So I'm kind of confused with the terms osmolarity and osmotic pressure when it refers to the kidney. For example, when it says the descending loop of henle is impermeable to salt, so filtrate osmolarity goes up. Isn't the filtrate inside the loop, which is permeable to water at the descending loop? Does that mean the water will go out of the loop but the salts stay in? I understand osmolarity on its own but for some reason when applied to the kidney it is confusing the crap out of me.