Heart valves

The tricuspid and pulmonary valves of the heart regulate blood flow from tissues to the lungs for oxygen enrichment, the mitral and aortic valves of the left heart control arterial blood flow to organs and tissues. The aortic and pulmonary valves are the outlet valves of the left and right ventricles, respectively. The mitral and tricuspid valves of the heart are the outlet valves of the left and right atria and at the same time the inlet valves of the left and right ventricles, respectively. The aortic and pulmonary valves of the heart are open during the contraction phase of the ventricles (systole) and closed during the relaxation phase of the ventricles (diastole). During the isovolumic contraction and relaxation phases, all four valves are closed. Closed pulmonary and tricuspid valves of the heart can withstand a pressure of 30 mm Hg, the aortic - about 100 mm Hg, the mitral - up to 150 mm Hg. Increased loads on the left heart valves determine their greater susceptibility to diseases. Hemodynamics can play an important role in the development of valve pathology

The aortic valves of the heart open at the beginning of systolic contraction of the left ventricle and close before diastolic relaxation of the ventricle. Systole begins at the moment of opening of the aortic valve (20-30 ms) and lasts about 1/3 of the cardiac cycle. Blood flow through the heart valves quickly increases and reaches its maximum velocity in the first third of systole after full opening of the cusps. Inhibition of blood flow through the heart valves occurs more slowly. The reverse pressure gradient inhibits low-velocity wall flow with the formation of backflow in the sinuses. During systole, the direct pressure difference, under the action of which blood moves through the aortic valves of the heart, does not exceed several mm Hg, while the reverse pressure difference on the valve normally reaches 80 mm Hg. The heart valves close at the end of the flow deceleration phase with the formation of an insignificant backflow. All heart valves are closed in the phases of isovolumic contraction and relaxation. The aortic valves of the heart change their size and shape during the contraction cycle of the heart, mainly in the direction of the aortic axis. The perimeter of the fibrous ring reaches a minimum at the end of systole and a maximum at the end of diastole. Studies on dogs have shown a 20% change in perimeter at an aortic pressure of 120/80 mm Hg. During systole, a vortex of fluid is formed in the sinuses. The vortices contribute to the rapid and effective closure of the valves. The volume of reverse flow is 5% of the direct flow. In a healthy organism, under the influence of a direct pressure difference, the blood flow velocity quickly increases to values of 1.4 ± 0.4 m / s. In children, even higher speeds are observed - 1.5 ± 0.3 m / s. At the end of systole, there is a short period of reverse blood flow, which is recorded by the ultrasound Doppler method. The source of the reverse flow can be either the actual reverse flow of blood through the valve orifice during the closure phase of the cusps, or the movement of already closed cusps towards the left ventricle.

The velocity profile in the plane of the fibrous ring is uniform, but with a slight slope towards the septal wall. In addition, the systolic blood flow through the aortic valves of the heart retains the spiral character formed in the left ventricle. Swirling of the blood flow in the aorta (0-10°) eliminates the formation of stagnant zones, increases the pressure near the walls, facilitating more effective blood collection into the outgoing vessels, and prevents injury to blood cells due to the unbroken flow. Opinions on the direction of rotation of the blood flow in the ascending aorta are ambiguous. Some authors pointed to the counterclockwise rotation of the systolic blood flow through the aortic valves of the heart, if you look along the flow, others - in the opposite direction, others do not mention the spiral character of the systolic blood ejection at all, and others are inclined to the hypothesis of the origin of the swirling flow in the aortic arch. The unstable, and in some cases multidirectional nature of the rotation of the blood flow in the ascending aorta and its arch is apparently associated with individual morphofunctional features of the outlet section of the left ventricle, aortic structures, sinuses of Valsalva, and the aortic wall.

The blood flow through the pulmonary valves of the heart is close to the aortic, but significantly smaller in magnitude. In a healthy adult organism, the speeds reach 0.8±0.2 m/s, in a child - 0.9±0.2 m/s. Behind the pulmonary structures, a swirl of the flow is also observed, which is directed counterclockwise in the phase of blood flow acceleration.

Relaxation of the ventricle is followed by deceleration of the blood flow, and the mitral structures partially close. During contraction of the atrium, the velocity in the A-wave is usually less than that of the E-wave. Initial studies were aimed at explaining the mechanism of mitral valve closure. B. J. Bellhouse (1972) was the first to suggest that vortices formed behind the cusps during ventricular filling contribute to partial closure of the cusps. Experimental studies have confirmed that without the formation of large vortices behind the cusps, the mitral structures would remain open until the onset of ventricular contraction, and its closure would be accompanied by significant regurgitation. J. Reul et al. (1981) found that the reverse pressure drop in mid-diastole of the ventricle provides not only fluid deceleration, but also initial closure of the cusps. Thus, the participation of vortices in the mechanism of cusp closure refers to the beginning of diastole. E. L. Yellin et al. (1981) clarified that the closure mechanism is influenced by the combined effect of chordal tension, flow inhibition, and ventricular vortices.

The diastolic blood flow from the left atrium through the mitral structures into the left ventricle is swirled clockwise when viewed downstream. Modern magnetic resonance imaging studies of the spatial velocity field in the left ventricle reveal vortex motion of blood both during the cusp closure phase and during the atrial contraction phase. The swirling of the flow is provided by the tangential blood supply from the pulmonary veins into the left atrium cavity, as well as by the direction of the blood flow by the anterior mitral valve leaflet to the spiral trabeculae of the inner wall of the left ventricle. It is appropriate to ask the question: what is the meaning of this phenomenon - the swirling of blood in the left ventricle of the heart and the aorta? In a swirling flow, the pressure at the walls of the left ventricle exceeds the pressure on its axis, which contributes to the stretching of its walls during the period of increased intraventricular pressure, the inclusion of the Frank-Starling mechanism in the process and a more effective systole. The swirling flow intensifies the mixing of blood volumes - oxygen-saturated with depleted. The increase in pressure near the walls of the left ventricle, the maximum value of which occurs at the final stage of diastole, creates additional forces on the mitral valve cusps and promotes their rapid closure. After the mitral valve closes, the blood continues its rotational motion. The left ventricle in systole only changes the direction of the forward movement of the blood, without changing the direction of the rotational motion, therefore, the sign of the swirl changes to the opposite, if we continue to look along the flow.

The velocity profile of the tricuspid valve is similar to the mitral valve, but the velocity is lower because the area of the passage opening of such a valve is larger. The tricuspid valves of the heart open earlier than the mitral valve and close later.

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