Control of heart rate

• Nodes involved in heart contraction

  • Sinoatrial node (SAN)
  • Atrioventricular node (AVN)
• Sinoatrial node (SAN)

  The pacemaker of the heart, located within the wall of the right atrium
• Atrioventricular node (AVN)

  Located near the lower end of the right atrium in the wall that separates the 2 atria
• How heartbeats are initiated & coordinated

  1. SAN initiates wave of depolarisation / impulses across atria, causing atrial systole

  2. Non-conducting tissue prevents immediate contraction of ventricles

  3. AVN delays impulse while ventricles fill & valves close

  4. AVN sends wave of depolarisation down Bundle of His, causing ventricles to contract from apex (base) upwards
• Cardiac output

  Stroke volume (V) x heart rate
• Autonomic nervous system

  Controls involuntary actions of glands & muscles, has 2 subdivisions: Sympathetic (involved in 'fight or flight' response, stimulates effectors to speed up activity) and Parasympathetic (involved in normal resting conditions, inhibits effectors to slow down activity)
• Receptors involved in changing heart rate

  • Baroreceptors (detect changes in blood pressure, located in the carotid body)
  • Chemoreceptors (detect changes in pH, located in the carotid body & aortic body)
• How the body responds to an increase in heart rate

  1. Baroreceptors detect rise in blood pressure OR chemoreceptors detect rise in CO2 / pH decrease

  2. Send impulse to cardiac centre / medulla oblongata

  3. Sends more impulses to SAN by parasympathetic nervous system (baroreceptors) OR sympathetic nervous system (chemoreceptors)

  4. Baroreceptors - Stimulates release of acetylcholine, which decreases heart rate

  5. Chemoreceptors increases heart rate; < CO2 removed (lungs); pH rises to normal; reduced impulses to SAN= > heart rate
• How the body responds to a decrease in blood pressure

  1. Baroreceptors send < impulses to the cardiac centre in the medulla oblongata

  2. More impulses to SAN via SNS

  3. Stimulates release of noradrenaline, which increases heart rate & strength of contraction