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Updated: Oct 8 2022

Glomerular Filtration Rate (GFR)

  • 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
          • acutely decreased after myocardial infarction
        • PBS is hydrostatic pressure in Bowman's space
          • PBS opposes filtration
        • πGC is oncotic pressure in glomerular capillaries
          • πGC opposes filtration
          • chronically decreased by hypoalbuminemia
        • 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%
          • Non-linear relationship between GFR and creatinine
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