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Glomerular Filtration Rate (GFR)

Topic updated on 08/29/17 11:31am

Overview
 

 

 
Glomerular Filtration Rate (GFR)
  • GFR is an important clinical tool used to assess patients with kidney disease
    • this is an index of functional renal mass
      • therefore, this data point can be used to determine the severity and course of renal disease
  • Fluid movement across the glomerulus follows Starling's law
    • GFR = Kf [(PGC - PBS) - (πGC - πBS)] 
      • Kf is filtration coefficient
        • water permeability or hydraulic conductance of glomerular capillary wall
      • PGC is hydrostatic pressure in glomerular capillaries
        • PGC favors filtration
      • PBS is hydrostatic pressure in Bowman's space
        • PBS opposes filtration
      • πGC is oncotic pressure in glomerular capillaries
        • πGC opposes filtration
      • πBS is oncotic pressure in Bowman's space
        • typically, πBS = 0
  • Glomerular filtration rate and arteriolar resistance
    • the glomerular capillaries are between the afferent (precapillary) and efferent (postcapillary) arteriole
      • e.g., a drop in renal perfusion pressure (e.g., hypotension) stimulates the activation of the renin-angiotensin system which produces angiotensin II
        • angiotensin II preferentially causes vessel constriction of the efferent arteriole
          • this in turn increases PGC
            • the idea is to prevent PGC from declining in the setting of hypotension
Measurement of Glomerular Filtration Rate (GFR)
  • GFR measurement (inulin
    • C(inulin) = GFR
      • inulin is freely filtered across glomerular capillaries and is neither reabsorbed nor secreted
      • inulin is a glomerular marker
    • C(inulin) = [U(inulin) x V] / P(inulin) = GFR
      • C(inulin) is clearance of inulin (mL/min)
      • U(inulin) is urine concentration of inulin (mg/mL)
      • V is urine flow rate (mL/min)
      • P(inulin) is plasma concentration of inulin (mg/mL)
    • normally, GFR ≈ 100 mL/min
  • GFR measurement (creatinine)
    • C(creatinine) ≈ GFR
      • creatinine is freely filtered across glomerular capillaries and is moderately secreted by peritubular capillaries into tubular lumen
        • C(creatinine) slightly overestimates GFR by 10-20%

 



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