Thoracic Aorta

The aorta can be subdivided into four parts — the aortic root, ascending aorta, aortic arch, and descending aorta.

The aortic root or bulb begins at the level of the aortic semilunar valve. The three leaflets of the aortic valve form the aortic sinuses, right, left, and posterior. The right and left aortic sinus house the openings to the coronary arteries, which provide the heart muscle with the necessary nutrients for survival.

The ascending aorta originates at the base of the heart with the root of the aorta. The aortic sinuses are found in the wall of the root of the aorta. These are related to the cusps of the aortic valve and are named for them—right, left, and posterior. Two of these sinuses—the right and the left—contain the orifices of the coronary arteries. The ascending aorta extends anterosuperior from the base of the heart and slightly to the right for approximately 5 cm and terminates by becoming the arch of the aorta, which is usually at the level of the sternal angle (Fig. 12-7).

The aortic arch courses from right to left as well as from anterior to posterior. It lies in almost a true sagittal plane (Fig. 12-8). From the sternal angle, the aortic arch ascends toward the left. As it ascends, it is directed posterior. The upper portion of the arch courses posterior to the left of the trachea and esophagus. At about the level of the fourth thoracic vertebra, it turns inferior and runs a short distance before becoming the descending aorta.

There are three major branches given off by the aortic arch—the brachiocephalic trunk, the left common carotid artery, and the left subclavian artery (Fig. 12-9). These branches supply the head and upper extremities with blood.

The descending aorta is a continuation of the aortic arch. It can be divided into a superior thoracic descending and an inferior abdominal descending portion. The thoracic descending aorta extends from its origin at the level of the intervertebral disk between the fourth and fifth thoracic vertebrae to the point at which it traverses the aortic hiatus in the diaphragm (Fig. 12-10), which is about the level of the twelfth thoracic vertebra. As the thoracic aorta passes through the opening in the diaphragm, it becomes the abdominal portion of the descending aorta.

The branches of the thoracic descending aorta can be classified as either parietal or visceral and may be summarized as follows:

The parietal branches supply the body wall of the thoracic cavity, whereas the visceral branches supply the organs contained within the thoracic cavity.

Coronary Vasculature

The coronary arteries are the source of blood supplying the heart. The aorta is the origin of the two main coronary arteries (Fig. 12-11). Three small dilations, or sinuses, are located in the root of the aorta. They lie opposite the corresponding cusps of the aortic valve. The coronary arteries arise from two of these, so they are considered coronary sinuses. The third sinus is considered a noncoronary sinus because it does not connect directly with any coronary arteries.

The left main coronary artery arises from the left posterior aortic sinus, and the right main coronary artery originates from the anterior, or right, aortic sinus. These sinuses are called the aortic sinuses, or Valsalva’s sinuses.

Left Main Coronary Artery

This artery originates in the left aortic sinus. It arises from a single opening in the upper portion of the sinus, and its length can vary from 2 to 3 mm to 3 to 4 cm. It divides to form the anterior interventricular branch (left anterior descending artery) and the left circumflex branch (Fig. 12-12). These branches occupy the anterior interventricular sulcus and the atrioventricular sulcus, respectively. This oversimplification of the bifurcation of the left main coronary artery is provided because usually no more than two large branches of this artery can be found.¹

The anterior interventricular branch is considered a direct continuation of the left main coronary artery. It courses toward the apex of the heart in the anterior interventricular sulcus. In the frontal view, the left anterior interventricular branch plus the left main coronary artery form a gently reversed “S” curve. After reaching the apex of the heart, the left anterior interventricular branch continues up into the posterior interventricular sulcus and anastomoses with the terminal branches of the posterior interventricular branch of the right coronary artery. Along its route, the left anterior interventricular branch gives off a variable number of arteries that course into the interventricular septum. At the level of the pulmonary valve, smaller branches are given off that supply the right ventricle. One or more of these branches form a curve around the heart and meet similar branches of the proximal right coronary artery. This circle is called Vieussens’ ring, and it is an important means of collateral circulation between the right and left coronary arteries. Some larger branches are given off to supply the free wall of the left ventricle. At times, one or more of these branches arise at the point of division of the left main coronary artery; these are usually called the diagonal left ventricular arteries.

The left circumflex coronary artery, which is the second major branch of the left main coronary artery, courses at a sharp angle from its origin toward the left atrioventricular sulcus. It travels in this groove around to the back of the heart toward the right coronary artery, where anastomosis of these arteries frequently occurs. The circumflex artery gives off a large branch called the left marginal artery. The left marginal artery courses along the left margin of the heart to supply the left ventricle. Branches of the left marginal artery course over the free wall of the left ventricle somewhat parallel to those given off by the left anterior interventricular branch (left anterior descending artery).

Right Main Coronary Artery

Frequently, the right main coronary artery has two aortic ostia, which may arise from the anterior aortic sinus and from a smaller adjacent ostium. When present, the second ostium is very small—about 1 mm in diameter. This is called the conus artery of the right coronary system (Fig. 12-13). It is this artery that frequently anastomoses with an opposing branch from the left anterior interventricular artery, forming Vieussens’ ring. When the right main coronary artery arises from a single ostium in the aortic sinus, the first ventricular branch is considered the conus artery.

From its origin in the aortic sinus, the right main coronary artery courses in the right atrioventricular sulcus (coronary sulcus) to the diaphragmatic surface of the heart. At the acute margin of the heart, the right coronary artery gives off an artery called the right marginal branch, which courses almost to the apex of the heart. On the diaphragmatic surface of the heart, the right coronary artery makes a U turn and courses toward the apex of the heart in the posterior interventricular sulcus. This terminal portion of the right coronary artery is called the posterior interventricular branch (posterior descending artery).

The U turn of the right coronary artery is located at the crux of the heart. This is the point at which the right and left atrioventricular sulci cross the posterior interatrial and interventricular sulci. It is also here that the atrioventricular node artery originates from the right coronary artery.

On reaching the apex of the heart, the posterior interventricular branch frequently anastomoses with the anterior interventricular branch of the left coronary artery.

Venous Drainage

The venous drainage of the heart is considered to consist of three separate groups, rather than one.

The first group from the area of the left ventricle drains into the coronary sinus. Some prominent veins in this system are the anterior interventricular, posterior interventricular (middle cardiac), and left marginal veins. As it enters the atrioventricular sulcus, the anterior interventricular vein becomes the great cardiac vein, which in turn becomes the coronary sinus. The posterior interventricular vein usually drains directly into the right atrium; however, it often joins the coronary sinus just before entering the atrium. The left marginal vein and associated smaller vessels drain from the left ventricle into the great cardiac vein and coronary sinus.

In the second group, several large veins called the anterior cardiac veins transport venous blood from the area of the right ventricle and drain directly into the right atrium. The most prominent of these veins—the right marginal vein—drains the lower margin of the heart.

The third group consists of many very small veins, thebesian veins, which are often found on the right side of the heart but seldom on the left side. These small venous tributaries begin in the myocardium and drain directly into the cardiac chambers. They are found primarily in the right atrium and right ventricle.

Pulmonary Circulation

The right portion of the heart receives oxygen-poor blood via the superior and inferior venae cavae and directs it to the pulmonary circulation for oxygenation. Some blood is also sent to the bronchial arteries to supply the trachea, bronchi, esophagus, and posterior mediastinum. The pulmonary vasculature is also supplied with nutrients by the small vessels called the vasa vasorum. The pulmonary circulation can be divided into three portions: the pulmonary arteries, the alveolar capillary system, and the pulmonary veins. The right atrium propels the blood through the main pulmonary artery (pulmonary trunk), which branches off into a right and left main pulmonary artery. Each of these arteries services its respective lung, entering at the level of the hilum.