<P> The arteries, when healthy, are capable of autoregulation to maintain coronary blood flow at levels appropriate to the needs of the heart muscle . These relatively narrow vessels are commonly affected by atherosclerosis and can become blocked, causing angina or a heart attack . The coronary arteries that run deep within the myocardium are referred to as subendocardial . </P> <P> The coronary arteries are classified as "end circulation", since they represent the only source of blood supply to the myocardium; there is very little redundant blood supply, which is why blockage of these vessels can be so critical . </P> <P> Coronary arteries supply blood to the myocardium and other components of the heart . Two coronary arteries originate from the left side of the heart at the beginning (root) of the aorta, just after the aorta exits the left ventricle . There are three aortic sinuses (dilations) in the wall of the aorta just superior to the aortic semilunar valve . Two of these, the left posterior aortic sinus and anterior aortic sinus, give rise to the left and right coronary arteries, respectively . The third sinus, the right posterior aortic sinus, typically does not give rise to a vessel . Coronary vessel branches that remain on the surface of the artery and follow the sulci of the heart are called epicardial coronary arteries . </P> <P> The left coronary artery distributes blood to the left side of the heart, the left atrium and ventricle, and the interventricular septum . The circumflex artery arises from the left coronary artery and follows the coronary sulcus to the left . Eventually, it will fuse with the small branches of the right coronary artery . The larger anterior interventricular artery, also known as the left anterior descending artery (LAD), is the second major branch arising from the left coronary artery . It follows the anterior interventricular sulcus around the pulmonary trunk . Along the way it gives rise to numerous smaller branches that interconnect with the branches of the posterior interventricular artery, forming anastomoses . An anastomosis is an area where vessels unite to form interconnections that normally allow blood to circulate to a region even if there may be partial blockage in another branch . The anastomoses in the heart are very small . Therefore, this ability is somewhat restricted in the heart so a coronary artery blockage often results in myocardial infarction causing death of the cells supplied by the particular vessel . </P>

When does blood flow from the aorta into the coronary arteries