The superior vena cava is a large vein that descends through the chest into the top of the heart. Its purpose is to bring de-oxygenated blood from the upper portions of the body back to the heart. Blood from the arms, neck, and head travel back to the superior vena cava, which then carries it into the right atrium. It is one of only two veins that bring de-oxygenated blood into the heart; the other is the inferior vena cava, which transports blood up from the lower body.
Two major veins, the left and right brachiocephalic veins, join in the upper chest to form the superior vena cava. From that point it is just a short distance down to the heart. One other blood vessel, the azygous vein, which returns de-oxygenated blood from the torso, also joins the superior vena cava just before it enters the right atrium. All of the blood enters the right atrium, then moves to the right ventricle, where it is then sent via the pulmonary artery to the lungs for re-oxygenation.
Several other structures surround the superior vena cava in the chest. It is considered to lie inside the mediastinum, or the central portion of the chest cavity located between the lungs. This puts it in close proximity to several major structures, including the sternum, trachea, and aorta. It also sits directly next to the the right lung's upper lobe.
The thin walls of the superior vena cava make it susceptible to pressure from the structures around it. When this happens, it blocks the de-oxygenated blood being returned to the heart and causes it to back up. This causes pressure to build up in the smaller veins of the upper body, which in turn leads to edema in the face and arms. This is a relatively rare but serious condition known as superior vena cava syndrome.
Superior vena cava syndrome can be caused by a variety of other issues. Cancer is the most common cause; tumors, most often in the lung but possibly in other areas such as the trachea, can compress the vessel. Certain diseases and infections including tuberculosis, syphilis, and histoplasmosis may lead to the problem. Cardiac and vascular issues, like aortic aneurysms or pericarditis, may put excessive pressure on the vein. Thromobosis, or blood clots, which are often the result of vein catheters, can also be a cause.