The Azygos System of Veins

Manubriosternal plane | Main Anatomy Index | Pleurae and pleural cavities

Last updated 30 March 2006

The Azygos System of Veins

• The azygos system consists of veins on each side of the vertebral column.
• These veins drain the back, thoracic and abdominal walls.
• It exhibits much variation, not only in its origin, but also its course, tributaries, anastomoses, and termination.

• The azygos vein and its main tributary, the hemiazygos vein, usually arise from the posterior aspect of the IVC and the renal vein respectively.
• These veins provide another means of venous drainage from the abdomen and thorax.

The Azygos Vein

• The azygos vein connects the superior and inferior venae cavae, either directly by joining the IVC or indirectly by the hemiazygos and accessory hemiazygos veins.
• The azygos vein drains blood from the posterior walls of the thorax and abdomen.

• It ascends in the posterior mediastinum, passing close to the right sides of the bodies of the inferior eight thoracic vertebrae (T4-T12).
• It is covered anteriorly by the oesophagus as it passes posterior to the root of the right lung.
• It then arches over the superior aspect of this root to join the SVC.

• In addition to the posterior intercostal veins, the azygos vein communicates with the vertebral venous plexuses.
• This vein also receives the mediastinal, oesophageal, and bronchial veins.

The Hemiazygos Vein

• This vein arises on the left side of the junction of the left subcostal and ascending lumbar veins.
• It ascends on the left side of the vertebral column, posterior to the thoracic aorta, as far as T9 vertebra.
• Here it crosses to the right, posterior to the aorta, thoracic duct, and oesophagus, and joins the azygos vein.

• The hemiazygos vein receives the inferior three intercostal veins, the inferior oesophageal veins, and several small mediastinal branches.

The Accessory Hemiazygos Vein

• This vein begins at the medial end of the 4^th or 5^th intercostal space and descends on the left side of the vertebral column from T5 to T8.

• It receives tributaries from veins in the 4^th to 8^th intercostal spaces and sometimes from the left bronchial veins.
• It crosses over T7 or T8 vertebrae, posterior to the thoracic aorta and thoracic duct, where it joins the azygos vein.
• Sometimes the accessory hemiazygos vein joins the hemiazygos vein and opens with it in the azygos.
• The accessory azygos vein is frequently connected to the superior intercostal vein.

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