Coronary Arteries: Branches, Territories, and Dominance

The coronary arteries are the first branches of the aorta. They arise from the aortic sinuses, the small bulges of the aortic wall just above the cusps of the aortic valve: the right coronary artery from the right aortic sinus and the left coronary artery from the left aortic sinus. The right coronary artery runs in the right side of the coronary (atrioventricular) groove, around the right border of the heart and onto its posterior surface. The left coronary artery is a short trunk, often called the left main, that passes behind the pulmonary trunk and divides into two large branches: the anterior interventricular artery, usually called the left anterior descending (LAD), and the circumflex artery. The grooves are the map. The atrioventricular groove carries the right coronary and circumflex arteries around the heart like a crown, and the interventricular grooves carry the anterior and posterior interventricular arteries down toward the apex. If you can picture those grooves on the surface of the heart, you can draw the whole system. One functional point is examined often: most coronary filling happens in diastole. During systole the contracting myocardium compresses the vessels running through it, most of all in the thick wall of the left ventricle, so a short diastole, as in a fast heart rate, cuts into the time the heart muscle gets to perfuse itself.

Close to its origin, the right coronary artery often gives off the sinuatrial (SA) nodal branch, which climbs to the SA node at the junction of the superior vena cava and the right atrium. Studies of dissected and imaged hearts put this origin at a little over half of people; in most of the rest the SA nodal artery comes from the circumflex. As it runs along the acute, inferior margin of the heart, the right coronary artery gives off the right marginal branch, which supplies the right ventricle toward the apex. It then turns onto the posterior surface in the atrioventricular groove. At the crux of the heart, where the atrioventricular and posterior interventricular grooves meet, it usually gives off the atrioventricular (AV) nodal branch and then turns down the posterior interventricular groove as the posterior interventricular artery, often called the posterior descending artery. The usual territory of the right coronary artery is therefore the right atrium, most of the right ventricle, the SA node in about half of hearts, and, in the common right-dominant pattern, the AV node, the inferior wall of the left ventricle and the posterior third of the interventricular septum.

The anterior interventricular artery (LAD) runs down the anterior interventricular groove toward the apex and often wraps around it onto the posterior surface. Two sets of branches leave it. Diagonal branches run onto the anterolateral wall of the left ventricle. Septal perforating branches dive into the interventricular septum and supply its anterior two-thirds, which includes the right bundle branch and much of the left bundle branch. In most hearts the LAD supplies the largest share of left ventricular muscle, which is why its proximal segment draws so much attention. The circumflex artery follows the left side of the atrioventricular groove around the left border of the heart. Its obtuse marginal branches supply the lateral wall of the left ventricle, and it also supplies the left atrium. In a left-dominant heart it continues to the crux and gives off the posterior interventricular artery and the AV nodal branch, taking over the territory the right coronary artery covers in most people. A quick way to hold the left side in memory: the LAD feeds the front and the septum, the circumflex feeds the side and the back, and dominance decides who feeds the bottom.

Dominance is defined by one branch: the artery that gives rise to the posterior interventricular artery is the dominant one. It does not mean that artery supplies more muscle. The left coronary artery, through the LAD and circumflex, usually supplies more of the left ventricle even in a right-dominant heart. Right dominance, with the posterior interventricular artery coming from the right coronary artery, is the most common pattern in every large series. Left dominance, with the posterior interventricular artery coming from the circumflex, and codominance, where both contribute, make up the rest. The reported percentages vary widely between studies, depending on the population and on whether dominance was judged by angiography, CT or dissection, so exams usually ask for the definition and the most common pattern rather than a figure. Dominance matters because it changes what a blockage does. In a right-dominant heart, a right coronary occlusion threatens the inferior wall and the AV node. In a left-dominant heart, the circumflex carries that territory instead, so a circumflex occlusion affects far more muscle than it would otherwise.

This is where coronary anatomy meets the conduction system, and it is a favorite source of exam questions. SA node: supplied by the SA nodal artery, from the right coronary artery in a little over half of hearts and from the circumflex in most of the rest. AV node: supplied by the AV nodal artery, which comes from the dominant artery at the crux, so usually from the right coronary artery. This is why an inferior infarction from a right coronary occlusion is classically linked with slow heart rates and AV block. Bundle of His and bundle branches: the bundle of His receives blood from the AV nodal artery with help from septal branches. The right bundle branch and the anterior fascicle of the left bundle branch run in the territory of the LAD's septal branches. The posterior fascicle of the left bundle branch usually has a dual supply, from both the anterior and posterior septal branches, which helps explain why it is the least often blocked. The pairing to remember: right coronary with the nodes, LAD with the bundle branches. Our cardiac conduction guide walks through the pathway itself, from the SA node to the Purkinje fibers.

Many coronary questions are territory questions in disguise. The usual exam pairings, for the common right-dominant heart, are these. Anterior wall and anterior septum: the LAD. In questions built around an ECG, these are the anterior leads V1 to V4. Lateral wall: the circumflex or the diagonal branches of the LAD, which match leads I, aVL, V5 and V6. Inferior wall: the right coronary artery in right-dominant hearts and the circumflex in left-dominant hearts, which match leads II, III and aVF. A question that adds a slow heart rate or AV block is pointing at the right coronary artery and the AV nodal branch. Right ventricle: a proximal right coronary occlusion, before the marginal branches leave. Work every question the same way: name the wall, name the artery that usually feeds it, then check whether the question has told you anything about dominance that changes the answer.

Most of the heart's venous blood returns through the coronary sinus, a wide venous channel in the posterior part of the atrioventricular groove that opens into the right atrium between the opening of the inferior vena cava and the tricuspid valve. Three named veins feed it, and each travels with an artery. The great cardiac vein starts at the apex, runs up beside the LAD in the anterior interventricular groove, and turns into the atrioventricular groove alongside the circumflex. The middle cardiac vein runs with the posterior interventricular artery. The small cardiac vein runs with the right marginal artery. Not all blood takes that route. The anterior cardiac veins drain much of the front of the right ventricle straight into the right atrium, and the smallest cardiac veins open directly into the chambers themselves, mostly on the right side. If you learn the veins as companions of the arteries you already know, the venous side takes minutes rather than hours.