Page 240 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice

CHAPTER • 9

Introduction to Acid-Base Disorders

Stephen P. DiBartola

To Faraday we are indebted for naming the products of dissociation, ions—and thus we came by “hydrogen ions,” a
term now synonymous with proton. Tiny though it is, I suppose no constituent of living matter has so much power to
influence biological behavior.. ..
A. Baird Hastings, Ann N Y Acad Sci 133:16, 1966.

Metabolic processes each day yield 50 to 100 mEq of H þ concentration of H is in the range of nanoequivalents
þ
ions (fixed or nonvolatile acid) from the metabolism of per liter. That is, hydrogen ions are present at one-
proteins and phospholipids and 10,000 to 15,000 mmol millionth the concentration of other electrolytes. What,
of CO 2 (volatile acid) from the metabolism of carbohy- then, accounts for the emphasis on hydrogen ions in biol-
drate and fat. Carbon dioxide is potentially an acid by ogy and medicine? The answer lies in the fact that hydro-
virtue of its ability to combine with H 2 O in the presence gen ions are highly reactive. The proteins of the body have
of carbonic anhydrase to form carbonic acid (H 2 CO 3 ). many dissociable groups. These may gain or lose protons
þ
Carbon dioxide is continuously removed by alveolar as [H ] changes, resulting in alterations in charge and
ventilation so that the partial pressure of CO 2 (PCO 2 )is molecular configuration that may adversely affect protein
þ
kept constant at approximately 40 mm Hg. structure and function. The [H ] of body fluids must be
kept constant so that detrimental changes in enzyme func-
CONCEPT OF ACIDITY tion andcellularstructuredonotoccur.Therangeof[H ]
þ
compatible with life is 16 to 160 nEq/L.
The most commonly used concept of acids and bases is
that of Bro ¨nsted and Lowry, who stated that an acid is CONCEPT OF PH
a proton donor and a base a proton acceptor. In the

following equation, HA is an acid and A is a base: The concept of pH was introduced by S rensen to allow
þ
easier notation for the wide range of [H ] found in chem-
þ
HA ⇄ H þ A ical systems. The term pH is defined as the negative base
10 logarithm of the hydrogen ion concentration
In aqueous solutions, protons or H ions are normally expressed in equivalents per liter or the base 10 logarithm
þ
bound by electrostatic interaction to H 2 O, resulting in of the reciprocal of the hydrogen ion concentration:
the formation of hydronium ions, designated H 3 O .Con-
þ

ventionally, however, the term hydrogen ion and the sym- 1
þ
pH ¼ log ½H Š¼ log
bol H are used to refer to protons in aqueous solutions. 10 10 ½H Š
þ
þ
The acidity of a solution refers to the chemical activity
þ
of its constituent H ions. Chemical activity is related to Thus, at the normal extracellular fluid (ECF) [H ]of
þ
chemical concentration by the activity coefficient, a factor 40 nEq/L (4 10 8 Eq/L):
that varies directly with temperature and inversely with
the ionic strength of the solution. Physiologic control ½H Š¼ 4 10 8 Eq=L
þ
of body temperature and osmolality, and the dilute nature 8
10
of body fluids, result in this factor being near unity, and pH ¼ log ð4 10 Þ
the difference between activity and concentration is ¼ log 4 log 10 8
10
10
negligible in body fluids.
¼ 0:602Þ ð 8Þ
ð
The concentrations of the most important electrolytes
¼ 8 0:602
þ
in body fluids (e.g., Na ,K ,Cl ,HCO 3 ) are in the
þ

range of milliequivalents per liter, whereas the ¼ 7:398
231