GFR divided by renal plasma flow, normally about 20 per cent
Expansion
Filtration fraction = GFR / renal plasma flow, normally about 125 / 625 = 20 per cent.
Rises when the efferent arteriole constricts, as with angiotensin II: GFR is maintained while plasma flow falls.
Falls when the afferent arteriole constricts, or with efferent dilatation.
The physiological importance lies downstream. A higher filtration fraction means a greater proportion of plasma has been filtered, so the blood entering the peritubular capillaries is more concentrated, with a higher oncotic pressure and a lower hydrostatic pressure.
Both changes favour reabsorption from the proximal tubule back into the capillary.
This is the mechanism of glomerulotubular balance in hypovolaemia: angiotensin II raises the filtration fraction, which automatically enhances proximal salt and water reabsorption without any change in hormone action on the tubule itself. It is a neat example of physical forces performing regulation.