Page 18 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

8 APPLIED PHYSIOLOGY

phosphate in ECF is 0.8 ( 2) þ 0.2 ( 1) ¼ 1.8. At Effective and Ineffective Osmoles
a normal plasma phosphate concentration of 4 mg/dL, In any fluid compartment, the osmotic effect of a solute is
the phosphate concentration expressed in mEq/L would
in part dependent on the permeability of the solute across
be:
the membranes separating the compartment. Consider
thetwo fluidcompartmentsina rigidbox in Figure 1-3.
4 10
1:8 ¼ 2:3 mEq=L Assumethatthemembranedividingthetwocompartments
31 is freely permeable to urea and water but is impermeable
to glucose. When urea is added to the left compartment
OSMOLALITY AND OSMOLARITY (top of figure), it moves down its concentration gradient
from left to right, and water moves down its concentration
Regardless of its weight, 1 mol of any substance contains gradient from right to left until there are equal concen-
23
the same number of particles (6.023 10 ; Avogadro’s trations of urea and water on both sides of the membrane.
law). Solutes exert an osmotic effect in solution that is No fluid rises in the column attached to the left fluid com-
dependent only on the number of particles in solution,
partment because urea is an ineffective osmole and does
not their chemical formula, weight, size, or valence.
not generate osmotic pressure. In biologic fluids, urea is a
One osmole (Osm) is defined as 1 g molecular weight
small molecule that freely diffuses across most cell
of any nondissociable substance; therefore, each osmole membranes and therefore does not contribute to effective
also contains 6.023 10 23 molecules. osmolality.
If a substance does not dissociate in solution (e.g., glu- When glucose is added to the left compartment (bot-
cose), 1 mol equals 1 Osm. If a substance dissociates in tom of figure), water moves down its concentration gra-
solution, the number of osmoles equals the number of
dient from right to left, but glucose cannot move across
dissociated particles. For example, assuming that NaCl
the membrane. This movement of water from a solution

completely dissociates into Na and Cl in solution, each
þ
of lesser solute concentration across a semipermeable
millimole of NaCl provides 2 milliosmoles (mOsm):
1 mOsm of Na þ and 1 mOsm of Cl . If a compound

in solution dissociates into three particles, the number
of osmoles in solution is increased three times (e.g.,
CaCl 2 ). The milliosmolar concentration of a solution
Urea
may be expressed as the solution’s milliosmolarity or H O
2
milliosmolality.
Osmolality refers to the number of osmoles per kilo-
gram of solvent. An aqueous solution with an osmolality
of 1.0 results when 1 Osm of a solute is added to 1 kg of
water. The volume of the resulting solution exceeds 1 L
= Urea Ineffective osmole
by the relatively small volume of the solute. In clinical vet-
erinary medicine, osmolality is expressed as milliosmoles Semipermeable
membrane
per kilogram.
Osmolarity refers to the number of osmoles per liter Osmotic pressure
of solution. If 1 Osm of a solute is placed in a beaker and
enough water is added to make the total volume 1 L, the
H 2 O
osmolarity of the resulting solution is 1. In clinical medi-
cine, osmolarity is expressed as milliosmoles per liter. In
biologic fluids, there is a negligible difference between
osmolality and osmolarity, and the term osmolality is used
in this discussion
= Glucose
In clinical medicine, osmolality is measured in serum, Effective osmole
because the addition of anticoagulants for plasma samples Figure 1-3 Effective and ineffective osmoles. Top, Effect of adding
would increase solute in the sample. Serum osmolality a permeable solute such as urea (small closed circles) to the fluid on
one side of a membrane. In this setting, equilibrium is reached by
usually is measured by freezing-point depression, which urea equilibration across the membrane rather than water
is more precise and accurate than vapor pressure movement into the urea compartment. Consequently, no osmotic
determinations. One osmole of a solute in 1 kg of water pressure is generated. Bottom, Effect of adding an impermeable
depresses the freezing point of the water by 1.86 C. 55 solute such as glucose (large open circles) to the fluid on one side of a

Average values for measured serum osmolality in the membrane. As water moves into the glucose compartment,
dog and cat are 300 and 310 mOsm/kg, respectively. 8,17 hydraulic pressure is generated (measured by the height of the
Measured osmolality may not be the same as calculated column of water above the glucose compartment), which at
osmolality (see later discussion). equilibrium equals the osmotic pressure of the solution.