Forced Respiration

 

In deep forced respiration, the capacity of the thorax during inspiration increases by a larger amount by greater movements of the ribs and diaphragm. They may have to move enough to allow inhalation of the maximum amount of air possible for that individual - the vital capacity.

Accessory muscles may assist the respiratory muscles. An accessory muscle is defined as one that normally carries out a particular action, but can be recruited to aid another action.

Examples of accessory respiratory muscles include:

  1. Pectoralis muscle

  2. Scalene Muscle

  3. Sternocleidomastoid Muscle

  4. External Intercostal Muscle

  5. Trapezius Muscle

Forced Inspiration

In forced inspiration, the first rib is raised a little by the contraction of scalene muscles that run from the cervical vertebrae to the first rib.

Contraction of both layers of the intercostal muscles moves the second rib in relation to the first, the third in relation to the second, and so on.

Continued contraction of the diaphragm after the point where it can descend no further causes the lower ribs to move upwards, but principally outwards because of the constraints imposed by the shape of the costovertebral joints.

The sternum is pulled up by sternocleidomastoid muscle running between the mastoid process on the base of the skull and the sternum and clavicle.

Forced Expiration

In forced expiration, the normal elastic recoil of ribs and anterior abdominal wall is reinforced considerably by contraction of the muscles of the abdominal wall acting as accessory muscles of respiration. The abdominal muscles are attached to the lower parts of the thoracic cage above and the pelvic bones below. Their effect is two fold:

  1. They pull down the lower ribs, thus decreasing the lateral and anteroposterior dimensions of the thorax

  2. They compress the abdominal contents, forcing them under the diaphragm which is displaced upwards, thus decreasing the vertical dimensions of the thorax.

In general, forced respiration is still rhythmical, but each cycle is deeper and faster than in quiet respiration.