The drugs furosemide, ethacrynic acid and bumetanide act on the medullary part of the ascending limb of the loop (loop of Henle) of the nephron. They inhibit the reabsorption of chloride and sodium ions from the loop into the interstitial ﬂuid. The result is that the interstitial ﬂuid becomes relatively hypotonic. If a high concentration of ions is present here, water will ﬂow from the adjacent collecting duct into the interstitial ﬂuid and back into the bloodstream. Good control of water balance is achieved by alterations in the permeability of the collecting duct to water by the presence of antidiuretic hormone (ADH) from the posterior pituitary gland. This is one of the major control systems for water balance, and slight interference here will completely upset the normal function of the kidney and result in a variation in urine output. A hypotonic interstitial ﬂuid will result in a diuresis.