The Respiratory System
closer together and reduces their contact with the dissimilar gas
molecules, and (2) resists any force that tends to increase the
surface area of the liquid.
Water is composed of highly polar molecules and has a very
high surface tension. As the major component of the liquid ﬁlm
that coats the alveolar walls, water is always acting to reduce the
alveoli to their smallest possible size, as we noted earlier.
If the ﬁlm were pure water, the alveoli would collapse be-
tween breaths. But the alveolar ﬁlm also contains
tant), a detergent-like complex of lipids and proteins
produced by the type II alveolar cells. Surfactant decreases the
cohesiveness of water molecules, much the way a laundry de-
tergent reduces the attraction of water for water, allowing water
to interact with and pass through fabric. As a result, the surface
tension of alveolar ﬂuid is reduced, and less energy is needed
to overcome those forces to expand the lungs and discourage
alveolar collapse. Breaths that are deeper than normal stimulate
type II cells to secrete more surfactant.
When too little surfactant is present, surface tension can col-
lapse the alveoli. Once this happens, the alveoli must be com-
pletely reinﬂated during each inspiration, an eﬀort that uses
tremendous amounts of energy. Tis is the problem faced by
infant respiratory distress syndrome (IRDS)
a condition peculiar to premature babies. Since fetal lungs do
not produce adequate amounts of surfactant until the last two
months of development, babies born prematurely oFen are un-
able to keep their alveoli inﬂated between breaths.
IRDS is treated by spraying natural or synthetic surfactant
into the newborn’s respiratory passageways. In addition, devices
that maintain positive airway pressure throughout the respira-
tory cycle can keep the alveoli open between breaths. Severe
cases require mechanical ventilators.
Many IRDS survivors suﬀer from
, a chronic lung disease, during childhood and beyond. Tis
condition is believed to result from inﬂammatory injury caused
by mechanically ventilating the premature newborn’s delicate
respiratory zone structures.
Healthy lungs are unbelievably stretchy, and this distensibility is
. Speciﬁcally, lung compliance (
) is a
measure of the change in lung volume (Δ
) that occurs with a
given change in transpulmonary pressure [Δ(
relationship is stated as
Te more a lung expands for a given rise in transpulmo-
nary pressure, the greater its compliance. Said another way, the
higher the lung compliance, the easier it is to expand the lungs
at any given transpulmonary pressure.
Lung compliance is determined largely by two factors: (1)
distensibility of the lung tissue and (2) alveolar surface tension.
Because lung distensibility is generally high and surfactant
keeps alveolar surface tension low, healthy lungs tend to have
high compliance, which favors eﬃcient ventilation.
Any decrease in the natural resilience of the lungs diminishes
lung compliance. Chronic inﬂammation, or infections such as
tuberculosis, can cause nonelastic scar tissue to replace normal
lung tissue (
). Decreased production of surfactant can
also impair lung compliance. Te lower the lung compliance,
the more energy is needed just to breathe.
Since the lungs are contained within the thoracic cavity, we
also need to consider the compliance (distensibility) of the tho-
racic wall. ±actors that reduce the compliance of the thoracic
wall hinder lung expansion. Te total compliance of the respira-
tory system is comprised of lung compliance and thoracic wall
Deformities of the thorax, ossiﬁed costal cartilages (common
during old age), and paralyzed intercostal muscles all hinder
thoracic expansion, reducing total respiratory compliance.
Respiratory Volumes and Pulmonary
Explain and compare the various lung volumes and
Deﬁne dead space.
Indicate types of information that can be gained from
pulmonary function tests.
Te amount of air ﬂushed in and out of the lungs depends on
the conditions of inspiration and expiration. Consequently, sev-
eral respiratory volumes can be described. Speciﬁc combinations
of these respiratory volumes, called
measured to gain information about a person’s respiratory status.
of interest are tidal, inspiratory
reserve, expiratory reserve, and residual. Te values recorded
(and used in the following text) represent
normal values for a healthy 20-year-old male weighing about
70 kg (155 lb). ±igure 22.16b provides average values for males
During normal quiet breathing, about 500 ml of air moves
into and then out of the lungs with each breath. Tis respiratory
volume is the
tidal volume (TV)
. Te amount of air that can be
inspired forcibly beyond the tidal volume (2100 to 3200 ml) is
inspiratory reserve volume (IRV)
expiratory reserve volume (ERV)
is the amount of air—
normally 1000 to 1200 ml—that can be expelled from the lungs
aFer a normal tidal volume expiration. Even aFer the most
strenuous expiration, about 1200 ml of air remains in the lungs;
this is the
residual volume (RV)
, which helps to keep the alveoli
patent (open) and prevent lung collapse.