azygos vein

The azygos vein arises from the union of the right subcostal vein and right ascending lumbar vein, ascending through the aortic hiatus (or occasionally through the diaphragmatic crura) alongside the aorta and thoracic duct. It travels along the right side of the vertebral bodies in the posterior mediastinum, receiving tributaries including the posterior intercostal veins (4th-11th on the right), the hemiazygos and accessory hemiazygos veins (which cross the midline to drain left-sided structures), the right superior intercostal vein, and esophageal, mediastinal, and bronchial veins. At the level of T4-T5, it arches over the root of the right lung (arching over the right main bronchus) to join the posterior aspect of the SVC just before it enters the right atrium. Clinically, the azygos system provides a vital anastomotic connection between the SVC and IVC, allowing venous blood to bypass an obstructed IVC (e.g., due to thrombosis or tumor compression) and drain into the SVC instead. This collateral pathway is important in IVC syndrome and can become dramatically dilated. The 'azygos vein sign' on chest CT/X-ray refers to a dilated azygos vein seen in conditions of IVC obstruction or elevated right heart pressures. Additionally, an azygos lobe is a congenital anatomic variant where the azygos vein invaginates into the right upper lobe of the lung during development, creating a fissure visible on chest imaging with no significant clinical consequence in most cases. The azygos vein also drains most of the mediastinal structures and is a landmark for identifying the right main bronchus during bronchoscopy and thoracic surgery.