Updated: 7/28/2020

Autoregulation

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Introduction
  • Autoregulation is a mechanism that ensures constant blood flow to organs, despite changing resistance (perfusion pressures) in the blood vessels
    • 2 hypotheses explain autoregulation
      • myogenic hypothesis
      • metabolic hypothesis
    • recall that blood flow (Q) = (change in pressure [P])/resistance (R)
Hypotheses of Autoregulation
  • Myogenic hypothesis
    • when the vascular smooth muscle is stretched (like when arterial pressure is increased), the smooth muscle contracts (increases resistance)
      • when there is an increase in blood pressure, the myogenic reflex causes smooth muscle contraction in order to maintain flow
  • Metabolic hypothesis
    • O2 delivery to a tissue is matched to O2 consumption of that tissue
      • this is accomplished by changing the resistance (and blood flow) of the arterioles
    • metabolic activity causes tissues to produce metabolites, including vasodilators
      • CO2, H+, K+, lactate, and adenosine
    • ↑ metabolic demand → ↑ O2 demand → vasodilation → ↓ resistance
      • this results in ↑ blood flow
 
Autoregulation of Organs
Organ Factors Determining Autoregulation
Heart
  • Local metabolites
    • pO2, adenosine, pCO2, and NO  
Brain
  • Local metabolites
    • pCO2 and pH
Kidneys
  • Myogenic hypothesis
    • tubuloglomerular feedback
Lungs
  • Local metabolites
    • pO2 → vasoconstriction
Skeletal muscles
  • Local metabolites
    • lactate, K+, and adenosine
Skin
  • Sympathetic innervation
 
Organ-Specific Autoregulation
  • Heart
    • most sensitive to
      • pO2, adenosine, pCO2, and NO
      • adenosine results in coronary vasodilation 
    • ↑ myocardial contractility → ↑ O2 demand and consumption → vasodilation → ↑ blood flow
  • Brain
    • most sensitive to
      • pCO2 and pH
    • ↑ pCO2 → ↓ pH → vasodilation→ ↑ blood flow to remove excess CO2
  • Kidneys
    • myogenic hypothesis
    • tubuloglomerular feedback
      • ↑ renal arteriole pressure → ↑ blood flow
        • ↑ glomerular filtration rate (GFR)
      • ↑ GFR increases delivery of solute and water to juxtaglomerular apparatus (JGA)
        • JGA secretes vasoactive substance
          • constriction of afferent arterioles
          • returns renal blood flow and GFR back to normal
  • Lungs
    • most sensitive to
      • low pO2, which causes vasoconstriction NOT vasodilation
        • the only organ in which low pO2 causes vasoconstriction
        • causes only well-ventilated areas to be perfused to maximize areas of gas exchange
  • Skeletal muscles
    • most sensitive to
      • vasoactive metabolites during exercise (e.g., lactate, K+, and adenosine)
      • sympathetic innervation at rest
        • recall that α1 receptors cause vasoconstrict and β2 receptors cause vasodilation
  • Skin
    • most sensitive to sympathetic innervation
      • ↓ body temperature → α1 receptors vasoconstrict
        • ↓ blood flow → retention of heat
      • ↑ body temperature  → inhibition of cutaneous vasoconstriction
        • ↑ blood flow from warm core to cutaneous vessels → dissipation of heat
    • plays a major role in the regulation of body temperature

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