The left coronary artery arises from the left aortic sinus of the ascending
aorta, passes between the left auricle and the left side
of the pulmonary trunk, and runs in the coronary sulcus. In approximately 40% of people, the sinoatrial nodal branch arises from the
circumflex branch of the left coronary artery and ascends on the posterior surface of the left atrium to the sinoatrial node.
As it enters the coronary sulcus, at the superior end of the anterior interventricular groove, the left coronary artery divides into two branches, the anterior interventricular branch and the circumflex branch.