pressure changes 2011 2003

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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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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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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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End of inspiration:

Patm = Palv

No air flow


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During expiration:

Decreased thoracic

volume = increasedpressure inside

alveoli

Palv > PatmAir flows out of lungs


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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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pressure changes 2011 2003

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