Fibrous skeleton of heart

The fibrous skeleton of the heart is a complex, dense connective tissue structure located at the base of the right and left venticles mass. It serves as providing structural support for the attachment of the cardiac valves, maintaining the integrity and shape of the heart, and electrically insulating the atria from the ventricles except at the atrioventricular (AV) node, thereby ensuring proper conduction of electrical impulses.

The fibrous skeleton consists of the right and left fibrous rings, the left fibrous trigone, the central fibrous body, the membranous septum, the fibrous area of aortic-mitral continuity, the subvalvar collar of the mitral valve, the interleaflet triangles, and the tendon of inferior pyramidal space (tendon of Todaro).

The annuli of cardiac valves may be included in the fibrous skeleton of the heart, but as the majority of the mitral annulus is fibrous, the tricuspid annulus contains true fibrous tissue only at the site of leaflet attachment to the central fibrous body and the aortic annulus is supported by fibrous elements only at the noncoronary and some parts of the right and left coronary sinuses.

The fibrous rings surround the atrioventricular and arterial orifices, and are stronger upon the left than on the right side of the heart.

The atrioventricular rings serve for the attachment of the muscular fibers of the atria and ventricles, and for the attachment of the bicuspid and tricuspid valves. The left atrioventricular ring is closely connected, by its right margin, with the aortic arterial ring (left trigonum fibrosum); between these and the right atrioventricular ring is a triangular mass of fibrous tissue, the right trigonum fibrosum, which represents the os cordis seen in the heart of some of the larger animals, as the ox and elephant. Lastly, there is the tendinous band, already referred to, the posterior surface of the conus arteriosus.

The fibrous rings surrounding the arterial orifices serve for the attachment of the great vessels and semilunar valves. Each ring receives, by its ventricular margin, the attachment of some of the muscular fibers of the ventricles; its opposite margin presents three deep semicircular notches, to which the middle coat of the artery is firmly fixed. The attachment of the artery to its fibrous ring is strengthened by the external coat and serous membrane externally, and by the endocardium internally. From the margins of the semicircular notches the fibrous structure of the ring is continued into the segments of the valves. The middle coat of the artery in this situation is thin, and the vessel is dilated to form the sinuses of the aorta and pulmonary artery.