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

GFR

GFR

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

GFR

GFR

GFR

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

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.