I have a doubt. If filtration fraction is the fraction of substance that enters the kidney that is filtered and it is 20% then how ca the glomerular filtrate be isotonic to plasma? Similarly after reabsorption from pct?

GF is not isotonic to plasma, in fact it is hypertonic esp in the PCT and that is the reason why most substances are reabsorbed at this point because it's the site of the highest [ ] gradient. FF is the fraction of volume and not of [ ]