ABSTRACT

The plasma concentration of sodium, or [Na+]p, and the plasma osmolality are very closely related measures. The correlation between them is so tight that [Na+]p is commonly used as a surrogate for plasma osmolality. Indeed, as we shall discuss below, after introducing the concept of effective osmoles, [Na+]p may be the physiologically more relevant variable. Disturbances in which [Na+]p is reduced, collectively referred to as hyponatremia, are virtually always accompanied by a reduction of plasma osmolality. Similarly, disturbances in which [Na+]p is increased, collectively referred to as hypernatremia, are indicative of an elevation of plasma osmolality. Properly understood, both hyponatremia and hypernatremia are not single diseases, but instead syndromes, and each has multiple potential causes. Correct diagnosis and management of these two syndromes depend critically on an understanding of the underlying physiology.