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What is hemiazygos vein

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Here is a detailed, textbook-based explanation of the hemiazygos vein:

Hemiazygos Vein

The hemiazygos vein (also called the inferior hemiazygos vein) is part of the azygos venous system, which drains the posterior thoracic and abdominal walls on both sides of the body.
Azygos System of Veins
Fig. 3.105 - Azygos System of Veins (Gray's Anatomy for Students)

Origin

The hemiazygos vein usually arises at the junction between the left ascending lumbar vein and the left subcostal vein. It may also arise from either of these veins alone, and often has a connection to the left renal vein.

Course

  • It usually enters the thorax through the left crus of the diaphragm (sometimes through the aortic hiatus).
  • It ascends through the posterior mediastinum on the left side of the vertebral column.
  • At approximately vertebral level T9 (TIX), it crosses the vertebral column posterior to the thoracic aorta, esophagus, and thoracic duct.
  • It then enters the azygos vein on the right side.

Tributaries

The hemiazygos vein receives:
  1. The lowest 4-5 left posterior intercostal veins
  2. Esophageal veins
  3. Mediastinal veins

Relation to the Accessory Hemiazygos Vein

The accessory hemiazygos vein (also called the superior hemiazygos vein) handles the upper left side. It descends on the left from the superior posterior mediastinum to approximately T8 (TVIII), where it crosses to join the azygos vein, or ends in the hemiazygos vein, or connects to both. It drains the 4th to 8th left posterior intercostal veins and sometimes the left bronchial veins.
Together, the hemiazygos and accessory hemiazygos veins are the left-sided counterparts of the azygos vein.

Summary Table

FeatureHemiazygos VeinAccessory Hemiazygos Vein
Also calledInferior hemiazygosSuperior hemiazygos
SideLeftLeft
Level of crossing~T9 (TIX)~T8 (TVIII)
OriginL. ascending lumbar + L. subcostal veinDescends from superior posterior mediastinum
Drains intoAzygos veinAzygos vein or hemiazygos
Intercostal veins drainedLowest 4-5 left4th-8th left posterior intercostal

Clinical Significance

The azygos system (including the hemiazygos) provides an important collateral venous pathway between the superior and inferior vena cava. If the portal vein or either main vena cava is obstructed, this system can carry blood as an alternative route. In portal hypertension, the azygos/hemiazygos system participates in forming esophageal varices through communication with esophageal veins.
  • Gray's Anatomy for Students, p. 268-269
  • Color Atlas of Human Anatomy Vol. 2, Internal Organs, p. 127
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