heart rate control

• cardiac muscle is myogenic; contracts of its own accord
• sinoatrial node (SAN): located in right atrium and known as the pacemaker
• atrioventricular node (AVN): located near border of right and left ventricle within atria
• bundle of HIS: runs through the septum
• purkyne fibres: in the walls of the ventricles
• heart contraction:

  • SAN releases wave of depolarisation across the atria, causing it to contract
  • AVN releases another wave of depolarisation when the first reaches it (non-conductive layer prevents from reaching ventricles)
  • bundle of HIS conduct the wave of depolarisation down the septum and the purkyne fibres in the walls of the ventricles, so walls of ventricles contract
  • short delay before this happens, whilst AVN transmits second waves of depolarisation
  • allows enough time for atria to pump all blood into the ventricles
• sympathetic nervous system increases frequency of electrical impulses
• parasympathetic nervous system decreases frequency of electrical impulses
• response to increased blood pressure:

  • increased pressure
  • baroreceptors in aorta are stretched
  • electrical impulses sent to medulla oblongata
  • next impulses sent via parasympathetic nervous system to SAN to decrease frequency of electrical impulses
  • reduces heart rate
• response to decreased blood pressure:

  • decreased pressure
  • baroreceptors in wall of aorta not stretched
  • electrical impulses sent to medulla oblongata
  • next impulses sent via sympathetic nervous system to SAN to increase frequency of electrical impulses
  • increases heart rate
• response to decrease in blood pH (more acidic):

  • decreased pH
  • chemoreceptor in walls of aorta
  • electrical impulses sent to medulla oblongata
  • next impulses sent via sympathetic nervous system to SAN to increase frequency of electrical impulses
  • increase heart rate to deliver blood to lungs rapidly to remove CO2