Vein has part(s) venous valve
Veins are blood vessels in the circulatory system of humans and most other animals that carry blood toward the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are those of the pulmonary and fetal circulations which carry oxygenated blood to the heart. In the systemic circulation arteries carry oxygenated blood away from the heart, and veins return deoxygenated blood to the heart, in the deep veins.There are three sizes of veins, large, medium, and small. Smaller veins are called venules, and the smallest the post-capillary venules are microscopic that make up the veins of the microcirculation. Veins are often closer to the skin than arteries.
Veins have less smooth muscle, and connective tissue, and wider internal diameters than arteries. Because of their thinner walls and wider lumens they are able to expand and hold more blood. This greater capacity gives them the term of capacitance vessels. At any time, nearly 70% of the total volume of blood in the human body is in the veins. In medium and large sized veins the flow of blood is maintained by one-way (unidirectional) venous valves  to prevent backflow. In the lower limbs this is also aided by muscle pumps, also known as venous pumps that exert pressure on intramuscular veins when they contract and drive blood back to the heart.Venous system
The venous system is the system of veins in the systemic and pulmonary circulations that return blood to the heart. In the systemic circulation the return is of deoxygenated blood from the organs and tissues of the body, and in the pulmonary circulation the pulmonary veins return oxygenated blood from the lungs to the heart. Almost 70% of the blood in the body is in the veins, and almost 75% of this blood is in the small veins and venules. All of the systemic veins are tributaries of the largest veins, the superior and inferior vena cava, which empty the oxygen-depleted blood into the right atrium of the heart. The thin walls of the veins, and their greater internal diameters (lumens) enable them to hold a greater volume of blood, and this greater capacitance gives them the term of capacitance vessels. This characteristic also allows for the accommodation of pressure changes in the system.  The whole of the venous system, bar the post-capillary venules is a large volume, low pressure system. The venous system is often asymmetric, and whilst the main veins hold a relatively constant position, unlike arteries, the precise location of veins varies among individuals.Venous valves
Blood flows back to the heart in the systemic deep veins, with the flow of blood maintained by one-way valves in the deep veins, superficial veins, and in the perforator veins. The venous valves serve to prevent regurgitation (backflow) due to the low pressure of veins, and the pull of gravity. They also serve to prevent the over-widening of  the vein.A venous valve is bicuspid (having two leaflets) and is formed by an infolding of part of the tunica intima on either side of the lumen of the veins. The leaflets are strengthened with collagen, and elastic fibres, and covered with endothelium. The endothelial cells  on the surfaces of the leaflets facing the vein wall, are arranged transversely. On the leaflet surfaces that open to let the blood flow, the cells are arranged longitudinally in the direction of the flow. The leaflets are attached to the venous wall at their convex edges. Their margins are concave and are directed with the flow lying against the wall. As the valve forms, the vein wall where the leaflets attach, becomes dilated on each side. These widenings form the pockets, hollow cup-shaped regions, on the cardial side, known as the valvular sinuses. The endothelial cells in the sinuses are able to stretch twice as much as those in areas without valves. When the blood tries to reverse its direction (due to low venous pressure and the pull of gravity), the sinuses fill first closing the leaflets and keeping them together.The deep veins of the lower limb include the common femoral vein, femoral vein, and the deep femoral vein; the popliteal vein, the tibial, and fibular veins. In the common femoral vein one valve is located above the saphenofemoral junction called the suprasaphenic valve. There are sometimes two valves in the same tract.  In the femoral vein there are often three valves, the most constantly found valve is just below the joining of the deep femoral vein. The deep femoral vein and its perforators have valves.  In the popliteal veins there are between one and three valves; in each posterior tibial vein there are between 8 and 19 valves, and in  the anterior tibial veins there are between 8 and 11 valves.In the superficial veins there are between one and seven valves along the thigh portion of the great saphenous vein (GSV); two to six below the knee and one to four in the marginal veins of the foot. There is a valve  at the termination of the GSV known as the terminal valve to prevent reflux from the femoral vein  A preterminal valve is located just below the openings of the tributaries to prevent reflux form these into the GSV. Incompetence of the GSV is a common cause of varicose veins.
The valves also divide the column of blood into segments which helps move the blood unidirectionally to the heart. Their action is supported by the action of skeletal muscle pumps that contract and compress the veins. A skeletal muscle is confined in its fascia and contraction of the muscle which makes it wider results In compression on the vein that pushes the blood forward. Valves in the perforating veins close when a calf muscle contracts, to prevent backflow from the deep veins to the superficial. There are more valves in the lower leg, due to increased gravitational pull, with the number decreasing as the veins travel to the hip. There are no valves in the veins of the thorax or abdomen.Approximately 95% of the venous valves are in the small veins of less than 300 micrometres.There is a valve at the junction of the inferior vena cava (one of the great vessels) and the right atrium known as the valve of inferior vena cava also known as the eustachian valve.Microanatomy
The three layers of the vein wall are the outer tunica externa, the middle tunica media and the inner tunica intima. There are also numerous valves present in many of the veins.
The outer tunica externa, also known as the tunica adventitia is a sheath of thick connective tissue. This layer is absent in the post-capillary venules.The middle tunica media is mainly of vascular smooth muscle cells, elastic fibers and collagen. This layer is much thinner than that in arteries  Vascular smooth muscle cells control the size of the vein lumens, and thereby help to regulate blood pressure.The inner tunica intima is a lining of endothelium comprising a single layer of extremely flattened epithelial cells, supported by delicate connective tissue. This subendothelium is a thin but variable connective tissue. The tunica intima has the most variation in blood vessels, in terms of their wall thickness and relative size of their lumen. The endothelial cells continuously produce nitric oxide a soluble gas, to the cells of the adjacent smooth muscle layer. This constant synthesis is carried out by the enzyme endothelial nitric oxide synthase (eNOS). Other endothelial secretions are endothelin, and thromboxane (vasoconstrictors), and prostacyclin a vasodilator.Superficial vein thrombosis
SVT is the development of a thrombus in a superficial vein. SVT is not normally clinically significant, but the thrombus can migrate into the deep venous system where it can also give rise to a pulmonary embolism. The main risk factor for SVT in the lower limbs is varicose veins.
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