How HR affects CO

• How HR affects CO:

  • ANS is the main regulator of HR
  • SAN receives dual innervation from SNS and PNS
  • dual innervation is always based on reciprocal inhibition
  • which means only one branch works at one time
• • at rest the parasympathetic NS is more dominant, inhibiting the SAN via the Vagus nerve, setting the Vagal tone to a HR less than 100 bpm
  • during physical or psychological stress, the sympathetic NS becomes more dominant, with the release of catecholamines (specifically epinephrine) as well, exciting the SAN, increasing HR
• Modulation of HR:

  • inherent HR is determined by the SAN - modulated according to tasks/body needs via:
  • SNS + catecholamines (epinephrine)
  • PNS
  • normal HR is dependent on ionic and hormonal homeostasis
• Chronotropes:

  • positive chronotropes increase HR, include:
  • SNS
  • epinephrine
  • thyroxine - hormone that increases metabolic rate and HR
  • pyrexia - fever - 1 degree increase in HR is equal to 13% increase in metabolic rate
  • infection
  • heat
  • hypercalcaemia - excess of calcium in blood
  • negative chronotropes decrease HR, include:
  • PNS
  • myocardial ischaemia/hypoxia
  • hyperkalaemia - excess potassium
  • hypothermia