Introduction

Thirst, hunger, and renal function control water and electrolyte balance. These systems compensate for losses through the skin, respiratory tract, and gastrointestinal (GI) tract. Hormones and (to a lesser extent) local cell modulatory molecules (eg, eicosanoids) are also critical. With age, all of these functions change, making the elderly more prone to problems with water and electrolyte balance. Imbalances are most likely to occur when acute illness, hospitalization, use of medications, or extremes of weather upset homeostasis.