764
UNIT FIVE
Carbon monoxide
(CO) is a toxic gas produced in gasoline engines and
some stoves as a result of incomplete combustion of fuels. It is also a
component of tobacco smoke. Carbon monoxide is toxic because it
binds hemoglobin many times more eF
ectively than does oxygen and
therefore does not readily dissociate from hemoglobin. Thus, when a
person breathes carbon monoxide, less hemoglobin is available for oxy-
gen transport, depriving cells of oxygen. Treatment for carbon monox-
ide poisoning is to administer oxygen in high concentration to replace
some of the carbon monoxide bound to hemoglobin molecules. Carbon
dioxide (CO
2
) is usually given simultaneously to stimulate the respiratory
center, which, in turn, increases breathing rate. Rapid breathing helps
reduce the concentration of carbon monoxide in the alveoli.
Oxygen
molecules
Alveolus
Diffusion
of oxygen
Alveolar
wall
Oxyhemoglobin
molecule
Diffusion
of oxygen
Hemoglobin
molecules
Capillary
Hemoglobin
molecules
Blood flow
(from body
tissues)
Blood flow
(to lungs)
Blood
P
O
2
= 95 mm Hg
Tissue
P
O
2
= 40 mm Hg
Blood
P
O
2
= 40 mm Hg
Tissue cells
(a)
(b)
10
20
30
40
50
60
70
80
90
100
% saturation of hemoglobin
10
04
0
5
0
6
0
7
0
9
0
80
100 110 120 130 140
20 30
P
O
2
(mm Hg)
Oxyhemoglobin dissociation at 38
°
C
20 mm Hg
40 mm Hg
80 mm Hg
P
CO
2
=
P
O
2
(mm Hg)
10
20
30
40
50
60
70
80
90
100
Oxyhemoglobin dissociation at 38
°
C
% saturation of hemoglobin
10
04
0
5
0
6
0
7
0
9
0
80
100 110 120 130 140
20
30
7.6
7.4
7.2
pH=
P
O
2
(mm Hg)
10
20
30
40
50
60
70
80
90
100
Oxyhemoglobin dissociation at various temperatures
% saturation of hemoglobin
10
04
0
5
0
6
0
7
0
9
0
80
100 110 120 130 140
20
30
43
°
C
38
°
C
30
°
C
20
°
C
10
°
C
0
°
C
FIGURE 19.37
Blood transports oxygen. (
a
) Oxygen molecules, entering the blood from the alveolus, bond to hemoglobin, forming
oxyhemoglobin. (
b
) In the regions of the body cells, oxyhemoglobin releases oxygen. Much oxygen is still bound to hemoglobin at the P
O
2
of
systemic venous blood.
FIGURE 19.38
The amount of oxygen released from oxyhemoglobin
increases as the P
CO
2
increases.
FIGURE 19.39
The amount of oxygen released from oxyhemoglobin
increases as the blood pH decreases.
FIGURE 19.40
The amount of oxygen released from oxyhemoglobin
increases as the blood temperature increases.
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