SECTION 6.3
The cardiac cycle and heart sounds
Explain the phases of the cardiac cycle, how pressure and valves move blood, and what produces each heart sound.
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One heartbeat is called a cardiac cycle, and it consists of a period of contraction, called systole, followed by a period of relaxation, called diastole. At a resting heart rate of about 75 beats per minute, one cycle lasts roughly 0.8 seconds, with diastole taking longer than systole. During the cycle, pressures inside the chambers rise and fall, and those pressure changes open and close the valves. Blood moves not because the heart pushes it continuously, but because pressure gradients open the correct valve at the correct moment. The sounds you hear through a stethoscope are produced by valves closing, and changes in those sounds or in the timing of the cycle can reveal valve disease or an arrhythmia. This section follows one cycle step by step, relating pressure, valve position and sound.
Key Concepts & Topics
One cycle, two phases
The cardiac cycle alternates between systole, when the myocardium contracts and ejects blood, and diastole, when it relaxes and refills. Both atria and ventricles have their own systole and diastole, but the term usually refers to the ventricles because they do the main pumping. At a normal resting rate, atrial systole comes first and completes ventricular filling; ventricular systole follows, closing the AV valves, opening the semilunar valves and ejecting blood; then ventricular diastole begins, the semilunar valves close and the ventricles refill. Because the cycle repeats continuously, the phases are best described by their pressure and valve events rather than by fixed times. When heart rate rises, diastole shortens more than systole, which reduces the time available for filling and for coronary perfusion.
For example: At 75 beats per minute each cycle lasts 0.8 seconds; at 150 beats per minute it lasts only 0.4 seconds, and most of the reduction falls on diastole.
The phases of the cycle
The cycle is usually described in five steps. During atrial systole, the atria contract and top up the ventricles, which are already mostly filled; the AV valves are open and the semilunar valves closed. During isovolumetric contraction, the ventricles begin to contract, pressure rises sharply, the AV valves snap shut (producing the first heart sound), and for a moment all valves are closed, so volume does not change. During ventricular ejection, pressure exceeds arterial pressure, the semilunar valves open and blood leaves through the pulmonary trunk and aorta. During isovolumetric relaxation, the ventricles relax, pressure falls, the semilunar valves snap shut (producing the second heart sound), and again all valves are briefly closed. During ventricular filling, pressure in the ventricles falls below atrial pressure, the AV valves open and blood flows in, completing the cycle.
For example: The phrase “all valves closed” describes both isovolumetric phases, when the ventricles change pressure without changing volume.
Pressure drives the valves
Valves do not open and close on a schedule; they respond to the pressure difference across them. When pressure in the chamber behind a valve exceeds pressure in front of it, the valve opens. When pressure in front becomes greater, the valve closes. This is why the AV valves close at the start of ventricular systole, when ventricular pressure exceeds atrial pressure, and why the semilunar valves open only when ventricular pressure exceeds aortic or pulmonary pressure. During the isovolumetric phases, both sets of valves are closed because the pressure has not yet crossed either threshold. Because the cycle is driven by pressure, anything that changes pressures, such as high blood pressure or a narrowed valve, changes when the valves open and close and therefore changes the sounds and the workload.
For example: In aortic stenosis the aortic valve opens less fully, so the left ventricle must generate higher pressure to eject the same volume.
Heart sounds and murmurs
The two normal heart sounds come from valves closing. The first sound, S1, is produced when the AV valves close at the start of ventricular systole; it is longer and lower-pitched. The second sound, S2, is produced when the semilunar valves close at the end of ejection; it is shorter and sharper. A third sound, S3, can sometimes be heard in children and in some conditions, and a fourth, S4, may be heard when the atria contract against a stiff ventricle. Murmurs are different: they are whooshing sounds caused by turbulent flow, usually because a valve is narrowed so blood is forced through a smaller opening, or because a valve leaks so blood flows backward. The timing of a murmur within the cycle, and the point on the chest where it is loudest, help identify which valve is affected.
For example: A murmur heard between S1 and S2 occurs during ejection; a murmur heard after S2 occurs during filling, which points to different valves.
Timing, the ECG and heart rate
The cardiac cycle is coordinated by electrical events that can be recorded on an electrocardiogram (ECG). The P wave represents atrial depolarisation, which precedes atrial systole. The QRS complex represents ventricular depolarisation, which triggers ventricular systole. The T wave represents ventricular repolarisation, during which the ventricles reset and relax. Because electrical events precede mechanical ones, the ECG is a useful way to see when each phase begins. Heart rate affects the cycle mainly by changing diastole: as rate rises, the time for filling and for coronary blood flow falls. This is why very fast rhythms can reduce cardiac output and cause ischaemia even when the heart is otherwise healthy, and why controlling rate is part of treating many cardiac conditions.
For example: On an ECG, the QRS complex is followed almost immediately by the first heart sound, because ventricular contraction closes the AV valves.
Key facts to remember
- A cardiac cycle is one heartbeat: systole (contraction) followed by diastole (relaxation).
- At about 75 beats per minute a cycle lasts roughly 0.8 s, with diastole longer than systole.
- The five phases are atrial systole, isovolumetric contraction, ventricular ejection, isovolumetric relaxation and ventricular filling.
- Valves open and close with pressure differences; S1 is AV valves closing and S2 is semilunar valves closing.
- Murmurs are turbulent sounds from narrowed or leaking valves; the ECG shows the electrical events that precede each phase.
Example · Follow one heartbeat at rest. The atria contract, topping up the ventricles. The ventricles begin to contract, the AV valves snap shut and the first sound is heard; for a moment no valve is open, so pressure rises without volume change. Pressure then exceeds aortic and pulmonary pressure, the semilunar valves open and blood is ejected. The ventricles relax, the semilunar valves snap shut and the second sound is heard. Once ventricular pressure falls below atrial pressure, the AV valves open and the ventricles refill. The cycle then repeats, driven entirely by pressure and valve position.
Read full notes
Read one topic at a time. These notes explain the mechanisms and relationships behind the key facts.
One cycle, two phases
The value of describing the cardiac cycle in phases is that each phase has a clear pressure pattern and a clear valve position, so the behaviour of the heart can be predicted rather than memorised. During systole, pressure rises, valves open or close according to the gradients, and blood is ejected. During diastole, pressure falls, the ventricles refill, and the coronary arteries receive their own supply. Because diastole is longer than systole at rest, the heart spends more time filling than ejecting. This balance changes with heart rate: as rate rises, diastole shortens more, so both filling and coronary supply are reduced. Understanding the phases therefore explains both normal function and the effects of a fast rhythm.
The phases step by step
Two of the five phases deserve special attention because they are counter-intuitive. In isovolumetric contraction, the ventricle is contracting but no blood is leaving, because the AV valves have closed and the semilunar valves have not yet opened. Pressure rises rapidly with no change in volume, which is why the phase is named for constant volume. In isovolumetric relaxation, the opposite happens: the ventricle is relaxing, but no blood is entering yet, because the semilunar valves have closed and the AV valves have not yet opened. Pressure falls rapidly at constant volume. These two brief phases are the moments when all valves are closed, and they ensure that the ventricle can build pressure before ejecting and drop pressure before filling.
Pressure drives the valves
Thinking of the cycle as a pressure story rather than a sequence of instructions makes it easier to predict what will happen in disease. High arterial pressure means the left ventricle must generate more pressure before the aortic valve opens, so ejection is delayed and systole is prolonged. A narrowed aortic valve means the ventricle must generate much more pressure for the same flow, which thickens the myocardium. A leaking mitral valve means blood flows backward when the ventricle contracts, so less goes forward. In each case the timing or the volume of the cycle changes, and those changes can be heard as murmurs and seen on imaging. The pressure story also explains why the first and second sounds occur when they do.
Heart sounds and murmurs
Auscultation is a pressure and timing exercise. The first sound is the AV valves closing, which happens when ventricular pressure crosses atrial pressure. The second sound is the semilunar valves closing, which happens when ventricular pressure falls below arterial pressure. Splitting of a sound, extra sounds and murmurs all carry information. A murmur between the first and second sounds occurs while blood is being ejected, so it suggests a problem with a semilunar valve or with the flow through it. A murmur after the second sound occurs during filling, so it suggests a problem with an AV valve. The location on the chest where the murmur is loudest corresponds to where the sound from that valve is transmitted, which is why clinicians learn specific listening points.
Timing, the ECG and heart rate
The ECG shows the electrical triggers of the cycle, so it lets you predict the mechanical events. Atrial depolarisation (P wave) precedes atrial systole; ventricular depolarisation (QRS complex) precedes ventricular systole; and ventricular repolarisation (T wave) accompanies relaxation. Because the mechanical events follow almost immediately, the ECG is used to time murmurs, to detect arrhythmias and to see whether the atria and ventricles are coordinated. Heart rate matters because it changes the duration of the cycle, mostly by changing diastole. At very high rates, filling time and coronary perfusion time become short, so cardiac output may fall even though the heart is beating faster. This is why rate control is a therapeutic goal in many conditions, including atrial fibrillation and heart failure.
Explore the diagram
Start with the whole view, then select a numbered structure. Compare the additional illustrations to connect what a structure looks like with what it does.
Pressure, valves and the cardiac cycle
Reduced motion · use the slider to explore each state.
AV open; semilunar closed. Blood enters relaxing ventricles.
Educational sequence · slowed and simplified, not real-time physiology
Where to listen · anterior view
Reduced motion · use the slider to explore each state.
Second RIGHT intercostal space, sternal border.
Educational sequence · slowed and simplified, not real-time physiology · Guided landmark tour
Connect the illustrations to the key ideas
FILLING · AV valves open, semilunar valves closed
Atrial systole completes filling; pressure in the ventricles is low.
CONTRACTION AND EJECTION · AV valves shut (S1), then semilunar valves open
Isovolumetric contraction raises pressure, then ejection sends blood out.
RELAXATION · semilunar valves shut (S2), then AV valves open
Isovolumetric relaxation lowers pressure before filling begins again.
Core Concepts Defined
- Cardiac cycle
- One complete heartbeat, from the start of one contraction to the start of the next.
- Systole
- The contraction phase of the cardiac cycle.
- Diastole
- The relaxation phase of the cardiac cycle, when chambers refill.
- Atrial systole
- Contraction of the atria that completes ventricular filling.
- Isovolumetric contraction
- Early ventricular contraction with all valves closed, so pressure rises without volume change.
- Ventricular ejection
- The phase when semilunar valves are open and blood leaves the ventricles.
- Isovolumetric relaxation
- Early ventricular relaxation with all valves closed, so pressure falls without volume change.
- Ventricular filling
- The phase when AV valves are open and blood flows into the ventricles.
- End-diastolic volume
- The volume of blood in a ventricle at the end of filling.
- End-systolic volume
- The volume of blood remaining in a ventricle after ejection.
- Stroke volume
- The volume ejected by one ventricle in one beat.
- S1 (first heart sound)
- The sound of the AV valves closing at the start of ventricular systole.
- S2 (second heart sound)
- The sound of the semilunar valves closing at the end of ejection.
- Murmur
- A turbulent sound caused by a narrowed or leaking valve.
- Electrocardiogram (ECG)
- A recording of the heart’s electrical activity.
- P wave
- The ECG wave representing atrial depolarisation.
- QRS complex
- The ECG complex representing ventricular depolarisation.
- T wave
- The ECG wave representing ventricular repolarisation.
Go deeper · worked example
Following one heartbeat
- Atrial systole. The atria contract, adding the final portion of blood to ventricles that are already mostly filled. The AV valves are open and the semilunar valves closed.
- Isovolumetric contraction. The ventricles begin to contract, ventricular pressure rises above atrial pressure, and the AV valves close, producing the first heart sound. All valves are briefly closed, so volume does not change.
- Ventricular ejection. Ventricular pressure exceeds pulmonary and aortic pressure, the semilunar valves open and blood is ejected into the pulmonary trunk and aorta.
- Isovolumetric relaxation. The ventricles relax, pressure falls below arterial pressure, and the semilunar valves close, producing the second heart sound. Again all valves are briefly closed.
- Ventricular filling. Ventricular pressure falls below atrial pressure, the AV valves open and blood flows in. The cycle is complete and ready to begin again, driven only by pressure and valve position.
Explain it yourself · Explain why the first heart sound occurs at the start of ventricular systole and not when the semilunar valves open.
Reveal a model explanation
Heart sounds are produced by valves closing, not opening. At the start of ventricular systole, ventricular pressure rises above atrial pressure, so the AV valves snap shut, producing the first sound. The semilunar valves do not open until ventricular pressure exceeds aortic and pulmonary pressure, which happens a little later. When they do open, blood flows smoothly at first and does not produce a normal sound. The second sound occurs later, when the semilunar valves close at the end of ejection. Sound therefore marks closure, and the two normal sounds mark the two sets of valves closing.
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OpenStax, Anatomy and Physiology 2e, 19.3 Cardiac Cycle; Tortora & Derrickson, Principles of Anatomy and Physiology, 12e, ch. 20. Author review only; medical and French-language expert review pending.
Original teaching schematics. English and French wording is author-reviewed for study; anatomy and French-language expert review is pending. Checks are practice only, not a graded examination.
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