Left azygos vein

The left azygos vein in animals (referred to as the hemiazygos vein in humans where both right and left components exist), is a venous structure that contributes to the drainage of the dorsal thoracic wall. In many domestic mammals such as dogs, the azygos system is predominantly right-sided with little or no functional left counterpart, whereas in ruminants and pigs a more developed left azygos vein may persist and drain into the coronary sinus or directly into the right atrium via the left side.

The left azygos vein begins, like the right azygos vein, in the lumbar region, from which it drains the first one or two pairs of veins. It lies to the left of the aorta, passes through the diaphragm in contact with it, then accompanies it in the thorax to the middle mediastinum, crossing laterally the origin of the left dorsal intercostal arteries. In the mediastinum, it crosses to the left of the aortic arch and then the pulmonary trunk, and reaches the dorsal surface of the left atrium. From this point, it shows exactly the arrangement already described for the terminal segment of the left cranial vena cava and drains the veins of the heart in the same manner. Along its course, it receives, in addition to the first lumbar veins, the dorsal costo-abdominal veins, the dorsal intercostal veins in a number and arrangement that vary with the species, the bronchial veins and oesophageal veins and the great and middle cardiac veins. In the humans, pig and often in ruminants, the costo-abdominal vein, the most caudal dorsal costo-abdominal and dorsal intercostal veins on the right side are collected by a right hemiazygos vein (V. hemiazygos dextra), which crosses the body of the tenth thoracic vertebra, or one of its neighboring vertebrae, to join the left azygos vein; it may even completely replace its caudal portion.

The anatomical configurationof the left azygos vein reflects species-specific patterns of embryologic venous development, particularly the persistence or regression of the supracardinal veins. For Barone, in reality, in ruminants and pigs, the left azygos vein represents, in addition to a left azygos vein proper, the terminal supra-atrial portion of the left cranial vena cava, which it prolongs.