E) Great Vessels

The great vessels are the large blood vessels which arise from the heart, and their immediate branches.

Systemic Arteries

The systemic vessels supply the body, and all arise from the thoracic aorta.

Ascending aorta

The ascending aorta arises from the left ventricle in the central mediastinum. It is 5 cm long and passes superiorly through the mediastinum, where it is continuous with the arch of the aorta. It has two important branches - the right and left coronary arteries. These vessels arise immediately above the aortic valve on their respective sides.
The ascending aorta is related:

  • Anteriorly with the pericardium (which coats the vessel), the thymus and the sternum.
  • Posteriorly with the left atrium. The right pulmonary artery and right main bronchus pass behind the vessel.
  • On the right (and posterior) with the superior vena cava and the right atrium
  • On the left with the left atrium

The pulmonary trunk and ascending aorta have an interesting relationship. The pulmonary trunk initially lies anterior, the ascending aorta posterior. As they ascend, the pulmonary trunk passes posterior and left lateral; the ascending aorta passes anterior and slightly right lateral. The pulmonary trunk is therefore related to the ascending aorta on the left at its highest extent.

Arch of Aorta

The arch of the aorta passes to the left, in front of the trachea and then along its left lateral wall, before descending as the descending thoracic aorta. It follows a curving path and gives off three significant branches, the brachiocephalic, left carotid and left subclavian arteries.

These branches may vary in number from 1 to 4. After the 'normal' branching (in 65% of people), the left common carotid arises from the brachiocephalic trunk in about 25% of people.

The arch is related on the left with the left mediastinal pleura and the lung. It is crossed on this aspect by the left phrenic nerve (anteriorly) and the vagus nerve (posteriorly), which gives off the left recurrent laryngeal, which hooks under the arch and ascends on its posterior surface between the oesophagus and the trachea. The left brachiocephalic vein passes above and anteriorly, in front of the origins of the three major branches. The pulmonary trunk is below; the space between the pulmonary trunk and the arch is the aortic window and an important lymph node station of the mediastinum.

Descending Thoracic Aorta

The descending aorta passes from the lateral border of T4 to the anterior border of T12, closely related to the oesophagus (initially posteromedial, then anterior, and finally anterolateral). It is continuous with the abdominal aorta, the point of interchange being the diaphragm. The descending aorta has numerous small branches, including paired posterior intercostal arteries for each intercostal space, the two bronchial arteries and various mediastinal and oesophageal branches.

Systemic Veins

The significant systemic veins of the mediastinum include the superior and inferior vena cava, and the azygos vein.

Superior Vena Cava

This large vessel is formed by the confluence of the two brachiocephalic veins at the level of the first costal cartilage. It descends within the mediastinum for about 7 cm, where it enters the right atrium. It also receives venous blood from the azygos vein, which enters just above the right main bronchus.
The SVC is related anteriorly, laterally and posteriorly with the right lung and pleura. It is crossed posteriorly by the right pulmonary artery and the right main bronchus, which divide it from the azygos vein. The ascending aorta lies medial and slightly anterior. The trachea is posterior and medial.

Inferior Vena Cava

The IVC enters the thorax anterior to, and at the level of the T8 - T9 intervertebral disc, and runs a very short course into the right atrium.

Azygos Vein

The azygos (unpaired) vein usually arises from the inferior vena cava in the abdomen, and passes superiorly into the mediastinum, lying posterior to most other structures and anterior of the vertebral bodies. Upon reaching the level of the 4th intercostal cartilage, it abruptly curves forward, superior to the left hilar structures (including right main bronchus) to terminate in the superior vena cava.
The azygos vein is an important collateral in the even of SVC or IVC obstruction.

Pulmonary Arteries

The pulmonary arteries carry unoxygenated blood to the lungs from the heart.

Pulmonary Trunk

The pulmonary trunk arises as the most anterior vessel from the heart, and is about 5 cm long. It passes posterior and left lateral, initially anterior to the aorta and finishing to its left. It divides into the right and left pulmonary arteries which continue to each lung.
The pulmonary trunk is related:

  • Posteriorly and to the right with the ascending aorta
  • Above is the carina
  • The lung and pleura lie anteriorly
  • The arch of the aorta lies in an imaginary line with the continuation of the vessel; during the foetal period the vessels communicate through the ductus arteriosus which closes at birth. This space is known as the aortic window due to its appearance on lateral radiographs.

Right Pulmonary Artery

In general, pulmonary arteries lie anterior to their associated bronchi.
The right pulmonary artery passes right lateral from the bifurcation of the pulmonary trunk. It passes posterior to the ascending aorta and the inferior vena cava to reach the hilum of the lung. The azygos vein lies posteriorly, separated by the right main bronchus. On entering the hilum, the right pulmonary artery divides into an upper lobar artery and an inferior artery which accompanies the bronchus intermedius.

Left Pulmonary Artery

The left pulmonary artery is shorter than the right, and passes to the left hilum. It lies inferior to the aortic arch and anterior to the descending aorta. The left main bronchus is posterior. The left pulmonary artery divides into upper and lower lobar arteries.

Pulmonary Veins

The pulmonary veins collect blood from the lungs and return it to the left atrium, the most posterior of the chambers of the heart. There are usually four pulmonary veins, two from each side. The veins lie behind most mediastinal vessels with the exception of the left pulmonary veins and the descending aorta, which lies further posterior.