Essentials of Anatomy and Physiology

rib cage. The base of each lung rests on the diaphragm
below; the apex (superior tip) is at the level of the clav-
icle. On the medial surface of each lung is an indenta-
tion called the hilus, where the primary bronchus and
the pulmonary artery and veins enter the lung.
The pleural membranes are the serous membranes
of the thoracic cavity. The parietal pleuralines the
chest wall, and the visceral pleurais on the surface of
the lungs. Between the pleural membranes is serous
fluid, which prevents friction and keeps the two mem-
branes together during breathing.

Alveoli

The functional units of the lungs are the air sacs called
alveoli. The flat alveolar type I cells that form most of
the alveolar walls are simple squamous epithelium. In
the spaces between clusters of alveoli is elastic con-
nective tissue, which is important for exhalation.
Within the alveoli are macrophages that phagocytize
pathogens or other foreign material that may not have
been swept out by the ciliated epithelium of the
bronchial tree. There are millions of alveoli in each
lung, and their total surface area is estimated to be 700
to 800 square feet (picture a sidewalk two and a half
feet wide that is as long as an American football field,
or a rectangle 25 feet by 30 feet). Each alveolus is sur-
rounded by a network of pulmonary capillaries (see
Fig 15–4). Recall that capillaries are also made of sim-
ple squamous epithelium, so there are only two cells
between the air in the alveoli and the blood in the pul-

monary capillaries, which permits efficient diffusion of

gases (Fig. 15–5).

Each alveolus is lined with a thin layer of tissue

fluid, which is essential for the diffusion of gases, be-

cause a gas must dissolve in a liquid in order to enter

or leave a cell (the earthworm principle—an earth-

worm breathes through its moist skin, and will suffo-

cate if its skin dries out). Although this tissue fluid is

necessary, it creates a potential problem in that it

would make the walls of an alveolus stick together

internally. Imagine a plastic bag that is wet inside; its

walls would stick together because of the surface ten-

sion of the water. This is just what would happen in

alveoli, and inflation would be very difficult.

This problem is overcome by pulmonary surfac-

tant, a lipoprotein secreted by alveolar type II cells,

also called septal cells. Surfactant mixes with the tissue

fluid within the alveoli and decreases its surface ten-

sion, permitting inflation of the alveoli (see Box 15–2:

Hyaline Membrane Disease). Normal inflation of the

alveoli in turn permits the exchange of gases, but

before we discuss this process, we will first see how air

gets into and out of the lungs.

MECHANISM OF BREATHING

Ventilationis the term for the movement of air to and

from the alveoli. The two aspects of ventilation are

inhalation and exhalation, which are brought about by

The Respiratory System 347

Epiglottis

Vocal cord

A B

Trachea

Glottis

Figure 15–3. Vocal cords and glottis, superior view. (A) Position of the vocal cords dur-

ing breathing. (B) Position of the vocal cords during speaking.

QUESTION:What makes the vocal cords vibrate?