T h e ne w e ngl a nd jou r na l o f m e dicine n engl j med 372;1 nejm.org January 1, 2015
55Review Article H uman cells dwell in salt water. Their well-being depends on the ability of the body to regulate the salinity of extracellular fluids. By controlling water intake and excretion, the osmoregulatory system normally prevents the plasma sodium concentration from straying outside its normal range (135 to 142 mmol per liter). Failure of the system to regulate within this range exposes cells to hypotonic or hypertonic stress. This review considers the causes and consequences of an abnormal plasma sodium concentration and offers a framework for correcting it.
Pl a sm a Sodium Concentr ation a nd E x tr acellul a r T onicit yThe plasma sodium concentration affects cell volume. The term "tonicity" describes the effect of plasma on cells -hypotonicity makes cells swell and hypertonicity makes them shrink. Hypernatremia always indicates hypertonicity. Hyponatremia usually indicates hypotonicity, but there are exceptions (e.g., hyperglycemic hyponatremia and pseudohyponatremia) that are not covered in this review.Pl a sm a Sodium Concentr ation a nd the Elec troly te a nd Water Content of the Body Solute concentrations (osmolalities) must be equal inside and outside of cells because water channels (aquaporins) make cell membranes permeable to water. 1,2The "sodium pump" (Na + /K + -ATPase) functionally excludes sodium from cells, exchanging it for potassium by means of active transport. Although sodium is largely extracellular and potassium is intracellular, body fluids can be considered as being in a single "tub" containing sodium, potassium, and water, because osmotic gradients are quickly abolished by water movement across cell membranes. As such, the concentration of sodium in plasma water should equal the concentration of sodium plus potassium in total body water. This theoretical relationship was validated empirically by Edelman et al., 3 who used isotopes to measure exchangeable body cations and water. Edelman and colleagues described the relation between these variables with the following equation: −25.6, (Na + e + K + e ) total body H 2 O [Na + ] in plasma H 2 O=1.11 ×where Na + e is exchangeable sodium, K + e exchangeable potassium, and H 2 0 water. This equation has an intercept (−25.6); the regression line relating plasma sodium