HUMAN BIOLOGY

the RespiRatoRy system 179

What are the main structures and functions

of the respiratory system?

• The lungs and airways are the main components of the
  respiratory system.


• The respiratory system’s basic functions are taking oxygen into
  the body and removing carbon dioxide.


• In alveoli inside the lungs, oxygen enters lung capillaries, and
  carbon dioxide leaves them to be exhaled.

takE-homE mEssaGE

alveolus Air sac where

gases diffuse between the

lungs and blood capillaries.

bronchiole Narrow pas-

sageway that ends in

clusters of alveoli.

bronchus Airway that leads

directly to a lung.

diaphragm Sheet of mus-

cle separating the thoracic

and abdominal cavities.

epiglottis Flap that closes

off the larynx during

swallowing.

larynx The voice box, the

airway where sound is

produced.

lungs Lobed organs where

gas exchange occurs.

pharynx The throat; it con-

nects to the larynx and

esophagus.

pleurae Paired membranes

that enclose each lung.

respiratory system Organ

system that consists of the

lungs and airways.

trachea The windpipe;

it branches into the two

bronchi.

vocal cords Horizontal

folds above the larynx

that vibrate as air passes

upward between them.

Inside each lung, the bronchi

narrow as they branch and form

“bronchial trees.” These narrowing

airways are bronchioles. Their nar-

rowest portions deep in the lungs are

respiratory bronchioles. In each lung,

about 150 million tiny air sacs bulge

out from their walls. Each sac is an

alveolus (plural: alveoli). Alveoli are

where gases diffuse between the

lungs and blood capillaries (Figure

10.1B and 10.1C). Together the mil-

lions of alveoli provide a huge sur-

face area for this exchange of gases.

If they were stretched out as a single

layer, they would cover the body

several times over—or the floor of a

racquetball court!

The flaplike epiglottis, attached to the larynx, points up
during breathing. When you swallow, the larynx moves up
so that the epiglottis partly covers the opening of the lar-
ynx. This helps prevent food from entering the respiratory
tract and causing choking.
From the larynx, air moves into the “windpipe” or
trachea (tray-kee-uh). Press gently at the lower front of
your neck, and you can feel some of the bands of cartilage
that ring the tube, adding strength and helping to keep it
open. The trachea branches into two airways, one lead-
ing to each lung. Each airway is a bronchus ( brong-cuss;
plural: brong-kee). The epithelial lining of bronchi includes
mucus-secreting cells and cilia. Figure 10.2 shows a close-
up of these cilia. The mucus traps bacteria and airborne
particles, then the upward-beating cilia sweep the debris-
laden mucus toward the mouth.
Just above the larynx, horizontal folds of an elastic
mucous membrane form the vocal cords (Figure 10.3).
When you exhale, air rushes through the glottis, a gap
between the cords that opens to the larynx. Air moving
through it makes the cords vibrate. By controlling the
vibrations we can make sounds. Using our lips, teeth,
tongue, and the soft roof of the mouth (the soft palate), we
can form these sounds into vocalizations such as speech.

Gases are exchanged in the lungs

You r lungs are cone-shaped organs separated from each
other by the heart. The left lung has two lobes, the right
lung three. The lungs are located inside the rib cage
above the diaphragm, a broad sheet of muscle between
the thoracic (chest) and abdominal cavities. The lungs are
soft, spongy, and elastic and don’t attach directly to the
chest wall. Instead, each lung is enclosed by a pair of thin
membranes called pleurae (singular: pleura). This arrange-
ment is a little like a fist pushed into an inflated balloon. A
lung occupies the same sort of position as your fist, and the
pleural membrane folds back on itself (as the balloon does)
to form a closed pleural sac. A narrow intrapleural space
(intra- means “between”) separates the membrane’s two
facing surfaces. A thin film of lubricating fluid in the space
reduces chafing between the membranes.

Figure 10.2 The lining of the airways includes mucus-

secreting cells (orange) and tufts of hairlike cilia.

vocal cords

glottis (closed)

epiglottis

tongue’s base

glottis

(open)

Figure 10.3 The vocal cords are at

the upper opening of the larynx.

Contraction of skeletal muscle in the

cords changes the width of the glottis,

the gap between them. The sketches

show what the glottis looks like when it

is closed and opened. (Images courtesy of Kay

Elemetrics Corporation; © Cengage Learning)

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