Urinary concentrating ability of the kidney is increased by

When there is an ECF volume expansion, there is increased renal blood flow and increased GFR. When renal blood flow is more, the solutes in renal medulla are washed off. And this reduces the medullary concentration. Similarly, when increased GFR, Na+ reabsorption power is decreased, this leads to a decrease of the medullary concentration. When ECF volume contraction occurs, the medullary concentration is increased.