fore, if water is in balance in all of the compartments,
the electrolytes are also in balance. Although water
and ions are constantly moving, their relative propor-
tions in the compartments remain constant; this is
fluid–electrolyte homeostasis, and its maintenance is
essential for life (see Box 19–1: Edema).
WATER INTAKE AND OUTPUT
Most of the water the body requires comes from the
ingestion of liquids; this amount averages 1600 mL
per day. The food we eat also contains water. Even
foods we think of as somewhat dry, such as bread,
contain significant amounts of water. The daily water
total from food averages 700 mL. The last source of
water, about 200 mL per day, is the metabolic water
that is a product of cell respiration. The total intake
of water per day, therefore, is about 2500 mL, or 2.5
liters.
Most of the water lost from the body is in the form
of urine produced by the kidneys; this averages 1500
mL per day. About 500 mL per day is lost in the form
of sweat, another 300 mL per day is in the form of
water vapor in exhaled air, and another 200 mL per
day is lost in feces. The total output of water is thus
about 2500 mL per day.
Naturally, any increase in water output must be
compensated for by an increase in intake. Someone
who exercises strenuously, for example, may lose 1 to
2 liters of water in sweat and must replace that water
by drinking more fluids. In a healthy individual, water
intake equals water output, even though the amounts
of each may vary greatly from the averages just men-
tioned (Fig. 19–2 and Table 19–1).REGULATION OF WATER
INTAKE AND OUTPUT
The hypothalamus in the brain contains osmorecep-
torsthat detect changes in the osmolarity of bodyFluid–Electrolyte and Acid–Base Balance 443BOX19–1 EDEMA
lower extremities (pulmonary edema was described
in Chapter 15).
Systemic bacterial infections may increase capil-
lary permeability, and loss of plasma to tissue spaces
is one aspect of septicemia. In this situation, how-
ever, the edema is of secondary importance to the
hypotension, which may be life-threatening.
Insufficient osmosis, the return of tissue fluid into
capillaries, is a consequence of a decrease in plasma
proteins, especially albumin. This may occur in
severe liver diseases such as cirrhosis, kidney disease
involving loss of protein in urine, malnutrition, or
severe burn injuries.
Because edema is a symptom rather than a dis-
ease, treatment is aimed at correcting the specific
cause. If that is not possible, the volume of tissue
fluid may be diminished by a low-salt diet and the
use of diuretics.Edemais an abnormal increase in the amount of
tissue fluid, which may be localized or systemic.
Sometimes edema is inapparent, and sometimes it
is apparent as swelling.
Localized edema follows injury and inflamma-
tion of a body part. Spraining an ankle, for
example, damages tissues that then release
histamine. Histamine increases the permeability of
capillaries, and more tissue fluid is formed. As
tissue fluid accumulates, the ankle may become
swollen.
Systemic edema is the result of an imbalance
between the movement of water out of and into
capillaries, that is, between filtration and osmosis.
Excessive filtration will occur when capillary pres-
sure rises. This may be caused by venous obstruc-
tion due to blood clots or by congestive heart
failure. Edema of this type is often apparent in theTable 19–1 WATER INTAKE AND OUTPUTAverage Amount
Form per 24 Hours (mL)
Intake
Liquids 1600
Food 700
Metabolic water 200
Output
Urine 1500
Sweat (and insensible 500
water loss)
Exhaled air (water vapor) 300
Feces 200