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
- PBS is hydrostatic pressure in Bowman's space
- πGC is oncotic pressure in glomerular capillaries
- πBS is oncotic pressure in Bowman's space
- 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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Level of Evidence 5 and Other Journal Articles (includes Case Reports, Expert Opinions,
Personal Observations, and Biomechanic Studies)
Snyder S, Pendergraph B. Detection and evaluation of chronic kidney disease. Am Fam Physician. 2005 Nov 1;72(9):1723-32. Review.
PMID:16300034 (Link to Abstract)
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