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The expansion of the lungs during inspiration follows the enlargement of the thoracic cavity in all its diameters by the contraction of the respiratory muscles.
These are mentioned below:
1. Diaphragm:
A dome-shaped muscle separating the abdominal wall from the thoracic cavity, with vertically running fibres at the periphery and horizontally running fibres at the centre ending in the central tendon. It is innervated by phrenic nerves (C3, C4 and C5). The muscle is attached behind to fixed structures like vertebrae and in front to sternum and to mobile structures like the lower ribs at the sides.
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Inspiration commences by contraction of the diaphragm-the central tendon of which moves downwards increasing the vertical diameter of the chest. At the same time the lower ribs are moved somewhat upwards increasing the lateral and anteroposterior diameter of the chest.
The descent of diaphragm is accompanied by displacement of abdominal contents downwards associated with relaxation of the abdominal muscles. It has been estimated that during quiet breathing the diaphragm is displaced downwards by about 1.5 cm and the area of the diaphragm being 350 sq cm the whole volume of inspired air during a single quiet breathing 500 ml can be accommodated by downward movement of the diaphragm only (Fig. 8.6).
During expiration the diaphragm relaxes, its dome moves upwards aided by contraction of the abdominal muscles pushing the diaphragm upwards. This results in expulsion of air from the lungs during expiration (Fig 8.7).
2. Intercostal Muscles:
The normal breathing is predominantly diaphragmatic. The relative proportions between diaphragmatic and intercostal contribution perhaps vary with different age groups and among different individuals. For example, the infant’s ribs are commonly horizontal. Both the external and inter-coastal groups of muscle fibres originate and insert on adjacent ribs.
The muscle fibres of the external intercostal group run forwards and downwards and those of the internal intercostal group run backwards and downwards from rib to rib (Fig. 8.8). The cephalid motion of the ribs indicates that the upper ribs are fixed relative to the lower ones.
When the contraction of the abdominal muscle fibres fixes the lower ribs during cough or forced inspiration the rib cage moves downwards. These intercostal muscle fibres are innervated by intercostal nerves (T1 to T11).
Electromyographic studies depict that the intercostal muscles become active during quiet inspiration, forced inspiration and forced expiration. Normal quiet expiration is due to passive relaxation of the intercostal muscles and is not attended with any electrical variation.
3. Motion of the Ribs:
The ribs are almost semicircular bones which articulate with the vertebrae at one end and with the sternum or costal cartilages at the other.
i. During inspiration the sternum is elevated and pushed forwards due to the elevation of the shaft of the ribs of which the sternal ends are lower than the vertebral ends during expiration. This is caused due to the contraction of internal intercostal muscles and results in an increase in the anteroposterior diameter of the chest. Depression of the shaft of the ribs during expiration causes the opposite effect (Fig. 8.9).
ii. Rib motion includes the rotation of the ribs around an axis joining two points of attachment at the vertebrae and the sternum (bucket handle motion). This rotation causes an increase in transverse diameter of the thorax. The changes in position of the diaphragm and thorax during inspiration and expiration are represented diagrammatically in the model as in Fig. 8.10.
During expiration both intercostal muscles which are responsible for movement of the ribs and sternum described above relax, the size of the thorax decreases in anteroposterior and in transverse diameter the lung volume consequently decreases.
The rib motion is not equal throughout the thorax. The upper ribs are more nearly horizontal than the lower ribs and are arcs of smaller circles. In inspiration the lower ribs are affected by displacement of all the superior ribs.
Their motion is somehow, the sum of the narrowing of many interspaces rather than only the adjacent one. Due to such differences, the changes in the lateral diameter of the thorax are greater in the lower chest than in the upper chest.
The contribution of costal motion to breathing depends on the force of intercostal contraction and on the position of the ribs prior to the intercostal contraction.
4. Accessory Muscles of Respiration:
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During forced inspiration and expiration many other muscles besides the diaphragm and intercostal muscles come into play to assist respiration. Most important of this group are the scalene and sternocleidomastoids of the anterior neck muscles which stabilise the first ribs and upper sternum during forced inspiration. The abdominal muscles contract violently during forced expiration and thus act as accessory muscles. During violent respiratory effort even the platysma and all other muscles of the trunk take part in respiration.
