Download The Respiratory System Dr.Muna Zuhair Lecture 3 Alveoli: Are sac

The Respiratory System
Dr.Muna Zuhair
Lecture 3
Alveoli:
Are sac like structures (about 200µm in diameter), they are the terminal
portion of the bronchial tree & are responsible for the spongy structure of the
lungs, within these alveoli O2 & CO2 will be exchanged between the blood
& the air & the structure of the alveolar wall will enhance this exchange
greatly.
Generally the alveolar wall lie between two adjacent alveoli called
interalveolar septum, this septum consist of two thin squamous epithelial
layers between which lie capillaries, fibroblasts, elastic & reticular fibers &
also macrophages, these structures lie within a connective tissue stroma
called “interstitium” which contains the most vascular capillary network in
the body.
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The Respiratory System
Dr.Muna Zuhair
Lecture 3
Air in the alveoli is separated from blood in the capillaries by three
components referred to as the blood – air barriers, the total thickness of this
barrier varies from (0.1-1.5) µm. & these are:

cells of the alveolar wall (type I & type II squamous alveolar cells).

basement membrane of the alveolar cells.
 endothelial cells of the pulmonary capillaries.
Inter-alveolar septum:
The interalveolar septum is composed of 5 main cell types:
a) capillary endothelial cells (30%).
b) type I squamous alveolar cells (8 %).
c) type II septal (great alveolar) cells (16 %).
d) alveolar macrophages (10 %).
e) interstitial cells including fibroblasts & mast cells (36%).
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The Respiratory System
Dr.Muna Zuhair
Lecture 3
within the interalveolar septum the anastomosing pulmonary capillaries &
different types of cells are supported by a meshwork of reticular and elastic
fibers and those are regarded as the primary means of structural support to
the alveoli.
1. Capillary endothelial cells:
 These are extremely thin cells, the clustering of the nuclei & other
cellular organelles allows the remaining areas of the cell to become
extremely thin thus increasing the efficacy of gaseous exchange.
 The capillary endothelium is continuous not fenestrated type
 the most prominent feature in these cells is the numerous
pinocytotic vesicles.
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The Respiratory System
Dr.Muna Zuhair
Lecture 3
2. Type I pneumocytes:(squamous alveolar cells)
 These cells will make up to 97% of the alveolar wall & they are
also extremely thin cells.
 all the cellular organelles are grouped around the nucleus reducing
the thickness of the rest of the cell, thus decreasing the thickness of
the blood –air barrier by leaving large areas of the cytoplasm free
of organelles.
 The main function of these cells is to provide a barrier of minimal
thickness through which gaseous exchange take place, the
cytoplasm of these cells contains many pinocytotic vesicles which
may play a role in the turn over of surfactant & in the removal of
small particles on the surface of the alveoli.
3. Type II pneumocytes: (great alveolar, septal cells)
 They are found in between type I pneumocytes, they are cuboidal
in shape and are usually found in groups of 2-3 cells along the
alveolar wall, they are found mainly where the alveolar walls unite
& form an angles.
 These cells divide by mitosis to form another type II pneumocytes
and also to form type I cells but the main function of type II
pneumocytes is the production of surfactant which is a substance
released from the cell produce reduction of the surface tension
thus preventing collapse of the alveoli during expiration &
facilitate expansion during inspiration.The reduction in the surface
tension means that less inspiratory force is needed to inflate the
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The Respiratory System
Dr.Muna Zuhair
Lecture 3
alveoli at the same time alveoli without surfactant would tend to
collapse during expiration.
 Type II pneumocytes are first detectable in the alveolar wall at
about 28 week gestation after this time the premature babies are
capable of respiratory survival, however between 28 week
gestation and term the amount of surfactant may be inadequate
leading to alveolar collapse and type I pneumocyte damage and in
this case some artificial respiratory assistance may be needed and
the condition is known as “respiratory distress syndrome”.
Recently surfactant has also been suggested to have bactericidal
effect aiding in the removal of potentially dangerous bacteria that
reach the alveoli.
4. Alveolar macrophages:
These cells are found within the interalveolar septum and are often seen
on the surface of the alveoli.These cells also called Dust cells they often
contain phagocytosed materials, particularly inhaled carbon particles, they
are also important defense mechanism against inhaled bacteria.after
phagocytosis these cells will enter the respiratory bronchioles or terminal
bronchioles where they are either pass into the lymphatic vessels and then
removed to the regional lymph node or they adhere to the ciliated mucous
coated epithelium which will eventually carried up to the trachea & cleared
by coughing. The phagocytosed debris within these cells was most likely
passed from the alveolar lumen to the interstitium by the pinocytotic activity
of type I pneumocytes.
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The Respiratory System
Dr.Muna Zuhair
Lecture 3
5. Interstitial cells:
The interstitium of the interalveolar septum also contains fibroblasts, mast
cells & other connective tissue cells.The fibroblasts will synthesize collagen
& elastic fibers which are important factors in giving resilience & elasticity
to the lung.
Alveolar pores:
The interalveolar septum contain many pores (10-15 µm. in diameter), they
connect the neighboring alveoli so will equalize air pressure in the alveoli
and also promote the collateral circulation of air when a bronchiole is
obstructed.
Pleura:
Is the serous membrane covering the lung.It consists of 2 layers
parietal & visceral that are continuous at the region of the hilum. Both
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The Respiratory System
Dr.Muna Zuhair
Lecture 3
membranes are composed of mesothelial cells resting on a fine connective
tissue layer that contains collagen and elastic fibers.The parietal & visceral
pleura enclose a cavity that is entirely lined with squamous mesothelial cells
and contains only thin film of liquid that acts as lubricant to facilitate the
smooth sliding of one surface over the other during respiration.
Defense mechanisms:
 the respiratory system has a large surface area that is exposed to both
blood and the external environment so it is very susceptible to
invasion by bacteria and other non-infective agents.
 particles larger then (10 µm.) are retained in the nasal passages,
particles of (2-10 µm.) are trapped by the mucous coated ciliated
epithelium and the cough reflex can eliminate these particles by
expectoration or swallowing. smaller particles are removed by
alveolar macrophages.
 in addition to the above mechanisms we have many lymphoid tissues
containing both B & T lymphocytes are found in association with the
bronchi, this important part of the immune system is called BALT
(bronchus- associated lymphatic tissue).
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