Although the pumping action of your heart is certainly effective at ejecting blood from its chambers, it also needs a method to guarantee that the pumped blood goes only in the desired direction. This task falls to four heart valves: the tricuspid, mitral, pulmonary and aortic. The valves are strong, thin leaflets of tissue anchored to the myocardium. The flaps consist of single sheets of fibrous tissue covered by endocardial cells. At the base of each valve leaflet, the fibrous layer merges with the myocardium to form a flexible hinge.
The tricuspid valve (on the right side of the heart) and the mitral valve (on the left side) regulate blood flow from the atria to the ventricles. The aortic and pulmonary valves guard the openings from the ventricles to the aortic and pulmonary arteries, respectively. The valves are designed to allow blood to pass in only one direction. The valves don't automatically open when blood is approaching. Instead, they function like a gate that opens only when it's pushed and is designed so that it can swing open in only one direction. The valves open and close due to the natural pressure differences that build up within the heart's chambers during the systolic and diastolic portions of each cardiac cycle.
For example, the aortic valve opens to allow blood to eject from the left ventricle into the aorta, because during systole (contraction) the pressure in the left ventricle is higher than in the aorta. This pressure difference forces the valve to open and allows blood to flow through it. During diastole, when the left ventricle relaxes, the pressure in the left ventricle becomes low again while the pressure in the aorta remains high. The valve is pushed closed by the pressure, and blood is prevented from leaking back into the heart.
The familiar sound of the heartbeat is caused by the heart valves slamming shut. The first thump is heard when the valves between the atria and ventricles close; the second when the valves between the ventricles and arteries close.