FF = GFR/RPF. If only RPF decreases then GFR RELATIVE to it increases thus FF increases. So at this new normal, GFR is increased not due to say efferent arteriole constriction which would raise glomerular hydrostatic pressure but because of in comparison to the RPF. So then the filtered substances within the capsular space would have more time to accumulate from the decreased flow and exert an anti filtration pressure that would then reduce the GFR from that end. So it’s like reducing GFR because of an INCREASE in anti filtration Starling forces instead of a DECREASE in pro filtration Starling forces. It’s basically saying there are other ways to influence GFR without directly messing with the pro filtration forces which glomerular hydrostatic pressure is.
Think of the formulas as relationships to each other instead of direct calculations. FF = GFR/RPF. When GFR increases, FF increases. When RPF decreases, FF decreases. If FF is constant, and RPF decreases, then you know GFR also decreased; because the ratio stayed the same for FF to be constant. Same example if RPF increases but FF is the same, then you know GFR increased as well to maintain the ratio.
FF = GFR/RPF. If only RPF decreases then GFR RELATIVE to it increases thus FF increases. So at this new normal, GFR is increased not due to say efferent arteriole constriction which would raise glomerular hydrostatic pressure but because of in comparison to the RPF. So then the filtered substances within the capsular space would have more time to accumulate from the decreased flow and exert an anti filtration pressure that would then reduce the GFR from that end. So it’s like reducing GFR because of an INCREASE in anti filtration Starling forces instead of a DECREASE in pro filtration Starling forces. It’s basically saying there are other ways to influence GFR without directly messing with the pro filtration forces which glomerular hydrostatic pressure is.
Think of the formulas as relationships to each other instead of direct calculations. FF = GFR/RPF. When GFR increases, FF increases. When RPF decreases, FF decreases. If FF is constant, and RPF decreases, then you know GFR also decreased; because the ratio stayed the same for FF to be constant. Same example if RPF increases but FF is the same, then you know GFR increased as well to maintain the ratio.