What are the mitral valve leaflets?
The mitral valve leaflets, also called cusps, are two flaps of thin fibrous tissue that attach to the mitral annulus and act as one-way swing doors between the atrium (upper chamber) and ventricle (lower chamber) on the heart’s left side. Unlike the tricuspid, aortic and pulmonary valves (the heart contains four valves – two atrioventricular valves and two semilunar valves) that each consist of three leaflets, the mitral valve only contains two, which is why it is commonly referred to as the “bicuspid” valve. When the bicuspid valve closes, the orientation of the two cusps make it resemble a mitre (a traditional hat that is often worn by bishops); this is why it is called the mitral valve.
What do the mitral valve leaflets look like?
The two pliable leaflets, the anterior and posterior cusps, are usually less than 5 mm thick and look like eyelids. A fine membrane called endocardium covers them and connective tissue reinforces them. The curved anterior (aortic) cusp is the larger one of the two and can be found on the opening’s aortic side. The posterior (mural) cusp is narrower and is situated somewhat towards the back, on the left. It features indentations (clefts) at two points, forming three ‘scallops’ or ‘segments’ (lateral, central and medial) along the free edge. Two smaller indentations can be found on each side, the anterolateral and posteromedial commissures (joining points); these dents are what separate the anterior and posterior leaflets. Each leaflet is attached to the mitral annulus and is divided into ‘zones’. The anterior has 2 zones, rough and clear, while the posterior leaflet has three, rough clear and basal. The chordae tendineae attach to the rough and basal zones in both cusps, as well as to the two commissures.
What is the function of the mitral leaflets? How do they function?
The cusps of the mitral valve have an important function. Opening and closing systematically as part of the cardiac cycle, they serve as a one-directional doorway for blood that flows from the left atrium into the left ventricle during the cardiac cycle. Once blood enters the lungs and undergoes oxygenation, it passes through to the left atrium via the pulmonary veins. The left ventricle relaxes (in diastole) and the bicuspid valve opens, allowing the oxygen-rich blood to enter it. Then, in systole, the mitral valve shuts as the left ventricle contracts and ejects the blood into the aorta by way of the aortic valve. When the bicuspid valve closes, the leaflet tips (which are controlled by the chordae tendineae and papillary muscles and overlap slightly on the side of the ventricle) shut and meet precisely (coapt), preventing the backflow of blood into the left atrial chamber. Ultimately, the cusps of the mitral valve regulate blood flow.
What happens when they do not function properly?
When the cusps do not coapt, which can happen for a number of reasons, the normal flow of blood during the cardiac cycle is interrupted, causing a variety of symptoms and complications. Blood may leak back into the atrium (mitral regurgitation) and cause congestive heart failure, atrial fibrillation, pulmonary hypertension, etc., and make one more prone to infective endocarditis. Alternatively, less blood may pass through into the ventricle during diastole (mitral stenosis) due to calcification (obstructive growths on the leaflets), leading to similar problems. Other conditions that affect the leaflets and structures of the mitral valve include mitral valve prolapse, mitral atresia, congenital valve disease, myocardial infarction (heart attack) and infection.