Respiratory Physiology
463
Transport of Oxygen in Blood
Table 13–8
summarizes the oxygen content of systemic arte-
rial blood, referred to simply as arterial blood. Each liter nor-
mally contains the number of oxygen molecules equivalent to
200 ml of pure gaseous oxygen at atmospheric pressure. The
oxygen is present in two forms: (1) dissolved in the plasma and
erythrocyte water and (2) reversibly combined with hemoglo-
bin molecules in the erythrocytes.
As predicted by Henry’s law, the amount of oxygen
dissolved in blood is directly proportional to the
P
O
2
of the
blood. Because the solubility of oxygen in water is relatively
low, only 3 ml can be dissolved in 1 L of blood at the normal
arterial
P
O
2
of 100 mmHg. The other 197 ml of oxygen in
a liter of arterial blood, more than 98 percent of the oxygen
content in the liter, is transported in the erythrocytes, revers-
ibly combined with hemoglobin.
Each
hemoglobin
molecule is a protein made up of four
subunits bound together. Each subunit consists of a molecu-
lar group known as
heme
and a polypeptide attached to the
heme. The four polypeptides of a hemoglobin molecule are
collectively called
globin.
Each of the four heme groups in
a hemoglobin molecule (
Figure 13–25
) contains one atom
of iron (Fe
2+
), to which oxygen binds. Because each iron
atom can bind one molecule of oxygen, a single hemoglobin
molecule can bind four oxygen molecules (see Figure 2–21).
However, for simplicity, the equation for the reaction between
oxygen and hemoglobin is usually written in terms of a single
polypeptide-heme chain of a hemoglobin molecule:
O
2
+ Hb
34
HbO
2
(13–8)
Thus, this chain can exist in one of two forms—
deoxyhemo-
globin (Hb)
and
oxyhemoglobin (HbO
2
).
In a blood sam-
ple containing many hemoglobin molecules, the fraction of all
the hemoglobin in the form of oxyhemoglobin is expressed as
the
percent hemoglobin saturation:
Percent Hb saturation =
O
2
bound to Hb
Maximal capacity of Hb to bind O
2
×
100
(13–9)
For example, if the amount of oxygen bound to hemo-
globin is 40 percent of the maximal capacity, the sample is said
to be 40 percent saturated. The denominator in this equation
is also termed the
oxygen-carrying capacity
of the blood.
What factors determine the percent hemoglobin satura-
tion? By far the most important is the blood
P
O
2
. Before turn-
ing to this subject, however, it must be stressed that the
total
amount
of oxygen carried by hemoglobin in the blood depends
not only on the percent saturation of hemoglobin but also on
how much hemoglobin is in each liter of blood. A signifi cant
decrease in hemoglobin in the blood is called
anemia
.
For
example, if a person’s blood contained only half as much hemo-
globin per liter as normal, then at any given percent saturation,
the oxygen content of the blood would be only half as much.
What Is the Effect of
P
O
2
on Hemoglobin Saturation?
Based on equation 13–8 and the law of mass action, it is evident
that increasing the blood
P
O
2
should increase the combination
of oxygen with hemoglobin. The experimentally determined
quantitative relationship between these variables is shown in
Figure 13–26
, which is called an
oxygen-hemoglobin dis-
sociation curve.
(The term
dissociate
means “to separate,” in
this case, oxygen from hemoglobin; it could just as well have
been called an oxygen-hemoglobin association curve.) The
curve is sigmoid because, as stated earlier, each hemoglobin
molecule contains four subunits. Each subunit can combine
with one molecule of oxygen, and the reactions of the four
subunits occur sequentially, with each combination facilitat-
ing the next one.
This combination of oxygen with hemoglobin is an exam-
ple of cooperativity, as described in Chapter 3. The explanation
in this case is as follows. The globin units of deoxyhemoglobin
Table 13–8
Oxygen Content of Systemic Arterial
Blood at Sea Level
1 liter (L) arterial blood contains
3 ml
O
2
physically dissolved (1.5%)
197 ml
O
2
bound to hemoglobin (98.5%)
Total
200 ml
O
2
Cardiac output = 5 L/min
O
2
carried to tissues/min = 5 L/min
×
200 ml O
2
/L
= 1000 ml O
2
/min
Globin
polypeptide
CH
3
CH
CH
2
CH
CH
2
CH
2
CH
2
CH
2
CH
2
COOH
COOH
CH
3
CH
3
C
C
CC
N
Fe
2
+
N
N
N
NO
2
CH
3
Figure 13–25
Heme. Oxygen binds to the iron atom (Fe
2+
). Heme attaches to
a polypeptide chain by a nitrogen atom to form one subunit of
hemoglobin. Four of these subunits bind to each other to make a
single hemoglobin molecule. See Figure 2–21 for the structure of
hemoglobin.
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