752
UNIT FIVE
inspiration. On the average, the same volume leaves during
a normal, resting expiration. Thus, the
resting tidal volume
is about 500 mL
(f
g. 19.26)
.
During forced maximal inspiration, a volume of air in
addition to the resting tidal volume enters the lungs. This
additional volume is called the
inspiratory reserve volume
(complemental air), and it equals about 3,000 mL.
During a maximal forced expiration, about 1,100 mL of
air in addition to the resting tidal volume can be expelled
from the lungs. This volume is called the
expiratory reserve
volume
(supplemental air). However, even after the most
forceful expiration, about 1,200 mL of air remains in the
lungs. This is the
residual volume.
Residual air remains in the lungs at all times, and conse-
quently, newly inhaled air always mixes with air already in the
lungs. This prevents the oxygen and carbon dioxide concentra-
tions in the lungs from fl uctuating greatly with each breath.
Once the respiratory volumes are known, four
respira-
tory capacities
can be calculated by combining two or more of
A person can exhale more air than normal by contracting
the posterior
internal
(
expiratory
)
intercostal muscles
. These
muscles pull the ribs and sternum downward and inward,
increasing the pressure in the lungs. Also, the
abdominal
wall muscles,
including the external and internal obliques,
the transversus abdominis, and the rectus abdominis,
squeeze the abdominal organs inward. Thus, the abdominal
wall muscles can increase pressure in the abdominal cavity
and force the diaphragm still higher against the lungs, push-
ing additional air out of the lungs
(f
g. 19.25)
.
Table 19.3
summarizes the steps in expiration.
PRACTICE
20
Describe the events in inspiration.
21
How does surface tension aid in expanding the lungs during
inspiration?
22
What forces are responsible for normal expiration?
Respiratory Air Volumes and Capacities
Different degrees of effort in breathing move different vol-
umes of air in or out of the lungs. The measurement of such
air volumes is called
spirometry,
and it describes four dis-
tinct
respiratory volumes.
One inspiration plus the following expiration is called a
respiratory cycle.
The volume of air that enters or leaves
during a respiratory cycle is termed the
tidal volume.
About
500 milliliters (mL) of air enter during a normal, resting
Sternocleidomastoid
elevates sternum
Pectoralis minor
elevates ribs
Diaphragm
contracts more
Sternum
moves
up and out
Diaphragm
contracts
External
intercostal
muscles pull
ribs up and out
(a)
(b)
FIGURE 19.24
Maximal inspiration. (
a
) Shape of the thorax at the end of normal inspiration. (
b
) Shape of the thorax at the end of maximal
inspiration, aided by contraction of the sternocleidomastoid and pectoralis minor muscles.
TABLE
19.3
|
Major Events in Expiration
1. The diaphragm and external respiratory muscles relax.
2. Elastic tissues of the lungs and thoracic cage, stretched during
inspiration, suddenly recoil, and surface tension collapses alveolar walls.
3. Tissues recoiling around the lungs increase the intra-alveolar
pressure.
4. Air is squeezed out of the lungs.
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