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Department of Basic Science/ College of Dentistry/ University of Baghdad
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General Human Histology
Lecture 6 Assist. Prof. Ahmed Anwar Albir
Respiratory System
2- The Respiratory Portion of the Respiratory System
Respiratory Bronchiole
Each terminal bronchiole (Figure 14) subdivides into 2 or
more respiratory bronchiole that serve as regions of transition
between the conducting and respiratory portions of the respiratory
system (Figure 15). The respiratory bronchiolar mucosa is structurally
identical to that of the terminal bronchioles, except that their walls
are interrupted by numerous saclike alveoli where gas exchange
occurs. Portions of the respiratory bronchioles are lined with ciliated
cuboidal epithelial cells and Clara cells, but at the rim of the alveolar
openings the bronchiolar epithelium becomes continuous with the
squamous alveolar lining cells (type I alveolar cells). Proceeding
distally along these bronchioles, the alveoli increase greatly in
number, and the distance between them is markedly reduced. Between
alveoli, the bronchiolar epithelium consists of ciliated cuboidal
epithelium; however, the cilia may be absent in more distal portions.
Smooth muscle and elastic connective tissue lie beneath the epithelium
of respiratory bronchioles (Figures 15 and 16).
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Figure 15. Photomicrograph of a thick section of lung showing
a terminal bronchiole dividing into 2 respiratory bronchioles, in
which alveoli appear. The Spongelike appearance of the lung is due
to the abundance of alveoli and alveolar sacs.
Figure 16. Respiratory bronchiole.
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Alveolar Ducts
Proceeding distally along the respiratory bronchioles, the number
of alveolar openings into the bronchiolar wall becomes ever greater
until the wall consists of nothing else, and the tube is now called
an alveolar duct (Figure 17). Both the alveolar ducts and the alveoli
(Figures 13 and 18) are lined with extremely attenuated squamous
alveolar cells. In the lamina propria surrounding the rim of the alveoli
is a network of smooth muscle cells. These sphincterlike smooth
muscle bundles appear as knobs between adjacent alveoli. Smooth
muscle disappears at the distal ends of alveolar ducts. A rich matrix
of elastic and collagen fibers provides the only support of the duct and
its alveoli.
Alveolar ducts open into atria that communicate with alveolar
sacs, 2 or more of which a rise from each atrium. Elastic and reticular
fibers form a complex network encircling the openings of atria, alveolar
sacs, and alveoli. The elastic fibers enable the alveoli to expand with
inspiration and to contract passively with expiration. The reticular
fibers serve as a support that prevents overdistention and damage to
the delicate capillaries and thin alveolar septa.
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Figure 17. Diagram of a portion of the bronchial tree. Note that the
smooth muscle in the alveolar duct disappears in the alveoli.
Figure 18. Alveolar walls (interalveolar septa)
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Alveoli
Alveoli are saclike evaginations (about 200µm in diameter) of
the respiratory bronchioles, alveolar ducts, and alveolar sacs. Alveoli
are responsible for the spongy structure of the lungs (Figure 15).
Structurally, alveoli resemble small pockets that are open on one
side, similar to the honeycombs of a beehive. Within these cuplike
structures, O2 and CO2 are exchanged between the air and the blood.
The structure of the alveolar walls is specialized for enhancing
diffusion between the external and internal environments. Generally,
each wall lies between 2 neighboring alveoli and is therefore called
an interalveolar septum or wall. An interalveolar septum (Figures 19
through 20) consists of 2 thin squamous epithelial layers between
which lie capillaries, elastic and reticular fibers, and connective tissue
matrix and cells. The capillaries and connective tissue constitute
the interstitium.
Within the interstitium of the interalveolar septum is found
the richest capillary network in the body.
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Figure 19. Three-dimensional schematic diagram of pulmonary
alveoli showing the structure of the interalveolar septum. Note the
capillaries, connective tissue, and macrophages. These cells can
also be seen in-or passing into-the alveolar lumen. Alveolar pores
are numerous. Type II cells are identified by their abundant
apical microvilli. The alveoli are lined with a continuous epithelial
layer of type I cells.
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Figure 20. Section of a lung fixed by intra-alveolar injection of
fixative. Observe in the interalveolar septum 3-laminar structures
(arrowheads) constituted by a central basement membrane and
2 very thin cytoplasmic layers. These layers are formed by the
cytoplasm of epithelial cell type I and the cytoplasm of capillary
endothelial cells. High magnification.
Walls of alveoli are composed of two types of cells: type I
pneumocytes and type II pneumocytes.
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1. Type I Pneumocytes
They are extremely attenuated cells that line the alveolar surfaces.
Type I cells composed of simple squamous epithelium (also called type I
alveolar cells and squamous alveolar cells) make up 97% of the alveolar
surfaces.
These cells are so thin (sometimes only 25nm) that the electron
microscope was needed to prove that all alveoli are covered with
an epithelial lining (Figures 19 and 21). Organelles such as the Golgi
complex, endoplasmic reticulum, and mitochondria are grouped
around the nucleus, reducing the thickness of the blood-air barrier
and leaving large areas of cytoplasm virtually free of organelles.
The cytoplasm in the thin portion contains abundant pinocytotic
vesicles, which may play a role in the turnover of surfactant
(described below) and the removal of small particulate contaminants
from the outer surface. In addition to desmosomes, all type I epithelial
cells have occluding junctions that prevent the leakage of tissue
fluid into the alveolar air space (Figure 22). The main role of these
cells is to provide a barrier of minimal thickness that is readily
permeable to gases.
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Figure 21. Electron micrograph of the interalveolar septum. Note the
capillary lumen, alveolar spaces, alveolar type I epithelial cells, fused
basal laminae, and a fibroblast.
Figure 22. Cryofracture preparation showing an occluding junction
between 2 type I epithelial cells of the alveolar lining.
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2. Type II Pneumocytes
They make up the remaining 3% of the alveolar surfaces (also
known as great alveolar cells, septal cells, and type II alveolar cells).
Type II cells are interspersed among the type I alveolar cells with which
they have occluding and desmosomal junctions (Figures 23 and 24).
Type II cells are rounded cells that are usually found in groups of
2 or 3 along the alveolar surface at points where the alveolar walls
unite and form angles. These cells, which rest on the basement
membrane, are part of the epithelium, with the same origin as the
type I cells that line the alveolar walls. They divide by mitosis to
replace their own population and also the type I population. In
histologic sections, they exhibit a characteristic vesicular or foamy
cytoplasm. These vesicles are caused by the presence of lamellar
bodies (Figures 23 and 24) that are preserved and evident in tissue
prepared for electron microscopy. Lamellar bodies, which average
1-2µm in diameter, contain concentric or parallel lamellae limited
by a unit membrane. Histochemical studies show that these bodies,
which contain phospholipids, glycosaminoglycans, and proteins,
are continuously synthesized and released at the apical surface of
the cells. The lamellar bodies give rise to a material that spreads
over the alveolar surfaces, providing an extracellular alveolar
coating, pulmonary surfactant, that lowers alveolar surface tension.
Pulmonary surfactant, synthesized on the rough endoplasmic
reticulum (RER) of type II pneumocytes, is composed primarily
of two phospholipids, dipalmitoyl phosphatidylcholine and
phosphatidylglycerol, and four unique proteins, surfactant proteins
A, B, C, and D. The surfactant is modified in the Golgi apparatus
and is then released from the trans- Glogi network into secretory
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vesicles, known as composite bodies, the immediate precursors of
lamellar bodies. Pulmonary surfactant serves several major functions
in the economy of the lung, but it primarily aids in reducing the
surface tension of the alveolar cells. The reduction of surface tension
means that less inspiratory force is needed to inflate the alveoli, and
thus the work of breathing is reduced. In addition, without surfactant,
alveoli would tend to collapse during expiration. In fetal development,
surfactant appears in the last weeks of gestation and coincides with
the appearance of lamellar bodies in the type II cells.
The surfactant layer is not static but is constantly being turned
over. The lipoproteins are gradually removed from the surface by the
pinocytotic vesicles of the squamous epithelial cells, by macrophages,
and by type II alveolar cells.
Alveolar lining fluids are also removed via the conducting
passages as a result of ciliary activity. As the secretions pass up
through the airways, they combine with bronchial mucus, forming
a bronchoalveolar fluid, which aids in the removal of particulate
and noxious components from the inspired air. The bronchoalveolar
fluid contains several lytic enzymes (e g, lysozyme, collagenase,
β-glucuronidase) that are probably derived from the alveolar
macrophages.
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Figure 23. Secretion of surfactant by a type II cell. Surfactant
is a protein-lipid complex synthesized in the rough endoplasmic
reticulum and stored in the lamellar bodies. It is continuously
secreted by means of exocytosis (arrows) and forms an overlying
monomolecular film of lipid covering an underlying aqueous
hypophase. Occluding junctions around the margins of the
epithelial cells prevent leakage of tissue fluid into the alveolar
lumen.
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Figure 24. Electron micrograph of a type II cell protruding into the
alveolar lumen. Arrows indicate lamellar bodies containing newly
synthesized pulmonary surfactant. RER, rough endoplasmic reticulum;
G, Golgi complex; RF, reticular fibers. Note the microvilli of the type II
cell and the junctional complexes (JC) with the type I epithelial cell.
Blood- Air Barrier
Air in the alveoli is separated from capillary blood by 3
components referred to collectively as the blood-air barrier: the surface
lining and cytoplasm of the alveolar cells; the fused basal laminae of the
closely apposed alveolar and endothelial cells; and the cytoplasm of the
endothelial cells (Figure 25). The total thickness of these layers varies
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from 0.1 to 1.5µm. During inspiration, oxygen-containing air enters the
alveolar spaces of the lung. Because the total surface area of all the
alveoli exceeds 140m2 and the total blood volume in all of the capillaries
at any one time is no more than 140ml, the space available for diffusion
of gases is enormous. Oxygen diffuses through the blood-air barrier
to enter the lumina of the capillaries and binds to the heme portion
of the erythrocyte hemoglobin, forming oxyhemoglobin. CO2 leaves
the blood (liberation of CO2 from H2CO3 is catalyzed by the enzyme
carbonic anhydrase present in erythrocytes), diffuses through the
blood-air barrier into the lumina of the alveolus, and exits the alveolar
spaces as the CO2-rich air is exhaled. The passage of O2 and CO2 across
the blood- air barrier is due to passive diffusion in response to the partial
pressures of these gases within the blood and alveolar lumina.
Figure 25. Portion of the interalveolar septum showing the blood-air
barrier.
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Lung (Alveolar) Macrophages
Alveolar macrophages, also called dust cells, are found in the
interior of the interalveolar septum and are often seen on the surface
of the alveolus (Figure 20).
These cells phagocytose particulate matter, such as dust and
bacteria, and thus maintain a sterile environment within the lungs. These
cells also assist type II pneumocytes in the uptake of surfactant.
Approximately 100 million macrophages migrate to the bronchi each
day and are transported from there by ciliary action to the pharynx to be
eliminated by being swallowed or expectorated. Some alveolar
macrophages, however, reenter the pulmonary interstitium and migrate
into lymph vessels to exit the lungs.
Medical Application
In congestive heart failure, the lungs become congested with blood,
and erythrocytes pass into the alveoli, where they are phagocytized by
alveolar macrophages. In such cases, these macrophages are called heart
failure cells when present in the lung and sputum; they are identified by
a positive histochemial reaction for iron pigment (hemosiderin).
Alveolar Pores
The interalveolar septum contains pores, 10-15µm in diameter, that
connect neighboring alveoli (Figure 19). These pores equalize air
pressure in the alveoli and promote the collateral circulation of air when
a bronchiole is obstructed.
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Alveolar- Lining Regeneration
Inhalation of NO2 destroys most of the cells lining the alveoli
(type 1 and type II cells). The action of this compound or other toxic
substances with the same effect is followed by an increase in the mitotic
activity of the remaining type II cells. The normal turnover rate of type II
cells is estimated to be 1% per day and results in a continuous renewal of
both its own population and that of type I cells.
Medical Application
Emphysema is a chronic lung disease characterized by
enlargement of the air space distal to the bronchioles, with destruction
of the interalveolar wall. Emphysema usually develops gradually and
results in respiratory insufficiency. The major cause of emphysema is
cigarette smoking. Even moderate emphysema is rare in nonsmokers.
Probably irritation produced by cigarette smoking stimulates the
destruction, or impairs the synthesis, of elastic fibers and other
components of the interalveolar septum.
Gross Structure of the Lungs
The left lung is subdivided into two lobes and the right lung into
three lobes. Each lung has a medial indentation, the hilus, where the
primary bronchi, bronchiolar arteries, and pulmonary arteries enter and
the bronchiolar veins, pulmonary veins, and lymph vessels leave the lung.
This group of vessels and the airway that enter the hilus make up the root
of the lung.
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Each lobe is subdivided into several bronchopulmonary segments
supplies by a tertiary intrapulmonary (segmental) bronchus. In turn,
bronchopulmonary segments are subdivided into many lobules, each
served by a bronchiole. Lobules are separated from one another by
connective tissue septa, in which lymph vessels and tributaries of
pulmonary veins travel. Branches of bronchial and pulmonary arteries
follow bronchioles in their passage through the center of the lobule.
Pulmonary Blood Vessels
Circulation in the lungs includes both nutrient (systemic) and
functional (pulmonary) vessels. Pulmonary arteries and veins represent
the functional circulation. Pulmonary arteries are thin-walled as a result
of the low pressures (25 mm Hg systolic, 5 mm Hg diastolic) encountered
in the pulmonary circuit. Within the lung the pulmonary artery
branches, accompanying the bronchial tree (Figure 26). Its branches are
surrounded by adventitia of the bronchi and bronchioles. At the level
of the alveolar duct, the branches of this artery form a capillary network
in the interalveolar septum and in close contact with the alveolar
epithelium. The lung has the best-developed capillary network in the
body, with capillaries between all alveoli, including those in the
respiratory bronchioles.
Venules that originate in the capillary network are found singly in
the parenchyma, somewhat removed from the airways; they are supported
by a thin covering of connective tissue and enter the interlobular septum
(Figure 26). After veins leave a lobule, they follow the bronchial tree
toward the hilum.
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Nutrient vessels follow the bronchial tree and distribute blood to
most of the lung up to the respiratory bronchioles, at which point they
anastomose with small branches of the pulmonary artery.
Figure 26. Blood and lymph circulation in a pulmonary lobule.
Both vessels and bronchi are enlarged out of proportion in this
drawing. In the interlobular septum, only one vein (on the left) and
one lymphatic vessel (on the right) are shown, although both actually
coexist in both regions. At the lower left, an enlargement of the
pleura shows its mesothelial lining.
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Pulmonary Lymphatic Vessels
The Lymphatic vessels (Figure 26) follow the bronchi and
the pulmonary vessels; they are also found in the interlobular
septum, and they all drain into lymph nodes in the region of the
hilum. This lymphatic network is called the deep network to
distinguish it from the superficial network, which includes the
lymphatic vessels in the visceral pleura. The lymphatic vessels of the
superficial network drain toward the hilum. They either follow the
entire length of the pleura or penetrate the lung tissue via the interlobular
septum.
Lymphatic vessels are not found in the terminal portions of the
bronchial tree or beyond the alveolar ducts.
Nerves
Both parasympathetic and sympathetic efferent fibers innervate
the lungs; general visceral afferent fibers, carrying poorly localized
pain sensations, are also present. Most of the nerves are found in the
connective tissues surrounding the larger airways.
Pleura
The pleura (Figure 26) is the serous membrane covering the
lung. It consists of 2 layers, parietal and visceral, that are continuous in
the region of the hilum. Both membranes are composed of mesothelial
cells resting on a fine connective tissue layer that contains collagen and
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elastic fibers. The elastic fibers of the visceral pleura are continuous with
those of the pulmonary parenchyma.
The parietal and visceral layers define a cavity entirely lined
with squamous mesothelial cells. Under normal conditions, this pleural
cavity contains only a film of liquid that acts as a lubricant, facilitating
the smooth sliding of one surface over the other during respiratory
movements.
In certain pathologic states, the pleural cavity can become
a real cavity, containing liquid or air. The walls of the pleural cavity,
like all serosal cavities (peritoneal and pericardial), are quite
permeable to water and other substances-hence the high frequency of
fluid accumulation (pleural effusion) in this cavity in pathologic
conditions. This fluid is derived from the blood plasma by exudation.
Conversely, under certain conditions, liquids or gases in the pleural
cavity can be rapidly absorbed.