Expansion
Azygos on the right, hemiazygos and accessory hemiazygos on the left
Expansion
- Azygos vein - the whole right side, ascending on the right of the vertebral bodies
- Hemiazygos vein - lower left side, crossing the midline at about T9 to join the azygos
- Accessory hemiazygos vein - upper left side, crossing at about T8
The system connects the inferior and superior venae cavae. That is its clinical importance: in superior vena cava obstruction or inferior vena cava obstruction, the azygos route enlarges to carry collateral flow, and a dilated azygos vein becomes visible at the right tracheobronchial angle on a chest radiograph.
An azygos lobe is a normal variant, formed when the vein migrates abnormally through the apex of the right lung, dragging a mesentery of pleura with it.