Heart Arterial Supply
The heart has a small margin for error in its physiology and function. Therefore, it requires a dense vascular system for regulation of blood gases and nutrients. The right and left coronary arteries provide the arterial supply of the heart. These are the first branches of the aorta. They arise from the ascending aorta immediately above the aortic valve and initially pass around the opposite sides of the pulmonary trunk (Fig. 5.36A; also see Figs. 5.30 and 5.34D). The coronary arteries and their major branches are distributed over the surface of the heart and lie within subepicardial connective tissue. Table 5.3 summarizes the typical pattern of distribution of the coronary arteries in most people.
Right Coronary Artery The right coronary artery arises from the right aortic sinus of the ascending aorta, runs forward between the right side of the pulmonary trunk and the right auricle, and descends almost vertically in the right atrioventricular groove (coronary sulcus) (see Figs. 5.30, 5.34D, and 5.36A). At the inferior border of the heart, it continues posteriorly along the atrioventricular groove to anastomose with the left coronary artery in the posterior interventricular groove.
Branches
1. The right conus artery supplies the anterior surface of the pulmonary conus (infundibulum of the right ventricle) and the upper part of the anterior wall of the right ventricle.
2. Two or three anterior ventricular branches supply the anterior surface of the right ventricle. The right marginal artery is the largest and runs along the right margin of the anterior surface toward the apex.
3. Two or three posterior ventricular branches supply the diaphragmatic surface of the right ventricle. One of these, the atrioventricular nodal branch, supplies the AV node.
4. The posterior interventricular (posterior descending) artery runs toward the apex in the posterior interventricular groove. It gives off branches to the right and left ventricles, including its inferior wall. It supplies branches to the posterior part of the ventricular septum but not to the apical part, which receives its supply from the anterior interventricular branch of the left coronary artery. The posterior interventricular artery usually is a branch of the right coronary artery. However, it may originate from the circumflex branch of the left coronary artery (see the description of coronary artery dominance below).
5. Atrial branches supply the anterior and lateral surfaces of the right atrium. One branch supplies the posterior surface of both the right and left atria. An early branch, the sinuatrial nodal artery, supplies the SA node and the right and left atria; in 35% of individuals, it arises from theleft coronary artery.
Left Coronary Artery
The left coronary artery, which is usually larger than the right coronary
artery, supplies the major part of the heart. It arises from the left aortic sinus
of the ascending aorta, passes forward between the left side of the pulmonary
trunk and the left auricle, enters the atrioventricular groove (coronary sulcus),
and divides into an anterior interventricular branch and a circumflex branch
(see Figs. 5.30, 5.34D, and 5.36A).
Branches
1. The anterior interventricular artery (left anterior descending artery,
LAD) runs downward along the ventricular septum in the anterior
interventricular groove to the apex of the heart (see Figs. 5.30 and
5.36A). In most individuals, it then passes around the apex of the heart
to enter the posterior interventricular groove and anastomose with the
terminal branches of the posterior interventricular branch of the right
coronary artery. In about one third of individuals, it ends at the apex of
the heart. The anterior interventricular branch supplies the right and left
ventricles with numerous branches that also supply the anterior two
thirds of the ventricular septum. One larger ventricular branch (lateral
or diagonal artery) may arise directly from the trunk of the left
coronary artery. A small left conus artery supplies the pulmonary
conus.
2. The circumflex artery winds around the left margin of the heart in the
atrioventricular groove. A left marginal artery is a large branch that
supplies the left margin of the left ventricle down to the apex. Anterior
and posterior ventricular branches supply the left ventricle. Atrial
branches supply the left atrium.
Variations in the Coronary Arteries
Variations in the pattern of blood supply to the heart occur commonly, and
most affect the blood supply to the diaphragmatic surface of both ventricles.
The condition known as coronary artery dominance refers to the variable
origin of the posterior interventricular artery (Fig. 5.37). In right dominance,the posterior interventricular artery is a large branch of the right coronary artery (Fig. 5.37A). Right dominance is present in most individuals (~67%). In left dominance, the posterior interventricular artery is a branch of the circumflex branch of the left coronary artery (~15%) (Fig. 5.37B). In codominance (~18%), both the right coronary and circumflex arteries contribute to the formation of the posterior interventricular artery or to multiple branches that substitute for that vessel.
Coronary Artery Anastomoses
Anastomoses (collateral circulation) between the terminal branches of the right and left coronary arteries do exist; however, they are usually not large enough to provide an adequate blood supply to the cardiac muscle should one of the large branches become blocked by disease. Thus, the coronary arteries are generally considered to be functional end arteries. A sudden block of one of the larger branches of either coronary artery usually leads to myocardial death (myocardial infarction), although sometimes the collateral circulation is enough to sustain the muscle.
Heart Venous Drainage
Most blood from the heart wall drains into the right atrium via the coronary sinus (see Figs. 5.31 and 5.36B). The coronary sinus is a large, dilated vein that lies in the posterior part of the atrioventricular groove (coronary sulcus). It empties into the right atrium just to the left of the inferior vena cava (see Fig. 5.33). Three main veins drain the heart wall and open into the coronary sinus:
the great cardiac vein, middle cardiac vein, and small cardiac vein. Each of these typically accompanies a specific coronary artery (Table 5.4). The great cardiac vein drains most of the areas of the heart supplied by the left coronary artery and joins the left end of the coronary sinus (see Fig. 5.36B). The middle and small cardiac veins drain most of the areas normally supplied by the right coronary artery and drain into the right end of the coronary sinus.
Table 5.4 Major Coronary Veins and Accompanying Coronary Arteries
Not all venous blood drains into the coronary sinus. A variable number of small anterior cardiac veins drain the anterior surface of the heart and empty directly into the right atrium. Other very small veins may drain directly into the chambers of the heart (usually the atria).
Clinical Notes
Coronary Artery Disease The myocardium receives its blood supply through the right and left coronary arteries. Although the coronary arteries have numerous anastomoses at the arteriolar level, they are essentially functional end arteries. A sudden block of one of the large branches of either coronary artery will usually lead to necrosis of the cardiac muscle (myocardial infarction) in that vascular area, and often, the patient dies. An acute thrombosis on top of a chronic atherosclerotic narrowing of the lumen causes most cases of coronary artery blockage. Arteriosclerotic disease of the coronary arteries may present in three
ways, depending on the rate of narrowing of the lumina of the arteries: (1) general degeneration and fibrosis of the myocardium occur over many years and are caused by a gradual narrowing of the coronary arteries. (2) Angina pectoris is cardiac pain that occurs on exertion and is relieved by rest. In this condition, the coronary arteries are so narrowed that myocardial ischemia occurs on exertion but not at rest. (3) Myocardial infarction occurswhen coronary flow is suddenly reduced or stopped and the cardiac muscle undergoes necrosis. Myocardial infarction is the major cause of death in industrialized nations.
Table 5.5 correlates individual coronary arteries with specific sites of
myocardial infarction and the electrocardiographic signature of each infarct. Coronary bypass surgery, coronary angioplasty, and coronary artery
stenting are now commonly accepted methods of treating coronary artery disease. Coronary bypass surgery involves harvesting a segment of blood vessel and using that to circumvent a blockage in a coronary artery. Many such procedures use the great saphenous vein from the lower limb as the donor vessel because of its size and surgical ease of access. Emerging techniques increasingly use the internal thoracic artery from the neighboring chest wall to revascularize the heart wall.