Cardiac Cycle Simulation
A Guided Interactive Inquiry
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 starting the presentation of the cardiac cycle, you may find it beneficial to first review the names and functions of the heart chambers, valves, and major vessels.
Click on a structure to reveal its name and function
Cardiac Cycle Events and Phases
The cardiac cycle consists of two main events (parts or periods), ventricular systole and ventricular diastole. Pacemaker cells in the heart walls regulate the timing and duration of these events.
- 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.
- 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 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 most resemble the list below.
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 ventricular isovolumetric contraction phase as a starting point because it marks the beginning of ventricular systole. Another commonly used starting point is the atrial systole phase. It is selected because it coincides with the P wave at the beginning of the electrocardiogram.
Heart Blood Flow Regulation
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 myocardiocytes (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.
The 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.
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.
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.
Aortic, Ventricular, and Atrial 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 chamber pressures, chamber volumes, and heart sounds.
Procedures and Assessments
For each phase, press the appropriate button to see a phase individually displayed along with the associated heart actions. Move the heart, at any time, to any area of the Wiggers diagram 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.
Assessment: Analyze the phase events by answering the following questions.
1. In which stage of systole/diastole are the ventricles during this phase?
Early systole.
2. What part of the ECG is associated with this phase?
QRS.
Why does this phase begin at this portion of the ECG? (Hint: show myocardiocyte action potentials)
The cardiomyocytes are depolarizing, which initiates contraction.
3. During this phase of contraction, do the ventricular walls move a lot or very little?
Very little.
4. Does ventricular pressure rise rapidly or slowly during this phase?
Rapidly.
5. What happens to ventricular volume during this phase?
It stays about the same.
6. Summarize the relationship between your answers to questions 3, 4, and 5.
During this phase, the ventricular walls begin to contract, which applies increasing pressure to the blood in the chambers. This occurs before any outward blood flow occurs, so the chamber volume doesn’t change. This is why the phase is called isovolumetric contraction.
7. What causes the AV valves to close during this phase?
They close when the ventricular pressure is greater than the atrial pressure
8. Where is this shown on the Wiggers diagram?
Where the ventricular and atrial pressure intersect.
9. Why do semilunar valves remain closed during this phase?
Because the ventricular pressure is less than the pressure in the major arteries?
10. What is the cause the heart sound during this phase?
The sound is produced by vibrations created when the AV valves close.
11. What is the approximate duration of this phase?
About 0.05 secs or 6% of the cycle.
Ventricular Ejection
Procedure: Click the ventricular ejection button to view a scan of this phase and the associated heart actions.
Assessment: Analyze the phase events by answering the following questions.
1. In which stage of systole/diastole are the ventricles during this phase?
Mid to late systole.
2. What part of the ECG is associated with this phase?
End of the QRS complex and the start of the T wave.
3. Do the ventricular walls appear to contract fully, partially, or very little?
Fully.
4. What is the maximum ventricular pressure reached during this phase?
About 120 mmHg
5. Why does the pressure decrease after reaching a peak? (Hint: show myocardiocyte action potentials)
The decreases when the cardiomyocytes start to repolarize and stop contracting.
6. Based on the ventricular volume graph, does the rate of blood ejection appear to be uniform throughout this phase or divided into rapid and slow parts? Why?
It is divided in to rapid and slow parts. The rate of ejection slows due to the decrease in ventricular pressure.
7. What causes the semilunar valves 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?
About 70 ml.
10. Approximately how much blood remains in the ventricle (end-systolic volume) at the end of this phase?
About 60 ml.
11. What is the approximate duration of this phase?
About 28% of the cycle or 0.22 secs.
Isovolumetric Relaxation
Procedure: Click the ventricular isovolumetric relaxation button to view a scan of this phase and the associated heart actions.
Assessment: Analyze the phase events by answering the following questions.
1. In which stage of systole/diastole are the ventricles during this phase?
Early diastole.
2. What part of the ECG is associated with this phase?
It takes place at the end of the T wave.
3. Why do the ventricular cardiomyocytes begin to relax during this phase? (Hint: show myocardiocyte action potentials)
The cardiomyocytes have repolarized, which initiates relaxation.
4. During this phase of relaxation, do the ventricular walls move a lot or very little?
Very little.
5. Does ventricular pressure decrease rapidly or slowly during this phase?
Rapidly.
6. What happens to ventricular volume during this phase?
It stays about the same.
7. What causes the semilunar valves to close during this phase?
The ventricular pressure drops below pressure in the large arteries.
8. Summarize the relationship between your answers to questions 4, 5, 6, and 7.
During this phase, the ventricular walls begin to relax, which applies decreasing pressure to the blood in the chambers. When both the AV and semilunar valves are closed, blood flow is prevented from flowing in or out of the chamber, and the volume doesn’t change. This is why the phase is called isovolumetric relaxation.
9. What is the cause the heart sound during this phase?
The sound is produced by vibrations created when the AV valves close.
What is the approximate duration of this phase?
It lasts about 10% of the cycle or about 0.08 secs.
Ventricular Filling
Procedure: Click the ventricular filling button to view a scan of this phase and the associated heart actions.
Assessment: Analyze the phase events by answering the following questions.
1. In which stage of systole (or diastole) are the ventricles during this phase?
Mid diastole
2. What part of the ECG is associated with this phase?
It takes place along the flat (isoelectric) line that runs between the end of the T wave and the following P wave when the myocardiocytes are in a resting state.
3. What does this indicate about the electrical activity of the heart? (Hint show myocardiocyte action potentials)
The myocardiocytes are in a resting state between action potentials.
4. At what point do the AV valves open? Why?
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 to be uniform throughout the phase or divided into fast and slow parts?
Initially, the volume increases rapidly as accumulated blood in the atria flows into the ventricles. The rate then slows as blood from the major veins enters the ventricles via the atria.
6. What are the 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 doesn’t 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?
About 70% to 80%
9. What is the approximate duration of this phase?
This phase lasts about 44% of the cycle or about .35 secs
Atrial Systole
Procedure: Click the atrial systole button to view a scan of this phase and the associated heart actions.
Assessment: Analyze the phase events by answering the following questions.
2. In which stage of systole/diastole are the ventricles during this phase?
They are in the late stage of diastole.
3. What part of the ECG is associated with this phase?
It takes place from the middle of the P wave to the middle of the QRS complex, the P-R segment.
4. What does this indicate about the electrical activity of the heart? (Hint show myocardiocyte action potentials)
The atrial myocardiocytes have depolarized to initiate contraction.
5. What does it indicate about the electrical activity of the heart?
6. Approximately how much blood volume is added to the ventricles during this phase?
About 10% to 30% of the ventricular volume is added or 15 ml to 30 ml.
7. What is the total ventricular volume at the end of this phase (end-diastolic volume)?
It is about 120 ml to 130 ml.
8. How much does the ventricular pressure change due to the blood volume added by atrial contraction?
The ventricular pressure slightly increases.
9. Why does the blood not flow back into the pulmonary veins and vena cava during this phase?
The blood doesn’t flow back into the major veins their pressures are higher than the atrial pressures.
10. What is the approximate duration of this phase?
It lasts about 14% of the cycle or 0.1 secs.
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References and Attributions
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 Oslo – From the action potential to the ECG
University of Texas Medical Branch – Cardiac Cycle
