Valves of the Heart – Atrioventricular and Semilunar Valves

The Valves of the Heart are present in 2 pairs: (a) a pair of atrioventricular valves, and (b) a pair of semilunar valves. The valves prevent regurgitation of the blood.

Atrioventricular Valves

The left and right atria communicate with the left and right ventricles via left and right atrioventricular orifices, respectively. The left and right atrioventricular orifices are guarded by the left and right atrioventricular valves respectively:

A. Right atrioventricular valve (also called tricuspid valve): As the name indicates it’s 3 cusps anterior, posterior and septal, which is located against the 3 walls of the ventricle. The tricuspid valve can admit the tips of 3 fingers.

B. Left atrioventricular valve (also called bicuspid/ mitral valve): As the name indicates it’s 2 cusps a larger anterior/aortic cusp and a smaller posterior cusp. The mitral/bicuspid valve can admit the tips of 2 fingers.

Structure

The atrioventricular valves are created from 2 components:

A. A fibrous ring.

B. Cusps.

The fibrous rings surround the orifice. The cusps are created by the fold of the endocardium enclosing some connective tissue inside it. Every cusp has a connected and free margin and atrial and ventricular surfaces. The atrial surfaces are smooth. The ventricular surfaces and free margins are rough and provide connection to the chordae tendinae. As discussed earlier, the chordae tendinae attach the apices of papillary muscles with margins and ventricular surfaces of the cusps. The chordae tendinae of every papillary muscle are connected to the contiguous halves of the 2 cusps.

The valves are closed during ventricular systole. The papillary muscles shorten and chordae tendinae are pulled upon to prevent the eversion of the cusps of tricuspid valve because of increased intraventricular pressure.

Points to be noted:

A. The nutrition to the fibrous ring and basal 1 -third of cusps is provided by the blood vessels.

B. The nutrition to the distal 2 -third of the cusps is provided directly by the blood inside the chambers of the heart.

C. The cusps of mitral valve are smaller but thicker than those of tricuspid valve.

Clinical Significance

Role Of Papillary Muscle In Acute Cardiac Failure

The papillary muscles prevent the prolapse of atrio-ventricular valves into the atria during ventricular systole. The rupture of a papillary muscle, following a nearby myocardial infarction, will enable the prolapse of the affected cusp to take place into the atrium at every systole. This may consequently result in acute cardiac failure.

Semilunar Valves

The left and right ventricles pump out blood via pulmonary and aortic orifices, respectively. Each of these orifices is guarded by 3 semilunar cusps thus they’re referred to as semilunar valves. Both aortic and pulmonary valves are quite similar to every other in structure and functions.

Every valve has 3 cusps that are connected directly to the wall of aorta/pulmonary trunk.

(Note: they don’t have fibrous ring quite similar to tricuspid and mitral valves)

The cusps create small pockets with their mouths directed upwards in the direction of the lumen of great vessels. Every cusp has a fibrous nodule in the midpoint of its free edge. On every side of the nodule the thickened crescentic edge is termed lunule, which extends up to the base. When the valve is closed, the nodules meet in the centre.

The cusps of semilunar valves are open and stretched during ventricular systole and closed during ventricular diastole to prevent regurgitation of the blood into the ventricle.

Points to be noted:

A. No chordae tendinae or papillary muscles are related to semilunar valves. The connection of the sides of cusps to the atrial wall prevents regurgitation of blood.

B. Opposite to the cusps, the roots of pulmonary trunk and ascending aorta present 3 dilatations termed sinuses. The blood in these types of sinuses prevents the cusps from sticking to the wall of great vessels. The anterior aortic sinus provides origin to the right coronary artery and left posterior aortic sinus provides origin to the left coronary artery.

Positions Of Cusps In The Pulmonary And Aortic Valves

The positions of cusps of pulmonary valves are: (a) right anterior, (b) left anterior, and (c) posterior.

The positions of cusps of aortic valve are just opposite to those of the pulmonary valve. They’re: (a) right posterior, (b) left posterior, and (c) anterior.

The aortic sinuses are also termed accordingly, i.e., right posterior aortic sinus, left posterior aortic sinus, and anterior aortic sinus. The right coronary artery originates from anterior aortic sinus and left coronary from left posterior aortic sinus. Since no coronary artery originates from right posterior aortic sinus, it’s referred to by some anatomists as non-coronary sinus.

Embryologically, pulmonary valve has anterior, left and right cusps on the other hand aortic valve has posterior, left and right cusps. Thus the left coronary artery originates from the left aortic sinus, the right coronary artery from the right aortic sinus and no artery originates from the posterior aortic sinus (non-coronary sinus).

Clinical Significance

Murmurs

The abnormal heart sounds are named murmurs. They’re produced because of regurgitation of blood heard when the valves are either stenosed or when the valves aren’t closed properly (leading to regurgitation).

In aortic and pulmonary stenosis the murmur is heard during systole and in insufficiency of these valves they’re heard during diastole.

In stenosis of mitral and tricuspid valves, the murmurs are heard during diastole and in their insufficiency during systole.

Mitral Stenosis (Narrowing Of Mitral Orifice)

It’s most common in young age. Generally there’s historyof rheumatic fever in the childhood in these types of cases. This leads to rise in the left atrial pressure and enlargement of left atrium which might press on the esophagus.

Medically features of mitral stenosis will probably be as follows:

A. Shortness of breath (dyspnea).

B. Dysphagia (difficulty in swallowing).

C. Hoarseness of voice (Ortners syndrome).

Tricuspid Stenosis

In tricuspid stenosis blood flow from right atrium to right ventricle is reduced. Theelevation of right atrial pressure leads to systemic venous congestion and right heart failure.

Aortic Stenosis

In aortic stenosis there accumulation of blood in left ventricle, causing its dilatationand hypertrophy. There’s low cardiac output which might manifest as syncope (fainting) on exertion.

Pulmonary Stenosis

It’s just about always congenital, generally a part of Fallots tetralogy. It leads to hypertrophy of right ventricle.