Quiet Respiration

Quiet Inspiration

The muscular fibres of the diaphragm contract in quiet inspiration. Their action is to pull the central tendon downwards since they are firmly anchored peripherally; this movement increases the vertical dimension and decreases pressure within the thoracic cavity.

The abdominal viscera are pushed down as the diaphragm contracts.

At the same time, the first rib is fixed by the scalene muscles in the neck and the upper intercostal muscles are active.

Comparatively little movement of the upper ribs occurs, but the sternum is displaced anteriorly, increasing the anteroposterior diameter of the thorax. The sternum is moved forwards and upwards during inspiration because of the obliquity of the ribs attached to it.

The lower ribs pivot at their posterior and anterior attachments and swing out, increasing the lateral dimensions of the thorax.

The middle ribs are able to move laterally because of the flexibility of the long inclined costal cartilages which connect them to the sternum. The costal angle between the costal cartilages of the lower ribs on each side increases appreciably during deep inspiration.The movements of the chest wall during respiration are followed by the lungs which, therefore, expand drawing air in or contract to force air out.

The pleural layers play an essential part in this process. The parietal pleura will follow the movements of the thoracic wall as they are directly attached to it. Visceral pleura will follow the movements of the parietal pleura because of the adhesion of the two layers brought about by the thin layer of pleural fluid between them. The pleural fluid enables the pleural layers to slide over each other wherever they are in contact and to resist the opposing forces generated by the movements of the chest wall and the elasticity of the lungs. The lungs will inflate because the visceral pleura are directly attached to the lungs.

Quiet Expiration

Quiet expiration is brought about by passive elastic recoil of the lungs and chest wall. The term ‘passive’ indicates that no muscular effort is required.

When sufficient air has been drawn into the lungs to meet the functional demands, the intercostal muscles and diaphragm relax.