3- Cardiac physiology

1. Myocardium (Heart Muscle)

  • Comprises 95% of the heart’s mass.

  • Cardiomyocytes (heart muscle cells):

    • Striated, branched, and contain 1-2 central nuclei.

    • Connected by intercalated discs that contain:

      • Desmosomes – Provide structural support.

      • Gap junctions – Allow ion flow for coordinated contraction (functional syncytium).

  • Muscle fibers are arranged diagonally, allowing the heart to wring blood out of the ventricles.

2. Cardiac Conduction System (Electrical System of the Heart)

  • Specialized cardiac muscle fibers (autorhythmic fibers) generate and propagate action potentials.

  • Functions:

    • Act as pacemakers.

    • Form the cardiac conduction system.

  • Abnormalities → Can lead to arrhythmias (irregular heart rhythms).

Conduction Pathway (Order of Impulse Transmission)

  1. Sinoatrial (SA) Node → Pacemaker of the heart (fires at 100 bpm, slowed to 75 bpm by parasympathetic input).

  2. Atrioventricular (AV) Node → Delays impulse slightly (40-60 bpm if SA node fails).

  3. AV Bundle (Bundle of His) → Electrical bridge between atria & ventricles.

  4. Right & Left Bundle Branches → Carries impulses through the interventricular septum.

  5. Purkinje Fibers → Spread impulse through ventricles, triggering contraction.

3. Action Potential of Contractile Fibers (Ventricular Myocytes)

  • Three Phases:

    1. Depolarization → Na⁺ influx.

    2. Plateau (Maintained depolarization) → Ca²⁺ influx and K⁺ efflux.

    3. Repolarization → K⁺ efflux continues, returning to resting potential.

  • Refractory Period:

    • Longer than contraction, preventing tetany (sustained contraction).

4. Electrocardiogram (ECG/EKG)

  • Measures summed electrical activity of heart muscle.

  • Three Main Waves:

    • P wave → Atrial depolarization (atria contract).

    • QRS complex → Ventricular depolarization (ventricles contract).

    • T wave → Ventricular repolarization (ventricles relax).

ECG Intervals & Segments

  • P-Q Interval → Time for impulse to travel from SA node to ventricles.

  • S-T Segment → Time when ventricles are fully depolarized.

  • Q-T Interval → Ventricular depolarization to repolarization.

  • Electrical events precede mechanical events.

5. Cardiac Cycle (Pressure & Volume Changes)

  • Systole → Contraction phase.

  • Diastole → Relaxation phase.

Heart Sounds

  • S1 ("Lubb") → AV valve closure (ventricular contraction).

  • S2 ("Dupp") → Semilunar valve closure (ventricular relaxation).

Isovolumetric Phases

  • Isovolumetric contraction → All valves closed, ventricles contract but no blood ejected yet.

  • Isovolumetric relaxation → All valves closed, ventricles relax, no blood entering.

Key Volume Measurements

  • End Diastolic Volume (EDV) → Blood in ventricle before contraction (~120mL).

  • End Systolic Volume (ESV) → Blood left in ventricle after contraction (~50mL).

  • Stroke Volume (SV) = EDV - ESV (~70mL per beat).

6. Cardiac Output (CO)

  • Definition: The amount of blood ejected by the heart per minute.

  • Formula: CO=HR×SVCO = HR \times SVCO=HR×SV

    • HR (Heart Rate) = Beats per minute.

    • SV (Stroke Volume) = Blood ejected per beat.

  • Normal Resting CO: ~5.25 L/min.

Cardiac Reserve

  • The difference between resting CO and maximum CO.

  • Higher in athletes, lower in heart disease patients.

7. Factors Regulating Stroke Volume (SV)

SV is influenced by three main factors:

  1. Preload (Degree of stretch before contraction)

    • Frank-Starling Law: Increased filling = Stronger contraction.

    • Proportional to EDV.

  2. Contractility (Strength of contraction)

    • Depends on Ca²⁺ levels.

    • Increased by: Epinephrine, norepinephrine, thyroxine.

    • Decreased by: Acidosis, hypoxia.

  3. Afterload (Resistance to ejection)

    • High afterload = Decreased SV (e.g., hypertension increases afterload).

    • Aortic pressure ~80 mmHg, Pulmonary pressure ~20 mmHg.

8. Factors Regulating Heart Rate (HR)

  • Normal HR without autonomic input = 100 bpm.

  • Resting HR = 75 bpm due to vagal tone.

Autonomic Regulation

  • Sympathetic stimulation → Increases HR (norepinephrine).

  • Parasympathetic stimulation → Decreases HR (acetylcholine).

Hormonal & Chemical Influences

  • Increased HR: Epinephrine, norepinephrine, thyroid hormones.

  • Decreased HR: Hypoxia, acidosis.

Temperature Effects

  • High body temperature = Increased HR.

  • Low body temperature = Decreased HR.

Summary

  • Cardiac conduction system: SA node → AV node → Bundle of His → Bundle Branches → Purkinje Fibers.

  • ECG: P wave (atria contract), QRS complex (ventricles contract), T wave (ventricles relax).

  • Cardiac cycle phases: Systole (contraction), Diastole (relaxation).

  • Stroke volume regulated by preload, contractility, and afterload.

  • Cardiac output (CO = HR × SV) determines how much blood the heart pumps per minute.

  • Heart rate is controlled by the autonomic nervous system, hormones, and body temperature.

Midterm Study Tips

✔ Memorize conduction pathway (SA → AV → Bundle of His → Purkinje fibers).
✔ Understand ECG waves and how they relate to heart contractions.
✔ Know the phases of the cardiac cycle and heart sounds (S1 = AV valves, S2 = Semilunar valves).
✔ Calculate Stroke Volume (SV = EDV - ESV) and Cardiac Output (CO = HR × SV).
✔ Understand factors affecting stroke volume (preload, contractility, afterload).
✔ Review how heart rate is regulated (autonomic nervous system, hormones, temperature).