C h a p t e r 2 1
Control of Respiratory Function
527
Efficiency and Work of Breathing
The efficiency of breathing is determined by matching
the TV and respiratory rate in a manner that provides
an optimal minute volume while minimizing the work
of breathing. The
minute volume,
or total ventilation,
is the amount of air that is exchanged in 1 minute (TV
multiplied by the respiratory rate). It is determined by
the metabolic needs of the body, which during normal
activity are about 6000 mL (500 mL TV × respiratory
rate of 12 breaths/min).
The work of breathing is determined by the amount of
effort required to move air through the conducting air-
ways and by the ease of lung expansion. Because expan-
sion of the lungs is difficult for persons with stiff and
noncompliant lungs, they usually find it easier to breathe
if they keep their TV low and breathe at a more rapid
rate (e.g., 300 mL × 20 breaths/min = 6000 mL/min)
to achieve their minute volume and meet their oxygen
needs. In contrast, persons with obstructive airway dis-
ease usually find it less difficult to inflate their lungs but
expend more energy in moving air through the airways.
As a result, these persons tend to take deeper breaths
and breathe at a slower rate (e.g., 600 mL × 10 breaths/
min = 6000 mL) to achieve their oxygen needs.
TABLE 21-2
Pulmonary FunctionTests
Test
Symbol
Measurement*
Maximal voluntary ventilation MVV
Maximum amount of air that can be breathed in a given time
Forced vital capacity
FVC
Maximum amount of air that can be rapidly and forcefully exhaled from the
lungs after full inspiration.The expired volume is plotted against time.
Forced expiratory volume
achieved in 1 s
FEV
1.0
Volume of air expired in the first second of FVC
Percentage of forced vital
capacity
(FEV
1.0
/FVC%)
× 100
Volume of air expired in the first second, expressed as a percentage of FVC
Forced midexpiratory flow
rate
FEF
25%–75%
The forced midexpiratory flow rate determined by locating the points on
the volume-time curve recording obtained during FVC corresponding to
25% and 75% of FVC and drawing a straight line through these points.
The slope of this line represents the average midexpiratory flow rate.
Forced inspiratory flow rate
FIF
25%–75%
FIF is the volume inspired from RV at the point of measurement. FIF
25%–75%
is the slope of a line between the points on the volume pressure tracing
corresponding to 25% and 75% of the inspired volume.
*By convention, all the lung volumes and rates of flow are expressed in terms of body temperature and
pressure and saturated with water vapor (BTPS), which allows for a comparison of the pulmonary function
data from laboratories with different ambient temperatures and altitudes.
SUMMARY CONCEPTS
■■
Breathing involves the movement of atmospheric
air into and out of the alveolar structures in the
lungs. It requires a system of open airways and
alternating pressure changes resulting from the
action of the respiratory muscles in changing the
volume of the chest cage.
■■
Lung compliance or ease with which the lungs
can be inflated reflects the elastic forces of
the lung tissue and the surface tension in the
alveoli. Surfactant molecules, produced by type
II alveolar cells, reduce the surface tension in the
lungs, thereby increasing lung compliance and
ease of inflation.
■■
Airway resistance refers to the impediment
to flow that the air encounters as it moves
through the airways. It varies with airway
radius and lung volume, being greatest in the
bronchi with medium-sized radii and lowest in
the bronchioles with their smaller radii. Airway
resistance decreases as the lungs expand
and pull the airways open during inspiration
and it increases as the lungs deflate during
expiration.
■■
Lung volumes reflect the amount of air that is
exchanged during normal and forced breathing.
The minute volume (tidal volume [TV] multiplied
by the respiratory rate) is determined by the
metabolic needs of the body.
■■
The efficiency of breathing is determined by
matching theTV and respiratory rate in a manner
that provides an optimal minute volume while
minimizing the work of breathing. Persons with
stiff and noncompliant lungs usually find it
easier to keep theirTV low and breathe at a more
rapid rate, whereas those with increased airway
resistance usually find it less difficult to inflate
their lungs and increase theirTV, while breathing
at a slower rate.
