Biomedical Engineering Reference
In-Depth Information
smaller alveoli will pass into the larger ones until only one large alveolus remains. In
addition, collapsed alveoli, or those with extremely small radii, would require extremely
high opening pressures to inflate. Thus, freshly inspired air would be directed toward
large-radii alveoli because the pressure required to expand them is lowest.
This does not occur because, unlike water, the surface tension of the fluid film lining
alveoli is not constant but varies in proportion to the alveolar size or exposed area. In fact,
the fluid film lining alveoli contains surfactants capable of lowering surface tension far
below that of water (70 dynes/cm).
The ability of surfactants to lower surface tension is related to their chemical structure.
Because lung surfactants have hydrophobic and hydrophilic groups at different ends of the
molecule, they preferentially accumulate at the liquid-air interface. At the interface, the
hydrophilic end of the molecule extends into the liquid, and the hydrophobic end projects
into the air. Surfactant naturally accumulates at the liquid-air interface and reduces the
number of water molecules that would normally occupy it. Their presence disrupts the
attracting forces between water molecules so the surface tension is reduced.
During expiration, as surface area is decreased the alveoli deflate and the relative
concentration of surfactant molecules per unit area increases so that the surface tension
is reduced further. As alveoli inflate, water molecules must be brought to the interface,
so surface tension increases as fewer surfactant molecules are present per unit area. Even
though the Young-Laplace relationship is still applicable, alveolar surface tension is not
constant but decreases as alveolar radius decreases. As a result, pressure in small-radii
alveoli is lower than that in large-radii alveoli.
During inspiration, air initially moves into smaller-radii alveoli or from larger to
smaller alveoli to ensure uniform filling. In addition, the presence of surfactant reduces the
opening and expanding pressures of small-radii alveoli, which enhances alveolar stability
and reduces the work of breathing (Fenn and Rahn, 1965).
The elastic properties of the lung relate to both geometric weave of the elastin and
collagen fibers and to the surface tension of the fluid film lining the alveoli. While alveolar
surface tension is low in alveoli of small radii, it increases as alveoli enlarge. Therefore,
surface tension forces contribute to lung elasticity, especially as the lung inflates. Over
the normal tidal volume range, the weave of elastic fibers and surface tension contribute
about equally to lung elasticity.
9.3.3 Frictional Forces
When the respiratory muscles contract, in addition to the elastic recoil of the lung, they
must also overcome two types of friction: (1) drag as air passes through the airways; and
(2) viscous friction as the lung and chest wall and abdominal organs slide over one another.
During normal breathing, airway resistance accounts for 80% of the friction and viscous
friction for the remaining 20%.
Viscous friction occurs as the outer surfaces of the lung slide over the inner chest
wall and the various lung lobes move over one another during breathing. Even though
the adjacent pleura of the lung and chest wall are lubricated with intrapleural fluid, some
frictional resistance is present. In addition, as the diaphragm descends with inspiration, it
compresses and displaces abdominal contents, and frictional resistance is encountered as
abdominal organs are displaced and move over one another.
The magnitude of the frictional resistance encountered by air as it moves between
mouth and alveoli depends on the linear velocity of airflow as well as the airflow pattern and
