In obstruction of the superior or inferior vena cava, venous blood is returned to the heart by an alternate route via the azygos vein, which becomes dilated in the process.
Thoracic duct-below the level of the sternal angle, the thoracic duct lies posterior to the esophagus, between the azygos vein and the descending aorta. So, if the azygos vein became dilated, it could impinge on the thoracic duct. The trachea is superior to the azygos vein, which loops over the right bronchus before emptying into the superior vena cava. This means that a dilated azygos vein would have little impact on the trachea. Because the azygos is on the right side of the body, it could not compress anything at the root of the left lung. The right phrenic nerve is anterior to the azygos vein. The descending aorta is much larger and more muscular than the azygos vein, so it is unlikely that it would be compressed by this much smaller structure.
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