Page 19 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Physiology of Body Fluids in Dogs and Cats 9

membrane to a solution of greater solute concentration is tonicity of a solution may be less than the measured
called osmosis. The influx of water into the left compart- osmolality if both effective and ineffective osmoles are
ment resulting from the osmotic effect of glucose causes present. Thus, the tonicity and osmolality of a solution
the solution to rise in the column. The height of fluid in are not necessarily equal—a circumstance that often is
the column is proportional to the osmotic pressure true in biologic solutions.
generated by glucose. In this example, glucose is an effec-
tive osmole because it generates osmotic pressure by caus- Measured Osmolality
ing a shift of water across the boundary membrane.
The osmolality determined with an osmometer is the
Glucose is an effective osmole in this setting because
measured osmolality, which typically is not the same as
the boundary membrane is impermeable to glucose but
the calculated osmolality estimated using various formulas.
permeable to water. In biologic fluids, glucose can
contribute to osmolality because it is not freely diffusible.
Calculated Osmolality
Tonicity The calculated osmolality is an estimate of serum osmolal-
The effective osmolality of a solution is referred to as the ity using various formulas. The formulas include solutes
tonicity of the solution. A freezing-point depression that have a major contribution to total osmolality. Calcu-
osmometer measures all osmotically active particles in lated osmolality often is less than measured osmolality
the solution. Thus, the measured osmolality of a solution because the formulas either exclude some osmotically
includes both effective and ineffective osmoles. The active particles or estimate their contribution.

Example 1 Determine how many millimoles, milliequivalents, and milliosmoles of sodium and chloride there are in 1 L of
a 0.9% solution of NaCl.
Concentration of 0.9% NaCl: 0.9 g NaCl/100 mL of solution ¼ 900 mg NaCl/dL
Convert milligrams to grams and deciliters to liters 900 mg NaCl/100 dL 1 g/1000 mg 10 dL/L ¼ 9 g NaCl/L
Formula weight of NaCl: Atomic mass of Na þ atomic mass of Cl
(use atomic weight from Table 1-3 or periodic table) ¼ 23 þ 35.5 ¼ 58.5
Molar mass of NaCl: 58.5 g

Convert grams to moles: 9 g NaCl (1 mol/58.5g) ¼ 0.154 mol NaCl
Convert moles to millimoles: 0.154 mol (1000 mmol/mol) ¼ 154 mmol NaCl
þ
þ

Determine millimoles of Na and Cl NaCl in solution dissociates into Na and Cl , yielding
þ
154 mmol/L of Na and 154 mmol/L of Cl
Determine milliequivalents of Na and Cl millimoles valence ¼ milliequivalents
þ
þ
Na and Cl each have a valence of 1

154 mmol 1 ¼ 154 mEq of Na þ
154 mmol 1 ¼ 154 mEq of Cl

þ
Determine milliosmoles of Na and Cl NaCl in solution dissociates into Na and Cl , so the mOsm/L in
þ
0.9% NaCl is the sum of the mOsm for each component:
154 mEq/L Na þ 154 mEq/L Cl
þ
þ
154 mOsm/L Na þ 154 mOsm/L Cl
¼ 308 mOsm/L
Example 2 Determine how many millimoles, milliequivalents, and milliosmoles of calcium and chloride there are in 1 L
of a 10% solution of CaCl 2 .
Concentration of 10% CaCl 2 : 10 g CaCl 2 /100 mL of solution ¼ 10 g CaCl 2 /dL
To convert deciliters to liters: 10 g CaCl 2 /dL 10 dL/L ¼ 100 g CaCl 2 /L
Formula weight of CaCl 2 : Atomic mass of Ca þ 2 (atomic mass of Cl)
(use atomic weight from Table 1-3 or periodic table) ¼ 40.1 þ (2 35.5) ¼ 111.1

Continued