Page 244 - Fluid, Electrolyte, and Acid-Base Disorders in Small Animal Practice
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Introduction to Acid-Base Disorders 235
dissolved CO 2 , the concentration of which normally is CO 2diss þ H 2 O ⇄ H 2 CO 3 ⇄ H þ HCO
þ
3
kept constant by alveolar ventilation.
However, the number of carbonic acid molecules is
THE BICARBONATE– negligible compared with the number of dissolved CO 2
CARBONIC ACID SYSTEM: molecules and HCO 3 ions. Therefore, this equation
PHYSICAL CHEMISTRY can be simplified as:
Gaseous CO 2 produced in the tissues is soluble in water, CO 2diss þ H 2 O ⇄ H þ HCO 3
þ
and the concentration of dissolved CO 2 in body fluids is
proportional to the partial pressure of CO 2 in the gas The law of mass action for this equilibrium can be
phase (PCO 2 ): expressed as:
½ CO 2diss ¼ a PCO 2 Þ þ
ð
½H ½HCO 3
K a ¼
½CO 2diss ½H 2 O
where a is a factor called the solubility coefficient of
CO 2 . The solubility coefficient of CO 2 has a value of
The concentration of water in dilute body fluids remains
0.0301 mmol/L/mm in arterial plasma at 37 C. Thus:
virtually unchanged by this reaction and can be
0
incorporated into K a to yield another constant, K a :
½ CO 2diss ¼ 0:0301PCO 2
þ
½H ½HCO 3
0
Dissolved CO 2 combines with water to form carbonic K ¼
a
acid: ½CO 2diss
Solving for [H ] yields:
þ
CO 2diss þ H 2 O ! H 2 CO 3
0
K ½CO 2diss
The uncatalyzed reaction proceeds slowly, but its rate is ½ a
H ¼
dramatically increased by the enzyme carbonic anhydrase, ½HCO 3
which is present in abundance in the body (e.g., red cells,
0
renal tubular cells). In the body, therefore, the hydration In body fluids at 37 C, K a is approximately equal to
7
0
of CO 2 to form H 2 CO 3 reaches equilibrium almost 8 10 mol/L and p K a equals 6.1. An approximate
0
instantaneously. Normally, the equilibrium is so far to value of 6.1 for this p K a is valid at temperatures ranging
from 30 to 40 C (86 to 104 F) and pH values ranging
the left that there are approximately 340 molecules of
42 from 7.0 to 7.6. 37
dissolved CO 2 for each molecule of carbonic acid.
þ
The dissociation of carbonic acid can be expressed A formula for [H ] in nanomoles per liter or
0
using the law of mass action: nanoequivalents per liter is obtained by expressing K a
in nanomoles per liter or nanoequivalents per liter:
þ
½H ½HCO 3
K a ¼ 800½CO 2diss
þ
½H 2 CO 3 ½H ¼
½HCO 3
K a for this reaction is 2.72 10 4 mol/L (pK a ¼3.57).
The ratio of bicarbonate to carbonic acid at the normal Using the solubility coefficient for carbon dioxide yields:
þ
[H ] of body fluids can be calculated by rearranging this
ð
equation: þ 800 0:0301ÞPCO 2 24PCO 2
½H ¼ ¼
½HCO 3 ½HCO 3
½HCO 3 K a
¼ This is the Henderson equation and has been used exten-
þ
½H 2 CO 3 ½H
sivelyin the clinical evaluation of acid-base disturbances. It
4
þ
¼ 2:72 10 =4 10 8 shows clearly that the [H ] (and thus pH) of body fluids is
¼ 6:8 10 3 determinedby the ratio ofPCO 2 to HCO 3 concentration.
The Henderson-Hasselbalch equation is derived by
expressing [H ] and K a in moles per liter or equivalents
þ
0
þ
Thus, at [H ] ¼ 40 nmol/L (pH 7.40), there are 6800
per liter and converting the equation to logarithmic form:
bicarbonate ions and 340 molecules of dissolved CO 2
for each molecule of carbonic acid.
0
K ½CO 2diss
a
The reaction of dissolved CO 2 in aqueous body fluids ½H ¼
þ
can be summarized as: ½HCO 3 
