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.'
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