Anatomy, Thorax, Heart Right Coronary Arteries

Structure and Function

Structural Overview

The RCA originates from the aorta just above the right (coronary) semilunar cusp of the aortic valve (see Image. Valves of the Heart). This artery courses between the pulmonary trunk and right atrium within the right atrioventricular groove and curves around the acute margin of the heart (see Image. Anatomy of the Heart). Major branches include the sinoatrial nodal branch, the right (acute) marginal branch, the posterior interventricular branch, commonly referred to as the posterior descending artery (PDA), and the posterolateral branch.[2][3]

The PDA and posterolateral arteries arise from the RCA in 70% to 80% of the population.[4] Individuals with this configuration have a right-dominant coronary circulation. If these arteries originate from the circumflex branch of the left coronary artery (LCx), the heart is considered left-dominant. When both the RCA and left coronary artery (LCA) contribute to these branches, the heart has a balanced coronary circulation.

Histology

The coronary arteries are composed of 3 layers. These layers are the tunica intima, tunica media, and tunica adventitia.

The tunica intima forms the innermost layer and consists of the endothelium, subendothelium, and internal elastic lamina. Simple squamous cells line the luminal surface, allowing smooth blood flow and enabling functions such as pinocytosis, diapedesis, and the secretion of molecules into the lumen or underlying smooth muscle to stimulate the production of nitric oxide. A break in any part of the endothelial lining exposes blood to the subendothelial layer, triggering the clotting cascade. Disruption of the simple squamous endothelium may initiate atherosclerosis, where lipid accumulation damages the endothelium, promotes clot formation, and leads to plaque buildup. The internal elastic lamina forms a thick layer beneath the subendothelium.

The tunica media comprises the middle layer and is composed of smooth muscle cells arranged in a hexagonal configuration to enhance contractility. Typically, this layer contains approximately 5 layers of smooth muscle, along with limited amounts of elastic fibers, collagen, and proteoglycans.[5] A well-developed external elastic lamina is usually present only in large arteries.

The tunica adventitia constitutes the outer connective tissue layer that provides structural support to the vessel. This layer consists of loose connective tissue containing fibroblasts, collagen, small vessels, and nerves. Vasa vasorum supply larger arteries, while vasomotor nerves (nervi vascularis) facilitate contraction of the vessel wall.[6]

Functional Overview

Approximately 5% of cardiac output supplies the myocardium through the coronary arteries.[7] The sinoatrial nodal branch delivers blood to the sinoatrial node, while the right marginal artery supplies the right ventricle. The PDA courses alongside the middle cardiac vein within the posterior interventricular sulcus and supplies the atrioventricular node, the posterior 1/3 of the interventricular septum, the posterior 2/3 of the ventricular walls, and serves as the sole arterial source for the posteromedial papillary muscle.

Blood flow

A direct relationship exists between myocardial oxygen consumption and coronary blood flow. At rest, total coronary perfusion ranges from 250 to 300 mL per minute and increases to 1,000 to 2,000 mL per minute during exercise. Regulation of coronary vessel tone depends primarily on local metabolic control, particularly the release of adenosine from myocardial cells. Adenosine production increases as cardiac energy demand rises during exertion, promoting vasodilation and enhancing blood flow to match metabolic needs. Coronary arteries also contain a high density of β2-adrenergic receptors, which mediate vasodilation when stimulated.[8]

Although both coronary arteries branch from the aorta and experience the same systemic blood pressure, the LCA and RCA demonstrate different patterns of blood flow. Most LCA flow occurs during diastole, when myocardial relaxation allows coronary pressure to exceed left ventricular pressure. During systole, pressure within the left ventricle closely matches or exceeds aortic pressure, which can result in minimal forward flow, or even transient flow reversal, referred to as the “throttle effect.” This diastolic dependence is primarily due to elevated intramyocardial pressure during systole, which compresses the intramural vessels, especially those coursing from the epicardium to the endocardium. Additional contributing factors include the distortion of flow by eddy currents within the sinus of Valsalva during systole, which reduces perfusion to the coronary ostia.

In contrast, RCA flow is not diastole-dependent. The right heart generates lower intraventricular pressures, so the pressure within the RCA typically remains higher than the pressure within the right ventricular wall during both systole and diastole. Consequently, flow through the RCA is more continuous throughout the cardiac cycle.[9]

Embryology

The coronary arteries develop within the atrioventricular and interventricular grooves. After the separation of the aorta and pulmonary artery, these vessels fuse with vascular buds arising from the right and left semilunar cusps (sinuses of Valsalva) to establish myocardial perfusion.

Blood Supply and Lymphatics

Blood supply to the coronary arteries is provided in part by luminal flow, while larger vessels receive additional nourishment from a network of vasa vasorum microvessels. The vasa vasorum is thought to contribute to the development of coronary atherosclerosis.[10]

Nerves

Parasympathetic fibers from the vagus nerve (cranial nerve X) may induce vasodilation of coronary resistance vessels. Sympathetic fibers may promote vasoconstriction through activation of α-adrenergic receptors on these vessels. However, this effect is typically overridden by local metabolic control, specifically, the release of adenosine from myocardial cells, which leads to net vasodilation during exertion.[11]

Muscles

The tunica media, the middle layer of the coronary arteries, consists of concentric layers of smooth muscle cells. Vascular resistance in the coronary circulation is largely modulated by the activity of these smooth muscle layers.

Physiologic Variants

Anatomic variations in coronary artery circulation include a shared common trunk between the LCA and RCA, an atrioventricular node supplied by the LCx, and, rarely, an atrioventricular nodal branch arising from both the RCA and the LCx. The PDA originates from the LCx in approximately 10% of the population and from both the LCx and RCA in about 20%. The sinoatrial nodal artery arises from the LCA in 34% of individuals.

Surgical Considerations

Revascularization of the heart may be achieved through coronary artery bypass grafting (see Image. Coronary Artery Bypass Graft).[12] This procedure restores blood flow to obstructed regions of the myocardium by grafting vessels from other parts of the body into the coronary circulation. Common graft sources include the left internal thoracic artery, also known as the left internal mammary artery (LIMA), and the great saphenous vein from the lower extremity. As a nonsurgical alternative, percutaneous coronary intervention may be used to address coronary artery stenosis. This technique often involves the placement of stents to widen partially occluded vessels.[13]

Clinical Significance

The posterior and anterior interventricular arteries typically lack anastomotic connections, so occlusion of a coronary artery can severely compromise myocardial perfusion. Ischemic heart disease is characterized by an imbalance between oxygen supply and the metabolic demand of cardiac tissue.[14] This condition often arises when blood flow to the myocardium is reduced, as in coronary artery atherosclerosis, where the lumen of a major epicardial artery becomes obstructed by atherosclerotic plaque and, in some cases, superimposed thrombosis. Ischemic syndromes include angina pectoris, acute myocardial infarction, and sudden cardiac death.[15]

Coronary artery occlusion most commonly affects the left anterior descending artery, also known as the left anterior interventricular artery, but approximately 25% of cases involve the right coronary arteries.[16] Occlusion of the PDA or distal branches of the RCA may cause a posterior wall infarction. The obstruction compromises blood supply to the posterior walls of the left and right ventricles and the posterior 1/3 of the interventricular septum.

Unlike the anteromedial papillary muscle, which receives blood from both the left anterior interventricular artery and the LCx, the posteromedial papillary muscle is supplied solely by the PDA.[17] This single-vessel dependence increases its vulnerability to ischemia and may result in mitral regurgitation.

An inferior wall infarction typically results from occlusion of the RCA (see Media. Conus Occlusion on Coronary Angiography with Aortography).[18] The endocardium, being the innermost layer of the heart wall, is generally the most severely affected by infarction. Obstruction of the RCA or any of its branches supplying the sinoatrial or atrioventricular node may impair pacemaking activity. Nodal dysfunction may present clinically as bradycardia or varying degrees of heart block.[19]