Porth's Essentials of Pathophysiology, 4e

161

Disorders of Fluid, Electrolyte, and Acid–Base Balance

C h a p t e r 8

Concentrations of Extracellular and Intracellular Electrolytes in Adults Extracellular Concentration*

TABLE 8-1

Intracellular Concentration*

Electrolyte

Conventional Units

SI Units

Conventional Units

SI Units

Sodium

135–145 mEq/L 3.5–5.0 mEq/L 98–106 mEq/L 24–31 mEq/L 8.5–10.5 mg/dL 2.5–4.5 mg/dL 1.3–2.1 mg/dL

135–145 mmol/L 3.5–5.0 mmol/L 98–106 mmol/L 24–31 mmol/L 2.1–2.6 mmol/L 0.8–1.45 mmol/L

10–15 mEq/L 140–150 mEq/L

10–15 mmol/L 140–150 mmol/L 3–4 mmol/L 7–10 mmol/L <0.25 mmol/L

Potassium

Chloride

3–4 mEq/L 7–10 mEq/L <1 mg/dL 4 mEq/kg † variable †

Bicarbonate

Calcium

Phosphorus Magnesium

75 mmol/L

0.65–1.1 mmol/L

variable †

*Values may vary among laboratories, depending on the method of analysis used. † Values vary among various tissues and with nutritional status.

Osmosis andTonicity Despite the remarkable difference in the concentration of individual molelcules in the ICF and ECF, the total concentration gradient is the same in both compartments because of the osmotic movement of water. Osmosis refers to the movement of water across a semiperme- able membrane (i.e., one that is permeable to water but impermeable to most solutes). 1,3 As with solute particles, water diffuses down its concentration gradient, moving Laboratory measurements of electrolytes in body fluids are expressed as a concentration or amount of solute in a given volume of fluid, such as milligrams per deciliter (mg/dL), milliequivalents per liter (mEq/L), or millimoles per liter (mmol/L). The use of milligrams (mg) per deciliter expresses the weight of the solute in one tenth of a liter (dL). The concentration of electrolytes such as calcium, phosphate, and magnesium is often expressed in mg/dL. The milliequivalent is used to express the charge equivalency for a given weight of an electrolyte: 1 mEq of sodium has the same number of charges as 1 mEq of chloride, regardless of molecular weight. The number of milliequivalents of an electrolyte in a liter of solution can be derived from the following equation: BOX 8-1 Measurement Units

from the side of the membrane with a greater concen- tration of water and lesser concentration of solute par- ticles to the side with a lesser concentration of water and greater concentration of solute particles (Fig. 8-2). As water moves across the semipermeable membrane, it generates a pressure called the osmotic pressure . The magnitude of the osmotic pressure represents the hydro- static pressure (measured in millimeters of mercury [mm Hg]) needed to oppose the movement of water across the membrane.

Osmotic pressure

mg/100 mL 10 valence atomic weight = × ×

mEq

Water

The Système Internationale (SI) units express electrolyte concentration in millimoles per liter (mmol/L). A millimole is one thousandth of a mole, or the molecular weight of a substance expressed in milligrams. The number of millimoles of an electrolyte in a liter of solution can be calculated using the following equation:

Semipermeable membrane

FIGURE 8-2. Movement of water across a semipermeable membrane. Water moves from the side that has fewer nondiffusible particles to the side that has more. The osmotic pressure is equal to the hydrostatic pressure needed to oppose water movement across the membrane.

mEq/L valence

=

mmol/L

Made with