1.5: Aorta, Pulmonary Arteries, and Coronary Arteries

The Aorta

The ascending aorta arises in right posterior position relative to the pulmonary trunk (Figure 1.1.1, upper panel). It ascends superiorly, obliquely to the right and slightly anterior toward the sternum. On the right is the medial wall of the right atrium. Anteriorly are the right atrial appendage, the right ventricular outflow tract and the pulmonary trunk. The transverse pericardial sinus separates the back of the aorta from the left atrium and right pulmonary artery. The arch of the aorta begins just above the cuff of pericardial reflection, proximal to the origin of the brachiocephalic artery. The arch passes superiorly for a short distance before passing posteriorly to the left and finally terminating on the lateral aspect of the vertebral column. In its course, the arch gives origin to the neck and arm arteries. The arterial duct, a patent channel in fetal life, connects the left pulmonary artery to the aorta just distal to the origin of the left subclavian artery. In the adult, the duct is represented by a fibrous ligament.

The Pulmonary Arteries

The pulmonary trunk is also covered with a cuff of serous pericardium at its origin. It arises from the anterior aspect of the heart, just behind the left lateral edge of the sternum. It swings diagonally to the left side of the ascending aorta (Figure 1.1.1, upper panel). Being a short vessel, it soon bifurcates into the left and right pulmonary arteries. The left pulmonary artery passes in front of the descending aorta and superior to the left main bronchus before branching in the lung hilum. The longer right pulmonary artery traverses the mediastinum under the aortic arch before passing behind the superior caval vein to reach the right lung hilum.

The Coronary Circulation

As mentioned previously, the left and right coronary arteries emerge from the left and right coronary sinuses respectively. Usually the arteries arise from within the sinus just beneath or at the level of the aortic bar (sinutubular junction). In the left sinus there is usually a single orifice but in the right sinus it is usual to find multiple orifices where the early branches of the right coronary artery take direct origin. The main coronary arteries pass within the fatty tissues of the atrioventricular and interventricular grooves. The left coronary has a short main stem that branches into the anterior descending and circumflex arteries (Figure 1.5.1). The circumflex runs in the left atrioventricular groove and the right coronary artery runs in the right atrioventricular groove to variable lengths. From the atrioventricular groove, the encircling arteries give origin to ventricular and atrial branches. An early atrial branch is the sinus node artery which arises slightly more frequently from the right than the left coronary artery. It usually ascends the interatrial musculature to reach the terminal groove but recent evidence has shown a more variable course. In the majority of hearts the posterior descending artery, which runs in the posterior interventricular groove, is a branch from the right coronary artery and this is termed 'right dominance'. In a little under 10% of hearts the posterior descending is a branch of the circumflex giving 'left dominance'. A 'balanced' circulation is seen when both right and left coronary arteries give rise to parallel posterior descending branches. The artery to the atrioventricular node arises from the dominant artery at the cardiac crux.

Figure 1.5.1: Diagram showing the right (RCA) and left (LCA) coronary arteries and their main ventricular branches. The left anterior descending (LAD) and posterior descending (PDA) coronary arteries mark the anterior and posterior margins of the ventricular septum.

After passing through the capillary network, coronary arterial blood is collected by venules which drain to the cardiac veins. The veins drain either to the coronary sinus or directly to the cardiac chambers. The great cardiac vein ascends along the anterior descending coronary and turns into the left atrioventricular groove. In the posterior atrioventricular groove it becomes the coronary sinus. It is joined near its entrance to the right atrium by the middle cardiac vein which ascends in the posterior interventricular groove and the small cardiac vein. The latter ascends along the marginal coronary artery before entering the posterior atrioventricular groove. Atrial veins also empty into the coronary sinus. A further group of veins, the anterior cardiac veins, run across the anterior aspect of the heart to drain directly into the right atrium. In addition to the coronary arteries and veins, the heart also has an extensive lymphatic network. These are divided into the deep, middle and superficial plexuses which drain into collecting channels accompanying the major arterial stems and finally into primary lymph nodes situated in the anterior mediastinum.