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

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Chapter | 17 Fluid, Electrolyte, and Acid-Base Balance
and physiology of the body fluids, the pathological mechanisms
by which normal processes become deranged, the
means by which these disturbances can be identified accurately,
and finally the procedures that can be used to correct
such disturbances in a prompt, safe, and effective manner
(Tasker, 1980 ).
II . PHYSIOLOGY OF FLUID AND
ELECTROLYTE BALANCE
A variety of units have been used in the quantitative evaluation
of biological specimens. To avoid confusion, this
chapter describes the units of measure that apply directly to
the body fluids and electrolytes. An international standard
for clinical chemistry units, the “ Systeme Internationale
d’Unites ” (SI units), was developed to provide consistent
terminology and usage. Although SI units are the international
standard, they may not be familiar to all students or
clinicians. All solute concentrations are expressed in moles
or millimoles per l, blood gas partial pressure in kilopascals,
and osmolality in millikelvins of freezing point depression.
Electrolytes are substances that exist as positive or negative
charged particles in aqueous solution. The positively
charged particles are “ cations, ” and the negatively charged
particles are “ anions. ” For univalent ions such as sodium,
potassium, chloride, and bicarbonate, 1 mole equals 1
equivalent. For multivalent ions, 1 equivalent is equal to
the molecular weight in grams (i.e., 1 mole) divided by the
charge on the particle. To maintain electrical neutrality in
biological fluids, there must be an equal number of equivalents
or milliequivalents of anions and cations in solution.
Electrolytes in solution combine equivalent for equivalent,
not on a gram for gram or mole for mole basis.
The osmotic properties of a solute in solution are related
to the number of particles in solution and not to its weight
or its charge. One osmole of a nondissociable substance is
equal to its molecular weight in grams. One osmole of any
substance that dissociates in solution into two or more particles
is equal to the molecular weight in grams divided by
the number of particles into which each molecule dissociates.
Osmolarity is defined as the number of osmoles per l of
final solution, whereas osmolality is the number of osmoles
per kilogram of water. Although the expressions are similar,
osmolality more correctly describes the osmotic properties
as measured in the clinical laboratory.
Most solutes in biological fluids are present in relatively
dilute concentrations, and it is more convenient to express these
concentrations as millimoles, milliequivalents, or milliosmoles.
These simply represent one-thousandth of the standard unit.
Conventional terms are milligrams per deciliter (mg/dl), millimoles
per l (mmol/l), milliequivalent/l (mEq/l), and milliosmole
per kg water (mOsm/kg). The concentrations of the
principal anions and cations in plasma are presented in Table
17-1 as expressed in these conventional terms.
III . BODY FLUID COMPARTMENTS
Before discussing the assessment of fluid deficits or imbalances,
it is necessary to consider the organization and composition
of the fluid compartments from which these losses
occur. An understanding of the forces that govern the relative
volume and composition of the body fluid compartments
is central to understanding both the clinical and
clinicopathological manifestations of altered fluid balance.
A . Total Body Water
Water is the most abundant compound in the body, and most
of life’s essential processes take place in this aqueous environment.
Although there is substantial variation, the total
body water (TBW) of most domestic animals is approximately
60% of body weight (0.60 l/kg). In a 500-kg horse,
this amounts to approximately 300 l ( Carlson, 1983c ),
whereas in a 20-kg dog, it amounts to just over 12 l ( Kohn
and DiBartola, 1992 ).
Adipose tissue contains little water, and the amount of
body fat has a major impact on the relative TBW. The average
body water content of women is 0.45 to 0.50 l/kg as
compared to 0.55 to 0.60 l/kg for men ( Edelman et al ., 1958 ;
Elkinton and Danowski, 1955 ). This difference is largely the
result of the larger fat deposits in the adult woman and the
larger muscle mass of the adult man ( Elkinton and Danowski,
1955 ). Clear sex-associated differences in body fat are not
appreciated in domestic animals. However, certain species
of domestic animals such as fattened swine or sheep have a
large amount of body fat. Although lighter sheep had a TBW
of near 0.65 l/kg ( Wade and Sasser, 1970 ), the TBW of these
fattened animals may be less than 0.50 l/kg ( English, 1966b ;
Hansard, 1964 ), whereas the TBW of the athletic horse is
generally greater than 0.65 l/kg ( Dieterich and Holleman,
1973 ; Judson and Mooney, 1983 ; Robb et al ., 1972 ). The
relative water content of newborn animals is much higher
than adults. Data in human infants, calves, foals, and lambs
suggest a water content in excess of 75% of body weight at
birth ( Bennett, 1975 ; Dalton, 1964 ; Edelman and Leibman,
1959 ; Phillips et al ., 1971 ; Pownall and Dalton, 1973 ). The
large TBW is primarily the result of the very large extracellular
fluid (ECF) volume, which exceeds 0.40 l/kg at birth
in most species ( Bennett, 1975 ; Kami et al ., 1984 ; Spensley
et al ., 1987 ; Tollertz, 1964 ). There is an initial rapid decline
during the first few days to weeks of life with TBW, and ECF
volumes approach adult levels by 6 months of age ( Spensley
et al ., 1987 ).
The TBW consists of two major compartments, the
intracellular fluid (ICF) volume and the ECF volume. The
distribution of body water is illustrated in Figure 17-1 indicating
the normal fluid balance of a 450-kg horse. The ICF
accounts for approximately one-half to two-thirds of the
TBW, and the ECF accounts for the remainder. Although
these two compartments differ markedly in electrolyte