The Lungs and Kidneys Can Help
Compensate for Changes in pH
The simple act of breathing also plays a role in maintain-
ing the pH of body fluids. As we have seen, the
carbonic
acid-bicarbonate system is a dynamic mechanism for buff-
ering the pH of the blood and body fluids. Carbonic acid
(H2CO3) can be eliminated by exhaling CO2, and the con-
centration of CO2
can be rapidly regulated by breathing
(
F ig u r e 1 5 . 1 5
). Increasing the rate and depth of breath-
ing lowers the CO2
concentration and shifts the equilib-
rium to the left, thereby reducing the H+ concentration
and increasing blood pH. Conversely, decreasing the rate
and depth of breathing shifts the equilibrium to the right,
thereby increasing the H+ concentration and decreasing
blood pH. The rate of breathing is influenced by chemo-
receptors in the carotid artery, aortic arch, and medulla.
This respiratory mechanism is powerful and can respond
rapidly, within minutes, to changes in blood pH, but it can
only alter the carbonic acid concentration. It cannot fully
compensate for any dramatic changes in pH.
The kidneys can respond to changes in blood pH by per-
manently removing excess H+ or HCO3- from the body. The
kidneys can alter the reabsorption of H+ or HCO3-, thereby
shifting the equilibrium of the carbonic acid-bicarbonate
buffer system, removing H+ or HCO3- from the body and
altering blood pH. The renal mechanism can take hours or
days, and is therefore slower than the respiratory mecha-
nism—but the removal of the excess H+ is permanent.
Large Changes in pH May Result
in Acidosis or Alkalosis
Acid-base
imbalances
lead
to
changes in blood pH, including
a c id o s is
and
a lk a lo s is
. Acidosis
depresses the central nervous sys-
tem through depression of synap-
tic transmission. If the systemic
arterial blood pH falls below 7, the
individual
becomes
disoriented,
then becomes comatose, and may
die. Alkalosis causes overexcitability in both the central
nervous system and peripheral nerves. Neurons conduct
impulses repetitively, even when not stimulated; the re-
sults are nervousness, muscle spasms, and even convul-
sions and death.
There are many causes and two forms of acidosis and
alkalosis: respiratory forms, which result from an altera-
tion of CO2
levels, and metabolic forms, which are due to
changes in the levels of H+ or HCO3-. Your body compen-
sates for these imbalances in various ways.
R e g u l a t i o n o f t h e c a r b o n i c a c i d - b i c a r b o n a t e
b u f f e r s y s t e m b y t h e r e s p i r a t o r y a n d r e n a l
s y s t e m s
F i g u r e 1 5 . 1 5 ______________________________
The respiratory system can rapidly alter the bicarbonate buffer
system in the blood to compensate for changes in pH, but only
the kidneys are capable of permanent compensation by excret-
ing H+ or HCO3-.
Some stimulus disrupts
homeostasis by
Decreasing
Blood pH (increase
in H+ concentration)
a c id o s is
(as-i-DO-sis)
A condition in which
blood pH is below 7.35.
a lk a lo s is
(al-ka-
LO-sis) A condition
in which blood pH is
higher than 7.45.
460 CHAPTER 15
The Urinary System and Fluid, Electrolyte, and Acid-Base Balance
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