Body Fluid Physiology

In the ECF, Na and Cl constitute 90% or more of the effective solutes. Serum Na concentration defines the relative amount of sodium and water in plasma; the maintenance of a normal Na concentration, thus, contributes to regulation of the volume of body fluids.

The size of the ECF and ICF compartments depends on the amount of water within each; the distribution of water depends on their osmolality. The osmolality of a solution is a function of the number of solute particles or osmoles per unit volume. In a given patient, the effective osmolality may be calculated as follows, using the values of 2.8 and 18 to convert values of blood urea nitrogen (BUN) and glucose, respectively, to mOsm/L.

Osmolality = 2[Na in mEq/L] + [BUN in mg/dL] / 2.8 + [Glucose in mg/dL] / 18

Normal serum osmolality (265 to 285 mOsm/L) is maintained by kidney function, which dilutes or concentrates urine. This is accomplished by a variety of mechanisms involving glomerular filtration, arterial pressure, blood flow, physical factors in the kidneys, the sympathetic nervous system, and hormones such as aldosterone, atrial natriuretic factor, vasopressin, and dopamine.

These systems converge to control water and electrolyte balance through glomerular ultrafiltration of the plasma followed by changes in the electrolyte content of this ultrafiltrate by tubular reabsorption and secretion. These mechanisms,together with thirst, control both plasma osmolality and plasma volume.

[Jospe, N. and G. Forbes. Fluids and Electrolytes -- Clinical Aspects. Pediatrics in Review (November, 1996), 17(11): 395-404.]