Mechanism of respiration The movement of air into and out of lungs is brought about by changes in the size of the thoracic cavity, the lungs following these changes passively. These changes takes place by the activity of a group of muscles called respiratory muscles. In normal respiration breathing movement consists of an active inspiration followed by passive expiration. The respiratory muscles: 1- The diaphragm Responsible for about 75% of respiration. المحاضر ة السابعة

During inspiration : the diaphragm contracts, it descends and increases the length of the thoracic cavity. The increase is about 1-5 cm during quite respiration and up to 7 cm during deep breathing. At the same time the abdominal muscles relax, the abdominal contents are passed down and the abdomen bulgs. On expiration : the diaphragm relaxes and the abdominal muscles contract (regain their tone) pushing the abdominal contents and the diaphragm up into the thorax region.

2- Intercostals muscles: During inspiration: The ribs raised by the contraction of the external intercostal and the intercartilaginous parts of the internal intercostals muscles. At the same time the ribs rolate. Since the ribs pass down wards and forward from their articulation with the vertebral columns, these movements produce an increase in the anterior-posterior and transverse diameters of the thorax. The intercostals muscles are supplied with the intercostal nerves arising from the 1-10 thoracic segments.

During expiration: Normal expiration is a passive process. Relaxation of the diaphragm and external intercostal muscles decreases all dimensions of the chest. The volume of the thoracic cavity decreases with increase in the intrapulmonary pressure to about +2 mm Hg leading to pump out of air aided by lung elasticity.

Forced respiration: 1- Forced inspiration In addition to the contractions of the diaphragm and external intercostal muscles, other thoracic muscles become involved during forced inspiration to increase the intra thoracic volume. These muscle called accessory muscles of respiration. Sternocleiomastoid, serratus anterior, scalene muscles, pectoralis minor and latismus dorssi muscles. 2- Forced expiration: muscles of forced expiration (abdominal muscles and intercostal muscles) are able to reduce the thoracic cavity even more, producing larger volumes of expelled air. Forced expiration is an active process.

InspirationExpiration Normal quit breathing Contraction of diaphragm and external intercostals muscles increases the thoracic and lung volume, decreasing intra pulmonary pressure to about 3 mm. Hg. Relaxation of the diaphragm and external intercostals, plus elastic recoil of lung decreases lung volume and increases intrapulmonary pressure to about +3 mm Hg. Forced ventilation Inspiration aided by contraction of accessory muscles such as the scalenes and sternocleidomastoid, decreases intra- plumonary pressure to about -20 mm Hg or less. Expiration, aided by contraction of abdominal muscles and internal intercostals muscles increases intrapulmonary pressure to about + 30 mm Hg or more.