Introduction Filtration fraction (FF) represents the ratio between glomerular filtration rate (GFR) and renal plasma flow (RPF) this in turn represents which fraction of RPF becomes filtered via the glomerular capillaries the normal filtration fraction is ~20% and thus implies that 80% of the RPF is not filtered the unfiltered RPF will then be part of the peritubular capillary blood flow after entering the efferent arteriole Filtration fraction (FF) = GFR / RPF Filtered load = GFR x plasma concentration Renal plasma flow (RPF) can be determined by using clearance of para-aminohippuric acid (PAH) RPF = [U(PAH) x V] / [RA(PAH) - RV (PAH)] U(PAH) = urine concentration of PAH (mg/mL) V = urine flow rate (mL/min) RA(PAH) = renal artery concentration of PAH (mg/mL) RV(PAH) = renal vein concentration of PAH (mg/mL) determining the RPF will allow you to determine the renal blood flow (RBF) RBF = RPF / (1 - hematocrit) Changes in Starling Force and Its Effect On RPF, GFR, and FF Condition RPF GFR FF (GFR/RPF) Afferent arteriole constriction ↓ ↓ Unchanged Efferent arteriole constriction ↓ ↑ ↑ ↑ plasma protein concentration Unchanged ↓ ↓ ↓ plasma protein concentration Unchanged ↑ ↑ Ureter constriction Unchanged ↓ ↓