Page 20 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

10 APPLIED PHYSIOLOGY

Molar mass of CaCl 2 : 111.1 g
Convert grams to moles: 100 g CaCl 2 (1 mol/111.1 g) ¼ 0.9 mol CaCl 2
Convert moles to millimoles 0.9 mol (1000 mmol/mol) ¼ 900 mmol CaCl 2

Determine millimoles of Ca þ2 and Cl : CaCl 2 in solution dissociates into Ca þ2 and 2Cl, yielding
900 mmol/L of Ca þ2 and 1800 mmol/L of Cl
Determine milliequivalents of Ca þ2 and Cl millimoles valence ¼ milliequivalents
Ca þ2 has a valence of 2; Cl has a valence of 1

900 mmol Ca þ2 2 ¼ 1800 mEq of Ca þ2

1800 mmol Cl 1 ¼ 1800 mEq of Cl
Determine milliosmoles of Ca þ2 and Cl : CaCl 2 in solution dissociates into Ca þ2 þ 2Cl

mOsm/L in 10% CaCl 2 is the sum of the milliosmoles for
each component:
1800 mEq/L Ca þ2 þ 1800 mEq/L Cl
900 mOsm/L Ca þ2 þ 1800 mOsm/L Cl
¼ 2700 mOsm/L

Osmolal Gap serum. Larger molecules like albumin contribute little
Osmolal gap is the difference between measured osmolal- to the osmolality.
ity and calculated osmolality. As mentioned above, osmotic activity depends on the
solute and its permeability across the membrane. Sodium
Colloid Osmotic Pressure (Oncotic Pressure) is the most abundant cation in the ECF. Although there is
variation among different types of cells, many cell
Colloids are large molecular weight (MW ¼ 30,000) membranes are impermeable to sodium. Sodium move-
particles present in a solution. The component of the ment across most cell membranes occurs by active
total osmotic pressure in plasma contributed by colloids transport. Consequently, Na and its associated anions
þ
is called the colloid osmotic pressure (oncotic pressure).
account for most of the osmotically active particles in
Plasma proteins are the major colloids present in normal
the ECF and as such are considered effective osmoles.
plasma. Although colloid osmotic pressure is only about
Glucose and urea are two other substances with poten-
0.5% of the total osmotic pressure, oncotic pressure is
tial osmotic activity. Many cell membranes are not freely
extremely important in transcapillary fluid dynamics.
permeable to glucose, in which case glucose would be
Oncotic pressure can be measured using a colloid
osmotically active. In contrast, urea does not make a
osmometer (oncometer).
major contribution to effective osmolality in the ECF
Several examples related to fluid therapy are included because it is a small molecule that is freely diffusible across
here to illustrate how these definitions may be used in most cell membranes. However, urea may have an impact
clinical veterinary medicine.
on serum osmolality if its concentration is increased.
Osmolality of the ECF may be estimated using various
EXCHANGE OF WATER formulas. This is called the calculated osmolality because
BETWEEN EXTRACELLULAR it is based on estimating the contribution of osmotically
active substances. Calculated osmolality by itself is not
AND INTRACELLULAR FLUID very useful because it is simply an estimate based on the
SPACES concentration of commonly measured solutes. Calculated
osmolality, which is an estimate, may not be the same as
The number of osmotically active particles in each space measured osmolality, which is determined by an
determines the volume of fluid in the ECF and ICF osmometer.
compartments. The osmolality of physiologic fluids is The formulas for calculated osmolality include various
dominated by small solutes that are present in high combinations of the most osmotically active solutes, but
concentrations. In serum, sodium, potassium, chloride, none includes all osmotically active solutes because not all
bicarbonate, urea, and glucose are present in high enough solutes are measured on routine biochemical profiles. The
concentrations to individually affect osmolality. Together formulas also assume complete dissociation of some
these make up more than 95% of the total osmolality of solutes, which may not be true in serum.