Electrical Activity of Heart: Complete Notes

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

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

Study Tip: The cardiovascular system should be studied as an integrated sequence: blood → blood groups → vessels → heart → electrical impulse → cardiac cycle → heart sounds → blood circulation . Understanding this sequence makes physiology-based questions much easier than memorizing isolated facts.

1. Introduction to Blood and Cardiovascular Physiology

The cardiovascular system is one of the major transport systems of the human body. It consists mainly of the heart, blood and blood vessels. The heart acts as a muscular pump, while blood vessels provide the pathways through which blood travels. Blood transports oxygen, carbon dioxide, nutrients, hormones, metabolic waste products and many other substances.

Cardiovascular physiology is closely connected with electrical activity. The heart is not simply a mechanical pump. Its pumping activity depends on the generation and propagation of electrical impulses through specialized cardiac tissues. These electrical events trigger contraction of cardiac muscle, producing coordinated pumping of blood.

The heart therefore demonstrates an important relationship between electrical and mechanical events. Electrical excitation occurs first, followed by mechanical contraction. Electrical relaxation and recovery are associated with subsequent relaxation of cardiac muscle.

Major functions of the cardiovascular system

Transport: Movement of oxygen, carbon dioxide, nutrients, hormones and waste products.
Protection: Blood components participate in immunity and hemostasis.
Regulation: Helps maintain temperature, pH, fluid balance and homeostasis.
Communication: Hormones and signaling molecules are transported through blood.
Perfusion: Maintains adequate blood supply to tissues and organs.

⭐ Central Concept

The heart functions through a coordinated sequence: electrical excitation → cardiac muscle contraction → blood ejection → electrical recovery → relaxation → filling .

2. Hemostasis

Hemostasis is the physiological process that prevents excessive blood loss after vascular injury while maintaining blood in a fluid state within intact vessels. It is a carefully regulated process involving blood vessels, platelets, plasma proteins and regulatory mechanisms.

Hemostasis should not be confused with uncontrolled thrombosis. Normal hemostasis is localized and appropriately regulated, whereas pathological clot formation can obstruct blood vessels and impair tissue perfusion.

Major stages of hemostasis

Vascular response
Primary hemostasis and platelet plug formation
Secondary hemostasis and coagulation
Clot stabilization
Clot retraction and repair
Fibrinolysis and removal of the clot

1. Vascular response

Immediately after vascular injury, the injured blood vessel undergoes vasoconstriction. This reduces local blood flow and helps limit blood loss. Several mechanisms contribute to vascular constriction, including local smooth muscle responses and mediators released during platelet activation.

Reduces blood flow at the site of injury.
Helps limit initial blood loss.
Provides time for platelet and coagulation mechanisms to act.

2. Primary hemostasis

Primary hemostasis is mainly associated with platelet adhesion, activation and aggregation. When the vascular endothelium is damaged, structures beneath the endothelium become exposed. Platelets can adhere to these exposed surfaces.

Platelet adhesion

von Willebrand factor, commonly abbreviated as vWF, plays an important role in platelet adhesion at sites of vascular injury. It helps connect platelets with exposed subendothelial structures.

Platelet activation

After adhesion, platelets undergo activation. They change shape and release or generate mediators that promote recruitment and activation of additional platelets.

Platelets change from a resting disc-like form to an activated form.
Activated platelets expose binding sites that support aggregation.
Platelet-derived mediators promote further platelet recruitment.
A temporary platelet plug is formed.

3. Secondary hemostasis

Secondary hemostasis involves the coagulation cascade. Plasma coagulation factors participate in a series of enzymatic reactions that ultimately generate thrombin. Thrombin converts soluble fibrinogen into insoluble fibrin strands.

Fibrin forms a mesh that reinforces the platelet plug and produces a more stable hemostatic clot.

4. Fibrinolysis

Once tissue repair progresses and the clot is no longer required, the fibrinolytic system contributes to clot removal. Plasmin is the major enzyme responsible for degradation of fibrin.

Exam Point: Primary hemostasis is mainly associated with platelet plug formation, whereas secondary hemostasis strengthens the plug through fibrin formation.

3. Blood Groups

Blood groups are classifications of blood based mainly on inherited antigens present on the surface of red blood cells and corresponding antibodies found in plasma. Blood group systems are important in blood transfusion, transplantation, pregnancy-related medicine and immunohematology.

The two blood group systems most commonly discussed in basic physiology and medical biology are the ABO system and the Rh system.

Why are blood groups important?

They determine compatibility during blood transfusion.
They are genetically determined.
They are useful in immunohematology.
They are relevant to maternal-fetal blood group incompatibility.
They are used in blood banking and transfusion medicine.

4. ABO Blood Group System

The ABO blood group system is based primarily on the presence or absence of A and B antigens on red blood cells. The corresponding naturally occurring antibodies are present in plasma.

Blood Group	RBC Antigen	Major Plasma Antibody
A	A antigen	Anti-B
B	B antigen	Anti-A
AB	A and B antigens	Neither anti-A nor anti-B
O	Neither A nor B antigen	Anti-A and anti-B

Genetic basis

The ABO locus has three commonly described alleles: I^A, I^B and i. I^A and I^B are codominant, whereas i is recessive to both.

Genotype	Phenotype
I^AI^A	A
I^Ai	A
I^BI^B	B
I^Bi	B
I^AI^B	AB
ii	O

Important: The AB phenotype is a classic example of codominance because both I^A and I^B alleles contribute to the phenotype.

5. Rh Blood Group System

The Rh blood group system is another clinically important blood group system. The D antigen is particularly important in routine transfusion terminology. Individuals who have the D antigen are commonly described as Rh-positive, while those lacking it are described as Rh-negative.

Rh-positive and Rh-negative

Rh-positive: D antigen is present on RBCs.
Rh-negative: D antigen is absent from RBCs.
The Rh system is genetically distinct from the ABO system.
Rh status is important during transfusion and pregnancy.

Rh incompatibility

Rh incompatibility can become clinically important when an Rh-negative mother is exposed to Rh-positive fetal red-cell antigen. Exposure may occur during pregnancy or delivery and can lead to maternal sensitization. In a subsequent pregnancy with an Rh-positive fetus, maternal antibodies can potentially affect fetal red blood cells.

Preventive anti-D immunoglobulin is used clinically in appropriate situations to reduce the risk of maternal sensitization. The exact clinical management depends on obstetric and transfusion protocols.

Exam Point: ABO antibodies are naturally occurring in individuals who lack the corresponding antigen, whereas Rh antibodies, particularly anti-D, generally develop following exposure to the antigen.

6. Cardiovascular System

The cardiovascular system consists of the heart and blood vessels. Blood vessels can be broadly classified into arteries, veins and capillaries. The heart provides the pressure required to move blood through the vascular system.

Major components

Heart

Muscular organ that generates pressure and maintains blood circulation.

Arteries

Carry blood away from the heart. Most systemic arteries carry oxygenated blood.

Veins

Carry blood toward the heart. Most systemic veins carry deoxygenated blood.

Capillaries

Small vessels where exchange of gases, nutrients and waste products occurs.

Pulmonary and systemic circulation

The cardiovascular system can be considered as two major circuits: pulmonary circulation and systemic circulation.

Pulmonary circulation

Pulmonary circulation carries deoxygenated blood from the right side of the heart to the lungs and returns oxygenated blood to the left side of the heart.

Systemic circulation

Systemic circulation carries oxygenated blood from the left side of the heart to body tissues and returns deoxygenated blood to the right side of the heart.

7. Anatomy of the Heart

The human heart is a four-chambered muscular organ. The chambers are divided into two atria and two ventricles. The right and left sides are separated by septa, preventing direct mixing of oxygenated and deoxygenated blood under normal conditions.

Four chambers

Right atrium: Receives deoxygenated blood from systemic veins.
Right ventricle: Pumps deoxygenated blood toward the lungs.
Left atrium: Receives oxygenated blood from pulmonary veins.
Left ventricle: Pumps oxygenated blood into systemic circulation.

Major valves

Valve	Location	Major function
Tricuspid valve	Right atrium → right ventricle	Prevents backflow into right atrium during ventricular contraction.
Mitral/Bicuspid valve	Left atrium → left ventricle	Prevents backflow into left atrium during ventricular contraction.
Pulmonary semilunar valve	Right ventricle → pulmonary trunk	Prevents backflow into right ventricle.
Aortic semilunar valve	Left ventricle → aorta	Prevents backflow into left ventricle.

8. Electrical Conduction System of the Heart

The heart contains specialized cardiac cells capable of generating and conducting electrical impulses. These specialized structures coordinate atrial and ventricular activity.

Major components of the conduction system

Sinoatrial node (SA node)
Atrioventricular node (AV node)
Atrioventricular bundle (Bundle of His)
Right and left bundle branches
Purkinje fibers

Sinoatrial node

The sinoatrial node is located in the right atrium and normally acts as the primary pacemaker of the heart. Its spontaneous electrical activity initiates the cardiac rhythm under normal conditions.

Atrioventricular node

The AV node receives the electrical impulse from the atria and conducts it toward the ventricles. Conduction through the AV node is relatively slow, allowing time for ventricular filling after atrial contraction.

Bundle of His and Purkinje system

After passing through the AV node, the impulse travels through the atrioventricular bundle, divides into right and left bundle branches, and then spreads through the Purkinje fiber network. This produces coordinated activation of ventricular myocardium.

Sequence to memorize:

SA Node → AV Node → Bundle of His → Bundle Branches → Purkinje Fibers

9. Action Potential in Cardiac Tissue

Cardiac action potentials are electrical changes in membrane potential generated by ion movement across cardiac cell membranes. Their exact characteristics differ between working myocardial cells and pacemaker cells.

Action potential of ventricular contractile cells

Ventricular myocytes have a characteristic prolonged action potential. The prolonged duration is largely related to calcium entry during the plateau phase and contributes to the long refractory period of cardiac muscle.

Phases of ventricular action potential

Phase	Main event
Phase 4	Resting membrane potential in ventricular contractile cells.
Phase 0	Rapid depolarization mainly due to opening of fast voltage-gated Na⁺ channels.
Phase 1	Initial brief repolarization.
Phase 2	Plateau phase involving Ca²⁺ entry through L-type calcium channels balanced by outward K⁺ currents.
Phase 3	Repolarization as K⁺ efflux predominates and calcium entry decreases.

Why is the plateau important?

It prolongs the cardiac action potential.
It contributes to the long refractory period of cardiac muscle.
It helps prevent sustained tetanic contraction of the myocardium.
It allows time for coordinated contraction and relaxation.

High-Yield Point: The cardiac muscle action potential has a prolonged plateau phase that distinguishes it from the typical brief action potential of many neurons and skeletal muscle fibers.

10. Pacemaker Potential and SA Node

Pacemaker cells, especially those in the SA node, differ from ventricular contractile cells because they do not maintain a stable resting membrane potential. Instead, their membrane potential gradually changes during diastole, producing spontaneous rhythmic activity.

Important characteristics

Pacemaker cells show spontaneous depolarization.
The SA node normally sets the rhythm of the heart.
Autonomic nervous system activity modifies heart rate.
Sympathetic stimulation generally increases heart rate.
Parasympathetic stimulation generally decreases heart rate.

Funny current

The spontaneous depolarization of sinoatrial nodal cells involves several ionic mechanisms. A current commonly called the funny current (I_f) contributes to the gradual pacemaker depolarization. Calcium currents and changes in potassium conductance also contribute to the generation of the pacemaker action potential.

Remember: Pacemaker cells generate rhythmic electrical activity automatically, whereas contractile cardiac cells primarily respond to propagated electrical excitation and generate force.

11. Electrical Activity of Heart

Electrical activity of the heart represents the coordinated generation, propagation and recovery of electrical impulses through cardiac tissue. This electrical activity ensures that the atria and ventricles contract in an organized sequence.

Basic sequence

The SA node spontaneously generates an electrical impulse.
The impulse spreads through the atrial myocardium.
Atrial depolarization leads to atrial contraction.
The impulse reaches the AV node.
Conduction through the AV node is delayed.
The impulse enters the Bundle of His.
The impulse travels through bundle branches.
Purkinje fibers distribute the impulse through ventricular myocardium.
Ventricular depolarization produces ventricular contraction.
Ventricular repolarization contributes to ventricular relaxation.

Electrical excitation and mechanical contraction

Electrical activity and mechanical activity are tightly coupled but are not identical events. Depolarization changes membrane potential and activates intracellular mechanisms that ultimately produce contraction. In cardiac myocytes, calcium entry through L-type calcium channels contributes to calcium-induced calcium release from the sarcoplasmic reticulum. The increase in intracellular calcium permits actin-myosin interaction and force generation.

12. ECG and Its Waves

An electrocardiogram, or ECG, is a recording of the electrical activity of the heart obtained from electrodes placed on the body surface. It provides information about cardiac rhythm, conduction and the timing of electrical events.

Main components of a normal ECG

ECG Component	Represents
P wave	Atrial depolarization.
QRS complex	Ventricular depolarization.
T wave	Ventricular repolarization.
PR interval	Time from onset of atrial depolarization to onset of ventricular depolarization.
ST segment	Period when ventricles are largely depolarized.

High-Yield Memory Trick:

P = Atria depolarize
QRS = Ventricles depolarize
T = Ventricles repolarize

13. Cardiac Cycle

The cardiac cycle is the complete sequence of electrical, mechanical and pressure changes that occur during one heartbeat. It includes atrial contraction, ventricular contraction and relaxation, as well as the filling and emptying of the heart chambers.

The cardiac cycle is traditionally divided into systole and diastole. Systole refers primarily to ventricular contraction, whereas diastole refers primarily to ventricular relaxation and filling.

Major phases

Ventricular filling
Atrial systole
Isovolumetric ventricular contraction
Ventricular ejection
Isovolumetric ventricular relaxation

14. Detailed Phases of the Cardiac Cycle

Phase 1: Ventricular filling

During ventricular diastole, the ventricles relax and blood enters them from the atria. When ventricular pressure is lower than atrial pressure, the atrioventricular valves open and ventricular filling occurs.

Ventricles are relaxed.
AV valves are open.
Semilunar valves are closed.
Blood flows from atria into ventricles.
Most ventricular filling occurs passively.

Phase 2: Atrial systole

Atrial systole occurs after atrial depolarization. Atrial contraction provides an additional contribution to ventricular filling. This is sometimes described as the atrial kick.

Triggered by atrial depolarization.
Atria contract.
AV valves remain open.
Additional blood enters the ventricles.

Phase 3: Isovolumetric ventricular contraction

Ventricular depolarization initiates ventricular contraction. Ventricular pressure rapidly rises. When ventricular pressure exceeds atrial pressure, the AV valves close.

For a brief period, both AV and semilunar valves are closed. Ventricular pressure rises, but ventricular volume remains constant because no blood enters or leaves the ventricle.

Key term: Isovolumetric means that ventricular volume remains constant. During isovolumetric contraction, all valves are closed.

Phase 4: Ventricular ejection

When ventricular pressure becomes greater than the pressure in the aorta or pulmonary trunk, the semilunar valves open. Blood is then ejected from the ventricles.

Aortic valve opens from the left ventricle.
Pulmonary valve opens from the right ventricle.
Blood is ejected into the great arteries.
Ventricular volume decreases.

Phase 5: Isovolumetric ventricular relaxation

Following ventricular repolarization, ventricular muscle relaxes and ventricular pressure falls. When ventricular pressure becomes lower than arterial pressure, the semilunar valves close.

For a short period all valves are closed again. Ventricular pressure continues to decrease without a change in ventricular volume.

Sequence:

Filling → Atrial Systole → Isovolumetric Contraction → Ejection → Isovolumetric Relaxation

15. Heart Sounds

Heart sounds are generated mainly by vibrations associated with blood flow and valve closure during the cardiac cycle. The two major normal heart sounds are S1 and S2.

First heart sound — S1

The first heart sound occurs at the beginning of ventricular systole and is associated primarily with closure of the atrioventricular valves. These are the tricuspid and mitral valves.

Occurs near the beginning of ventricular systole.
Associated with AV valve closure.
Often described as the "lub" sound.

Second heart sound — S2

The second heart sound occurs near the end of ventricular systole and beginning of ventricular diastole. It is associated primarily with closure of the semilunar valves.

Associated with aortic and pulmonary valve closure.
Marks the beginning of ventricular diastole.
Often described as the "dub" sound.

Sound	Valve closure	Associated phase	Memory clue
S1	Mitral + Tricuspid	Beginning of ventricular systole	Lub
S2	Aortic + Pulmonary	Beginning of ventricular diastole	Dub

Easy memory trick:

S1 = AV valves close = "LUB"
S2 = Semilunar valves close = "DUB"

16. Integration of Electrical Activity and Cardiac Cycle

One of the most important concepts in cardiovascular physiology is the relationship between electrical activity and mechanical activity. The electrical event precedes the corresponding mechanical event.

Electrical event	Mechanical consequence
SA node depolarization	Initiates atrial electrical activation.
Atrial depolarization / P wave	Atrial contraction follows.
Ventricular depolarization / QRS	Ventricular contraction follows.
Ventricular repolarization / T wave	Ventricular relaxation follows.

Excitation-contraction coupling

In cardiac muscle, an action potential travels along the cell membrane and into the transverse tubules. Calcium enters through L-type calcium channels and contributes to calcium release from the sarcoplasmic reticulum. The rise in intracellular calcium allows interaction between contractile proteins and produces contraction.

Calcium is subsequently removed from the cytosol through several mechanisms, including reuptake into the sarcoplasmic reticulum and extrusion from the cell. Cytosolic calcium concentration decreases and the muscle relaxes.

17. Important Clinical and Exam Concepts

1. Arrhythmia

An arrhythmia is an abnormality of cardiac rhythm. It may involve abnormal impulse generation, abnormal impulse conduction, or both. Electrical abnormalities can therefore alter the normal relationship between atrial and ventricular activity.

2. Tachycardia

Tachycardia refers to an abnormally rapid heart rate. The underlying cause can vary and may involve physiological responses, abnormal pacemaker activity or other rhythm disturbances.

3. Bradycardia

Bradycardia refers to a slower-than-usual heart rate. A slow rate can occur physiologically in some individuals, while pathological bradycardia may result from abnormalities involving impulse generation or conduction.

4. AV block

Atrioventricular block occurs when conduction of electrical impulses from the atria to the ventricles is delayed or interrupted to varying degrees. The severity depends on the degree of conduction impairment.

5. ECG as a physiological tool

Helps assess cardiac rhythm.
Provides information about electrical conduction.
Allows identification of timing abnormalities.
Can provide evidence of myocardial electrical abnormalities.
Is useful for studying the relationship between cardiac electrical events and the cardiac cycle.

Important distinction: ECG records electrical activity at the body surface. It does not directly record the force of cardiac contraction or blood pressure.

18. Important Comparisons for Competitive Exams

Concept	Key Point
SA node	Normally the primary pacemaker of the heart.
AV node	Introduces a conduction delay and conducts impulses toward the ventricles.
Purkinje fibers	Rapidly distribute excitation through ventricular myocardium.
P wave	Atrial depolarization.
QRS complex	Ventricular depolarization.
T wave	Ventricular repolarization.
S1	Primarily associated with AV valve closure.
S2	Primarily associated with semilunar valve closure.
Systole	Primarily refers to ventricular contraction.
Diastole	Primarily refers to ventricular relaxation and filling.
Primary hemostasis	Platelet adhesion, activation and aggregation.
Secondary hemostasis	Coagulation and fibrin formation.
AB blood group	A and B antigens are present; I^A and I^B are codominant.
Rh-positive	D antigen is present.

19. Quick Revision Notes

⭐ Must-Remember Points

Hemostasis prevents excessive blood loss after vascular injury.
Primary hemostasis is mainly associated with platelet plug formation.
Secondary hemostasis strengthens the plug through fibrin formation.
Thrombin converts fibrinogen into fibrin.
Plasmin is the major enzyme involved in fibrin degradation.
ABO blood groups depend on A and B antigens on RBCs.
Group A has A antigen and anti-B antibody.
Group B has B antigen and anti-A antibody.
Group AB has both A and B antigens.
Group O lacks A and B antigens on RBCs.
I^A and I^B are codominant alleles.
The D antigen is particularly important in the Rh blood group system.
The cardiovascular system consists of the heart and blood vessels.
The right side of the heart pumps blood toward the lungs.
The left side of the heart pumps blood into systemic circulation.
The SA node normally acts as the primary cardiac pacemaker.
The AV node introduces a delay in conduction.
The conduction sequence is SA node → AV node → Bundle of His → bundle branches → Purkinje fibers.
P wave represents atrial depolarization.
QRS complex represents ventricular depolarization.
T wave represents ventricular repolarization.
Electrical excitation normally precedes mechanical contraction.
Cardiac ventricular action potentials have a prolonged plateau phase.
Calcium plays a central role in cardiac excitation-contraction coupling.
Cardiac cycle includes ventricular filling, atrial systole, ventricular contraction, ejection and relaxation.
S1 is mainly associated with closure of the mitral and tricuspid valves.
S2 is mainly associated with closure of the aortic and pulmonary valves.
Systole is mainly associated with ventricular contraction.
Diastole is mainly associated with ventricular relaxation and filling.
Isovolumetric contraction occurs when all four valves are closed and ventricular pressure rises.
Isovolumetric relaxation occurs when all four valves are closed and ventricular pressure falls.

21. Final Exam-Oriented Summary

Electrical activity of the heart is the foundation of coordinated cardiac contraction. The sinoatrial node normally initiates the electrical impulse, which spreads through the atria and reaches the atrioventricular node. After the AV nodal delay, the impulse passes through the Bundle of His, bundle branches and Purkinje fibers to activate the ventricles.

SA node: primary pacemaker under normal conditions.
AV node: delays conduction and connects atrial excitation with ventricular conduction.
Purkinje fibers: distribute excitation rapidly through ventricular myocardium.
P wave: atrial depolarization.
QRS complex: ventricular depolarization.
T wave: ventricular repolarization.
S1: mainly associated with closure of mitral and tricuspid valves.
S2: mainly associated with closure of aortic and pulmonary valves.
Cardiac cycle: consists of coordinated filling, contraction, ejection and relaxation.
Hemostasis: prevents excessive blood loss following vascular injury.
ABO blood groups: are based primarily on A and B RBC antigens and corresponding antibodies.
Rh system: D antigen is particularly important in determining Rh-positive or Rh-negative status.
Action potential: cardiac ventricular cells show a prolonged plateau phase.
Calcium: plays an essential role in excitation-contraction coupling.

For competitive examinations such as CSIR-NET, GATE Biotechnology, DBT-BET, ICMR-JRF and MSc-level physiology examinations, focus on understanding the sequence rather than memorizing isolated facts:

SA Node → Atrial Depolarization → AV Node → Bundle of His → Purkinje Fibers → Ventricular Depolarization → Ventricular Contraction → Ejection → Ventricular Repolarization → Relaxation

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Sunday, 23 August 2026