Despite losing 20%-25% of their original kidney volume, older individuals maintain body fluid hemostasis under most circumstances; however, their ability to withstand environmental, disease-related, or iatrogenic stresses becomes progressively narrowed. Glomerular filtration rates fall with each decade, accompanied by limitations on sodium conservation, potassium ion secretion, and acid excretion. Medications used by older patients are a common cause of hyperkalemia through a number of pathophysiologic mechanisms. In addition, water homeostasis frequently fails due to defects in thirst, urinary concentrating ability, and free water excretion, resulting in hypernatremia or hyponatremia in many sick older patients.