The cardiac skeleton, or fibrous skeleton of the heart, is the structure of dense connective tissue that separates the atria from the ventricles. The fibrous skeleton provides critical support for the heart and separates the flow of electrical impulses through the heart.

Fibrous Ring Structure

The primary structure of cardiac skeleton consists of four dense bands of tough elastic tissue called fibrous rings that encircle the bases of the heart valves. The fibrous skeleton is composed primarily of collagen, a protein found in every type of connective tissue in the human body. There are four fibrous rings:

• The aortic ring encircles the aortic valve. It provides support for the aortic valve so that it is open, yet does not have backflow.
• The pulmonary ring encircles the pulmonary valve. Similar to the aortic ring, it provides structural support for the pulmonary valve.
• The left fibrous ring encircles the bicuspid valve. This ring is the thickest and strongest of all the fibrous rings due to the thickness of the left ventricle, which requires more structural support than the other chambers of the heart. It also surrounds the coronary arteries and AV node.
• The right fibrous ring encircles the tricuspid valve. It also surrounds the coronary arteries and AV node.

The fibrous skeleton provides a great amount of structural and functional support for the valves of the heart by enabling them to stay open and provides a point of attachment for the valves to the wall of the heart.  

Fibrous Rings of the Heart

Transverse section of the heart showing the fibrous rings surrounding the valves.

This transverse section of the heart indicates the aortic valve, bicuspid valve, tricuspid valve, pulmonary valve, and left and right coronary arteries.

Electrical Functions

The fibrous skeleton of the heart acts as an insulator for the flow of electrical current across the heart. It stops the flow of electricity between the different chambers of the heart so that electrical impulses do not flow directly between the atria and ventricles. The sinoatrial (SA) node lies on the top of the heart, while the AV node is located at the center of the fibrous skeleton, the only path by which electricity can flow through the heart.

This electrical separation is essential for cardiac function, because electrical impulses flow from the top of the heart to the bottom of the heart. The separation allows the AV node and AV bundle to delay the wave of depolarization so that the atria can contract and assist in ventricular filling before the ventricles themselves depolarize and contract. Without the fibrous skeleton of the heart, the heart's ability to pump blood would be considerably less efficient since the ventricles would contract before filled to capacity. The fibrous skeleton of the heart also protects against cardiac arrhythmias.