737
CHAPTER NINETEEN
Respiratory System
Besides producing ATP, the aerobic reactions produce
CO
2
, a metabolic waste that combines with water to form
carbonic acid, helping to maintain blood pH. Too much CO
2
,
however, lowers blood pH, compromising homeostasis. The
respiratory system both provides oxygen for aerobic reac-
tion, and eliminates CO
2
rapidly enough to maintain the pH
of the internal environment.
19.3
ORGANS OF THE
RESPIRATORY SYSTEM
The organs of the respiratory system can be divided into two
groups, or tracts. Those in the
upper respiratory tract
include
the nose, nasal cavity, sinuses, and pharynx. Those in the
lower respiratory tract
include the larynx, trachea, bronchial
tree, and lungs
(f
g. 19.1)
.
Nose
The nose is covered with skin and is supported internally by
muscle, bone, and cartilage. Its two
nostrils
(external nares)
provide openings through which air can enter and leave the
nasal cavity. Many internal hairs in these openings prevent
entry of large particles carried in the air.
Nasal Cavity
The
nasal cavity,
a hollow space behind the nose, is divided
medially into right and left portions by the
nasal septum.
This cavity is separated from the cranial cavity by the cribri-
form plate of the ethmoid bone and from the oral cavity by
the hard palate.
The nasal septum may bend during birth or shortly before adoles-
cence. Such a
deviated septum
may obstruct the nasal cavity, making
breathing diF
cult.
As
f
gure 19.2
shows,
nasal conchae
(turbinate bones)
curl out from the lateral walls of the nasal cavity on each
side, forming passageways called the
superior, middle,
and
inferior meatuses
(see chapter 7, pp. 212 and 214). The nasal
chonchae support the mucous membrane that lines the nasal
cavity and help increase its surface area.
The upper posterior portion of the nasal cavity, below the
cribriform plate, is slitlike, and its lining contains the olfac-
tory receptors that provide the sense of smell. The remainder
of the cavity conducts air to and from the nasopharynx.
The mucous membrane lining the nasal cavity has pseu-
dostratiF
ed ciliated epithelium rich in mucous-secreting gob-
let cells (see chapter 5, p. 146). It also includes an extensive
network of blood vessels and normally appears pinkish. As
air passes over the membrane, heat radiates from the blood
and warms the air, adjusting its temperature to that of the
body, as evaporation of water from the mucous lining moist-
ens the air. The sticky mucus the mucous membrane secretes
entraps dust and other small particles entering with the air.
As the cilia of the epithelial cells move, a thin layer of
mucus and any entrapped particles are pushed toward the
pharynx
(f g. 19.3)
. When the mucus reaches the pharynx,
it is swallowed. In the stomach, gastric juice destroys micro-
organisms in the mucus, including pathogens. Thus, the F l-
tering from the mucous membrane prevents particles from
reaching the lower air passages and consequently prevents
respiratory infections. Clinical Application 19.1 discusses
how cigarette smoking impairs the respiratory system, begin-
ning with the cleansing mucus and cilia.
PRACTICE
1
What is respiration?
2
Which organs constitute the respiratory system?
3
What is the function of the mucous membrane that lines the nasal
cavity?
4
What is the function of the cilia on the cells that line the nasal cavity?
Frontal
sinus
Nasal
cavity
Oral
cavity
Larynx
Bronchus
Hard
palate
Nostril
Right lung
Left lung
Trachea
Soft palate
Pharynx
Epiglottis
Esophagus
FIGURE 19.1
Organs of the respiratory system.
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