Page 181 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Body Fluid Homeostasis infants is 0.75 compared to only 0.64 (0.53) in
young men (women) and 0.53 (0.46) in elderly
Life cannot exist without water. Water is the men (women). Gender-related differences
initial and final product of countless biochemi- (and interindividual differences) are mainly
cal reactions. It serves as a solvent, transport due to differences in a person’s total body fat
vehicle, heat buffer, and coolant, and has a content. The average fraction of water in most
variety of other functions. Water is present in body tissues (in young adults) is 0.73 com-
cells as intracellular fluid, and surrounds them pared to a fraction of only about 0.2 in fat
as extracellular fluid. It provides a constant en- (! B).
Kidneys, Salt, and Water Balance (! p. 2). other 2/5 (0.25 BW) is extracellular. Extracellu-
Fluid compartments. In a person with an
vironment (internal milieu) for cells of the
average TBW of ca. 0.6, about 3/5 (0.35 BW) of
body, similar to that of the primordial sea sur-
rounding the first unicellular organisms
the TBW is intracellular fluid (ICF), and the
The volume of fluid circulating in the body
lar fluid (ECF) is located between cells (in-
terstice, 0.19), in blood (plasma water (0.045)
remains relatively constant when the water
and in “transcellular” compartments (0.015)
balance (! A) is properly regulated. The aver-
age fluid intake of ca. 2.5 L per day is supplied
such as the CSF and intestinal lumen (! C). The
by beverages, solid foods, and metabolic oxida-
protein concentration of the plasma is signifi-
tion (! p. 229 C). The fluid intake must be high
cantly different from that of the rest of the ECF.
in the ionic composition of the ECF and the ICF
nation, respiration, perspiration, and defecation
(! p. 265 C). The mean daily H 2O turnover is
(! p. 93 B). Since most of the body’s supply of
7 enough to counteract water losses due to uri- Moreover, there are fundamental differences
2.5 L/70 kg (1/30 th the body weight [BW]) in
Na ions are located in extracellular compart-
+
adults and 0.7 liters/7 kg (1/10th the BW) in in- ments, the total Na content of the body deter-
+
fants. The water balance of infants is therefore mines its ECF volume (! p. 170).
more susceptible to disturbance. Measurement of fluid compartments. In
Significant rises in the H 2O turnover can clinical medicine, the body’s fluid compart-
occur, but must be adequately compensated ments are usually measured by indicator dilu-
for if the body is to function properly (regula- tion techniques. Provided the indicator sub-
tion, ! p. 170). Respiratory H 2O losses occur, stance, S, injected into the bloodstream
for example, due to hyperventilation at high spreads to the target compartment only (! C),
altitudes (! pp. 106 and 136), and perspiration its volume V can be calculated from:
losses (! p. 222) occur due to exertion at high V[L] = injected amount of
temperatures (e.g., hiking in the sun or hot S [mol]/C S [mol/L] [7.12]
work environment as in an ironworks). Both where C S is the concentration of S after it
can lead to the loss of several liters of water per spreads throughout the target compartment
hour, which must be compensated for by in- (measured in collected blood specimens). The
creasing the intake of fluids (and salt) accord- ECF volume is generally measured using inulin
ingly. Conversely, an increased intake of fluids as the indicator (does not enter cells), and the
will lead to an increased volume of urine being TBW volume is determined using antipyrine.
excreted (! p. 170). The ICF volume is approximately equal to the
Water loss (hypovolemia) results in the antipyrine distribution volume minus the in-
stimulation of thirst, a sensation controlled by ulin distribution volume. Evans blue, a sub-
the so-called thirst center in the hypothalamus. stance entirely bound by plasma proteins, can
Thirst is triggered by significant rises in the be used to measure the plasma volume. Once
osmolality of body fluids and angiotensin II this value is known, the blood volume can be
concentration of cerebrospinal fluid (! p. 170). calculated as the plasma volume divided by
Body water content. The fraction of total [1 – hematocrit] (! p. 88), and the interstitial
body water (TBW) to body weight (BW = 1.0) volume is calculated as the ECF volume minus
ranges from 0.46 (46%) to 0.75 depending on a the plasma volume.
168 person’s age and sex (! B). The TBW content in
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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