The azygos system is a crucial part of the body's circulatory system and drains most of the thorax. It comprises the azygos, hemiazygos, and accessory hemiazygos veins.

The azygos vein, positioned just right of the midline and anterior to the vertebral column, begins at the junction of the right ascending lumbar and subcostal veins, terminating in the superior vena cava. This vein drains blood from the right side of the thoracic wall, thoracic viscera, and posterior abdominal wall.

The hemiazygos vein lies on the left of the midline and anterior to the vertebral column. It starts at the intersection of the left ascending lumbar and subcostal veins, ending by merging with the azygos vein. It drains blood from the left lower thoracic wall, thoracic viscera, and the left posterior abdominal wall.

The accessory hemiazygos vein also lies on the left of the midline, draining the left side of the upper thoracic wall and thoracic viscera before terminating in the azygos vein.

Additionally, the left and right brachiocephalic veins merge into the superior vena cava, draining the head, neck, upper limbs, and upper thorax. These veins are essential for returning deoxygenated blood back to the heart, maintaining the efficiency and balance of the circulatory system.