Heart valves prevent the return of systolic blood
The anatomy of the aortic and pulmonary valves must be constructed with a particularly strong fibrous tissue but must also be very flexible to withstand the additional physical burden.
The atrioventricular (AV: tricuspid and mitral) valves prevent blood from returning to the atria from the ventricles during systole, and the semilunar valves (aortic and pulmonary valves) prevent blood flow. return to the ventricles from the aorta and pulmonary arteries during diastole. These valves, which are of the left atrium, open and close passively. They close when an opposing pressure gradient pushes blood back, and they open when a gradient pushes blood forward. Anatomically, a thin AV valve requires almost no backflow to close, while a thicker semilunar valve requires a fairly rapid reverse flow of a few milliseconds.
Figure. Mitral and aortic valve (left ventricular valve)
Functions of muscle papillae
The muscular papillae are attached to the leaflets of the AV valve by tendons. The papillary muscles contract when the ventricular septum contracts, but, contrary to expectations, they do not help the valves close. Instead, they pull the leaflets inward to the ventricles to prevent them from bulging too much into the atria when the ventricles contract. If a tendon is ruptured or paralyzed, the valve will inflate sharply as the ventricle contracts, sometimes too much to leak forcefully and lead to severe cardiac impotence and even death.
The aortic valve and pulmonary valve
The semilunar valves of the aorta and pulmonary artery function quite differently from the AV valves. First, the high pressure in the end-systolic artery causes the semilunar valve to close abruptly, as opposed to the gentle closing of the AV valve. Second, because of the smaller opening, the ejection rate through the aortic and pulmonary valves is much greater than through the wider AV valve. Similarly, because of the rapid closing and ejection rates, the margins of the aortic and pulmonary valves are subject to much greater mechanical friction than the AV valves. Finally, the AV valve has tendon support, while the semilunar valve does not. It is clear from the anatomy of the aortic and pulmonary valves that they must be constructed with a particularly strong fibrous tissue but must also be very flexible to withstand the additional physical burden.