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.