1. GFR Renal Physiology Series No. 3 Charles J. Foulks, M.D. 9/14/2011

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4. GFR • GFR = Kf x Pnet • Kf= leakiness of capillaries • PNet= net hydrostatic pressure = • PG - PB - πG • Kf of glomerulus is 400-fold higher than • Kf of any other capillaries • Capillary

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8. GFR • GFR cannot exceed 20% of RPF UNLESS • Increase in Pgc or • Increase in RPF • Why? • Filtration equilibrium has occurred at GFR=0.2xRPF • RPF is an important determinant of GFR

9. GFR • If RPF falls and Pgc is unchanged, GFR will still fall by the proportional amount of decrease in RPF. • What determines Πgc? • Π in the AA and • By removal of filtrate (high uf rate or filtration fraction)

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12. GFR ↑ Afferent Constriction = ↓ Filtration

13. GFR ↑ Efferent Constriction = ↑ Filtration

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17. GFR • GFR is relatively constant over arterial • BPs of 80-170 mm Hg • Persists in isolated kidney • Independent of nervous system • No autoregulation would create 46 liters/day of urine if BP = 125 mm Hg • = 6 liters/day with autoregulation

18. GFR • GFR Increases with: • Increased glomerular blood flow • Decreased afferent arteriolar resistance • Increased efferent arteriolar resistance • Sympathetic stimulation (extreme situations only) lowers GFR • NE and Epi lower GFR

19. GFR • Autoregulation of GFR • Mediated by Tubuloglomerular Feedback • Low NaCl (flow) at Macula Densa: • Lowers afferent arteriolar resistance (?) • Raises efferent arteriolar resistance (AII) • Macula Densa also regulates renal BP via renin-angiotensin-aldosterone

20. Renal Clearance • Renal Clearance (CS) is the volume of plasma completely cleared of a substance (S) per minute • Units are ml/min • CS = (US x V)/PS • US is [S] in urine, V is urine flow rate, PS is [S] in plasma

21. Renal Clearance • CS = (US x V)/PS • Filtered-Only substances (no secretion or reabsorption) have CS = GFR • Example: Inulin • Secreted substances have CS > GFR • Example: PAH • Reabsorbed substances have CS < GFR • Example: glucose

22. Renal Clearance • Renal clearance of inulin allows clinical determination of GFR • GFR = (UI x V)/PI • PAH is 90% secreted. Renal clearance of PAH allows clinical determination of Renal Plasma Flow • RPF = (UPAH x V)/(PPAH x 0.9)

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24. GFR • What does a normal GFR really mean? • Determined by measuring GFR in normal people with no active diseases, normal urinalysis, normal electrolytes and minerals. No symptoms of renal disease either (nocturia). • If your GFR is normal then we know to expect • Normal EPO • Normal electrolytes, minerals, volume, water • Normal drug metabolism or clearance

25. GFR • The GFR is a guide • It tells you when to expect certain things to happen or to see certain perturbations. • It tells you when to alter medications. • It tells you when to plan for RRT. • It tells you when to place vascular access or dialysis catheter and when to discuss transplantation.