Page 16 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

6 APPLIED PHYSIOLOGY

ATOMIC MASS (ALSO REFERRED TOAS The molecular weights of important compounds in body
RELATIVE ATOMIC MASS OR ATOMIC fluids are shown in Table 1-3.
WEIGHT)
FORMULA WEIGHT
Most naturally occurring elements consist of one or more
Ionic compounds do not really form molecules, and a
isotopes of that element, each of which has a different
more appropriate term for the mass of these substances
mass. For example, carbon in the environment consists
of approximately 99% 12 Cand 1% 13 C. The atomic mass is formula weight. For example, the formula weight of
CaCl 2 is the atomic mass of Ca 2þ (40) plus two times
of an element is an average mass based on the distribution
the atomic mass of Cl (2 35.5) ¼ 111.

of stable isotopes for that element, and is determined by
the weight of that element relative to the weight of the MOLE
12
C isotope of carbon, which is defined as 12.000. 23
Atomic mass usually is reported with no units or as atomic A mole is defined as 6.023 10 particles. Some physi-
mass units. The atomic mass is shown in most periodic ology texts define a mole as the molecular (or atomic)
weight of a substance in grams, but a mole really just
tables of the elements. The atomic weights of some bio- 23
describes 6.023 10 (Avogadro’s number) particles.
logically important elements in body fluids are listed in
It is defined as the number of atoms in exactly 12 g of
Table 1-3.
12
C. One mole of a substance weighs its molecular weight
in grams (see section on Molecular Mass).
MOLECULAR MASS (MOLECULAR
WEIGHT) MOLAR MASS
Many elements combine to form physiologically impor- The molar mass is the mass in grams of 1 mol of a
tant compounds. The molecular mass of a compound is substance. By definition, 1 mol of carbon has a mass of
the sum of the atomic masses of the atoms that form 12 g. Molar masses are numerically equivalent to atomic
the compound. For example, the molecular mass of water or molecular weights but are reported in grams. For
(H 2 O) is 18 and represents two times the atomic mass of example, 1 mol Na weighs 23 g. Molar mass and gram
þ
hydrogen (2 1) plus the atomic mass of oxygen (16). molecular weight often are used interchangeably.

TABLE 1-3 Atomic or Molecular Weights of Physiologically Important Substances

Substance Symbol or Formula Atomic or Molecular Weight Valence

Calcium ion Ca 40.1 þ2
Carbon C 12.0 0
Chloride ion Cl 35.5 1
Hydrogen ion H 1.0 þ1
Magnesium ion Mg 24.3 þ2
Nitrogen N 14.0 0
Oxygen O 16.0 0
Phosphorus P 31.0 0
Potassium ion K 39.1 þ1
Sodium ion Na 23.0 þ1
Sulfur S 32.1 0
Ammonia NH 3 17.0 0
Ammonium ion NH 4 18.0 þ1
Bicarbonate ion HCO 3 61.0 1
Carbon dioxide CO 2 44.0 0
Glucose C 6 H 12 O 6 180.0 0
Lactate ion C 3 H 5 O 3 89.0 1
Phosphate ion PO 4 95.0 3
96.0 2
HPO 4
97.0 1
H 2 PO 4
Sulfate ion SO 4 96.1 2
Urea NH 2 CONH 2 60.0 0
Water H 2 O 18.0 0
Adapted from Rose BD. Clinical physiology of acid-base and electrolyte disorders, 3rd ed. New York: McGraw-Hill, 1989, with permission of the McGraw-Hill
Companies.