The Cardiac Cycle Made Easy: A Step-by-Step Guide for Nursing Students

The cardiac cycle sounds complicated until you see that it is just a pattern: one squeeze, one release, repeated tens of thousands of times a day. Your heart fills with blood, then pumps it out. The confusion comes from trying to memorize every phase, pressure, and valve movement at once instead of learning the logical sequence. Get the sequence first — diastole then systole, valves opening and closing on pressure differences — and the details (volumes, heart sounds, ECG correlations) slot into place naturally.

Why the cardiac cycle trips up nursing students

Most students hit a wall because textbooks present the cycle as a list of simultaneous events — atrial contraction, ventricular filling, valve positions, pressure changes, and ECG correlations all at once. That is too much to hold. Your brain needs the framework first, the way you learn to drive before you learn engine mechanics. The cardiac cycle is one complete heartbeat, and two phases drive everything: diastole (relaxation and filling) and systole (contraction and ejection). Every valve movement, pressure change, and heart sound stems from those two. For a deeper reference on the underlying physiology, see the NCBI StatPearls cardiac cycle overview.

Diastole: the filling phase

Diastole is when the heart relaxes and fills. It takes up roughly two-thirds of the cycle because most filling is passive — the heart simply opens and lets blood flow in.

Early diastole — passive filling. The ventricles have just emptied and relaxed, so their pressure drops below the atria, which have been collecting blood from the veins. The AV valves (mitral and tricuspid) open, and blood flows passively from atria into ventricles — about 70–80% of filling happens here with no active work. On the ECG this falls during the T wave and the baseline that follows; the heart is electrically recovering.

Late diastole — the atrial kick. With the ventricles already mostly full, the atria contract to push in the final 20–30%, bringing the ventricles to their maximum volume (end-diastolic volume, EDV). This matters clinically: patients in atrial fibrillation lose this coordinated contraction, and losing the atrial kick can meaningfully reduce cardiac output (commonly cited as up to roughly 20–25%), which is why they often feel weak or short of breath. On the ECG, the atrial kick corresponds to the P wave. It is usually silent, but an S4 heart sound ("atrial gallop") occurs here when a stiff ventricle resists filling — seen in hypertension or heart failure.

Systole: the ejection phase

Systole is the squeeze — shorter than diastole but far more forceful. It runs in three quick beats.

Isovolumetric contraction — building pressure. The ventricles start contracting but blood has nowhere to go yet, so pressure rises sharply while volume stays the same (that is what "isovolumetric" means). As ventricular pressure exceeds atrial pressure, the AV valves snap shut — producing S1, the "lub." The semilunar valves stay closed because ventricular pressure has not yet beaten aortic and pulmonary pressure. Think of squeezing a capped water bottle: it gets harder, but nothing moves until the cap opens. On the ECG this is the QRS complex into the ST segment.

Ventricular ejection — blood leaves the heart. Ventricular pressure finally exceeds aortic and pulmonary pressure, the semilunar (aortic and pulmonic) valves open, and blood rushes out. Roughly 60–70% of the ventricle's contents are ejected (the stroke volume); the rest remains (end-systolic volume, ESV). Ejection is usually silent, but a murmur such as the harsh systolic murmur of aortic stenosis can be heard here as blood forces through a narrowed valve.

Isovolumetric relaxation — pressure drops. The ventricles relax and pressure falls fast (volume momentarily constant again). Once ventricular pressure drops below aortic and pulmonary pressure, the semilunar valves slam shut — producing S2, the "dub." Then diastole begins again and the cycle repeats.

The full sequence, in order

1. Ventricles relax and fill passively (early diastole).

2. Atria contract and top off the ventricles (atrial kick / late diastole).

3. Ventricles contract, AV valves close (isovolumetric contraction) → S1.

4. Semilunar valves open, blood ejects (ventricular ejection).

5. Ventricles relax, semilunar valves close (isovolumetric relaxation) → S2, then repeat.

At a resting heart rate of about 75 beats per minute, one full cycle takes roughly 0.8 seconds — about 0.5 seconds of diastole and 0.3 seconds of systole. That diastole-heavy split is why a very fast heart rate is a problem: shorten diastole too much and the ventricles cannot fill.

Key concepts you need to know

Preload, afterload, and contractility determine how well the heart pumps. Preload is how much the ventricles are stretched at the end of diastole (EDV) — more stretch means a more forceful contraction up to a point (the Frank-Starling mechanism). Afterload is the resistance the ventricles must overcome to eject; high blood pressure means high afterload and harder work. Contractility is the intrinsic strength of the contraction, independent of preload and afterload, and it falls when heart muscle is damaged (after an MI or in heart failure).

Cardiac output (CO) is how much blood the heart pumps per minute: CO = stroke volume × heart rate. With a typical stroke volume of about 60–70 mL and a rate of 60–100 bpm, resting CO is roughly 4–6 L/min. If stroke volume drops (heart failure, valve disease), the rate rises to compensate; if the rate climbs too high, diastole shortens, filling suffers, and CO falls anyway.

Extra heart sounds. S3 (the "ventricular gallop," remembered as "Ken-tuck-y") occurs in early diastole as blood rushes into a dilated or failing ventricle — abnormal in adults but often normal in children and young adults. S4 (the "atrial gallop," "Tenn-es-see") occurs in late diastole when the atria contract against a stiff ventricle and is always abnormal, pointing to hypertension, heart failure, or ventricular hypertrophy. Murmurs are turbulent-flow sounds that occur in systole or diastole depending on which valve is involved.

How this shows up on the NCLEX

You will not be asked to recite the cycle in order — you will be tested on whether you understand why things happen and what they mean for the patient. Expect items like: identifying that the "lub-dub" at the apex is S1 and S2; recognizing that aortic stenosis increases afterload; applying Frank-Starling to see that an IV fluid bolus increases preload by raising venous return; and knowing that a patient in atrial fibrillation has lost the atrial kick. The skill is application, so practice with NGN-style case study questions rather than memorizing definitions in isolation.

How to actually remember this

• Draw it. Sketch the heart, label chambers and valves, and draw arrows for blood flow. Drawing forces deeper processing than rereading. The same tactic powers the broader TEAS anatomy and physiology and HESI A2 A&P content too.

• Understand over memorize. If you grasp why the AV valves close, you do not need a trick to recall it. Use mnemonics only for true lists.

• Watch it move. A good animated cardiac-cycle video makes the sequence click faster than text.

• Tie it to assessment. In clinicals, listen for S1 and S2, feel pulses, watch the monitor. The cycle stops being abstract when you can hear it on a real chest.

Cardiac cycle FAQ

What are the two main phases of the cardiac cycle?

Diastole (relaxation and filling) and systole (contraction and ejection). Diastole takes up roughly two-thirds of the cycle because most ventricular filling is passive; systole is shorter but far more forceful. Every valve movement and heart sound follows from these two phases.

What causes the S1 and S2 heart sounds?

S1 (the "lub") is the closing of the AV valves — mitral and tricuspid — at the start of ventricular contraction. S2 (the "dub") is the closing of the semilunar valves — aortic and pulmonic — when the ventricles relax. Both are valves snapping shut as pressure gradients reverse.

What is the atrial kick and why does it matter?

The atrial kick is the atrial contraction in late diastole that pushes the final 20–30% of blood into the ventricles. Patients in atrial fibrillation lose this coordinated contribution, which can meaningfully lower cardiac output and leave them feeling weak or short of breath.

How do you calculate cardiac output?

Cardiac output equals stroke volume multiplied by heart rate (CO = SV × HR). With a stroke volume around 60–70 mL and a heart rate of 60–100 bpm, resting cardiac output is about 4–6 L/min. A very fast heart rate can actually lower it by shortening diastole and reducing filling time.

What's the difference between S3 and S4?

S3 occurs in early diastole as blood fills a dilated or failing ventricle ("Ken-tuck-y"); it can be normal in children and young adults but is abnormal in older adults. S4 occurs in late diastole when the atria contract against a stiff ventricle ("Tenn-es-see") and is always abnormal, suggesting hypertension, heart failure, or hypertrophy.

Final thoughts

The cardiac cycle is not as complicated as nursing school makes it feel: the heart fills, then squeezes, and valves open and close on pressure differences. Everything else — EDV, ESV, stroke volume, cardiac output, Frank-Starling — builds on that simple rhythm. Master diastole and systole first, then layer in valves, then pressures and volumes. Once the sequence makes sense, the details stick because they finally have context.

Medically reviewed for clinical accuracy. This article is educational and is not a substitute for professional medical advice.