C h a p t e r 8
Disorders of Fluid, Electrolyte, and Acid–Base Balance
161
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
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
Semipermeable
membrane
Water
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.
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:
mEq
mg/100 mL 10 valence
atomic weight
=
× ×
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:
mmol/L
mEq/L
valence
=
BOX 8-1
Measurement Units
TABLE 8-1
Concentrations of Extracellular and Intracellular Electrolytes in Adults
Extracellular Concentration*
Intracellular Concentration*
Electrolyte
Conventional Units
SI Units
Conventional Units
SI Units
Sodium
135–145 mEq/L
135–145 mmol/L
10–15 mEq/L
10–15 mmol/L
Potassium
3.5–5.0 mEq/L
3.5–5.0 mmol/L
140–150 mEq/L
140–150 mmol/L
Chloride
98–106 mEq/L
98–106 mmol/L
3–4 mEq/L
3–4 mmol/L
Bicarbonate
24–31 mEq/L
24–31 mmol/L
7–10 mEq/L
7–10 mmol/L
Calcium
8.5–10.5 mg/dL
2.1–2.6 mmol/L
<1 mg/dL
<0.25 mmol/L
Phosphorus
2.5–4.5 mg/dL
0.8–1.45 mmol/L
4 mEq/kg
†
75 mmol/L
Magnesium
1.3–2.1 mg/dL
0.65–1.1 mmol/L
variable
†
variable
†
*Values may vary among laboratories, depending on the method of analysis used.
†
Values vary among various tissues and with nutritional status.
