172
U N I T 2
Integrative Body Functions
gastrointestinal fluids such as occurs with severe vom-
iting, diarrhea, or gastrointestinal suction; excessive
urinary losses, such as occurs with osmotic diuresis or
injudicious use of diuretic therapy; excessive sweat-
ing due to fever and exercise; or endocrine disorders,
such as adrenal insufficiency, in which reduced levels
of aldosterone cause excessive sodium loss in the urine
(see Chapter 32). Third-space losses cause sequestering
of ECF in the serous cavities or extracellular spaces of
injured tissue.
Isotonic fluid volume deficit is manifested by a
decrease in ECF volume, as evidenced by a decrease
in body weight. A mild ECF deficit exists when weight
loss equals 2% of body weight. A moderate deficit
equates to a 5% loss in weight and a severe deficit to
an 8% or greater loss in weight (Table 8-3).
3
Because
the ECF is trapped in the transcellular compartment of
persons with third-space losses, their body weight may
not decrease.
Thirst is a common symptom of fluid deficit, although
it is not always present in early stages of isotonic fluid
deficit. Urine output decreases and urine osmolality and
specific gravity increase as ADH levels rise because of a
decrease in vascular volume. Although there is an iso-
tonic loss of fluid from the vascular compartment, blood
components such as red blood cells and BUN become
more concentrated.
The fluid content of body tissues decreases as
fluid is removed from the interstitial spaces. The eyes
assume a sunken appearance and feel softer than nor-
mal as the fluid content in the anterior chamber of the
eye decreases. Fluids add resiliency to the skin and
underlying tissues that is referred to as
tissue turgor
.
Tissue turgor is assessed by pinching a fold of skin
between the thumb and forefinger (Fig. 8-9). The skin
should immediately return to its original configuration
when the fingers are released. A loss of 3% to 5% of
body water in children causes the resiliency of the skin
to be lost, and the tissue remains elevated for several
seconds.
3
Decreased tissue turgor is less predictive of
fluid deficit in older persons (>65 years) because of the
loss of tissue elasticity. In infants, fluid deficit may be
evidenced by depression of the anterior fontanel due to
a decrease in cerebrospinal fluid.
Arterial and venous volumes decline during periods
of fluid deficit, as does filling of the capillary circula-
tion, which can be assessed by applying pressure to a
fingernail for 5 seconds and then releasing the pres-
sure and observing the time (normally 1 to 2 seconds)
that it takes for the color to return to normal (capil-
lary refill time).
3
As the volume in the arterial system
declines, the blood pressure decreases, the heart rate
increases, and the pulse becomes weak and thready.
Postural hypotension (a drop in blood pressure on
standing) is an early sign of fluid deficit. On the venous
side of the circulation, the veins become less promi-
nent. When volume depletion becomes severe, signs of
hypovolemic shock and vascular collapse appear (see
Chapter 20).
Treatment of fluid volume deficit consists of fluid
replacement and measures to correct the underlying
cause. Usually, isotonic electrolyte solutions are used
for fluid replacement. Acute hypovolemia and hypovo-
lemic shock can cause renal damage; therefore, prompt
Proportionate changes in
sodium and water
Loss of sodium
and water
Isotonic fluid deficit
in ECF compartment
Contraction of fluids in
interstitial and vascular
compartments of the ECF
Gain of sodium
and water
Isotonic fluid excess
in ECF compartment
Expansion of fluids in the
interstitial and vascular
compartments of the ECF
Disproportionate changes
in sodium and water
Loss of sodium
or gain of water
Hyponatremia
Water movement from
extracellular to intracellular
compartment
Gain of sodium
or loss of water
Hypernatremia
Water movement from
intracellular to extracellular
compartment
280 mOsm
Intracellular fluid (ICF)
Extracellular fluid (ECF)
280
mOsm
280 mOsm
260
mOsm
280 mOsm
280 mOsm
280
mOsm
300
mOsm
Water
Water
FIGURE 8-8.
Effect of isotonic fluid excess and deficit and of hyponatremia and hypernatremia on
movement of water between the extracellular fluid (ECF) and intracellular fluid (ICF) compartment.
