Overview of Heart Rate Control Mechanisms

Oct 10, 2024

Learn A-Level Biology: Control of Heart Rate

Introduction

  • Control of heart rate links to the stages in the cardiac cycle.
  • Cardiac Muscle: Myogenic - contracts and relaxes independently but the rate is controlled by the nervous system.

Key Structures in the Heart

  • Sinoatrial Node (SAN):
    • Located in the right atrium.
    • Known as the pacemaker.
    • Releases waves of depolarization causing muscle contractions.
  • Atrioventricular Node (AVN):
    • Located between the atria and ventricles.
    • Releases wave of depolarization with a slight delay.
  • Bundle of His:
    • Conductive tissues running down the heart's septum.
  • Purkinje Fibres:
    • Conductive tissues that branch through the ventricle walls.

Control of the Cardiac Cycle

  1. SAN Activity:
    • Releases wave of depolarization causing atrial systole.
    • Atria contract.
  2. AVN Activity:
    • Releases second wave of depolarization.
    • Depolarization travels through Bundle of His due to non-conductive tissue separating atria and ventricles.
    • Travels through Purkinje fibres leading to contraction starting from the apex of the ventricle.

Importance of Non-Conductive Tissue

  • Causes delay for atria to fully contract and ventricles to fill before contraction.
  • Ensures efficient blood ejection from the heart.

Nervous System Control

  • Part of the Autonomic Nervous System (ANS), controlled subconsciously by the medulla oblongata.
  • Sympathetic Nervous System: Increases heart rate.
  • Parasympathetic Nervous System: Decreases heart rate.

Homeostasis and Heart Rate

Key Stimuli

  • Blood pH: Affects the heart's response during high respiration (exercise).
    • Low pH during high CO2 or lactic acid.
    • Detected by chemoreceptors.
  • Blood Pressure: Detected by pressure receptors (baroreceptors).
    • High pressure can damage artery walls.
    • Low pressure affects oxygen delivery and waste removal.

Response to Stimuli

  • Increase in Pressure:
    • Detected by baroreceptors.
    • More impulses to medulla oblongata.
    • Activates parasympathetic nerves reducing heart rate.
  • Decrease in Pressure:
    • Detected by baroreceptors.
    • More impulses activate sympathetic nerves increasing heart rate.
  • Decrease in pH:
    • Detected by chemoreceptors.
    • More impulses to sympathetic nerves increase heart rate.

Key Points for Exam Questions

  • Importance of specifying "more" electrical impulses.
  • Distinction between sympathetic and parasympathetic responses.

Conclusion

  • Understanding the mechanics and control of the heart rate is crucial for explaining cardiovascular responses to various physiological conditions.