Autoregulation

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

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