Page 17 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

Applied Physiology of Body Fluids in Dogs and Cats 7

MOLALITY AND MOLARITY negative one (e.g., Cl ); a divalent cation has a charge

of positive two (Ca 2þ ). One atom of Ca 2þ combines with
Molality refers to the number of moles of solute per kilo-
two atoms of Cl to form CaCl 2 . It is useful to express

gram of solvent, whereas molarity refers to the number of
concentrations of solutes in body fluids in equivalents
molesofsoluteperliterofsolution.Themolarityandmolal-
per liter (Eq/L) or milliequivalents per liter (mEq/L)
ity of most biologic solutions are approximately equal
to reflect the charge or valence of the solute. The equiva-
becausethedensityofwateris1 kg/L.Theslightdifference
lent weight of a substance is the atomic, molecular, or
between molarity and molality of a substance in plasma is
formula weight of a substance divided by the valance.
because of nonaqueous proteins and lipids, which make
up about 6% of the total volume. In body fluids, this differ- ELECTROCHEMICAL EQUIVALENCE
ence is relatively unimportant, and the terms molality and 49
molarity often are used interchangeably. Rose defines electrochemical equivalence as follows:
One equivalent is defined as the weight in grams of an
MILLIMOLE AND MILLIGRAM element that combines with or replaces 1 g of hydrogen
þ
þ
The prefix “milli” refers to 1 one-thousandth. A milli- ion (H ). Because 1 g of H is equal to 1 mol of H þ
mole is 1 10 3 mol; a milligram is 1 10 3 g. Many (containing approximately 6.023 10 23 particles),

biologic substances in body fluids are measured in 1 mol of any univalent anion (charge equals 1 ) will
þ
millimoles or milligrams. combine with this H and is equal to 1 equivalent (Eq).

For example, 1 mol (1 equivalent) of Cl combines with
CONCENTRATION 1 mol of H ; 1 mol (1 equivalent) of Na could replace
þ
þ
þ
Concentration refers to the amount of a substance that is 1 mol of H ; 1 mol (2 equivalents) of Ca 2þ combines with

present in a specified volume. The amount of a substance 2 mol(2equivalents)ofCl toform1 molofCaCl 2 .There-
can be expressed as mass (grams or milligrams), moles (or fore, it is useful to express concentrations of solutes in body
millimoles), or equivalents (or milliequivalents). Volume fluids in equivalents per liter (Eq/L), thus reflecting the
usually isexpressed as liters (L), deciliters (dL), or milliliters charge or valence of the solute.
(mL). A deciliter is one tenth of a liter (i.e., 100 mL).
Many solutions used for fluid therapy are percent EQUIVALENT WEIGHT
solutions. Percent concentration refers to a number of The equivalent weight of a substance is the atomic, molec-
parts in 100 parts of solution. This may be used to express ular, or formula weight divided by the valence. The
concentration in terms of weight per unit weight, weight milliequivalent (mEq) weight is 10 3 times the equivalent
per unit volume, or volume per unit volume. For example, weight.Foranelementsuchassodium,whichhasavalence
a 0.9% solution of NaCl contains 0.9 g of NaCl per of þ1, the milliequivalent weight is equal to its atomic
100 mL of solution, because 100 mL of H 2 O is equal to weight. Therefore, each millimole of Na þ provides
100 g of H 2 O (0.9 g NaCl/100 g H 2 O). Because a gram 1 mEq. In contrast, the milliequivalent weight of Ca 2þ
is equal to 1000 mg and a deciliter is equal to 100 mL of is one half its atomic weight because its valence is þ2.
solution, a 0.9% solution of NaCl contains 900 mg of Each millimole of Ca 2þ provides 2 mEq (0.5 mmol pro-
NaCl per deciliter (9 g NaCl/L). Similarly, a 10% solution vides 1 mEq). These relationships may be summarized as:
of CaCl 2 contains 10 g of CaCl 2 per 100 mL of solution,
or 10 g of CaCl 2 per deciliter (100 g/L), and 5% dextrose Millimolecular weight=valence ¼ milliequivalent weight
contains 5 g of dextrose per deciliter (50 g/L).
ð
Millimoles valence ¼ milliequivalents mEqÞ
CATION
A cation is an atom or molecule with a positive charge. A To convert concentrations:
þ
monovalent cation has one positive charge (e.g., Na ),
mEq=L ¼ mmol=L valence
and a divalent cation has two positive charges (e.g., Ca 2þ ).
mg=dL 10
ANION mEqL ¼ molecular weight valence
An anion is an atom or molecule with a negative charge. A
monovalent anion has one negative charge (e.g., Cl ), and Note: Multiplication by 10 in the numerator converts

2 mg/dL to mg/L. Dividing by the molecular weight
a divalent anion has two negative charges (e.g., SO 4 ).
converts milligrams to millimoles. Multiplying by the
VALENCE valence converts to milliequivalents.
Ions in body fluids combine according to ionic charge Phosphate can exist in body fluids in three different
(valence) rather than weight. The number of cations ionic forms: H 2 PO 4 , HPO 4 2 , and PO 4 3 (see

(positively charged ions) in a solution always equals the Chapter 7). At the normal pH of ECF, approximately
number of anions (negatively charged ions) to maintain 80% of phosphate is in the HPO 4 2 form and 20% is in
electroneutrality. A univalent anion has a charge of the H 2 PO 4 form. Therefore, the average valence of