The myocardium of the heart has a high metabolic demand because the muscle requires energy to undergo constant rhythmic contractions. In order to meet this demand, the heart has its own circulatory system, called coronary circulation, which is part of the systemic circuit. Coronary circulation is the blood flow to the heart tissues via coronary arteries and supplies the myocardium with oxygen and nutrients. Two major coronary arteries arise from the aorta: the left coronary artery, which supplies blood to the left side of the heart, and the right coronary artery, which supplies blood to the right side of the heart. The left anterior descending artery (also known as the anterior interventricular artery) branches off the left coronary artery and sends blood to the front and bottom of the left ventricle and front of the septum. The circumflex artery also branches off the left coronary artery and supplies blood to the left atrium and the side and back of the left ventricle. The right coronary artery branches to the right marginal artery, which delivers blood to the right side of the heart, and the posterior interventricular artery, which supplies blood to the posterior region of the heart. Blood is returned to the right side of the heart via coronary (cardiac) veins.Because of the continued contractions and relaxations of the heart muscle, the coronary arteries are under high stress as they are continuously compressed. Because of this pulsation, they only move blood during relaxation of the heart. If coronary arteries are not functioning properly, the heart will not receive sufficient oxygen supply. This can cause serious heart problems, such as cardiac ischemia, in which a blood clot obstructs blood flow within the heart. A myocardial infarction (or heart attack) occurs when heart muscle dies due to the lack of blood flow, usually from prolonged ischemia.