Respiratory Physiology
467
This reaction is aided by the fact that deoxyhemoglobin, formed
as blood fl ows through the tissue capillaries, has a greater affi n-
ity for carbon dioxide than does oxyhemoglobin.
The remaining 60 percent of the carbon dioxide mol-
ecules entering the blood in the tissues is converted to bicar-
bonate ions:
carbonic
anhydrase
CO
2
+ H
2
O
34
H
2
CO
3
34
HCO
3
+ H
+
(13–11)
carbonic
bicarbonate
acid
The fi rst reaction in equation 13–11 is rate-limiting and is very
slow unless catalyzed by the enzyme
carbonic anhydrase.
This enzyme is present in the erythrocytes but not in the
plasma; therefore, this reaction occurs mainly in the erythro-
cytes. In contrast, carbonic acid dissociates very rapidly into a
bicarbonate ion and a hydrogen ion without any enzyme assis-
tance. Once formed, most of the bicarbonate moves out of the
erythrocytes into the plasma via a transporter that exchanges
one bicarbonate for one chloride ion (this is called the “chlo-
ride shift”).
The reactions shown in equation 13–11 also explain why,
as mentioned earlier, the H
+
concentration in tissue capillary
blood and systemic venous blood is higher than that in arterial
blood and increases as metabolic activity increases. The fate of
these hydrogen ions will be discussed in the next section.
conditions associated with inadequate oxygen supply to the tis-
sues and helps to maintain oxygen delivery. For example, the
increase in DPG is important during exposure to high altitude
when the
P
O
2
of the blood is decreased because DPG increases
the unloading of oxygen in the tissue capillaries.
Transport of Carbon Dioxide
in Blood
In a resting person, metabolism generates about 200 ml of
carbon dioxide per minute. When arterial blood fl ows through
tissue capillaries, this volume of carbon dioxide diffuses from
the tissues into the blood (
Figure 13–30a
). Carbon dioxide
is much more soluble in water than is oxygen, so blood carries
more dissolved carbon dioxide than dissolved oxygen. Even
so, only 10 percent of the carbon dioxide entering the blood
dissolves in the plasma and erythrocytes. In order to transport
all of the CO
2
produced in the tissues to the lung, CO
2
in the
blood must be carried in other forms.
Another 30 percent of the carbon dioxide molecules
entering the blood react reversibly with the amino groups of
hemoglobin to form
carbamino hemoglobin.
For simplicity,
this reaction with hemoglobin is written as:
CO
2
+ Hb
34
HbCO
2
(13–10)
(a)
(b)
P
l
a
s
m
a
Cells
Dissolved
CO
2
Dissolved
O
CO
CO
CO
C
2
d
ed
ed
ed
ed
ed
ved
ved
Dissolved
O
CO
CO
CO
CO
CO
CO
C
2
T
is
is
s
u
su
e
e c
a
p
ap
il
ill
a
r
ary
ry
Interstitial fluid
Tissue
C
O
2
C
O
2
CO
2
A
CA
H
2
O
CO
CO
3
H
C
O
3
C
l
C
l
Cl
Cl
C
+
H
2
O
+ Hb
b
Hb
H
O
CO
C
2
Begin
Capillary
wall
E
r
y
t
h
r
o
c
y
t
yte
P
l
a
s
m
a
E
r
y
t
yth
th
r
o
c
oc
y
t
e
d
ed
e
ve
ve
v
l
ol
o
s
s
i
Di
C
O
2
d
Dissolved
Dissolved
issolved
ssolved
ssolved
solved
olved
olved
C
O
2
s
us
us
u
lu
lu
ol
o
Alveo
Hb
H
2
O
CO
CO
3
A
CA
HCO
HCO
HCO
HCO
HCO
HCO
HCO
3
+
H
+
H
2
O
Hb
Hb
Hb
Hb
Hb
Hb
Hb
Hb
Hb
O
CO
CO
CO
CO
CO
CO
CO
CO
CO
C
2
C
l
C
l
Cl
Cl
Cl
C
L
u
n
g
c
a
p
i
l
la
r
y
Lung
Capillary
wall
CO
2
osphere
m
t
A
ired CO
p
xp
x
e
(e
i
2
)
H
C
O
3
HCO
HCO
HCO
HCO
HCO
HCO
HCO
HCO
HC
3
+
H
+
Figure 13–30
Summary of CO
2
movement. Expiration of CO
2
is by bulk fl ow, whereas all movements of CO
2
across membranes are by diffusion. Arrows
refl ect relative proportions of the fates of the CO
2
. About two-thirds of the CO
2
entering the blood in the tissues ultimately is converted to
HCO
3
in the erythrocytes because carbonic anhydrase (CA) is located there, but most of the HCO
3
then moves out of the erythrocytes
into the plasma in exchange for chloride ions (the “chloride shift”). See Figure 13–31 for the fate of the hydrogen ions generated in the
erythrocytes.
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