It is the transportation of NaCl (Sodium Chloride) in the high division of the ascending limb of the loop of Henle.
The Flat Ascending Limb of the Loop of Henle is penetrable to the chemical element NaCl (Sodium Chloride) which is not water. It causes tubular fluid throughout the descending limb in the body to the thick ascending limb of the body. It is impenetrable to water and urea without maybe perforable to Chlorine.