“Azygos” = single (no companion). Azygos system = azygos vein + hemiazygos vein + accessory hemiazygos vein; lie in front of thoracic vertebral column.

Azygos Vein

• Right side only; upper posterior abdominal wall + posterior mediastinum
• Connects IVC ↔ SVC; has valves; may be tortuous

Functions: drains thoracic wall + upper lumbar region; collateral channel connecting SVC ↔ IVC

Formation (variable):

1. Union of right subcostal vein + right ascending lumbar vein at T12 (most common)
2. Continuation of posterior IVC near renal veins
3. Continuation of right subcostal vein
4. Occasionally from right renal vein or right 1st lumbar vein

Course/Termination:

1. Ascends, leaves abdomen via aortic opening of diaphragm → posterior mediastinum
2. Ascends in front of vertebral column to T4
3. At T4, arches forward above right lung hilum
4. Terminates in SVC at 2nd costal cartilage level

Relations:

Arch relations:

Tributaries:

1. Lower 7 right posterior intercostal veins (5th–11th, except 1st)
2. Right superior intercostal vein (2nd+3rd+4th right posterior intercostal veins)
3. Hemiazygos vein (joins T7/T8)
4. Accessory hemiazygos vein (joins T8/T9)
5. Right subcostal vein
6. Right bronchial vein
7. Right ascending lumbar vein
8. Esophageal veins (except lower end)
9. Mediastinal veins
10. Pericardial veins

Hemiazygos Vein (Inferior Hemiazygos)

• Left side only; mirror image of lower azygos vein

Formation: left ascending lumbar vein + left subcostal vein union; may arise from posterior left renal vein

Course: pierces left crus → ascends front of left vertebral column to T8 → turns right, crosses posterior to aorta, esophagus, thoracic duct → terminates in azygos vein

Tributaries: lower 3 (9th–11th) left posterior intercostal veins; left subcostal vein; left ascending lumbar vein; small esophageal/mediastinal veins

Accessory Hemiazygos Vein (Superior Hemiazygos)

• Left side only; mirror image of upper azygos vein

Course: begins at medial end of left 4th/5th ICS → descends left vertebral column → at T8, turns right, crosses posterior to aorta, esophagus, thoracic duct → terminates in azygos vein

Hemiazygos + accessory hemiazygos terminal parts sometimes join as common trunk before crossing to azygos

Tributaries: 5th–8th left posterior intercostal veins; left bronchial veins (sometimes)

Comparison

Applied Anatomy

• SVC obstruction — azygos vein = main collateral channel, shunts blood from upper body to IVC bypassing obstruction; hemiazygos + accessory hemiazygos also participate

The term “azygos” means single — i.e., without a companion. The azygos system of veins consists of the azygos vein, hemiazygos vein, and accessory hemiazygos vein. These veins lie in front of the thoracic part of the vertebral column and play an important role in the venous drainage of the thorax.

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Azygos Vein

The azygos vein is present only on the right side, in the upper part of the posterior abdominal wall and the posterior mediastinum. It connects the inferior vena cava with the superior vena cava and is provided with valves; it may appear tortuous.

Functions

1. Drains venous blood from the thoracic wall and upper lumbar region
2. Forms an important collateral channel connecting the SVC and IVC

Formation

The formation of the azygos vein is variable. It may form in one of the following ways:

1. Union of the right subcostal vein and right ascending lumbar vein at the level of T12 vertebra (most common)
2. As a continuation of the posterior aspect of the inferior vena cava near the renal veins
3. As a continuation of the right subcostal vein
4. Occasionally arises from the right renal vein or right 1st lumbar vein

Course and Termination

After formation, the azygos vein:

1. Ascends and leaves the abdomen through the aortic opening of the diaphragm → enters the posterior mediastinum
2. Ascends vertically in front of the vertebral column up to the level of T4 vertebra
3. At T4, arches forwards above the hilum of the right lung
4. Terminates in the superior vena cava at the level of the 2nd costal cartilage

Relations

Relations of the Arch of Azygos Vein

Tributaries

1. Lower 7 right posterior intercostal veins (i.e., 5th–11th, except the 1st)
2. Right superior intercostal vein (formed by union of 2nd, 3rd, and 4th right posterior intercostal veins)
3. Hemiazygos vein (joins at the level of T7 or T8 vertebra)
4. Accessory hemiazygos vein (joins at the level of T8 or T9 vertebra)
5. Right subcostal vein
6. Right bronchial vein
7. Right ascending lumbar vein
8. Esophageal veins (except those at its lower end)
9. Mediastinal veins
10. Pericardial veins

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Hemiazygos Vein

The hemiazygos vein (syn. inferior hemiazygos vein) lies on the left side only and corresponds to the lower part of the azygos vein (i.e., it is the mirror image of the lower part of the azygos vein).

Formation

Formed, similar to the azygos vein, by the union of the left ascending lumbar vein and left subcostal vein. May arise from the posterior surface of the left renal vein.

Course and Termination

1. Pierces the left crus of the diaphragm, ascends vertically in front of the left side of the vertebral column up to T8 vertebra
2. At T8, turns right and crosses in front of the vertebral column, posterior to the aorta, esophagus, and thoracic duct
3. Terminates in the azygos vein

Tributaries

1. Lower three (9th–11th) left posterior intercostal veins
2. Left subcostal vein
3. Left ascending lumbar vein
4. Small esophageal and mediastinal veins

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Accessory Hemiazygos Vein

The accessory hemiazygos vein (syn. superior hemiazygos vein) lies on the left side only and corresponds to the upper part of the azygos vein (mirror image of the upper part of the azygos vein).

Course and Termination

1. Begins at the medial end of the left 4th or 5th intercostal space
2. Descends along the left side of the vertebral column
3. At T8 vertebra, turns right and passes in front of the vertebral column, posterior to the aorta, esophagus, and thoracic duct
4. Terminates in the azygos vein

N.B. Sometimes the terminal parts of the hemiazygos and accessory hemiazygos veins join together to form a common trunk, which crosses the vertebral column to open into the azygos vein.

Tributaries

1. 5th–8th left posterior intercostal veins
2. Left bronchial veins (sometimes)

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Comparison — Azygos, Hemiazygos, and Accessory Hemiazygos Veins

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Applied Anatomy

Azygos Vein as Collateral Channel in SVC Obstruction

In superior vena cava (SVC) obstruction, the azygos vein serves as the main collateral channel to shunt blood from the upper half of the body to the inferior vena cava, bypassing the obstruction. The hemiazygos and accessory hemiazygos veins, draining into the azygos vein, also participate in this collateral circulation.

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