Читать книгу Disorders of Fluid and Electrolyte Metabolism - Группа авторов - Страница 21

In 1958, Edelman et al. [4] demonstrated that the major determinants of plasma sodium concentration are total exchangeable body sodium, total exchangeable body potassium, and total body water. A simplified version of what has come to be known as the Edelman equation is:

where PNa is the sodium concentration in plasma water (in mmol/L), Na_E is the total exchangeable body sodium (in mmol), K_E is the total exchangeable body potassium (in mmol), and TBW is the total body water (in L).

Normal sodium concentration in whole plasma is 140 mmol/L. However, plasma is composed of aqueous (93%) and non-aqueous (7%) fractions. Sodium is present only in the aqueous fraction. Therefore, sodium concentration in plasma water is 140/0.93 or 150.5 mmol/L [3]. Total body sodium, measured by isotope dilution analysis, is made up of a pool that exchanges within 24 h and accounts for 70% of total body sodium. This pool is estimated at 40 mmol/kg [4], or 2,800 mmol in a 70 kg man. The nonexchangeable Na resides primarily in bone. When similarly measured, 85% of total body potassium is exchangeable, estimated at 53.3 mmol/kg [5], or 3,731 mmol in a 70 kg man. At a plasma water sodium concentration of 150.5 mmol/L such an individual would have 43.4 L of total body water (equation 2).