Blood sodium is the concentration of serum sodium ion, which is the most abundant cation in the extracellular fluid. It plays a very important role in maintaining the volume of extracellular fluid, regulating acid-base balance and maintaining normal plasma osmolality, as well as normal cellular physiological functions, and it is also involved in the excitation transmission of nerve and muscle junctions. The normal value of serum sodium is between 135 and 145 mmol/L. High serum sodium may be seen in patients with severe vomiting, dehydration, aldosteronism, or patients who have consumed foods high in salt. Low serum sodium may be seen in patients with impaired renal tubular function or loss of large amounts of gastric juice by gastrointestinal drainage, as well as in patients on large amounts of anti-urinary hormone drugs.