Mechanism of Breathing – Class 11 | Chapter – 17 | Biology Short Notes Series PDF

Mechanism of Breathing: The process in which air moves in and out of the lungs is known as breathing. This is carried out through various respiratory organs. In other words, breathing is a simple give and take process.,


The air that we breathe in and out of the lungs varies in its pressure. So basically when there is a fall in air pressure the alveolar spaces fall and the air enters the lungs (inspiration) and as the pressure of the alveoli within exceeds the atmospheric pressure, the air is blown from the lungs (expiration). The flow rate of air is in proportion to the magnitude of the pressure difference.

Process in Mechanism of Breathing

  • Inspiration
  • Expiration

Mechanism of Breathing


In the process of inspiration, there would be a contraction of muscles attached to the ribs on the outer side which pulls out the ribs and results in the expansion of the chest cavity.

Later, the diaphragm, contracts, moves downwards and expands the chest cavity resulting in the contraction of the abdominal muscles.

The expansion of the chest cavity produces a partial vacuum which sucks air into the lungs and fills the expanded alveoli.

Mechanism Of Inspiration

  • The process of intake of atmospheric air is known as inspiration. It is an active process.
  • When the volume of the thoracic cavity increases and the air pressure decreases, inspiration takes place.
  • Contraction of external intercostal muscles increases the volume of the thoracic cavity.
  • Contraction of the diaphragm further increases the size of the thoracic activity. Simultaneously, the lungs expand.
  • With the expansion of the lungs, the air pressure inside the lungs decreases.
  • The pressure equalizes and the atmospheric air rushes inside the lungs.


The expiration process is considered once after the gaseous exchange occurs in the lungs and the air is expelled. This expulsion of air is called expiration.

During this process, muscles attached to the ribs contract, the muscles of the diaphragm and the abdomen relax which leads to a decrease in the volume of the chest cavity and increases the pressure of the lungs, causing the air in the lungs to be pushed out through the nose.

Mechanism of Expiration

  • The process of exhaling carbon dioxide is called expiration. It is a passive process.
  • It occurs when the size of the thoracic activity decreases and the air pressure outside increases.
  • Now the external intercostal muscles relax and the internal intercostal muscles contract.
  • As a result, the ribs are pulled inwards and the size of the thoracic cavity is reduced.
  • The diaphragm is relaxed and the lungs get compressed.
  • Consequently, the pressure increases and the air is forced outside.

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By Team Learning Mantras