The cardiac skeleton is the structure of dense connective tissue in the heart which electrically isolates the atria from the ventricles, forming four fibrous rings that surround the ostia of the valves, a right and left fibrous trigone, formed by connecting the rings, and the membranous portions of the interatrial and interventricular septa. The fibrous structure of the cardiac skeleton also provides rigidity to prevent the dilatation of individual valves or outflow tracts, as well as a point of attachment for valve leaflets and the myocardium. The myocardium extends from the fibrous skeleton in a series of spirals that progress to the apex. The atrioventricular conducting system is the only electrical connection between the atria and the ventricles in a normal heart. The majority of the skeleton lies within the plane of the base of the ventricles, roughly parallel to the coronary sinus.
The cardiac skeleton consists of four dense bands of tough elastic tissue that encircle the bases of the pulmonary trunk, aorta, and heart valves. While not a true skeleton, it does provide structure and support for the heart, as well as isolating the atria from the ventricles. This allows the atrioventricular node and atrioventricular bundle to delay the wave of depolarization such that the atria can contract and assist in ventricular filling before the ventricles themselves depolarize and contract (the AV bundle is the only part of the conduction system that passes from the atria to the ventricles in a normal heart). It also allows the valves (bicuspid, tricuspid, semilunar) to keep open by giving them structural support.