Mechanism of respiration
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‫السابعة‬
•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.
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.
Inspiration
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.
Forced
Inspiration aided by
ventilation contraction of accessory
muscles such as the
scalenes
and
sternocleidomastoid,
decreases
intraplumonary pressure to
about -20 mm Hg or less.
Expiration
Relaxation
of
the
diaphragm and external
intercostals, plus elastic
recoil of lung decreases
lung volume and increases
intrapulmonary pressure
to about +3 mm Hg.
Expiration,
aided
by
contraction of abdominal
muscles and internal
intercostals
muscles
increases intrapulmonary
pressure to about + 30
mm Hg or more.