Coronary and Pulmonary Circulations
The major blood vessels which constitute this circulation are the left main coronary artery and the right main coronary artery, both of which originate at the base of the aorta from openings called the coronary Ostia located behind the aortic valve leaflets (Klabunde, “Cardiovascular Physiology Concepts”). The left main coronary artery is usually larger than the right one and divides into the left anterior descending and circumflex branches (Figure-1). The right coronary artery gives out branches like the marginal artery and posterior interventricular artery. The coronary arteries and their branches lie on the surface of the heart and hence are also known as epicardial coronary vessels. The branches of the main arteries further branch out into arterioles and finally form a microvascular network of capillaries. These capillaries lie adjacent to cardiac myocytes. There will be many capillaries adjacent to each cardiomyocyte so that the capillary-to-cardiomyocyte ratio is high and this enables adequate oxygen supply to the heart cells. The arteries and arterioles have much less vascular resistance than the microvascular bed in healthy persons so that there is a free flow of blood to the capillary bed. The capillary bed enters venules which join together and form cardiac veins. Removal of waste products from the heart cells is done through this circuit. Most of the cardiac veins drain into the coronary sinus located on the posterior side of the heart. The coronary sinus drains into the right atrium. This completes the coronary circulation. The anterior cardiac veins and the thespian veins drain directly into the cardiac chambers (Klabunde, “Cardiovascular Physiology Concepts”). Most of the coronary flow occurs during diastole because, during systole, there is marked extravascular compression which affects coronary flow (Levick, “Introduction to Cardiovascular Physiology”).There is good autoregulation between 60 and 200 mmHg perfusion pressure that maintains normal coronary blood flow whenever coronary perfusion pressure changes due to changes in aortic pressure.