20
40
60
80
100
0
20
40
60
80
100
0
Percent O
2
saturation of hemoglobin
P
O
2
(mm Hg)
20
40
60
80
100
0
20
40
60
80
100
0
Percent O
2
saturation of hemoglobin
P
O
2
(mm Hg)
In the lungs
In the tissues
40%
75%
98%
95%
At sea level, there is lots of O
2
.
At a P
O
2
in the lungs of 100 mm Hg,
Hb is 98% saturated.
At high altitude, there is less O
2
.
At a P
O
2
in the lungs of only 80
mm Hg, Hb is still 95% saturated.
In resting tissues, at a P
O
2
of 40 mm Hg,
Hb is 75% saturated—only 23% of O
2
carried by Hb is released.
In metabolically active tissues (e.g.,
exercising muscle), the P
O
2
is even lower.
At a P
O
2
of 20 mm Hg, Hb is only 40%
saturated—an additional 35% of O
2
has
been unloaded for tissue use.
At high P
O
2
, large changes in P
O
2
cause only
small changes in Hb saturation
. Notice that the
curve is relatively flat here. Hb’s properties
produce a
safety margin
that ensures that Hb is
almost fully saturated even with a substantial P
O
2
decrease. As a result, Hb remains saturated even
at high altitude or with lung disease.
At low P
O
2
, large changes in P
O
2
cause large
changes in Hb saturation.
Tissues other than
lungs have a low P
O
2
because they consume O
2
.
Notice that the curve is relatively steep at low P
O
2
.
Hb’s properties ensure that oxygen is delivered
where it is most needed—when tissues need more,
they get more.
831
previous page 865 Human Anatomy and Physiology (9th ed ) 2012 read online next page 867 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off