C h a p t e r 8
Disorders of Fluid, Electrolyte, and Acid–Base Balance
167
lymphatic drainage systems, use the same mechanisms
for interstitial fluid exchange as other areas of the body.
The milking action of the moving structures, such as the
lungs, continually forces fluid and plasma proteins back
into lymphatic channels, helping to keep these cavities
empty. Thus any obstruction of lymph flow causes fluid
to accumulate in the serous cavities. Although the accu-
mulation of third-space fluids produces a gain in body
weight, it does not contribute to the body's fluid reserve
or function.
The prefix
hydro-
may be used to indicate the pres-
ence of excessive fluid in one of the serous cavitis, with
hydrothorax
referring to fluid in the pleural cavity. An
accumulation of fluid in the peritoneal cavity as
asci-
tes
and the transudation of fluid into the serous cavities
is referred to as
effusion
. Effusion can contain blood,
plasma proteins, inflammatory cells (i.e., pus), and
extracellular fluid.
Water and Sodium Balance
The distribution of body fluids between the ICF and
ECF compartments relies on the concentration of ECF
water and sodium. Water provides approximately
90% to 93% of the volume of body fluids and sodium
salts approximately 90% to 95% of ECF solutes.
Normally, equivalent changes in sodium and water are
such that the volume and osmolality of ECF are main-
tained within a normal range. Positive water balance
(intake greater than output) results in a decrease in
body fluid osmolality and ECF sodium concentration
due to diluting effects of the excess water.
7
Likewise, a
negative water balance (intake less than output) results
in an increase in body fluid osmolality and ECF sodium
concentration.
Regulation of Water Balance
Total body water (TBW) accounts for a large percent-
age of body weight. In young men, TBW accounts for
approximates 60% of body weight and decreases to
approximately 52% in elderly men. Because women
usually have less lean muscle mass than men, TBW
accounts for only about 50% of body weight in young
women and decreases to approximately 46% in elderly
women.
1,3
Obesity decreases TBW, with levels some-
times as low as 30% to 40% of body weight in adults
(Fig. 8-6).
Infants normally have more TBW than older chil-
dren or adults. Total body water constitutes approxi-
mately 75% to 80% of body weight in full-term
infants and an even greater percentage in premature
infants. In addition to having a greater percentage of
body water than adults, infants have more than half
of their TBW in their ECF compartment, as compared
SUMMARY CONCEPTS
■■
Body fluids, which contain water and electrolytes,
are distributed between the intracellular fluid
(ICF) and extracellular fluid (ECF) compartments
of the body, with two thirds being contained in
the ICF and one third in the ECF.
■■
The cell membrane serves as a selective barrier
to the movement of substances between the ICF
and ECF. Lipid-soluble substances (e.g., oxygen
[O
2
] and carbon dioxide [CO
2
]), which dissolve
in the lipid layer of the cell membrane, pass
directly through the membrane. Electrolytes,
such as sodium [Na
+
] and potassium [K
+
], rely on
transport proteins located in the cell membrane
for movement across the membrane, accounting
for the compartmental difference in their
concentrations.
■■
Water moves across a semipermeable
membrane by osmosis moving from the side
with the greater concentration of water and
lesser concentration of solute particles to the
side having the lesser water concentration
and greater solute concentration. Osmolarity
refers to the osmotic activity that nondiffusible
particles exert in pulling water from one side of
a semipermeable membrane to the other and
tonicity to the tension or effect that the osmotic
pressure of a solution with nondiffusible solutes
exerts on cell size because of water movement.
Cells remain the same size when placed in an
isotonic solution with the same osmolarity
as the ICF; swell when placed in a hypotonic
solution that has an osmolality less than the
ICF; and shrink when placed in a hypertonic
solution that has an osmolality greater than
the ICF.
■■
Edema represents the accumulation of fluid
volume in the interstitial spaces of the ECF
resulting from: (1) an increase in capillary
filtration pressure, (2) a decrease capillary
colloidal osmotic pressure, (3) an increase in
capillary permeability, or (4) obstructed lymphatic
flow.The effect that edema exerts on body
function is determined by its location, with edema
of the brain, larynx, or lungs representing an
acute life-threatening situation.
■■
Third spacing represents the loss or trapping
of ECF in the transcellular space, such as in
the pericardial sac, the peritoneal cavity, or the
pleural cavity.
