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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