Diastole (/daɪˈæstəli/ dy-AST-ə-lee) is the relaxed phase of the cardiac cycle when the chambers of the heart are refilling with blood. The contrasting phase is systole when the heart chambers are contracting. Atrial diastole is the relaxing of the atria, and ventricular diastole the relaxing of the ventricles.

The term originates from the Greek word διαστολή (diastolē), meaning "dilation", from διά (diá, "apart") + στέλλειν (stéllein, "to send").

A typical heart rate is 75 beats per minute (bpm), which means that the cardiac cycle that produces one heartbeat, lasts for less than one second. The cycle requires 0.3 sec in ventricular systole (contraction)—pumping blood to all body systems from the two ventricles; and 0.5 sec in diastole (dilation), re-filling the four chambers of the heart, for a total of 0.8 sec to complete the cycle.

During early ventricular diastole, pressure in the two ventricles begins to drop from the peak reached during systole. When the pressure in the left ventricle falls below that in the left atrium, the mitral valve opens due to a negative pressure differential (suction) between the two chambers. The open mitral valve allows blood in the atrium (accumulated during atrial diastole) to flow into the ventricle (see graphic at top). Likewise, the same phenomenon runs simultaneously in the right ventricle and right atrium through the tricuspid valve.

The ventricular filling flow (or flow from the atria into the ventricles) has an early (E) diastolic component caused by ventricular suction, and then a late one created by atrial systole (A). The E/A ratio is used as a diagnostic measure as its diminishment indicates probable diastolic dysfunction, though this should be used in conjunction with other clinical characteristics and not by itself.

Early diastole is a suction mechanism between the atrial and ventricular chambers. Then, in late ventricular diastole, the two atrial chambers contract (atrial systole), causing blood pressure in both atria to increase and forcing additional blood flow into the ventricles. This beginning of the atrial systole is known as the atrial kick—see Wiggers diagram. The atrial kick does not supply the larger amount of flow (during the cardiac cycle) as about 80 percent of the collected blood volume flows into the ventricles during the active suction period.

At the beginning of the cardiac cycle the atria, and the ventricles are synchronously approaching and retreating from relaxation and dilation, or diastole. The atria are filling with separate blood volumes returning to the right atrium (from the vena cavae), and to the left atrium (from the lungs). After chamber and back pressures equalize, the mitral and tricuspid valves open, and the returning blood flows through the atria into the ventricles. When the ventricles have completed most of their filling, the atria begin to contract (atrial systole), forcing blood under pressure into the ventricles. Now the ventricles start to contract, and as pressures within the ventricles rise, the mitral and tricuspid valves close producing the first heart sound (S1) as heard with a stethoscope.

As pressures within the ventricles continue to rise, they exceed the "back pressures" in the aorta, and the pulmonary trunk. The aortic and pulmonary valves known as the semilunar valves open, and a defined fraction of blood within the heart is ejected into the aorta and pulmonary trunk. Ejection of blood from the heart is known as systole. Ejection causes pressure within the ventricles to fall, and, simultaneously, the atria begin to refill (atrial diastole). Finally, pressures within the ventricles fall below the back pressures in the aorta and the pulmonary arteries, and the semilunar valves close. Closure of these valves give the second heart sound (S2). The ventricles then start to relax, the mitral and tricuspid valves begin to open, and the cycle begins again.