Describe the mechanism of breathing

The movement of air into and out of the lungs is carried out by creating a pressure

gradient between the lungs and the atmosphere.

- The diaphragm and a specialised set of muscles – external and internal intercostals between the ribs, help in generation of such gradients.

- Breathing involves two stages

(i) Inspiration, during which atmospheric air is drawn into the lungs.

(ii) Expiration during which the alveolar air is released out.

(i) Inspiration

- Inspiration occur when the intra-pulmonary pressure is less than the atmospheric pressure, i.e., the intra-pulmonary pressure is negative pressure in compared to the atmospheric pressure.

- Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero-posterior axis.

- The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.

- The overall increase in the thoracic volume causes a similar increase in

pulmonary volume.

- 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.

(ii) Expiration

- Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.

- 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.

- This leads to an increase in intra-pulmonary pressure to slightly above the atmospheric pressure causing the expulsion of air from the lungs.