Page 663 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

650 SPECIAL THERAPY

they closely resemble a colloid solution and thus are TABLE 27-1 Colloid Osmotic
referred to and treated as such. The osmotic pressure Pressure in Normal
exerted by the naturally occurring colloids in plasma is
higher than that calculated for an ideal solution in vitro. Cats and Dogs
One of the main reasons for this discrepancy is that nega- Colloid Osmotic
tively charged proteins (such as albumin, which has a net Pressure
negative charge of 17 at physiologic pH) retain cations Mean SD Reference
within the intravascular space by electrostatic attraction Species (mm Hg) Number
(termed the Donnan effect). 71 These cations contribute
to the effective plasma protein osmotic pressure because Canine (plasma) 20.8 1.8 185
osmotic pressure is proportional to the number of Canine (plasma) 17.5 3.0 108
molecules present rather than their size. Therefore col- Canine (whole 19.9 2.1 44
loid osmotic pressure (COP) is the most correct term blood)
when referring to the osmotic pressure exerted by plasma Feline (plasma) 19.8 2.4 185
proteins and their associated electrolyte molecules. For Feline (whole 24.7 3.7 44
blood)
comparison, the oncotic pressure exerted by an albumin
solution of 7 g/dL is 19.8 mm Hg, whereas the in vivo
COP is actually 28 mm Hg, and the total osmotic pres-
sure of all plasma solutes is 5400 mm Hg. 71 muscle or subcutaneous tissue is relatively impermeable
By virtue of its relatively high concentration in the vas- to protein, whereas the pulmonary capillary endothelium
cular space, albumin usually accounts for 60% to 70% of is more permeable with a reflection coefficient to albumin
the plasma COP with globulins making up the remain- of approximately 0.5 to 0.64. 113 Consequently, the nor-
der. 108,168,176 Interestingly, the variation in COP in dogs mal protein concentration in lymph from skin or skeletal
may be because of differences in globulin concentration muscle is approximately 50% that of plasma compared
than in albumin concentration. 65,108 Red blood cells with 65% in pulmonary lymph. 113 Hyaluronan and its
and platelets do not contribute significantly to plasma associated cations also may contribute to interstitial
5
COP. 118 Serum albumin concentration is determined COP. Because of the volume occupied by the interstitial
by the relative rates of synthesis, degradation, and loss matrix, interstitial albumin is distributed in a volume that
from the body and its distribution between the extravas- is less than the total interstitial volume. This phenome-
cular and interstitial spaces. Albumin synthesis, which is non is called the volume exclusion effect, and the
unique to the liver, appears to be regulated, at least in “excluded volume” with respect to albumin may be as
part, by the hepatic plasma COP. 53,117,130 Increases of high as one half to two thirds of the total interstitial vol-
plasma COP independent of albumin concentration, such ume. 13,112,175 Consequently, in a normally hydrated
as in hyperglobulinemia, are associated with decreased interstitium, much less protein is required to exert a given
serum albumin concentration. 18,131,132 The main site osmotic pressure, and relatively smaller volumes of
of albumin degradation is uncertain, but the reticuloen- extravasated fluid result in greater decrements in intersti-
dothelial system has been suggested. Equations have tial COP. This effect maintains the intravascular-to-extra-
been derived to estimate plasma COP from plasma pro- vascular COP gradient in early edema formation.
tein concentrations, 108,160 but direct measurement with Conversely, when interstitial volume is overexpanded by
a colloid osmometer is more accurate. 7,28,160,176 fluid in edematous states, a dramatic increase occurs in
COPs measured in normal dogs and cats are given in the volume available for albumin sequestration. 71 The
Table 27-1. 44,108,186 increase in interstitial COP that occurs with dehydration
acts to restrict mobilization of interstitial fluid. 76
INTERSTITIAL COLLOID
OSMOTIC PRESSURE INTRAVASCULAR
HYDROSTATIC PRESSURE
Capillaries are permeable to protein, despite the fact that
the microvascular barrier greatly restricts macromolecular Intravascular hydrostatic pressure is the main force that
flux. Of the total quantity of albumin present in the body, determines fluid egress from the vasculature. It may vary
40% is intravascular and 60% is extravascular. 133 Further- in different tissues and at different levels within each cap-
more, all of the albumin present in plasma circulates illary bed. The normal hydrostatic pressure in the capil-
through the interstitium every 24 hours. 114 The intersti- lary bed is controlled by local myogenic, neurogenic,
tial COP varies from tissue to tissue depending on such and humoral modulation of the arterial and venous
factors as the permeability of the capillary wall to protein, resistances. Precapillary arteriolar constriction may
the rate of transvascular solvent flow, the retention of pro- reduce flow, and therefore hydrostatic pressure, through
tein in the interstitial matrix, and the rate of lymphatic a capillary bed or shunt flow away from that bed, resulting
clearance of protein. The microvascular barrier of skeletal in changes in the total surface area available for