This is the process by which the lungs expand to take in air then contract to expel it. The cycle of respiration, which occurs about 15 times per minute, consists of three phases:
Inspiration, Expiration and Pause
Proper breathing involves all the muscles of the head, neck, thorax and abdomen, in addition to the involuntary musculature of the larynx, trachea and bronchi.
The main muscles of respiration in normal quite breathing are the intercostals muscles and diaphragm.
Inspiration
External intercostals muscles
Actively contract
- Ribs and sternum move upwards and outwards
- Width of chest increases from side to side, from front to back and
from top to bottom
Diaphragm contracts
- Descends
- Depth of chest increases
Capacity of thorax is increased
Pressure between pleural surfaces is reduced.
Elastic tissue of lungs is stretched.
Lungs Expand to fill thoracic cavity
Air pressure within alveoli is now less than atmospheric pressure
Air is sucked into alveoli from atmosphere |
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Expiration
External intercostals muscles relax
- Rib and sternum move downwards and inwards
- Width of chest diminishes
Diaphragm relaxes
- Ascends
- Depth of chest diminishes
Capacity of thorax is decreased
Pressure between pleural surfaces is increased
Elastic tissue of lungs recoils
Air pressure within alveoli is now greater than atmospheric pressure
Air is forced out of alveoli to atmosphere. |
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