464
Chapter 13
are tightly held by electrostatic bonds in a conformation with
a relatively low affi nity for oxygen. The binding of oxygen to a
heme molecule breaks some of these bonds between the glo-
bin units, leading to a conformation change that leaves the
remaining oxygen-binding sites more exposed. Thus, the bind-
ing of one oxygen molecule to deoxyhemoglobin increases the
affi nity of the remaining sites on the same hemoglobin mol-
ecule, and so on.
The shape of the oxygen-hemoglobin dissociation curve
is extremely important in understanding oxygen exchange.
The curve has a steep slope between 10 and 60 mmHg
P
O
2
and a relatively fl at portion (or plateau) between 70 and 100
mmHg
P
O
2
. Thus, the extent to which oxygen combines with
hemoglobin increases very rapidly as the
P
O
2
increases from 10
to 60 mmHg, so that at a
P
O
2
of 60 mmHg, 90 percent of the
total hemoglobin is combined with oxygen. From this point
on, a further increase in
P
O
2
produces only a small increase in
oxygen binding.
This plateau at higher
P
O
2
values has a number of impor-
tant implications. In many situations, including at high altitude
and with pulmonary disease, a moderate reduction occurs in
alveolar and therefore arterial
P
O
2
. Even if the
P
O
2
decreased
from the normal value of 100 to 60 mmHg, the total quantity
of oxygen carried by hemoglobin would decrease by only 10 per-
cent because hemoglobin saturation is still close to 90 percent
at a
P
O
2
of 60 mmHg. The plateau provides an excellent safety
factor so that even a signifi
cant limitation of lung function still
allows almost normal oxygen saturation of hemoglobin.
The plateau also explains why, in a healthy person at sea
level, increasing the alveolar (and therefore the arterial)
P
O
2
either by hyperventilating or by breathing 100 percent oxygen
adds very little additional oxygen to the blood. A small addi-
tional amount dissolves, but because hemoglobin is already
almost completely saturated with oxygen at the normal arte-
rial
P
O
2
of 100 mmHg, it simply cannot pick up any more oxy-
gen when the
P
O
2
is elevated beyond this point. This applies
only to healthy people at sea level. If a person initially has a
low arterial
P
O
2
because of lung disease or high altitude, then
there would be a great deal of deoxyhemoglobin initially pres-
ent in the arterial blood. Therefore, raising the alveolar and
thereby the arterial
P
O
2
would result in signifi cantly more oxy-
gen transport.
The steep portion of the curve from 60 down to 20
mmHg is ideal for unloading oxygen in the tissues. That is,
for a small decrease in
P
O
2
due to diffusion of oxygen from the
blood to the cells, a large quantity of oxygen can be unloaded
in the peripheral tissue capillary. Also, small shifts in the
position of the curve due to various factors can signifi cantly
increase oxygen unloading as we will see for hydrogen ions in
Figure 13–29.
We now retrace our steps and reconsider the movement
of oxygen across the various membranes, this time includ-
ing hemoglobin in our analysis. It is essential to recognize
that the oxygen bound to hemoglobin does
not
contribute
directly to the
P
O
2
of the blood; only dissolved oxygen does so.
Therefore, oxygen diffusion is governed only by the dissolved
portion, a fact that permitted us to ignore hemoglobin in dis-
cussing transmembrane partial pressure gradients. However,
the presence of hemoglobin plays a critical role in determining
the
total amount
of oxygen that will diffuse, as illustrated by a
simple example (
Figure 13–27
).
Two solutions separated by a semipermeable membrane
contain equal quantities of oxygen. The gas pressures in both
solutions are equal, and no net diffusion of oxygen occurs.
Addition of hemoglobin to compartment B disturbs this equi-
librium because much of the oxygen combines with hemo-
globin. Despite the fact that the total
quantity
of oxygen in
20
40
60
80
100
20
0
40
60
80
100
Hemoglobin saturation (%)
Systemic
venous
P
O
2
P
O
2
(mmHg)
Systemic
arterial
P
O
2
140
120
Amount of O
2
unloaded in
tissue
capillaries
Figure 13–26
Oxygen-hemoglobin dissociation curve. This curve applies to blood at
37°C and a normal arterial hydrogen ion concentration. At any given
blood hemoglobin concentration, the
y
-axis could also have plotted
oxygen content, in milliliters of oxygen. At 100 percent saturation,
the amount of hemoglobin in normal blood carries 200 ml of oxygen.
AB
AB
New
equilibrium
Add Hb to
right side
Hb
Pure H
2
O
with O
2
O
2
AB
P
O
2
=
P
O
2
P
O
2
>
P
O
2
P
O
2
=
P
O
2
Figure 13–27
Effect of added hemoglobin on oxygen distribution between two
compartments containing a fi xed number of oxygen molecules and
separated by a semipermeable membrane. At the new equilibrium,
the
P
O
2
values are again equal to each other but lower than before
the hemoglobin was added. However, the total oxygen, in other
words, the oxygen dissolved plus that combined with hemoglobin, is
now much higher on the right side of the membrane.
Adapted from Comroe.
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