Cardiac cycle

The cardiac cycle covers everything that happens in the heart during a single beat. Each chamber goes through systole, when it contracts and pushes blood out, and diastole, when it relaxes and fills. The atria contract first and the ventricles a fraction of a second later, and valves that open and close with changes in pressure keep blood moving in one direction. At a resting rate of about 75 beats a minute, one cycle lasts roughly 0.8 seconds, and the ventricles spend more of that time relaxing than contracting.

During diastole the atrioventricular valves, the tricuspid and mitral, are open and the ventricles fill mostly passively; atrial contraction then tops them up. As the ventricles start to contract, rising pressure shuts the atrioventricular valves, and for a moment all four valves are closed, a phase called isovolumic contraction. Once ventricular pressure exceeds the pressure in the aorta and pulmonary trunk, the semilunar valves open and blood is ejected. As the ventricles relax, the semilunar valves close, pressure falls, and filling begins again.

Closure of the atrioventricular valves makes the first heart sound, S1 or 'lub', and closure of the semilunar valves makes the second, S2 or 'dub', which clinicians listen for with a stethoscope.