The azygos system of veins

• The azygos vein: commences as the union of the right subcostal vein and one or more veins from the abdomen. It passes through the aortic opening in the diaphragm, ascends on the posterior chest wall to the level of T4 and then arches over the right lung root to enter the superior vena cava. It receives tributaries from the: lower eight posterior intercostal veins, right superior intercostal vein and hemiazygos veins.

• The hemiazygos vein: arises on the left side in the same manner as the azygos vein. It passes through the aortic opening in the diaphragm and up to the level of T9 from where it passes diagonally behind the aorta and thoracic duct to drain into the azygos vein at the level of T8. It receives venous blood from the lower four left posterior intercostal veins.

• The accessory hemiazygos vein: drains blood from the middle posterior intercostal veins (as well as some bronchial and mid-oesophageal veins). The accessory hemiazygos crosses to the right to drain into the azygos vein at the level of T7.

• The upper four left intercostal veins drain into the left brachio-cephalic vein via the left superior intercostal vein.

The mediastinum II—the vessels of the thorax 13

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