AZIGOS-HEMIAZIGOS VENOUS SYSTEM
Anatomy:
The azygos-hemiazygos system is a paired paravertebral venous pathway in the posterior thorax with a H-shaped configuration,
comprised of the azygos,
hemiazygos,
accessory hemiazygos veins and superior intercostal veins.
It is responsible for draining the thoracic wall and upper lumbar region via the lumbar veins and posterior intercostal veins.
The azygos vein is a unilateral vessel that arises at the union of the right ascending lumbar vein and the right subcostal vein around T12-L2,
enters the thorax via the aortic hiatus in the diaphragm and ascends along the anterolateral surface of the thoracic vertebrae,
arching over the right main bronchus at T5-T6 and enters the SVC at T4.
This arch of the azygos vein is an important anatomical landmark,
and contains a valve halfway,
which may led to accumulation of the intravenously injected contrast agent.
Although there is much variability,
the azygos vein receives the accessory hemiazygos vein and the hemiazygos vein at the level of T8 and T9 respectively.
The hemiazygos vein is a similar structure on the opposite side of the vertebral column and similar to the azygos vein is formed by the confluence of the left ascending lumbar and left subcostal veins.
It enters the thorax either through the aortic hiatus or directly through the diaphragmatic crura,
ascends along the left anterolateral aspect of the thoracic vertebrae and at T8–T9 crosses dorsal to the descending thoracic aorta to join the azygos vein.
The accessory hemiazygos vein which drains the superior left hemithorax and left bronchial vein,
arises from the 4-8thleft posterior intercostal veins,
lies longitudinally on the left side of the vertebral bodies and joins the azygos vein behind the oesophagus at different levels,
usually at the level of T8.
The right and left superior intercostal veins drains the left 2nd-4thintercostal spaces and join respectively the azygos vein and the left brachiocephalic vein.
Embryology:
The azygos venous system arises from the supracardinal veins embryologically: the azygos vein is considered to derive from the upper right supracardinal vein,
the hemiazygos vein from the upper left supracardinal vein and the azygos arch from an upper segment of the right posterior cardinal vein.
In the abdominal cavity,
the supracardinal veins become the infrarenal portion of the definitive inferior vena,
as thus a comunication between both systems is possible.
|