The upper mediastinum contains the aortic arch with the origins of its major branches. These include the innominate (brachiocephalic) artery, proximal left common carotid artery, and proximal left subclavian artery. The left and right innominate (brachiocephalic) veins join to become the superior vena cava (SVC).

The thymic remnant and surrounding mediastinal fat are the first tissues encountered when entering the upper mediastinum. These tissues lie over the left innominate vein and the aortic arch.

The left innominate vein is approximately 6–7 cm long and it transverses the upper mediastinum under the manubrium sterni and over the superior border of the aortic arch. It joins the right innominate vein just to the right of the sternum at the level of the first to second intercostal space to form the SVC.

The right innominate vein is approximately 3 cm in length and it courses vertically downward and joins the left innominate vein at a 90° angle to form the SVC.

The SVC is approximately 6–7 cm in length and is located lateral and parallel to the ascending aorta. A small segment is enclosed within the pericardium.

The ascending aorta is contained within the pericardium. The aortic arch begins at the superior attachment of the pericardium. The first branch of the aortic arch is the innominate artery, which then branches into the right subclavian and right common carotid arteries. The next branch of the arch is the left common carotid artery, followed by the left subclavian artery. The innominate artery and the left common carotid artery originate relatively anteriorly, while the left subclavian artery originates more posteriorly. Anatomical variants include a common origin for the left common carotid artery and innominate artery, as well as a common origin for the left subclavian and left common carotid artery.

The left vagus nerve travels between the left common carotid and subclavian arteries just anterior to the arch and branches off into the recurrent laryngeal nerve, which loops around and behind the aortic arch, ascending along the tracheoesophageal groove.

The right vagus nerve crosses over the right subclavian artery, immediately gives off the recurrent laryngeal nerve, which loops behind the subclavian artery and ascends behind the common carotid artery along the tracheoesophageal groove.

The thoracic or descending aorta begins at the fourth thoracic vertebra on the left side of the vertebral column. Below the root of the lung, it courses to a position anterior to the vertebral column as it passes into the abdominal cavity through the aortic hiatus in the diaphragm at the twelfth thoracic vertebra.

The esophagus lies on the right side of the aorta proximally. Distally, as it enters the diaphragm, it courses in front of the aorta.

The aorta has nine pairs of aortic intercostal arteries that arise from the posterior aspect of the aorta and travel to the associated intercostal spaces. The bronchial and esophageal arteries are additional branches of the aorta as it descends in the thorax.