Page 12 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 1

Applied Physiology of Body Fluids in

Dogs and Cats

Maxey L. Wellman, Stephen P. DiBartola, and Catherine W. Kohn

Normal body weight 0:8 ¼ Lean body mass
Appropriate treatment of fluid and electrolyte
Obese body weight 0:7 ¼ Lean body mass
abnormalities requires a basic understanding of the phys-
Thin body weight 1:0 ¼ Lean body mass
iology of fluid balance. The purpose of this chapter is to
provide an overview of the principles of body fluid
Water is the major component of all body fluids, which
homeostasis, beginning with a brief review of body fluid
are distributed into several physically distinct
compartments. This is followed by a discussion of mea-
compartments. Body fluids in each compartment equili-
surement of solutes in body fluids and the concepts of
brate with fluids in other compartments by multiple
anion gap, osmolal gap, and zero balance.
mechanisms across a wide variety of membranes to main-
tain homeostasis. The volume of fluid in each of these
DISTRIBUTION OF compartments has been estimated using various isotope
BODY FLUIDS or dye dilution techniques and calculating their volumes
ofdistribution. Results areexpressedeither asapercentage
In health, approximately 60% of an adult animal’s body of body weight, which is easy to measure when calculating
weight is water. Estimates of total body water in adult fluid therapy needs, or as a percentage of total body water,
dogs that are neither very thin nor obese are 534 to which is a useful conceptualization of body fluid
660 mL/kg. 26,59 Total body water of adult cats also compartments. Studies of body fluid compartments often
was determined to be approximately 60%. 56 There are are performed in experimental animals that have been
some species and individual variations in total body water, anesthetized, splenectomized, or nephrectomized. Data
likely related to age, sex, and body composition. In from these kinds of studies vary with the protocol used
humans, total body water decreases with age and is lower and thus provide only approximations of fluid compart-
in women than in men. 13 Neonatal dogs and cats have ment sizes in healthy awake animals. The second edition
higher total body water content (80% of body weight) of this book contains a more detailed discussion of the
than adults (60% of body weight), 30 and an age-related techniques involved in determination of total body water
decrease in total body water has been described in puppies and the amount of fluid in the various compartments.
and kittens during the first 6 months of life. 35 Total body As shown in Figure 1-1, the largest volume of fluid in
water was approximately 70% of total body weight in the body is inside cells. The intracellular fluid (ICF)
racing Greyhounds, likely due to low body fat content. 21 compartment comprises approximately 40% of body
Because fat has a lower water content than lean tissue, weight (approximately two thirds of total body water).
fluid needs should be estimated on the basis of lean body The composition of ICF is very different from extracellu-
mass to avoid overhydration, especially in patients with lar fluid (ECF) (Fig. 1-2). Intracellular homeostasis is
cardiac or renal insufficiency or in those with maintained by shifts in water, solutes, and numerous
hypoproteinemia. Formulas for estimating lean body other substances across the cell membrane.
mass are based on the assumptions that (1) in normal Any fluid not contained inside a cell is in the extracellu-
small animal patients, approximately 20% of body weight lar fluid compartment (approximately one third of total
is due to fat, (2) morbid obesity increases body fat body water). Fluid shifts that occur during changes in
to approximately 30% of body weight, and (3) body hydration can havea marked effect onthe ECF, andinmost
weight is a reasonable estimate of lean body mass in thin disease states, loss of fluids occurs initially from the ECF.
patients: For example, in diarrhea, a large volume of gastrointestinal

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