G5i: Describe the compensatory mechanisms in a fit person moving from supine to standing position

Effects of Moving Supine to Standing

1) MAP

    • Circulation is a column of fluid ∴subject to forces of gravity
    • ↑ hydrostatic P at feet
    • ↓hydrostatic P at head
    • Pooling of blood in distal limb veins
    • ↓SV
    • ↓CO by F – S mechanism
    • ∴↓MAP

2) ↓Cerebral Perfusion Pressure

  • ↑vertical distance from heart → brain
  • Brain is 30cm above head
  • ∴↓MAP by 30cm H2O = 22mmHg
  • ∴↓MAP from 90 → 68mmHg

CPP = MAP – CVP (ICP)

  • ∴↓CPP
  • ↓CBF
  • Cerebral ischaemia/fainting if no compensation

Aim of Compensatory Mechanisms

  1. Maintain MAP
  2. Maintain CBF

Compensatory Mechanisms

Map Maintenance

    • Baroreceptor reflex
      • ↓venous return
      • ↓MAP
      • Detected by high pressure BaroReceptors
      • ↓firing
      • At Vasomotor Centre Medulla
        • ↓symp. inhibition
        • ↓parasymp. outflow
      • ↑symp & ↓parasymp outflow
      1. Heart → ↑HR, ↑FoC
      2. Resistance Vessels → VC → ↑SVR
      3. Capacitance Vessels → VC → ↑VR
    • ↑Venous return
      • Venoconstriction
      • Muscle pump → ↑VR
      • Thoracic pump (with respiration) → ↑VR
      • One way venous valves → propels blood forward
    • NET EFFECT → Restore MAP

CBF Maintenance

CBF
  • CBF remains constant with autoregulation between MAP 60 – 160mmHg

CPP

CPP = MAP – CVP (ICP)

↓CVP

  • Upright posture
  • Blood drains from vessels → RA

ICP

    • Cranium is fixed volume
    • ↓CVP → ↓ICP
    • Standing also allows CSF pooling in spinal cord → ↓ICP
    • ∴CPP maintained

CVR

  • Metabolic autoregulation
    • ↓CBF → build up of metabolites: CO2, H+, K+, adenosine → VD → ↓CVR → ↑CBF
  • Pressure autoregulation
    • Myogenic mechanism
    • ↓MAP → ↓CPP → ↓arteriole stretch → vessels dilate → ↓CVR → ↑CBF
  • NET EFFECT: Maintain CBF
  • OVERALL: on standing MAP ↓20% → lasts a few seconds → compensatory mechanisms restore MAP & CBF