Chapter 22
The Respiratory System
803
22
Te respiratory system (Figure 22.1) includes the
nose, nasal
cavity
, and
paranasal sinuses
; the
pharynx
; the
larynx
; the
tra-
chea
; the
bronchi
and their smaller branches; and the
lungs
,
which contain tiny air sacs called
alveoli
. Functionally, the sys-
tem consists of two zones.
Te
respiratory zone
, the actual site of gas exchange, is com-
posed of the respiratory bronchioles, alveolar ducts, and al-
veoli, all microscopic structures.
Te
conducting zone
includes all other respiratory passage-
ways, which provide fairly rigid conduits for air to reach the
gas exchange sites. Te conducting zone organs also cleanse,
humidify, and warm incoming air. As a result, air reaching
the lungs has fewer irritants (dust, bacteria, etc.) than when
it entered the body, and it is warm and damp, like the air of
the tropics.
Table 22.1
summarizes the functions of the major organs
of the respiratory system. In addition to these organs, some au-
thorities also include the respiratory muscles (diaphragm, etc.)
as part of this system. Although we will consider how these
skeletal muscles bring about the volume changes that promote
ventilation, we continue to classify them as part of the
muscular
system
.
The Nose and Paranasal Sinuses
Te nose is the only externally visible part of the respiratory sys-
tem. Unlike the eyes and lips, facial features o±en referred to po-
etically, the nose is usually an irreverent target. We are urged to
keep our nose to the grindstone and keep it out of other people’s
business. Considering its important functions, however, it de-
serves more esteem. Te nose (1) provides an airway for respira-
tion, (2) moistens and warms entering air, (3) filters and cleans
inspired air, (4) serves as a resonating chamber for speech, and
(5) houses the olfactory (smell) receptors.
Te structures of the nose are divided into the
external nose
and the internal
nasal cavity
for ease of consideration.
External Nose
Te surface features of the external nose include the
root
(area be-
tween the eyebrows),
bridge
, and
dorsum nasi
(anterior margin),
the latter terminating in the
apex
(tip of the nose)
(Figure 22.2a)
.
Te external openings of the nose, the
nostrils
or
nares
(na
9
rēz),
are bounded laterally by the flared
alae
.
Te skeletal framework of the external nose is fashioned by
the nasal and frontal bones superiorly (forming the bridge and
root, respectively), the maxillary bones laterally, and flexible
plates of hyaline cartilage (the alar and septal cartilages, and the
lateral processes of the septal cartilage) inferiorly (Figure 22.2b).
Noses vary a great deal in size and shape, largely because of dif-
ferences in nasal cartilages. Te skin covering the nose’s anterior
and lateral aspects is thin and contains many sebaceous glands.
Nasal Cavity
Te internal
nasal cavity
lies in and posterior to the external
nose. During breathing, air enters the cavity by passing through
the
nostrils
or
nares
(Figure 22.2a and
Figure 22.3b
). Te na-
sal cavity is divided by a midline
nasal septum
, formed anteri-
orly by the septal cartilage and posteriorly by the vomer bone
and perpendicular plate of the ethmoid bone (see Figure 7.13b,
p. 214). Te nasal cavity is continuous posteriorly with the nasal
portion of the pharynx through the
posterior nasal apertures
,
also called the
choanae
(ko-a
9
ne; “funnels”).
Te roof of the nasal cavity is formed by the ethmoid and
sphenoid bones of the skull. Te floor is formed by the
palate
,
which separates the nasal cavity from the oral cavity below. An-
teriorly, where the palate is supported by the palatine bones and
processes of the maxillary bones, it is called the
hard palate
.
Te unsupported posterior portion is the muscular
sof palate
.
Te part of the nasal cavity just superior to the nostrils, called
the
nasal vestibule
, is lined with skin containing sebaceous and
sweat glands and numerous hair follicles. Te hairs, or
vibrissae
(vi-bris
9
e;
vibro
5
to quiver), filter coarse particles (dust, pol-
len) from inspired air. Te rest of the nasal cavity is lined with
two types of mucous membrane.
Frontal bone
Nasal bone
Septal cartilage
Maxillary bone
(frontal process)
Lateral process of
septal cartilage
Minor alar cartilages
Major alar
cartilages
Dense fibrous
connective tissue
(b) External skeletal framework
Epicranius,
frontal belly
Ala of nose
Root and bridge
of nose
Dorsum nasi
Apex of nose
Naris (nostril)
(a) Surface anatomy
Figure 22.2
The external nose.
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