Circulatory System and Electrical Activity of the Heart

The Conducting System of the Heart

• Electrical Signaling

  • Begins in the Sinoatrial (SA) Node.
  • Signal travels to the Atrioventricular (AV) Node.

• Depolarization Sequence:

  1. SA Node Depolarizes: Generates action potential (AP).
  2. Conducts Rapidly: Through internodal pathways to the AV node.
  3. Atrial Depolarization: Slower conduction across the atria.
  • Characterized by purple shading in depolarization steps.
  1. Slow Conduction at AV Node: Ensures adequate filling time for ventricles.
  2. Rapid Transmission to Ventricles: Through AV bundle, bundle branches, and Purkinje fibers.
  3. Depolarization Wave: Moves upward from the apex of the heart following the coordinated contraction.

• Key Figures:

  • Electrocardiogram (ECG/EKG): Maps electrical events in the heart:
  • P Wave: Atrial depolarization (contraction).
  • QRS Complex: Ventricular depolarization (contraction and atrial repolarization).
  • T Wave: Ventricular repolarization (relaxation).

• Significant Intervals in ECG:

  • PR Interval: Time from SA node depolarization to ventricular contraction (~150 ms).
  • QT Interval: Time for ventricles to depolarize and repolarize (~400 ms).
  • S-T Segment: Indicates time between ventricular depolarization and repolarization (~120 ms).

• Conduction Malfunction Consequences:

  • If the AV node fails to depolarize, it can disrupt the effective transfer of electrical impulses, leading to arrhythmias.

Regulation of Heart Function

• Stroke Volume & Heart Rate are influenced by:

  • Electrical activity of the heart.
  • Sympathetic nervous system (SNS) input increases heart rate.

• Frank-Starling Mechanism:

  • The heart pumps all the blood that returns to it (contractility based law).

• Cardiac Output (CO):

  • CO = Stroke Volume x Heart Rate
  • Influenced by factors such as:
  • Contractility (influenced by SNS).
  • End-diastolic volume (preload) - affected by venous return.

• Vascular Resistance:

  • Total Peripheral Resistance (TPR) determines blood flow opposition.

• Blood Pressure:

  • Mean Arterial Pressure (MAP) determines blood flow throughout the body.
  • MAP = CO × TPR
  • Normal values: 120 mmHg (systolic) / 80 mmHg (diastolic).

Vascular Composition and Function

• Composition of Blood Vessels:

  • Arteries: Thick walls, high elasticity to withstand pressure.
  • Veins: Thinner walls, larger diameter, serve as blood reservoirs.

• Capillaries:

  • Site of gas and nutrient exchange; consist of a single endothelial layer for efficient exchange.

• Resistance Factors:

  • Blood viscosity, vessel length, and diameter influence resistance and blood flow.

• Regulatory Mechanisms:

  • Sympathetic stimulation increases heart rate and constricts blood vessels.
  • Parasympathetic stimulation reduces heart rate.

Summary of Important Points

• The heart’s conduction system is vital for maintaining coordinated contractions of the heart chambers.
• ECG provides insights into heart function by depicting electrical activities related to contraction and relaxation phases.
• The balance of stroke volume, heart rate, and vascular resistance is essential for maintaining adequate blood flow and pressure throughout the body.