The two heart valves that respectively connect the left atrium to the left ventricle and the right atrium to the right ventricle are called leaflet valves for anatomical reasons. The two leaflet valves function according to the recoil principle and, together with the other two heart valves, which are so-called semilunar valves, ensure orderly blood circulation, which is kept going by the individual phases of the heartbeat.
What is the leaflet valve?
Two of the total of four heart valves are formed as so-called leaflet valves. In their dual function as inlet and outlet valves, they respectively form the connection between the left atrium and the left ventricle or the right atrium and the right ventricle. From a functional point of view, the two valves are also called atrioventricular valves or AV valves. The leaflet valve in the right side of the heart has three leaflets (cuspis), which is already indicated by its name, tricuspid valve. Its counterpart in the left side of the heart has only two leaflets and is called the mitral valve or bicuspid valve. The name mitral valve comes from its resemblance to a bishop’s mitre. The two leaflet valves open during the relaxation phase of the ventricles (diastole), which occurs almost simultaneously with the contraction phase of the atria. Blood thus passes from the atria into the ventricles and fills them. During the subsequent contraction phase of the ventricles (systole), the two leaflet valves close so that blood is pumped from the right ventricle into the pulmonary artery. In parallel, the left ventricle also contracts and pumps blood into the aorta, the body’s artery from which all the arteries of the great circulatory system branch.
Anatomy and structure
The two leaflet valves are also called atrioventricular valves, or AV valves for short, because of their function. The AV valve of the right side of the heart consists of three leaflets called cuspis, which earned it the name tricuspid valve. The leaflet valve of the left side of the heart consists of only two leaflets, from which it derives its name bicuspid valve. However, it is more commonly known as the mitral valve because its appearance is somewhat reminiscent of the mitre, the headdress worn by Catholic bishops. The individual leaflets are connected at their edges by partially branched chordae tendineae with papillary muscles. These are small muscular elevations that arise from the cardiac muscles of the ventricles and have the ability to contract so that the chordae tendineae are tightened and prevent leaflets from passing through into the corresponding atrium when the leaflets close. Since each leaflet is connected to its “own” papillary muscle, three of them are found in the right ventricle and two in the left ventricle. The leaflets each consist of four layers. A layer of endothelial cells, formed from the endocardium of the atrium or ventricle, serves as the final layers. Below this is a thin layer of connective tissue cells, which also contains smooth muscle cells on the side facing the atrium. Beneath the connective tissue layer is the sponge layer with intercalated collagen fibers and elastic fibers.
Function and tasks
The purpose of the leaflet valves in their valvular function is to regulate blood flow between the left atrium and left ventricle or between the right atrium and right ventricle. During the contraction phase of the atria, which coincides almost simultaneously with the relaxation phase (diastole) of the ventricles, the leaflet valves are open so that both ventricles fill with blood. During the subsequent contraction phase (systole) of the ventricles, the leaflet valves close – similar to a check valve – preventing blood from flowing back into the respective atria. To prevent the leaflets from leaking into the atria due to the pressure buildup in the ventricles, the papillary muscles also contract, so that the tightened tendon cords virtually “hold” the leaflets in place. The closed leaflet valves thus allow the right ventricle to pump the oxygen-depleted and carbon dioxide-enriched blood from the systemic circulation into the pulmonary artery, and the left ventricle to pump oxygen-rich blood from the pulmonary circulation into the aorta, the large artery of the body, and thus into the systemic circulation.However, orderly blood flow requires not only the proper functioning of the two leaflet valves but also that of the two pocket valves, which are located in the left ventricle at the entrance to the aorta and in the right ventricle at the entrance to the pulmonary artery.
Diseases
In principle, two different functional defects can occur in both leaflet valves. If one of the leaflet valves releases an insufficiently large opening for blood flow from the respective atrium into the ventricle during the opening phase, stenosis exists with more or less severe effects. If a closed leaflet valve does not close completely during systole of the ventricles, there is valve insufficiency, which can be divided into different insufficiency classes according to severity. In this case, part of the blood flows back into the corresponding atrium, so that the cardiac output is limited by the “pumping” in the circuit. Depending on the severity of the valve insufficiency, there is a noticeable to severe loss of performance and shortness of breath. In special cases, a combination of both valve defects can occur on the same valve. The valve defects that occur may be acquired or may exist from birth due to a genetic defect. In most cases, an acquired valve defect on one of the leaflet valves is due to endocarditis, an inflammation of the inner lining of the heart, because the inflamed epithelial layer continues on the leaflets of the valves. Usually, endocarditis causes scarring or adhesions of the leaflets, resulting in stenosis or insufficiency, or even a combination of both dysfunctions. Similar symptoms may result from an inherited valve defect. In rare cases, for example, the anlage of the tricuspid valve is completely absent at birth, resulting in dangerous mixing of blood from the two atria through the then still open foramen ovalis, which connects the two atria prenatally.
