CHAPTER 36Gas Transport & pH in the Lung 623HYPOCAPNIA
Hypocapnia is the result of hyperventilation. During volun-
tary hyperventilation, the arterial PCO 2 falls from 40 to as low
as 15 mm Hg while the alveolar PO 2 rises to 120 to 140 mm Hg.
The more chronic effects of hypocapnia are seen in neurotic
patients who chronically hyperventilate. Cerebral blood flow
may be reduced 30% or more because of the direct constrictor
effect of hypocapnia on the cerebral vessels. The cerebral
ischemia causes light-headedness, dizziness, and paresthesias.
Hypocapnia also increases cardiac output. It has a direct con-
strictor effect on many peripheral vessels, but it depresses the
vasomotor center, so that the blood pressure is usually
unchanged or only slightly elevated.
Other consequences of hypocapnia are due to the associ-
ated respiratory alkalosis, the blood pH being increased to 7.5
or 7.6. The plasma HCO 3 – level is low, but HCO 3 – reabsorp-
tion is decreased because of the inhibition of renal acid secre-
tion by the low PCO 2. The plasma total calcium level does not
change, but the plasma Ca2+ level falls and hypocapnic indi-
viduals develop carpopedal spasm, a positive Chvostek sign,
and other signs of tetany.
CHAPTER SUMMARY
■ Partial pressure differences between air and blood for O 2 and
CO 2 dictate a net flow of O 2 into the blood and CO 2 out of the
blood in the pulmonary system. However, this flow is greatly en-
hanced by the ability for hemoglobin to bind O 2 and chemical re-
actions that increase CO 2 in the blood (eg, carbonic anhydrase).
■ The amount of O 2 in the blood is determined by the amount dis-
solved (minor) and the amount bound (major) to hemoglobin.
Each hemoglobin molecule contains four subunits that each can
bind O 2. Binding of the first O 2 to hemoglobin increases the affin-
ity for the second O2, and this pattern is continued until four O 2
are bound. Hemoglobin O 2 binding is also affected by pH, tem-
perature, and the concentration of 2,3-biphospholycerate
(2,3-BPG).
■ CO 2 in blood is rapidly converted into H 2 CO 3 due to the activity
of carbonic anhydrase. CO 2 also readily forms carbamino com-
pounds with blood proteins (including hemoglobin). The rapid
net loss of CO 2 allows more CO 2 to dissolve in blood.
■ The pH of plasma is 7.4. A decrease in plasma pH is termed
acidosis and an increase of plasma pH is termed alkalosis.
Acid and base shifts in the blood are controlled by proteins,
including hemoglobin, and principally by the carbonic acid-
bicarbonate buffering system. The carbonic acid-bicarbonate
buffering system is effective because dissolved CO 2 can be
controlled by respiration.
■ A short-term change in arterial PCO 2 due to decreased ventila-
tion results in respiratory acidosis. A short-term change in arte-
rial PCO 2 due to increased ventilation results in respiratory
alkalosis. Metabolic acidosis occurs when strong acids are added
to the blood, and metabolic alkalosis occurs when strong bases
are added to (or strong acids are removed from) the blood.
■ Respiratory compensation to acidosis or alkalosis involves quick
changes in ventilation. Such changes effectively change the PCO 2in the blood plasma. Renal compensation mechanisms are much
slower and involve H+ secretion or HCO 3 – reabsorption.
■ Hypoxia is a deficiency of O 2 at the tissue level. Hypoxia has
powerful consequences at the cellular, tissue, and organ level: It
can alter cellular transcription factors and thus protein expres-
sion; it can quickly alter brain function and produce symptoms
similar to alcohol (eg, dizziness, impaired mental function,
drowsiness, headache); and it can affect ventilation. Long-term
hypoxia results in cell and tissue death.MULTIPLE-CHOICE QUESTIONS
For all questions, select the single best answer unless otherwise directed.- Most of the CO 2 transported in the blood is
A) dissolved in plasma.
B) in carbamino compounds formed from plasma proteins.
C) in carbamino compounds formed from hemoglobin.
D) bound to Cl–.
E) in HCO 3 –. - Which of the following has the greatest effect on the ability of
blood to transport oxygen?
A) capacity of the blood to dissolve oxygen
B) amount of hemoglobin in the blood
C) pH of plasma
D) CO 2 content of red blood cells
E) temperature of the blood - Which of the following is not true of the system?
CO 2 + H 2 O ←→^1 H 2 CO 3 ←→^2 H+ + HCO 3 –
A) Reaction 1 is catalyzed by carbonic anhydrase.
B) Because of reaction 2, the pH of blood declines during
breath holding.
C) Reaction 1 occurs in the kidneys.
D) Reaction 1 occurs primarily in plasma.
E) The reactions move to the left when there is excess H+ in the
tissues. - Uncompensated respiratory acidosis differs from uncompen-
sated metabolic acidosis in that
A) plasma pH change is always greater in uncompensated res-
piratory acidosis compared to uncompensated metabolic
acidosis.
B) there are no compensation mechanisms for respiratory
acidosis, whereas there is respiratory compensation for
metabolic acidosis.
C) uncompensated respiratory acidosis involves changes in
plasma [HCO 3 – ], whereas plasma [HCO 3 – ] is unchanged in
uncompensated metabolic acidosis.
D) uncompensated respiratory acidosis is associated with a
change in PCO2, whereas uncompensated metabolic acidosis
occurs along the isobar line for PCO 2. - O 2 delivery to the tissues would be reduced to the greatest
extent in
A) a normal subject breathing 100% O 2 on top of Mt. Everest.
B) a normal subject running a marathon at sea level.
C) a patient with carbon monoxide poisoning.
D) a patient who has ingested cyanide.
E) a patient with moderately severe metabolic acidosis.