Mitral valve

The mitral valve is an anatomical functional structure of the funnel-shaped heart, consisting of a fibrous ring, wings with chords, papillary muscles, functionally associated with adjacent areas 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) emerging from them. The medial (anterior) branches connected to each other form a so-called mitral-aortic contact, or a subaortic curtain dividing the inlet and outlet openings of the left ventricle. The lateral (posterior) cords of both fibrous triangles form the posterior "semicircle" of the left fibrous ring, often thin and poorly defined by its posterior third. A fibrous ring that has a mitral valve is part of the fibrous carcass of the heart.

The main valves forming the mitral valve are the anterior (aortic or septal) and posterior (mural) valves. The line of attachment of the anterior valve takes less than half the circumference of the fibrous ring. The greater part of its circumference is occupied by the posterior sash. The front leaf of a square or triangular shape has a larger area than the rear. A wide and movable anterior valve has a primary role in the mitral valve closure function, and the back flap has a predominantly supporting function. The number of valves varies - two valves in 62% of people, three in 19%, four in 11% and five in 8%. The sections of the connection of the wings between themselves are called commissure. Distinguish the anterolateral and posteromedial commissures. Usually commissures are located at a distance of 3-8 mm from the fibrous ring, which forms the mitral valve. Inside the atrial topographical landmark for the right fibrous triangle is the posterior internal commissure of the mitral valve, and vice versa, to determine the pathologically changed commissures are guided along the deepening of the left atrial wall in this zone. The anterior commissure of the mitral valve corresponds to the area of the left fibrous triangle, where the envelope artery is sufficiently close. Chords connect the valves with the papillary muscles and the number of chords can reach several tens. From the anterior papillary muscles depart from 5 to 20 chords, from the posterior papillary muscles from 5 to 30. There are chords of the 1st (marginal), the second (supporting, or ventricular) and the third (annular or basal) chords , attached, respectively, to the free edge, the ventricular surface and the base of the valves. Edge chords can be divided into several terminal branches. In addition, commissural (fan-like) chords are distinguished, representing small marginal chords (up to 5-7) and departing from one central commissural chord. Fan-shaped chords are attached to the free edge of the commissural segment of each leaf. Paracommassural and paramedial chords are also distinguished, attached at an angle to the corresponding half of the anterior valve. The most powerful second-order chords are usually attached to the boundary between the rough and chord-free central zone of the front flap. On the posterior leaf, in addition to the chords of the 1st and 2nd order, basal and muscular chords are found that extend directly from the wall of the left ventricle.

Tendon chords of both valves depart from two groups of papillary (muscle) muscles - anterior (anterolateral) and posterior (posterior medial). The number of papillary muscles in the left ventricle ranges from 2 to 6. In this case, the chords depart from each muscle group to both the anterior and posterior valves. Both muscles are located perpendicular to the plane of such formation as the mitral valve and begin near the boundary between the apical and middle third free wall of the left ventricle. The anterior papillary muscle begins from the anterior wall of the ventricle, and the posterior muscle - from the posterior wall of it near the connection with the interventricular septum. The right and left papillary muscles are blood supply, mainly by the septal branches, respectively, of the right and left coronary arteries.

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

Vortices formed behind the valve flaps, ensure their convergence in the diastole. Atrial contraction increases the effect of the cover, the valves due to their tension atrial muscle tufts.

At the beginning of the systole, the valves that form the mitral valve are joined by free margins due to contraction of the left ventricle and the occurrence of an inverse gradient on the valve. The posterior wing is shifted forward towards the septal valve as a result of 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 the atrial systole, and the remaining narrowing is due to a reduction in the basal segments of the left ventricular myocardium. At the same time, the anteroposterior (by 6%) and mediolateral (by 13%) sizes of the mitral orifice decrease, the coaptation zone of the valves increases and the reliability of the valve closure increases. The size of the anterior segment of the fibrous ring that forms the mitral valve almost does not change during the cardiac cycle. Dilation of the left heart, reduction of contractility of their myocardium, disturbance of rhythm and conduction may affect the reduction of the fibrous ring. In the phase of early banishment with a rapid increase in pressure in the left ventricle, an isometric contraction of the papillary muscles supports the closure of the valves. During the late-ejection phase, shortening of the papillary muscles (on average by 34%) helps prevent the prolapse of the valves to the left atrial cavity as the distance between the mitral valve and the apex of the heart decreases.

During the ejection phase, the support chords and the fibrous ring stabilize the mitral valve in one plane, and the main stress is in the rough coaptation zone of the valves. However, the pressure on the coaptation zone of both closed valves is balanced, which presumably ensures the formation of a moderate stress along the rough edge. The front flap, which forms the mitral valve, adjoins the aortic root at an angle of 90 °, which ensures that the systole is positioned parallel to the blood stream, thereby contributing to a reduction in stress on it.

The mitral valve is opened not only under the influence of hemodynamic mechanisms, but also with the active participation of all structures of the mitral apparatus. The opening of the valve begins in the phase of isovolumeic relaxation of the left ventricle due to the increase in the distance between its apex and the base (when the shape of the left ventricle changes), as well as due to the continued reduction of papillary muscles. This contributes to the early divergence of the valves. In the diastole, an unobstructed passage of blood from the atrium into the ventricle is facilitated by the eccentric expansion of the posterior part of the fibrous ring and the corresponding displacement of the mural leaf.