Osmolarity throughout the nephron?

Hi,

Can anyone explain to me how osmolarity changes in the different parts of the nephron tubule, how it compares to the cortical/medullary interstitial fluid’s osmolarity, and the osmolarity of the blood? Why is the osmolarity of the glomerular filtrate the same as the filtrate in the blood despite the presence of proteins in the plasma?

Also, why doesn’t the osmolarity in the proximal convoluted tubule change when solutes are actively secreted into it?

Thanks