Control of Extracellular Fluid Osmolarity and Sodium Concentration
Regulation of extracellular fluid osmolarity and sodium concentration are closely linked because sodium is the most abundant ion in the extracellular compartment. Plasma sodium concentration is nor-mally regulated within close limits of 140 to 145 mEq/ L, with an average concentration of about 142 mEq/L. Osmolarity averages about 300 mOsm/L (about 282 mOsm/L when corrected for interionic attraction) and seldom changes more than ±2 to 3 per cent. As discussed, these variables must be pre-cisely controlled because they determine the distribu-tion of fluid between the intracellular and extracellular compartments.
Estimating Plasma Osmolarity from Plasma Sodium Concentration
In most clinical laboratories, plasma osmolarity is not routinely measured. However, because sodium and its associated anions account for about 94 per cent of the solute in the extracellular compartment, plasma osmo-larity (Posm) can be roughly approximated as
Posm = 2.1 x Plasma sodium concentration
For instance, with a plasma sodium concentration of 142 mEq/L, the plasma osmolarity would be estimated from the formula above to be about 298 mOsm/L. To be more exact, especially in conditions associated with renal disease, the contribution of two other solutes, glucose and urea, should be included. Such estimates of plasma osmolarity are usually accurate within a few percentage points of those measured directly.
Normally, sodium ions and associated anions (pri-marily bicarbonate and chloride) represent about 94 per cent of the extracellular osmoles, with glucose and urea contributing about 3 to 5 per cent of the total osmoles. However, because urea easily permeates most cell membranes, it exerts little effective osmotic pressure under steady-state conditions. Therefore, the sodium ions in the extracellular fluid and associated anions are the principal determinants of fluid move-ment across the cell membrane. Consequently, we can discuss the control of osmolarity and control of sodium ion concentration at the same time.
Although multiple mechanisms control the amount of sodium and water excretion by the kidneys, two primary systems are especially involved in regulating the concentration of sodium and osmolarity of extra-cellular fluid: (1) the osmoreceptor-ADH system and (2) the thirst mechanism.
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