Cardiac cycle

Cardiac cycle

Main articles: Cardiac cycle, Systole and Diastole

The cardiac cycle as correlated to the ECG

The cardiac cycle refers to a complete heartbeat which includes systole and diastoleand the intervening pause. The cycle begins with contraction of the atria and ends with relaxation of the ventricles. Systole is when the ventricles of the heart contract to pump blood to the body. Diastole is when the ventricles relax and fill with blood. The atria and ventricles work in concert, so in systole when the ventricles are contracting, the atria are relaxed and collecting blood. When the ventricles are relaxed in diastole, the atria contract to pump blood to the ventricles. This coordination ensures blood is pumped efficiently to the body.^[7]

At the beginning of the cardiac cycle, in early diastole, both the atria and ventricles are relaxed. Since blood moves from areas of high pressure to areas of low pressure, when the chambers are relaxed, blood will flow into the atria (through thecoronary sinus and the pulmonary veins). As the atria begin to fill, the pressure will rise so that the blood will move from the atria into the ventricles. In late diastole the atria contract pumping more blood into the ventricles. This causes a rise in pressure in the ventricles, and in ventricular systole blood will be pumped into the pulmonary artery.

When the atrioventricular valves (tricuspid and mitral) are open, during blood flow to the ventricles, the semilunar valves are closed to prevent backflow into the ventricles. When the ventricular pressure is greater than the atrial pressure the tricuspid and mitral valves will shut. When the ventricles contract the pressure forces the semilunar aortic and pulmonary valves open. As the ventricles relax the semilunar valves will close in response to decreased pressure.