Natrium (plasma, serum) 137-145 mmol/L
The body's sodium content is about 4 mol (92 g) and is distributed 45 % as unorganic sodium compounds in the skeleton and 55 % as monovalent cations (Na+) both extracellularly (50 %) og intracellularly (5 %). Sodium is the dominating cation in the extracellular fluid and is thereby decisive for osmolality. Sodium is filtered freely in the glomeruli and 99.5 % is reabsorbed in the tubules. Reabsorption of sodium is stimulated by aldosterone. The concentration of Na+ i plasma is regulated primarily by ADH and thirst mechanisms.
Regulating the volume is prioritised over osmolality. Big changes in volume can therefore make it difficult to interpret concentrations of Na in the blood. The symptoms of hyper- or hyponatremia are dependent on how big the changes are and how quickly they occurred. Hyponatremia is characterised by nausea, headache, fatigue, cramps, coma, and death if the sodium falls from 125 to 100 mmol/L. Hypernatremia leads primarily to thirst, and with increasing concentration comes fatigue, irritability, weakness, cramps, and eventually coma and death.
Read more about sodium i The handbook on medical biochemistry