Systole is the phase when your heart muscle contracts and pumps blood out. Diastole is the phase when it relaxes and fills back up. Together, these two phases make up one complete heartbeat, called the cardiac cycle, which lasts about 0.8 seconds at a normal resting heart rate. Every time you see a blood pressure reading like 120/80, the top number measures pressure during systole and the bottom number measures pressure during diastole.
What Happens During Systole
Systole is the working phase of the heartbeat. The heart’s lower chambers (ventricles) contract, building pressure until it’s high enough to push open the valves leading to the lungs and the rest of the body. Blood is then ejected out of the heart through these exit valves, while the valves between the upper and lower chambers stay shut to prevent blood from flowing backward. The entire ventricular contraction lasts about 270 milliseconds.
The upper chambers (atria) have their own brief systole as well, lasting roughly 100 milliseconds. Atrial contraction actually happens at the tail end of diastole, giving the ventricles a final top-off of blood right before they squeeze. This “atrial kick” contributes a meaningful portion of filling, which is one reason conditions like atrial fibrillation (where the atria quiver instead of contracting) can reduce the heart’s pumping efficiency.
What Happens During Diastole
Diastole is the refilling phase. Once the ventricles finish pumping, pressure inside them drops rapidly. The exit valves snap shut, and for a brief moment all four valves are closed while the muscle relaxes. Once ventricular pressure falls low enough, the valves between the atria and ventricles pop open, and blood that has been pooling in the atria rushes in.
Most of the filling happens passively and quickly, driven simply by the pressure difference between the blood-filled atria and the now-relaxed ventricles. A slower trickle continues as the ventricles keep relaxing. Then the atria contract at the very end of diastole to squeeze in that last bit of blood before the cycle starts over.
At a resting heart rate, diastole is actually longer than systole. That’s important because the heart muscle itself gets most of its own blood supply during diastole, when the muscle is relaxed and blood can flow through the small arteries embedded in the heart wall. When your heart rate climbs very high, diastole shortens more than systole does, which is one reason an extremely fast heart rate can strain the heart.
The Sounds You Hear
The classic “lub-dub” of a heartbeat comes from valves slamming shut at the transitions between systole and diastole. The “lub” (called S1) happens when the valves between the atria and ventricles close at the start of systole. The “dub” (called S2) happens when the exit valves close at the start of diastole. These sounds are what a doctor listens for with a stethoscope, and abnormal extra sounds or murmurs can signal valve problems.
Electrical Signals That Drive Each Phase
If you’ve ever seen a heart monitor tracing (an ECG), each wave corresponds to a specific part of the cycle. The small P wave represents the electrical signal that triggers atrial contraction. The tall, spiky QRS complex triggers ventricular systole. The rounded T wave marks the ventricles resetting their electrical charge, which corresponds to the beginning of ventricular relaxation and diastole. When doctors look at an ECG strip, they’re reading the timing and shape of these waves to determine whether systole and diastole are happening in the right sequence and at the right speed.
Pressures Inside the Heart
The pressure difference between systole and diastole inside the heart is dramatic. In the left ventricle, peak systolic pressure averages about 130 mmHg, with a normal range of 90 to 140 mmHg. That’s the force needed to push blood through the aorta and out to the entire body. At the end of diastole, once the ventricle is relaxed and filled, pressure drops to an average of just 9 mmHg (range of 5 to 12). That enormous swing, from roughly 130 down to 9, happens with every single heartbeat.
What Blood Pressure Numbers Mean
When you get a blood pressure reading, you’re measuring the pressure in your arteries, not inside the heart itself, but the same systole/diastole concept applies. The top number (systolic) captures peak arterial pressure when the heart contracts. The bottom number (diastolic) captures the lowest pressure between beats, when the heart is relaxed.
The 2025 guidelines from the American Heart Association and American College of Cardiology define the categories as follows:
- Normal: below 120/80 mmHg
- Elevated: systolic 120 to 129 with diastolic still below 80
- Stage 1 hypertension: systolic 130 to 139 or diastolic 80 to 89
- Stage 2 hypertension: systolic 140 or higher, or diastolic 90 or higher
Notice that either number being too high is enough to place you in a higher category. Isolated systolic hypertension, where only the top number is elevated, becomes increasingly common with age as arteries stiffen. Elevated diastolic pressure is more typical in younger adults. Both matter for long-term cardiovascular risk, which is why both numbers appear on every reading.
Why the Balance Between Systole and Diastole Matters
Problems with either phase can lead to heart failure, but in different ways. When the heart can’t contract forcefully enough during systole, it fails to pump adequate blood out. This is sometimes called heart failure with reduced ejection fraction. When the heart muscle becomes stiff and can’t relax properly during diastole, it fails to fill adequately even though its squeezing strength is preserved. This is heart failure with preserved ejection fraction, and it accounts for roughly half of all heart failure cases.
The distinction matters because the symptoms can look similar, including shortness of breath, fatigue, and fluid retention, but the underlying problem and treatment approach differ. Imaging tests like echocardiography can measure how well the heart contracts during systole and how well it relaxes during diastole, helping to distinguish between the two.