746
UNIT FIVE
During gas exchange, oxygen diffuses through the alveo-
lar walls and enters the blood in nearby capillaries. Carbon
dioxide diffuses from the blood through these walls and
enters the alveoli
(f gs. 19.17
and
19.18)
.
Several techniques enable a person who has stopped breathing
to survive. In
artif
cial respiration,
a person blows into the mouth of
a person who has stopped breathing. The oxygen in the rescuer’s
exhaled breath can keep the victim alive.
In
extracorporeal membrane oxygenation,
blood is pumped out of
the body and across a gas-permeable membrane that adds oxygen
and removes CO
2
, simulating lung function. Such a device can keep a
person alive until he or she recovers from other problems, but is too
costly and cumbersome to maintain life indeF
nitely.
A lung assist device, called an
intravascular oxygenator,
consists
of hundreds of tiny porous hair-thin F
bers surgically implanted in the
inferior vena cava. Here, deoxygenated blood returning to the heart
receives oxygen and is rid of CO
2
—but only at about 30% the capac-
ity of a healthy respiratory system.
PRACTICE
11
What is the function of the cartilaginous rings in the tracheal wall?
12
How do the right and left bronchi di±
er in structure?
13
List the branches of the bronchial tree.
14
Describe structural changes in the respiratory tubes as their
diameters decrease.
15
How are gases exchanged in the alveoli?
Blood flow
Blood flow
Arteriole
Alveolus
Capillary
Venule
Alveolar
wall
Air
O
2
CO
2
O
2
CO
2
Blood vessel
Capillary
Alveolus
Bronchiole
Alveolus
FIGURE 19.16
²Oxygen²(O
2
) di±
uses from the air in the alveolus into
the capillary, while carbon dioxide (CO
2
) di±
uses from blood in the
capillary into the alveolus.
FIGURE 19.17
³alsely colored scanning electron micrograph of
casts of alveoli and associated capillary networks. These casts were
prepared by F
lling the alveoli and blood vessels with resin and
later removing the soft tissues by digestion, leaving only the resin
casts (420×).
Tissues and Organs: A Text-Atlas oF Scanning Electron
Microscopy,
by Richard D. Kessel and Randy Kardon.© 1979 W. H.
³reeman and Company.
FIGURE 19.18
³alsely colored scanning electron micrograph of
lung alveoli and a bronchiole (70×).
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