A 35-year-old female is admitted to the emergency department because of cardiac arrhythmia. ECG examination reveals that the patient suffers from atrial fibrillation. Where is the mass of specialized conducting tissue that initiates the cardiac cycle located?

The SA node, the primary pacemaker of the heart, is a mass of specialized cardiac cells within the myocardium at the upper end of the crista terminalis, near the opening of the superior vena cava into the right atrium. The AV node is at the junction of the coronary sinus and the right atrium upon the right fibrous trigone (central fibrous body). The eustachian valve directs blood from the inferior vena cava and through the right atrium toward the tricuspid valve ostium. The interatrial septum is located between the left and right atria. The septomarginal trabeculum (moderator band) arises from the muscular portion of the interventricular septum and passes to the base of the anterior papillary muscle in the right ventricle. The moderator band carries the right bundle branch of the conduction system just beneath its endocardial layer.