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 = K_f [(P_GC – P_BS) – (π_GC – π_BS)] 
    • K_f is filtration coefficient
      • water permeability or hydraulic conductance of glomerular capillary wall
    • P_GC is hydrostatic pressure in glomerular capillaries 
      • P_GC favors filtration
      • acutely decreased after myocardial infarction
    • P_BS is hydrostatic pressure in Bowman’s space
      • P_BS opposes filtration
    • π_GC is oncotic pressure in glomerular capillaries
      • π_GC opposes filtration
      • chronically decreased by hypoalbuminemia
    • π_BS is oncotic pressure in Bowman’s space
      • typically, π_BS = 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 P_GC
          • the idea is to prevent P_GC 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%