The human bodys ability to regulate plasma osmolality within a very narrow and well-defined physiological range underscores the vital importance of preserving water and sodium balance at rest and during exercise.The principle endocrine regulator of whole body fluid homeostasis is the posterior pituitary hormone, arginine vasopressin (AVP).Inappropriate AVP secretion may perpetuate either slow or rapid violation of these biological boundaries, thereby promoting pathophysiology, morbidity and occasional mortality.