The cardiac cycle is everything that happens in the heart during one heartbeat — and it confuses students because the textbook describes four chambers, four valves, and a wall of pressure numbers all at once. The cycle is far simpler once you hold on to a single rule: valves open and close because of pressure differences, and blood always flows from higher pressure to lower pressure. Get that rule, and every valve movement in the cardiac cycle becomes predictable instead of memorized.
The One Rule: Pressure Moves the Blood and the Valves
The heart has no tiny doormen deciding when valves open. A valve opens when the pressure behind it is higher than the pressure in front of it, and it slams shut when that relationship reverses. Blood is just following pressure downhill.
There are two valve pairs to track. The atrioventricular (AV) valves — the tricuspid on the right, the mitral on the left — sit between each atrium and ventricle. The semilunar valves — the pulmonary and the aortic — sit at the exit of each ventricle into its great artery. Throughout the cycle, exactly one valve pair is open at a time, or briefly neither.
Two terms label the phases: systole is contraction, diastole is relaxation. The cycle alternates between them. One more useful fact before the walkthrough: the two atria always act together, and the two ventricles always act together, so although the right and left sides of the heart pump to different places — the right side to the lungs, the left side to the body — they move through the cycle in lockstep. That means you can follow the cycle by tracking just one side and trust the other to mirror it.
Atrial Systole: Topping Off the Ventricles
The cycle begins with the atria contracting — atrial systole.
At this point the ventricles are relaxed and the AV valves are already open, so blood has been draining passively from atria to ventricles. In fact about 70–80% of ventricular filling happens passively, before the atria contract at all. Atrial contraction adds only the final 20–30% — the "atrial kick" that tops the ventricles off.
The ventricles are now full. The volume of blood in a ventricle at the end of filling is the end-diastolic volume (EDV), roughly 120–130 mL.
Ventricular Systole: The Pump Pushes
Now the ventricles contract — ventricular systole — and it happens in two parts.
Isovolumetric contraction. The ventricles begin to squeeze, and ventricular pressure rises above atrial pressure. That pressure reversal slams the AV valves shut (this is the first heart sound, "lub"). But pressure is not yet high enough to push open the semilunar valves. So for a brief moment every valve is closed, the ventricle is squeezing a sealed volume of blood, and pressure climbs fast. No blood moves — hence isovolumetric, "same volume."
Ventricular ejection. Ventricular pressure finally exceeds the pressure in the aorta and pulmonary artery. The semilunar valves are pushed open and blood is ejected. The amount pushed out per beat is the stroke volume, about 70 mL. What remains behind is the end-systolic volume (ESV), roughly 50–60 mL. Note that the heart never empties completely.
Ventricular Diastole: Relax and Refill
The ventricles stop contracting and relax — ventricular diastole.
Isovolumetric relaxation. As the ventricles relax, their pressure drops below aortic and pulmonary pressure. Blood in the great arteries starts to fall back toward the ventricles and catches in the semilunar valve cusps, snapping them shut (the second heart sound, "dub"). Ventricular pressure is now falling fast, but it is still above atrial pressure, so the AV valves remain closed too. Again, briefly, all four valves are shut.
Ventricular filling. Ventricular pressure finally drops below atrial pressure. The AV valves open and blood pours from atria into ventricles. This passive filling is most of the cycle — and the next round of atrial systole will simply top it off. The cycle is back to the start.
Putting the Numbers Together
The volumes connect into the heart's key output measures. Stroke volume = EDV − ESV — what went in minus what stayed, about 120 − 50 = 70 mL. Cardiac output, the blood pumped per minute, is stroke volume × heart rate: 70 mL × 75 beats per minute ≈ 5.25 L/min, close to the body's entire blood volume every minute. The two heart sounds map directly onto the cycle: "lub" is the AV valves closing as systole begins, "dub" is the semilunar valves closing as diastole begins.
Getting Help
The cardiac cycle is mechanical, but the heartbeat is triggered electrically — the electrical events behind each contraction build on action potentials explained. For more cardiovascular walkthroughs, browse the full set of Anatomy & Physiology study guides.
Conclusion
The cardiac cycle is one heartbeat broken into four phases: atrial systole tops off the ventricles, ventricular systole builds pressure and ejects blood, and ventricular diastole relaxes and refills. Every valve movement follows the same rule — pressure higher behind a valve opens it, pressure higher in front shuts it. Track the pressures and the whole cycle, sounds and volumes included, falls into place.
