992
UNIT 4
Maintenance of the Body
26
Electrolyte concentrations of body fluids are usually ex-
pressed in
milliequivalents per liter (mEq/L)
, a measure of the
number of electrical charges in 1 liter of solution. We can com-
pute the concentration of any ion in solution using the equation
no. of
mEq/L
5
ion concentration (mg/L)
electrical
atomic weight of ion (mg/mmol)
3
charges on
one ion
To calculate the mEq/L of sodium or calcium ions in solu-
tion in plasma, we would determine the normal concentration
of these ions in plasma, look up their atomic weights in the
periodic table (see Appendix E), and plug these values into the
equation:
3300 mg/L
Na
1
:
23 mg/mmol
3
1
5
143 mEq/L
100 mg/L
Ca
2
1
:
40 mg/mmol
3
2
5
5 mEq/L
Notice that for ions with a single charge, 1 mEq is equal
to 1 mmol, which, when dissolved in 1 kg of water, produces
1 mOsm (see p. 973). On the other hand, 1 mEq of ions with a
double charge (like calcium) is equal to ½ mOsm. In either case,
1 mEq provides the same amount of charge.
Comparison of Extracellular and Intracellular Fluids
A quick glance at the bar graphs in
Figure 26.2
reveals that
each fluid compartment has a distinctive pattern of electrolytes.
Except for the relatively high protein content in plasma, how-
ever, the extracellular fluids are very similar. ±eir chief cation
is sodium, and their major anion is chloride. However, plasma
contains somewhat fewer chloride ions than interstitial fluid,
because the nonpenetrating plasma proteins are normally an-
ions and plasma is electrically neutral.
In contrast to extracellular fluids, the ICF contains only small
amounts of Na
1
and Cl
2
. Its most abundant cation is potas-
sium, and its major anion is HPO
4
2
2
. Cells also contain substan-
tial quantities of soluble proteins (about three times the amount
found in plasma).
Total solute concentration (mEq/L)
0
Na
+
K
+
Ca
2+
Protein
anions
Mg
2+
Cl
HCO
3
Na
+
HPO
2–
4
SO
2–
4
20
40
60
80
100
120
140
160
Sodium
K
+
Potassium
Ca
2+
Calcium
Mg
2+
Magnesium
HCO
3
Bicarbonate
Cl
Chloride
HPO
2–
4
SO
2–
4
Hydrogen
phosphate
Sulfate
Blood plasma
Interstitial fluid
Intracellular fluid
Figure 26.2
Electrolyte composition of blood plasma, interstitial fluid, and
intracellular fluid.
The very low intracellular Ca
2
1
concentration (10
2
7
M
) does not include
Ca
2
1
stores sequestered inside organelles. The high concentration of intracellular HPO
4
2
2
includes large amounts bound to intermediate metabolites, proteins, and lipids.
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