3.6.1.3 Control of heart rate

• When a wave of electrical activity reaches the AVN, there is a short delay before a new wave leaves the AVN. Explain the importance of this short delay (2)

  • to allow atria to empty
  • before ventricles contract
• The heart muscle is referred to as myogenic because of its ability to initiate its own contraction without needing external electrical stimulation.
• In the wall of the right atrium, there is a region of specialised fibres called the sinoatrial node (SAN) which is the pacemaker of the heart.
• The sinoatrial node (SAN) initiates a wave of electrical stimulation which causes the atria to contract.
• Describe the roles of the sinoatrial node (SAN) and atrioventricular node (AVN) in coordinating the heartbeat.

  • The SAN acts as the pacemaker, initiating the heartbeat by generating electrical impulses.
  • These impulses spread across the atria, causing them to contract (atrial systole).
  • The impulses reach the AVN, which introduces a slight delay.
  • This delay ensures the ventricles contract after the atria have fully emptied.
  • Impulses are then conducted through the Bundle of His and Purkyne fibres, resulting in coordinated ventricular contraction.
• The ventricles do not start contracting until the atria have finished contracting and emptied due to the presence of tissues at the atria which is unable to conduct the wave of excitation.
• The atrioventricular node (AVN) is located between the two atria and it passes the wave of excitation to ventricles, down the bundle of His to the apex of the heart.
• The bundle of His branches into Purkyne fibres which carry the electrical wave upwards. This causes the ventricles to contract, this empties them into the vessels.
• Autonomic Nervous System (ANS): Controls involuntary actions. It consists of two divisions:

  • Sympathetic Nervous System (SNS): Speeds up heart rate (fight-or-flight response).
  • Parasympathetic Nervous System (PNS): Slows down heart rate (rest-and-digest response).
• The medulla oblongata in the brainstem contains two key centres: the cardioacceleratory centre and the cardioinhibitory centre.
• The Cardioacceleratory Centre is linked to the heart via the sympathetic nervous system.

  • Increases the frequency of impulses sent to the sinoatrial node (SAN) to raise the heart rate.
• The Cardioinhibitory Center is linked to the heart via the parasympathetic nervous system (vagus nerve).

  • Decreases the frequency of impulses sent to the SAN to slow the heart rate.
• Changes in blood pressure and changes in pH are factors which increase the heart rate.
• Changes in pH caused by high carbon dioxide concentration, are detected by chemoreceptors located in carotid arteries, the aorta and the brain.

  • receptors send impulses to the medulla oblongata more frequently as a result via the sympathetic pathway
  • more frequent impulses are sent to the SAN, which increases heart rate
  • increases blood flow to the lungs where carbon dioxide can be expelled
• Changes in blood pressure are detected by baroreceptors in the sinus.

  • receptors send impulses to medulla oblongata more frequently via the parasympathetic pathway to the SAN
  • causes heart rate to decrease, lowering the blood pressure
• Cardiac output (CO) = heart rate (R) x stroke volume (SV)
• When the heart beats, both ventricles contract at the same time. Explain how this is coordinated in the heart after the initiation of the heartbeat by the SAN (2)

  • Electrical activity only through Bundle of His / AVN
  • Wave of electrical activity passes over / through both ventricles at the same time
• Describe how a heartbeat is initiated and coordinated (5)

  • SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction
  • Non-conducting tissue prevents immediate contraction of ventricles / prevents impulses reaching the ventricles
  • AVN delays impulse whilst blood leaves atria / ventricles fill
  • AVN sends wave of electrical activity / impulses down the Bundle of His
  • Causing ventricles to contract from base up;
• The heart controls and coordinates the regular contraction of the atria and ventricles. Describe how. (5)

  • SAN → AVN → bundle of His / Purkyne fibres
  • Impulses / electrical activity (over atria)
  • Atria contract
  • Non-conducting tissue (between atria and ventricles)
  • Delay (at AVN) ensures atria empty / ventricles fill before ventricles contract
  • Ventricles contract from the apex upwards
• Increased intensity of exercise leads to an increased heart rate. Explain how (3)

  • Carbon dioxide detected by chemoreceptors / pressure detected by baroreceptors
  • Medulla / cardiac centre involved
  • More impulses to SAN / along sympathetic nerve
• The cardiac cycle is controlled by the sinoatrial node (SAN) and the atrioventricular node (AVN). Describe how. (5)

  • SAN initiates heartbeat / acts as a pacemaker / myogenic
  • SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction
  • AVN delays (electrical activity / impulses)
  • Allowing atria to empty before ventricles contract / ventricles to fill before the contract
  • AVN sends wave of electrical activity / impulses down Bundle of His / Purkyne fibres
  • Causing ventricles to contract (from base up) / ventricular systole
• The sinoatrial node (SAN) is in the right atrium of the heart. Describe the role of the sinoatrial node. (2)

  • Send out electrical activity / impulses
  • Initiates the heartbeat / acts as a pacemaker / (stimulates) contraction of atria
• Explain how nervous control in a human can cause increased cardiac output during exercise. (4)

  • Coordination via medulla (of brain) / cardiac centre
  • Increased impulses along sympathetic / cardiac accelerator nerve
  • To SAN / pacemaker
  • More impulses sent from SAN / pacemaker
• Explain why increased cardiac output is an advantage during exercise (3)

  • in exercise - more energy release for aerobic respiration
  • higher cardiac output - increases oxygen supply to muscles; Increases glucose supply (to muscles)
  • Increases CO2 removal (from muscles) / lactate removal; Increases heat removal (from muscles) / for cooling
• Explain how a rise in blood pressure results in a decrease in the rate of heartbeat (6)

  • baroreceptors in aorta / carotid arteries / carotid sinus
  • send impulses to medulla / cardio-inhibitory centre
  • impulses via parasympathetic nerves
  • to SAN
  • release of ACh / inhibits SAN / decreases impulses from SAN
  • decreases impulses to AVN
• Describe how an impulse reaches the base of the ventricles of the heart from the sinoatrial node (3)

  • it spreads through the atria
  • to the atrioventricular node
  • then through bundle of His/Purkyne fibres
• An increase in muscle activity causes an increase in heart rate. Describe and explain how (4)

  • increase in CO2 detected by chemoreceptors
  • send more impulses to cardiac centre / medulla
  • more impulses from cardiac centre / medulla along sympathetic pathway
  • to SAN
• How does myogenic control coordinate the heartbeat?

  • The sinoatrial node (SAN) generates electrical impulses that spread across the atria.
  • The atrioventricular node (AVN) introduces a delay before transmitting impulses to the ventricles via the Bundle of His and Purkyne fibres.
• The Bundle of His conducts electrical impulses from the AVN to the ventricles, ensuring coordinated contraction.
• What is the role of the autonomic nervous system in heart rate control?

  • The sympathetic nervous system increases heart rate.
  • The parasympathetic nervous system decreases heart rate.