Sometimes a person will describe a habit as being

“as natural as breathing.” Indeed, what could be more
natural? We rarely think about breathing, and it isn’t
something we look forward to, as we might look for-
ward to a good dinner. We just breathe, usually at the
rate of 12 to 20 times per minute, and faster when
necessary (such as during exercise). You may have
heard of trained singers “learning how to breathe,”
but they are really learning how to make their breath-
ing more efficient.
Most of the respiratory systemis concerned with
what we think of as breathing: moving air into and out
of the lungs. The lungs are the site of the exchanges of
oxygen and carbon dioxide between the air and the
blood. Both of these exchanges are important. All
of our cells must obtain oxygen to carry out cell respi-
ration to produce ATP. Just as crucial is the elimina-
tion of the CO 2 produced as a waste product of cell
respiration, and, as you already know, the proper func-
tioning of the circulatory system is essential for the
transport of these gases in the blood.

DIVISIONS OF THE

RESPIRATORY SYSTEM

The respiratory system may be divided into the upper
respiratory tract and the lower respiratory tract. The
upper respiratory tractconsists of the parts outside
the chest cavity: the air passages of the nose, nasal cav-
ities, pharynx, larynx, and upper trachea. The lower
respiratory tractconsists of the parts found within
the chest cavity: the lower trachea and the lungs them-
selves, which include the bronchial tubes and alveoli.
Also part of the respiratory system are the pleural
membranes and the respiratory muscles that form the
chest cavity: the diaphragm and intercostal muscles.
Have you recognized some familiar organs and
structures thus far? There will be more, because this
chapter includes material from all of the previous
chapters. Even though we are discussing the body
system by system, the respiratory system is an excel-
lent example of the interdependent functioning of all
the body systems.

NOSE AND NASAL CAVITIES

Air enters and leaves the respiratory system through
the nose, which is made of bone and cartilage covered

with skin. Just inside the nostrils are hairs, which help

block the entry of dust.

The two nasal cavitiesare within the skull, sepa-

rated by the nasal septum, which is a bony plate made

of the ethmoid bone and vomer. The nasal mucosa

(lining) is ciliated epithelium, with goblet cells that

produce mucus. Three shelf-like or scroll-like bones

called conchaeproject from the lateral wall of each

nasal cavity (Figs. 15–1 and 6–6). Just as shelves in a

cabinet provide more flat space for storage, the con-

chae increase the surface area of the nasal mucosa. As

air passes through the nasal cavities it is warmed and

humidified, so that air that reaches the lungs is warm

and moist. Bacteria and particles of air pollution are

trapped on the mucus; the cilia continuously sweep

the mucus toward the pharynx. Most of this mucus is

eventually swallowed, and most bacteria present will

be destroyed by the hydrochloric acid in the gastric

juice.

In the upper nasal cavities are the olfactory recep-

tors, which detect vaporized chemicals that have been

inhaled. The olfactory nerves pass through the eth-

moid bone to the brain.

You may also recall our earlier discussion of the

paranasal sinuses, air cavities in the maxillae, frontal,

sphenoid, and ethmoid bones (see Figs. 15–1 and 6–9).

These sinuses are lined with ciliated epithelium, and

the mucus produced drains into the nasal cavities. The

functions of the paranasal sinuses are to lighten the

skull and provide resonance (more vibrating air) for

the voice.

PHARYNX

The pharynxis a muscular tube posterior to the nasal

and oral cavities and anterior to the cervical vertebrae.

For descriptive purposes, the pharynx may be divided

into three parts: the nasopharynx, oropharynx, and

laryngopharynx (see Fig. 15–1).

The uppermost portion is the nasopharynx, which

is behind the nasal cavities. The soft palateis elevated

during swallowing to block the nasopharynx and pre-

vent food or saliva from going up rather than down.

The uvula is the part of the soft palate you can see at

the back of the throat. On the posterior wall of the

nasopharynx is the adenoid or pharyngeal tonsil, a

lymph nodule that contains macrophages. Opening

into the nasopharynx are the two eustachian tubes,

which extend to the middle ear cavities. The purpose

344 The Respiratory System