GFR

Renal Physiology Series No. 3Charles J. Foulks, M.D.

9/14/2011

GFR

GFR

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

GFR

GFR

GFR

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

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)

GFR

GFR

GFR

↑ Afferent Constriction = ↓ Filtration

GFR

↑ Efferent Constriction = ↑ Filtration

GFR

GFR

GFR

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

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

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

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

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

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)

GFR

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

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.