The cardiac cycle is the complete sequence of events that occurs each time your heart beats, from the moment it fills with blood to the moment it pumps that blood out to your body and lungs. At a resting heart rate of about 75 beats per minute, one full cycle takes roughly 0.8 seconds. Every cycle has two main phases: diastole, when the heart relaxes and fills, and systole, when it contracts and ejects blood.
How One Heartbeat Breaks Down
Each cardiac cycle moves through a precise sequence. Diastole comes first and takes up the majority of the cycle. During this phase, the ventricles (the heart’s two lower, more powerful chambers) relax, and blood flows in from the atria (the two upper chambers). Systole follows, compressing the ventricles to push blood into the lungs and the rest of the body. Then the cycle resets.
Within those two broad phases, there are six distinct stages. Diastole includes four of them: a brief period where the ventricles relax but no blood enters yet, a rapid filling stage where most of the blood rushes in, a slower filling stage where the flow tapers off, and finally a contraction of the atria that tops off the ventricles with the last bit of blood. Systole includes two stages: a brief buildup of pressure inside the ventricles while all four valves are closed, followed by the ejection of blood once pressure is high enough to force the outflow valves open.
What Happens During Diastole
Diastole begins the moment the ventricles stop contracting and start to relax. At first, all four heart valves are shut, and the ventricles are simply loosening up without any blood moving in or out. This is sometimes called the relaxation phase. Once the pressure inside the ventricles drops below the pressure in the atria, the valves between the atria and ventricles (the mitral valve on the left, the tricuspid valve on the right) swing open.
Blood then pours rapidly into the ventricles. This passive rush accounts for the majority of ventricular filling, driven purely by the pressure difference between the chambers. As the ventricles fill and the pressure equalizes, the flow slows to a trickle. At the very end of diastole, the atria contract and squeeze their remaining blood downward, adding a final boost. In a healthy heart, this atrial “kick” contributes roughly 20 to 30 percent of the total filling volume. That percentage becomes more important during exercise or in certain heart conditions where passive filling is impaired.
By the end of diastole, each ventricle holds its maximum amount of blood, known as the end-diastolic volume. In a healthy adult, that volume is typically around 65 to 70 milliliters per square meter of body surface area, which translates to roughly 120 to 130 milliliters of blood in an average-sized person.
What Happens During Systole
Systole begins when the ventricles start contracting. For a split second, all four valves are closed again. The ventricles are squeezing, but because no valves are open, the blood has nowhere to go. Pressure inside the chambers rises sharply. This pressure-building phase is very brief but essential.
Once the pressure inside the left ventricle exceeds the pressure in the aorta (and the right ventricle exceeds the pressure in the pulmonary artery), the outflow valves pop open and blood surges out. The left ventricle sends blood to the body through the aortic valve, while the right ventricle sends blood to the lungs through the pulmonic valve. This ejection phase is the heart’s main job, the moment that creates a pulse you can feel in your wrist or neck.
A healthy heart doesn’t empty completely. After ejection, each ventricle retains a small reserve of blood called the end-systolic volume. In healthy adults, the left ventricle ejects about 60 percent of the blood it held at the end of diastole. That percentage is the ejection fraction, one of the most commonly used measures of heart function. A normal ejection fraction sits around 60 percent for men and 62 percent for women, though anything above 50 percent is generally considered healthy.
The Electrical Signals That Drive Each Phase
The mechanical events of the cardiac cycle don’t happen on their own. Each contraction is triggered by an electrical signal that spreads through the heart muscle in a specific pattern, and this pattern shows up on an electrocardiogram (ECG or EKG) as a series of recognizable waves.
The P wave is the first deflection on an ECG tracing. It represents the electrical activation of the atria, which triggers atrial contraction at the end of diastole. A fraction of a second later, the QRS complex appears, a sharp, tall set of peaks that marks the electrical activation of the ventricles. This is the signal that kicks off ventricular systole. Finally, the T wave appears as the ventricles reset their electrical charge and begin to relax, marking the transition back into diastole.
The timing matters. The electrical signal always precedes the mechanical event by a tiny margin. So the QRS complex fires just before the ventricles actually squeeze, and the T wave finishes just as the ventricles begin to relax and refill. When doctors look at an ECG, they’re essentially watching the cardiac cycle unfold in electrical form, checking that each phase fires in the correct order and with the right timing.
The Heart Sounds You Can Hear
The two familiar heart sounds, often described as “lub-dub,” are produced by valves slamming shut at specific points in the cycle. The first sound (S1) occurs right at the start of systole. It’s caused primarily by the mitral valve closing, with a smaller contribution from the tricuspid valve. This closure prevents blood from flowing backward into the atria as the ventricles begin to squeeze.
The second sound (S2) marks the beginning of diastole. It’s produced by the aortic and pulmonic valves snapping shut once the ventricles finish ejecting blood. This prevents blood in the aorta and pulmonary artery from falling back into the ventricles. Together, S1 and S2 give a doctor a real-time audio window into the mechanical cycle. Any extra sounds, clicks, or murmurs between or alongside these two beats can signal valve problems, abnormal blood flow, or changes in heart structure.
How the Cycle Changes With Heart Rate
At rest, the 0.8-second cycle splits unevenly between its two phases. Diastole is the longer portion, roughly twice the duration of systole. This gives the heart plenty of time to fill and, just as importantly, gives the coronary arteries time to deliver oxygen to the heart muscle itself, since the heart receives most of its own blood supply during diastole.
When your heart rate increases during exercise or stress, the cycle compresses. The phase that shortens most is diastole. Systole stays relatively constant because the ventricles still need a minimum amount of time to contract effectively. At very high heart rates, filling time shrinks so much that each beat pumps a slightly smaller volume of blood, although the faster rate more than compensates by delivering more total blood per minute.
This is also why a chronically elevated resting heart rate can strain the heart over time. Less diastolic filling time means less oxygen delivery to the heart muscle per beat and less efficient filling of the ventricles. Conversely, athletes with low resting heart rates (sometimes in the 40s or 50s) have long, luxurious diastolic periods, allowing efficient filling and excellent coronary blood flow with each cycle.