The Clinical Physiology of Water Metabolism

IN normal man osmolality, or concentration of osmotically active solutes in body fluids or total body water, is remarkably constant despite large variations in water and solute intake and excretion. Every kilogram of body water contains 285 to 290 milliosmoles of solute consisting primarily of salts of sodium in extracellular and of potassium in intracellular fluids. The identical osmolality of intracellular and extracellular fluids is produced by free movement of water across all cellular and subcellular membranes, governed only by the physical forces of osmosis and diffusion. The single exception is control of water permeability by antidiuretic hormone (ADH) at . . .