This chapter contains sections titled:

• Introduction

• Total Body Water and Plasma Volume

• Hormonal and Physical Effects on Brain Adaptation

• Vasopressin

• Estrogen, Progesterone, and Androgens

• Physical Factors

• Age-Related Changes in Sodium and Water Homeostasis

• Thirst and Water Metabolism in the Elderly

• Regulation of Vasopressin

• Diabetes Insipidus

• Hyponatremia

• Hypernatremic States

• Disorders of Potassium

• Hypokalemia

• Key References

• References