Complete guide to know human vascular system

Vascular System 
Blood Vessels 
The blood vessels comprise the vascular system. Blood ves­sels are of three types: arteries, capillaries, and veins.
Arteries and Arterioles 
Arteries transport blood away from the heart. They have thick walls composed of an inner endothelium layer (tunica interna), an outer connective tissue layer (tu­nica externa), and also a thick middle layer (tunica media) of elastic fibers and smooth muscle. The elastic fibers en­able an artery to expand and accommodate the sudden increase in blood volume that results after each heartbeat. Ar­terial walls are sometimes so thick that they are supplied with blood vessels.
Arterioles are small arteries just visible to the naked eye. The middle layer of these vessels has some elastic tis­sue but is composed mostly of smooth muscle whose fibers encircle the arteriole. Contraction of smooth muscle cells is under involuntary control by the autonomic ner­vous system. If the muscle fibers contract, the lumen (cav­ity) of the arteriole decreases; if the fibers relax, the lumen of the arteriole enlarges. Whether arterioles are constricted or dilated affects blood pressure. The greater the number of vessels dilated, the lower the resistance to blood flow, and hence, the lower the blood pressure, and vice versa.
Arteriosclerosis The plaques associated with atheroscle­rosis may also restrict nutrition of the smooth muscle tis­sue and elastic fibers comprising the artery wall, causing these tissues to deteriorate and lesions to form. This is ac­companied by the deposition of calcium salts and the for­mation of nonelastic scar tissue, resulting in increased rigidity of the vessel wall. This process of hardening of the arteries, or arteriosclerosis, not only contributes to hyper­tension but also increases the risk of a heart attack or stroke.
A stroke, also called a cerebrovascular accident (CVA) , occurs when a portion of the brain is deprived of oxygen. Two reasons for strokes are hypertension, which can cause a cerebral artery to burst, or a blood clot, which can prevent blood flow to the brain.
Capillaries 
Arterioles branch into capillaries, which are ex­tremely narrow, microscopic blood vessels with a wall composed of only one layer of endothelial cells. Capillary beds (a network of many capillaries) are present in all re­gions of the body; consequently, a cut in any body tissue draws blood. Capillaries are an important part of the circu­latory system because nutrient and waste molecules are ex­changed only across their thin walls. Oxygen and glucose diffuse out of capillaries into the tissue fluid that surrounds cells, and carbon dioxide and other wastes diffuse into the capillaries. Since capillaries serve the needs of the cells, the heart and other vessels of the circulatory system can be considered a means by which blood is con­ducted to and from the capillaries.
Not all capillary beds are open or in use at the same time. For instance, after a meal, the capillary beds of the digestive tract are usually open, and during muscular exercise, the capillary beds of the skeletal muscles are open.
Most capillary beds have a shunt that allows blood to move directly from arteriole to a venule (a small vessel leading to a vein) when the capillary bed is closed. Sphinc­ter muscles, called precapillary sphincters, encircle the en­trance to each capillary. When the capillary bed is closed, the capillary sphincters are constricted, preventing blood from entering the capillaries; when the capillary bed is open, the capillary sphincters are relaxed. As would be ex­pected, the larger the number of capillary beds open, the lower the blood pressure.
Veins and Venules
Veins and smaller vessels called venules carry blood from the capillary beds to the heart. First, the venules drain the blood from the capillaries and then join together to form a vein. The wall of a vein is much thinner than that of an ar­tery because the middle layer of muscle and elastic fibers is thinner. Within some veins, especially in the major veins of the arms and legs, valves allow blood to flow only toward the heart when they are open and prevent the backward flow of blood when they are closed.
At any given time, more than half of the total blood volume is found in the veins and venules. If blood is lost due to, for example, hemorrhaging, sympathetic nervous stimulation causes the veins to constrict, providing more blood to the rest of the body. In this way, the veins act as a blood reservoir.
Arteries and arterioles carry blood away from the heart, veins and venules carry blood to the heart, and capillaries join arterioles to venules.
Varicose Veins and Phlebitis Varicose veins are abnor­mal and irregular dilations in superficial (near the surface) veins, particularly those in the lower legs. Varicose veins in the rectum, however, are commonly called piles, or more properly, hemorrhoids. Varicose veins develop when the valves of the veins become weak and ineffective due to backward pressure of the blood. The problem can be ag­gravated when venous blood flow is obstructed by crossing the legs or by sitting in a chair so that its edge presses against the back of the knees.
Phlebitis (fli'-bi'tus), or inflammation of a vein, is a more serious condition, particularly when a deep vein is involved. When blood in a large, unbroken vein clots, thromboembolism can occur. In this instance the embolus, which is a blood clot moving along in the bloodstream, may eventually come to rest in a pulmonary arteriole, blocking circulation through the lungs. This condition, termed pulmonary embolism, can result in death.