Medically reviewed by [Name], Department of Physiology, Narayana College of Health Sciences | Last updated: [Month, Year]
Quick answer: The cardiac cycle is the complete sequence of electrical and mechanical events between the start of one heartbeat and the start of the next. At a resting heart rate of 75 beats per minute, one full cycle takes 0.8 seconds, divided into atrial systole (0.1s), ventricular systole (0.3s) and complete diastole (0.4s).
The cardiac cycle at a glance
| Phase | Duration | AV valves (mitral, tricuspid) | Semilunar valves (aortic, pulmonary) | Heart sound |
| Atrial systole | 0.10 s | Open | Closed | — |
| Isovolumetric contraction | 0.05 s | Snap shut | Closed | S1 — “LUB” |
| Isovolumetric relaxation | 0.06 s | Closed | Snap shut | S2 — “DUB” |
| Ventricular filling | 0.34 s | Open | Closed | — |
Your heart beat somewhere around 100,000 times yesterday. It will do the same today, and tomorrow, and for every day after that, without ever demanding the recovery your quadriceps start begging for two flights up a staircase.
What makes that possible isn’t strength. It’s timing.
Most students can recite “systole and diastole” by the second week of physiology. Far fewer can tell you what the pressure inside the left ventricle is doing at the 0.15-second mark, or why every single beat involves a moment when all four valves are shut, twice. This guide is about that second layer.
What is the cardiac cycle?
The cardiac cycle is the full sequence of electrical and mechanical events that takes the heart from the beginning of one heartbeat to the beginning of the next. Every cycle moves blood in one direction only, through four chambers and four valves, using pressure gradients rather than suction.
It splits into two mechanical periods:
- Systole — active contraction and ejection of blood
- Diastole — relaxation and refilling of the chambers
That much is standard. The part worth getting right is that “systole” and “diastole” mean different things depending on which chamber you’re talking about. The atria and the ventricles are never contracting at the same time, which is the entire point of the design.
How long does one cardiac cycle last?
One cardiac cycle lasts 0.8 seconds at a heart rate of 75 beats per minute. The arithmetic is straightforward: 60 seconds divided by 75 beats gives 0.8 seconds per beat.
But that 0.8 seconds is not a fixed quantity, and this is where the clinically interesting bit lives.
When heart rate climbs, the cycle shortens. Systole shortens a little. Diastole shortens a lot. Push someone into a sustained tachycardia, and you compress the filling window far more aggressively than the ejection window, so the ventricle has less time to load before it has to fire again. The left ventricle also perfuses its own muscle mainly during diastole, when the myocardium isn’t squeezing its own coronary vessels closed. Shorten diastole enough and you’ve simultaneously reduced filling and reduced coronary blood supply to a heart that is now working harder.
That trade-off is why a fast heart is not automatically an efficient one.
What are the phases of the cardiac cycle?
1. Atrial systole — 0.1 seconds
The cycle opens with electrical depolarization of the atria, which shows up as the P wave on an ECG. Both atria contract and push the last 20–30% of blood into ventricles that are already mostly full.
At this point the AV valves are wide open and the semilunar valves are firmly closed.
One detail most guides skip: there are no valves guarding the entry of the venae cavae into the right atrium or the pulmonary veins into the left. So, when the atria contract, a small volume of blood does travel backwards into those veins. That retrograde bump is what producing the ‘a’ wave you can see in a jugular venous pressure trace, and it’s the reason JVP waveforms are readable at the bedside at all.
2. Ventricular systole — 0.3 seconds
The impulse reaches the ventricles through the bundle of His, the bundle branches and the Purkinje network, and the ventricles contract from the apex upward. This phase has two clean sub-stages.
Isovolumetric contraction (0.05 s). Ventricular pressure rises sharply. The moment it crosses atrial pressure, the mitral and tricuspid valves slam shut, and that closure produces the first heart sound, S1 — the “LUB.” Now every valve in the heart is closed. The muscle is contracting hard, pressure is climbing steeply, and not one millilitre of blood has moved. Hence isovolumetric: same volume.
Ventricular ejection (0.25 s). Pressure keeps building until it exceeds the pressure sitting in the aorta and pulmonary artery. For the left ventricle that means clearing roughly 80 mmHg of aortic diastolic pressure. The semilunar valves are forced open and blood surges out, with left ventricular pressure peaking near 120 mmHg.
The right ventricle does exactly the same work at the same time, but against a far lower load. Peak right ventricular pressure sits around 25 mmHg, because the pulmonary circuit is a low-pressure system. Same volume of blood, roughly a fifth of the pressure. That’s why the left ventricular wall is three times thicker.
3. Complete diastole — 0.4 seconds
Isovolumetric relaxation (0.06 s). The ventricles begin to relax and pressure falls. When it drops below aortic and pulmonary pressure, blood starts sliding backwards, catches the cusps of the semilunar valves and snaps them closed. That’s the second heart sound, S2 — the “DUB.” The small blip this backflow leaves on the aortic pressure trace is the dicrotic notch. And once again, briefly, all four valves are shut and ventricular volume doesn’t change.
Ventricular filling (0.34 s). Ventricular pressure continues to drop until it falls below atrial pressure. The AV valves open and blood pours through. Filling itself has a rhythm to it: a rapid phase first, then a slow middle phase called diastasis, and finally the atrial contraction that starts the next cycle. By the time the atria kick in, the ventricles are already about 70–80% full.
A precision point worth marking: complete (or joint) diastole lasts 0.4 seconds, meaning the period when all four chambers are relaxed together. Ventricular diastole lasts 0.5 seconds, because the ventricles are still relaxed during the 0.1 seconds of atrial systole. Examiners ask this deliberately.
What controls the timing of the cardiac cycle?
The cardiac cycle is myogenic — it starts inside the heart itself, with no instruction from the brain required. The trigger is the sinoatrial (SA) node, a patch of self-depolarising tissue in the wall of the right atrium near the opening of the superior vena cava, and the reason it’s called the natural pacemaker.
The impulse travels:
- SA node → depolarises both atria
- AV node → holds the signal for roughly 0.1 seconds
- Bundle of His → down the interventricular septum
- Right and left bundle branches
- Purkinje fibres → rapid spread through the ventricular myocardium
That pause at the AV node is deliberate and essential. It buys the atria enough time to finish emptying before the ventricles start squeezing. Remove the delay and the two would contract nearly together, which would make the atrial contribution to filling pointless.
The autonomic nervous system doesn’t create the rhythm. It only speeds it up or slows it down.
Why does the heart make a “lub-dub” sound?
The heart sounds come from valve closure and the vibration it sets up in the surrounding blood and chamber walls — not from the muscle contracting. Muscle contraction is silent.
- S1 (“LUB”) — closure of the mitral and tricuspid valves at the start of ventricular systole. Lower in pitch, longer in duration.
- S2 (“DUB”) — closure of the aortic and pulmonary valves at the start of ventricular diastole. Sharper and shorter.
Two more sounds exist and are worth knowing:
- S3 occurs during rapid ventricular filling. It can be entirely normal in children, young adults and pregnancy. In an older patient it often points toward volume overload or heart failure.
- S4 is produced by atrial contraction forcing blood into a stiff, poorly compliant ventricle. It’s almost always abnormal.
Stroke volume, ejection fraction and cardiac output
Stroke volume (SV) is the volume of blood ejected by the left ventricle in one contraction. It is the difference between what was in the ventricle before it contracted and what remained after.
- End-diastolic volume (EDV): ~120 ml
- End-systolic volume (ESV): ~50 ml
- Stroke volume (SV) = EDV − ESV = ~70 ml
Two figures follow from that:
Ejection fraction (EF) = SV ÷ EDV = 70 ÷ 120 ≈ 58%. A normal resting EF sits between 55% and 70%. Note what this means: a healthy heart empties a little over half of what it holds, and never comes close to emptying completely. The residual volume is a reserve, and it’s what allows the heart to increase output sharply during exercise.
Cardiac output (CO) = SV × heart rate = 70 ml × 72 bpm ≈ 5 litres per minute. Which is, give or take, the entire blood volume of an adult. Your whole circulation makes a complete lap roughly once a minute while you’re sitting still.
Where students usually lose marks
After enough viva sessions, the same five errors keep surfacing.
- Confusing ventricular diastole (0.5 s) with complete diastole (0.4 s). Read the question carefully.
- Attributing the heart sounds to muscle contraction. It’s valve closure. Every time.
- Forgetting that all four valves are closed twice per cycle, not once, during isovolumetric contraction and isovolumetric relaxation.
- Overstating the atrial contribution to filling. At rest, atrial systole is a top-up of 20–30%, not the main event. It matters far more at high heart rates, when passive filling time collapses.
- Writing that the semilunar valves open when ventricular pressure exceeds “arterial pressure.” It’s arterial diastolic pressure. The distinction is the whole mechanism.
Frequently asked questions
What causes the lub-dub sound of the heart?
The sounds are produced by the sudden closure of the heart valves and the vibrations that follow, not by the heart muscle contracting. S1, the “LUB,” is the atrioventricular valves closing at the start of ventricular systole. S2, the “DUB,” is the semilunar valves closing at the start of ventricular diastole.
What is stroke volume and how is it calculated?
Stroke volume is the amount of blood pumped out of the left ventricle in a single contraction. It equals end-diastolic volume minus end-systolic volume, or roughly 120 ml minus 50 ml. The average resting stroke volume in a healthy adult is about 70 ml.
What initiates the cardiac cycle?
The sinoatrial (SA) node initiates the cardiac cycle. It sits in the wall of the right atrium near the superior vena cava and generates electrical impulses on its own, without nervous stimulation. This is why the heartbeat is described as myogenic.
What happens to the cardiac cycle when heart rate increases?
The total cycle duration shortens, but not evenly. Diastole is compressed far more than systole, which reduces ventricular filling time and cuts coronary perfusion of the left ventricle. This is the main reason sustained tachycardia reduces cardiac efficiency.
Why are all four heart valves closed twice in every cycle?
All four valves close during isovolumetric contraction and again during isovolumetric relaxation. In both moments, ventricular pressure is between atrial pressure and arterial pressure, so no valve has a gradient sufficient to open. Ventricular volume stays constant while pressure changes rapidly.
Do the left and right ventricles follow the same cycle?
Yes. Both ventricles contract and relax simultaneously and eject the same stroke volume. The difference is pressure, not timing: the left ventricle peaks near 120 mmHg against systemic resistance, while the right peaks near 25 mmHg against the low-resistance pulmonary circuit.
What is the dicrotic notch?
The dicrotic notch is a small dip and rebounds on the aortic pressure trace, produced when blood briefly flows backwards at the end of ejection and closes the aortic valve. It marks the boundary between the ventricular systole and diastole on an arterial pressure waveform.
From the diagram to the ward
A pressure-volume loop on a whiteboard is one kind of learning. Placing a stethoscope on a real chest and picking out an S3 gallop is another entirely, and the gap between them is where most healthcare training either succeeds or quietly fails.
If you’re heading into allied health sciences, nursing or physiotherapy, the question worth asking of any programme is simple: how far is the classroom from the patient?
At Narayana College of Health Sciences, the answer is a short walk. NCHS sits within the Narayana Health campus at Bommasandra, Bengaluru, a network built on one of India’s largest volumes of cardiac care. Students learn the cardiac cycle in a lecture hall and then see it, hear it and monitor it in a hospital where cardiac medicine is the daily work rather than a rotation on a schedule.
