Phase 4 - Isovolumetric Relaxation

This stage follows on from early ventricular diastole, where the ventricles repolarise and expand. Blood briefly flows back from the aorta and pulmonary trunk into their respective ventricles through the semilunar valves. This slight backflow causes the cusps of the valves to quicky close, represented by the 'dicrotic notch' on the aortic pressure curve. The pressure in the aorta and pulmonary arteries rise, represented by the "dicrotic wave", proceeded by relaxation of the muscles causing a reduction in pressure, lusitropy. The sarcoplamic reticulum is the regulator of this relaxtion stage.

Neither the semilunar valves or the atrioventricular valves are open so the ventricles are not taking in blood. Blood volume is constant, whilst the ventricular pressure reduces, left atrial pressure rises and reaches it peak.

The cycle returns to phase 1 once the atrioventricular valves re-open, allowing blood into the ventricles.

Heart Sounds:

Heart sound S2 can be heard as the blood rebounds off the closed valves.

Did you know..? 

In a resting person, atrial systole lasts 0.1 seconds, ventricular systole 0.3 seconds and the period when all your chambers are in diastole 0.4 seconds. This would give a total duration of 0.8 seconds (800msec) in a heart beating 75bpm.