Chapter 22
The Respiratory System
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22
Te paired
lungs
occupy all of the thoracic cavity except the
mediastinum, which houses the heart, great blood vessels, bron-
chi, esophagus, and other organs
(Figure 22.10)
.
Gross Anatomy of the Lungs
Each cone-shaped lung is surrounded by pleurae and connected
to the mediastinum by vascular and bronchial attachments, col-
lectively called the lung
root
. Te anterior, lateral, and posterior
lung surfaces lie in close contact with the ribs and form the con-
tinuously curving
costal surface
. Just deep to the clavicle is the
apex
, the narrow superior tip of the lung. Te concave, inferior
surface that rests on the diaphragm is the
base
.
On the mediastinal surface of each lung is an indentation,
the
hilum
, through which pulmonary and systemic blood ves-
sels, bronchi, lymphatic vessels, and nerves enter and leave the
lungs. Each main bronchus plunges into the hilum on its own
side and begins to branch almost immediately. All conduct-
ing and respiratory passageways distal to the main bronchi are
found in the lungs.
Te two lungs differ slightly in shape and size because the
apex of the heart is slightly to the leF of the median plane. Te
leF lung is smaller than the right, and the
cardiac notch
—a
concavity in its medial aspect—is molded to and accommodates
the heart (±igure 22.10a). Te leF lung is subdivided into supe-
rior and inferior
lobes
by the
oblique fissure
, whereas the right
lung is partitioned into superior, middle, and inferior lobes by
the
oblique
and
horizontal fissures
.
Each lobe contains a number of pyramid-shaped
bron-
chopulmonary segments
separated from one another by
connective tissue septa. Te right lung has 10 bronchop-
ulmonary segments, but the leF lung is more variable and
consists of 8 to 10 segments
(Figure 22.11)
. Each segment
is served by its own artery and vein and receives air from an
individual segmental (tertiary) bronchus.
Te bronchopulmonary segments are clinically important
because pulmonary disease is oFen confined to one or a few
segments. Teir connective tissue partitions allow diseased seg-
ments to be surgically removed without damaging neighboring
segments or impairing their blood supply.
Te smallest subdivisions of the lung visible with the naked
eye are the
lobules
, which appear at the lung surface as hexagons
ranging from the size of a pencil eraser to the size of a penny (±ig-
ure 22.10b). A large bronchiole and its branches serve each lobule.
In most city dwellers and in smokers, the connective tissue that
separates the individual lobules is blackened with carbon.
As we mentioned earlier, the lungs consist largely of air
spaces. Te balance of lung tissue, or its
stroma
(literally “mat-
tress” or “bed”), is mostly elastic connective tissue. As a result,
the lungs are soF, spongy, elastic organs that together weigh
just over 1 kg (2.2 lb). Te elasticity of healthy lungs reduces the
work of breathing, as we will describe shortly.
Blood Supply and Innervation of the Lungs
Te lungs are perfused by two circulations, the pulmonary and
the bronchial, which differ in size, origin, and function.
Pulmonary Circulation of the Lungs
Systemic venous blood that
is to be oxygenated in the lungs is delivered by the
pulmonary
arteries
, which lie anterior to the main bronchi (±igure 22.10c).
In the lungs, the pulmonary arteries branch profusely along with
the bronchi and finally feed into the
pulmonary capillary net-
works
surrounding the alveoli (see ±igure 22.9a).
Right
superior
lobe (3
segments)
Right
middle
lobe (2
segments)
Right
inferior lobe
(5 segments)
Left superior
lobe
(4 segments)
Left inferior
lobe
(5 segments)
Right lung
Left lung
Figure 22.11
A cast of the bronchial tree.
The individual bronchopulmonary segments
have been painted different colors.
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