Chapter 26
Fluid, Electrolyte, and Acid-Base Balance
991
26
In this chapter, we first examine the composition and distri-
bution of fluids in the internal environment and then consider
the roles of various body organs and functions in establishing,
regulating, and altering this balance.
Body Fluids
List the factors that determine body water content and
describe the effect of each factor.
Indicate the relative fluid volume and solute composition
of the fluid compartments of the body.
Contrast the overall osmotic effects of electrolytes and
nonelectrolytes.
Describe factors that determine fluid shifts in the body.
Body Water Content
Not all bodies contain the same amount of water. Total body
water is a function not only of age and body mass, but also of
sex and the relative amount of body fat. Infants, with their low
body fat and low bone mass, are 73% or more water. (±is high
level of hydration accounts for their “dewy” skin, like that of a
freshly picked peach.) AFer infancy total body water declines
throughout life, accounting for only about 45% of body mass
in old age.
A healthy young man is about 60% water, and a healthy
young woman about 50%. ±is difference between the sexes
reflects the fact that females have relatively more body fat and
less skeletal muscle than males. Of all body tissues, adipose tis-
sue is
least
hydrated (containing up to 20% water)—even bone
contains more water than does fat. By contrast, skeletal muscle
is about 75% water, so people with greater muscle mass have
proportionately more body water.
Fluid Compartments
Water occupies two main
fluid compartments
within the body
(Figure 26.1)
. Almost two-thirds by volume is in the
intra-
cellular fluid (ICF)
compartment
, which actually consists of
trillions of tiny individual “compartments”: the cells. In an adult
male of average size (70 kg, or 154 lb), IC² accounts for about
25 L of the 40 L of body water.
±e remaining one-third or so of body water is outside
cells, in the
extracellular fluid (ECF)
compartment
. ±e EC²
constitutes the body’s “internal environment” referred to by
Claude Bernard and is the external environment of each cell. As
²igure 26.1 shows, the EC² compartment is divisible into two
subcompartments: (1)
plasma
, the fluid portion of blood, and
(2)
interstitial fluid (IF)
, the fluid in the microscopic spaces
between tissue cells. ±ere are numerous other examples of
EC² that are distinct from both plasma and interstitial fluid—
lymph, cerebrospinal fluid, humors of the eye, synovial fluid,
serous fluid, gastrointestinal secretions—but most of these are
similar to I² and are usually considered part of it.
Composition of Body Fluids
Water serves as the
universal solvent
in which a variety of solutes
are dissolved. Solutes may be classified broadly as
electrolytes
and
nonelectrolytes
.
Electrolytes and Nonelectrolytes
Nonelectrolytes
have bonds (usually covalent bonds) that pre-
vent them from dissociating in solution. ²or this reason, no
electrically charged species are created when nonelectrolytes
dissolve in water. Most nonelectrolytes are organic molecules—
glucose, lipids, creatinine, and urea, for example.
In contrast,
electrolytes
are chemical compounds that
do
dissociate into ions in water. (See Chapter 2 if necessary to
review these concepts of chemistry.) Because ions are charged
particles, they can conduct an electrical current—and so have
the name
electrolyte
. Typically, electrolytes include inorganic
salts, both inorganic and organic acids and bases, and some
proteins.
Although all dissolved solutes contribute to the osmotic ac-
tivity of a fluid, electrolytes have much greater osmotic power
than nonelectrolytes because each electrolyte molecule dissoci-
ates into at least two ions. ²or example, a molecule of sodium
chloride (NaCl) contributes twice as many solute particles as
glucose (which remains undissociated), and a molecule of mag-
nesium chloride (MgCl
2
) contributes three times as many:
NaCl
S
Na
1
1
Cl
2
(electrolyte; two particles)
MgCl
2
S
Mg
2
1
1
2Cl
2
(electrolyte; three particles)
glucose
S
glucose
(nonelectrolyte; one particle)
Regardless of the type of solute particle, water moves accord-
ing to osmotic gradients—from an area of lesser osmolality to
an area of greater osmolality. ²or this reason, electrolytes have
the greatest ability to cause fluid shiFs.
Plasma
Volume = 3 L, 20% of ECF
Total body water
Volume = 40 L
60% of body weight
Extracellular fluid (ECF)
Volume = 15 L
20% of body weight
Intracellular fluid (ICF)
Volume = 25 L
40% of body weight
Interstitial fluid (IF)
Volume = 12 L
80% of ECF
Figure 26.1
The major fluid compartments of the body.
[Values are for a 70-kg (154-lb) male.]
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