control of heart rate

• the cardiac muscle is myogenic meaning it can contract and relax without thinking about it.
• how heartbeat is initated & coordinated:

  1. the SAN sends a wave of impulses across the atria causing them to contract.
  2. there is a delay to allow all blood to leave the atria & enter the ventricles.
  3. the AVN sends impulses down the bundle of his.
  4. the bundle of his divides into purkyne fibres which carry the impulse across the ventricles.
  5. causing the ventricles to contract.
  6. pushing blood up and out of the heart.
• SAN = sinoatrial node
• AVN = atrioventricular node
• the bottom of the ventricle is called the apex
• the rate at which the SAN fires impulses is controlled by the medulla oblongata in the brain.
• stimuli that may require heart rate changes are detected by pressure receptors and chemoreceptors.
• there are pressure receptors called baroreceptors in the aorta and the carotid arteries.

  • they are stimulated by high & low blood pressure.
• there are chemical receptors called chemoreceptors in the aorta, carotid arteries & medulla.

  • they monitor oxygen & CO2 levels in the blood
  • they also monitor pH of the blood which is affected by CO2 concentration.
• The autonomic nervous system controls involuntary actions such as breathing, digestion and heartbeat.
• electrical impulses from receptors are sent to the medulla along sensory neurones.

  • the medulla processes the information & sends impulses to the SAN along the sympathetic or parasympathetic neurones.
• the sinoatrial node is in the top right atrium.
• the atrioventricular node is found in the right artium just below the SAN and towards the middle.
• the bundle of His is located between the AVN and ventricles
• Purkyne fibres run through the walls of both ventricles
• high blood pressure:

  • detected by baroreceptors in the aorta / carotid arteries.
  • impulses are sent to the medulla.
  • impulses are then sent to the SAN via the parasympathetic neurones.
  • acetylcholine is released & binds to receptors on the SAN.
  • heart rate slows to reduce blood pressure back to normal.
• low blood pressure:

  • detected by baroreceptors in the aorta / carotid arteries.
  • impulses are sent along the medulla.
  • impulses are then sent to the SAN via the sympathetic neurone.
  • noradrenaline is released & binds to receptors on the SAN.
  • heart rate increases to increase blood pressure to normal.
• high blood O2 & low CO2 / high pH:

  • detected by chemoreceptors in the aorta / carotid arteries.
  • impulses are sent to the medulla.
  • impulses are then sent to the SAN via the parasympathetic neurone.
  • releasing acetylcholine which binds to receptors on SAN.
  • heart rate decreases to return O2 , CO2 & pH to normal.
• low blood O2 & high CO2 / low pH:

  • detected by chemoreceptors in the aorta / carotid arteries.
  • impulses are sent to the medulla.
  • impulses are then sent to the SAN via sympathetic neurone.
  • releasing noradrenaline which binds to receptors on SAN.
  • heart rate increases and O2, CO2 & pH return to normal.
• high CO2 concentrations means the blood pH is low / acidic