This model is more close to what actually happens in the various parts of certain tubes in a kidney. A kidney has a bunch of nephrons and a nephron has a part called ‘loop of Henle’. Different parts of the loop, selectively reabsorb different substances. 

Read a little about the loop of Henle and adjust the parameters such that you are modeling an ascending limb or a descending limb of the loop. Explain your answers.