Heart valve

In anatomy, the heart valves are valves in the heart that prevent blood from flowing the wrong way.

There are four valves of the heart:

Two atrioventricular (AV) valves - ensure blood flows from the atria to the ventricles, and not the other way.
Two semilunar valves - these are present in the arteries leaving the heart, and they prevent blood flowing back from the artery into the ventricle.

Heart valves open and shut depending on the difference in pressure on each side. The sound of the heart valves shutting causes the heart sounds.

AV valves

These are large, multicusped valves that prevent backflow from the ventricles into the atria during systole. They are anchored to the wall of the ventricle by chordae tendinae, that prevent the valve from inverting.

The chordae tendinae are attached to papillary muscles that cause tension to better hold the valve. Together, the papillary muscles and the chordae tendinae are known as the subvalvular apparatus. The function of the subvalvular apparatus is to keep the valves from prolapsing into the atria when they close. The subvalvular apparatus have no effect on the opening and closure of the valves, however. This is caused entirely by the pressure gradient across the valve.

Mitral valve

Also known as the bicuspid valve, the mitral valve gets its name from the resemblance to a bishop's mitre (a type of hat). It prevents blood flowing from the left ventricle into the left atrium. As it is on the left side of the heart, in must cope with a lot of strain and pressure, this is why it is made of only two cusps, as there is less to go wrong.

A common complication of rheumatic fever is thickening and stenosis of the mitral valve.

Tricuspid valve

The tricuspid valve is on the right side of the heart, between the right atrium and the right ventricle. Being the first valve after the venae cavae, and thus the whole venous system, it is the most common valve to be infected (endocarditis) in IV drug users.

Semilunar valves

These are positioned on the pulmonary artery and the aorta. These valves to not have chordae tendinae, but are more similar to valves in veins.

Aortic valve

The aortic valve lies between the left ventricle and the aorta. The aortic valve has three cusps. During ventricular systole, pressure rises in the left ventricle. When the pressure in the left ventricle rises above the pressure in the aorta, the aortic valve opens, allowing blood to exit the left ventricle into the aorta. When ventricular systole ends, pressure in the left ventricle rapidly drops. When the pressure in the left ventricle decreases, the aortic pressure forces the aortic valve to close. The closure of the aortic valve contributes the A2 component of the second heart sound (S2).

The most common congenital abnormality of the heart is the bicuspid aortic valve. In this condition, instead of three cusps, the aortic valve has two cusps. This condition is often undiagnosed until the person develops calcific aortic stenosis. Aortic stenosis occurs in this condition usually in patients in their 40s or 50s, an average of 10 years earlier than in people with normal aortic valves.

Pulmonic valve

The pulmonic valve lies between the right ventricle and the pulmonary artery. Similar to the aortic valve, the pulmonic valve opens in ventricular systole, when the pressure in the right ventricle rises above the pressure in the pulmonary artery. At the end of ventricular systole, when the pressure in the right ventricle falls rapidly, the pressure in the pulmonary artery will close the pulmonic valve.

The closure of the pulmonic valve contributes the P2 component of the second heart sound (S2). The right heart is a low pressure system, so the P2 component of the second heart sound is usually softer than the A2 component of the second heart sound.

Pathology of the valves

Endocarditis
Stenosis - a constricture of the heart valve, making it hard for blood to get through.
Insufficiency - the inability of the heart valve to close properly, meaning some blood can flow the wrong way.
et cetera