The vena cava is the name given to the two large veins, the superior vena cava (superior vena cava) and the inferior vena cava (inferior vena cava), in which the blood of the large systemic circulation is collected and directed to the right atrium in the common inflow sinus venarum cavarum. These are the two veins with the largest internal diameter, reaching two to three centimeters, depending on requirements.
What is a vena cava?
The return flow of deoxygenated blood from the systemic circulation to the heart occurs through the superior vena cava (superior vena cava) and the inferior vena cava (inferior vena cava). The two vena cavae open into a common inflow pathway (sinus venarum cavarum) into the right atrium, from where the blood is pumped via the right ventricle into the pulmonary circulation to be re-oxygenated. The two vena cavae reach a variable cross-section of two to three centimeters, making them the body veins with the largest cross-section. The superior vena cava collects venous blood from the upper part of the body, i.e., from the head–neck-thoracic region and the upper extremities. This also includes the organs located above the diaphragm, such as the lungs. However, this does not include the blood of the closed pulmonary circulation, which does not serve to supply energy and nutrients to the lung tissue. The inferior vena cava receives venous blood return from the abdomen and lower extremities.
Anatomy and structure
The superior vena cava is formed at the level of the first rib on the right edge of the sternum by the union of several veins that collect venous blood from the head, neck, and arms. Continuing to the right atrium of the heart, the azygos vein enters, which, together with the hemiazygos vein, forms a system of cavocaval anastomoses, that is, a connection between the venous vascular system of the superior and inferior vena cava. The inferior vena cava is formed by the union of the two great iliac veins and runs upward to the right of the descending branch of the aorta. Several vein orifices provide direct blood drainage from the lower organs and other body tissues. This does not apply to the intestinal area because the blood, which is enriched with numerous substances, is first directed from the portal vein to the liver and only enters the inferior vena cava after being processed in the liver just below the diaphragm. Unlike the other veins, the two vena cavae do not have venous valves. The walls of the vena cavae, like all blood vessels, are composed of three layers; however, the walls of the veins are much thinner compared to the walls of the arteries because of the much lower blood pressure within the venous vasculature. The innermost layer of the vena cava walls, the intima, consists of endothelial cells that originate from a fine connective tissue membrane called the basement membrane. The middle layer, called the media, consists of elastic fibers and smooth muscle cells. Connected to the outside is the externa or adventitia, which contains connective tissue and elastic fibers. Blood vessels and nerve fibers also run in the externa to supply the vessels with oxygenated arterial blood.
Function and tasks
The two vena cavae act as a central collecting basin for the “used”, deoxygenated blood from the large body circulation. Their main function is to empty the collected venous blood during the relaxation phase of the atria into the right atrium, from where it is pumped via the right ventricle into the small systemic circulation, also known as the pulmonary or pulmonary circulation. In another function, the vena cavae, by virtue of their large volume in conjunction with their partially elastic vascular walls, provide pressure equalization in the venous vasculature so that the central venous blood pressure within the large body circulation does not rise above 15 mm Hg. The cavocaval anastomoses, which correspond to a connection between the vascular system of the superior and inferior vena cava, can provide back-up functions to a certain extent in an emergency, which can come into play when stenosis or even blockage of a vein occurs. The two vena cavae offer an ideal opportunity to insert catheters into the right atrium for diagnostic or therapeutic purposes without having to bypass a heart valve, for example.
Diseases
The most common health complaints associated with the two vena cavae are caused by temporary or permanent functional restriction. The functional restriction may be caused by external action, such as compression of the vessel, or by internal stenosis or blockage. The best-known form of compression of the vena cava is vena cava syndrome, which can affect heavily pregnant women in particular. The syndrome can occur when the expectant mother is in the supine position and the baby compresses the inferior vena cava, obstructing the return of venous blood from regions below the diaphragm. This can cause an acute drop in blood pressure, which can even lead to unconsciousness in the heavily pregnant woman. However, vena cava syndrome can also be triggered by swelling and by tumors if the growths occupy appropriate space. If the superior vena cava is affected, the condition is known as superior influence congestion (superior vena cava syndrome), which is usually manifested by specific symptoms such as a feeling of pressure in the neck. Similar symptoms may be seen when either vena cava is blocked by injury or by narrowing (stenosis) or by blood clots (thrombi).
