azygos vein

  Explanation

The azygos vein is outlined in orange.  It joins the superior vena cava from posteriorly and provides an important collateral pathway for blood return to the heart in cases of superior or inferior vena cava obstruction.  The superior vena cava is outlined in green.  It forms at the junction of the right and left brachiocephalic veins, which are indicated in blue.  

Image A and the accompanying movie show a different patient with an abnormality of the azygos.

The aorta is shown in red, the superior vena cava in dark blue, the azygos and other posterior collaterals in light blue and the top of the main pulmonary artery in purple.