Cardiac Cycle & Electrical Activity of Heart

Cardiovascular System • Cardiac Cycle • Heart Sounds • Action Potential in Cardiac Tissue • Electrical Activity of Heart

CSIR-NET • GATE • DBT • ICMR • MSc Biotechnology

Study Tip: To understand cardiac physiology, connect the concepts in this sequence: electrical activity → action potential → excitation-contraction coupling → atrial systole → ventricular systole → ventricular diastole → valve movement → blood flow → heart sounds → cardiac output.

Do not study the cardiac cycle and electrical activity as separate topics. The electrical events initiate the mechanical events of the heart. The mechanical events then produce changes in pressure, valve movement, blood flow and heart sounds.

📚 Table of Contents / Index

Introduction to the Cardiovascular System
Basic Organization of the Heart
Pathway of Blood Through the Heart
Cardiac Cycle
Atrial Systole
Ventricular Systole
Isovolumetric Contraction
Ventricular Ejection
Ventricular Diastole
Isovolumetric Relaxation
Ventricular Filling
Cardiac Volumes and Cardiac Output
Heart Sounds
Electrical Activity of the Heart
Cardiac Conduction System
SA Node
AV Node
Bundle of His and Purkinje Fibres
Action Potential in Cardiac Tissue
Action Potential of Ventricular Contractile Cells
Pacemaker Action Potential
Electrical Activity and ECG
Autonomic Regulation of the Heart
Important Comparisons
Quick Revision Notes
10 MCQs with Hidden Answers
Final Exam-Oriented Summary

1. Introduction to the Cardiovascular System

The cardiovascular system is a transport system responsible for the continuous movement of blood throughout the body. It consists mainly of the heart, blood and blood vessels. The heart functions as a muscular pump, while arteries, veins and capillaries provide the vascular pathway through which blood circulates.

The cardiovascular system is essential for maintaining homeostasis. Blood transports oxygen from the respiratory system to tissues and carries carbon dioxide from tissues toward the lungs. It also transports nutrients, hormones, metabolic waste products, electrolytes and other substances.

Major functions of the cardiovascular system

Transport of oxygen: Hemoglobin-containing erythrocytes transport oxygen from the lungs to peripheral tissues.
Transport of carbon dioxide: Carbon dioxide produced by cellular metabolism is transported toward the lungs for elimination.
Nutrient transport: Glucose, amino acids, lipids, vitamins and other nutrients are transported through the blood.
Hormonal transport: Endocrine hormones are transported from endocrine glands to their target tissues.
Waste removal: Metabolic products are transported toward organs such as the kidneys, liver and lungs.
Temperature regulation: Blood distribution contributes to the regulation of body temperature.
Protection: Blood participates in immune defence and hemostasis.
Homeostasis: The cardiovascular system helps maintain appropriate pH, fluid balance, electrolyte distribution and tissue perfusion.

⭐ Key Point

The heart is not simply a pump. It is an electrically controlled muscular organ in which electrical excitation precedes and coordinates mechanical contraction.

2. Basic Organization of the Heart

The human heart is a muscular organ located in the thoracic cavity between the lungs. It is divided into four chambers: right atrium, right ventricle, left atrium and left ventricle.

Right Atrium

Receives deoxygenated blood from the systemic circulation through the superior and inferior vena cava.

Right Ventricle

Pumps deoxygenated blood toward the lungs through the pulmonary artery.

Left Atrium

Receives oxygenated blood from the lungs through the pulmonary veins.

Left Ventricle

Pumps oxygenated blood into the systemic circulation through the aorta.

Heart valves

Valves maintain unidirectional blood flow through the heart. Their opening and closing are primarily determined by pressure differences across the valves.

Valve	Location	Main function
Tricuspid valve	Right atrium → right ventricle	Prevents backflow from right ventricle into right atrium during ventricular systole.
Pulmonary semilunar valve	Right ventricle → pulmonary artery	Prevents backflow from pulmonary artery into right ventricle.
Mitral / bicuspid valve	Left atrium → left ventricle	Prevents backflow from left ventricle into left atrium during ventricular systole.
Aortic semilunar valve	Left ventricle → aorta	Prevents backflow from aorta into left ventricle.

Major divisions of circulation

Pulmonary circulation: Carries blood between the heart and lungs.
Systemic circulation: Carries blood between the heart and the rest of the body.
Coronary circulation: Supplies oxygen and nutrients to the cardiac muscle itself.

3. Pathway of Blood Through the Heart

Blood moves through the heart in a highly organized sequence. The right side of the heart primarily handles deoxygenated blood, whereas the left side handles oxygenated blood.

Body

→

Venae Cavae

→

Right Atrium

→

Right Ventricle

→

Pulmonary Artery

→

Lungs

→

Pulmonary Veins

→

Left Atrium

→

Left Ventricle

→

Aorta

→

Body

Exam Point:

The pulmonary artery carries deoxygenated blood, whereas the pulmonary veins carry oxygenated blood. Therefore, arteries and veins are defined according to the direction of blood flow relative to the heart, not according to oxygen concentration.

4. Cardiac Cycle

The cardiac cycle refers to the complete sequence of electrical, mechanical and pressure events occurring in the heart during one heartbeat. It includes atrial contraction, ventricular contraction and relaxation.

At a heart rate of approximately 75 beats per minute, one complete cardiac cycle lasts about 0.8 second.

Main phases of the cardiac cycle

Atrial systole
Ventricular systole
Ventricular diastole
Isovolumetric contraction
Ventricular ejection
Isovolumetric relaxation
Rapid ventricular filling
Reduced filling / diastasis

5. Atrial Systole

Atrial systole is the contraction of the atria. It occurs after the electrical excitation of the atrial myocardium and contributes to the final filling of the ventricles.

Events during atrial systole

The atrial myocardium contracts.
Blood is pushed from the atria into the ventricles.
The atrioventricular valves are open.
The semilunar valves remain closed.
Ventricular end-diastolic volume is established.

Atrial contraction contributes an additional amount of ventricular filling, sometimes called the atrial kick. This contribution becomes particularly important when ventricular filling is impaired.

Key Point: Atrial systole occurs before ventricular systole because atrial electrical activation occurs before ventricular electrical activation.

6. Ventricular Systole

Ventricular systole is the period during which the ventricles contract. It follows ventricular depolarization and produces a rise in ventricular pressure.

Ventricular systole can be divided into two important mechanical phases: isovolumetric contraction and ventricular ejection.

Major events

Ventricular myocardium depolarizes.
Ventricular muscle contracts.
Ventricular pressure rises.
Atrioventricular valves close.
Semilunar valves eventually open when ventricular pressure exceeds arterial pressure.
Blood is ejected into the pulmonary artery and aorta.

7. Isovolumetric Contraction

Isovolumetric contraction is the initial phase of ventricular systole. During this phase, the ventricles contract and ventricular pressure increases rapidly.

The term "isovolumetric" means that ventricular volume remains constant during this short interval.

Why does ventricular volume remain constant?

At this stage, both the atrioventricular valves and semilunar valves are closed. Since no blood enters or leaves the ventricles, ventricular volume remains unchanged even though pressure rises sharply.

Parameter	During isovolumetric contraction
Ventricular contraction	Present
Ventricular pressure	Rapidly increasing
AV valves	Closed
Semilunar valves	Closed
Blood ejection	Absent
Ventricular volume	Constant

Exam Point: Isovolumetric contraction is associated with closure of the atrioventricular valves and contributes to the beginning of the first heart sound, S1.

8. Ventricular Ejection

When ventricular pressure becomes greater than the pressure in the pulmonary artery or aorta, the semilunar valves open and ventricular ejection begins.

Phases of ejection

Rapid ejection: A large fraction of stroke volume is ejected during the early part of ventricular ejection.
Reduced ejection: The rate of blood ejection decreases as ventricular contraction progresses and ventricular repolarization begins.

Right and left ventricular ejection

The right ventricle ejects blood into the pulmonary circulation through the pulmonary artery. The left ventricle ejects blood into the systemic circulation through the aorta.

Important:

The left ventricle normally generates a much higher pressure than the right ventricle because it pumps blood through the systemic circulation, which has substantially greater vascular resistance than the pulmonary circulation.

9. Ventricular Diastole

Ventricular diastole is the period during which the ventricles relax and refill with blood. It is essential because the myocardium requires relaxation between contractions and the ventricles must refill before the next systolic event.

Major events during diastole

Ventricular muscle relaxes.
Ventricular pressure decreases.
Semilunar valves close.
AV valves eventually open.
Blood flows from the atria into the ventricles.
Ventricular volume increases.

Ventricular filling occurs partly passively and is completed by atrial systole.

10. Isovolumetric Relaxation

Isovolumetric relaxation occurs immediately after ventricular ejection. During this period, ventricular muscle relaxes rapidly and ventricular pressure falls.

The semilunar valves have closed, while the atrioventricular valves have not yet opened. Consequently, all four cardiac valves are closed during this short phase.

Parameter	Isovolumetric relaxation
Ventricular muscle	Relaxing
Ventricular pressure	Rapidly decreasing
AV valves	Closed
Semilunar valves	Closed
Ventricular volume	Constant

Key Point: The ventricular volume remains constant during isovolumetric relaxation because all valves are closed.

11. Ventricular Filling

Once ventricular pressure falls below atrial pressure, the atrioventricular valves open. Blood then flows from the atria into the ventricles.

Stages of ventricular filling

Rapid filling: Blood enters the relaxed ventricle rapidly.
Reduced filling / diastasis: The rate of ventricular filling decreases.
Atrial systole: Atrial contraction provides additional ventricular filling.

At the end of ventricular filling, the ventricular volume reaches the end-diastolic volume.

Remember the sequence:

Ventricular relaxation → ventricular pressure falls → AV valves open → ventricular filling → atrial systole → end-diastolic volume

12. Cardiac Volumes and Cardiac Output

End-Diastolic Volume (EDV)

End-diastolic volume is the volume of blood present in a ventricle at the end of ventricular filling, immediately before ventricular systole.

A typical resting ventricular EDV is approximately 120 mL, although actual values vary with body size, physiological state and measurement method.

End-Systolic Volume (ESV)

End-systolic volume is the volume of blood remaining in the ventricle after ventricular ejection.

A typical resting ESV may be around 50 mL.

Stroke Volume

Stroke volume is the amount of blood ejected by one ventricle during one cardiac cycle.

Stroke Volume = EDV − ESV

Example:
If EDV = 120 mL and ESV = 50 mL:
SV = 120 − 50 = 70 mL

Cardiac Output

Cardiac output is the volume of blood pumped by one ventricle per minute.

Cardiac Output = Heart Rate × Stroke Volume

Example:
Heart rate = 75 beats/min
Stroke volume = 70 mL/beat
Cardiac output ≈ 5250 mL/min = 5.25 L/min

Ejection Fraction

Ejection fraction represents the fraction of ventricular end-diastolic volume that is ejected during systole.

Ejection Fraction = (Stroke Volume / End-Diastolic Volume) × 100

13. Heart Sounds

Heart sounds are produced mainly by vibrations associated with cardiac structures and blood flow during valve closure and other mechanical events. The two major sounds heard during routine cardiac auscultation are S1 and S2.

S1 – First Heart Sound

S1 is associated primarily with closure of the atrioventricular valves at the beginning of ventricular systole.

Occurs near the beginning of ventricular systole.
Associated mainly with mitral and tricuspid valve closure.
Marks the transition from ventricular filling toward ventricular contraction.
Often described acoustically as "lub".

S2 – Second Heart Sound

S2 is associated primarily with closure of the semilunar valves at the end of ventricular systole and beginning of ventricular diastole.

Associated with aortic and pulmonary valve closure.
Marks the end of ventricular ejection.
Occurs near the beginning of ventricular diastole.
Often described acoustically as "dub".

S3 and S4

Additional heart sounds may sometimes be detected. S3 is associated with rapid ventricular filling, whereas S4 is associated with atrial contraction against a relatively stiff ventricle. Their interpretation depends on age, physiological context and clinical circumstances.

Sound	Main association	Timing
S1	AV valve closure	Beginning of ventricular systole
S2	Semilunar valve closure	Beginning of ventricular diastole
S3	Rapid ventricular filling	Early diastole
S4	Atrial contraction against a stiff ventricle	Late diastole

Exam Shortcut:

S1 = AV valves close = ventricular systole begins
S2 = semilunar valves close = ventricular diastole begins

14. Electrical Activity of the Heart

The heart has an intrinsic electrical system capable of generating and conducting electrical impulses. These electrical signals coordinate contraction of the atria and ventricles.

Cardiac electrical activity begins normally in specialized pacemaker cells of the sinoatrial node. The impulse then travels through the atria, reaches the atrioventricular node and proceeds through the ventricular conduction system.

Importance of electrical activity

Initiates cardiac muscle contraction.
Coordinates atrial contraction.
Coordinates ventricular contraction.
Ensures an appropriate sequence of chamber activation.
Allows efficient pumping of blood.
Creates electrical signals that can be detected using an ECG.

15. Cardiac Conduction System

The cardiac conduction system is composed of specialized cardiac cells that generate and conduct electrical impulses.

SA Node

→

Atrial Myocardium

→

AV Node

→

AV Bundle

→

Bundle Branches

→

Purkinje Fibres

→

Ventricular Myocardium

Major components

Sinoatrial node: Primary pacemaker of the normal heart.
Atrioventricular node: Receives the atrial impulse and delays conduction before ventricular activation.
Atrioventricular bundle: Conducts the impulse from the AV node into the ventricular conduction system.
Right and left bundle branches: Conduct electrical excitation toward the respective ventricles.
Purkinje fibres: Rapidly distribute the impulse throughout ventricular myocardium.

16. Sinoatrial Node (SA Node)

The sinoatrial node is a specialized group of cardiac cells located in the right atrium near the entry of the superior vena cava. It normally acts as the primary pacemaker of the heart.

Why is the SA node called the pacemaker?

SA nodal cells have spontaneous electrical activity. Their membrane potential gradually depolarizes during diastole until threshold is reached and another action potential is generated.

Generates spontaneous rhythmic electrical impulses.
Normally determines the heart rhythm.
Initiates atrial depolarization.
Sets the timing of the cardiac cycle under normal conditions.
Its activity is influenced by the autonomic nervous system.

Exam Point: The SA node is the normal pacemaker of the human heart.

17. Atrioventricular Node (AV Node)

The atrioventricular node is located in the lower part of the right atrium near the atrioventricular junction.

Functions of AV node

Receives the electrical impulse from the atria.
Conducts the impulse toward the ventricular conduction system.
Introduces a physiological delay in conduction.
Allows time for atrial contraction and ventricular filling before ventricular contraction.

Why is AV nodal delay important?

If the ventricles contracted immediately after atrial depolarization, atrial contraction would have less time to contribute to ventricular filling. The AV nodal delay helps preserve the proper atrium-to- ventricle sequence.

18. Bundle of His, Bundle Branches and Purkinje Fibres

Bundle of His

The atrioventricular bundle, commonly called the bundle of His, conducts the electrical impulse from the AV node into the ventricular conduction system.

Bundle branches

The AV bundle divides into right and left bundle branches that conduct electrical activity toward the corresponding ventricles.

Purkinje fibres

Purkinje fibres are specialized conducting fibres that distribute the electrical impulse rapidly through the ventricular myocardium.

Allow rapid spread of ventricular excitation.
Promote coordinated ventricular contraction.
Help ensure that ventricular myocardium contracts in an efficient sequence.

19. Action Potential in Cardiac Tissue

An action potential is a rapid change in membrane potential produced by changes in ion permeability across the cell membrane. Cardiac action potentials differ from typical neuronal action potentials because cardiac muscle cells can have a prolonged depolarization or plateau phase.

This prolonged action potential is particularly important in cardiac contractile cells because it contributes to a long refractory period. The long refractory period helps prevent sustained tetanic contraction of the heart.

Central concept:

The heart must contract rhythmically rather than remaining in a continuously contracted state. The prolonged cardiac action potential and refractory period are important physiological mechanisms that help prevent tetanus.

Major phases of ventricular contractile-cell action potential

Phase	Name	Main ionic events
Phase 4	Resting membrane potential	Stable resting potential in ventricular contractile cells, maintained by ionic gradients and membrane conductances.
Phase 0	Rapid depolarization	Rapid opening of voltage-gated fast Na⁺ channels and Na⁺ influx.
Phase 1	Initial repolarization	Na⁺ channels inactivate and transient outward K⁺ current contributes to early repolarization.
Phase 2	Plateau	Ca²⁺ entry through L-type Ca²⁺ channels is balanced substantially by outward K⁺ currents.
Phase 3	Repolarization	Ca²⁺ channels inactivate while K⁺ efflux becomes dominant.

20. Action Potential of Ventricular Contractile Cells

Ventricular contractile cells have a characteristic action potential with five conventionally numbered phases: 0, 1, 2, 3 and 4.

Phase 0 – Rapid depolarization

Fast voltage-gated Na⁺ channels open.
Na⁺ rapidly enters the cell.
Membrane potential becomes rapidly more positive.
This phase produces rapid depolarization.

Phase 1 – Initial repolarization

Fast Na⁺ channels become inactivated.
Transient outward K⁺ current contributes to repolarization.
The membrane potential briefly becomes less positive.

Phase 2 – Plateau

The plateau phase is one of the most characteristic features of the ventricular cardiac action potential.

L-type Ca²⁺ channels remain open.
Ca²⁺ enters the cell.
Outward K⁺ currents also occur.
The balance between inward Ca²⁺ and outward currents prolongs depolarization.
Ca²⁺ entry contributes to excitation-contraction coupling.

Phase 3 – Repolarization

Ca²⁺ channels progressively inactivate.
K⁺ outward current becomes dominant.
Membrane potential returns toward the resting level.

Phase 4 – Resting membrane potential

Ventricular contractile cells remain at a relatively stable resting membrane potential between action potentials.

21. Pacemaker Action Potential

Pacemaker cells such as those of the SA node differ from ventricular contractile cells. They do not maintain a stable resting membrane potential in the same way. Instead, they undergo spontaneous diastolic depolarization.

Important features

No stable resting membrane potential equivalent to ventricular contractile cells.
Gradual spontaneous depolarization during diastole.
Threshold is eventually reached.
Action potential is generated.
Pacemaker activity determines cardiac rhythm under normal conditions.

Phase 4 – Pacemaker potential

The membrane potential gradually becomes less negative during diastole. This spontaneous depolarization brings the cell toward threshold.

Phase 0 – Depolarization

In SA nodal cells, the upstroke of the action potential depends mainly on Ca²⁺ entry through voltage-gated calcium channels rather than the fast Na⁺-dependent upstroke characteristic of ventricular contractile cells.

Phase 3 – Repolarization

Repolarization is mainly associated with increased K⁺ conductance and outward K⁺ current.

Exam Difference:

Ventricular contractile cell: phases 0, 1, 2, 3 and 4 with a prominent plateau.

SA nodal pacemaker cell: spontaneous phase-4 depolarization and no prominent phase-2 plateau like the ventricular contractile-cell action potential.

22. Electrical Activity and Electrocardiogram (ECG)

The electrocardiogram, or ECG, records electrical activity associated with the heart from the body surface. It does not directly record mechanical contraction. Instead, it reflects electrical events associated with cardiac depolarization and repolarization.

Major ECG components

ECG component	Represents
P wave	Atrial depolarization.
QRS complex	Ventricular depolarization.
T wave	Ventricular repolarization.

Very Important:

The P wave represents atrial depolarization, not atrial contraction itself. Similarly, the QRS complex represents ventricular depolarization, while the subsequent mechanical contraction follows the electrical event.

23. Autonomic Regulation of the Heart

Although the heart possesses intrinsic rhythmicity, its activity is continuously influenced by the autonomic nervous system.

Sympathetic stimulation

Generally increases heart rate.
Increases cardiac contractility.
Increases conduction through the AV node.
Helps increase cardiac output during physiological stress or exercise.

Parasympathetic stimulation

Generally decreases heart rate.
Strongly influences the SA and AV nodes.
Slows AV nodal conduction.
Helps reduce cardiac activity during resting conditions.

Feature	Sympathetic	Parasympathetic
Heart rate	Generally increases	Generally decreases
Contractility	Increases	Limited direct ventricular effect compared with sympathetic input
AV conduction	Generally increases	Generally decreases
Main neurotransmitter at cardiac targets	Norepinephrine	Acetylcholine

24. Important Comparisons for Competitive Exams

Concept	Key Feature
Systole	Period of cardiac contraction, especially ventricular contraction when discussing ventricular systole.
Diastole	Period of cardiac relaxation and ventricular filling.
Atrial systole	Contraction of atria that contributes to ventricular filling.
Ventricular systole	Ventricular contraction and blood ejection.
Isovolumetric contraction	All valves closed; ventricular pressure rises; volume remains constant.
Isovolumetric relaxation	All valves closed; ventricular pressure falls; volume remains constant.
S1	Associated mainly with AV valve closure.
S2	Associated mainly with semilunar valve closure.
SA node	Normal primary pacemaker.
AV node	Provides important conduction delay.
Purkinje fibres	Rapid distribution of ventricular excitation.
P wave	Atrial depolarization.
QRS complex	Ventricular depolarization.
T wave	Ventricular repolarization.
Cardiac output	Heart rate × stroke volume.
Stroke volume	EDV − ESV.
Phase 0 ventricular AP	Rapid Na⁺-dependent depolarization.
Phase 2 ventricular AP	Plateau involving Ca²⁺ entry and K⁺ currents.
Phase 3 ventricular AP	Repolarization dominated by outward K⁺ currents.

25. Quick Revision Notes

⭐ Must-Remember Points

The cardiovascular system consists mainly of the heart, blood and blood vessels.
The heart has four chambers: right atrium, right ventricle, left atrium and left ventricle.
The right side primarily pumps deoxygenated blood toward the lungs.
The left side primarily pumps oxygenated blood toward the systemic circulation.
The cardiac cycle is the complete sequence of events during one heartbeat.
At approximately 75 beats/min, one cardiac cycle lasts about 0.8 second.
Atrial systole contributes to final ventricular filling.
Ventricular systole includes isovolumetric contraction and ejection.
During isovolumetric contraction, all four valves are closed.
During ventricular ejection, semilunar valves are open.
During isovolumetric relaxation, all four valves are again closed.
Ventricular filling occurs after AV valves open.
EDV is the ventricular volume at the end of diastolic filling.
ESV is the ventricular volume remaining after systolic ejection.
Stroke volume = EDV − ESV.
Cardiac output = heart rate × stroke volume.
S1 is associated mainly with AV valve closure.
S2 is associated mainly with semilunar valve closure.
The SA node is the normal pacemaker.
The AV node introduces a conduction delay.
The AV bundle conducts the impulse into the ventricular conduction system.
Purkinje fibres rapidly distribute excitation through ventricular myocardium.
The P wave represents atrial depolarization.
The QRS complex represents ventricular depolarization.
The T wave represents ventricular repolarization.
Ventricular contractile-cell action potentials have phases 0, 1, 2, 3 and 4.
Phase 0 is rapid Na⁺-dependent depolarization.
Phase 2 is the characteristic plateau phase.
Ca²⁺ entry during the plateau contributes to excitation-contraction coupling.
Phase 3 is mainly associated with K⁺-mediated repolarization.
SA nodal cells show spontaneous phase-4 depolarization.
The long cardiac refractory period helps prevent tetanic contraction.
Electrical excitation precedes mechanical contraction.

27. Final Exam-Oriented Summary

The cardiac cycle represents the coordinated mechanical events of the heart, while electrical activity provides the timing signal that initiates and coordinates these mechanical events.

SA node: normal pacemaker.
AV node: provides important conduction delay.
AV bundle: carries excitation into the ventricular conduction system.
Purkinje fibres: rapidly distribute ventricular excitation.
Atrial systole: contributes to ventricular filling.
Ventricular systole: consists mainly of isovolumetric contraction followed by ejection.
Isovolumetric contraction: all valves closed and ventricular pressure rises.
Ventricular ejection: semilunar valves are open.
Isovolumetric relaxation: all valves closed and ventricular pressure falls.
Ventricular filling: occurs when AV valves open.
S1: associated mainly with AV valve closure.
S2: associated mainly with semilunar valve closure.
P wave: atrial depolarization.
QRS complex: ventricular depolarization.
T wave: ventricular repolarization.
Stroke volume: EDV − ESV.
Cardiac output: heart rate × stroke volume.
Ventricular action potential: includes a prolonged plateau phase.
Phase 0: rapid Na⁺-dependent depolarization.
Phase 2: Ca²⁺-dependent plateau with opposing outward currents.
Phase 3: repolarization, largely due to outward K⁺ currents.
Pacemaker cells: show spontaneous diastolic depolarization.

For CSIR-NET, GATE, DBT-BET, ICMR-JRF and MSc examinations, the most important strategy is to connect the electrical and mechanical events. Remember the sequence:

SA node activation → atrial depolarization → atrial systole → AV nodal delay → ventricular depolarization → ventricular systole → ejection → ventricular repolarization → ventricular relaxation → filling

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Sunday, 23 August 2026