Special Circulations

Coronary Circulation

The two major branches of the coronary circulation are the left main and right main coronary arteries (Fig. 7-6). These vessels arise from coronary ostia, which are small openings in the wall of the ascending aorta just distal to the aortic valve. The left main coronary artery is relatively short in length (~1 cm). After coursing behind the pulmonary artery trunk, it divides into the left anterior descending artery, which travels along the in-terventricular groove on the anterior surface of the heart, and the circumflex artery, which travels posteriorly along the groove between the left atrium and ventricle. These branches of the left coronary artery supply blood primarily to the left ventricle and atrium. The right main coronary artery travels between the right atrium and ventricle (left atrioventricular groove) toward the posterior regions of the heart. This vessel and its branches serve the right ventricle and atrium, and in most individuals, the inferoposterior region of the left ventricle. Significant variation is possible among individuals in the anatomical arrangement and distribution of flow by the coronary vessels.

The major coronary vessels lie on the epicardial surface of the heart and serve as low-resistance distribution vessels. These

FIGURE 7-6 Anterior view of the heart showing the major coronary arteries. The left main artery arises from the aorta (Ao) just distal to the aortic valve, travels behind the pulmonary artery (PA), and then branches into the circumflex artery (courses along the left atrioventricular groove) and left anterior descending artery (courses along the interventricular groove), both of which primarily supply blood to the left ventricle. The right coronary artery arises from the aorta and travels between the right atrium and ventricle toward the posterior regions of the heart to supply the right ventricle and atrium and the inferoposterior wall of the left ventricle. SVC, superior vena cava; IVC, inferior vena cava.

FIGURE 7-6 Anterior view of the heart showing the major coronary arteries. The left main artery arises from the aorta (Ao) just distal to the aortic valve, travels behind the pulmonary artery (PA), and then branches into the circumflex artery (courses along the left atrioventricular groove) and left anterior descending artery (courses along the interventricular groove), both of which primarily supply blood to the left ventricle. The right coronary artery arises from the aorta and travels between the right atrium and ventricle toward the posterior regions of the heart to supply the right ventricle and atrium and the inferoposterior wall of the left ventricle. SVC, superior vena cava; IVC, inferior vena cava.

epicardial arteries give off smaller branches that dive into the myocardium and become the microvascular resistance vessels that regulate coronary blood flow. The resistance vessels give rise to a dense capillary network so that each cardiac myocyte is closely associated with several capillaries. The high capillary-to-fiber density ensures short diffusion distances to maximize oxygen transport into the cells and removal of metabolic waste products (e.g., CO2, H+) (see Chapter 8).

Coronary veins are located adjacent to coronary arteries. These veins drain into the coronary sinus located on the posterior aspect of the heart. Blood flow from the coronary sinus empties into the right atrium. Some drainage also occurs directly into the cardiac chambers through the anterior cardiac veins and thebesian vessels.

Coronary blood flow is not steady as in most other organs. When flow is measured within a coronary artery, it is found to decrease during cardiac systole and increase during diastole (Fig. 7-7). Therefore, most of the blood flow to the myocardium occurs during diastole. The reason that coronary flow is influenced by the cardiac cycle is that during systole, the contraction of the myocardium compresses the microvasculature within the ventricular wall, thereby increasing resistance and decreasing flow. During systole, blood flow is reduced to the greatest extent within the innermost regions of the ventricular wall (i.e., in the subendocardium) because this is where the compressive forces are greatest. (This results in the subendocardial regions being more susceptible to ischemic injury when coronary artery disease or reduced aortic pressure is present.) As the ventricle begins to relax in early diastole, the compressive forces are removed and blood flow is permitted to increase. Blood flow reaches a peak in early diastole and then falls passively as the aortic pressure falls toward its diastolic value. Therefore, it is the aortic pressure during diastole that is most crucial for perfusing the coronaries. This explains why increases in heart rate can reduce coronary perfusion. At high heart rates, the length of diastole is greatly shortened, which reduces the time for coronary perfusion. This is not a problem when the coronary arteries are normal, because they dilate with increased heart rate and metabolism; however, if the coronaries are diseased and their vasodilator reserve is limited, increases in heart rate can limit coronary flow and lead to myocardial ischemia and anginal pain.

The mechanical forces affecting coronary flow are greatest within the left ventricle because this chamber develops pressures that are several-fold greater than those developed by

Diastole Systole Diastole

Diastole Systole Diastole

FIGURE 7-7 Pulsatile nature of coronary blood flow measured in the left coronary artery. Flow is lower during systole because of mechanical compression of intramuscular coronary vessels. Flow is maximal early in diastole, and then it falls as aortic pressure declines.

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