Sodium is concerned with the physiology, pathophysiology, and clinical consequences of altered physiology involving the sodium ion.The first section focuses on the presence and handling of sodium in the normal state. In chapter one, Drs. Solomon and Galey deal with the fundamentals of transport and energy metabolism as they relate to sodium. This is followed by a chapter in which Drs. Gardenswartz and Schrier consider in detail the normal body economy of sodium, and especially the factors (particularly extracellular fluid volume) that regulate the renal handling of sodium and the responses of the various portions of the nephron to these influences.