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 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.
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