mechanism of breathing

Description
The External Intercostal Muscles present between the Ribs contract pulling the ribs, by which ribs make an angular movement, expanding the thoracic area.
- At the same time, the diaphragm present below the lungs also contract increasing the thoracic area.

As the thoracic area increases, the negative pressure between the pleural membranes force the lungs (which are elastic in nature) to expand. As a result, the air through the nasal cavity and bronchus (and bronchioles) gushes inside the lungs filling the alveoli with fresh air.

Fresh air (with oxygen) then diffuses into the blood vessels through the alveolar walls. At the same time carbon-di-oxide also diffuses from blood vessels into the alveoli.
All this happens within the time period between inhalation and exhalation.

When we exhale,
- The Diaphragm muscles are relaxed and diaphragm returns to its normal structure. This decreases the thoracic volume.
- The Internal Intercostal Muscles contract, making ribs to return back to there normal position. This also decreases the Thoracic volume.

As the thoracic volume decreases, the lungs are forced contracted. By this the air in alveoli are pushed outside through the bronchioles, bronchus and finally out by nasal cavity.