ABSTRACT
The symptom of increased frequency of micturition arises due to either increased urine volume or the frequent passage of small volumes of urine. The kidneys, via a countercurrent exchange mechanism, maintain an increasing concentration gradient between the outer and inner medulla such that the extracellular fluid is more concentrated in the inner medulla. A good response to exogenous vasopressin indicates cranial diabetes insipidus, whereas a poor response occurs with nephrogenic diabetes insipidus and conditions that cause an impaired medullary concentration gradient. In the presence of Antidiuretic hormone, water channels move into the walls of the collecting duct, allowing water to move from the tubule to the renal interstitium, resulting in more concentrated urine and the retention of water within the body.
