The right atrium is the chamber of the heart that gets deoxygenated blood returning from the body. It is located on the right side of the midline, and the 2 major veins, the superior and inferior venae cavae, that drain into it are also situated on the right side of the body. This means that, to get to the right side of the body, all blood coming from the left side has to cross the midline. This left-to-right shunting is performed by a number of significant and, in certain cases, quite large veins, several of that are in the thorax.
In adults, the left brachiocephalic vein crosses the midline immediately posterior to the manubrium and produces blood from the left side of the head and neck, the left upper limb, and part of the left thoracic wall into the superior vena cava.
The hemiazygos and accessory hemiazygos veins drain posterior and lateral parts of the left thoracic wall, pass immediately anterior to the bodies of thoracic vertebrae, and flow into the azygos vein on the right side, which finally attaches together with the superior vena cava.