Respiratory Mechanics

The movement of gas in and out of the lungs is a mechanical process, which is dependent on the following factors:

The respiratory muscles and their actions The compliance of the chest wall and the lungs

• The gas flow in the airways

The Respiratory Muscles and their Actions

Respiration can be divided into inspiration and

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Figure RR.7. Movement of the chest wall during inspiration expiration. Inspiration is normally active while expiration is passive. Inspiration

During inspiration the lungs can be expanded with two degrees of freedom (Figure RR.7):

• Displacement of the abdominal contents by contraction of the diaphragm

• Radial expansion of the thoracic cage by the accessory respiratory muscles Diaphragm

This is the principal muscle of breathing, accounting for about 75% of the air that enters the lungs during spontaneous inspiration. Contraction of the diaphragm moves abdominal contents downward and forward during each inspiration. Two-thirds of the diaphragmatic fibres are slow twitch, making it relatively resistant to fatigue.

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Accessory Respiratory Muscles

These comprise the external intercostal and strap muscles (sternocleidomastoid, anterior serrati, scalenes). During quiet breathing their contribution to inspiration is small. They act mainly to stabilize the upper rib cage and preventing indrawing. As respiration deepens, the contribution of these muscles increases by elevating the rib cage and expanding it in the lateral and antero-posterior directions.

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