The superior vena cava is one of two vena cavae in which all venous blood from the systemic circulation is collected and delivered centrally to the right atrium via the common sinus venarum cavarum. In the superior vena cava, deoxygenated venous blood from the head, neck, and upper extremities collects and flows into the right atrium during the brief relaxation phase of the two atria.
What is the superior vena cava?
The superior vena cava, also known as the superior vena cava, is the collecting basin for venous, deoxygenated blood from the head and neck and upper extremities. The superior vena cava thus receives venous blood from almost all regions of the body located above the diaphragm. The counterpart of the superior vena cava is the inferior vena cava or inferior vena cava, which receives venous blood from the body regions below the diaphragm. Both vena cavae open into the common sinus venarum cavarum in the right atrium. The deoxygenated blood passes from the right atrium into the right ventricle, from where it is pumped into the pulmonary circulation and re-oxygenated. Both vena cavae, with a variable cross-section of two to three centimeters, embody the veins with the largest diameter. The name vena cava, which corresponds to the Latin vena cava, derives from the phenomenon that both vena cavae in deceased people do not contain blood, so they are hollow.
Anatomy and structure
The superior vena cava is formed at the level of the first rib by the union of the left and right brachiocephalic veins. With a length of only five to six centimeters, it runs directly to the right atrium or sinus venarum cavarum. At the level of the third rib, the azygos vein joins the superior vena cava. The azygos vein deserves special mention because, together with the hemiazygos vein, it forms so-called cavocaval anastomoses, connections between the two venous systems of the superior and inferior vena cava, so that in the event of stenosis or blockage in one of the two venous plexuses, the other venous system can serve to some extent as a back-up. With the exception of the absence of venous valves, the histological structure of the walls of the superior vena cava corresponds in principle to that of the other blood vessels. The innermost of the three layers that make up the vessel walls is called the intima and consists of a single-celled layer of epithelial cells only. The middle layer, the media, adjoins the intima on the outside. It is mainly composed of a meshwork of elastic and collagen fibers. The outermost layer, the adventitia, which adjoins the media on the outside, is formed mainly of connective tissue and, in the case of the superior vena cava, also contains smooth muscle cells and blood vessels to supply the vein walls.
Function and Tasks
The main function of the superior vena cava is to receive venous blood, which is deoxygenated, from body structures located above the diaphragm. Together with its counterpart, the inferior vena cava, the superior vena cava conveys the “used”, deoxygenated blood of the systemic circulation to the right atrium. From there, the blood enters the right ventricle and is pumped into the pulmonary circulation during the beating phase of the ventricles (ventricular systole). In the lungs, re-oxygenation takes place and carbon dioxide is excreted. Central venous blood pressure varies from 0 to about 15 mm Hg, which is much lower than arterial blood pressure. Similar to the large-volume main artery of the body, the aorta, which with its windkessel function provides mitigation of systolic pressure peaks and maintenance of residual diastolic pressure in the arteries, the two vena cavae provide a similar stabilizing influence on the venous side of the great circulatory system. The elastic fibers in the media of their vessel walls make it possible for the lumen of the vena cavae to passively adapt to requirements. Connections between the venous systems of the superior and inferior vena cava (cavocaval anastomoses) allow the superior vena cava to serve a back-up function for the inferior vena cava and also vice versa.
Diseases
The most common health complaints associated with the superior vena cava are due to mechanical dysfunction of the vena cava.Either it is compressed, so that its full cross-section is no longer available for the passage of venous blood, or internal vascular stenoses or thrombi obstruct the blood flow. The symptoms that occur are similar in both complexes of causes and are referred to as vena cava syndrome. The functional impairment of the vena cava can either be temporary as it is often observed in heavily pregnant women when the child compresses the inferior vena cava and leads to sometimes severe symptoms or it can lead to permanent problems in case of space occupation due to tissue proliferation. When the superior vena cava is compressed or otherwise obstructed in its flow, symptoms of so-called superior influence congestion present themselves. Affected individuals initially experience congestion of the jugular veins and an uncomfortable feeling of pressure in the neck and head area. As the condition progresses, the veins in the head and arms may also become congested and clearly visible. Causes of the upper influence congestion are mostly compressions, which result from space occupation of tumors or other tissue growths. High-frequency atrial fibrillation can also cause symptoms of upper influence congestion.
