Acid Base Physiology
Under normal conditions, body fluids are maintained at a pH between 7.35
and 7.45; excursions out of this range constitute acidosis
(low pH) and alkalosis (high pH). The
main buffer present in the body is bicarbonate. Bicarbonate exists
in equilibrium with carbonic acid which in turn can be converted to carbon
dioxide and water.
| H2O + CO2 |
 |
H2CO3 |
|
H+ + HCO3- |
Other, weaker buffers exist in equilibrium with the bicarbonate system,
so if you know where you are with bicarbonate, you have a good handle on
the acid-base state of the body. Bicarbonate is such a good buffer
(i.e., it holds the pH steady despite addition or subtraction of protons) for
several reasons. A buffer is strongest near its pKa. The pKa
of bicarbonate is 6.1 which is closer to 7.4 than most other major bases
in the body. Also, at the left side of the bicarbonate system is carbon
dioxide, a gas which can be dumped via the lungs. Thus, the body
can fine tune its acid-base status by affecting either bicarbonate levels
(via the kidneys) or carbon dioxide (via the lungs). When something
goes wrong with either system, it is reflected in an acid-base abnormality.
The relative amounts of carbonic acid and bicarbonate ion determine
the pH according to the Henderson-Hasselbach equation; if any two terms
are known, the third can be calculated. This equation addresses the
right-half of the above equilibrium:
| H2CO3 |
|
H+ + HCO3- |
The right half of the equation is the dissociation of carbonic acid
into bicarbonate and a proton. The rate of dissociation, Ka, is expressed
as the ratio of products (protons and bicarbonate) over reactants (carbonic
acid):
| Ka = [H+]*[HCO3-]/[H2CO3] |
|
The Henderson-Hasselbach Equation |
| [H+] = Ka*[HCO3-]/[H2CO3] |
|
Rearrangement |
| 1/log[H] = 1/logKa - log ([HCO3-]/[H2CO3]) |
|
Take the log of each term |
| pH = pKa + [H2CO3] / [HCO3-] |
|
pH is the negative log of [H+] |
| pH = 6.1 + [H2CO3] / [HCO3-] |
|
The known pKa of carbonic acid is 6.1 |
| pH = 6.1 + [CO2]*0.03/[HCO3-] |
|
CO2 used as a surrogate for H2CO3 |
The last statement is a bit tricky. Since carbonic acid is itself
in equilibrium with carbon dioxide, it is possible to substitute carbon
dioxide into the equation. This is of great utility because carbon
dioxide and bicarbonate are both readily measurable in the blood.
Carbon dioxide is quantitated on blood gases, and bicarbonate is routinely
measured in standard chemistry or electrolyte panels.
Normal HCO3- Levels
| Adults |
23-25 mEq/Liter |
| Infants |
21.5-23.5 mEq/Liter |
In maintenance of acid-base homeostasis, the carbon dioxide level is
regulated by alveolar ventilation. To lower the pCO2 (partial
pressure of CO2 in the blood), carbon dioxide is "blown off"
by increased alveolar ventilation; conversely, to increase the pCO2,
carbon dioxide is retained. Balance is also regulated by altering
the amount of HCO3- in the blood. The kidney
can regulate the amount of bicarbonate that it retains or excretes.
The rate of production of HCO3- also varies according
to a number of factors. Either protons or HCO3-
can be lost in bodily secretions, also altering pH balance. For instance,
vomit often contains stomach acid, while diarrhea contains HCO3-.
Finally, addition of exogenous alkali or acids can affect this system.
When the primary problem is either increased or decreased alvelolar
ventilation, the condition is designated "respiratory" alkalosis or acidosis,
respectively. When the problem is primarily related to bicarbonate
excess or deficit, the condition is designated "metabolic" alkalosis or
acidosis, respectively. If there is both a respiratory and metabolic
component leading to altered pH homeostatis, the condition is referred
to as a "mixed" acidosis or alkalosis.
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Last modification: April 30, 1998
