| Heart valve | |
|---|---|
 Valves of the heart in motion. The front wall of the heart is removed in this image. | |
| Details | |
| System | Cardiovascular |
| Identifiers | |
| MeSH | D006351 |
| TA2 | 3973 |
| FMA | 7110 |
| Anatomical terminology | |
Aheart valve (cardiac valve) is a biologicalone-way valve that allowsblood to flow in one direction through the chambers of theheart. Amammalian heart usually has four valves. Together, the valves determine the direction ofblood flow through the heart. Heart valves are opened or closed by a difference inblood pressure on each side.[1][2][3]
The mammalian heart has twoatrioventricular valves separating the upperatria from the lowerventricles: themitral valve in the left heart, and thetricuspid valve in the right heart. The twosemilunar valves are at the entrance of the arteries leaving the heart. These are theaortic valve at theaorta, and thepulmonary valve at thepulmonary artery.
The heart also has acoronary sinus valve and aninferior vena cava valve, not discussed here.
The heart valves and thechambers are lined withendocardium. Heart valves separate theatria from theventricles, or the ventricles from ablood vessel. Heart valves are situated around thefibrous rings of thecardiac skeleton. The valves incorporate flaps calledleaflets orcusps, similar to aduckbill valve orflutter valve, which are pushed open to allow blood flow and which then close together to seal and prevent backflow. Themitral valve has two cusps, whereas the others have three. There are nodules at the tips of the cusps that make the seal tighter.
Thepulmonary valve has left, right, and anterior cusps.[4] Theaortic valve has left, right, and posterior cusps.[5] Thetricuspid valve has anterior, posterior, and septal cusps; and the mitral valve has just anterior and posterior cusps.
The valves of the human heart can be grouped in two sets:[6]
| Valve | Number of flaps/cusps | location | prevent backflow of blood |
|---|---|---|---|
| Atrioventricular valves | 3 (right), 2 (left) | From the ventricles into the atria | |
| Tricuspid valve | 3 | between the right atrium and right ventricle. | |
| Bicuspid or mitral valve | 2 | between the left atrium and left ventricle. | |
| Semilunar valves | 3 (half-moon shaped) flaps | into the ventricle | |
| Pulmonary semilunar valve | 3 (half-moon shaped) flaps | at the opening between the right ventricle and the pulmonary trunk | |
| Aortic semilunar valve | 3 (half-moon shaped) flaps | at the opening between the left ventricle and the aorta |
The atrioventricular valves are themitral valve, and thetricuspid valve, which are situated between theatria and theventricles, and preventbackflow from the ventricles into the atria duringsystole. They are anchored to the walls of the ventricles bychordae tendineae, which prevent them from inverting.
The chordae tendineae are attached topapillary muscles that cause tension to better hold the valve. Together, the papillary muscles and the chordae tendineae 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.[7] The subvalvular apparatus has no effect on the opening and closure of the valves, however, which is caused entirely by the pressure gradient across the valve. The peculiar insertion of chords on the leaflet free margin, however, provides systolic stress sharing between chords according to their different thickness.[8]
The closure of the AV valves is heard aslub, thefirst heart sound (S1). The closure of the SL valves is heard asdub, thesecond heart sound (S2).
The mitral valve is also called thebicuspid valve because it contains two leaflets or cusps. The mitral valve gets its name from the resemblance to abishop'smitre (a type of hat). It is on the left side of the heart and allows the blood to flow from theleft atrium into theleft ventricle.
Duringdiastole, a normally-functioning mitral valve opens as a result of increased pressure from the left atrium as it fills with blood (preloading). As atrial pressure increases above that of the left ventricle, the mitral valve opens. Opening facilitates the passive flow of blood into the left ventricle. Diastole ends with atrial contraction, which ejects the final 30% of blood that is transferred from the left atrium to the left ventricle. This amount of blood is known as the end diastolic volume (EDV), and the mitral valve closes at the end of atrial contraction to prevent a reversal of blood flow.
The tricuspid valve has three leaflets or cusps and is on the right side of the heart. It is between theright atrium and theright ventricle, and stops the backflow of blood between the two.
The aortic and pulmonary valves are located at the base of the aorta and thepulmonary trunk respectively. These are also called the "semilunar valves". These two arteries receive blood from the ventricles and their semilunar valves permit blood to be forced into the arteries, and prevent backflow from the arteries into the ventricles. These valves do not have chordae tendineae, and are more similar to the valves in artery than they are to the atrioventricular valves. The closure of the semilunar valves causes thesecond heart sound.
Theaortic valve, which has three cusps, lies between theleft ventricle and theaorta. During ventricularsystole, pressure rises in the left ventricle and when it is greater than 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 and the pressure in the aorta forces the aortic valve to close. The closure of the aortic valve contributes the A2 component of the second heart sound.
Thepulmonary valve (sometimes referred to as the pulmonic valve) lies between theright ventricle and thepulmonary artery, and has three cusps. Similar to the aortic valve, the pulmonary 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 pulmonary valve. The closure of the pulmonary valve contributes the P2 component of the second heart sound. 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. However, it is physiologically normal in some young people to hear both components separated during inhalation.
In the developing heart, the valves between the atria and ventricles, the bicuspid and the tricuspid valves, develop on either side of theatrioventricular canals.[9] The upward extension of the bases of the ventricles causes the canal to become invaginated into the ventricle cavities. The invaginated margins form the rudiments of the lateral cusps of the AV valves. The middle and septal cusps develop from the downward extension of theseptum intermedium.
The semilunar valves (the pulmonary and aortic valves) are formed from four thickenings at the cardiac end of thetruncus arteriosus.[9] These thickenings are calledendocardial cushions.[citation needed] The truncus arteriosus is originally a single outflow tract from the embryonic heart that will later split to become theascending aorta andpulmonary trunk. Before it has split, four thickenings occur. There are anterior, posterior, and two lateral thickenings. Aseptum begins to form between what will later become the ascending aorta and pulmonary tract. As the septum forms, the two lateral thickenings are split, so that the ascending aorta and pulmonary trunk have three thickenings each (an anterior or posterior, and half of each of the lateral thickenings). The thickenings are the origins of the three cusps of the semilunar valves. The valves are visible as unique structures by the ninth week. As they mature, they rotate slightly as the outward vessels spiral, and move slightly closer to the heart.[9]
In general, the motion of the heart valves is determined using theNavier–Stokes equation, using boundary conditions of the blood pressures, pericardial fluid, and external loading as the constraints.The motion of the heart valves is used as a boundary condition in the Navier–Stokes equation in determining the fluid dynamics of blood ejection from the left and right ventricles into the aorta and the lung.
The pressure drop,, across an open heart valve relates to the flow rate, Q, through the valve:
If:
Usually, the aortic and mitral valves are incorporated in valve studies within a single degree of freedom. These relationships are based on the idea of the valve being a structure with a single degree of freedom. These relationships are based on theEuler equations.
Equations for the aortic valve in this case:
where:
Atrioventricular valve
Valvular heart disease is a general term referring to dysfunction of the valves, and is primarily in two forms, eitherregurgitation, (alsoinsufficiency, orincompetence) where a dysfunctional valve lets blood flow in the wrong direction,[10] orstenosis, when a valve is narrow.[11]
Regurgitation occurs when a valve becomes insufficient and malfunctions, allowing some blood to flow in the wrong direction. This insufficiency can affect any of the valves as inaortic insufficiency,mitral insufficiency,pulmonary insufficiency andtricuspid insufficiency. The other form of valvular heart disease isstenosis, a narrowing of the valve. This is a result of the valve becoming thickened and any of the heart valves can be affected, as inmitral valve stenosis,tricuspid valve stenosis,pulmonary valve stenosis andaortic valve stenosis. Stenosis of the mitral valve is a common complication ofrheumatic fever. Inflammation of the valves can be caused byinfective endocarditis, usually a bacterial infection but can sometimes be caused by other organisms. Bacteria can more readily attach to damaged valves.[12] Another type ofendocarditis which doesn't provoke an inflammatory response, isnonbacterial thrombotic endocarditis. This is commonly found on previously undamaged valves.[12] A major valvular heart disease ismitral valve prolapse, which is a weakening of connective tissue calledmyxomatous degeneration of the valve. This sees the displacement of a thickened mitral valve cusp into the left atrium during systole.[11]
Disease of the heart valves can be congenital, such asaortic regurgitation or acquired, for exampleinfective endocarditis. Different forms are associated withcardiovascular disease,connective tissue disorders andhypertension. The symptoms of the disease will depend on the affected valve, the type of disease, and the severity of the disease. For example, valvular disease of theaortic valve, such asaortic stenosis oraortic regurgitation, may cause breathlessness, whereas valvular diseases of thetricuspid valve may lead to dysfunction of the liver andjaundice. When valvular heart disease results from infectious causes, such asinfective endocarditis, an affected person may have afever and unique signs such assplinter haemorrhages of the nails,Janeway lesions,Osler nodes andRoth spots. A particularly feared complication of valvular disease is the creation ofemboli because of turbulent blood flow, and the development ofheart failure.[11]
Valvular heart disease is diagnosed byechocardiography, which is a form ofultrasound. Damaged and defective heart valves can berepaired, orreplaced withartificial heart valves. Infectious causes may also require treatment withantibiotics.[11]
The most common form of valvular anomaly is acongenital heart defect (CHD), called abicuspid aortic valve. This results from the fusing of two of the cusps duringembryonic development forming a bicuspid valve instead of a tricuspid valve. This condition is often undiagnosed until calcific aorticstenosis has developed, and this usually happens around ten years earlier than would otherwise develop.[13][14]
Less common CHD's aretricuspid andpulmonary atresia, andEbstein's anomaly. Tricuspid atresia is the complete absence of the tricuspid valve which can lead to an underdeveloped or absent right ventricle. Pulmonary atresia is the complete closure of the pulmonary valve. Ebstein's anomaly is the displacement of the septal leaflet of the tricuspid valve causing a larger atrium and a smaller ventricle than normal.
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Heart valves were first documented byLeonardo da Vinci over 500 years ago.[15] Da Vinci achieved this by doingdissections on cows, pigs, and humans and studying the dissections. Da Vinci also performedvivo studies on pigs, by using small metallic tracers to analyze the movement of blood in the heart. Da Vinci made wax casts of the bull heart to construct glass models of the bull heart to study the hydraulic characteristics of blood flowing through the heart and heart valves. This was done to make a circulation model that would mimic human circulation. Da Vinci used seeds to visualize turbulences and blood flow.[16]
The first medically proven and adoptedartificial heart valve was the Star-Edwards valve, invented by Miles "Lowell" Edwards. The valve was first implanted in a patient in 1960 and remained in use, globally, until it was replaced in 2007 by Edwards' company,Edwards Lifesciences, with a new ring design.[17] Types ofartificial heart valve includepericardial heart valves and theBjork–Shiley valve.
This article incorporates text in thepublic domain from the 20th edition ofGray's Anatomy(1918)
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