Work of breathing is the work done by respiratory muscles during breathing to overcome the resistance

in thorax and respiratory tract.


During respiratory processes, inspiration is active process and the expiration is a passive process. So,

during quiet breathing, respiratory muscles perform the work only during inspiration and not during expiration.


During the work of breathing, the energy is utilized to overcome three types of resistance:

1. Airway resistance

2. Elastic resistance of lungs and thorax

3. Nonelastic viscous resistance.

1. Airway Resistance

Airway resistance is the resistance offered to the passage of air through respiratory tract. Resistance

increases during bronchiolar constriction, which in creases the work done by the muscles during breathing.

Work done to overcome the airway resistance is called airway resistance work.

2. Elastic Resistance of Lungs and Thorax

Energy is required to expand lungs and thorax against the elastic force. Work done to overcome this elastic resistance is called compliance work.

3. Non-elastic Viscous Resistance

Energy is also required to overcome the viscosity of lung tissues and tissues of thoracic cage. Work done

to overcome this viscous resistance is called tissue resistance work.

Above factors are explained by the curve that shows the relation between lung volume and pleural


 Increase in Compliance

Compliance increases due to loss of elastic property of lung tissues, which occurs both in physiological and pathological conditions:

1. Physiological condition: Old age

2. Pathological condition: Emphysema.

Decrease in Compliance

Compliance decreases in several pathological conditions such as:

1. Deformities of thorax like kyphosis and scoliosis

2. Fibrotic pleurisy (inflammation of pleura resulting in fibrosis)

3. Paralysis of respiratory muscles

4. Pleural effusion

5. Abnormal thorax such as pneumothorax, hydrothorax, hemothorax and pyothorax



Intraalveolar pressure is the pressure existing in the alveoli of the lungs. It is also known as intrapulmonary pressure.

Normal Values

Normally, intraalveolar pressure is equal to the atmospheric pressure, which is 760 mm Hg. It becomes negative during inspiration and positive during expiration.

Normal values are:

1. During normal inspiration:

–1 mm Hg (760 – 1 = 759 mm Hg)

2. During normal expiration:

+1 mm Hg (760 + 1 = 761 mm Hg)

3. At the end of inspiration and expiration:

Equal to atmospheric pressure (760 mm Hg)

4. During forced inspiration with closed glottis

(Müller maneuver): –80 mm Hg

5. During forced expiration with closed glottis

(Valsalva maneuver): +100 mm Hg.


Intraalveolar pressure is measured by using plethysmograph.

Significance of Intra-alveolar Pressure

1. Intraalveolar pressure causes flow of air in and out of alveoli. During inspiration, the

Intraalveolar pressure becomes negative, so the atmospheric air enters the alveoli. During expiration, intraalveolar pressure becomes positive. So, air is expelled out of alveoli.

2. Intraalveolar pressure also helps in exchange of gases between the alveolar air and the blood.

Transpulmonary Pressure

Transpulmonary pressure is the pressure difference between intraalveolar

pressure and intrapleural pressure. It is the measure of elastic forces in lungs, which is responsible for collapsing tendency of lungs.

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