766
UNIT FIVE
The resulting carbonic acid dissociates almost immediately,
releasing hydrogen ions (H
+
) and bicarbonate ions (HCO
3
):
H
2
CO
3
H
+
+ HCO
3
These new hydrogen ions might be expected to lower blood
pH, but this reaction occurs in the systemic capillaries,
where deoxyhemoglobin is generated. Deoxyhemoglobin is
an excellent buffer because hydrogen ions readily bind it.
The bicarbonate ions diffuse out of the red blood cells and
enter the blood plasma. As much as 70% of the carbon diox-
ide transported in the blood is carried in this form.
As the bicarbonate ions leave the red blood cells and
enter the plasma,
chloride ions,
which also have negative
charges, are electrically repelled, and they move from the
plasma into the red blood cells. This exchange in position
of the two negatively charged ions, shown in
f gure 19.42
,
maintains the ionic balance between the red blood cells and
the plasma. It is termed the
chloride shift.
As blood passes through the capillaries of the lungs, the
dissolved carbon dioxide diffuses into the alveoli, in response
to the relatively low P
CO
2
of the alveolar air. As the plasma
P
CO
2
drops, hydrogen ions and bicarbonate ions in the red
blood cells recombine to form carbonic acid, and under the
infl uence of carbonic anhydrase, the carbonic acid quickly
yields new molecules of CO
2
and water:
H
+
+ HCO
3
H
2
CO
3
CO
2
+ H
2
O
Carbaminohemoglobin also releases its CO
2
, and both of
these events contribute to the P
CO
2
of the alveolar capillary
blood. CO
2
diffuses out of the blood until an equilibrium is
established between the P
CO
2
of the blood and the P
CO
2
of the
Carbon Dioxide Transport
Blood fl owing through capillaries gains CO
2
because the tis-
sues have a high P
CO
2
. This CO
2
is transported to the lungs in
one of three forms: as CO
2
dissolved in plasma, as part of a
compound formed by bonding to hemoglobin, or as part of a
bicarbonate ion
(f g. 19.41)
.
The amount of carbon dioxide that dissolves in plasma
is determined by its partial pressure. The higher the P
CO
2
of
the tissues, the more carbon dioxide will go into solution.
However, only about 7% of the carbon dioxide is transported
in this form.
Unlike oxygen, which binds the iron atoms of hemoglo-
bin molecules, carbon dioxide bonds with the amino groups
(—NH
2
) of these molecules. Consequently, oxygen and carbon
dioxide do not directly compete for binding sites—a hemoglo-
bin molecule can transport both gases at the same time.
Carbon dioxide binding hemoglobin forms a loosely
bound compound called
carbaminohemoglobin
(kar-
bam
ı˘-no-he
mo-globin). This molecule readily decomposes
in regions where the P
CO
2
is low, releasing its carbon diox-
ide. Although this method of transporting carbon dioxide is
theoretically quite effective, carbaminohemoglobin forms
relatively slowly. Only about 15% to 25% of the total CO
2
is
carried this way.
In the most important CO
2
transport mechanism
bicar-
bonate ions
(HCO
3
) form. Recall that carbon dioxide reacts
with water to form carbonic acid (H
2
CO
3
). This reaction
occurs slowly in the blood plasma, but much of the CO
2
dif-
fuses into the red blood cells. These cells contain an enzyme,
carbonic anhydrase
(kar-bon
ik an-hi
dra
¯s), which speeds
the reaction between CO
2
and water.
Tissue cell
Tissue
P
CO
2
= 45 mm Hg
Cellular CO
2
CO
2
dissolved
in plasma
P
CO
2
= 40 mm Hg
CO
2
combined with
hemoglobin to form
carbaminohemoglobin
Blood
flow from
systemic
arteriole
Plasma
CO
2
H
2
O
H
2
CO
3
+
HCO
3
HCO
3
H
+
H
+
combines
with hemoglobin
+
Red blood cell
Capillary wall
Blood
flow to
systemic
venule
P
CO
2
= 45 mm Hg
FIGURE 19.41
Carbon dioxide produced by cells is transported in the blood plasma in a dissolved state, bound to hemoglobin, or in the form of
bicarbonate ions (HCO
3
).
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