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Osmolarity; The Relation Between Compartment Solute Content and Solvent Volume
The osmolarity of the body fluids is a function of the number of solute particles present and the volume of total body water in which those particles are dissolved. All body fluid compartments are isosmotic with each other despite differences in the distribution of chemical species among the compartments. To maintain a normal osmolarity and volume of body fluid the daily losses of water and Na+ must be balanced by ingestion of water and Na+. Body fluid osmolarity is regulated by renal control of water retention or excretion mediated by the mechanisms described above, and by control of water ingestion by the thirst mechanism.
Since Na+ is the major osmotically effective solute of the ECF conditions of excess water are characterized by hyponatremia and conditions of water deficit are associated with hypernatremia.
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