Download Architecture of the Lung

SYMPOSIUM
Architecture of the Lung
Morphology and Function
Harumi Itoh, MD, PhD,* Mizuki Nishino, MD,† and Hiroto Hatabu, MD, PhD†
Abstract: The architecture of the lung is discussed with special focus on lung parenchyma. The lung parenchyma is mainly comprised
of numerous air-containing passages and intervening fine structures,
corresponding to alveolar ductal lumens and alveoli, as well as alveolar septa and small pulmonary vessels occupying 10% of total parenchymal volume. The shapes and relative arrangements of alveolar
ducts and alveoli are discussed in detail, which is followed by a brief
description of bronchial circulation and pulmonary lymphatics.
(J Thorac Imaging 2004;19:221–227)
T
he purpose of this article is to demonstrate the architecture
of the lung with special focus on lung parenchyma. The
term “architecture” simply describes the notion of morphologic and functional correlation.1,2 There are two kinds of lung
structures, parenchymal and non-parenchymal structures. The
lung parenchyma resembles a sponge, and occupies 90% of
total lung volume. Non-parenchymal structures consist of the
bronchial tree, pulmonary vessels, and interlobular septa (Fig.
1). As shown in Figure 1B, the bronchi and pulmonary arteries
run together, alternated by pulmonary veins.
LUNG PARENCHYMA
A closer look at the lung parenchyma reveals numerous
air-containing passages and intervening fine structures. Both
are distributed evenly as seen in the 0.5 mm thick lung slice
shown in Figure 2. The passage and intervening structures correspond to alveolar ductal lumens and alveoli, respectively.
Alveolar septa and small pulmonary vessels occupy 10% of the
total parenchymal volume, that is, the mean density of lung
parenchyma is 0.1g/ml, which corresponds to −900HU of CT
attenuation.3
ALVEOLAR DUCT AND ALVEOLI
An alveolar duct is best viewed in the short axis diameter. Seven or 8 alveoli surround the alveolar ductal lumen.
From the *Department of Radiology, University of Fukui Faculty of Medical
Sciences, Matsuoka-cho, Yoshida-gun, Fukui, Japan; and †Department of
Radiology, Beth Israel Deaconess Medical Center, Boston, MA.
Reprints: Hiroto Hatabu MD, PhD, Department of Radiology, Beth Israel Deaconess Medical Center, 330 Brookline Ave., Boston, MA 02215 (e-mail:
[email protected]).
Copyright © 2004 by Lippincott Williams & Wilkins
J Thorac Imaging • Volume 19, Number 4, October 2004
The interalveolar septum is a thin membrane, and the overall
shape of the alveolus is polyhedral (Fig. 3A). When we look at
the alveolar duct on histology, every alveolar septal membrane
appears as a line (Fig. 3B). The 2-dimensional histologic image shown in Figure 3B is more frequently referenced than the
3-dimensional view shown in Figure 3A, which leads to a lack
of 3-dimensional understanding of lung parenchyma. It is possible to distinguish alveolar ductal lumen, alveolar entrance,
lateral wall of alveolus, and dome of alveolus on both Figures
3A and 3B. The diameter of the ductal lumen is 0.3 mm, and
the mean size of the alveolus is 0.2 mm. The alveolar duct
length is about 1 mm in the long axis. The inner surface of the
alveolar duct is covered by a sheet of alveoli. The shape of each
alveolar entrance is not round but polygonal, like a honeycomb. At higher magnification, a small hole in the alveolar
dome can be seen, which is Kohn pore (Fig. 4).
The photographs of a real honeycomb show the entrance
of each cell as hexagonal in shape (Fig. 5). Mathematically, the
overall shape is ideal for maximum cell volume in a limited
space. The honeycomb structure is composed of a single layer
of alveoli but in the lung parenchyma, the alveoli walls are
double-layered.
When we look at a model of a honeycomb (Fig. 6A),
each cell is derived from a rhombic dodecahedron originally
described by the German astronomer, Johannes Kepler. Note
the imaginary alveolar entrance, lateral wall, and alveolar
dome. Each dome is composed of 3 planes. The important geometric feature of this honeycomb model is that exactly the
same sheet of cells can be formed on the other side holding the
dome in common. The red line in each dome indicates where
the lateral wall stands and extends toward the other side. When
we compare the model with the magnified view of the alveoli,
white lines corresponding to our red lines are seen in the dome
of alveoli (Fig. 6B).
A double-layered alveolar sheet can be demonstrated in
a lateral view, as shown in Figure 7A. Every lateral wall of the
alveolus joins to the apex of the alveolar dome. On histology,
it is emphasized that the double-layered alveolar sheets hold
alveolar domes in common (Fig. 7B). This common histologic
image defines the 2-dimensional architectural unit of lung parenchyma. Now we take a histologic look at the architectural
unit forming a network in the parenchymal space and sur-
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branching is different from that of a bronchiole, as there is no
spur.
Next, we discuss the overall shape of the alveolar duct.
In Figure 9A, the contact radiograph shows alveolar ducts
coated with barium sulfate demonstrating a round radiolucent
area, which corresponds to alveolar ductal lumen, as compared
with the magnified view of the lung specimen. The lumen is
surrounded by polygonal alveoli. Note the characteristic zigzag shaped domes of alveoli and that the overall shape of the
alveolar duct is polygonal. The contact radiograph shows the
overall shape of the similarly sized bronchiole and alveolar
ducts (Fig. 9B). The bronchiole is cylindrical in shape, but the
alveolar duct is polygonal. This implies that the alveolar duct
has an ideal overall shape for peak lung function. In fact, the
histologic image shows the number of alveolar ducts is much
greater than that of the bronchioles (Fig. 9C).
RESPIRATORY BRONCHIOLE
The respiratory bronchiole is called the transitional zone
because part of the bronchiolar wall is replaced by alveoli. The
number of alveoli increases as the respiratory bronchioles
branch out (Fig. 10A). The distance from the respiratory bronchiole to the nearest septal structures of the secondary lobule is
constant. For example, in the case shown in Figure 10B, the
respiratory bronchiole is separated from the pulmonary vein by
lung parenchyma. The distance between the two is maintained
at 2 mm. On histologic examination of the respiratory bronchiole, the bronchiolar wall, which is remote from the pulmonary
artery, is replaced by a sheet of alveoli (Fig. 10C). A close-up
image reveals these alveoli form a double sheet of alveoli
where they abut the recurrent branch of the alveolar duct (Fig.
10D).
ALVEOLAR CAPILLARY BEDS AND VENULES
FIGURE 1. A, Inflated and fixed lung specimens show bronchial tree, pulmonary vessels, and interlobular septa, which are
known as non-parenchymal structures. Lung parenchyma occupies 90% of total lung volume. B, A schematic drawing of
the lung based on a contact radiograph of the specimen is
shown. Note that pulmonary artery and vein run alternatively
in the lung.
rounding the alveolar ductal lumen (Fig. 8A). Every alveolar
duct appears isolated in this image. However, on a 3-dimensional photograph of the lung specimen, the alveolar ducts are
characterized by frequent branching (Fig. 8B). The pattern of
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The important structural component of the interalveolar
septum is the alveolar capillary. The capillary beds are calculated to comprise 50% of the volume of alveolar septum, as
demonstrated in Figure 11A. The alveolar capillary is a dense
network consisting of a number of irregular polygons. According to Weibel, 10% of alveoli come into contact with nonparenchymal structures, such as pulmonary vessels.4 A number of alveoli abut the pulmonary vein (Figs. 11B and 11C).
Such alveoli do not form the usual double-layered sheets of
alveoli. The alveolar dome contiguous to the vessel is a singlefaced alveolar wall because gas diffusion does not occur toward the pulmonary vessel. In contrast, the interalveolar septum is a double-faced alveolar wall. Gas exchange is possible
in both sides of the double-faced alveolar wall. Alveolar capillaries are connected to post- or pre-capillary small vessels
(Fig. 12A). Such small pulmonary vessels occupy part of the
limited interstitial space between the alveolar ducts (Fig. 12B).
© 2004 Lippincott Williams & Wilkins
J Thorac Imaging • Volume 19, Number 4, October 2004
Architecture of the Lung
FIGURE 2. A, Details of lung parenchyma consisting of numerous aircontaining passages and intervening
fine structures, corresponding to alveolar ducts and alveoli. B, A magnified view of Fig. 2A.
FIGURE 3. A, Short axis view of alveolar duct surrounded by 7–8 alveoli. Note a thin membranous interalveolar septum and polyhedronshaped alveolus. B, Histologic image
of alveolar duct showing alveolar
septal membrane as a line.
FIGURE 4. An enlarged view of the alveolar dome shows Kohn
pore as small hole (arrow).
FIGURE 5. Photograph of a real honeycomb. Note the hexagonal shape of the entrance of each cell.
FIGURE 6. A, Model of a honeycomb. Note the exact same sheet of
cells formed on the other side holding the dome in common. B, A magnified view of alveoli showing white
lines similar to the red lines in the
honeycomb model.
© 2004 Lippincott Williams & Wilkins
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J Thorac Imaging • Volume 19, Number 4, October 2004
FIGURE 7. A, A lateral view of a
double-layered alveolar sheet. Note
every lateral wall of the alveolus adjoins the apex of the alveolar dome.
B, Histology confirms that the doublelayered alveolar sheets hold alveolar
domes in common.
FIGURE 8. A, Histologic specimen
shows a network in the parenchymal
space that surrounds the alveolar
ductal lumen. Note that every alveolar duct appears isolated. B, Magnified view of the inflated and fixed
lung specimen showing frequent
branching of the alveolar duct.
FIGURE 9. A, Contact radiograph of
alveolar ducts with barium sulfate.
Note the round radiolucent area corresponding to alveolar ductal lumen
surrounded by polygonal alveoli. B,
Note the cylindrically-shaped bronchioles and polygonal shape of the
alveolar duct. C, On histology, a
greater number of alveolar ducts are
shown compared with those seen in
a bronchiole.
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J Thorac Imaging • Volume 19, Number 4, October 2004
Architecture of the Lung
FIGURE 10. A, Contact radiograph
of respiratory bronchiole with
barium sulfate, showing an increase
in the number of alveoli as the respiratory bronchiole branches out. B,
Note the constant distance from respiratory bronchiole to the nearest
septal structures of the secondary
lobule. C, Histology of the respiratory bronchiole. D, Magnified view of
the respiratory bronchiole demonstrating the double sheet of alveoli
abutting the recurrent branch of alveolar duct.
Typically, small pulmonary vessels are located in the corner
where 4 alveolar ducts gather (Fig. 12C). This corner is called
a ridge in solid geometry and is ideal for blood vessel distribution. However, we do not know the specific details of how the
arteriole and venule are arranged in lung parenchyma. For this
purpose, radiologic analysis combined with 3-dimensional reconstructions may be necessary. The rough arrangement of the
arteriole and venule within the secondary lobule is shown in
Figure 12D. However, the size of these vessels is still too large
to study at an alveolar ductal level.
FIGURE 11. A, Micrograph demonstrating a dense network of alveolar
capillaries consisting of a number of
irregular polygons. B, Magnified
view of the lung parenchyma shows
a number of alveoli abutting the pulmonary vein (arrow). C, Histologic
specimen shows contact between alveoli and pulmonary vein.
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Itoh et al
FIGURE 12. A, Micrograph shows
connection between alveolar capillaries and post- or pre- capillary
small vessels. B, Magnified view of
the lung shows small pulmonary
vessels occupying part of the limited
interstitial space between alveolar
ducts. C, Histology shows typical location of small pulmonary vessel. D,
The contact radiograph shows
rough arrangement of arteriole and
venule within the secondary lobule.
FIGURE 13. A, Macroscopic specimen of the left lower lobe inflated
with air. B, 3D CT of the specimen.
Note the rich network pattern especially in the lower portion. C, HRCT
shows subpleural structure as thin
lines along visceral pleura. D, Histology confirms subpleural lymphatics.
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J Thorac Imaging • Volume 19, Number 4, October 2004
BRONCHIAL CIRCULATION
There is a rich blood supply from bronchial arteries in
the bronchi and hilar lymph nodes as well as communicating
vessels between the pulmonary vein and the bronchial venous
plexus.5 Bronchial veins are located around a bronchoarterial
sheath, which communicates directly with the adjacent pulmonary vein. The pulmonary vein gives off a small branch to the
neighboring airways. This special route may be responsible for
peribronchial cuffing seen in the abnormal condition where
pulmonary venous pressure elevates.
PULMONARY LYMPHATICS
Subpleural lymphatic structures are sandwiched between air and lung parenchyma. As shown in Figure 13, 3-dimensional CT shows these structures to be a rich network, especially in the lower portion of the specimen. On HRCT, they
appear as thin lines along the visceral pleura (Fig. 13C). Finally, these subpleural lymphatics were proved on histology
(Fig. 13D).
CONCLUSIONS
Knowledge of the architecture of lung parenchyma is essential for understanding the morphologic-functional relation-
© 2004 Lippincott Williams & Wilkins
Architecture of the Lung
ship of the lung to elucidate the gas exchange process. The
lung parenchyma is mainly comprised of numerous aircontaining passages and intervening fine structures, corresponding to alveolar ductal lumens and alveoli, whose shapes
and relative arrangements in 3 dimensions were discussed in
detail.
ACKNOWLEDGMENT
The authors thank Ms. Donna Wolfe, Mr. Michael Larson, and Mr. Ronald J. Kukla for their assistance in manuscript
preparation.
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5. Murata K, Itoh H, Todo G, et al. Bronchial venous plexus and its communication with pulmonary circulation. Invest Radiol. 1986;21:24–30.
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