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Overview
  • Local control of blood flow
    • autoregulation
      • mechanism by which blood flow to an organ remains constant in response to changing arterial (perfusion) pressures
  • Metabolic hypothesis
    • oxygen delivery to a tissue is matched to oxygen consumption by that tissue
    • in response to metabolic activity, a tissue produces various vasodilatng metabolites that produce vasodilation of arterioles
      • vasodilation decreases resistance and increases blood flow
        • blood flow increases to meet increased oxygen demand
Organ
Factors determining autoregulation
Heart
Local metabolites — O2, adenosine, NO  
Brain
Local metabolites — CO2 (pH)
Kidneys
Myogenic and tubuloglomerular feedback
Lungs
Hypoxia → vasoconstriction
Skeletal muscle
Local metabolites — lactate, adenosine, K+
Skin
Sympathetic stimulation — temperature control

Regulation by Organ
  • Kidney  
    • local metabolic control 
      • myogenic hypothesis
        • ↑ renal arteriole pressure → ↑ renal blood flow (RBF) and ↑ GFR
        • increase in renal arterial pressure stretches blood vessel walls of afferent arterioles, which respond by contracting via stretch-induced contraction
          • opens stretch-activated Ca2+ channels in smooth muscle cell membranes
        • afferent arteriole contraction increases afferent arteriole resistance
          • balances increase in afferent arteriole pressure
            • maintains constant renal blood flow (RBF)
      • tubuloglomerular feeback
        • ↑ renal arteriole pressure → ↑ renal blood flow (RBF), ↑ GFR
        • ↑ GFR increases delivery of solute and water to macula densa
          • macula densa is component of juxtaglomerular apparatus (JGA)
        • in response to increased delivered load, JGA secretes a vasoactive substance that constricts afferent arterioles
          • returns RBF, GFR to normal
  • Lung
    • local metabolic control
      • hypoxia (↓ PO2) → vasoconstriction
        • shunts blood away from areas of the lung that are poorly ventilated
          • helps to match ventilation (air flow) and perfusion (blood flow)
        • pulmonary vasculature is unique in that hypoxia causes vasoconstriction such that only well-ventilated areas are perfused
  • Skeletal muscle
    • sympathetic control (rest)
      • at rest, circulation is regulated primarily by sympathetic innervation
      • skeletal muscle
        • arteriole vascular smooth muscle is innervated by sympathetic nerve fibers
          • vasoconstricting, α1-adrenergic receptors
      • NE → (+) α1 receptors → ↑ vasoconstriction → ↑ resistance → ↓ blood flow
    • local metabolic control (exercise)
      • during exercise, circulation is regulated primarily by local metabolites
        • lactate, adenosine, potassium (K+)
  • Skin
    • overview
      • cutaneous blood vessels exhibit sympathetic innervation that controls blood flow
        • principal function is regulation of body temperature
    • Sympathetic Control
      • ↑ body temperature
        • ↓ sympathetic tone → ↑ vasodilation → ↑ flow → dissipation of heat
      • ↓ body temperature
        • ↑ sympathetic tone → ↑ vasoconstriction → ↓ flow → retention of heat
 

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