Contradictory information concerning the formation of the azygos and hemiazygos veins and their relation to the diaphragm is given in anatomical textbooks. Often, they are described to be a direct continuation of the right resp. left ascending lumbar veins. The aim of this study was to re-evaluate their caudal commencement and course. We hypothesize that both veins originate from several tributaries and are not a direct continuation of the ascending lumbar veins.

20 formalin-embalmed human anatomic specimens were dissected. Caudal commencement and relation to the diaphragm of the azygos and hemiazygos veins were examined.

The azygos (100 %) and hemiazygos veins (90 %) were mostly formed by dorsal and ventral roots within the thorax. The dorsal root was almost always formed by the confluence of the ascending lumbar and subcostal veins and coursed dorsal to the psoas major muscle, thus far dorsally to the diaphragm. The ventral root was formed variably from the upper lumbar veins or from the inferior vena cava for the azygos vein, and from the left renal or upper lumbar veins for the hemiazygos vein. It passed the diaphragm variably either through the aortic hiatus or through its crura.

The azygos and hemiazygos veins stem almost exclusively from the confluence of a dorsal and ventral root uniting ventrolaterally to the body of the 12th thoracic vertebra cranial to the diaphragm. Due to their position, only the ventral root passes the diaphragm whereas the dorsal root has no contact to the diaphragm.

Azygos vein

Hemiazygos vein

Lumbar ascending vein

Subcostal vein

Posterior intercostal vein

© 2026 The Author(s). Published by Elsevier GmbH.