Atrial systole is the brief contraction of the heart’s two upper chambers (the atria) that pushes a final portion of blood into the lower chambers (the ventricles) just before they contract. It accounts for roughly 20% to 30% of the total blood that fills each ventricle, topping off the 70% to 80% that flows in passively during the earlier relaxation phase. Though it contributes the smaller share of filling, this “atrial kick” plays a meaningful role in keeping the heart pumping efficiently.
Where Atrial Systole Fits in the Cardiac Cycle
The heart cycles through two broad phases: diastole (relaxation and filling) and systole (contraction and ejection). Atrial systole is technically the very last phase of diastole, occurring just before the ventricles begin their own powerful contraction. By this point, the ventricles have already been passively filling with blood for most of the relaxation period. Atrial systole finishes the job.
Here’s the sequence in real time. After the ventricles relax, pressure inside them drops low enough for the valves between the atria and ventricles to swing open. Blood flows passively from the atria into the ventricles, driven only by gravity and the small pressure difference between the chambers. This passive filling handles most of the work. Then the atria contract, squeezing the remaining blood downward. Once atrial systole ends and the atria relax, atrial pressure drops below ventricular pressure, and the valves between the chambers begin to close. That closure marks the true start of ventricular systole.
What Happens During the Contraction
During atrial systole, the muscle fibers of both atria contract in a coordinated wave that moves from the top of the atria downward toward the ventricles. This creates a brief rise in atrial pressure, producing a small pressure gradient that pushes blood through the open atrioventricular valves (the tricuspid valve on the right side, the mitral valve on the left). Meanwhile, the valves leading out to the lungs and the aorta remain closed, so all the blood goes in one direction: into the ventricles.
Because the ventricular muscle is still relaxed at this point, the pressure inside the ventricles stays close to zero and doesn’t change much as the extra blood enters. By the end of atrial systole, each ventricle holds about 130 milliliters of blood. This volume is called the end-diastolic volume, and it represents the maximum amount of blood in the ventricle before it contracts.
The Electrical Signal That Triggers It
Atrial systole doesn’t happen spontaneously. It’s triggered by an electrical signal that spreads across both atria, causing the muscle cells to depolarize and contract. On an electrocardiogram (ECG), this electrical activity shows up as the P wave, the first small deflection you see before the larger spike of the QRS complex. The P wave typically appears 120 to 200 milliseconds before the QRS complex, giving the atria just enough time to contract and finish filling the ventricles before the ventricles fire.
This timing matters. The brief delay between atrial and ventricular contraction is built into the heart’s electrical wiring, controlled by a structure called the AV node that deliberately slows the signal. Without that pause, the atria and ventricles would try to contract at the same time, and the atrial kick would be lost.
Why the “Atrial Kick” Matters
At rest, a healthy heart can function reasonably well even without a proper atrial kick, since passive filling handles the majority of ventricular volume. But the extra 20% to 30% becomes increasingly important under certain conditions. During exercise, when the heart rate rises and each filling phase gets shorter, that atrial boost helps maintain an adequate volume of blood per beat. It also matters more in people whose ventricles are stiffer or less compliant, such as those with heart failure or thickened heart walls, because passive filling alone can’t move enough blood into a resistant ventricle.
The clearest example of what happens without atrial systole is atrial fibrillation, a common rhythm disorder in which the atria quiver chaotically instead of contracting in a coordinated way. With the atrial kick gone, stroke output (the amount of blood pumped per beat) drops by 20% to 30% in otherwise healthy people. In patients who already have heart disease, the decline is considerably larger, sometimes enough to cause noticeable fatigue, shortness of breath, or worsening heart failure symptoms.
Pressure Changes You Can Trace
The rise in atrial pressure during atrial systole is small but measurable. Normal mean pressure in the right atrium sits around 4 to 5 mmHg, while left atrial pressure ranges from about 10 to 25 mmHg in the pulmonary circuit. When the atria contract, this pressure briefly spikes, creating what’s called the “a wave” in the venous pulse. You can sometimes see this wave in the jugular veins of the neck during a physical exam, and it’s a direct visual sign that the atria are contracting normally.
Compare those numbers to what happens moments later: when the left ventricle contracts, it generates pressures up to 120 mmHg to push blood into the aorta. The right ventricle generates up to about 25 mmHg to send blood to the lungs. Atrial systole operates at far lower pressures because it only needs to move blood a short distance through already-open valves into relaxed chambers.
Atrial vs. Ventricular Systole
The term “systole” on its own usually refers to ventricular systole, the forceful contraction that sends blood out to the body and lungs. Atrial systole is a much gentler event. The atrial walls are thinner, the pressures involved are lower, and the purpose is different: topping off the ventricles rather than ejecting blood into the arteries.
In clinical shorthand, “systole” is formally defined as the period from mitral valve closure to aortic valve closure, which describes ventricular activity only. Atrial systole, despite its name, actually falls within the broader period classified as diastole. This can be confusing, but the key distinction is simple: atrial systole is about filling, while ventricular systole is about pumping.