Chapter 22
The Respiratory System
829
22
CO poisoning are given hyperbaric therapy (if available) or
100% O
2
until the CO has been cleared from the body.
Carbon Dioxide Transport
Describe carbon dioxide transport in the blood.
Normally active body cells produce about 200 ml of CO
2
each
minute—exactly the amount excreted by the lungs. Blood
transports CO
2
from the tissue cells to the lungs in three forms
(
Figure 22.22
on p. 833):
1.
Dissolved in plasma
(7–10%). Te smallest amount of CO
2
is transported simply dissolved in plasma.
2.
Chemically bound to hemoglobin
(just over 20%). Dis-
solved CO
2
is bound and carried in the RBCs as
carbami-
nohemoglobin
(kar-bam
0
ĭ-no-he
0
mo-glo
9
bin):
CO
2
1
Hb
m
HbCO
2
carbaminohemoglobin
Tis reaction is rapid and does not require a catalyst. Car-
bon dioxide transport in RBCs does not compete with
oxyhemoglobin transport because carbon dioxide binds
directly to the amino acids of globin (not to the heme).
CO
2
loading and unloading are directly influenced by
the P
CO
2
and the degree of Hb oxygenation. Carbon di-
oxide rapidly dissociates from hemoglobin in the lungs,
where the P
CO
2
of alveolar air is lower than that in blood.
Carbon dioxide readily binds with hemoglobin in the tis-
sues, where the P
CO
2
is higher than that in blood. Deoxy-
genated hemoglobin combines more readily with carbon
dioxide than does oxygenated hemoglobin, as we will see
in the discussion of the Haldane effect below.
3.
As bicarbonate ions in plasma
(about 70%). Most car-
bon dioxide molecules entering the plasma quickly enter
RBCs, where the reactions that prepare carbon dioxide for
transport as
bicarbonate ions (HCO
3
2
)
in plasma mostly
occur. As illustrated in Figure 22.22a, when dissolved CO
2
diffuses into RBCs, it combines with water, forming car-
bonic acid (H
2
CO
3
). H
2
CO
3
is unstable and dissociates
into hydrogen ions and bicarbonate ions:
CO
2
1
H
2
O
m
H
2
CO
3
m
H
1
1
HCO
3
carbon
water
carbonic
hydrogen
bicarbonate
dioxide
acid
ion
ion
Although this reaction also occurs in plasma, it is
thousands of times faster in RBCs because they (and
not plasma) contain
carbonic anhydrase
(kar-bon
9
ik
an-hi
9
drās), an enzyme that reversibly catalyzes the con-
version of carbon dioxide and water to carbonic acid. Hy-
drogen ions released during the reaction (as well as CO
2
itself) bind to Hb, triggering the Bohr effect. In this way
CO
2
loading enhances O
2
release. Because of the buffering
effect of Hb, the liberated H
1
causes little change in pH
under resting conditions. As a result, blood becomes only
slightly more acidic (the pH declines from 7.4 to 7.34) as it
passes through the tissues.
change shi±s the dissociation curve to the le± (as the blue lines
show in Figure 22.21).
If you give a little thought to how these factors are related,
you’ll realize that they all tend to be highest in the systemic cap-
illaries, where oxygen unloading is the goal. As cells metabolize
glucose and use O
2
, they release CO
2
, which increases the P
CO
2
and H
1
levels in capillary blood. Both declining blood pH (aci-
dosis) and increasing P
CO
2
weaken the Hb-O
2
bond, a phenom-
enon called the
Bohr effect
. Tis enhances oxygen unloading
where it is most needed.
Additionally, heat is a by-product of metabolic activity, and
active tissues are usually warmer than less active ones. A rise in
temperature affects hemoglobin’s affinity for O
2
both directly
and indirectly (via its influence on RBC metabolism and BPG
synthesis). Collectively, these factors see to it that Hb unloads
much more O
2
in the vicinity of hard-working tissue cells.
Homeostatic Imbalance
22.14
Inadequate oxygen delivery to body tissues is called
hypoxia
(hi-pok
9
se-ah). Hypoxia is more visible in fair-skinned people
because their skin and mucosae take on a bluish cast (become
cyanotic
) when Hb saturation falls below 75%. In dark-skinned
individuals, this color change can be observed only in the mu-
cosae and nail beds.
Hypoxia is classified based on cause:
Anemic hypoxia
reflects poor O
2
delivery resulting from
too few RBCs or from RBCs that contain abnormal or too
little Hb.
Ischemic (stagnant) hypoxia
results from impaired or
blocked blood circulation. Congestive heart failure may
cause bodywide ischemic hypoxia, whereas emboli or
thrombi block oxygen delivery only to tissues distal to the
obstruction.
Histotoxic hypoxia
occurs when body cells are unable to use
O
2
even though adequate amounts are delivered. Metabolic
poisons, such as cyanide, can cause histotoxic hypoxia.
Hypoxemic hypoxia
is indicated by reduced arterial P
O
2
.
Possible causes include disordered or abnormal ventilation-
perfusion coupling, pulmonary diseases that impair ventila-
tion, and breathing air containing scant amounts of O
2
.
Carbon monoxide poisoning
is a unique type of hy-
poxemic hypoxia, and a leading cause of death from fire.
Carbon monoxide (CO) is an odorless, colorless gas that
competes vigorously with O
2
for heme binding sites. More-
over, because Hb’s affinity for CO is more than 200 times
greater than its affinity for oxygen, CO is a highly success-
ful competitor. Even at minuscule partial pressures, carbon
monoxide can displace oxygen.
CO poisoning is particularly dangerous because it does
not produce the characteristic signs of hypoxia—cyanosis
and respiratory distress. Instead the victim is confused and
has a throbbing headache. In rare cases, fair skin becomes
cherry red (the color of the Hb-CO complex), which the eye
of the beholder interprets as a healthy “blush.” Patients with
(Text continues on p. 832.)
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