Overview

Azygos vein. The azygos vein begins at about L1–L2, where the right ascending lumbar and right subcostal veins join. Sometimes it arises from the back of the IVC instead. It enters the thorax through the aortic hiatus or through the right crus of the diaphragm. It then ascends in the posterior mediastinum on the right of the vertebral bodies, beside the thoracic duct and aorta. At T4 it arches forward over the right main bronchus and ends in the SVC.

Hemiazygos and accessory hemiazygos veins. These are the left-sided equivalents. The hemiazygos drains the lower left posterior intercostal veins and crosses behind the aorta, oesophagus and thoracic duct at about T8–T9 to join the azygos. The accessory hemiazygos drains the middle left intercostal spaces and crosses at about T7–T8.

Valves and connections. The azygos system has few or no effective valves. It connects freely with the valveless vertebral venous plexus (Batson plexus). Cancers of the prostate and breast can spread to the spine through this route.

Anatomy

Tributaries: The right posterior intercostal veins (spaces 2–4 drain through the right superior intercostal vein), the hemiazygos and accessory hemiazygos veins, and oesophageal, bronchial, mediastinal and pericardial veins.

Function

Drains the posterior thoracic and upper abdominal walls, the oesophagus and the bronchi.
Provides a bypass route between the IVC and SVC territories.

Clinical significance

Caval obstruction

If the IVC is blocked, blood from the lower body can reach the SVC through the lumbar veins and the azygos system. If the SVC is blocked below the point where the azygos joins it, blood flows the opposite way, down the azygos to the IVC.

Oesophageal varices

In portal hypertension, blood from the left gastric vein is diverted into the oesophageal veins and from there into the azygos system. The oesophageal veins dilate to form varices.

Azygos lobe

In about 1% of people the azygos vein runs through the apex of the right lung instead of over it. On chest radiograph this shows as a curved fissure, the azygos fissure, and the trapped part of the lung is called the azygos lobe. It is a harmless variant.

Test yourself

Oesophageal varices in portal hypertension develop at an anastomosis between which veins?

Left gastric vein and oesophageal tributaries of the azygos vein
Superior rectal vein and inferior rectal veins
Paraumbilical veins and superficial epigastric veins
Splenic vein and left renal vein

Answer: Left gastric vein and oesophageal tributaries of the azygos vein

The left gastric (coronary) vein belongs to the portal system and the oesophageal veins drain into the azygos (systemic) system. When portal pressure rises they dilate into oesophageal varices, which can bleed catastrophically. The other options describe the anorectal, paraumbilical and retroperitoneal anastomoses.