Topography and Relations of the Thoracic Aorta
The thoracic aorta is located in the posterior mediastinum and lies directly against the vertebral bodies. At its origin in the upper thoracic cavity, the vessel is situated slightly to the left of the midline of the vertebral column. As it descends, the aorta gradually shifts to the right, coming to lie strictly in the center. At the level of the twelfth thoracic vertebra (Th12), it leaves the thoracic cavity through the aortic hiatus of the diaphragm and continues as the abdominal aorta.
The surrounding anatomical relations (syntopy) of the thoracic aorta are of great clinical significance. Anteriorly and to the right of the vessel runs the azygos vein (v. azygos), while to the left lie the hemiazygos veins (vv. hemiazygos). The thoracic duct is also in close contact with it. Anteriorly, the vessel is crossed by the left main bronchus.
Of particular note is the relationship between the aorta and the esophagus, which changes along its descending course:
- In the upper third of the thoracic region, the esophagus lies to the right of the aortic trunk.
- In the middle third, the esophagus passes anterior to the aorta.
- In the lower third, just before passing through the diaphragm, the esophagus shifts to the left.
Parietal Branches
These branches supply the walls of the thoracic cavity, including the muscles, skin, and diaphragm. They include the superior phrenic and posterior intercostal arteries.
1. Superior phrenic arteries (aa. phrenicae superiores) These are 1–2 thin arterial branches originating from the lowest part of the thoracic aorta. They course toward the superior surface of the diaphragm, where they anastomose with the inferior phrenic arteries.
2. Posterior intercostal and subcostal arteries Arising from the posterolateral surface of the aorta are the posterior intercostal arteries (aa. intercostales posteriores, III to XI) and one subcostal artery (a. subcostalis, XII). In total, 10 intercostal and 1 subcostal artery arise from the aorta on each side. Notably, the first and second posterior intercostal arteries typically arise from the a. intercostalis suprema (a branch of the costocervical trunk).
The posterior intercostal arteries course laterally and enter their respective intercostal spaces, running along the costal groove on the inferior margin of the rib. At the angle of the rib, each artery divides into several branches:
- Collateral branch (r. collateralis) — runs along the superior margin of the rib below and anastomoses with the anterior intercostal arteries (from the internal thoracic artery).
- Lateral cutaneous branch (r. cutaneus lateralis) — pierces the intercostal musculature, supplying the skin of the lateral chest and abdominal wall. In females, it also gives off branches to the breast.
- Dorsal branch (r. dorsalis) — courses toward the deep back muscles and overlying skin. It gives rise to the crucial spinal branch that enters the vertebral canal.
The terminal parts of the lower left intercostal arteries cross the costal margin, pass between the broad abdominal muscles, and reach the rectus abdominis muscle, forming anastomoses with the superior and inferior epigastric arteries.
Visceral Branches
This group of vessels supplies the internal organs located in the thoracic cavity. All of them arise from the anterior wall of the aorta.
- Bronchial branches (rr. bronchiales): typically 2–4 in number, originating from the anterior wall of the initial thoracic aorta. They supply the bronchi, lymph nodes of the root of the lung, pleura, and partially the esophagus.
- Esophageal branches (rr. esophagei): 3 to 6 vessels supplying the esophageal wall. They form important anastomoses with the esophageal branches of the left gastric artery (from the celiac trunk system) and the inferior phrenic arteries.
- Mediastinal branches (rr. mediastinales): numerous small branches supplying the connective tissue of the posterior mediastinum and regional lymph nodes.
- Pericardial branches (rr. pericardiaci): course toward the posterior surface of the serous pericardium.