The AV node resides in the right atrium in the lower portion of the interatrial septum. It is located under a thin strip of atrial muscle just above the septal cusp of the tricuspid valve and anterior to the ostium of the coronary sinus. The upper portion of the AV node is in continuity with the atrial myocardium and the fibers of the internodal tracts. At the lower end of the AV node, the cells become larger in size and are arranged in longitudinal parallel fibers to form the common His bundle. These fibers pass through the fibrous trigone along the upper portion of the membranous interventricular septum and then divide into the right and left bundle branches.