Page 22 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

12 APPLIED PHYSIOLOGY

165 Na' = 1.11 (Na + K )/ TBW –25.6
s
e
e
S Y.X = 5.6
160 r = 0.83
155
150

145

140
(Na' s ) "Corrected" serum sodium concentration (mEq / L serum water) 135

130

125
120
No edema
115 Edema

110

110 115 120 125 130 135 140 145 150 155 160 165 170 175 180
(Na +K ) / TBW (mEq/L)
e
e
Figure 1-4 Relationship of plasma [Na ] to ([Na ] e [K ] e )/TBW. [Na ] e , total exchangeable Na ;

þ
þ
þ
þ
[K ] e , total exchangeable K ; TBW, total body water. (From Edelman IS, Leibman J, O'Meara MP, et al.
þ
þ
Interrelations between serum sodium concentration, serum osmolarity and total exchangeable sodium, total
exchangeable potassium, and total body water. J Clin Invest 1958;37:1236–1256.)
of isotonic fluid increases the ECF volume. Isotonic fluid
TABLE 1-4 Effect of Water and loss, if of sufficient magnitude, results in hypovolemia and
Solute Losses from Body shock. These concepts are discussed further in Chapter 3.
Fluids EXCHANGE OF WATER
Theoretical Replacement BETWEEN PLASMA AND
Loss ECF Fluid INTERSTITIAL SPACES
Hypotonic Hypertonic Hypotonic Most of the ECF is in either the interstitial compartment
Isotonic Isotonic Isotonic
(approximately three quarters of the ECF) or the intra-
Hypertonic Hypotonic Isotonic/hypertonic
vascular compartment, most of which is plasma (approxi-
mately one quarter of the ECF). The partitioning of fluid
between plasma and ISF spaces is critically important for
During water deprivation, the tonicity of ECF maintenance of the effective circulating blood volume.
increases relative to that of the ICF. Water shifts out of The effective blood volume has been defined as “the com-
cells and into ECF until the osmolalities of the two ponent of blood volume to which the volume-regulatory
compartments are equal. The osmolalities of both ICF system responds by causing renal sodium and water reten-
and ECF are greater than those during the state of normal tion in the setting of cardiac and hepatic failure even
hydration. This water shift augments the ECF volume, though measured total blood and plasma volume may
thus helping to preserve the effective circulating blood be increased.” 42,51
volume and protecting against the development of shock. Exchange of solutes and fluid between plasma and
Loss or gain of isotonic fluid from the ECF results in no interstitial spaces occurs at the capillary level. The volume
change in ECF osmolality, and no osmotically mediated of the vascular space is controlled by a balance between
water shifts between the ICF and ECF occur. Loss of iso- forces that favor filtration of fluid through the vascular
tonicfluidresultsinadecreaseinECFvolume,whereasgain endothelium into the interstitial space (capillary