Physiology and Pathophysiology of Water Homeostasis

In the evolutionary process, the successful adaptation of living organisms initially to an aqueous and thereafter to an arid terrestrial environment posed radically different challenges to the maintenance of water balance. Whereas the former required defense against water excess, the latter called for water conservation. To meet such challenges, the mammalian nephron evolved mechanisms for increasing both water excretion by diluting and water conservation by concentrating the urine. This chapter reviews the process whereby the osmosensors control thirst and the secretion of the antidiuretic hormone (vasopressin) to allow for either urinary dilution or concentration and thereby delicately maintain tonicity of body fluids within a very narrow range. Central to this process is the now well-defined cellular pathway whereby vasopressin renders the collecting duct, water permeable. Disorders of vasopressin secretion and action result in disturbances of body fluids tonicity, which are clinically recognized as abnormalities in reduced plasma sodium concentration or hyponatremia.