Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 17 Fluid, Electrolyte, and Acid-Base Balance
horses must meet specific guidelines for venous blood pH
and bicarbonate or risk disqualification.
VI . EVALUATION OF IMBALANCES
It is important to understand the difference between volume
regulation and osmoregulation. Osmoregulation is
governed by osmoreceptors influencing ADH and thirst,
whereas volume disturbances are sensed by multiple volume
receptors that activate effectors such as aldosterone.
Antidiuretic hormone increases water resorption (and
therefore urine osmolality), but it does not affect sodium
transport directly. Aldosterone enhances sodium reabsorption
but not directly that of water. Thus, osmoregulation is
achieved by changes in water balance and volume regulation
mostly by changes in sodium balance.
Water balance is achieved when water intake from all
sources is equal to water output by all routes. Water is
available as drinking water, as water content of feedstuffs,
and as metabolic water derived by oxidative metabolism.
Oxidation of 1g of fat, carbohydrate, or protein results in
the production of 1.07, 0.06, or 0.41g of water, respectively.
Water is normally lost from the body by four basic
routes: urine, feces, insensible loss (respiratory and cutaneous
evaporation), and sensible perspiration (sweat) in
some animal species. Water intake and output may vary
considerably from day to day, but normal animals are able
to maintain water balance within remarkably narrow limits
and, at the same time, maintain the critical interrelationship
between water balance and electrolyte balance.
For human subjects, there are well-established normal
values for water intake and output via various routes.
Although there is a substantial amount of data on water
balance for many domestic animals ( English, 1966a ;
Fonnesbeck, 1968 ; Hinton, 1978 ; Kamal et al ., 1972 ;
Leitch and Thomson, 1944 ; Sufit et al ., 1985 ; Tasker,
1967a ; Yoshida et al ., 1967 ), these data vary markedly
from species to species and are valid only for the specific
experimental conditions under which they were collected.
Animals eat to meet their caloric requirements. The nursing
or grazing animal may have a feed intake that is greater
than 90% water as compared to animals on dry hay or dried
prepared pet food, which may contain less than 10% water.
Some desert rodents are so well adapted that they are able
to maintain water balance without water intake and rely on
the water content of feedstuffs and metabolic water derived
from oxidative metabolism. The koala in its native state in
Australia obtains virtually all its water from the leaves of
specific species of eucalyptus trees, which constitute its
entire diet. Dehydration because of water restriction with
and without heat stress has been studied widely in a variety
of animal species ( Bianca et al ., 1965 ; Brobst and Bayly,
1982 ; Carlson et al ., 1979a ; Elkinton and Taffel, 1942 ;
Genetzky et al ., 1987 ; Hix et al ., 1953 ; Kamal et al ., 1972 ;
Rumbaugh et al ., 1982 ; Rumsey and Bond, 1976 ; Schultze
et al ., 1972 ; Tasker, 1967b ).
Quantitative assessment of the compartmental distribution
of TBW between the ECF and the ICF has largely
been based on the volume dilution of certain compounds
or isotopic tracers. These studies require steady-state
conditions and take a substantial period of time; as such,
they are not well suited to the dynamic and often rapidly
changing fluid balance characteristic of many clinical situations.
Bioelectrical impedance analysis (BIA) may prove
a useful tool in this regard. Bioelectrical impedance has
been used for the assessment of body fat and fluid balance
in humans and many animal species. Multifrequency
bioelectrical impedance analysis has been successfully
employed to monitor the rapid fluid changes in small animal
patients undergoing dialysis. Algorithms for multifrequency
BIA have been developed for use in the horse
( Fielding et al ., 2004 ).
A . Water
1 . Depletion-Dehydration
Dehydration is a relatively common problem in domestic
animals. Dehydration results from inadequate fluid intake
in the face of normal to increased fluid losses. When water
losses occur with little or no electrolyte losses (i.e., panting
or feed and water restriction), serum sodium concentration
and osmolality increase. This situation is called a hypertonic
dehydration and occurs when water losses exceed
losses of the exchangeable cations sodium and potassium
( Carlson, 1987 ). This imbalance between total body water
and exchangeable cations is best characterized as a relative
water deficit ( Scribner, 1969 ). The effects of a pure water
loss of 30l in a 450-kg horse are illustrated in Figure 17-2 .
In this theoretical example, there has been a 10% loss of
body water but no change in electrolyte balance. Plasma
FIGURE 17-2 Body fluid compartments in a 450-kg horse with a pure
water loss of 30l. Hypertonic fluid volume contraction is indicated by the
increase in serum sodium concentration from 140 mEq/l (140 mmol/l) to
155mEq/l (155mmol/l). Fluid losses are shared by the extracellular fluid
(ECF) volume and the intracellular fluid (ICF) volume.