526
U N I T 6
Respiratory Function
Many of these volumes can be measured using an
instrument called a
spirometer
. The person is asked first
to breathe normally into the spirometer, during which
the tidal volume is measured, and then to inhale and
exhale maximally, during which the IRV and ERV are
measured. The RV cannot be measured with the spirom-
eter because this air cannot be expressed from the lungs.
It is measured by indirect methods, such as the helium
dilution methods, the nitrogen washout methods, or
body plethysmography.
Lung capacities include two or more lung volumes.
The
vital capacity
(VC) equals the IRV plus the TV and
the ERV and is the amount of air that can be exhaled
from the point of maximal inspiration. The
inspira-
tory capacity
(IC) equals the TV plus the IRV. It is the
amount of air a person can breathe in beginning at the
normal expiratory level. The
functional residual capac-
ity
(FRC) is the sum of the RV and ERV; it is the volume
of air that remains in the lungs at the end of normal
expiration. The
total lung capacity
(TLC) is the sum of
all the volumes in the lungs. Lung volumes and capaci-
ties are summarized in Table 21-1.
The previously described lung volumes and capacities
are anatomic and static measurements, determined by
spirometry and recorded without relation to time. The
spirometer also is used to measure dynamic lung volumes
(i.e., ventilation with respect to time); these tests often are
used in assessing pulmonary function (Table 21-2). The
maximum voluntary ventilation
measures the volume of
air that a person can move into and out of the lungs dur-
ing maximum effort lasting for a specific period of time.
This measurement usually is converted to liters per min-
ute. Two other useful tests are the forced vital capacity
and the forced expiratory volume. The
forced vital capac-
ity
(FVC) involves full inspiration to total lung capacity
followed by forceful maximal expiration. Obstruction
of airways produces an FVC that is lower than that
observed with more slowly performed vital capacity
measurements. The
forced expiratory volume
(FEV) is
the expiratory volume achieved in a given time period.
The FEV
1.0
is the forced expiratory volume that can be
exhaled in 1 second. The FEV
1.0
frequently is expressed
as a percentage of the FVC. The FEV
1.0
and FVC are used
in the diagnosis of obstructive lung disorders.
6000
5000
4000
3000
2000
1000
0
Inspiratory reserve
volume 3100 mL
Expiratory reserve
volume 1200 mL
Residual volume
1200 mL
Lung volumes
Lung capacities
Inspiratory
capacity
3600 mL
Total lung
capacity
6000 mL
Functional
residual capacity
2400 mL
Vital capacity 4800 mL
Tidal volume 500 mL
mL
FIGURE 21-15.
Spirometry recording of respiratory
volumes (left) and diagram of lung capacities
(right).The tidal volume (TV, yellow) is the volume
of air inhaled and exhaled during normal breathing;
the inspiratory reserve volume (IRV, pink), the
maximal volume of air that can be forcefully inhaled
in excess of theTV; the expiratory reserve volume
(ERV, blue), the maximal volume of air that can be
exhaled in excess of theTV; and the residual volume
(RV, green), the air that continues to remain in the
lung after maximal respiratory effort.The functional
residual capacity (FRC) is the sum of the ERV and
the RV.The vital capacity is the IRV,TV, and ERV.
TABLE 21-1
LungVolumes and Capacities
Volume
Symbol
Measurement
Tidal volume (about 500 mL at rest)
TV
Amount of air that moves into and out of the lungs with each breath
Inspiratory reserve volume
(about 3000 mL)
IRV
Maximum amount of air that can be inhaled from the point of
maximal expiration
Expiratory reserve volume
(about 1100 mL)
ERV
Maximum volume of air that can be exhaled from the resting end-
expiratory level
Residual volume (about 1200 mL)
RV
Volume of air remaining in the lungs after maximal expiration.This
volume cannot be measured with the spirometer; it is measured
indirectly using methods such as the helium dilution method, the
nitrogen washout technique, or body plethysmography.
Functional residual capacity
(about 2300 mL)
FRC
Volume of air remaining in the lungs at end-expiration (sum of RV and
ERV)
Inspiratory capacity (about 3500 mL)
IC
Sum of IRV andTV
Vital capacity (about 4600 mL)
VC
Maximal amount of air that can be forcibly exhaled from the point of
maximal inspiration
Total lung capacity (about 5800 mL)
TLC
Total amount of air that the lungs can hold; it is the sum of all the
volume components after maximal inspiration.This value is about
20% to 25% less in females than in males.
