The Respiratory System
Principal Organs of the Respiratory System
DESCRIPTION, GENERAL AND DISTINCTIVE FEATURES
nose and nasal
Jutting external portion is supported by bone and cartilage. Internal
nasal cavity is divided by midline nasal septum and lined with mucosa.
Roof of nasal cavity contains olfactory epithelium.
Produces mucus; ﬁlters, warms, and moistens
incoming air; resonance chamber for speech
Receptors for sense of smell
Mucosa-lined, air-ﬁlled cavities in cranial bones surrounding nasal
Same as for nasal cavity except no receptors for
smell; also lighten skull
Passageway connecting nasal cavity to larynx and oral cavity to
esophagus. Three subdivisions: nasopharynx, oropharynx, and
Houses tonsils (lymphoid tissue masses involved in protection against
Passageway for air and food
Facilitates exposure of immune system to inhaled
Connects pharynx to trachea. Has framework of cartilage and dense
connective tissue. Opening (glottis) can be closed by epiglottis or vocal
Houses vocal folds (true vocal cords).
Air passageway; prevents food from entering
lower respiratory tract
Flexible tube running from larynx and dividing inferiorly into two
main bronchi. Walls contain C-shaped cartilages that are incomplete
posteriorly where connected by trachealis.
Air passageway; cleans, warms, and moistens
Consists of right and left main bronchi, which subdivide within the
lungs to form lobar and segmental bronchi and bronchioles. Bronchiolar
walls lack cartilage but contain complete layer of smooth muscle.
Constriction of this muscle impedes expiration.
Air passageways connecting trachea with alveoli;
cleans, warms, and moistens incoming air
Microscopic chambers at termini of bronchial tree. Walls of simple
squamous epithelium overlie thin basement membrane. External
surfaces are intimately associated with pulmonary capillaries.
Special alveolar cells produce surfactant.
Main sites of gas exchange
Reduces surface tension; helps prevent lung
Paired composite organs that ﬂank mediastinum in thorax. Composed
primarily of alveoli and respiratory passageways. Stroma is ﬁbrous elastic
connective tissue, allowing lungs to recoil passively during expiration.
House respiratory passages smaller than the
Serous membranes. Parietal pleura lines thoracic cavity; visceral pleura
covers external lung surfaces.
Produce lubricating ﬂuid and compartmentalize
Te small patch of
lines the slitlike supe-
rior region of the nasal cavity and contains smell receptors in
(see p. 565).
lines most of the nasal cavity. Te
respiratory mucosa is a pseudostratiﬁed ciliated columnar epi-
thelium, containing scattered
, that rests on a lamina
propria richly supplied with seromucous
Seromucous nasal glands contain mucus-secreting mucus
cells and serous cells that secrete a watery ﬂuid containing en-
zymes. Each day, these glands secrete about a quart (a liter) of
, an antibacterial enzyme. Te sticky
mucus traps inspired dust, bacteria, and other debris, while lys-
ozyme attacks and chemically destroys bacteria. Te epithelial
cells of the respiratory mucosa also secrete
antibiotics that help kill invading microbes. Additionally, the
high water content of the mucus ﬁlm humidiﬁes incoming air.
Te ciliated cells of the respiratory mucosa create a gentle
current that moves the sheet of contaminated mucus posteriorly
toward the throat, where it is swallowed and digested. We are
usually unaware of this important action of our nasal cilia, but
when exposed to cold air they become sluggish, allowing mucus
to accumulate in the nasal cavity and dribble out the nostrils.
Tis, along with the fact that water vapor in expired air tends
to condense at lower temperatures, explains why you have a
“runny” nose on a crisp wintry day.
Te nasal mucosa is richly supplied with sensory nerve end-
ings, and contact with irritating particles (dust, pollen, and the
like) triggers a sneeze reﬂex. Te sneeze forces air outward in a
violent burst—a crude but eﬀective way to expel irritants.
Rich plexuses of capillaries and thin-walled veins underlie
the nasal epithelium and warm incoming air as it ﬂows across
the mucosal surface. When the inspired air is cold, the vascu-
lar plexus becomes engorged with blood, intensifying the air-
heating process. Because these blood vessels are abundant and
located superﬁcially, nosebleeds are common and oFen profuse.
Protruding medially from each lateral wall of
the nasal cavity are three scroll-like mucosa-covered projec-
inferior nasal conchae