168
U N I T 2
Integrative Body Functions
to adults who have only about a third in their ECF
compartment.
1,3
The greater ECF water content of an
infant can be explained in terms of a higher metabolic
rate, larger surface area in relation to its body mass,
and inability to concentrate urine because of imma-
ture kidney structures. Because ECF is more readily
lost from the body, infants are more vulnerable to
fluid deficit than are older children and adults. As an
infant grows older, TBW decreases, and by the second
year of life, the percentages and distribution of body
water approach those of an adult.
The main source of water gain is through oral intake
and metabolic processes. Oral intake, including that
obtained from liquids and solid foods, is absorbed from
the gastrointestinal tract. The amount of water gained
from metabolic processes is much less than from oral
intake, varying from 150 to 300 mL/day, depending on
the metabolic rate.
Normally, the largest loss of water occurs through
the kidneys, with lesser amounts being lost through the
skin, lungs, and gastrointestinal tract. Even when oral
or parenteral fluids are withheld, the kidneys continue
to produce urine as a means of ridding the body of met-
abolic wastes. The urine output that is required to elimi-
nate these wastes is called the
obligatory urine output
.
Water losses that occur through evaporative losses from
skin and to moisten the air in the respiratory system are
referred to as
insensible water losses
because they occur
without a person’s awareness. The amount of water lost
from the skin through sweating varies depending on
physical activity and environmental temperature. The
sources of body water gains and losses are summarized
in Table 8-2.
Regulation of Sodium Balance
Sodium is the most plentiful electrolyte in the ECF
compartment, with a concentration ranging from
135 to 145 mEq/L (135 to 145 mmol/L).
3
Sodium does
not readily cross the cell membrane; as a result, only
a small amount (10 to 15 mEq/L [10 to 15 mmol/L])
is located in the ICF compartment.
1
As the major cat-
ion in the ECF compartment, Na
+
and its attendant Cl
–
and HCO
3
–
anions account for approximately 90% to
95% of the osmotic activity in the ECF. Thus, serum
osmolality usually varies with changes in serum sodium
concentration.
Sodium normally enters the body through the gas-
trointestinal tract, being derived from dietary sources.
Although body needs for sodium usually can be met by
as little as 500 mg/day, dietary intake frequently exceeds
that amount.
3
As package labels indicate, many com-
mercially prepared foods and soft drinks contain con-
siderable amounts of sodium.
Most sodium losses occur through the kidney. The
kidneys are extremely efficient in regulating sodium
output. When sodium intake is limited or conservation
of sodium is needed, the kidneys are able to reabsorb
almost all the Na
+
that has been filtered in the glomeru-
lus, resulting in essentially sodium-free urine.
Usually, less than 10% of sodium intake is lost
through the gastrointestinal tract and skin. Sodium
losses increase with conditions such as vomiting, diar-
rhea, and gastrointestinal suction, all of which can
remove sodium from the stomach or small intestine.
Sodium leaves the skin by way of the sweat glands,
which secrete a hypotonic solution containing both
sodium and chloride. Although sodium losses due to
sweating are usually negligible, they can increase greatly
during heavy exercise and periods of exposure to a hot
environment.
2
Mechanisms of Water and Sodium
Regulation
There are two major physiologic mechanisms for regu-
lating body levels of water: thirst, which is primarily a
regulator of water intake, and the antidiuretic hormone
(ADH), which controls the output of water by the kid-
ney. Thirst and ADH function in the maintenance of
Total body
water = 60%
Total body
water = 30%
220 lbs
220 lbs
FIGURE 8-6.
Body composition of a lean and an obese
individual. (Adapted with permission from Statland H. Fluids
and Electrolytes in Practice. 3rd ed. Philadelphia, PA: J.B.
Lippincott; 1963.)
TABLE 8-2
Sources of BodyWater Gains and
Losses in the Adult
Gains
(approximate) Losses (approximate)
Oral intake
Urine
1500 mL
As water
In food
Water of
oxidation
Total
1000 mL
1300 mL
200 mL
2500 mL
Insensible
losses
Lungs
300 mL
Skin
500 mL
Feces
200 mL
Total
2500 mL
