Respiratory Physiology
455
vo
lume
is
termed
the
expiratory reserve volume (ERV)
and is about 1200 ml. Even after a maximal active expiration,
approximately 1200 ml of air still remains in the lungs; this is
termed the
residual volume (RV).
Thus, the lungs are never
completely emptied of air.
The
vital capacity (VC)
is the maximal volume of air
a person can expire after a maximal inspiration. Under these
conditions, the person is expiring both the resting tidal vol-
ume and inspiratory reserve volume just inspired, plus the
expiratory reserve volume (see Figure 13–18). In other words,
the vital capacity is the sum of these three volumes.
A variant on this method is the
forced expiratory volume
in 1 s,
(
FEV
1
),
in which the person takes a maximal inspiration
and then exhales maximally as fast as possible. The important
value is the fraction of the total “forced” vital capacity expired
in 1 s. Normal individuals can expire approximately 80 per-
cent of the vital capacity in one second.
Measurement of vital capacity and FEV are useful diag-
nostically and are known as
pulmonary function tests
.
For
example, people with
obstructive lung diseases
(increased
airway resistance) typically have an FEV
1
that is less than 80
percent of the vital capacity because it is diffi
cult for them
to expire air rapidly through the narrowed airways. In con-
trast to obstructive lung diseases,
restrictive lung diseases
are characterized by normal airway resistance but impaired
respiratory movements because of abnormalities in the lung
tissue, the pleura, the chest wall, or the neuromuscular
machinery. Restrictive lung diseases are thus characterized
by a reduced vital capacity, but a normal ratio of FEV
1
to
vital capacity.
Tidal volume (TV)
Inspiratory reserve volume (IRV)
Expiratory reserve volume (ERV)
Residual volume (RV)
Respiratory Volumes and Capacities for an Average Young Adult Male
Respiratory Volumes
Respiratory Capacities
Measurement
500 ml
3000 ml
1200 ml
1200 ml
Typical Value
Amount of air inhaled or exhaled in one breath during relaxed, quiet breathing
Amount of air in excess of tidal inspiration that can be inhaled with maximum effort
Amount of air in excess of tidal expiration that can be exhaled with maximum effort
Amount of air remaining in the lungs after maximum expiration; keeps alveoli inflated
between breaths and mixes with fresh air on next inspiration
Vital capacity (VC)
4700 ml
Amount of air that can be exhaled with maximum effort after maximum inspiration
(ERV + TV + IRV); used to assess strength of thoracic muscles as well as
pulmonary function
Maximum amount of air that can be inhaled after a normal tidal expiration (TV + IRV)
Amount of air remaining in the lungs after a normal tidal expiration (RV + ERV)
Maximum amount of air the lungs can contain (RV + VC)
Definition
Inspiratory reserve
volume
Maximum possible inspiration
Tidal volume
Maximum
voluntary
expiration
Expiratory reserve volume
Residual volume
Functional residual capacity
Total lung capacity
Inspiratory
capacity
Vital
capacity
Expiration
Lung volume (ml)
Inspiration
1000
2000
3000
4000
5000
6000
0
3500 ml
2400 ml
5900 ml
Inspiratory capacity (IC)
Functional residual capacity (FRC)
Total lung capacity (TLC)
1
3
4
8
8
3
2
2
4
5
5
7
7
6
6
1
Figure 13–18
Lung volumes and capacities recorded on a spirometer, an apparatus for measuring inspired and expired volumes. When the subject inspires,
the pen moves up; with expiration, it moves down. The capacities are the sums of two or more lung volumes. The lung volumes are the four
distinct components of total lung capacity. Note that residual volume, total lung capacity, and functional residual capacity cannot be measured
with a spirometer.
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