Fluids and Electrolytes Introduction

This section is designed as both a tutorial and reference on the subject of fluids and electrolytes. In a healthy individual fluid volume and electrolyte concentrations are maintained within strict homeostatic limits through the interaction of several organ systems. Before considering pathology affecting this delicate balance, the basic physiology of fluid and electrolytes is reviewed here.

Distribution of Fluids

Total body water (TBW) can be divided by cell membranes into two main compartments: the fluid outside cells (extracellular fluid, ECF) and the fluid inside cells (intracellular fluid, ICF). After the first year of life, the ICF comprises about two-thirds of total body water, and the remaining one-third is found in the ECF compartment. The ECF is further divided into the intravascular (or plasma) volume and the interstitial fluid. These two ECF compartments are separated by a capillary membrane. TBW changes as a function of age, going from 3/4 of body weight at birth to about 2/3 of body weight at adolescence. An accurate estimation of the TBW is required for a number of calculations:

Adapted from Feld. (1988)
Age	TBW as % of body weight	ECF as % of body weight	ICF as % body weight
Premature	75-80
Newborn	70-75	50	35
1 Year Old	65	25	40-45
Adolescent Male	60	20	40-45
Adolescent Female	55	18	40

Body Weight
	Total Body Water
	ICF	ECF
		Intravascular Volume	Interstitial Volume

Electrolyte composition of body fluids

The electrolyte composition of these different compartments is different. The major cationic electrolyte in plasma is sodium with smaller contributions from Ca⁺², K⁺ and Mg⁺². The anions in plasma include Cl^-, HCO₃^-, and protein. In addition, there are a number of anions which are present but are not routinely detected, these undetermined anions constitute the "anion gap". By contrast, the dominant cation in intracellular fluid is potassium, with small amounts of Na⁺ and Mg⁺². There is little bicarbonate, and most of the anions are organic phosphates, cellular protein, organic acids and sulfates. In the table below, the mOsm/kg of each solute are given in parentheses:

	ICF	ECF
Cations:	K⁺(154), Na⁺(6), Mg⁺² (40)	Na⁺(142) ,Ca⁺²(5), K⁺(5), Mg^{+2 (3)}
Anions:	Organic PO₄^-3(106), protein (60), SO₄^-2 (17), HCO₃^- (13), organic acids (4)	Cl^-(105), HCO₃^- (24), protein (15), PO₄^-3 (5), SO₄^-2 (4), Organic acids (2)

There are three key concepts in consideration of fluid and electrolyte management: cell membrane permeability, osmolarity, and electroneutrality. Cell membrane permeability refers to the ability of a cell membrane to allow certain substances such as water and urea to pass freely, while charged ions such as sodium cannot cross the membrane and are trapped on one side of it. Osmolarity is a property of particles in solution. If a substance can dissociate in solution, it may contribute more than one equivalent to the osmolarity of the solution. For instance, NaCl will dissociate into two osmotically active ions: Na and Cl. One millimolar NaCl yields a 2 milliosmolar solution. Finally, the principle of electroneutrality means that the overall number of positive and negative charges balances. For instance, in conditions like renal tubular acidosis where HCO₃^- is lost, chloride is retained leading to a hyperchloremic state.

The expected osmolarity of plasma can be calculated according to the following formula. As is evident, the concentration of sodium is the major determinant:

Osmolarity (mOsm/kg) = 2*[mEq/L Na⁺] + (mg/dL glucose)/18 + (mg/dL BUN)/2.8

Normal serum osmolarity ranges from about 280 to 295 mOsm/kg. This equation is valid only when other significant particles are not present (for instance, alcohol, ethylene glycol, mannitol).

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Last modification: April 30, 1998