Filtration Fraction = GFR/(1-hematocrit)RBF
The filtration fraction therefore represents the proportion of the fluid reaching the kidneys which passes into the renal tubules.
The GFR on its own is the most common and important measure of renal function. However, in a condition such as renal 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.
Catecholamines (Norepinephrine and Epinephrine) increase the filtration fraction by vasoconstriction of Afferent and Efferent Arterioles which poss alpha 1 adrenergic receptors.