468
Chapter 13
Because carbon dioxide undergoes these various fates in
blood, it is customary to add up the amounts of dissolved car-
bon dioxide, bicarbonate, and carbon dioxide in carbamino
hemoglobin to arrive at the
total blood carbon dioxide,
which is measured as a component of routine blood chemistry
testing.
Just the opposite events occur as systemic venous blood
fl ows through the lung capillaries (
Figure 13–30b
). Because
the blood
P
CO
2
is higher than alveolar
P
CO
2
, a net diffusion of
CO
2
from blood into alveoli occurs. This loss of CO
2
from
the blood lowers the blood
P
CO
2
and drives reactions 13–10
and 13–11 to the left. HCO
3
and H
+
combine to produce
H
2
CO
3
, which then dissociates to CO
2
and H
2
O. Similarly,
HbCO
2
generates Hb and free CO
2
. Normally, as fast as CO
2
is generated from HCO
3
and H
+
and from HbCO
2
, it dif-
fuses into the alveoli. In this manner, all the CO
2
delivered
into the blood in the tissues is now delivered into the alveoli,
from where it is eliminated during expiration.
Transport of Hydrogen Ions
Between Tissues and Lungs
As blood fl ows through the tissues, a fraction of oxyhemo-
globin loses its oxygen to become deoxyhemoglobin, while
simultaneously a large quantity of carbon dioxide enters the
blood and undergoes the reactions that generate bicarbonate
and hydrogen ions. What happens to these hydrogen ions?
Deoxyhemoglobin has a much greater affi nity for H
+
than does oxyhemoglobin, so it binds (buffers) most of the
hydrogen ions (
Figure 13–31
). Indeed, deoxyhemoglobin can
be abbreviated HbH rather than Hb to denote its binding of
H
+
. In effect, the reaction is HbO
2
+ H
+
34
HbH + O
2
. In
this manner, only a small number of the hydrogen ions gener-
ated in the blood remain free. This explains why venous blood
(pH = 7.36) is only slightly more acidic than arterial blood
(pH = 7.40).
As the venous blood passes through the lungs, this reac-
tion is reversed. Deoxyhemoglobin becomes converted to oxy-
hemoglobin and, in the process, releases the hydrogen ions it
picked up in the tissues. The hydrogen ions react with bicar-
bonate to produce carbonic acid, which, under the infl uence
of carbonic anhydrase, dissociates to form carbon dioxide
and water. The carbon dioxide diffuses into the alveoli to be
expired. Normally all the hydrogen ions that are generated in
the tissue capillaries from the reaction of carbon dioxide and
water recombine with bicarbonate to form carbon dioxide and
water in the pulmonary capillaries. Therefore, none of these
hydrogen ions appear in the
arterial
blood.
What happens when a person is hypoventilating or has a
lung disease that prevents normal elimination of carbon diox-
ide? Not only would arterial
P
CO
2
increase as a result, but so
would arterial H
+
concentration. Increased arterial H
+
con-
centration due to carbon dioxide retention is termed
respira-
tory acidosis
.
Conversely, hyperventilation would lower the
arterial values of both
P
CO
2
and H
+
concentration, producing
respiratory alkalosis
.
The factors that infl uence the binding of CO
2
and O
2
by
hemoglobin are summarized in
Table 13–9
.
Plasma
Erythrocyte
Cell
Tissue capillary
Interstitial fluid
CO
2
CO
2
CO
2
+
H
2
O
H
2
CO
3
HbO
2
HCO
3
H
+
O
2
O
2
O
2
Hb
HbH
Figure 13–31
Binding of hydrogen ions by hemoglobin as blood fl ows through
tissue capillaries. This reaction is facilitated because deoxyhemoglobin,
formed as oxygen dissociates from hemoglobin, has a greater affi nity
for hydrogen ions than does oxyhemoglobin. For this reason, Hb
and HbH are both abbreviations for deoxyhemoglobin.
Table 13–9
Effects of Various Factors on
Hemoglobin
The affi nity of hemoglobin for oxygen is decreased by:
1. Increased hydrogen ion concentration
2. Increased
P
CO
2
3. Increased temperature
4. Increased DPG concentration
The affi nity of hemoglobin for both hydrogen ions and carbon
dioxide is decreased by increased
P
O
2
; that is, deoxyhemoglobin
has a greater affi nity for hydrogen ions and carbon dioxide
than does oxyhemoglobin.
Another aspect of the remarkable hemoglobin molecule
is its ability to bind and transport
nitric oxide.
A present
hypothesis is that as blood passes through the lungs, hemo-
globin picks up and binds not only oxygen but nitric oxide
that is synthesized there, carries it to the peripheral tissues,
and releases it along with oxygen. Simultaneously, via a differ-
ent binding site, hemoglobin picks up and catabolizes nitric
oxide produced in the peripheral tissues. Theoretically this
cycle could play an important role in determining the periph-
eral concentration of nitric oxide and, thereby, the overall
effect of this vasodilator agent. For example, by supplying net
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