The term cardiac cycle refers to a complete heartbeat that includes systole, diastole, and pause.[11] The cycle begins with the contraction of the atria and ends with the relaxation of the ventricles. The term systole refers to the contraction of the ventricles. As for the term diastole, it refers to the relaxation of the ventricles and their filling with blood. The atria and ventricles work in unison, as when the ventricles contract, the atria relax and fill with blood. In diastole, when the ventricles relax, the atria contract to pump blood into the ventricles. This consistency ensures that blood is pumped effectively to all parts of the body.[9]
At the beginning of the heart cycle, at the beginning of diastole, the atria and ventricles are relaxed, and when blood moves from areas of high pressure to areas of low pressure, where all the chambers of the heart are relaxed, blood will flow towards the atria (through the coronary sinus and pulmonary veins). When the atria begin to fill, the pressure inside them will rise, and blood will begin to move from the atria towards the ventricles. Late in diastole, the atria contract to pump more blood into the ventricles. This leads to an increase in pressure inside the ventricles to a certain point at which the phase of systole begins and the ventricles contract to pump blood towards the pulmonary trunk (right ventricle) and the aorta (left ventricle).[11]
When the atrioventricular valves (mitral and tricuspid) are open during blood flow into the ventricles, the pulmonary and aortic valves are closed to prevent the return of blood from the ventricles toward the atria. When the ventricular pressure becomes greater than the atrial pressure, the mitral and tricuspid valves will close. When the ventricles contract, the increased pressure forces the pulmonary and aortic valves open, and when the ventricles relax, the valves will close in response to the decrease in pressure.[1]
