Abstract
The maintenance of osmolar constancy of the body fluids is dependent upon the recognition of osmolar disequilibrium and its correction by modifying the ingestion and excretion of fluid and solute. Osmolar changes are sensed by the hypothalamus which regulates the secretion of antidiuretic hormone to modify the renal excretion of water. The integrity of the system depends upon the renal ability to vary the solute concentration of urine.