Heart Physiology

Heart Physiology Key Concepts

Cardiac Cycle

• Phases: Systole (contraction) and Diastole (relaxation)

• Each chamber goes through contraction/relaxation:

  1. Atrial contraction (systole) begins, injecting blood into ventricles.

  2. Atria relax (diastole), valves close.

  3. Isovolumetric contraction of ventricles.

  4. Ventricular ejection begins when semilunar valves open.

  5. Ventricular diastole starts as ventricles relax, pressure drops.

Cardiac Muscle Physiology

• Cardiac Myocytes: Connected via intercalated discs with gap junctions (allow action potential spread) and desmosomes (structural integrity).

• Excitation-Contraction Coupling: Involves calcium ions to trigger contraction. Sequence:

  1. Action potential spreads.

  2. Voltage-gated Ca2+ channels open.

  3. Ca2+ triggers release from the sarcoplasmic reticulum.

  4. Ca2+ binds to troponin, initiating contraction.

Autonomic Modulation

• Pacemaker Activity: Spontaneous depolarization in SA node (80-100 bpm); AV node (40-60 bpm).

• Influenced by:

  • K+ permeability.

  • Na+ influx/efflux.

  • Hormones (e.g., epinephrine increases heart rate).

Cardiac Output (CO)

• Formula: CO = SV (stroke volume) x HR (heart rate)

• Factors Affecting CO:

  • Autonomic innervation

  • Hormones

  • End-diastolic volume (EDV)

  • End-systolic volume (ESV)

• Cardiac reserve is the difference between resting and maximal CO.

Heart Sounds and Auscultation

• Heart Sounds: S1 (AV valves close) and S2 (semilunar valves close).

• Important for diagnosing heart conditions through stethoscope.

Common Cardiac Arrhythmias

• Ectopic Pacemakers: Cause timing disruptions (e.g., Atrial flutter, PVCs, Ventricular fibrillation).

• Atrial flutter: Atria beats excessively (200-400 bpm).

• Ventricular fibrillation: Erratic electrical signals, severe arrhythmia.

Stroke Volume Factors

• Preload: Initial stretching of cardiomyocytes before contraction (dependent on venous return).

• Contractility: Strength of contraction depending on calcium availability.

• Afterload: Resistance encountered during ventricular contraction (blood pressure).

Regulation of Heart Rate

• Controlled by autonomic nervous system (sympathetic and parasympathetic inputs).

• Influenced by baroreceptors and chemoreceptors (detect changes in blood pressure and gases).

Study Key Terms: Isovolumic contraction/relaxation, action potential, neurohormonal modulation, cardiac reserve, ejection fraction, Frank-Starling Law.