Class 11 Biology Breathing and Exchange of Gases Mechanism of breathing

Mechanism of breathing

  • Breathing involves two stages :
  • Inspiration- during which atmospheric air is drawn in.
  • Expiration- during which the alveolar air is released out.
  • Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure.
  • Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.
  • The diaphragm and a specialised set of muscles – external and internal intercostals between the ribs.
  • The contraction of external inter-costal muscles lifts up the ribs and the sternum causes an increase in the volume of the thoracic chamber in the dorso-ventral axis.
  • An increase in pulmonary volume decreases the intra-pulmonary pressure to less than the atmospheric pressure which forces the air from outside to move into the lungs.
  • Relaxation of the diaphragm and the inter-costal muscles returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume, which leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs.
  • A healthy human breathes 12-16 times/minute.
  • The volume of air involved in breathing movements can be estimated by using a spirometer.


Fig. Spirometer

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