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

2. GFR

3. GFR

4. GFR GFR = Kf x Pnet PNet = net hydrostatic pressure = PG - PB - πG
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

5. GFR

6. GFR

7. GFR

8. GFR GFR cannot exceed 20% of RPF UNLESS Why?
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)

10. GFR

11. GFR

12. GFR
↑ Afferent Constriction = ↓ Filtration

13. GFR
↑ Efferent Constriction = ↑ Filtration

14. GFR

15. GFR

16. GFR

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)

23. GFR

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.