Respiratory Physiology
465
compartment B is still the same, the number of
dissolved
oxy-
gen molecules has decreased. Therefore, the
P
O
2
of compart-
ment B is less than that of A, and so there is a net diffusion
of oxygen from A to B. At the new equilibrium, the oxygen
pressures are once again equal, but almost all the oxygen is in
compartment B and has combined with hemoglobin.
Let us now apply this analysis to capillaries of the lungs
and tissues (
Figure 13–28
). The plasma and erythrocytes
entering the lungs have a
P
O
2
of 40 mmHg. As we can see from
Figure 13–26, hemoglobin saturation at this
P
O
2
is 75 percent.
The alveolar
P
O
2
—105 mmHg—is higher than the blood
P
O
2
and so oxygen diffuses from the alveoli into the plasma. This
increases plasma
P
O
2
and induces diffusion of oxygen into the
erythrocytes, elevating erythrocyte
P
O
2
and causing increased
combination of oxygen and hemoglobin. Most of the oxygen
diffusing into the blood from the alveoli does not remain dis-
solved but combines with hemoglobin. Therefore, the blood
P
O
2
normally remains less than the alveolar
P
O
2
until hemoglo-
bin is virtually 100 percent saturated. Thus, the diffusion gra-
dient favoring oxygen movement into the blood is maintained
despite the very large transfer of oxygen.
In the tissue capillaries, the procedure is reversed. Because
the mitochondria of the cells all over the body are utilizing
oxygen, the cellular
P
O
2
is less than the
P
O
2
of the surrounding
interstitial fl
uid. Therefore, oxygen is continuously diffusing
into the cells. This causes the interstitial fl
uid
P
O
2
to always
be less than the
P
O
2
of the blood fl owing through the tissue
capillaries, so net diffusion of oxygen occurs from the plasma
within the capillary into the interstitial fl uid. As a result,
plasma
P
O
2
becomes lower than erythrocyte
P
O
2
, and oxygen
diffuses out of the erythrocyte into the plasma. The lower-
ing of erythrocyte
P
O
2
causes the dissociation of oxygen from
hemoglobin, thereby liberating oxygen, which then diffuses
out of the erythrocyte. The net result is a transfer, purely by
diffusion, of large quantities of oxygen from hemoglobin to
plasma to interstitial fl uid to the mitochondria of tissue cells.
In most tissues under resting conditions, hemoglobin is
still 75 percent saturated as the blood leaves the tissue capillar-
ies. This fact underlies an important mechanism by which cells
can obtain more oxygen whenever they increase their activity.
For example, an exercising muscle consumes more oxygen,
thereby lowering its tissue
P
O
2
. This increases the blood-to-
tissue
P
O
2
gradient. As a result, the rate of oxygen diffusion
from blood to cells increases. In turn, the resulting reduction
in erythrocyte
P
O
2
causes additional dissociation of hemoglo-
bin and oxygen. In this manner, the extraction of oxygen from
blood in an exercising muscle is almost maximal and is much
greater than the usual 25 percent. In addition, an increased
blood fl ow to the muscles, called active hyperemia (Chapter 12),
also contributes greatly to the increased oxygen supply.
Effect of Carbon Monoxide on Oxygen Carriage
Carbon monoxide
is a colorless, odorless gas that is a prod-
uct of the incomplete combustion of hydrocarbons, such as
gasoline. It is a common cause of sickness and death due to
poisoning, both intentional and accidental. Its most striking
pathophysiological characteristic is its extremely high affi n-
ity—210 times that of oxygen—for the oxygen-binding sites
in hemoglobin. For this reason, it reduces the amount of oxy-
gen that combines with hemoglobin in pulmonary capillaries
by competing for these sites. It also exerts a second deleterious
P
l
a
s
m
a
d
ed
ed
ed
e
ve
ve
ve
ve
lv
lv
Dissolv
O
2
D
i
s
issol
v
e
d
O
2
(
<
2
%
O
2
d
i
s
s
o
l
v
e
d
in p
la
la
s
as
m
a
)
b
Hb
Hb
Hb
Hb
Hb
Hb
O
2
Alveolus
I
n
Inspired
O
2
O
2
P
u
lmonar
y
c
a
p
i
l
l
a
r
y
Begin
Capillary
wall
Lung
E
r
y
t
h
th
r
o
cyte
P
l
a
s
m
a
Cells
O
2
consumed in cell
mitochondria
Interstitial
fluid
d
ed
ed
ed
e
ve
ve
ve
ve
lv
lv
Dissolv
O
2
D
i
s
s
o
so
l
ved
O
2
Dissolved
O
2
Hb +
b
Hb
Hb
Hb
Hb
Hb
Hb
O
2
Capillary
wall
T
i
s
s
u
e
c
a
p
i
l
l
a
r
y
Tissue
E
r
y
ry
t
h
r
o
c
y
t
yt
e
Hb +
Figure 13–28
Oxygen movement in the lungs and tissues. Movement of inspired air into the alveoli is by bulk fl ow; all movements across membranes
are by diffusion.
previous page 493 Vander's Human Physiology The Mechanisms of Body Function read online next page 495 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off