In cardiology, the cardiac skeleton, also called the fibrous skeleton of the heart, is a thick, uniform structure made of connective tissue. It helps hold the heart valves in place and controls the forces they experience. The cardiac skeleton keeps the upper chambers (atria) separate from the lower chambers (ventricles). It includes four thick rings of connective tissue that surround the mitral and tricuspid atrioventricular (AV) canals and extend to where the pulmonary trunk and aorta begin. This structure provides support to the heart and prevents electrical signals from passing directly between the atria and ventricles.

The special arrangement of connective tissue in the cardiac skeleton stops electrical signals from moving freely between the chambers. In a normal heart, there is only one path for electrical signals to travel from the upper to the lower chambers, called the atrioventricular node. The cardiac skeleton acts as a barrier that controls electrical signals until they reach the bundle of His, which then directs the signals to the ventricles. In this way, the cardiac skeleton ensures that electrical signals from the atria are directed efficiently to the ventricles.

Structure

The structure of the heart's parts has become a topic of growing interest. The cardiac skeleton connects several bands of dense connective tissue, such as collagen, which surround the bases of the pulmonary trunk, aorta, and all four heart valves. Though not a rigid skeleton, this structure supports the heart and separates the atria from the ventricles. This separation helps prevent atrial fibrillation from spreading to ventricles. In young people, the collagen structure is flexible and free of calcium buildup. As people age, calcium and other minerals collect in this structure. The ability of the ventricles to stretch is affected by these mineral deposits, which also slow the electrical signal that travels from the AV node to the bundle of His in older individuals.

The right and left fibrous rings of the heart (annuli fibrosi cordis) surround the openings between the atria and ventricles and the arteries. The right ring is called the annulus fibrosus dexter cordis, and the left is called the annulus fibrosus sinister cordis. The right fibrous trigone connects to the central fibrous body, which is the strongest part of the fibrous cardiac skeleton.

The atria (upper chambers) and ventricles (lower chambers) are separated by the properties of collagen in the fibrous rings. These rings, along with the central body and heart skeleton made of collagen, block electrical signals. The only path for electrical signals through this barrier is a channel that connects to the atrioventricular node and leads to the bundle of His. The muscle fibers of many heart cells attach to opposite sides of the valve rings.

The atrioventricular rings help attach the muscle fibers of the atria and ventricles, as well as the bicuspid and tricuspid valves. The left atrioventricular ring connects to the aortic arterial ring on its right side. Between the aortic ring and the right atrioventricular ring is a triangular area of fibrous tissue called the fibrous trigone. This structure is similar to the os cordis found in some larger animals, like oxen.

The tendinous band, located on the back side of the conus arteriosus, is also part of the fibrous structure. The fibrous rings around the arteries attach the major blood vessels and semilunar valves. These rings are called the aortic annulus. Each ring connects to the ventricles on one side and has three deep notches on the other side, where the artery's middle layer is attached. The artery's connection to its fibrous ring is reinforced by the outer layer of the artery and the inner lining of the heart. From the notches, the fibrous structure extends into the valve segments. The middle layer of the artery is thin in these areas, allowing the aorta and pulmonary artery to expand into sinuses.

In some animals, the fibrous trigone may develop significant mineral deposits with age, forming a structure called the os cordis (heart bone) or two separate ones (os cordis sinistrum and os cordis dextrum, the latter being larger). The os cordis is believed to have mechanical functions. In humans, two fibrous trigones (left and right) are present. These structures are important in heart surgery because they influence the spread of electrical signals through the atrioventricular node.

The os cordis has been known since ancient times in animals like deer and oxen, where it was believed to have medicinal or mystical properties. It is sometimes found in goats, otters, and recently in chimpanzees, the only great ape known to have it. In the past, Galen claimed that elephants also had the os cordis, a belief that remained in medical texts like Gray's Anatomy until the 19th century. However, this is not true.

Function

Electrical signals from the sinoatrial node and the autonomic nervous system must travel from the upper chambers to the lower ones so the ventricles can push blood through the body. The heart acts like a pump that sends blood in short bursts to the lungs, body, and brain.

The cardiac skeleton directs the electrical and autonomic signals from the upper part of the heart to the lower part and stops them from moving back up. It does this by creating a boundary that blocks electricity from passing through. This boundary is made of dense connective tissue that does not conduct electricity, and its placement in the heart is not random.

The strong, non-conductive collagen framework of the heart valves allows the atrioventricular node (AV node) to be positioned in the center of the heart. The AV node is the only path for electrical signals to travel from the atria to the ventricles through the cardiac skeleton. This is why atrial fibrillation cannot spread to the ventricles.

Over a person’s lifetime, the collagen structure of the heart changes. When collagen decreases with age, calcium often builds up in its place. This creates clear signs visible in medical images that help measure how much blood the heart pumps during each beat. The non-conductive nature of collagen also makes it hard to get accurate imaging signals unless the ratio of collagen to calcium is carefully considered.

History

Boundaries within the heart were first described and greatly expanded by Drs. Charles S. Peskin and David M. McQueen at the Courant Institute of Mathematical Sciences.