In all
multicellular organisms, the fluid within the cell and the fluids surrounding
the cells have a characteristic and nearly constant pH. This pH is maintained
in a number of ways, and one of the most important is through buffer systems.
Two important biological buffer systems are the dihydrogen phosphate system and
the carbonic acid system.
The condition called respiratory
acidosis occurs when blood pH falls as a result of decreased respiration.
When respiration is restricted, the concentration of dissolved carbon dioxide
in the blood increases, making the blood too acidic. Such a condition can be
produced by asthma, pneumonia, emphysema, or inhaling smoke.
Metabolic
acidosis is the decrease in blood pH that results when excessive amounts of
acidic substances are released into the blood. This can happen through
prolonged physical exertion, by diabetes, or restricted food intake. The normal
body response to this condition is increases breathing to reduce the amount of
dissolved carbon dioxide in the blood. This is why we breathe more heavily
after climbing several flights of stairs.
Respiratory
alkalosis results from excessive breathing that produces an increase in
blood pH. Hyperventilation causes too much dissolved carbon dioxide to be
removed from the blood, which decreases the carbonic acid concentration, which
raises the blood pH. Often, the body of a hyperventilating person will react by
fainting, which slows the breathing.
Metabolic
alkalosis is an increase in blood pH resulting from the release of alkaline
materials into the blood. This can result from the ingestion of alkaline
materials, and through overuse of diuretics. Again, the body usually responds
to this condition by slowing breathing, possibly through fainting.
The carbonic
acid-hydrogen carbonate ion buffer works throughout the body to maintain the pH
of blood plasma close to 7.40. The body maintains the buffer by eliminating
either the acid (carbonic acid) or the base (hydrogen carbonate ions). Changes
in carbonic acid concentration can be effected within seconds through increased
or decreased respiration. Changes in hydrogen carbonate ion concentration,
The phosphate buffer system operates in
the internal fluid of all cells. This buffer system consists of dihydrogen
phosphate ions (H2PO4-) as hydrogen-ion donor
(acid) and hydrogen phosphate ions (HPO42-) as
hydrogen-ion acceptor (base). These two ions are in equilibrium with each other
as indicated by the chemical equation below.
H2PO4-(aq)
H+(aq)
+ HPO42-(aq)
If additional hydrogen ions enter the
cellular fluid, they are consumed in the reaction with HPO42-,
and the equilibrium shifts to the left. If additional hydroxide ions enter the
cellular fluid, they react with H2PO4-,
producing HPO42-, and shifting the equilibrium to the
right.
The value of Ka for
this equilibrium is 6.23 × 10-8 at 25°C. From this equation, the
relationship between the hydrogen-ion concentration and the concentrations of
the acid and base can be derived.
Thus, when the concentrations of H2PO4-
and HPO42- are the same, the value of the molar
concentration of hydrogen ions is equal to the value of the equilibrium
constant, and the pH is equal to the pKa (-log Ka),
namely 7.21. Buffer solutions are most effective at maintaining a pH near the
value of the pKa. In mammals, cellular fluid has a pH in the
range 6.9 to 7.4, and the phosphate buffer is effective in maintaining this pH
range.
Another
biological fluid in which a buffer plays an important role in maintaining pH is
blood plasma. In blood plasma, the carbonic acid and hydrogen carbonate ion
equilibrium buffers the pH. In this buffer, carbonic acid (H2CO3)
is the hydrogen-ion donor (acid) and hydrogen carbonate ion (HCO3-)
is the hydrogen-ion acceptor (base).
H2CO3(aq)
H+(aq)
+ HCO3-(aq)
This buffer functions in exactly the
same way as the phosphate buffer. Additional H+ is consumed by HCO3-
and additional OH- is consumed by H2CO3. The
value of Ka for this equilibrium is 7.9 × 10-7,
and the pKa is 6.1 at body temperature. In blood plasma, the
concentration of hydrogen carbonate ion is about twenty times the concentration
of carbonic acid. The pH of arterial blood plasma is 7.40. If the pH falls
below this normal value, a condition called acidosis is produced. If the
pH rises above the normal value, the condition is called alkalosis.
The
concentrations of hydrogen carbonate ions and of carbonic acid are controlled
by two independent physiological systems. Carbonic acid concentration is
controlled by respiration, that is through the lungs. Carbonic acid is in
equilibrium with dissolved carbon dioxide gas.
H2CO3(aq)
CO2(aq)
+ H2O(l)
An enzyme called carbonic anhydrase
catalyzes the conversion of carbonic acid to dissolved carbon dioxide. In the
lungs, excess dissolved carbon dioxide is exhaled as carbon dioxide gas.
CO2(aq) CO2(g)
The concentration of hydrogen carbonate
ions is controlled through the kidneys. Excess hydrogen carbonate ions are
excreted in the urine.
The much
higher concentration of hydrogen carbonate ion over that of carbonic acid in
blood plasma allows the buffer to respond effectively to the most common
materials that are released into the blood. Normal metabolism releases mainly
acidic materials: carboxylic acids such as lactic acid (HLac). These acids
react with hydrogen carbonate ion and form carbonic acid.
HLac(aq) + HCO3-(aq)
Lac-(aq)
+ H2CO3(aq)
The carbonic acid is converted through
the action of the enzyme carbonic anhydrase into aqueous carbon dioxide.
H2CO3(aq)
CO2(aq)
+ H2O(l)
An increase in CO2(aq) concentration
stimulates increased breathing, and the excess carbon dioxide is released into
the air in the lungs.
