Vein
A vein is a blood vessel in the circulatory system of humans and most other animals that carries blood towards 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 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.
Structure
There are three sizes of vein, large, medium, and small. Smaller veins are called venules. The smallest veins are the post-capillary venules. Veins have a similar three-layered structure to arteries. The layers known as tunicae have a concentric arrangement that forms the wall of the vessel. The outer layer, is a thick layer of connective tissue called the tunica externa or adventitia; this layer is absent in the post-capillary venules. The middle layer, consists of bands of smooth muscle and is known as the tunica media. The inner layer, is a thin lining of endothelium known as the tunica intima. The tunica media in the veins is much thinner than that in the arteries as the veins are not subject to the high systolic pressures that the arteries are. There are valves present in many veins that maintain unidirectional flow.Unlike arteries, the precise location of veins varies among individuals.
Veins close to the surface of the skin appear blue for a variety of reasons. The factors that contribute to this alteration of color perception are related to the light-scattering properties of the skin and the processing of visual input by the visual cortex, rather than the actual colour of the venous blood which is dark red.
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 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.File:Diagram of the human heart.svg|thumb|Positions of venae cavae and vessels of the pulmonary circulation
Veins vary in size from the smallest post-capillary venules, and more muscular venules, to small veins, medium veins, and large veins. The thickness of the walls of the veins varies as to their location – in the legs the vein walls are much thicker than those in the arms. In the circulatory system, blood first enters the venous system from capillary beds where arterial blood changes to venous blood.
Large arteries such as the thoracic aorta, subclavian, femoral and popliteal arteries lie close to a single vein that drains the same region. Other arteries are often accompanied by a pair of veins held in a connective tissue sheath. The accompanying veins are known as venae comitantes, or satellite veins, and they run on either side of the artery. When an associated nerve is also enclosed, the sheath is known as a neurovascular bundle. This close proximity of the artery to the veins helps in venous return due to the pulsations in the artery. It also allows for the promotion of heat transfer from the larger arteries to the veins in a counterflow exchange that helps to preserve normal body heat.
;Venules
The first entry of venous blood is from the convergence of two or more capillaries into a microscopic, post-capillary venule. Post-capillary venules have a diameter of between 10 and 30 micrometres, and are part of the microcirculation. Their endothelium is made up of flattened oval or polygon shaped cells surrounded by a basal lamina. Post-capillary venules are too small to have a smooth muscle layer and are instead supported by pericytes that wrap around them. Post-capillary venules become muscular venules when they reach a diameter of 50 μm, and can reach a diameter of 1 mm. These larger venules feed into small veins.
;Small, medium, and large veins
The small veins merge to feed as tributaries into medium-sized veins. The medium veins feed into the large veins which include the internal jugular, and renal veins, and the venae cavae that carry the blood directly into the heart. The venae cavae enter the right atrium of the heart from above and below. From above, the superior vena cava carries blood from the arms, head, and chest to the right atrium of the heart, and from below, the inferior vena cava carries blood from the legs and abdomen to the right atrium. The inferior vena cava is the larger of the two. The inferior vena cava is retroperitoneal and runs to the right and roughly parallel to the abdominal aorta along the spine.
;Deep, superficial, and perforator veins
The three main compartments of the venous system are the deep veins, the superficial veins, and the perforator veins. Superficial veins are those closer to the surface of the body, and have no corresponding arteries. Deep veins are deeper in the body and have corresponding arteries. Perforator veins drain from the superficial to the deep veins. These are usually referred to in the lower limbs and feet. Superficial veins include the very small spider veins of between 0.5 and 1 mm diameter, and reticular or feeder veins.
;Venous plexuses
There are a number of venous plexuses where veins are grouped or sometimes combined in networks at certain body sites. The Batson venous plexus, runs through the inner vertebral column connecting the thoracic and pelvic veins. These veins are noted for being valveless, believed to be the reason for metastasis of certain cancers.
A subcutaneous venous plexus is continuous, and a high rate of flow is supplied by small arteriovenous anastomoses. The high rate of flow ensures heat transfer to the vein wall.
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 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 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, the sinuses fill first closing the leaflets and keeping them together. Approximately 95% of the venous valves are in the small veins of less than 300 micrometres.
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 ; 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.
There is a valve at the junction of the inferior vena cava and the right atrium known as the valve of inferior vena cava also known as the eustachian valve. This valve is an embryological remnant and is insignificant in the adult. However, when persistent it can cause problems.