Your heart generates its own electrical signals — it doesn't need your brain to tell it to beat. This intrinsic electrical system ensures your heart beats about 100,000 times per day without you ever thinking about it.

The Conduction Pathway:

1. SA Node (Sinoatrial Node): The natural pacemaker. Located in the right atrium, it fires 60-100 times per minute. Each impulse causes atrial contraction.
2. Internodal Pathways: Carry the impulse across the atria to the AV node.
3. AV Node (Atrioventricular Node): Delays the impulse by about 0.1 seconds. This crucial delay allows atria to finish contracting and fully fill the ventricles before they contract.
4. Bundle of His: The electrical highway into the ventricular septum.
5. Right and Left Bundle Branches: Carry impulses down the septum to the apex.
6. Purkinje Fibers: Spread throughout the ventricles, causing them to contract from the bottom up — efficiently squeezing blood upward toward the arteries.

Intercalated Discs: Specialized junctions between cardiac muscle cells that allow electrical impulses to spread rapidly. This makes cardiac muscle behave as a functional syncytium — all cells contract together as one unit.

An electrocardiogram (ECG or EKG) records the electrical activity of your heart. It's like a graph of the electrical storms happening inside your heart with each beat.

The Waveforms:

• P Wave: Atrial depolarization (electrical activation causing contraction). Small, rounded wave.
• QRS Complex: Ventricular depolarization. Large, spiked — the ventricles have much more muscle mass, so the electrical signal is bigger.
• T Wave: Ventricular repolarization (electrical recovery, preparing for next beat). Note: Atrial repolarization is hidden in the QRS complex.

Important Intervals:

• PR Interval: Time from SA node firing to ventricular activation (0.12-0.20 seconds). Includes the AV node delay.
• QT Interval: Total time for ventricular depolarization and repolarization. Varies with heart rate.
• QRS Duration: Should be less than 0.12 seconds. Widened QRS suggests conduction problems.

Clinical uses: Diagnosing arrhythmias, heart attacks (ST elevation), electrolyte imbalances, and drug effects.

The cardiac cycle is one complete heartbeat — all the events from one heartbeat to the next. At 75 beats per minute, one cycle takes about 0.8 seconds.

Phases of the Cardiac Cycle:

1. Atrial Systole (contraction): Atria contract, pushing the final 20% of blood into ventricles. Takes about 0.1 seconds.
2. Isovolumetric Contraction: Ventricles begin contracting. All valves are closed — pressure is building but no blood moves yet.
3. Ventricular Ejection: Pressure exceeds arterial pressure, semilunar valves open, blood is ejected.
4. Isovolumetric Relaxation: Ventricles relax. All valves closed again. Pressure drops.
5. Ventricular Filling: AV valves open. Blood passively flows from atria to ventricles (about 70% of filling).

Heart Sounds:

• S1 ("Lub"): AV valves closing (tricuspid and mitral) — beginning of systole
• S2 ("Dub"): Semilunar valves closing (aortic and pulmonary) — beginning of diastole
• S3 and S4: Abnormal sounds indicating possible heart problems

Cardiac Output (CO) is the volume of blood pumped by each ventricle per minute. It's a key measure of heart function.

The Formula:

Cardiac Output = Heart Rate × Stroke Volume

Normal values:

• Heart Rate (HR): 60-100 beats/minute at rest
• Stroke Volume (SV): About 70 mL/beat at rest
• Cardiac Output: About 5 L/min at rest (can increase to 20-25 L/min during exercise)

Factors affecting Stroke Volume:

• Preload: How much the ventricle is stretched before contraction (more stretch = stronger contraction — Frank-Starling Law)
• Afterload: The pressure the ventricle must overcome to eject blood (high blood pressure increases afterload)
• Contractility: The intrinsic strength of contraction (increased by sympathetic stimulation, calcium)

Blood Pressure Relationship:

Blood Pressure = Cardiac Output × Peripheral Resistance

This is why anything that increases heart rate, stroke volume, or vessel constriction raises blood pressure.