Right azygos vein

The right azygos vein in domestic animals is a major thoracic vein responsible for draining the dorsal thoracic wall and adjacent structures. In most domestic mammals, including dogs and horses, the right azygos vein is typically present and well developed, whereas the left azygos vein is variably present depending on the species.

Anatomically, the right azygos vein originates from the lumbar region, often as a continuation of the right ascending lumbar vein and the right subcostal vein. It ascends cranially along the right side of the vertebral column, within the thoracic cavity, dorsal to the aorta. As it courses cranially, it receives blood from the dorsal intercostal veins (usually from the 4th or 5th intercostal space caudally), as well as from the vertebral venous plexuses and sometimes from the esophageal and bronchial veins.

The right azygos vein typically arches ventrally over the root of the right lung near the level of the 6th or 7th thoracic vertebra and empties into the cranial vena cava (precardial vein) just before the latter enters the right atrium. In some species, such as the dog, the right azygos vein is the only azygos vein present, while in ruminants and pigs, both right and left azygos veins may persist, with the left draining into the coronary sinus or left brachiocephalic vein.

Developmentally, the right azygos vein is derived from the cranial segment of the right caudal cardinal vein, as demonstrated in comparative mammalian studies, and its topography is influenced by the expansion of adjacent organs during embryogenesis.[1-2] The course and connections of the right azygos vein can show considerable anatomical variation, which is clinically relevant for thoracic surgery and diagnostic imaging.[3-8]