Veins are vessels designed to collect and return blood, including deoxygenated hemoglobin, from tissues to the heart. In humans, veins and the venous vascular system can be divided in to three separate systems depending on anatomical relationships and function. Initially, veins can be divided into systemic and pulmonary systems. The veins that drain the heart, comprising the coronary venous system, may be described as an independent venous system, or be considered a subset of the systemic vascular system. The systemic veins transport venous blood—deoxygenated when compared with arterial blood—from the body to the heart. The pulmonary veins return freshly oxygenated blood from the lungs to the heart so that it may be pumped into the systemic arterial system.
Veins can also be described by their anatomical position. Deep veins run in organs or connective tissue that supports organs, muscle, or bone. Superficial veins are those that drain the outer skin and fascia.
In contrast to arteries, veins often run a more convoluted course, with frequent branching and fusions with other veins (anastomoses) that make the tracing of the venous system less straightforward than mapping the arterial system. In addition, there are reservoirs or pools (sinus) that collect venous return from multiple sources. Many veins contain valves that assure a unidirectional (one way) flow of venous blood toward the heart.
The systemic venous system can be roughly divided into groups depending on the region they drain, and the vessel through which they return blood to the heart.
The first systemic venous group consists of veins that drain the head, neck, thorax, and upper limbs. These veins ultimately return blood to the heart through the superior vena cava.
Veins that drain the abdomen, pelvis, and lower limbs return blood through the inferior vena cava. Both the superior and the inferior vena cava return deoxygenated blood to the right atrium of the heart. The coronary sinus collects blood from a number of cardiac veins before returning blood to the right atrium near the point where the inferior vena cava enters the right atrium.
The pulmonary veins return blood oxygenated in the lungs to the left atrium. There are four major pulmonary veins, each lung being drained by a pair of pulmonary veins. Akin to the drainage of a land basin from streams into a larger river system, smaller venules arise from the lung alveolar capillary bed, the venules fuse to form single veins that separately drain isolated lobes of the lung. The veins from the upper and middle lobes of the three lobed right lung fuse to create a pair of veins—a superior and inferior pulmonary vein that separately transport blood to the left atrium.
At a microscopic or histological level, veins have thinner walls than do arteries. They are more elastic and capable of a wider range of lower pressure volume transformations. The elasticity is a result of the fact that veins have far less subendothelial connective tissue in their vascular walls. In addition the tunica media and tunica adventitia are often indistinguishable layers or are poorly developed when compared with arterial linings.
Venules drain capillaries and capillary beds. The venules ultimately fuse (coalesce) into veins that, as they increase in size, also increase in organization and differentiation of their vascular walls. In general, the larger the vein the more likely it is to be invested or surrounded with smooth muscle tissue. The values with the venous system are formed from a cusp forming multiple folding of the tunica intima. Valves are generally absent from the largest veins and the pulmonary veins.
Veins also serve in fluid uptake and can receive lymph fluid from lymphatic vessels. The major lymphatic duct, for example, drains into the fused vein formed from the fusion of the subclavian and left internal jugular vein.