822
UNIT FIVE
basic (alkaline). Buffer systems convert stronger acids into
weaker acids or convert stronger bases into weaker bases, as
table 21.3
summarizes.
In addition to minimizing pH fl
uctuations, acid-base buf-
fer systems in body fl
uids buffer each other. Consequently,
whenever the hydrogen ion concentration begins to change,
the chemical balances in all of the buffer systems change too,
resisting the drift in pH.
Neurons are sensitive to changes in the pH of body F
uids. If the inter-
stitial F
uid becomes more alkaline than normal (alkalosis), neurons
become more excitable, and seizures may result. Conversely, acidic
conditions (acidosis) depress neuron activity and level of conscious-
ness may decrease.
PRACTICE
22
What is the di±
erence between a strong acid or base and a weak
acid or base?
23
How does a chemical bu±
er system help regulate pH of body
F
uids?
24
List the major bu±
er systems of the body.
Chemical buffer systems only temporarily solve the
problem of acid-base balance. Ultimately, the body must
eliminate excess acid or base. The lungs (controlled by the
respiratory center) and the kidneys accomplish this task.
Respiratory Excretion of Carbon Dioxide
The
respiratory center
in the brainstem helps regulate
hydrogen ion concentrations in the body fl
uids by control-
ling the rate and depth of breathing. Specifically, if body
cells increase their production of carbon dioxide, as occurs
during periods of physical exercise, carbonic acid production
increases. As the carbonic acid dissociates, the concentra-
ions and become —NH
2
groups again. These hydrogen
ions then combine with hydroxyl ions to form water
molecules. Once again, pH change is minimized.
The “R”-groups of certain amino acids (histidine and
cysteine) can also function as buffers. Thus, protein
molecules can function as acids by releasing hydrogen
ions under alkaline conditions or as bases by accepting
hydrogen ions under acid conditions. This special
property allows protein molecules to operate as an acid-
base buffer system.
Hemoglobin is an especially important protein
that buffers hydrogen ions. As explained in chapter 19
(p. 766), carbon dioxide, produced by cellular oxidation
of glucose, diffuses through the capillary wall and
enters the plasma and then the red blood cells. The red
blood cells contain an enzyme,
carbonic anhydrase,
that
speeds the reaction between carbon dioxide and water,
producing carbonic acid:
CO
2
+ H
2
O
H
2
CO
3
The carbonic acid quickly dissociates, releasing
hydrogen ions and bicarbonate ions:
H
2
CO
3
H
+
+ HCO
3
In the peripheral tissues, where CO
2
is generated,
oxygen is used in the metabolism of glucose.
Hemoglobin gives up much of its oxygen and is in the
form of deoxyhemoglobin. In this form, hemoglobin
can bind the hydrogen ions generated in red blood cells,
thus acting as a buffer to minimize the pH change that
would otherwise occur.
The above two reactions can be written as a single
reversible reaction:
CO
2
+ H
2
O
H
2
CO
3
H
+
+ HCO
3
Thus, in the peripheral tissues, where CO
2
levels
are high, the reaction equilibrium shifts to the right,
generating H
+
, buffered by hemoglobin, and HCO
3
,
which becomes a plasma electrolyte. In the lungs, where
oxygen levels are high, hemoglobin is no longer a good
buffer, and it releases its H
+
. However, the released
H
+
combines with plasma HCO
3
, shifting the reaction
equilibrium to the left, generating carbonic acid, which
quickly dissociates to form CO
2
and water. The water
is added to the body fl
uids, and the CO
2
is exhaled.
Carbonic acid is sometimes called a
volatile acid
because
of this relationship to CO
2
(see F
gs. 19.41 and 19.43).
Individual amino acids in body fl
uids can also
function as acid-base buffers by accepting or releasing
hydrogen ions. This is possible because every amino
acid has an amino group (—NH
2
) and a carboxyl group
(—COOH).
To summarize, acid-base buffer systems take up hydro-
gen ions when body fl
uids are becoming more acidic and
give up hydrogen ions when the fl
uids are becoming more
TABLE
21.3
|
Chemical Acid-Base
Buf
er Systems
Buf
er System
Constituents
Actions
Bicarbonate
system
Bicarbonate ion
(HCO
3
)
Converts a strong acid into
a weak acid
Carbonic acid
(H
2
CO
3
)
Converts a strong base into
a weak base
Phosphate system
Monohydrogen
phosphate ion
(HPO
4
–2
)
Converts a strong acid into
a weak acid
Dihydrogen phosphate
(H
2
PO
4
)
Converts a strong base into
a weak base
Protein system
(and amino acids)
—NH
3
+
group of an
amino acid or protein
Releases a hydrogen ion in
the presence of excess base
—COO
group of an
amino acid or protein
Accepts a hydrogen ion in
the presence of excess acid
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