Cardiac Cycle Simulation
Background Information
Introduction
The cardiac cycle is the sequence of mechanical and electrical events during a single heartbeat. When the heart beats 75 times per minute, one cardiac cycle lasts 0.8 seconds. The duration shortens as the heart rate increases and lengthens as the heart rate decreases. Because it is recurring, the end of one cycle immediately precedes and prepares the heart for the start of the next cycle.
Heart Structures Review
Before continuing the presentation of the cardiac cycle, it may be beneficial to first review the names and functions of the heart chambers, valves, and major vessels.
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Cardiac Cycle Segments
The cardiac cycle consists of two main periods (parts or divisions): ventricular systole and ventricular diastole. Pacemaker cells in the heart walls regulate the timing and duration of these periods.
â—‰ Ventricular systole makes up about 1/3 of the cardiac cycle. During this time, the chamber walls contract and eject blood from the heart into large arteries that unite with the pulmonary and systemic circulatory systems.
â—‰ Ventricular diastole makes up the remaining 2/3 of the cardiac cycle. During this time, the walls of the chambers relax, and blood enters the heart through large veins from the pulmonary and systemic circulatory systems.
Physiologists typically subdivide ventricular systole and diastole periods into several phases (or stages) to better study the events in the heart chambers and major blood vessels. The approach and number of phases vary but generally resemble the scheme below.
Phases and Basic Events
Ventricular Systole:
â—‰ Ventricular Isovolumetric Contraction
The blood-filled ventricles start contracting during this phase, increasing the pressures in the chambers. When the ventricle pressures rise above the atria pressures, it closes the atrioventricular valves. Because all the heart valves are closed, blood cannot exit the ventricles, so the blood volume in these chambers remains unchanged or “isovolumetric.”
â—‰ Ventricular Ejection Phase
The ventricular walls continue to contract, causing the blood pressure in these chambers to increase. When the ventricular blood pressures rise above those in the large heart arteries, it forces the semilunar valves to open, allowing blood to flow into the aorta and pulmonary trunk arteries. Initially, the blood flows rapidly out of the ventricles but slows as the phase continues.
Ventricular Diastole:
â—‰ Ventricular Isovolumetric Relaxation
The ventricles start relaxing, decreasing the blood pressure in the chambers. When the ventricular pressures fall below the pressures in the large arteries, it closes the semilunar valves. Because all the heart valves are closed, blood cannot enter the ventricles, so the blood volume in these chambers remains unchanged or “isovolumetric.”
â—‰ Passive Ventricular Filling
The pressure on the blood in the ventricular chambers decreases as the ventricular walls continue relaxing. When the ventricular pressures fall below the atrial pressures, it forces the atrioventricular valves to open. Initially, blood in the atria rapidly enters the ventricles. Filling slows as blood from the heart’s large veins flows into the ventricles after passing through the atria.
â—‰ Atrial Systole
The previous phase nearly fills the ventricles with blood. To complete the filling process, the atria contract to actively inject additional blood into the ventricles just before they contract.
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Cardiac Cycle Starting Points
Because the phases are part of a cyclic process, there is no set (or standardized) starting point for the cardiac cycle. Physiologists often select the atrial systole phase as the starting point of the cardiac cycle because it coincides with the P wave at the beginning of the electrocardiogram. Another commonly used starting point is the ventricular isovolumetric contraction phase. It is selected because it marks the beginning of ventricular systole.
Atrial Systole Start Point
Isovolumetric Contraction Start Point
Regulation of Blood Flow
Blood flows through the heart from areas of higher pressure to areas of lower pressure, which changes with each cardiac cycle phase.
The pressure increases in the heart chambers when cardiomyocytes (myocardiocytes or heart muscle cells) contract and decreases when the cardiomyocytes relax.
Electrochemical activity (action potentials) in cardiomyocyte membranes controls their rhythmic contraction and relaxation. The pace of cardiomyocyte action potentials is, in turn, regulated by the heart’s conduction system.
Heart valves direct the blood flow by allowing the blood only to enter the proper low-pressure areas.
As the blood moves from areas of higher pressure to lower pressure, it produces corresponding changes in heart chamber blood volumes.
Wiggers Diagram
The Wiggers diagram, named after its developer, Carl Wiggers, is a composite of several graphs related to the cardiac cycle. Physiologists use the information the Wiggers diagram provides to interpret and comprehend the changing events associated with each part of a heartbeat.
Wiggers Diagram
X-Axis Components
The X-axis (horizontal axis) of the Wiggers diagram displays the sequence and durations of the main divisions and subdivisions (phases) of the cardiac cycle.
X-Axis
Y-Axis Components
The Y-axis (vertical axis) displays the amplitudes of several heart events associated with each part of the cardiac cycle, including chamber pressures, chamber volumes, electrical activity, and sounds. The recordings are taken from the left side of the heart because the ventricle is thicker and produces more forceful contractions than the right.
Chamber and Arterial Pressures
Ventricular Volumes
Electrical Activity
Heart Sounds
Interactive Display
Use the interactive display below to put the cardiac cycle events in continuous motion. As the sequence of phases sweeps across the screen, notice how the heart’s electrical and mechanical activities are related. Also, observe how the heart’s mechanical activities affect heart pressures, volumes, and sounds.
Procedures and Assessments
Press the appropriate button for each phase to see it individually displayed along with the associated heart actions.
View the animated heart to compare the phase event graphs and heart actions.
To assess the phase events, use the portion of the Wiggers diagram highlighted by the phase scan. Use the hide/show buttons to select individual graphs.
Isovolumetric Contraction
Procedure: Click the ventricular isovolumetric contraction button to view a scan of this phase and the associated heart actions.
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Assessment: Analyze the phase events by answering the following questions.
1. The ventricles are in which stage of systole / diastole during this phase?
Ventricular isovolumetric contraction takes place during early systole.
2. What part of the ECG is associated with this phase?
The QRS complex.
3. Why does this phase begin at this portion of the ECG?
(Hint: show cardiomyocyte action potentials)
The cardiomyocytes are depolarizing, which initiates contraction.
4. What causes the AV valves to close during this phase?
The ventricular walls start contracting, which applies pressure to the blood in the chambers. The AV valves close when the pressure in the ventricles becomes higher than in the atria.
5. Where is this shown on the Wiggers diagram?
The AV valves close at the point where the ventricular and atrial pressures intersect.
6. Why do semilunar valves remain closed during this phase?
They remain closed during this phase because the ventricular pressure is less than the pressure in the major arteries.
7. What happens to ventricular volume during this phase? Why does this occur?
The volume remains relatively stable because all the valves are closed. The volume remains stable because all the valves are closed. These factors are the reason why the phase is called isovolumetric contraction.
8. Does ventricular pressure rise rapidly or slowly during this phase? Why?
The pressure rises rapidly because the closed heart valves block blood movement out of the chambers.
9. What is the cause of the heart sound during this phase?
The source of the heart sounds are vibrations produced by the closing AV valves.
10. What is the approximate duration of this phase?
This phase lasts 0.05 secs or 4% of the cycle.
Ventricular Ejection
Procedure: Click the ventricular ejection button to view a scan of this phase and the associated heart actions.
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Assessment: Analyze the phase events by answering the following questions.
1. The ventricles are in which stage of systole / diastole during this phase?
Ventricular ejection takes place during mid to late systole.
2. What part of the ECG is associated with this phase?
This phase runs from the QRS complex’s end to the T wave’s mid-portion.
3. How much movement occurs in the ventricular walls during this phase?
The ventricular walls fully contract.
4. What is the maximum ventricular pressure reached during this phase?
The peak ventricular pressure is about 120 mmHg.
5. Why does the pressure decrease after reaching a peak?
(Hint: show cardiomyocyte action potentials)
The pressure decreases when the cardiomyocytes start repolarizing and stop contracting.
6. Based on the ventricular volume graph, does the blood ejection rate appear uniform throughout this phase or divided into rapid and slow parts? Why does this occur?
The volume initially decreases rapidly and then slows. The rate of ejection slows due to the decrease in ventricular pressure.
7. What causes the semilunar valves to open?
The semilunar valves open when ventricular pressure exceeds the pressure in the major arteries.
8. How is the aortic pressure affected by the ventricular pressure?
The aortic pressure rises and falls in response to changes in the ventricular pressure.
9. Approximately how much blood is ejected (stroke volume) during this phase?
The amount of blood ejected is approximately 70 ml.
10. Approximately how much blood remains in the ventricle (end-systolic volume) at the end of this phase?
About 60 ml of blood remains in the ventricles.
11. What is the approximate duration of this phase?
This phase lasts about 0.22 secs or 28% of the cycle.
Isovolumetric Relaxation
Procedure: Click the ventricular isovolumetric relaxation button to view a scan of this phase and the associated heart actions.
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Assessment: Analyze the phase events by answering the following questions.
1. The ventricles are in which stage of systole / diastole during this phase?
The ventricles are in early diastole.
2. What part of the ECG is associated with this phase?
This phase takes place at the end of the T wave.
3. Why do the ventricular cardiomyocytes begin to relax during this phase?
(Hint: show cardiomyocyte action potentials)
More cardiomyocytes repolarize and enter a state of relaxation.
4. Does ventricular pressure decrease rapidly or slowly during this phase?
The ventricular pressure decreases rapidly.
5. Why do the AV valves remain closed during this phase?
The ventricular pressure remains higher than the atrial pressure during this short phase.
6. What causes the semilunar valves to close during this phase?
The ventricular pressures drop below pressures in the large arteries.
7. Where is this shown on the Wiggers diagram?
The semilunar valves close at the point where the ventricular and atrial pressures intersect.
8. What happens to ventricular volume during this phase? Why does this occur?
The volume remains relatively steady. With closed AV and semilunar valves, blood cannot flow in or out of the ventricles as the ventricles start to relax. These factors are the reason why the phase is called isovolumetric relaxation.
9. What is the cause of the heart sound during this phase?
The source of the heart sounds are vibrations produced by the closing semilunar valves.
10. What is the approximate duration of this phase?
It lasts about 0.08 secs or 10% of the cycle.
Passive Ventricular Filling
Procedure: Click the passive ventricular filling button to view a scan of this phase and the associated heart actions.
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Assessment: Analyze the phase events by answering the following questions.
1. In which stage of systole / diastole are the ventricles during this phase?
The ventricles are in the mid-portion of diastole.
2. What part of the ECG is associated with this phase?
This phase occurs along the flat (isoelectric) line between the end of the T wave and the mid-portion of the following P wave when the cardiomyocytes are at rest.
3. What does this indicate about the electrical activity of the heart?
(Hint: show cardiomyocyte action potentials)
The cardiomyocyte membranes are in a state of rest between action potentials.
4. At what point do the AV valves open? Why does this occur?
The AV valves open at the start of this phase when the ventricular pressures fall below the atrial pressures.
5.Does the rate at which the ventricles fill with blood (ventricular volume) appear uniform throughout the phase?
No. Initially, the volume increases rapidly as accumulated blood in the atria enters the ventricles. The rate slows as blood from the major veins passively flows through the atria into the ventricles.
6. What are the approximate ventricular and atrial pressures during most of this phase?
The pressures stay near 0 mmHg.
7. Why does the ventricular pressure not rise as the ventricular volume increases?
The pressure does not increase because the ventricles are fully relaxed and have space for the incoming blood.
8. Approximately what percentage of the total ventricle filling is added during this phase?
The ventricles add about 70% to 80% of their total filling during this phase.
9. What is the approximate duration of this phase?
This phase lasts about .35 secs or 44% of the cycle.
Atrial Systole (Active Ventricular Filling)
Procedure: Click the atrial systole button to view a scan of this phase and the associated heart actions.
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Assessment: Analyze the phase events by answering the following questions.
1. In which stage of systole / diastole are the ventricles during this phase?
The ventricles are in the late stage of diastole.
2. What part of the ECG is associated with this phase?
This phase takes place from the middle of the P wave to the middle of the QRS complex, the P-R segment.
3. What does this indicate about the electrical activity of the heart?
(Hint: show cardiomyocyte action potentials)
The atrial cardiomyocyte membranes depolarize to initiate contraction.
4. This phase adds approximately how much blood volume to the ventricles?
This phase adds 10% to 30% (15 ml to 30 m) to the total filling of the ventricles.
5. What is the total ventricular volume at the end of this phase (end-diastolic volume)?
The ventricles fill to about 120 ml to 130 ml.
6. How much does the ventricular pressure change due to the blood volume added by atrial contraction?
The ventricular pressure slightly increases because they have space for the added blood.
Why does the blood not flow back into the pulmonary veins and vena cava during this phase?
The blood does not flow back into the major veins because their pressures are higher than the atrial pressures.
8. What is the approximate duration of this phase?
This phase lasts about 0.1 secs or 14% of the cycle.
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References and Attributions
Anesthesia Key – Cardiac Cycle
Advances in Physiological Education (American Physiological Society) – Constructing the Wiggers diagram using core concepts: a classroom activity
Physiological Reviews (American Physiological Society) – Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior
N. I. H. National Library of Medicine – The Cardiac Cycle and the Physiological Basis of Left Ventricular Contraction, Ejection, Relaxation, and Filling
N. I. H. National Library of Medicine – Physiology, Cardiac Cycle
Researchgate – Transmembrane ionic currents underlying cardiac action potential in mammalian hearts
Rice University (OpenStax) – Cardiac Cycle
Science Direct – Isovolumetric Contraction
University of California Cardiovascular Imaging Lab – Cardiac Cycle
University of California Davis (LibreTexts) – Heart Sounds
University of Cape Town Clinical Research Centre – The Cardiac Cycle, Wiggers Diagram
University of British Columbia – Basic Physiology and Approach to Heart Sounds
University of Oslo – From the action potential to the ECG
University of Texas Medical Branch – Cardiac Cycle
University of Utah Medical School – HyperHeart