Posterior Mediastinum - LO 3

3. Describe the azygos venous system.

The azygos venous system is a series of unpaired (a- = not, zygous = yoke) veins into which the posterior intercostal, subcostal, and lumbar vv. of the thoracic and abdominal walls drain.

Intercostal neurovascular bundles consist of a vein, artery, and nerve (vein nearest to the rib: VAN). Anterior intercostal vv. drain into the internal thoracic vv., whereas posterior intercostal vv. drain into a variety of sources. See table below for details.

The azygos system of veins can exhibit a great deal of variation. The azygos v. typically runs along the right side of the vertebral column in this region, arching over the root of the R. lung to drain into the superior vena cava.

The hemi-azygos and accessory hemi-azygos vv. are inconstant. Both vessels may be present; however, each vessel or both vessels may be absent. The accessory hemi-azygos v. is the superior partner of the azygos v. (‘a goes first’). It typically runs along the left side of the vertebral column between T5-T8 and drains into the azygos v.

The hemi-azygos v. is the main tributary of the azygos v. and is often referred to as the inferior partner of the azygos v. It typically ascends as far as T9 and drains into the azygos v.