Page 23 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Physiology of Body Fluids in Dogs and Cats 13

Cations Anions
Vessel
P cap π p

Arteriole Venule
P if π if
–
Cl 110 mEq/L
Interstitium
Figure 1-5 Factors affecting fluid movement at the level of the
+
capillary. P cap , Capillary hydrostatic pressure; P if , interstitial Na 145 mEq/L
hydrostatic pressure; p p , capillary oncotic pressure; p if , interstitial
oncotic pressure.
–
HCO 3 21 mEq/L
hydrostatic pressure and tissue oncotic pressure) and HPO 4 , H 2 PO 4 2 mEq/L
2–
–
2–
forces that tend to retain fluid within the vascular space SO 4 2 mEq/L UA
–
+
(plasma oncotic pressure and tissue hydrostatic pressure). K 4 mEq/L Lactate 2 mEq/L
–
Other 3 mEq/L
Oncotic pressure is the osmotic pressure generated by Ca 5 mEq/L Proteins 16 mEq/L
2+
–n
UC
2+
plasma proteins in the vascular space. Starling’s law Mg 2 mEq/L
describes these relationships (Fig. 1-5): UA - UC = Anion gap
Figure 1-6 Relative concentrations of unmeasured anions (UAs)
Net filtration ¼ K f ½ðP cap P if Þ ðp p p if ފ and cations (UCs) in extracellular fluid (ECF).
where K f represents the net permeability of the capillary
between the forces favoring filtration and those favoring
wall, P represents the hydrostatic pressure generated by
reabsorption in the capillary.
the heart (P cap ) or tissues (P if ), and p represents the oncotic
pressure generated by plasma proteins (p p ) or filtered ELECTRONEUTRALITY AND
proteins and mucopolysaccharides in the interstitium (p if ).
The net filtration pressure in healthy capillaries is THE ANION GAP
about 0.3 to 0.5 mm Hg at the proximal (arteriolar) In body fluids, the sum of all cations must equal the sum
end of the capillary. 49 Near the venule, the forces favoring
of all anions to fulfill the law of electroneutrality. In the
filtration are less than the forces favoring reabsorption of
clinical setting, however, all anions and cations in body
fluid into the vascular space, because capillary hydrostatic
fluids are not routinely measured. Figure 1-6 compares
pressure decreases along the length of the capillary, but
the concentrations of the commonly measured anions
capillary oncotic pressure remains approximately the and the commonly measured cations in a gamblegram.
same. 49 Some of the fluid that is filtered into the The commonly measured cations are Na and K , and
þ
þ
interstitium at the proximal end of the capillary is the commonly measured anions are Cl and HCO 3 .
reabsorbed distally; the remainder of the filtered fluid is
The sum of the concentrations of commonly measured
transported by lymphatics in the interstitium. The hydro- anions is less than the sum of the concentrations of com-
static pressure transferred from arterioles to the capillaries monly measured cations. In other words, there are more
is controlled by autoregulation of the precapillary sphinc- unmeasured anions (UAs) than unmeasured cations
ter. Autoregulation protects the capillary from increases (UCs). From this observation, the concept of the anion
in hydrostatic pressure caused by systemic hypertension, gap was developed. It is important to remember that
which otherwise could cause a dangerous loss of vascular there is no real difference between the total number of
fluid into the ISF by filtration.
anions and the total number of cations in the body.
During water depletion, capillary oncotic pressure
In the clinical setting, the anion gap is used to predict
increases and hydrostatic pressure may decrease if deple-
changes in the UAs or less commonly in the UCs.
tion is severe enough to cause hypovolemia. These
The anion gap is defined as the difference between
alterations in Starling’s forces favor a decrease in net filtra-
the UAs and the UCs. According to the law of
tion of fluid into the interstitium at the level of the capil-
electroneutrality,
lary. Increased reabsorption of ISF augments effective
circulating blood volume, thus decreasing plasma protein Na þ K þ UC ¼ Cl þ HCO þ UA

þ
þ
concentration and increasing hydrostatic pressure. Con- 3
versely, loss of plasma protein decreases plasma oncotic
Rearranging this equation,
pressure and increases the net force favoring filtration
of fluid out of the capillary. Loss of intravascular fluid
þ
ðNa K Þ ðCl þ HCO 3 Þ ¼ UA UC ¼ anion gap

þ
increases plasma oncotic pressure, but filtration of fluid
into the interstitium produces the edema observed in
hypoproteinemic states. Thus, in the healthy animal, The range for the normal anion gap varies by species
maintenance of plasma volume depends on a fine balance and is approximately 12 to 24 mEq/L in dogs and 13