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U N I T 6
Respiratory Function
of gestation; consequently, many premature infants have
difficulty producing sufficient amounts of surfactant.
This can lead to alveolar collapse and severe respiratory
distress. This condition, called
infant respiratory distress
syndrome
, is the single most common cause of respira-
tory disease in premature infants. Recombinant forms of
surfactant are now available for use in treatment of the
disorder (see Chapter 22). The synthesis of surfactant
can also be impaired in the adult. This usually occurs as
the result of severe injury or infection and can contrib-
ute to the development of a condition called the
acute
respiratory distress syndrome
(see Chapter 24).
Airway Resistance
During breathing, the flow or volume of air that moves
into and out of the lungs is directly related to the pres-
sure difference between the lungs and the atmosphere
and inversely related to the resistance that the air
encounters as it moves through the conducting airways.
Airway radius.
The primary determinant of airway
resistance to airflow is the radius of the conducting
airway. Normally, the radius is large enough so that a
gradient of less than 1 cm/H
2
O pressure is needed for
sufficient airflow during quiet breathing. The site of
most of the resistance occurs in the larger bronchioles
and bronchi near the trachea, with the smallest air-
ways contributing very little to the total airway resis-
tance. The low resistance of the smaller bronchioles can
be explained in terms of their large number and their
parallel arrangements. Many airway diseases, such as
emphysema and chronic bronchitis, begin in the small
airways. Early detection of these diseases is often dif-
ficult because a considerable amount of damage must
be present before the usual diagnostic measurements of
airway resistance can detect them.
Lung Volume.
Airway resistance is also affected by
lung volume, being less during inspiration than during
expiration. This is because elastic-type fibers connect
the outside of the airways to the surrounding lung tis-
sues. As a result, these airways are pulled open as the
lungs expand during inspiration, and they become nar-
rower as the lungs deflate during expiration (Fig. 21-13).
This is one of the reasons persons with conditions that
increase airway resistance, such as bronchial asthma,
usually have less difficulty during inspiration than dur-
ing expiration.
Neural and Local Control of Airway Diameter.
Constriction of bronchial smooth muscle, which controls
airway diameter, also contributes to airway resistance.
The smooth muscles in the airways are under autonomic
nervous system control. Stimulation of the parasympa-
thetic nervous system produces bronchial constriction as
well as increased mucus secretion, whereas sympathetic
stimulation has the opposite effect. Parasympathetic
nerves can be stimulated by reflexes that originate in the
lungs, most of which begin with irritation of the epi-
thelial cells by cigarette smoke, dust, noxious gases, or
bronchial infections. Inflammatory mediators such as
the leukotrienes, histamine, and prostaglandins released
by resident mast cells and airway epithelial cells can also
cause bronchial constriction.
Many of the drugs used in the treatment of bron-
chial asthma and other respiratory conditions that
produce bronchial constriction act at the level of the
autonomic nervous system or inflammatory mediators
to relieve airway obstruction. For example,
β
-adrenergic
stimulants are often used to achieve short-term relief of
asthma symptoms. Anti-inflammatory agents, such as
the corticosteroids or leukotriene antagonists, are used
to achieve long-term relief (see Chapter 3, Fig. 3-4).
Laminar and Turbulent Airflow.
Depending on the
velocity and pattern of flow, airflow can be laminar or
turbulent.
Laminar
, or
streamlined
,
airflow
occurs at
low flow rates in which the air stream is parallel to the
sides of the airway. With laminar flow, the air at the
periphery must overcome the resistance to flow; and as
a result, the air in the center of the airway moves faster.
In the bronchial tree with its many branches, laminar
airflow probably occurs only in the very small airways,
where the velocity of flow is low. Because the small air-
ways contribute little resistance to airflow, they consti-
tute a silent zone in terms of respiratory sounds.
Turbulent airflow
is disorganized flow in which the
molecules of the gas move laterally, collide with one
Low lung volume
High lung volume
FIGURE 21-13.
Interaction of tissue forces on airways during
low and high lung volumes. At low lung volumes, the tissue
forces promote folding or collapsing and place less tension on
the airways, which become smaller; during high lung volumes,
the tissue forces stretch and pull the airways open.
