The thoracic wall and upper lumbar region are drained by the posterior intercostal and lumbar veins into the azygous veins. These consist of longitudinal trunks on right and left sides, similar to the posterior cardinal veins of the embryo. There is a single trunk on the right. On the left side the posterior cardinal vein disappears and is replaced by longitudinal veins that persist from the embryonic prevertebral venous plexuses; these are the hemiazygous veins.
This vessel represents the persistent right posterior cardinal vein of the embryo. Thus it begins embryo- logically at the inferior vena cava just above the renal veins, but this part is usually an avascular fibrous cord. The union of the ascending lumbar vein with the subcostal vein of the right side is the functional (blood- containing) commencement of the azygous vein. The vessel goes through the aortic opening of the diaphragm under shelter of the right crus and passes upwards lying on the sides of the vertebral bodies, on a plane posterior to that of the esophagus. Descent of the heart and pericardium in the embryo caused the right posterior cardinal vein to arch over the right bronchus. This the azygous vein does at the level of T4 vertebra and passing forward of the esophagus the vessel enters the superior vena cava. It receives the lower eight posterior intercostal veins and at its convexity the superior intercostal vein joins it. It receives the bronchial veins from the right lung and some veins from the middle third of the esophagus. The two hemiazygous veins join it.
These two veins lie longitudinally on the left side of the bodies of the thoracic vertebrae. They may communicate with each other, but characteristically drain separately from their adjoining ends behind the esophagus into the azygous vein. They receive the lower eight posterior intercostal veins, four each. The inferior vein is the hemiazygous vein, and the superior one is the accessory hemiazygous vein. The latter receives the bronchial veins from the left lung and some veins from the middle third of the esophagus.