filtration fraction filtration fraction in renal physiology, the filtration fraction is the ratio of the glomerular filtration rate (gfr) over the renal plasma flow (rpf). parameter value renal blood flow rbf = 1000 ml/min hematocrit hct = 40% glomerular filtration rate gfr = 120 ml/min renal plasma flow rpf = 600 ml/min filtration fraction ff = 20% urine flow rate v = 1 ml/min sodium inulin creatinine pah sna = 150 meq/l sin = 1 mg/ml scr = 0.01 mg/ml spah = una = 710 meq/l uin = 150 mg/ml ucr = 1.25 mg/ml upah = cna = 5 ml/min cin = 150 ml/min ccr = 125 ml/min cpah = 420 ml/min er = 90% erpf = 540 ml/min filtration fraction, ff = gfr/rpf, or f f = g f r r p f . the filtration fraction, therefore, represents the proportion of the fluid reaching the kidneys that passes into the renal tubules. it is normally about 20%. gfr on its own is the most common and important measure of renal function. however, in conditions such as renal artery stenosis, blood flow to the kidneys is reduced. filtration fraction must therefore be increased in order to perform the normal functions of the kidney. loop diuretics and thiazide diuretics decrease filtration fraction. catecholamines (norepinephrine and epinephrine) increase filtration fraction by vasoconstriction of afferent and efferent arterioles, possibly through activation of alpha-1 adrenergic receptors. severe hemorrhage will also result in an increased filtration fraction.