An intercepting trunk for the lower left posterior intercostal veins: The hemiazygos vein arises from the ascending lumbar vein, passes up on the left side of the vertebrae to the eighth thoracic vertebra, where it may receive the accessory branch, and crosses over the vertebral column to open into the azygos vein.

The descending intercepting trunk for the upper, often the fourth through the eighth, left posterior intercostal veins: The accessory hemiazygos vein lies on the left side, and at the eighth thoracic vertebra it joins the hemiazygos vein, or crosses to the right side to join the azygos vein directly.