Human Physiology, 14th edition (2016)

536 Chapter 16

CLINICAL APPLICATION

A tracheotomy is the surgical procedure of creating a hole

in the trachea, which is then called a tracheostomy and

into which various types of tubes are often inserted. Trache-

ostomies are usually done in a hospital to allow the patient

to breathe with the aid of a mechanical ventilator, but they

may be performed by a surgeon in an emergency to allow a

person to breathe when the trachea becomes occluded by

aspiration of a foreign object, trauma, or inflammation.

Air enters the trachea from the pharynx, which is the cav-
ity behind the palate that receives the contents of both the oral
and nasal passages. In order for air to enter or leave the trachea
and lungs, however, it must pass through a valvelike opening
called the glottis between the vocal folds. The ventricular and
vocal folds are part of the larynx, or voice box, which guards the
entrance to the trachea ( fig. 16.6 ). The projection at the front of
the throat, commonly called the “Adam’s apple,” is formed by
the largest cartilage of the larynx.
The conducting zone of the respiratory system, in summary,
consists of the mouth, nose, pharynx, larynx, trachea, primary
bronchi, and all successive branchings of the bronchioles up to and
including the terminal bronchioles. In addition to conducting air
into the respiratory zone, these structures serve additional func-
tions: warming and humidification of the inspired air, and filtration
and cleaning.
Regardless of the temperature and humidity of the ambi-
ent air, when the inspired air reaches the respiratory zone it is
at a temperature of 37 8 C (body temperature), and it is satu-
rated with water vapor as it flows over the warm, wet mucous
membranes that line the respiratory airways. This ensures that
a constant internal body temperature will be maintained and
that the lung tissue will be protected from desiccation.
Mucus secreted by cells of the conducting zone structures
serves to trap small particles in the inspired air and thereby per-
forms a filtration function. This mucus is moved along at a rate of
1 to 2 cm per minute by cilia projecting from the tops of epithe-
lial cells that line the conducting zone. There are about 300 cilia
per cell that beat in a coordinated fashion to move mucus toward
the pharynx, where it can either be swallowed or expectorated.
As a result of this filtration function, particles larger than
about 6 m m do not normally enter the respiratory zone of the
lungs. The importance of this function is evidenced by black
lung, a disease that occurs in miners who inhale large amounts
of carbon dust from coal, which causes them to develop pulmo-
nary fibrosis. The alveoli themselves are normally kept clean by
the action of resident macrophages (see fig. 16.1 ). The cleansing

Figure 16.6 A photograph of the larynx showing the
true and false vocal cords and the glottis. The vocal folds
(true vocal cords) function in sound production, whereas the
ventricular folds (false vocal cords) do not.

Ventricular fold

(false vocal cord)

Vocal fold

(true vocal cord)

Glottis

action of cilia and macrophages in the lungs is diminished by

cigarette smoke.

Thoracic Cavity

The diaphragm, a dome-shaped sheet of striated muscle, divides

the anterior body cavity into two parts. The area below the dia-

phragm, the abdominopelvic cavity, contains the liver, pancreas,

gastrointestinal tract, spleen, genitourinary tract, and other

organs. Above the diaphragm, the thoracic cavity contains the

heart, large blood vessels, trachea, esophagus, and thymus in the

central region, and is filled elsewhere by the right and left lungs.

The structures in the central region—or mediastinum —are

enveloped by two layers of wet epithelial membrane collectively

called the pleural membranes. The superficial layer, or parietal

pleura, lines the inside of the thoracic wall. The deep layer, or

visceral pleura, covers the surface of the lungs ( fig. 16.7 ).

The lungs normally fill the thoracic cavity so that the visceral

pleura covering each lung is pushed against the parietal pleura

lining the thoracic wall. There is, thus, under normal conditions,

little or no air between the visceral and parietal pleura. There is,

however, a “potential space”—called the intrapleural space —

that can become a real space if the visceral and parietal pleurae

separate when a lung collapses. The normal position of the lungs

in the thoracic cavity is shown in the radiographs in figure 16.8.

| CHECKPOINT

1a. Describe the structures involved in gas exchange in

the lungs and explain how gas exchange occurs.

1b. Describe the structures and functions of the

conducting zone of the respiratory system.

2. Describe how each lung is compartmentalized by the
   pleural membranes. What is the relationship between
   the visceral and parietal pleurae?

16.2 PHYSICAL ASPECTS

OF VENTILATION

The movement of air into and out of the lungs occurs as a

result of pressure differences induced by changes in lung

volumes. Ventilation is influenced by the physical properties