Page 14 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

4 APPLIED PHYSIOLOGY

(approximately 24% of total body water). About one estimate the solute space. There are limited data in the lit-
fourth of the ECF is within blood vessels and is called erature from cadaver and isotope dilution studies of body
the intravascular compartment (plasma). Intravascular solute content in small animals, and most of the following
fluids are approximately 5% of body weight (approxi- discussion is based on data from studies in humans. 13,48
mately 8% to 10% of total body water). Most of the intra- Solutes are not distributed homogeneously through-
vascular fluid is plasma. Plasma volume estimates range out body fluids. Vascular endothelium and cell
from 42 to 58 mL/kg in adult dogs that are neither very membranes have different permeabilities for various
thin nor obese. 26 Estimates for plasma volume in cats are solutes. Healthy vascular endothelium is relatively imper-
37 to 49 mL/kg. 26 Blood volume, which includes meable to the cellular components of blood and to plasma
erythrocytes, is a function of lean body mass, and proteins. Consequently, the volume of distribution of
estimated blood volume in dogs is 77 to 78 mL/kg cells and proteins is the plasma space itself. However,
(8% to 9% of body weight) and in cats is 62 to 66 mL/kg the vascular endothelium is freely permeable to ionic
(6% to 7% of body weight). 24 Racing Greyhounds may solutes, and the concentration of these ions is almost
have higher blood volumes (110 to 114 mL/kg) than the same in ISF as in plasma. Cell membranes maintain
other breeds, possibly related to higher lean body mass. 21 intracellular solutes at very different concentrations
Fluids produced by specialized cells to form cerebro- from those of the ECF. The compositions of solutes in
spinal fluid, gastrointestinal fluid, bile, glandular the ECF and ICF are compared in Figure 1-2, and
secretions, respiratory secretions, and synovial fluid are concentrations of solutes in plasma and in ISF and ICF
in the transcellular fluid compartment, which is estimated are listed in Table 1-1.
as approximately 1% of body weight (approximately 2% of The slightly increased concentration of cations and
total body water). Dense connective tissues, bone, and anions in ISF compared with plasma water occurs primar-
cartilage contain approximately 15% of total body water. ily because of the presence of negatively charged proteins
However, these tissues exchange fluids slowly with other in plasma. The equilibrium concentrations of permeable
compartments. Because this fluid usually is not taken into anions and cations across the vascular endothelium are
account for routine fluid therapy, this compartment is not determined by the Gibbs-Donnan equilibrium, which
shown in Figure 1-1. Thus, a more simplified distribution occurs because negatively charged, nondiffusible proteins
of total body water often used for fluid therapy is: affect the distribution of other small charged solutes. In
2 clinical practice, the difference in concentrations of
ICF is approximately / 3 of total body water
1 anions and cations across the vascular endothelium is neg-
ECF is approximately / 3 of total body water
3 ligible, and the effects of the Gibbs-Donnan equilibrium
ISF is approximately / 4 of ECF
Intravascular fluid is approximately ¼ of ECF are usually ignored. Thus, in clinical practice, plasma
concentrations of solutes are considered to reflect solute
Although body fluids traditionally are conceptualized
concentrations throughout the ECF. Average values for
anatomically within these various compartments, water
plasma concentrations of important solutes in dogs and
and solutes in these spaces are in dynamic equilibrium
cats are given in Table 1-2.
across the cell membrane, capillary endothelium, and
Table 1-1 shows that, although the solute
specialized lining cells. Fluids and electrolytes shift among
compositions of ECF and ICF are quite different, the
compartments to maintain homeostasis within each com-
total numbers of cations and anions in all body fluids
partment. In health, the concentration of a particular
are equal to maintain electroneutrality. The most abun-
substance may be similar or very different among the var- þ
dant cation in the ECF is sodium (Na ). Most of the body
ious fluid compartments. During disease, fluid volumes þ
Na is in the extracellular space. Approximately 70% of
and solute concentrations may change dramatically. Loss þ
body Na in humans is exchangeable, and 30% is fixed
or gain of fluid or electrolytes from one compartment 48
as insoluble salts in bone. The percentage of exchange-
likely will alter the volume and solute concentrations of able sodium is important because only exchangeable
other compartments.
solutes are osmotically active. Cell membranes are perme-
able to Na , which tends to diffuse into cells. In health,
þ
DISTRIBUTION OF BODY however, cell membrane sodium, potassium-adenosine-
SOLUTES triphosphatase (Na ,K -ATPase) actively removes Na þ
þ
þ
from cells, thus maintaining a steep extracellular-to-intra-
In addition to water, body fluids contain various cellular concentration gradient for Na . The ECF also
þ
concentrations of solutes. Total body content of solutes contains a small but physiologically important concentra-
may be measured by cadaver analysis (desiccation) or by tion of K . For example, alterations in ECF K þ
þ
isotope dilution studies. Every solute has a space or appar- concentrations may result in muscle weakness (hypokale-
ent volume of distribution. Dilution studies of body mia) orcardiotoxicity (hyperkalemia). Themost abundant
solute content yield variable results, depending on the anions in ECF are chloride (Cl ) and bicarbonate

volume of distribution of the particular tracer used to (HCO 3 ). The volume of distribution of Cl is primarily