By altering the rate and depth of breathing, the lungs can alter the
CO
2
concentration, thereby shifting the equilibrium of the carbonic
acid-bicarbonate buffer system and altering blood pH. This
respiratory mechanism is powerful and can respond rapidly (within
minutes) but can only alter the carbonic acid concentration.
T
CM
o
o
+
H2O
-------
h
2
c o
3
-------
H+
+
HCO3-
Carbon
dioxide
Water
Carbonic acid
Hydrogen
ion
Bicarbonate
ion
M
The kidneys can alter the reabsorption or secretion of H+ or HCO3- ,
thereby shifting the equilibrium of the carbonic acid-bicarbonate buffer
system, removing H+ or HCO3- from or adding it back into the body, and
altering blood pH. The renal mechanism is slower (within hours or days)
than the respiratory mechanism, but the changes are permanent.
Kidneys excrete
hydrogen ions (H+)
into the urine and
send bicarbonate
ions (HCO3-) back
to the blood
back to normal
1
Blood pH decreases,
Return to
homeostasis
Blood pH increases,
blood too acidic (acidosis)
when response
brings blood pH
blood too basic (alkalosis)
Return to
homeostasis
when response
brings blood pH
back to normal
Kidneys return
hydrogen ions (H+)
to the blood and
excrete bicarbonate
ions (HCO3-) in
the urine
Patients with unregulated diabetes mellitus often de-
velop a pH imbalance because they lack insulin that
allows the cells to utilize glucose. As a result, the
body is forced to use other materials, many of which
are acids, as energy sources. This causes the H+
level to increase in the blood. Which of the following
would occur as the body tries to regulate blood pH?
a. Kidneys increase H+
reabsorption.
b. Breathing rate decreases.
c. pH decreases.
d. Kidneys increase HCO3-
secretion.
e. Urinary system increases
excretion of H+.
y
The Kidneys Help Maintain the Acid-Base Balance of Body Fluids
461
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