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U N I T 6
Respiratory Function
Obstructive Airway Disorders
Obstructive airway disorders are caused by conditions
that limit expiratory airflow. Bronchial asthma repre-
sents an acute and reversible form of airway disease
caused by narrowing of the airways due to broncho-
spasm, inflammation, and increased airway secretions.
Chronic obstructive disorders include a variety of
airway diseases, such as bronchial asthma, chronic
obstructive pulmonary disease, bronchiectasis, and cys-
tic fibrosis.
Physiology of Airway Disease
Air moves through the upper airways (i.e., trachea
and major bronchi) into the lower or pulmonary air-
ways (i.e., bronchi and alveoli), which are located in
the lung.
1,2
In the pulmonary airways, the cartilaginous
layer that provides support for the trachea and major
bronchi gradually disappears and is replaced with
crisscrossing strips of smooth muscle (see Chapter 21).
The contraction and relaxation of the smooth muscle
layer, which is innervated by the autonomic nervous sys-
tem, controls the diameter of the bronchial airways and
consequent resistance to airflow. Parasympathetic stimu-
lation through the vagus nerve and cholinergic receptors
produces bronchoconstriction, whereas sympathetic
stimulation, through
β
2
-adrenergic receptors, produces
bronchodilation. At rest, a slight vagal-mediated bron-
choconstrictor tone predominates. When there is need
for increased airflow, as during exercise, the broncho-
dilator effects of the sympathetic nervous system are
stimulated and the bronchoconstrictor effects of the
parasympathetic nervous system are inhibited. Bronchial
smooth muscle also responds to inflammatory media-
tors, such as histamine, that act directly on bronchial
smooth muscle cells to produce bronchoconstriction.
Bronchial Asthma
Bronchial asthma is a common obstructive airway disease
that affects adults and children and occurs in all popula-
tions and locations throughout the world. It has been
estimated that 25.7 million people in the United States
suffer from asthma, 7.0 million of them children under
18 years of age.
17,18
The disease continues to be costly,
both in terms of emergency room visits and lost work
days. Close to 2.1 million emergency room visits were
attributed to asthma in 2009, and in 2008 it accounted
for an estimated 14.2 million lost work days for adults.
18
Asthma is a chronic inflammatorydisease of the airways
involving recurring symptoms of airflow obstruction and
bronchial hyper-responsiveness.
19–22
Airway obstruction
is characterized by episodic wheezing, difficulty breath-
ing, feeling of chest tightness, and a cough that often is
worse at night and in the early morning. These episodes,
which usually are reversible either spontaneously or with
treatment, also cause an associated increase in bronchial
responsiveness to a variety of stimuli.
Etiology and Pathogenesis
Asthma is commonly categorized into two types: extrin-
sic or allergic, due to a type I hypersensitivity reaction,
and intrinsic or
non-atopic,
that occurs without an aller-
gic component. In either type, episodes of bronchospasm
can be triggered by diverse nonimmune mechanisms,
including respiratory tract infections, exercise, ingestion
of aspirin, emotional upset, and exposure to bronchial
irritants such as cigarette smoke.
15,16
Asthma may also
be classified according to the agents or events that trig-
ger an attack. These include seasonal, exercise-induced,
drug-induced (e.g., aspirin), and occupational asthma.
The common denominator underlying all forms of
asthma is an exaggerated hypersensitivity response to a
variety of stimuli. After exposure to an inciting factor
(allergens, drugs, cold, or exercise), inflammatory media-
tors released by activated macrophages, eosinophils, mast
cells, and basophils induce bronchoconstriction, increased
SUMMARY CONCEPTS
■■
Disorders of the pleura include pleuritis, pleural
effusion, and pneumothorax. Pleuritis, or
inflammation of the pleura, characteristically
causes unilateral pain that is abrupt in onset and
exaggerated by respiratory movements. Pleural
effusion refers to the abnormal accumulation
of fluid in the pleural cavity.The fluid may be
a transudate (i.e., hydrothorax), exudate (i.e.,
empyema), chyle (i.e., chylothorax), or blood
(hemothorax).
■■
Pneumothorax refers to an accumulation of air in
the pleural cavity that causes partial or complete
collapse of the lung. Pneumothorax can result
from rupture of an air-filled bleb on the lung
surface or from penetrating or nonpenetrating
injuries. A tension pneumothorax is a life-
threatening event in which air accumulates in the
thorax, collapsing the lung on the injured side
and progressively shifting the mediastinum to
the opposite side of the thorax, producing severe
cardiac and respiratory impairment.
■■
Atelectasis refers to an incomplete expansion of
the lung. Primary atelectasis occurs most often
in premature and high-risk infants. Acquired
atelectasis occurs mainly in adults and is caused
most commonly by a mucus plug in the airway
or by external compression by fluid, tumor mass,
exudate, or other matter in the area surrounding
the airway.
