Descending Thoracic Aorta

The descending thoracic aorta is the continuation of the aortic arch as it passes downward through the thoracic cavity. It lies within the posterior mediastinum and is responsible for supplying oxygenated blood to the thoracic walls, lungs, esophagus, and diaphragm. It extends from the level of the T4 vertebra to the T12 vertebra, where it enters the abdominal cavity through the aortic hiatus of the diaphragm and becomes the abdominal aorta.

Origin and Termination

• Begins: At the lower border of the T4 vertebra, continuing from the aortic arch
• Ends: At the lower border of the T12 vertebra, where it passes through the diaphragm to become the abdominal aorta

Course

The descending thoracic aorta descends along the left side of the vertebral bodies within the posterior mediastinum. As it moves inferiorly, it gradually shifts toward the midline, so that by the time it reaches the diaphragm, it lies anterior to the vertebral column. It travels in close relation to several key thoracic structures.

Anterior Relations

• Root of the left lung (superiorly)
• Pericardium and left atrium (through the esophagus)
• Esophagus (mid to lower thorax)

Posterior Relations

• Thoracic vertebral bodies (T5 to T12)

Right-Sided Relations

• Azygos vein
• Thoracic duct
• Esophagus (initially on the right, then anterior)

Left-Sided Relations

• Mediastinal pleura and left lung
• Left sympathetic trunk

Branches of the Descending Thoracic Aorta

The thoracic aorta gives off several paired and unpaired branches that supply thoracic structures:

1. Posterior Intercostal Arteries

Nine pairs (3rd to 11th intercostal spaces). Each artery runs laterally along the intercostal space and supplies the intercostal muscles, overlying skin, and parietal pleura. The first two intercostal spaces are supplied by the superior intercostal artery (branch of costocervical trunk).

2. Subcostal Arteries

One pair of arteries below the 12th rib, which follow a course similar to intercostal arteries but run beneath the last rib.

3. Bronchial Arteries

Usually two on the left and one on the right. They supply the bronchi, connective tissue of the lungs, and part of the esophagus. The right bronchial artery may arise from the aorta or from a posterior intercostal artery.

4. Esophageal Arteries

Four to five small branches supplying the middle third of the esophagus. They anastomose with esophageal branches of the inferior thyroid artery (superiorly) and left gastric artery (inferiorly).

5. Pericardial Branches

Small vessels supplying the posterior aspect of the pericardium.

6. Superior Phrenic Arteries

Two small arteries that supply the superior surface of the diaphragm. They may anastomose with musculophrenic and pericardiacophrenic arteries.

Relations at the Diaphragm

At the level of T12, the descending thoracic aorta passes through the diaphragm via the aortic hiatus, located posterior to the median arcuate ligament. This hiatus also transmits the thoracic duct and sometimes the azygos vein.

Histological Structure

The aortic wall is composed of three layers:

• Tunica intima: Endothelial lining and thin connective tissue
• Tunica media: Thick layer of elastic fibers and smooth muscle, accommodating high-pressure blood flow
• Tunica adventitia: Connective tissue containing vasa vasorum and sympathetic nerves

Topographic Summary

• Start: T4 vertebral level
• End: T12 vertebral level (becomes abdominal aorta)
• Location: Posterior mediastinum
• Length: Approximately 20 cm
• Branches: Posterior intercostal, subcostal, bronchial, esophageal, pericardial, superior phrenic arteries

Clinical Notes (Minimal)

• Aneurysm: Descending thoracic aortic aneurysms can compress adjacent structures like the esophagus or left recurrent laryngeal nerve.
• Coarctation of the Aorta: A congenital narrowing, often near the ligamentum arteriosum, may affect blood flow in distal structures.