Essentials of Anatomy and Physiology

the adenoid and the lingual tonsils on the base of the
tongue, they form a ring of lymphatic tissue around
the pharynx to destroy pathogens that penetrate the
mucosa.
The laryngopharynxis the most inferior portion
of the pharynx. It opens anteriorly into the larynx and
posteriorly into the esophagus. Contraction of the
muscular wall of the oropharynx and laryngopharynx
is part of the swallowing reflex.

LARYNX

The larynxis often called the voice box, a name that
indicates one of its functions, which is speaking. The
other function of the larynx is to be an air passageway
between the pharynx and the trachea. Air passages
must be kept open at all times, and so the larynx is
made of nine pieces of cartilage connected by liga-
ments. Cartilage is a firm yet flexible tissue that pre-
vents collapse of the larynx. In comparison, the
esophagus is a collapsed tube except when food is pass-
ing through it.
The largest cartilage of the larynx is the thyroid
cartilage(Fig. 15–2), which you can feel on the ante-
rior surface of your neck. The epiglottisis the upper-
most cartilage. During swallowing, the larynx is
elevated, and the epiglottis closes over the top, rather

like a trap door or hinged lid, to prevent the entry of

saliva or food into the larynx.

The mucosa of the larynx is ciliated epithelium,

except for the vocal cords (stratified squamous epithe-

lium). The cilia of the mucosa sweep upward to

remove mucus and trapped dust and microorganisms.

The vocal cords(or vocal folds) are on either side

of the glottis, the opening between them. During

breathing, the vocal cords are held at the sides of the

glottis, so that air passes freely into and out of the tra-

chea (Fig. 15–3). During speaking, the intrinsic mus-

cles of the larynx pull the vocal cords across the glottis,

and exhaled air vibrates the vocal cords to produce

sounds that can be turned into speech. It is also phys-

ically possible to speak while inhaling, but this is not

what we are used to. The cranial nerves that are motor

nerves to the larynx for speaking are the vagus and

accessory nerves. You may also recall that for most

people, the speech areas are in the left cerebral hemi-

sphere.

TRACHEA AND BRONCHIAL TREE

The tracheais about 4 to 5 inches (10 to 13 cm) long

and extends from the larynx to the primary bronchi.

The wall of the trachea contains 16 to 20 C-shaped

pieces of cartilage, which keep the trachea open. The

gaps in these incomplete cartilage rings are posterior,

to permit the expansion of the esophagus when food is

swallowed. The mucosa of the trachea is ciliated

epithelium with goblet cells. As in the larynx, the cilia

sweep upward toward the pharynx.

The right and left primary bronchi(Fig. 15–4) are

the branches of the trachea that enter the lungs. Their

structure is just like that of the trachea, with C-shaped

cartilages and ciliated epithelium. Within the lungs,

each primary bronchus branches into secondary

bronchi leading to the lobes of each lung (three right,

two left). The further branching of the bronchial tubes

is often called the bronchial tree. Imagine the trachea

as the trunk of an upside-down tree with extensive

branches that become smaller and smaller; these

smaller branches are the bronchioles. No cartilage is

present in the walls of the bronchioles; this becomes

clinically important in asthma (see Box 15–1: Asthma).

The smallest bronchioles terminate in clusters of alve-

oli, the air sacs of the lungs.

LUNGS AND PLEURAL MEMBRANES

The lungsare located on either side of the heart in

the chest cavity and are encircled and protected by the

346 The Respiratory System

Epiglottis

Hyoid

bone

Thyroid

cartilage

Vocal

cords

Cricoid

cartilage

Tracheal

cartilages

A B

Figure 15–2. Larynx. (A) Anterior view. (B) Midsagittal
section through the larynx, viewed from the left side.
QUESTION:What is the function of the epiglottis?