Sumit Kumar Tomas Berl

The maintenance of the tonicity of body fluids within a very narrow physiologic range is made possible by homeostatic mechanisms that control the intake and excretion of water. Critical to this process are the osmoreceptors in the hypothalamus that control the secretion of antidiuretic hormone (ADH) in response to changes in tonicity. In turn, ADH governs the excretion of water by its end-organ effect on the various segments of the renal collecting system. The unique anatomic and physiologic arrangement of the nephrons brings about either urinary concentration or dilution, depending on prevailing physiologic needs. In the first section of this chapter, the physiology of urine formation and water balance is described.

The kidney plays a pivotal role in the maintenance of normal water homeostasis, as it conserves water in states of water deprivation, and excretes water in states of water excess. When water homeostasis is deranged, alterations in serum sodium ensue. Disorders of urine dilution cause hyponatremia. The pathogenesis, causes, and management strategies are described in the second part of this chapter.

When any of the components of the urinary concentration mechanism is disrupted, hypernatremia may ensue, which is universally characterized by a hyperosmolar state. In the third section of this chapter, the pathogenesis, causes, and clinical settings for hyperna-tremia and management strategies are described.

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