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No. 10
1.
2.
3.
4.
5.
Trachea
Bronchi
Lungs
Pleura
mediastinum
Section 4 The Trachea
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The trachea is a tube approximately 2.0 cm in
diameter and 11 cm long.
Location:
It is located in the midline of neck and upper
thorax, and in front of the esophagus.
It extends from the larynx at the lower border of
the cricoid cartilage at the level of sixth cervical
vertebra to the level of the sternal angle
(corresponding) to the lower border of the fourth
thoracic vertebra, where it divides into the right
and left principal bronchi.
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Composition:
The wall of trachea is mainly composed of
about 15～30 C-shaped rings of hyaline
cartilage keep the lumen of the trachea
open.
But the posterior wall of the trachea is
closed by the tracheal muscle and lies
against the anterior surface of the
esophagus permitting the esophagus to
expand during swallowing.
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Carina of trachea:
It is a sagittal semilunar ridge inside
the bifurcation of trachea, which can
be seen through bronchoscope and
used as a guide to the bronchi.
Section 5 The Bronchi
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The trachea terminates at the level of
fourth thoracic vertebra and divides into
the right and left principal bronchi.
The right principal bronchus is about
2～3 cm long and is shorter, wider and
more vertical in position than the left.
The left principal bronchus is longer,
about 4～5 cm long, finer and less vertical.
So the foreign objects from the trachea
usually pass to the right bronchus.
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Bronchial tree:
As entering the lungs, the principal
bronchus branches to form lobar bronchi
which enter the lobes of lung.
The lobar bronchi continue to branch,
forming still smaller bronchi called the
segmental bronchi, which are
subdivided into the bronchioles.
ramify into even smaller tubes called the
terminal bronchioles.
These branches resemble a tree and so
are called the bronchial tree.
Section 6 The Lungs
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The lungs are the essential organs of
respiration.
Healthy lungs always contain some air so
they may float in water. In the newborn
which has not breath, the lungs will not
float in water. At birth the lungs are
pinkish-white but they turn dark trey with
age and become almost black due to
carbonaceous deposits.

Location:
The lungs are situated one on each
side within the thorax, and separated
from each other by the heart and
other contents of the mediastinum.
Ⅰ. The External Features of Lungs
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Each lung is shaped somewhat like a
cone, with an apex, a base, two
surfaces and three borders.
The right lung is shorter than the left
one because the right dome of the
diaphragm is higher, and it is wider
because the heart and pericardium
bulge more to the left.
1. One apex and one base
1) The apex of lung is rounded and
extends to about 2～3 above the level of
the medial one-third of the clavicle.
2) The base of lung is concave and
related to the diaphragm which separates
the right lung from the liver and the left
lung from the stomach, spleen and liver,
so the base of lung is also called the
diaphragmatic surface.
2. Two surfaces
The lung possesses a costal surface
and a medial surface.
1) The costal surface is smooth,
convex and related to the inner
surface of the ribs, costal cartilages
and intercostal spaces.
2) The medial surface is related to
the mediastinum, so it is also called
the mediastinal surface.
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The hilum of lung and root of lung:
Near the center of the medial surface, there is a
depression called the hilum of lung (hilus). The
hilus is the region where the structures that form
the root of the lung-that is, the bronchus, blood
vessels, lymphatics and nerves-enter or leave the
lung.
The structures entering and emerging the hilum
is called the root of lung, which is short broad
pedicle and consists of the bronchi, pulmonary
artery and veins, nerves, bronchial vessels,
lymphatics and lymph nodes.
3. Three borders
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The borders of lung includes anterior,
posterior and inferiors.
The anterior border is thin and sharp,
having a deep notch at the forth and fifth
intercostals spaces in the left lung, called
the cardiac notch of left lung, beneath
which is the lingula of left lung.
The posterior border is round.
The inferior border is also sharp and
separates the base of lung from the costal
and medial surfaces.
Ⅱ. The Lobes and Segments of
Lungs
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Fissures and lobes:
Each lung is divided into superior
and inferior lobes by an oblique
fissure. The right lung is further
divided by a horizontal fissure, which
bounds a middle lobe.
The right lung therefore has three
lobes, whereas the left has only two.
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Bronchopulmonary segments:
Each lung is subdivided by connective
tissue partitions into smaller units called
bronchopulmonary segments.
Each bronchopulmonary segment
represents the portion of the lung that is
supplied by a specific tertiary bronchus.
Each lung has ten segments.
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The bronchopulmonary segments are
important surgically because a diseased
segment can be removed without having
to remove an entire lobe or the entire lung.
Also, disease does not spread so easily
across the partitions that separate the
segments, so pathology tends to be
confined to one or several segments
rather than spreading freely throughout
the lungs.
Section 7 The Pleura
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Each lung is enclosed in a double-walled
the pleura. Both layers of the pleura are
formed of serous membrane that lines the
inner surface of the thorax and the surface
of the thorax and the surfaces of lungs.
The portion of the pleura that adheres
firmly to the lungs is the visceral pleura.
The portion that lines the walls of the
thoracic cavity is the parietal pleura.
Ⅰ. The Parietal Pleura
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The serous membrane lining the
inner surface of chest wall is called
the parietal pleura.
According to the regions the parietal
pleura is divided into four portions:
① the costal pleura,
② he diaphragmatic pleura,
③ the mediastinal pleura ,
④ the cupola of pleura.
Ⅱ. The Visceral Pleura
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The pleura is reflected from the
mediastinum to the surface of lung, where
it is called visceral pleura covering the
lungs and extending into the fissures of
lung.
Below the root of lung the mediastinal
pleura extends as a double layer to the
mediastinal surface of lung. This double
layer is called the pulmonary ligament.
Ⅲ. The Pleural Cavity and
Recesses
Ⅰ) The Pleural Cavity
 The visceral and parietal layers are
continuous at the hilus of the lung.
between the two layers of the pleura is an
extremely narrow pleural cavity, which is
filled with pleural fluid. The pleural fluid
is secreted by the pleura, and it acts as a
lubricant to reduce the friction between
the two layers during respiratory
movements.
 The two pleural cavities are separated
from each other by the mediastinum.

Ⅱ) The Pleural Recesses
Costodiaphragmatic recess:
In quiet breathing the inferior border of
the lung does not completely extend the
inferior margin of the pleural reflexion, so
that the costal and diaphragmatic pleurae
are in contact with each other here, the
intervening narrow slit termed the
costodiaphragmatic recess. In quiet
respiration the lower limit of lung is about
5 cm above the lower limit of the pleura.
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Costomediastinal recess:
A similar condition present behind
the sternum, between the costal and
mediastinal pleurae it is termed the
costomediastinal recess.
Ⅳ. The Projection of the Inferior
Margins of Lungs and Pleurae
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On the surface of the body, the
projection of the inferior margins of
the lungs and pleurae are shown in
table 1:
The inferior margins of pleurae is the
costodiaphragmatic lines of reflexion
of the pleurae
Table 1 The inferior margins of
lungs and pleurae
Midclavicular
line
Midaxillary
line
Posterior median
line
Inferior
margin of
lungs
6th fib
8th rib
On the level of T10
spinal process
Inferior
margin of
pleurae
8th rib
10th rib
On the level of T12
spinal process
Section 8 The Mediastinum
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The mediastinum is generally defined as
the interval between the right and left
pleural sacs.
Location:
It extends from the sternum in front to
vertebral column behind, and from the
thoracic inlet above to the diaphragm
below.
Its lateral wall is the mediastinal pleura of
both sides.
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Division:
The mediastium is divided into superior and
inferior mediastina by the line drawn
horizontally from the sternal angle to the lower
border of 4th thoracic vertebra.
The inferior mediastinum is subdivided into an
anterior mediastinum in front of pericardium, a
middle mediastinum containing the
pericardium with heart and great vessels, and the
posterior mediastinum between pericardium
and vertebral column.
The posterior mediastinum extends to the lower
border of the 8th thoracic vertebra, its main
contents are the bronchi, esophagus, vagus and
phrenic nerves, thoracic duct etc.