood to the heart
The function and structure of each segment of the peripheral vascular system vary depending on the organ it supplies. Aside from capillaries, blood vessels are all made of three layers:
- The adventitia or outer layer which provides structural support and shape to the vessel
- The tunica media or a middle layer composed of elastic and muscular tissue which regulates the internal diameter of the vessel
- The tunic intima or an inner layer consisting of an endothelial lining which provides a frictionless pathway for the movement of blood
Within each layer, the amount of muscle and collagen fibrils varies, depending on the size and location of the vessel.
Arteries play a major role in nourishing organs with blood and nutrients. Arteries are always under high pressure. To accommodate this stress, they have an abundance of elastic tissue and less smooth muscle. The presence of elastin in the large blood vessels enables these vessels to increase in size and alter their diameter. When an artery reaches a particular organ, it undergoes a further division into smaller vessels that have more smooth muscle and less elastic tissue. As the diameter of the blood vessels decreases, the velocity of blood flow also diminishes. Estimates are that about 10% to 15% of the total blood volume is contained in the arterial system. This feature of high systemic pressure and low volume is typical of the arterial system.
There are two main types of arteries found in the body: (1) the elastic arteries, and (2) the muscular arteries. Muscular arteries include the anatomically named arteries like the brachial artery, the radial artery, and the femoral artery, for example. Muscular arteries contain more smooth muscle cells in the tunica media layer than the elastic arteries. Elastic arteries are those nearest the heart (aorta and pulmonary arteries) that contain much more elastic tissue in the tunica media than muscular arteries. This feature of the elastic arteries allows them to maintain a relatively constant pressure gradient despite the constant pumping action of the heart.
Arterioles provide blood to the organs and are chiefly composed of smooth muscle. The autonomic nervous system influences the diameter and shape of arterioles. They respond to the tissue's need for more nutrients/oxygen. Arterioles play a significant role in the systemic vascular resistance because of the lack of significant elastic tissue in the walls.
The arterioles vary from 8 to 60 micrometers. The arterioles further subdivide into meta-arterioles.
Capillaries are thin-walled vessels composed of a single endothelial layer. Because of the thin walls of the capillary, the exchange of nutrients and metabolites occurs primarily via diffusion. The arteriolar lumen regulates the flow of blood through the capillaries.
Venules are the smallest veins and receive blood from capillaries. They also play a role in the exchange of oxygen and nutrients for water products. There are post-capillary sphincters located between the capillaries and venules. The venule is very thin-walled and easily prone to rupture with excessive volume.
Blood flows from venules into larger veins. Just like the arterial system, three layers make up the vein walls. But unlike the arteries, the venous pressure is low. Veins are thin-walled and are less elastic. This feature permits the veins to hold a very high percentage of the blood in circulation. The venous system can accommodate a large volume of blood at relatively low pressures, a feature termed high capacitance. At any point in time, nearly three-fourths of the circulating blood volume is contained in the venous system. One can also find one-way valves inside veins that allow for blood flow, toward the heart, in a forward direction. Muscle contractions aid the blood flow in the leg veins. The forward blood flow from the lower extremities to the heart is also influenced by respiratory changes that affect pressure gradients in the abdomen and chest cavity. This pressure differential is highest during deep inspiration, but a small pressure differential is observable during the entire respiratory cycle.
Structure and Function
Vessels transport nutrients to organs/tissues and to transport wastes away from organs/tissues in the blood. A primary purpose and significant role of the vasculature is its participation in oxygenating the body. Deoxygenated blood from the peripheral veins is transported back to the heart from capillaries, to venules, to veins, to the right side of the heart, and then to the lungs. Oxygenated blood from the lungs is transported to the left side of the heart into the aorta, then to arteries, arterioles, and finally capillaries where the exchange of nutrients occurs. Loading and unloading of oxygen and nutrients occur mostly in the capillaries.
Blood vessels arise from the mesodermal embryonic layer. Embryonic development of vessels and the heart begins in the middle of the third week of life. Fetal circulation through this vasculature system begins around the eighth week of development.
Blood vessel formation occurs via two main mechanisms: (1) vasculogenesis and (2) angiogenesis.
Vasculogenesis is the process by which blood vessels form in the embryo. Interactions between precursor cells and various growth factors drive the cellular differentiation seen with vasculogenesis. Precursor mesodermal cells and their receptors respond to FGF2 to become hemangioblasts. Hemangioblast receptors then respond to VEGF, inducing further differentiation into endothelial cells. These endothelial cells then coalesce, forming the first hollow blood vessels. The first blood vessels formed by vasculogenesis include the dorsal aorta and the cardinal veins.
All other vasculature in the human body forms by angiogenesis. Angiogenesis is the process in which new blood vessels derive from the endothelial layer of a pre-existing vessel. Interactions involving VEGF drive angiogenesis. This process is the predominant form of neovascularization in the adult.
Blood Supply and Lymphatics
The walls of large blood vessels, like the aorta and the vena cava, are supplied with blood by vasa vasorum. This term translates to mean "vessel of a vessel."
Three types of vasa vasorum exist (1) vasa vasorum internae, (2) vasa vasorum externae, and (3) venous vasa vasorae. Vasa vasorum internae originate from the lumen of a vessel and penetrate the vessel wall to supply oxygen and nutrients. Vasa vasorum externae originate from a nearby branching vessel and feedback into the larger vessel wall. Some infections, such as late-stage manifestations of tertiary syphilis may lead to endarteritis of the vasa vasorum of the ascending aorta. Venous vasa vasorae originate within the vessel wall and drain into a nearby vein to provide venous drainage for vessel walls.
The sympathetic nervous system primarily innervates blood vessels. The smooth muscles of vasculature contain alpha-1, alpha-2, and beta-2 receptors. A delicate balance between the influence of the sympathetic and parasympathetic nervous systems is responsible for the underlying physiological vascular tone. Specialized receptors located in the aortic arch and the carotid arteries acquire information regarding blood pressure (baroreceptors) and oxygen content (chemoreceptors) from passing blood. This information is then relayed to the nucleus of the solitary tract via the vagus nerve. Blood vessel constriction or relaxation then ensues accordingly, determined by the body's sympathetic response.
Blood vessels contain only smooth muscle cells. These muscle cells reside within the tunica media along with elastic fibers and connective tissue. Although vessels only contain smooth muscles, the contraction of skeletal muscle plays an important role in the movement of blood from the periphery towards the heart in the venous system.
Injury to many blood vessels could have potentially serious implications. A rule of successful surgery is that a surgical site must have both adequate arterial supply and adequate venous drainage. Lack of either will result in suboptimal outcomes and complications for the patient. Special consideration must be given to avoid injury to the larger vessels (IVC, aorta, etc.) and any vessel particularly susceptible during specific surgical procedures.
Damage or disease of the blood vessels causes a variety of diseases including hypertension, aneurysm formation, aneurysm rupture, peripheral vascular disease, deep venous thrombosis, pulmonary embolism, transient ischemic attack, stroke, and many others. Some diseases are directly related to inherent vessel disease, while others are side effects of vessel disease. Clinically, vascular disease is an important problem. The CDC attributes $1 billion per day in cost to cardiovascular disease and stroke in the United States.