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chapter39
Water, Electrolytes,
and
Acid-Base Balance
O bligatory lo s se s
(skin a n d lungs)
I
K idney
I— - Urine w a ter (am o u n t
e x creted controlled by ADH)
F I G U R E 3 9 - 5
Regulation of osmolality in the body.
summarized in Figure 39-5. Body water is derived from
2-4 L of water consumed daily in food and drink and
300 mL of metabolic water formed daily by oxidation of
lipids and carbohydrates. Water loss occurs by perspira-
tion and expiration of air (~ 1
L/d), in feces (~200 mL)
(Chapter 12), and in urine (1-2 L/d).
Water balance is regulated to maintain constant osmo-
lality of body fluids. This osmolality is directly related to
the number of particles present per unit weight of solvent.
A solution that contains 1
mol of particles in 22.4 kg of
water (22.4 L at 4°C) exerts an osmotic pressure of 1
atm
and has an osmolality of 0.0446. Conversely, the osmotic
pressure of an osmolal solution
(1
mol of particles/kg of
water) is 22.4 atm. In this sense, “number of particles” is
roughly defined as the number of noninteracting molecu-
lar or ionic groups present. Since glucose does not readily
dissociate,
1
mol dissolved
1
kg of water (a molal solu-
tion) produces
1
mol of “particles” and has an osmolality
of 1. Sodium chloride dissociates completely in water to
form two particles from each molecule of NaCl so that a
molal solution of NaCl is a 2-osmolal solution. Similarly,
a molal solution of Na
2
SC
>4
or (NFLj^SCL is a 3-osmolal
solution. In practice, the milliosmole (mosm) is the unit
used.
With aqueous solutions, osmolarity is sometimes used
interchangeably with osmolality. Although this practice is
not strictly correct (moles of particles per liter of solution
versus moles of particles per kilogram of solvent), in water
at temperatures of biological interest the error is fairly
small unless solute concentrations are high (i.e., when an
appreciable fraction of the solution is not water). Thus,
with urine the approximation is acceptable, whereas with
serum it is not because of the large amount of protein
present. Although osmolarity is more readily measured, it
is temperature dependent, unlike osmolality.
Osmolality is commonly measured by freezing point
or vapor pressure depression. In terms of vapor pressure
(
P
v), the osmotic pressure (jt) is defined as
J T =
pv
_ Dv
^pure solvent
^solution
As defined above, osmolality =
j t
/22.4,
where
j t
is mea-
sured in atmospheres. In one instrument, solution and sol-
vent vapor pressures are measured by use of sensitive ther-
mistors to detect the difference in temperature decrease
caused by evaporation of solvent from a drop of pure
solvent and from a drop of solution. Because the rate of
evaporation (vapor pressure) of the solution is lower, the
temperature change will be less and the vapor pressure
difference can be calculated.
The freezing point of a solution is always lower than
that of the solvent. The exact difference depends on the
solvent and the osmolality of the solution. For water,
Osmolality =
AT
L86
where AT is the freezing point depression in degrees
Celsius. Instruments that measure the freezing point of a
sample are used in clinical laboratories to determine serum
and urine osmolality.
Since water passes freely through most biological mem-
branes, all body fluids are in osmotic equilibrium so that
the osmolality of plasma is representative of the osmolality
of other body fluids.
The osmotic pressure of extracellular fluid is due pri-
marily to its principal cation Na+ and the anions Cl
and HCCFj". Taking twice the Na+ concentration gives a
good estimate of serum osmolality. Thus, normal plasma
contains 135-145 mEq of Na+/L (3.1-3.3g/L) and nor-
mal plasma osmolality is about 270-290 mosm/kg (this
corresponds to an osmotic pressure of 6.8-7.3 atm and a
freezing point depression of 0.50-0.54°C). Glucose pro-
vides only 5-6 mosm/kg (or 0.1 atm to the osmotic pres-
sure). Plasma protein contributes about 10.8 mosm/kg. Be-
cause of their size and general inability to pass through
biological membranes, proteins are important determi-
nants of fluid balance between intravascular and ex-
travascular spaces. That portion of the osmotic pres-
sure which is due to proteins is often referred to as the
oncotic pressure.
Since many molecules in plasma interact, the measured
osmolality of a sample is an effective osmolality and is
lower than the value calculated from the concentrations
