Filtration fraction

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Parameter Value
renal blood flow RBF=1000 ml/min
hematocrit HCT=40%
renal plasma flow RPF=600 ml/min
filtration fraction FF=20%
glomerular filtration rate GFR=120 ml/min
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
ERPF=540 ml/min

In renal physiology, the filtration fraction is the ratio of the glomerular filtration rate (GFR) to the renal plasma flow (RPF).

Filtration Fraction, FF = GFR/RPF

The filtration fraction, therefore, represents the proportion of the fluid reaching the kidneys which passes into the renal tubules. It is normally about 20%.

The GFR on its own is the most common and important measure of renal function. However, in a condition such as renal artery stenosis, the blood flow to the kidneys is reduced. The filtration must therefore be increased in order to perform the normal tasks of the kidney in balancing fluid and electrolytes in the body. This would be reflected by a high filtration fraction, showing that the kidneys have to do more work with the fluid they are receiving. Diuretics such as loops and thiazides decrease the filtration fraction.

Catecholamines (Norepinephrine and Epinephrine) increase the filtration fraction by vasoconstriction of afferent and efferent arterioles, which is possibly activated by alpha 1 adrenergic receptors.

Severe haemorrhage will also result in an increased filtration fraction.