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Cardiac Cycle Simulation

A Guided Interactive Inquiry

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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.

Cardiac cycle and circulatory system animation

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.

Wiggers diagram phase durations

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.

Link to Animation
Terms of Use

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.

Link to Animation
Terms of Use

Assessment: Analyze the phase events by answering the following questions.

Early systole.

QRS.

The cardiomyocytes are depolarizing, which initiates contraction.

Very little.

Rapidly.

It stays about the same.

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.

They close when the ventricular pressure is greater than the atrial pressure

Where the ventricular and atrial pressure intersect.

Because the ventricular pressure is less than the pressure in the major arteries?

The sound is produced by vibrations created when the AV valves close.

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.

Link to Animation
Terms of Use

Assessment: Analyze the phase events by answering the following questions.

Mid to late systole.

End of the QRS complex and the start of the T wave.

Fully.

About 120 mmHg

The decreases when the cardiomyocytes start to repolarize and stop contracting.

It is divided in to rapid and slow parts. The rate of ejection slows due to the decrease in ventricular pressure.

The semilunar valves open when ventricular pressure exceeds the pressure in the major arteries.

The aortic pressure rises and falls in response to changes in the ventricular pressure.

About 70 ml.

About 60 ml.

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.

Early diastole.

It takes place at the end of the T wave.

The cardiomyocytes have repolarized, which initiates relaxation.

Very little.

Rapidly.

It stays about the same.

The ventricular pressure drops below pressure in the large arteries.

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.

The sound is produced by vibrations created when the AV valves close.

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.

Mid diastole

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.

The myocardiocytes are in a resting state between action potentials.

The AV valves open at the start of this phase when the ventricular pressures fall below the atrial pressures.

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.

The pressures stay near 0 mmHg.

The pressure doesn’t increase because the ventricles are fully relaxed and have space for the incoming blood.

About 70% to 80%

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.

They are in the late stage of diastole.

It takes place from the middle of the P wave to the middle of the QRS complex, the P-R segment.

The atrial myocardiocytes have depolarized to initiate contraction.

About 10% to 30% of the ventricular volume is added or 15 ml to 30 ml.

It is about 120 ml to 130 ml.

The ventricular pressure slightly increases.

The blood doesn’t flow back into the major veins their pressures are higher than the atrial pressures.

It lasts about 14% of the cycle or 0.1 secs.

Link to Animation
Terms of Use

Terms of Use

Human Bio Media materials are open-source and can be adapted and shared by anyone according to the Creative Commons Attribution 4.0 License guidelines.

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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

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