The heart operates through the cardiac cycle, which is divided into systole (contraction and ejection) and diastole (relaxation and filling). Ventricular diastole is the period when the main pumping chambers—the ventricles—relax and expand, preparing to receive the next volume of blood. This period allows the heart muscle to rest and refill, ensuring the chambers are adequately prepared for the next forceful contraction that will send blood to the lungs and the rest of the body. The entire process of ventricular filling is managed through distinct phases driven by pressure differences within the heart chambers.
Isovolumetric Relaxation
Diastole begins immediately following the ejection of blood from the ventricles during the preceding systole. Once the ventricles have pushed blood into the great arteries, the pressure inside these chambers begins to drop rapidly as the muscle fibers start to relax. This reduction in pressure causes the semilunar valves—the aortic and pulmonary valves—to snap shut, which marks the precise beginning of ventricular diastole and produces the second heart sound, often identified as S2.
For a brief moment, all four of the heart’s valves are closed: the semilunar valves are closed, and the atrioventricular (AV) valves have not yet opened. This specific period is called isovolumetric relaxation because the volume of blood inside the ventricles remains constant while the ventricular muscle is actively relaxing. The ventricular pressure continues its steep decline during this phase, falling below the pressure in the great arteries but remaining higher than the pressure in the atria. This active relaxation is necessary to create the pressure gradient required to draw in the new volume of blood.
Passive Ventricular Filling
The phase of isovolumetric relaxation ends when the rapidly decreasing ventricular pressure falls below the pressure within the atria. This pressure reversal across the atrioventricular valves—the mitral and tricuspid valves—causes them to open, initiating the main period of ventricular filling. Since the ventricles are relaxing and the atria have been collecting blood returning from the body and lungs, a significant pressure gradient is established, causing a rush of blood to flow from the atria into the ventricles.
The initial influx of blood is termed the rapid ventricular filling phase. During this period, the heart’s elastic recoil, or suction effect, helps to accelerate blood flow toward the apex of the relaxing ventricle. This phase accounts for the majority of the blood the ventricles will receive, typically about 70 to 80 percent of the final volume in a resting heart.
As the ventricles expand and fill, the pressure gradient between the atria and the ventricles begins to equalize. The rate of blood flow slows considerably once the pressures approach equilibrium. This slower, sustained filling period is known as diastasis. During diastasis, blood continues to trickle into the ventricles, maintained primarily by the continuous venous return of blood to the atria. The entire passive filling process is dependent on the compliance, or stretchiness, of the ventricular muscle to accommodate the incoming blood volume at a low pressure.
Atrial Contraction
The final stage of ventricular diastole is the atrial contraction, often referred to as the “atrial kick.” This phase begins when an electrical signal causes the atrial muscle to contract, actively pushing the remaining blood into the relaxed ventricles. This contraction contributes the last portion of the ventricular filling volume, which typically ranges from 10 to 30 percent of the total volume at rest.
This active push ensures the ventricles are maximally stretched and filled just before the next systole begins. When the heart rate increases significantly, the time available for the passive filling phases (rapid filling and diastasis) becomes much shorter. Consequently, the contribution of the atrial kick becomes more substantial, potentially accounting for up to 40 percent of the total ventricular volume. This final emptying of the atria marks the end of ventricular diastole, and the ventricles reach their maximum volume, known as the end-diastolic volume. The cycle then transitions back into systole as the ventricles contract and close the AV valves.