828
UNIT 4
Maintenance of the Body
22
simple diffusion. Tey are driven by the partial pressure gra-
dients of O
2
and CO
2
that exist on the opposite sides of the ex-
change membranes.
Check Your Understanding
13.
You are given a sealed container of water and air. The
P
CO
2
and
P
O
2
in the air are both 100 mm Hg. What are the
P
CO
2
and
P
O
2
in the water? Which gas has more molecules
dissolved in the water? Why?
14.
P
O
2
in the alveoli is about 56 mm Hg lower than in the
inspired air. Explain this difference.
15.
Suppose a patient is receiving oxygen by mask. Are the
arterioles leading into the O
2
-enriched alveoli dilated or
constricted? What is the advantage of this response?
For answers, see Appendix H.
Transport of Respiratory
Gases by Blood
We have considered external and internal respiration consecu-
tively to emphasize their similarities, but keep in mind that it
is blood that transports O
2
and CO
2
between these two ex-
change sites.
Oxygen Transport
Describe how oxygen is transported in blood, and explain
how temperature, pH, BPG, and
P
CO
2
affect oxygen loading
and unloading.
Molecular oxygen is carried in blood in two ways: bound to he-
moglobin within red blood cells and dissolved in plasma. Oxy-
gen is poorly soluble in water, so only about 1.5% of the oxygen
transported is carried in the dissolved form. Indeed, if this were
the
only
means of oxygen transport, a P
O
2
of 3 atm or a cardiac
output of 15 times normal would be required to provide the
oxygen levels needed by body tissues! Hemoglobin, of course,
solves this problem—98.5% of the oxygen is carried from lungs
to tissues in a loose chemical combination with hemoglobin.
Association of Oxygen and Hemoglobin
As we described in Chapter 17, hemoglobin (Hb) is composed
of four polypeptide chains, each bound to an iron-containing
heme group (see Figure 17.4). Because the iron atoms bind oxy-
gen, each hemoglobin molecule can combine with four mol-
ecules of O
2
, and oxygen loading is rapid and reversible.
Te hemoglobin-oxygen combination, called
oxyhemo-
globin
(ok
0
sĭ-he
0
mo-glo
9
bin), is written
HbO
2
. Hemoglobin
that has released oxygen is called
reduced hemoglobin
, or
de-
oxyhemoglobin
, and is written
HHb
. A single reversible equa-
tion describes the loading and unloading of O
2
:
Lungs
HHb
1
O
2
m
HbO
2
1
H
1
Tissues
A±er the first O
2
molecule binds to iron, the Hb molecule
changes shape. As a result, it more readily takes up two more
O
2
molecules, and uptake of the fourth is even more facilitated.
When one, two, or three oxygen molecules are bound, a hemo-
globin molecule is
partially saturated
. When all four of its heme
groups are bound to O
2
, the hemoglobin is
fully saturated
.
By the same token, unloading of one oxygen molecule en-
hances the unloading of the next, and so on. In this way, the
affinity
(binding strength) of hemoglobin for oxygen changes
with the extent of oxygen saturation, and both loading and un-
loading of oxygen are very efficient.
Te rate at which Hb reversibly binds or releases O
2
is regu-
lated by P
O
2
, temperature, blood pH, P
CO
2
, and blood concen-
tration of an organic chemical called BPG. Tese factors interact
to ensure that adequate O
2
is delivered to tissue cells.
Influence of P
O
2
on Hemoglobin Saturation
Te
oxygen-
hemoglobin dissociation curve
shows how local P
O
2
controls
oxygen loading and unloading from hemoglobin.
Focus on the
Oxygen-Hemoglobin Dissociation Curve
(Figure 22.20)
walks
you through this graph step by step, explaining how hemoglobin
ensures adequate oxygen delivery under a variety of conditions.
Under normal resting conditions (P
O
2
5
100 mm Hg), arte-
rial blood hemoglobin is 98% saturated, and 100 ml of systemic
arterial blood contains about 20 ml of O
2
. Tis
oxygen content
of
arterial blood is written as 20 vol % (volume percent). As arte-
rial blood flows through systemic capillaries, it releases about
5 ml of O
2
per 100 ml of blood, yielding an Hb saturation of 75%
and an O
2
content of 15 vol % in venous blood. Tis means that
substantial amounts of O
2
are normally still available in venous
blood (the
venous reserve
), which can be used if needed.
Te nearly complete saturation of Hb in arterial blood ex-
plains why breathing deeply increases both the alveolar and
arterial blood P
O
2
but causes very little increase in the O
2
satu-
ration of hemoglobin. Remember, P
O
2
measurements indicate
only the amount of O
2
dissolved in plasma, not the amount
bound to hemoglobin. However, P
O
2
values are a good index
of lung function, and when arterial P
O
2
is significantly less than
alveolar P
O
2
some degree of respiratory impairment exists.
Influence of Other Factors on Hemoglobin Saturation
²em-
perature, blood pH, P
CO
2
, and the amount of BPG in the blood
all influence hemoglobin saturation at a given P
O
2
. Red blood
cells (RBCs) produce BPG (2,3-bisphosphoglycerate) as they
break down glucose by the anaerobic process called glycolysis.
BPG binds reversibly with hemoglobin, and its levels rise when
oxygen levels are chronically low.
All of these factors influence Hb saturation by modifying
hemoglobin’s three-dimensional structure, thereby changing its
affinity for O
2
. Generally speaking, an
increase
in temperature,
P
CO
2
, H
1
, or BPG levels in blood lowers Hb’s affinity for O
2
, en-
hancing oxygen unloading from the blood. Tis is shown by the
rightward shi± of the oxygen-hemoglobin dissociation curve in
Figure 22.21
on p. 832. (Te purple lines represent normal body
conditions, and the red lines show the shi± to the right.)
Conversely, a
decrease
in any of these factors increases hemo-
globin’s affinity for oxygen, decreasing oxygen unloading. Tis
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