Page 13 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Physiology of Body Fluids in Dogs and Cats 3

Dry matter 40%
Intracellular fluid
(ICF) 40%
4 L

Interstitial fluid 15%
10 kg 1500 ml

Total body water 60% Extracellular fluid Plasma 5%
6 L (ECF) 20% 500 ml
Total body water
= 2 L
60% of body weight Dry matter 40%
Intracellular fluid
(ICF) 40%
2 L

Interstitial fluid 15%
750 ml
5 kg

Total body water 60% Extracellular fluid Plasma 5%
3 L (ECF) 20% 250 ml
1 L
Figure 1-1 Compartments of total body water expressed as percentage of body weight and total body
water for a 10-kg dog and a 5-kg cat.

Unfortunately, data from dye dilution studies of ECF
Extracellular fluid Intracellular fluid volume are difficult to interpret because no indicator is
truly confined to the ECF space. Estimates of ECF vary
Na + 145 12 dramatically with the indicator used. ECF volumes
reported for adult, healthy dogs and cats vary between
K + 4 140
15% and 30% of body weight. The wide range in estimates
Ca 2+ 2.5 4 of ECF volume likely results from the variety of
Mg 2+ 1 34 techniques used to measure this space and the heteroge-
neity of ECFs, which include interstitial fluid (ISF),
Cl – 110 4
plasma, and transcellular fluids. Dense connective tissue,
HCO 3 – 24 12 cartilage, and bone also contain a small amount of ECF.
HPO 4 2– , H PO 4 – 2 40 From a physiologic perspective and based on multiple
2
Protein – 15 * 50 studies using various indicators, the most accurate esti-
mate of the ECF in adult small animals is 27% of lean body
weight. However, an easier distribution of body fluids to
mEq/L remember is the 60:40:20 rule: 60% of body weight is
*0 in interstitial fluid, 15 in plasma water, 40% of body weight is ICF, and 20% of body
Figure 1-2 Average values for electrolyte concentrations in weight is ECF (see Fig. 1-1). Many clinicians use 20%
extracellular and intracellular fluid. Note the marked concentration as an estimate for ECF when calculating fluid therapy
differences for many electrolytes. needs for their patients.
As mentioned above and as shown in Figure 1-1, ECF
is distributed among several different subcompartments.
fluid is lost; in renal failure, a large volume of ECF may be Most ECF (about three fourths) is in spaces surrounding
excreted. Fluid losses often are treated using parenteral cells and is called interstitial fluid. Although accurate
fluids, which initially enter the ECF. Therefore, it is impor- studies of the size of the ISF compartment in dogs and
tant to be able to estimate the volume of the ECF compart- cats have not been reported, estimates derived from mea-
ment and the volume of fluid lost to initiate appropriate surement of fluids in other compartments indicate that
fluid replacement and monitor fluid therapy. the ISF comprises approximately 15% of body weight