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Pressure changes duringrespiratory cycle
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The pleural sacIt is a thin closed serous sac which invests the lung exceptat the hilum.
The layer which is attached to the outervisceral pleura:
surface of the lung.
The layer which is attached to the chestparietal pleura:
wall, mediastinum and diaphragm .
In between, there is a potential space filled with a thin filmof serous fluid with mucoid characteristics (pleural
fluid).
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Functions of pleural fluid:
1-Lubricant.
2-Adheres the 2 layers of pleura.
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Normally, no air exists in pleural space but rupture of the
lung by a penetrating wound can cause air, fluid, blood
or pus to enter.
If water is collected hydrothorax.
If pus is collected pyothorax.
If blood is collected haemothorax.
If air is collected pneumothorax.
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Pneumothorax
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Pleura and negative pressure
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Intra-pleural pressure (IPP)
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Intra-pleural pressure (IPP):
Definition: It is the pressure between thetwo layers of the pleura. It is always a
negative pressure i.e subatmospheric.
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Intra-pleural pressure (IPP)
Normal values: (During normal quiet breathing)At the beginning of inspiration(-5 cm H2O ) .
At the end of inspiration(-7.5 cm H2O ) .
At the beginning of expiration(-7.5 cm H2O ) .
At the end of expiration(-5 cm H2O ) .
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Intra-pleural pressure (IPP)
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1-Lack of
air in the
pleural
cavity
2- Recoil
forces of
the lung
3-Elastic
properties
of
the chest
wall
Causes of the negative IPP
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Recoil forces of the lungs
1-Elastic recoil force(1/3 of the recoil
forces).
2-Surface tension of the fluid lining
the alveoli(2/3 of the recoil forces) .
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Causes of the negative IPP
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Significance of negative IPP:
1-It maintains the lung inflated and prevents itscollapse.2-It helps expansion of the lung during inspiration.3-It helps venous return.4-It helps lymphatic drainage through thoracicduct.
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Alveolar pressure
(intrapulmonary pressure)
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Alveolar pressure (intrapulmonary pressure):
It is the pressure of air inside the lung alveoli.
When the glottis is opened and no air is flowing
into or out of the lungs, the pressure in all partsof the respiratory tree till the alveoli is equal to
atmospheric pressure which is considered to be
zero cm H2O
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Pressure changes & Airflow
During inspiration:
increased thoracic
volume decreasesPalv
Palv < Patm
Air flows into alveoli
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Pressure changes & Airflow
End of inspiration:
Patm = Palv
No air flow
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Pressure changes & Airflow
During expiration:
Decreased thoracic
volume = increasedpressure inside
alveoli
Palv > PatmAir flows out of lungs
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Pressure changes & Airflow
At end of expiration:
Patm = Palv
No air flow
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Changes in Intra-Alveolar Pressure
During Respiration
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Transpulmonary pressure:
It is the pressure difference between alveolar
pressure and the pleural pressure.
(I Alv P - IPP)
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Transpulmonary pressure
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Compliance
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Compliance is a measure for distensability
of elastic tissue.
It is expressed as the change in lung volume per unit
change in distending pressure.
Compliance =V/P =ml/cmH2O
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Compliance of the lungs (CL): It measures the change in lung volume against
the change in distending pressure.
.
It equals 200 ml/cm H2O pressurei.e. the increase in transpulmonary pressure by 1 cm
H2O, increases the lung volume 200 ml.
(when they are outside the thorax).
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Pulmonary compliance(Total compliance):
Compliance of the lungs and chest wall combinedtogether .
It is much less because lung expansibility in the chest is
limited by the rigid thoracic cage.
It is normally about 130 ml/cm H2O
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Factors affecting lung compliance:
The initial lung volume.
Presence of elastic fibers .
Surface tension of alveolar fluid .
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Causes of decreased lung compliance:
Diseases that cause stiffness of the lung aspulmonaryfibrosis, congestion and oedema.
Collection of air or fluid ( water,blood or pus) in theintra-pleural space.
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Deformities of vertebral column e.g. kyphosis(antro-posterior bending) and scoliosis (lateral
bending).
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Scoliosis
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Skeletal muscle disease as poliomyelitis .
Arthritis of the joints of the thoracic cage or
vertebral column.
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Causes ofincreased compliance:
Old age.
Emphysema.
E h
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Emphysema
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E h
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Emphysema
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Cigarette Smoking
Smoking
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Smoking
Increases pulmonary alveolar macrophages
Release of a chemical substance attractsleukocytes to the lungs.
The leukocytes in turn release proteases as elastase,
which attacks the elastic tissue in the lungs.
Plasma protein 1 -antitrypsin that inactivates
elastase, is inhibited.
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Chronic bronchitis
Irritation of bronchi and bronchioles.Excessive mucous secretion .
Decreased ciliary movements.
Mucous cannot pass out leading to airwayobstruction .
Over inflation of air spaces distal to the terminal
bronchioles.
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Chronic bronchitis
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