Problem:

In the normal function of the kidney, the bloodstream is near emptied of fluids as it moves through the glomerulus, in which glomerular filtrate is formed.

As it proceeds through the Nephron, a significant amount of this material is returned to the bloodstream, and only a comparatively time amount goes into the urine.

Question: Why, in evolutionary terms, is it not more favorable energetically to remove only the small number of molecules, perhaps using specialized channels made of protein and activated only by a certain molecules which need removing, at a potentially huge energy saving?

Please explain in detail.