Lecture 7

• The more preload, higher stroke volume, however excessive amounts can diminish the volume (frank-starling)
• Preload best measured as end diastolic volume - ECHO
• Preload increased by:

  1. circulating volume
  2. central venous pressure
  3. resp pump
  4. atrial filling or contraction
  5. slower heart rate as prolongs diastole
  6. ventricular compliance
• Bowditch effect:

  increase HR increase force contraction and vice versa

  high levels HR eventually lead decreased force contraction

  length muscle not affected, thus independent of frank-starling
• Bainbridge reflex:

  1. increased venous return
  2. baroreceptors atria detect increased stretch
  3. HR increased via symphathetic stimulation to SAN
  4. anatgonistic to carotid baroreceptor response (BP)
  5. involved in sinus arrhythmia
• Sympathetic actions (direct cardiac)

  Positive chronotropy (increase HR) - SAN

  Positive dromotropy (increases conduction velocity) - AVN

  Positive inotropy (increases contraction) - ventricles and atria

  Positive lusitropy (increases relaxation) - ventricles and atria
• Sympathetic actions (systemic)

  activation of RAAS

  suprarenal stimulation - catecholamines
• Action of RAAS

  1. angiotensin II, vasoconstriction, increased sodium ions and water retention
  2. aldosterone, increased sodium ions and water retention
  3. vasopressin (ADH), promotes water retention
• catecholamine release into bloodstream is regulated by sympathetic nervous system
• parasympathetic actions

  1. decreased chronotropy
  2. decreased dromotropy
  3. decreased inotropy
  4. ventricular contractility (no significant effect)
  5. RAAS (no significant effect)
• ANS regulation

  baroreceptors

  local e.g renal juxtaglomerular apparatus

  'higher' e.g fear
• Baroreceptors

  located in carotid sinuses and aortic arch

  decreased arterial pressure, decreased firing

  increased arterial pressure, increased firing
• increased baroreceptor firing decreases sympathetic tone and increases parasympathetic tone
• decreased baroreceptor firing increases sympathetic tone and decreases parasympathetic tone