Fibrous Skeleton and Valves of Heart

Fibrous Skeleton of Heart

The fibrous skeleton of the heart is formed by dense fibrous connective tissue that provides structural support to the cardiac musculature. It is concentrated around the valvular orifices, where it supports and anchors the valve cusps and leaflets.

Components of Fibrous Skeleton

The fibrous skeleton consists of the following components:

• Four fibrous rings surrounding the right and left atrioventricular, aortic, and pulmonary orifices.
• Tendon of the infundibulum
  • A fibrous band connecting the pulmonary ring with the aortic ring.
• Right fibrous trigone (trigonum fibrosum dextrum)
  • A thickened area of fibrous tissue formed at the junction of the right and left atrioventricular rings.
• Left fibrous trigone (trigonum fibrosum sinistrum)
  • A fibrous thickening located at the junction of the aortic and left atrioventricular rings.

Position of rings

The four fibrous rings do not lie in a single plane. The aortic ring occupies a central position within the fibrous skeleton. It is connected to the mitral and tricuspid rings on one side and to the pulmonary ring on the other through the tendon of the infundibulum.

Aortic Annulus

The aortic annulus is formed by three crescent-shaped scallops, designated as right, left, and posterior.

Each scallop has the following features:

• Ascending limbs: The two limbs extend upward and meet the corresponding limbs of adjacent scallops.
• Concave zone: This region provides attachment for the aortic valve cusp.
• Nadir: The lowest point of the scallop.

Left fibrous trigone

• The nadir of the left scallop is thickened to form the left fibrous trigone.
• It provides attachment to the anterior filum coronarium of the mitral annulus.

Right fibrous trigone

• The nadir of the right scallop forms a fibrous thickening known as the right fibrous trigone or central fibrous body.
• It provides attachment to the posterior filum coronarium of the mitral annulus and both fila coronaria of the tricuspid annulus.
• The right fibrous trigone is continuous with the membranous part of the interventricular septum.

Tendon of Todaro

• A fibrous band extends from the right side of the right fibrous trigone to the left horn of the valve of the inferior vena cava. This band is known as the tendon of Todaro.
• It lies immediately beneath the endocardium and is therefore subendocardial in position.

Subaortic spans

• These are triangular gaps located between adjacent aortic scallops.

Subaortic curtain

• The subaortic curtain is a fibrous sheet occupying the subaortic span between the left and right aortic scallops.
• Inferiorly, it continues with the anterior leaflet of the mitral valve.

Mitral Annulus

The mitral annulus consists of:

1. Right fibrous trigone
2. Left fibrous trigone
3. Two fila coronaria, which are thick bands of connective tissue

The anterior filum coronarium is attached to the left fibrous trigone, whereas the posterior filum coronarium is attached to the right fibrous trigone.

The region between the free ends of the two fila coronaria contains relatively weak connective tissue and is termed the sulcal connective tissue.

Tricuspid Annulus

The tricuspid annulus consists of two fila coronaria, anterior and posterior, both of which are attached to the right fibrous trigone.

A zone of weak connective tissue, termed sulcal connective tissue, lies between the free ends of the two fila coronaria.

Pulmonary Annulus

The pulmonary annulus consists of:

1. Three scallops: right anterior, left anterior, and posterior
2. Subvalvular spans: gaps located between adjacent scallops

Tendon of the infundibulum

• The tendon of the infundibulum is a band of fibrous connective tissue.
• It extends from the posterior scallop of the pulmonary annulus to the nadir of the anterior scallop of the aortic annulus.
• It provides a fibrous connection between the pulmonary and aortic annuli.

Functions of Skeleton of Heart

• Structural support: The fibrous skeleton provides attachment and support for the cardiac musculature.
• Electrical separation of atria and ventricles: It separates the atrial myocardium from the ventricular myocardium. Consequently, impulses from the atria reach the ventricles through the specialized cardiac conduction system.
• Maintenance of valvular function: By supporting and anchoring the valve cusps and leaflets, the fibrous skeleton contributes to effective valve closure and unidirectional blood flow.

Arrangements of cardiac muscles

Cardiac muscle fibers are arranged in characteristic patterns that facilitate effective contraction and complete emptying of the heart chambers.

Atrial Muscle Fibers

Atrial muscle fibers are arranged into three groups:

1. Superficial fibers: These fibers run mainly in a transverse direction.
2. Deep fibers: These form looped bundles that arch over the roof of the atria and are attached to the atrioventricular rings.
3. Annular fibers: These fibers encircle the openings of the veins entering the atria.

Functions

• Contraction of the superficial fibers reduces the transverse and anteroposterior dimensions of the atria.
• Contraction of the deep looped fibers decreases the vertical dimension of the atria.
• Annular fibers constrict the venous openings and help prevent reflux of blood into the veins during atrial systole.

Ventricular Muscle Fibers

Ventricular muscle fibers are arranged in two principal layers:

1. Superficial fibers: These fibers follow a spiral course from the fibrous skeleton of the heart toward the papillary muscles.
2. Deep fibers: These fibers connect the papillary muscles of the two ventricles and pass through the interventricular septum.

Functions

• Contraction of the superficial fibers reduces the longitudinal dimension of the ventricles.
• Contraction of the deep fibers decreases the transverse dimension of the ventricles, thereby assisting ventricular emptying.

Valves of Heart

The heart contains two pairs of functional valves:

1. Atrioventricular valves: right and left
2. Semilunar valves: aortic and pulmonary

These valves maintain unidirectional blood flow and prevent regurgitation.

Atrioventricular (AV) Valves

The atrioventricular valves guard the atrioventricular orifices. They permit blood to pass from the atria into the ventricles and prevent its return to the atria during ventricular contraction.

The two atrioventricular valves are:

1. Right atrioventricular valve (tricuspid valve)
2. Left atrioventricular valve (mitral valve)

Structure of valve

Each atrioventricular valve is supported by a fixed fibrous ring to which the valve cusps are attached.

• Cusps: These are thin, flat flaps projecting into the ventricular cavity.
• Each cusp has an attached margin, a free margin, an atrial surface, and a ventricular surface.
• The free margins of the cusps are connected to the papillary muscles by chordae tendineae.
• During ventricular systole, the atrioventricular valves close. Contraction of the papillary muscles keeps the chordae tendineae tense and prevents prolapse or eversion of the cusps into the atria.

Tricuspid Valve

The right atrioventricular orifice is guarded by the tricuspid valve. The orifice is directed toward the apex of the right ventricle.

The tricuspid valve consists of the tricuspid annulus, three leaflets, and chordae tendineae.

Tricuspid annulus

• It is a collagenous ring that provides attachment to the valve leaflets.
• It contains two fila coronaria, which are bands of dense collagenous tissue.
• The fila coronaria do not form a complete ring.
• Inferolaterally, they are continuous with the sulcal connective tissue.

Leaflets

Each leaflet consists of a fold of endocardium enclosing a collagenous fibrous core. The tricuspid valve has three leaflets:

1. Anterior leaflet: Attached to the upper part of the annulus and is the largest leaflet.
2. Septal leaflet: Extends along the interventricular septum from its muscular to membranous part.
3. Posterior leaflet: Attached to the inferior part of the annulus.

Commissures are the indented junctions between adjacent leaflets.

The chordae tendineae connect the tricuspid leaflets to the anterior, inferior, and septal papillary muscles.

Mitral Valve

The left atrioventricular orifice is guarded by the mitral valve, also known as the left atrioventricular valve.

The mitral valve consists of a fibrous annulus and two leaflets.

Mitral annulus

• It is a collagenous ring that provides attachment to the mitral leaflets.
• It includes the anterior and posterior fila coronaria and the right and left fibrous trigones.
• The fila coronaria extend from the fibrous trigones toward the sulcal connective tissue.

Subaortic curtain

The subaortic curtain is a fibrous sheet occupying the interval between adjacent aortic scallops and contributes to the continuity of the mitral annulus.

Leaflets of the mitral valve

The mitral valve has two leaflets:

1. Anterior leaflet: Attached to the anteromedial part of the annulus between the right and left fibrous trigones. Its central portion is continuous with the subaortic curtain.
2. Posterior leaflet: Attached to the posterolateral part of the annulus.

Chordae tendineae

The free margins of both leaflets are attached to chordae tendineae, which anchor them to the two papillary muscles, namely the anterior and posterior papillary muscles.

Semilunar Valves

The aortic and pulmonary orifices are guarded by the semilunar valves. Each valve consists of three semilunar cusps.

• Each cusp has an attached margin and a free margin.
• The free margin bears a central fibrous nodule, with a thin crescent-shaped lunule on either side.
• A small dilatation of the arterial wall lies above each cusp and is termed a sinus. These are known as the pulmonary sinuses in the pulmonary trunk and the aortic sinuses in the ascending aorta.
• Unlike the atrioventricular valves, the semilunar valves are not supported by a complete fibrous annulus of the same configuration. Their cusps are attached to fibrous tissue at the roots of the great arteries.
• The semilunar valves close during ventricular diastole and prevent backflow of blood into the ventricles.

Pulmonary Valve

The pulmonary valve has three cusps:

1. Anterior cusp
2. Right cusp
3. Left cusp

Aortic Valve

The aortic valve also has three cusps:

1. Right coronary cusp
2. Left coronary cusp
3. Posterior non-coronary cusp

The right and left coronary arteries arise from the aortic sinuses associated with the corresponding right and left coronary cusps.

Aortic Sinuses

Three aortic sinuses are present immediately above the cusps of the aortic valve:

1. Right coronary sinus: Gives origin to the right coronary artery.
2. Left coronary sinus: Gives origin to the left coronary artery.
3. Non-coronary sinus: Does not give rise to a coronary artery.

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