Vena Azygos: Structure, Function & Diseases

The azygos vein begins above the diaphragm and is a branch of the lumbar vein (ascending lumbar vein). It transports deoxygenated blood to the heart. In the event of a drainage disorder, the azygos vein may contribute to a bypass circulation because of its connections to other veins.

What is the azygos vein?

The azygos vein is an ascending vein in the trunk of the human body that originates from the lumbar vein (vena lumbalis ascendens) and joins the superior vena cava (vena cava superior). The azygos vein is part of the systemic circulation, also known as the great circulation, and transports deoxygenated blood. The name “vein” is derived from the Latin verb “venire,” which means “(to approach).” The blood flow of veins leads toward the heart – arteries, on the other hand, transport blood away from the heart. These designations are independent of whether the veins carry deoxygenated blood (as in the systemic circulation) or oxygenated blood (as in the pulmonary circulation). The azygos vein represents an unpaired vein because it has no exact counterpart in the left side of the body. The vena hemiazygos, located on the left, follows a different course than the vena azygos on the right. The azygos vein also owes its name to this circumstance, which comes from the Greek word for “unpaired.”

Anatomy and structure

The origin of the azygos vein is in the right lumbar vein (ascending lumbar vein). From this blood vessel, the azygos vein branches off above the diaphragm (diaphragm). The azygos vein runs along the right side of the spine, while the hemiazygos vein extends along the left side. The bronchial veins (venae bronchiales) and the intercostal veins (venae intercostales posteriores) flow into the vena azygos. In addition, blood from the esophageal veins (venae oesophageales) and from the vena hemiazygos flows into the vena azygos, which in turn ends in the superior vena cava. Before the azygos vein merges into the superior vena cava, it runs in an arc, which medicine also calls the arcus venae azygos. The wall of the azygos vein consists of three layers, with the tunica intima forming the innermost of them. The tunica media lies in the middle, but is not clearly demarcated from the outer tunica externa. In general, the walls of veins are thinner than the walls of arteries and, in particular, have weaker (smooth) ring muscles in the tunica media.

Function and Tasks

The function of the azygos vein is to receive deoxygenated blood from various confluent vessels and carry it to the superior vena cava. From there, the blood flows on to the right atrium. The vital organ then pumps the blood into the right ventricle, which brings it to the pulmonary circulation, also known as the small circulation. Via the pulmonary trunk (truncus pulmonalis), the deoxygenated blood finally reaches the lungs, where oxygen can bind to the red blood cells (erythrocytes). The blood of the azygos vein comes from the bronchial veins, among others, which drain blood from the bronchi and bronchial lymph nodes. Lymph nodes belong to the lymphatic organs and, as such, embody part of the immune system dedicated to fighting diseases and pathogens. The posterior intercostal veins also form tributaries to the azygos vein. These blood vessels form a group of intercostal veins, they are the counterpart of the intercostal arteries. Here, the venae intercostales posteriores represent the posterior intercostal veins and drain blood from the intercostal space, which anatomy also refers to as the intercostal space or spatium intercostale. Not all intercostal veins drain into the azygos vein; instead, some of them also drain into the hemiazygos vein and the internal thoracic veins. The oesophageal veins surround the esophagus and supply blood to the azygos vein that has previously reached the esophagus oxygenated via the aorta, intercostal artery, thyroid artery, and gastrica sinistra artery.

Diseases

Outflow obstruction affects blood flow in venous vessels. When such outflow obstruction affects the superior or inferior vena cava, the azygos vein may contribute to the bypass circulation-similar to the function of an artificial bypass. Connections between blood vessels are referred to in medicine as anastomoses.Various causes are possible for the development of outflow obstruction, which also determine possible treatment options. One potential cause is tumors that constrict the veins. Thrombi and other deposits within the vein can also lead to narrowing. Venous weakness or venous insufficiency particularly commonly affects the lower extremities and often manifests itself in symptoms such as swollen legs or feet, visible veins and spider veins, skin changes and pain in the legs. However, symptoms may differ if there is a drainage disorder in the upper veins. In addition, dilatation of the azygos vein is considered one indicator (of several) of pulmonary hypertension. This is increased blood pressure in the pulmonary circulation due to increased vascular resistance. Dilatation of the azygos vein is one of the radiographic signs of the condition: a vein diameter of more than 7 mm is considered critical. Pulmonary hypertension can be caused by various chronic and acute processes, for which, for example, structural changes or physiological stress reactions can be responsible. In this case, treatment also depends on the specific cause.