<P> Cardiac diastole is the period of the cardiac cycle when, after contraction, the heart relaxes and expands while refilling with blood returning from the circulatory system . Both atrioventricular (AV) valves open to facilitate the' unpressurized' flow of blood directly through the atria into both ventricles, where it is collected for the next contraction . This period is best viewed at the middle of the Wiggers diagram--see the panel labeled "Diastole". Here it shows pressure levels in both atria and ventricles as near - zero during most of the diastole . (See gray and light - blue tracings labeled "Atrial pressure" and "Ventricular pressure"--Wiggers diagram .) Here also may be seen the red - line tracing of "Ventricular volume", showing increase in blood - volume from the low plateau of the "Isovolumic relaxation" stage to the maximum volume occurring in the "Atrial systole" sub-stage . </P> <P> Atrial systole is the contracting of cardiac muscle cells of both atria following electrical stimulation and conduction of electrical currents across the atrial chambers (see above, Physiology). While nominally a component of the heart's sequence of systolic contraction and ejection, atrial systole actually performs the vital role of completing the diastole, which is to finalize the filling of both ventricles with blood while they are relaxed and expanded for that purpose . Atrial systole overlaps the end of the diastole, occurring in the sub-period known as ventricular diastole--late (see cycle diagram). At this point, the atrial systole applies contraction pressure to' topping - off' the blood volumes sent to both ventricles; this atrial kick closes the diastole immediately before the heart again begins contracting and ejecting blood from the ventricles (ventricular systole) to the aorta and arteries . </P> <P> Notably, atrial kick is absent or disrupted if there is loss of normal electrical conduction in the heart, such as caused by atrial fibrillation, atrial flutter, or heart block . Atrial kick may also be degraded by any deterioration in the condition of the heart, such as "stiff heart" found in patients with diastolic dysfunction . </P> <P> Ventricular systole is the contractions, following electrical stimulations, of the ventricular syncytium of cardiac muscle cells in the left and right ventricles . Contractions in the right ventricle provides pulmonary circulation by pulsing oxygen - depleted blood through the pulmonary valve then through the pulmonary arteries to the lungs . Simultaneously, contractions of the left ventricular systole provide systemic circulation of oxygenated blood to all body systems by pumping blood through the aortic valve, the aorta, and all the arteries . (Blood pressure is routinely measured in the larger arteries off the left ventricle during the left ventricular systole). </P>

When are both the av and semilunar valves closed