747
CHAPTER NINETEEN
Respiratory System
PRACTICE
16
Where are the lungs located?
17
What is the function of the serous F
uid in the pleural cavity?
18
How does the structure of the right lung di±
er from that of the left
lung?
19
What types of structures make up a lung?
19.4
BREATHING MECHANISM
Breathing, also called ventilation, is the movement of air from
outside the body into the bronchial tree and alveoli, followed
by a reversal of this air movement. The actions responsible for
these air movements are termed
inspiration
(in
spı˘-ra
shun),
or inhalation, and
expiration
(ek
spi-ra
shun), or exhalation.
Inspiration
Atmospheric pressure due to the weight of the air is the force
that moves air into the lungs. At sea level, this pressure is
sufF
cient to support a column of mercury about 760 millime-
ters high in a tube. Thus, normal air pressure equals 760 mil-
limeters (mm) of mercury (Hg). (Other units are in common
usage: 760 mm Hg = 760 Torr = 1 atmosphere.)
Air pressure is exerted on all surfaces in contact with
the air, and because people breathe air, the inside surfaces
of their lungs are also subjected to pressure. In other words,
when the respiratory muscles are at rest, the pressures on
the inside of the lungs and alveoli and on the outside of the
thoracic wall are about the same
(f
g. 19.21)
.
Pressure and volume are related in an opposite, or
inverse, way (this is known as Boyle’s law). ±or example,
pulling back on the plunger of a syringe increases the vol-
ume inside the barrel, lowering the air pressure inside.
Atmospheric pressure then pushes outside air into the
syringe
(f
g. 19.22
a
)
. In contrast, pushing on the plunger
of a syringe reduces the volume inside the syringe, but the
Lungs
The lungs are soft, spongy, cone-shaped organs in the tho-
racic cavity. The right and left lungs are separated medially
by the heart and the mediastinum, and they are enclosed by
the diaphragm and the thoracic cage (see
f gs.
1.9,
19.19
and
reference plates 16, 17, and 21).
Each lung occupies most of the thoracic space on its side
and is suspended in the cavity by a bronchus and some large
blood vessels. These tubular structures enter the lung on its
medial surface through a region called the
hilum.
A layer of
serous membrane, the
visceral pleura,
is F
rmly attached to
the surface of each lung, and this membrane folds back at
the hilus to become the
parietal pleura.
The parietal pleura,
in turn, forms part of the mediastinum and lines the inner
wall of the thoracic cavity
(f
g. 19.20)
.
No significant space exists between the visceral and
parietal pleurae, because they are essentially in contact with
each other. The potential space between them, the
pleural
cavity,
contains only a thin F lm of serous fl
uid that lubri-
cates the adjacent pleural surfaces, reducing friction as they
move against one another during breathing. This fl uid also
helps hold the pleural membranes together.
The right lung is larger than the left lung, and F
ssures
divide it into three parts, called the superior, middle, and
inferior lobes. The left lung is similarly divided and consists
of two parts, a superior and an inferior lobe.
A lobar bronchus of the bronchial tree supplies each
lobe. A lobe also has connections to blood and lymphatic
vessels and is enclosed by connective tissues. Connective
tissue further subdivides a lobe into
lobules,
each of which
contains terminal bronchioles together with their alveolar
ducts, alveolar sacs, alveoli, nerves, and associated blood
and lymphatic vessels.
Table 19.1
summarizes the characteristics of the major
parts of the respiratory system. Clinical Application 19.2
considers substances that irritate the lungs.
TABLE
19.1
|
Parts of the Respiratory System
Part
Description
Function
Nose
Part of face centered above the mouth and inferior to the
space between the eyes
Nostrils provide entrance to nasal cavity; internal hairs begin to ²
lter incoming air
Nasal cavity
Hollow space behind nose
Conducts air to pharynx; mucous lining ²
lters, warms, and moistens incoming air
Sinuses
Hollow spaces in various bones of the skull
Reduce weight of the skull; serve as resonant chambers
Pharynx
Chamber posterior to the nasal cavity, oral cavity, and
larynx
Passageway for air moving from nasal cavity to larynx and for food moving from
oral cavity to esophagus
Larynx
Enlargement at the top of the trachea
Passageway for air; prevents foreign objects from entering trachea; houses vocal
cords
Trachea
³lexible tube that connects larynx with bronchial tree
Passageway for air; mucous lining continues to ²
lter air
Bronchial tree
Branched tubes that lead from the trachea to the alveoli
Conducts air to the alveoli; mucous lining continues to ²
lter incoming air
Lungs
Soft, cone-shaped organs that occupy a large portion of
the thoracic cavity
Contain the air passages, alveoli, blood vessels, connective tissues, lymphatic
vessels, and nerves of the lower respiratory tract
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