Pre-Load

• Pre-load:

  • is a degree of the heart filling
  • consists of/determined by 2 factors:
  • filling time (HR)
  • increase HR = shorter diastole = less filling time
  • decrease HR = longer diastole = longer filling time = more filling
  • volume of venous return
  • increase VR = increase filling of heart = increase SV
  • factors increasing VR
  • increase skeletal muscle pump work
  • increase respiratory muscle pump work
  • venoconstriction via SNS
• Pre-load:

  • increase filling/pre-load -> increase emptying/SV
  • this is because pre-load generates a myocyte stretch
  • an increased myocyte stretch -> increase force of contraction -> increase SV -> increase CO
  • within physiological limits, stretching fibres:
  • optimises actin/myosin overlap -> optimises crossbridge opportunities -> increase force -> increase SV
• The Frank-Starling Law of the heart:

  • the heart has the intrinsic ability to adjust its force production, and therefore its SV, in proportion to myocardial fibre length
  • emptying is always matched to filling
  • i.e. SV is always ~ filling amount at rest