Azygos Venous System

The azygos venous system is a network of longitudinal veins located along the posterior thoracic wall. It is responsible for draining the thoracic wall and some abdominal structures and forms an important collateral pathway between the superior and inferior vena cava. This system consists of three main veins: the azygos vein on the right, and the hemiazygos and accessory hemiazygos veins on the left.

Functions

• Drains venous blood from the posterior thoracic wall, mediastinal structures, and upper lumbar region
• Provides a collateral pathway between the superior and inferior vena cava in case of obstruction

Azygos Vein

Origin

The azygos vein usually originates from the union of the right ascending lumbar vein and the right subcostal vein. In some cases, it may arise directly from the posterior aspect of the inferior vena cava at the level of L1.

Course

The azygos vein enters the thoracic cavity through the aortic hiatus of the diaphragm (T12 level). It ascends along the right side of the thoracic vertebral bodies, arching anteriorly over the root of the right lung at the level of T4 to empty into the superior vena cava.

Tributaries

• Right posterior intercostal veins (except 1st)
• Hemiazygos and accessory hemiazygos veins
• Right superior intercostal vein
• Esophageal veins
• Mediastinal veins
• Right bronchial vein
• Pericardial veins (occasionally)

Hemiazygos Vein

Origin

The hemiazygos vein is typically formed by the union of the left ascending lumbar vein and left subcostal vein. It may also receive tributaries from abdominal structures.

Course

It ascends on the left side of the vertebral column up to about the level of T8 or T9, where it crosses over the vertebral column posterior to the aorta, esophagus, and thoracic duct to join the azygos vein.

Tributaries

• Lower left posterior intercostal veins (9th–11th)
• Esophageal veins
• Mediastinal veins

Accessory Hemiazygos Vein

Origin and Course

The accessory hemiazygos vein begins at the 4th or 5th intercostal space and descends along the left side of the vertebral column. Around the level of T7 or T8, it crosses to the right to join the azygos vein or hemiazygos vein.

Tributaries

• Left posterior intercostal veins (4th to 8th)
• Sometimes communicates with the left superior intercostal vein

Left Superior Intercostal Vein

This vein drains the 2nd and 3rd (sometimes 4th) left posterior intercostal veins. It passes over the aortic arch and typically drains into the left brachiocephalic vein. It may communicate with the accessory hemiazygos vein inferiorly.

Anatomical Variations

• The number and pattern of tributaries can vary between individuals
• Hemiazygos and accessory hemiazygos veins may be incomplete or replaced by a single vein
• Sometimes all intercostal veins drain into a common left vein that crosses to join the azygos

Topographic Relationships

Azygos Vein

• Anterior: Esophagus, right lung root
• Posterior: Vertebral bodies
• Right: Right lung and pleura
• Left: Thoracic duct, aorta

Hemiazygos and Accessory Hemiazygos

• Anterior: Esophagus, descending thoracic aorta
• Posterior: Vertebral column
• Lateral: Left lung and pleura

Clinical Significance

• Collateral circulation: In cases of inferior or superior vena cava obstruction, the azygos system serves as a vital alternative pathway for venous return
• Visible enlargement: Dilated azygos vein may be seen on chest imaging in right heart failure or obstruction syndromes

Topographic Summary

• Azygos vein: Right side, drains into SVC
• Hemiazygos vein: Left side, crosses over to join azygos around T8–T9
• Accessory hemiazygos: Drains mid-left thoracic wall
• Supplies: Posterior thoracic wall, vertebrae, mediastinum
• Important role: Collateral drainage between SVC and IVC