Sodium and its accompanying anions, chloride and bicarbonate, are the principal solutes in the extracellular fluid (ECF). Because sodium and chloride are nearly completely excluded from the cells, they behave as impermeant solutes and account for most of the osmolality and tonicity of the ECF. Sodium chloride is therefore the main determinant of ECF volume, since changes in its concentration cause changes in water intake and in renal water excretion, due to stimulation or suppression of thirst and antidiuretic hormone secretion, respectively. Except for some relatively unusual pathological states such as diabetic hyperglycaemia and the presence of exogenous mannitol in the ECF, it is also the main determinant of the distribution of water between the ECF and the cells; this is because the cell membrane is highly permeable to water and any tonicity gradient between the two compartments is immediately abolished by osmotic water movement. The ECF sodium concentration is normally held within narrow limits, and deviations from this normal range are classified as hyponatraemia and hypernatraemia.

OSMOLALITY AND TONICITY

Osmolality is a physical property of a solution and is proportionate to the total concentration of solute—that is, the number of dissolved particles per kg of solvent (water) irrespective of their chemical nature. It is measured in the laboratory by methods such as freezing point depression or vapour pressure estimation—this is the true or measuredosmolality. Apart from sodium chloride, other solutes contributing significantly to ECF osmolality are urea and glucose, normally at about 5 mmol/l (equivalent to 5 mOsm/kg water) each. At physiological concentration, sodium chloride is not fully ionised, with a dissociation factor of about 1.86, thus contributing rather less to total osmolality than if it were fully dissociated. However, a good approximation to the osmolality of the ECF is obtained by doubling the sodium concentration and adding the urea …