Mitral valve

The mitral valve is an anatomical and functional structure of the heart of a funnel shape, consisting of a fibrous ring, cusps with chords, papillary muscles, functionally connected with the adjacent parts of the left atrium and ventricle.

The fibrous ring of the mitral valve is formed by the left and right fibrous triangles and the fibrous strands (branches) extending from them. The medial (anterior) branches, connected to each other, form the so-called mitral-aortic contact, or subaortic curtain, separating the inlet and outlet openings of the left ventricle. The lateral (posterior) strands of both fibrous triangles form the posterior "semicircle" of the left fibrous ring, often thinned and poorly defined by its posterior third. The fibrous ring that the mitral valve has is part of the fibrous framework of the heart.

The main cusps that form the mitral valve are the anterior (aortic or septal) and posterior (mural). The line of attachment of the anterior cusp occupies less than half the circumference of the fibrous ring. Most of its circumference is occupied by the posterior cusp. The anterior cusp, square or triangular in shape, has a larger area than the posterior cusp. The wide and mobile anterior cusp plays the main role in the closing function of the mitral valve, while the posterior cusp plays a predominantly supporting function. The number of cusps varies: two cusps in 62% of people, three in 19%, four in 11%, and five in 8%. The areas where the cusps connect to each other are called commissures. There are anterolateral and posteromedial commissures. Commissures are usually located at a distance of 3-8 mm from the fibrous ring that forms the mitral valve. The intra-atrial topographic landmark for the right fibrous triangle is the posterointernal commissure of the mitral valve, and vice versa, to determine the pathologically altered commissure, they are guided by the depression on the wall of the left atrium in this area. The anterolateral commissure of the mitral valve corresponds to the area of the left fibrous triangle, where the circumflex artery passes quite close. The chords connect the cusps with the papillary muscles and the number of chords can reach several dozen. From 5 to 20 chords extend from the anterior papillary muscles, from 5 to 30 from the posterior papillary muscles. There are chords of the 1st (marginal), 2nd (supporting, or ventricular) and 3rd (annular, or basal) order, attached, respectively, to the free edge, ventricular surface and base of the cusps. Marginal chords can divide into several terminal branches. In addition, commissural (fan-shaped) chords are distinguished, which are small marginal chords (up to 5-7) and extend from one central commissural chord. Fan-shaped chords are attached to the free edge of the commissural segment of each valve. Paracommissural and paramedian chords are also distinguished, which are attached at an angle to the corresponding half of the anterior valve. The most powerful chords of the 2nd order are usually attached at the border between the rough and chord-free central zone of the anterior valve. On the posterior valve, in addition to the chords of the 1st and 2nd order, there are basal and muscular chords extending directly from the wall of the left ventricle.

The tendinous chordae of both cusps originate from two groups of papillary muscles - anterior (anterolateral) and posterior (posteromedial). The number of papillary muscles in the left ventricle varies from 2 to 6. In this case, the chordae originate from each group of muscles both to the anterior and to the posterior cusps. Both muscles are located perpendicular to the plane of such a formation as the mitral valve and begin near the border between the apical and middle third of the free wall of the left ventricle. The anterior papillary muscle originates from the anterior wall of the ventricle, and the posterior muscle - from its posterior wall near the junction with the interventricular septum. The right and left papillary muscles are supplied with blood mainly by the septal branches of the right and left coronary arteries, respectively.

The mitral valve closes and opens with an active movement in which most components of the mitral apparatus participate simultaneously. Closure of the mitral valve begins in diastole (early diastolic closure phase of the cusps) during rapid filling of the ventricle.

Vortices formed behind the valve cusps ensure their convergence in diastole. Contraction of the atria increases the effect of covering the cusps due to their tension by the atrial muscle bundles.

At the beginning of systole, the cusps that form the mitral valve close with their free edges due to the contraction of the left ventricle and the occurrence of a reverse gradient on the valve. The posterior cusp shifts forward toward the septal cusp as a result of the narrowing of the opening (by 20-40%) along the mural part of the fibrous ring. More than half of the narrowing of the fibrous ring occurs during atrial systole, and the remaining narrowing is due to the contraction of the basal segments of the left ventricular myocardium. In this case, the anteroposterior (by 6%) and mediolateral (by 13%) sizes of the mitral orifice decrease, the coaptation zone of the cusps increases, and the reliability of valve closure increases. The size of the anterior segment of the fibrous ring that forms the mitral valve remains almost unchanged during the cardiac cycle. Dilation of the left chambers of the heart, decreased contractility of their myocardium, rhythm and conduction disturbances can affect the contraction of the fibrous ring. During the early ejection phase, as left ventricular pressure rapidly increases, isometric contraction of the papillary muscles maintains closure of the leaflets. During the late ejection phase, shortening of the papillary muscles (by an average of 34%) helps prevent leaflet prolapse into the left atrium as the distance between the mitral valve and the apex of the heart decreases.

In the expulsion phase, the supporting chords and the fibrous ring stabilize the mitral valve in one plane, and the main stress falls on the rough zone of coaptation of the cusps. However, the pressure on the coaptation zone of both closed cusps is balanced, which presumably ensures the formation of moderate stress along the rough edge. The anterior cusp, which forms the mitral valve, adjoins the aortic root at an angle of 90°, which ensures its position in systole parallel to the blood flow, thereby reducing the stress on it.

The mitral valve opens not only under the influence of hemodynamic mechanisms, but also with the active participation of all structures of the mitral apparatus. The valve opens in the phase of isovolumic relaxation of the left ventricle due to an increase in the distance between its apex and base (with a change in the shape of the left ventricle), as well as due to the continued contraction of the papillary muscles. This contributes to the early divergence of the cusps. In diastole, the unimpeded passage of blood from the atrium to the ventricle is facilitated by the eccentric expansion of the posterior part of the fibrous ring and the corresponding displacement of the mural cusp.