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The cardiac cycle consists of alternating periods of contraction (systole) and relaxation (diastole).
Second heart sound: Represents the beginning of ventricular systole.
Ventricular diastole: Interval between the first and second heart sounds, allowing for heart relaxation.
The cycle is characterized by polarization (depolarization and repolarization) of heart cells.

Systole: Refers to the contraction phase when the heart pumps blood. Specifically, ventricular contraction is emphasized as the critical phase for circulation.
Diastole: Refers to the relaxation phase when the heart fills with blood.
Atria and Ventricles: Atria contract first to push blood into the ventricles, followed by ventricular contraction to send blood to the arteries.
Four chambers of the heart cannot contract simultaneously to prevent blood flow interference.
The AV node (atrioventricular node) plays a role in controlling the timing of contractions between the atria and ventricles.

The heart muscle depolarizes and repolarizes; cardiac cycle relies on this alternate state.
Contractility: Focus on ventricular contraction; atrial contraction is non-essential for survival in some patients.
Intercalated discs: Specialized structures in cardiac muscle that facilitate rapid electrical conduction between cells.
An external stimulus can cause sodium ions to enter cardiac cells, leading to depolarization (becoming positively charged inside) and generating an action potential.

The action potential occurs when a cell reaches a threshold after sodium influx, leading to depolarization.
Depolarization spreads from one cell to another through intercalated discs, allowing the heart muscle to contract as a unit quickly.
Upon firing of the SA node (sinoatrial node), impulses travel through specialized fibers, ensuring coordinated contraction of atria and subsequent ventricular contraction.
Cardiac cells can relay impulses without external stimulation, leading to synchronized contractions.

SA node: Primary pacemaker with an intrinsic rate of 60-100 bpm.
AV node: Secondary pacemaker that delays impulses allowing atrial contraction to complete before ventricular contraction.
Bundle of His: Serves as a pathway from the AV node to the ventricles, dividing into right and left bundle branches to efficiently conduct impulses.
Purkinje fibers: Distribute electrical impulses throughout the ventricular muscle, leading to contraction.

Automaticity: Heart generates its own electrical impulses, functioning independently.
Excitability: Ability to respond and adapt to external stimuli (like adrenaline) affecting heart rate.
Conductivity: Effectiveness of the cardiac conduction system in transmitting impulses between neighboring cells.
Contractility: Refers to the muscle fibers contracting to push blood into circulation.

Heart rates can vary (60-100 bpm typically) based on input from the medulla oblongata, which regulates autonomic nervous responses, affecting sympathetic (increase heart rate) and parasympathetic (decrease heart rate) systems.
Factors influencing heart rate: emotions, anxiety, fear, exercise, and various stress responses.
Cardiac arrest risks: Arrhythmias or irregular electrical activity can lead to life-threatening conditions if impulse pathways are disrupted.