Human Physiology, 14th edition (2016)

562 Chapter 16

saturation for arterial and venous blood), you will see that a

shift to the right of the curve indicates a greater unloading of

oxygen. A shift to the left, conversely, indicates less unloading

but slightly more oxygen loading in the lungs.

When oxyhemoglobin dissociation curves are constructed

at different temperatures, the curve moves rightward as the

temperature increases. The rightward shift of the curve indi-

cates that the affinity of hemoglobin for oxygen is decreased

by a rise in temperature. An increase in temperature weakens

the bond between hemoglobin and oxygen and thus has the

same effect as a fall in pH. At higher temperatures, therefore,

more oxygen is unloaded to the tissues than would be the case

if the bond strength were constant. This effect can significantly

enhance the delivery of oxygen to muscles that are warmed

during exercise.

that changes in P^ O 2 within this range have little effect on the load-
ing reaction. One would have to ascend as high as 10,000 feet, for
example, before the oxyhemoglobin saturation of arterial blood
would decrease from 97% to 93%. At more common elevations,
the percent oxyhemoglobin saturation would not be significantly
different from the 97% value at sea level.
At the steep part of the sigmoidal curve, however, small
changes in P^ O 2 values produce large differences in percent satu-
ration. A decrease in venous P^ O 2 from 40 mmHg to 30 mmHg,
as might occur during mild exercise, corresponds to a change
in percent saturation from 75% to 58%. Since the arterial per-
cent saturation is usually still 97% during exercise, the lowered
venous percent saturation indicates that more oxygen has been
unloaded to the tissues. The difference between the arterial and
venous percent saturations indicates the percent unloading. In
the preceding example, 97%  2  75%  5  22% unloading at rest,
and 97%  2  58%  5  39% unloading during mild exercise. During
heavier exercise the venous P^ O 2 can drop to 20 mmHg or lower,
indicating a percent unloading of about 80%.

Effect of pH and Temperature

on Oxygen Transport

In addition to changes in P^ O 2 , the loading and unloading reac-
tions are influenced by changes in the affinity (bond strength) of
hemoglobin for oxygen. Such changes ensure that active skeletal
muscles will receive more oxygen from the blood than they do
at rest. This occurs as a result of the lowered pH and increased
temperature in exercising muscles.
The affinity is decreased when the pH is lowered and
increased when the pH is raised; this is called the Bohr effect.
When the affinity of hemoglobin for oxygen is reduced, there is
slightly less loading of the blood with oxygen in the lungs but
greater unloading of oxygen in the tissues. The net effect is that
the tissues receive more oxygen when the blood pH is lowered
( table 16.8 ). Since the pH can be decreased by carbon dioxide
(through the formation of carbonic acid), the Bohr effect helps
to provide more oxygen to the tissues when their carbon diox-
ide output is increased by a faster metabolism.
When you look at oxyhemoglobin dissociation curves
graphed at different pH values, you can see that the disso-
ciation curve is shifted to the right by a lowering of pH and
shifted to the left by a rise in pH ( fig. 16.34 ). If you calculate
percent unloading (by subtracting the percent oxyhemoglobin

Table 16.8 | Effect of pH on Hemoglobin Affinity for Oxygen and Unloading
of Oxygen to the Tissues

pH Affinity

Arterial O 2 Content

per 100 ml

Venous O 2 Content

per 100 ml

O 2 Unloaded to

Tissues per 100 ml

7.40 Normal 19.8 ml O 2 14.8 ml O 2 5.0 ml O 2

7.60 Increased 20.0 ml O 2 17.0 ml O 2 3.0 ml O 2

7.20 Decreased 19.2 ml O 2 12.6 ml O 2 6.6 ml O 2

Figure 16.34 The effect of pH on the

oxyhemoglobin dissociation curve. A decrease in blood

pH (an increase in H^1 concentration) decreases the affinity of

hemoglobin for oxygen at each PO 2 value, resulting in a “shift to

the right” of the oxyhemoglobin dissociation curve. This is called

the Bohr effect. A curve that is shifted to the right has a lower

percent oxyhemoglobin saturation at each PO 2.

See the Test Your Quantitative Ability section of the Review

Activities at the end of this chapter.

100

90

80

70

60

50

40

30

20

10

0

Percent oxyhemoglobin saturation

0 102030405060708090100110120130140

PO 2 (mmHg)

pH 7.60

7.40

7.20