The azygos vein is a vein running up the right side of the thoracic

vertebral column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordate ...

draining itself towards the

superior vena cava The superior vena cava (SVC) is the superior of the two venae cavae, the great venous trunks that return deoxygenated blood from the systemic circulation to the right atrium of the heart. It is a large-diameter (24 mm) short length vein th ...

. It connects the systems of superior

vena cava In anatomy, the venae cavae (; singular: vena cava ; ) are two large veins (great vessels) that return deoxygenated blood from the body into the heart. In humans they are the superior vena cava and the inferior vena cava, and both empty into the ...

and inferior vena cava and can provide an alternative path for blood to the

right atrium The atrium ( la, ātrium, , entry hall) is one of two upper chambers in the heart that receives blood from the circulatory system. The blood in the atria is pumped into the heart ventricles through the atrioventricular valves. There are two at ...

when either of the venae cavae is blocked.

Structure

The azygos vein transports deoxygenated blood from the posterior walls of the

thorax The thorax or chest is a part of the anatomy of humans, mammals, and other tetrapod animals located between the neck and the abdomen. In insects, crustaceans, and the extinct trilobites, the thorax is one of the three main divisions of the cre ...

and

abdomen The abdomen (colloquially called the belly, tummy, midriff, tucky or stomach) is the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. The abdomen is the front part of the abdominal segment of the to ...

into the

. It is formed by the union of the

ascending lumbar vein The ascending lumbar vein is a vein that runs up through the lumbar region on the side of the vertebral column. Structure The ascending lumbar vein is a paired structure (i.e. one each for the right and left sides of the body). It starts at the c ...

s with the right subcostal veins at the level of the 12th thoracic vertebra, ascending to the right of the

descending aorta In human anatomy, the descending aorta is part of the aorta, the largest artery in the body. The descending aorta begins at the aortic arch and runs down through the chest and abdomen. The descending aorta anatomically consists of two portions o ...

and

thoracic duct In human anatomy, the thoracic duct is the larger of the two lymph ducts of the lymphatic system. It is also known as the ''left lymphatic duct'', ''alimentary duct'', ''chyliferous duct'', and ''Van Hoorne's canal''. The other duct is the righ ...

, passing behind the right crus of diaphragm, anterior to the vertebral bodies of T12 to T5 and right posterior intercostal arteries. At the level of T4 vertebrae, it arches over the root of the right lung from behind to the front to join the superior vena cava. The trachea and oesophagus is located medially to the arch of the azygous vein. The "arch of the azygos vein" (''arcus venae azygos'') is an important anatomic landmark. As an anatomical variation in 1–2% of the population, the arch can be displaced laterally, thereby creating a

pleura The pulmonary pleurae (''sing.'' pleura) are the two opposing layers of serous membrane overlying the lungs and the inside of the surrounding chest walls. The inner pleura, called the visceral pleura, covers the surface of each lung and dips b ...

l septum separating an

azygos lobe In human anatomy, an azygos lobe is a normal anatomical variation of the upper lobe of the right lung. It is seen in 0.3% of the population. Embryologically, it arises from an anomalous lateral course of the azygos vein, in a pleural septum withi ...

from the upper lobe of the right lung. The origin and anatomical course of the azygos vein are quite variable. Usually, there is a singular azygos vein on the right side of the body. However, the azygos vein is occasionally located in the midline or two independent veins may be present like in early embryonic development. In some rare variations, for example, it also drains thoracic veins, bronchial veins, and even gonadal veins. The vein is so named because it has no symmetrically equivalent

vein Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenat ...

on the left side of the body.

Development

It has been proposed that the azygos vein develops by originally draining to the posterior cardinal vein and then to the longitudinal venous channel. Following retrogression of the left common cardinal vein, the left azygos vein loses contact with the posterior cardinal vein. Thus, the blood drains into the right azygos line.

Tributaries

Major tributaries are the

hemiazygos vein The hemiazygos vein (vena azygos minor inferior) is a vein running superiorly in the lower thoracic region, just to the left side of the vertebral column. Structure The hemiazygos vein and the accessory hemiazygos vein, when taken together, essen ...

and

accessory hemiazygos vein The accessory hemiazygos vein, also called the superior hemiazygous vein, is a vein on the left side of the vertebral column that generally drains the fourth through eighth intercostal spaces on the left side of the body. Structure The accessor ...

, draining into the azygous vein at the midthoracic level. These two veins are located at the opposite side of the vertebral column. The azygous vein also receives drainage from all the

intercostal vein The intercostal veins are a group of veins which drain the area between the ribs ("costae"), called the intercostal space. They can be divided as follows: * Anterior intercostal veins * Posterior intercostal veins ** Posterior intercost vein tha ...

s on the right, except for

supreme intercostal vein The supreme intercostal vein (highest intercostal vein) is a paired vein that drains the first intercostal space on its corresponding side. It usually drains into the brachiocephalic vein. Alternatively, it drains into the superior intercostal v ...

(first intercostal vein). Other tributaries include the right

bronchial vein The bronchial veins are small vessels that return blood from the larger bronchi and structures at the roots of the lungs. The right side drains into the azygos vein, while the left side drains into the left superior intercostal vein or the accesso ...

s and veins from

pericardium The pericardium, also called pericardial sac, is a double-walled sac containing the heart and the roots of the great vessels. It has two layers, an outer layer made of strong connective tissue (fibrous pericardium), and an inner layer made o ...

, mediastinum, and

oesophagus The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to th ...

. It communicates with the vertebral venous plexuses.

Function

The azygos system of veins is considered to be the azygos vein, along with its left-sided counterparts, the

and the

. It also creates a cavo-caval anastomosis by offering an alternative, collateral blood flow from the lower half of the body to the superior vena cava, bypassing the inferior vena cava. This can have clinical significance in any blood flow restriction of the inferior vena cava.

Clinical significance

Azygos vein abnormalities can be suggested on chest radiograph by enlargement of the azygos shadow to greater than 1 cm. False positives can occur in heart failure causing increased pressures on the right side of the heart, or adjacent lymphadenopathy. Azygos and hemiazygos continuation of the inferior vena cava (IVC) was not common in daily life. It is very hard to observe, particularly when it is not associated with congenital heart disease or deep venous thrombosis. Thus, it is crucial to diagnose the enlarged azygos vein at the confluence with the superior vena cava and in the retrocrural space to prevent misdiagnosis as a right-sided paratracheal mass. The loss of the intrahepatic segment of IVC with azygous and hemiazygos continuation happens in 0.6% of patients diagnosed for congenital heart disease and usually occurs simultaneously with situs inversus, asplenia, or polysplenia, persistent left superior vena cava (SVC), and congenital pulmonary venolobar syndrome. Azygos and hemiazygos continuation of the IVC is rare especially when it is not associated with congenital heart disease.

History

The Greek root ''zyg'' refers to a pair. 'A-' means ''not''. Thus, ''azygos'' means ''unpaired''. The azygos vein is unpaired in that there is only one in the body, mostly on the right side. While there is the

and its accessory on the left side of the body, they are considered tributaries of the azygos vein rather than its left-side equivalent. This terminology is only accurate in some species, such as the human, dog, and cat. Ruminants (such as sheep and cows) have paired azygos veins.

Additional images

Image:Gray480.png, Diagram showing completion of development of the parietal veins. Image:Gray577.png, The venæ cavæ and azygos veins, with their tributaries. Image:Gray972.png, Mediastinal surface of right lung. File:Slide3zzzz.JPG, Azygos vein File:Slide6zzzz.JPG, Azygos vein File:CT of port-a-cath in azygos vein.jpg, CT scan showing a

port A port is a maritime facility comprising one or more wharves or loading areas, where ships load and discharge cargo and passengers. Although usually situated on a sea coast or estuary, ports can also be found far inland, such as H ...

whose tip goes into the azygos vein.

References

External links

* – "Right side of the mediastinum."
Dissection at lumc.edu

{{Authority control Veins of the torso