832
UNIT 4
Maintenance of the Body
22
Once generated, HCO
3
2
moves quickly from the RBCs
into the plasma, where it is carried to the lungs. To counter-
balance the rapid outrush of these anions from the RBCs,
chloride ions (Cl
2
) move from the plasma into the RBCs.
±is ion exchange process, called the
chloride shif
, occurs
via facilitated diffusion through a RBC membrane protein.
In the lungs, the process is reversed (Figure 22.22b). As
blood moves through the pulmonary capillaries, its P
CO
2
de-
clines from 45 mm Hg to 40 mm Hg. For this to occur, CO
2
must first be freed from its “bicarbonate housing.” HCO
3
2
reenters the RBCs (and Cl
2
moves into the plasma) and
binds with H
1
to form carbonic acid. Carbonic anhydrase
then splits carbonic acid to release CO
2
and water. ±is
CO
2
, along with that released from hemoglobin and from
solution in plasma, then diffuses along its partial pressure
gradient from the blood into the alveoli.
The Haldane Effect
±e amount of carbon dioxide transported in blood is markedly
affected by the degree to which blood is oxygenated. ±e lower
the P
O
2
and the lower the Hb saturation with oxygen, the more
CO
2
that blood can carry. ±is phenomenon, called the
Hal-
dane effect
, reflects the greater ability of reduced hemoglobin
to form carbaminohemoglobin and to buffer H
1
by combining
with it. As CO
2
enters the systemic bloodstream, it causes more
oxygen to dissociate from Hb (Bohr effect). ±e dissociation of
O
2
allows more CO
2
to combine with Hb (Haldane effect).
±e Haldane effect encourages CO
2
exchange in both the tis-
sues and lungs. In the pulmonary circulation, the situation that
we just described is reversed—uptake of O
2
facilitates release of
CO
2
(Figure 22.22b). As Hb becomes saturated with O
2
, the H
1
released combines with HCO
3
2
, helping to unload CO
2
from
the pulmonary blood.
Influence of CO
2
on Blood pH
Typically, the H
1
released during carbonic acid dissociation is
buffered by Hb or other proteins within the RBCs or in plasma.
±e HCO
3
2
generated in the red blood cells diffuses into the
plasma, where it acts as the
alkaline reserve
part of the blood’s
carbonic acid–bicarbonate buffer system.
±e
carbonic acid–bicarbonate buffer system
is very im-
portant in resisting shi²s in blood pH, as shown in the equation
in point 3 concerning CO
2
transport (p. 829). For example, if
the hydrogen ion concentration in blood begins to rise, excess
H
1
is removed by combining with HCO
3
2
to form carbonic
acid (a weak acid). If H
1
concentration in blood drops below
desirable levels, carbonic acid dissociates, releasing hydrogen
ions and lowering the pH again.
Changes in respiratory rate or depth can alter blood pH
dramatically by altering the amount of carbonic acid in blood.
Slow, shallow breathing allows CO
2
to accumulate in blood. As
a result, carbonic acid levels increase and blood pH drops. Con-
versely, rapid, deep breathing quickly flushes CO
2
out of blood,
reducing carbonic acid levels and increasing blood pH.
In this way, respiratory ventilation provides a fast-acting sys-
tem to adjust blood pH (and P
CO
2
) when it is disturbed by meta-
bolic factors. Respiratory adjustments play a major role in the
acid-base balance of the blood, as we will discuss in Chapter 26.
Check Your Understanding
16.
Rapidly metabolizing tissues generate large amounts of CO
2
and H
1
. How does this affect O
2
unloading? What is this
effect called?
17.
List the three ways CO
2
is transported in blood and state
approximate percentages of each.
18.
What is the relationship between CO
2
and pH in the blood?
Explain.
For answers, see Appendix H.
20
40
60
80
100
0
Percent O
2
saturation of hemoglobin
20
40
60
80
100
20
40
60
80
0
Percent O
2
saturation of hemoglobin
P
O
2
(mm Hg)
Normal body
temperature
10
°
C
20
°
C
38
°
C
43
°
C
Normal arterial
carbon dioxide
(P
CO
2
40 mm Hg)
or H
+
(pH 7.4)
Increased carbon dioxide
(P
CO
2
80 mm Hg)
or H
+
(pH 7.2)
Decreased carbon dioxide
(P
CO
2
20 mm Hg) or H
+
(pH 7.6)
(a)
(b)
100
Figure 22.21
Effect of temperature,
P
CO
2
, and blood pH on
the oxygen-hemoglobin dissociation curve.
Oxygen unloading is
enhanced at conditions of
(a)
increased temperature,
(b)
increased
P
CO
2
, and/or hydrogen ion concentration (decreased pH), causing
the dissociation curve to shift to the right. This response is called the
Bohr effect.
previous page 866 Human Anatomy and Physiology (9th ed ) 2012 read online next page 868 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off