How is salt-water balance maintained in our body?

Salt-water balance is maintained by counter current mechanism in kidney. The counter current mechanism helps to concentrate the filtrate which occurs in the loop of Henle and vasa recta in the medulla region of the kidney. The flow of filtrate in the two limbs of loop of Henle is in opposite directions and thus forms a counter current. The flow of blood in the two limbs of vasa recta is also in opposite direction and therefore forms a counter current. The proximity between the loop of Henle's and vasa recta as well as counter current in them help in maintaining an increasing osmolarity towards the inner medullary interstitial fluid, that is from 300 mOsmolL^-1 in the cortex to about 1200 mOsmolL^-1 in the inner medulla. This gradient is mainly caused by NaCl and urea. NaCl is transported by the ascending limb of loop of Henle which is exchanged with the descending limb of vasa recta. NaCl is returned to the medullary interstitial fluid by the ascending portion of vasa recta. Similarly, small amounts of urea enter the thin segment of the ascending limb of loop of Henle which is transported back to the medullary interstitial fluid by the collecting duct. The overall function of counter current mechanism is to concentrate sodium chloride in the interstitial fluid and thereby cause water to diffuse out of the collecting ducts and concentrate the urine.