The lower half of the superior vena cava is within the pericardial sac and is therefore contained in the middle mediastinum.
The superior vena cava receives the azygos vein immediately before entering the pericardial sac and may also receive pericardial and mediastinal veins.
The superior vena cava can be easily visualized forming part of the right superolateral border of the mediastinum on a chest radiograph (see Fig. 3.67A).
Arch of aorta and its branches
The thoracic portion of the aorta can be divided into ascending aorta, arch of the aorta, and thoracic (descending) aorta. Only the arch of the aorta is in the superior mediastinum. It begins when the ascending aorta emerges from the pericardial sac and courses upward, backward, and to the left as it passes through the superior mediastinum, ending on the left side at vertebral level TIV/V (see Fig. 3.86). Extending as high as the midlevel of the manubrium of the sternum, the arch is initially anterior and finally lateral to the trachea.
Three branches arise from the superior border of the arch of the aorta; at their origins, all three are crossed anteriorly by the left brachiocephalic vein.
The first branch
Beginning on the right, the first branch of the arch of the aorta is the brachiocephalic trunk (Fig. 3.90). It is the largest of the three branches and, at its point of origin behind the manubrium of the sternum, is slightly anterior to the other two branches. It ascends slightly posteriorly and to the right. At the level of the upper edge of the right sternoclavicular joint, the brachiocephalic trunk divides into: the right common carotid artery, and the right subclavian artery (see Fig. 3.86).
The arteries mainly supply the right side of the head and neck and the right upper limb, respectively.
Occasionally, the brachiocephalic trunk has a small branch, the thyroid ima artery, which contributes to the vascular supply of the thyroid gland.
The second branch
The second branch of the arch of the aorta is the left common carotid artery (Fig. 3.90). It arises from the arch immediately to the left and slightly posterior to the brachiocephalic trunk and ascends through the superior mediastinum along the left side of the trachea.
The left common carotid artery supplies the left side of the head and neck.
The third branch
The third branch of the arch of the aorta is the left subclavian artery (Fig. 3.90). It arises from the arch of the aorta immediately to the left of, and slightly posterior to, the left common carotid artery and ascends through the superior mediastinum along the left side of the trachea.
The left subclavian artery is the major blood supply to the left upper limb.
The ligamentum arteriosum is also in the superior mediastinum and is important in embryonic circulation, when it is a patent vessel (the ductus arteriosus). It connects the pulmonary trunk with the arch of the aorta and allows blood to bypass the lungs during development (Fig. 3.90). The vessel closes soon after birth and forms the ligamentous connection observed in the adult.
The trachea is a midline structure that is palpable in the jugular notch as it enters the superior mediastinum.
Posterior to it is the esophagus, which is immediately anterior to the vertebral column (Fig. 3.92, and see Figs. 3.86 and 3.87). Significant mobility exists in the vertical positioning of these structures as they pass through the superior mediastinum. Swallowing and breathing cause positional shifts, as may disease and the use of specialized instrumentation.
As the trachea and esophagus pass through the superior mediastinum, they are crossed laterally by the azygos vein on the right side and the arch of the aorta on the left side.