Final answer:
The azygos vein system, including the hemiazygos and accessory hemiazygos veins, alongside the internal thoracic veins, primarily handles the venous drainage of the thoracic wall, flowing into the superior vena cava.
Step-by-step explanation:
The principal venous drainage of the thoracic wall is primarily by the azygos vein system and the internal thoracic veins, also known as internal mammary veins. The azygos vein originates in the lumbar region and passes through the diaphragm into the thoracic cavity on the right side of the vertebral column. It drains blood from the intercostal veins, esophageal veins, bronchial veins, and other veins draining the mediastinal region, eventually leading to the superior vena cava. Meanwhile, each internal thoracic vein drains the anterior surface of the chest wall and flows into the brachiocephalic vein.
Moreover, each intercostal vein drains muscles of the thoracic wall, each esophageal vein delivers blood from the inferior portions of the esophagus, and each bronchial vein drains the systemic circulation from the lungs. These veins all contribute to the blood flow into the azygos vein. Furthermore, the inferior vena cava drains the areas largely inferior to the diaphragm but does not directly contribute to the venous drainage of the thoracic wall.