Mechanism of Breathing

· Breathing involves two stages: Inspiration during which atmospheric air is drawn in and expiration by which the alveolar air is released out.

· The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere.

· Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure.

· Similarly, 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, help in generation of such gradients.

Mechanism of Inspiration:

· 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, i.e., inspiration.

Mechanism of Expiration:

· 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, i.e., expiration.

· We have the ability to increase the strength of inspiration and expiration with the help of additional muscles in the abdomen.

· On an average, a healthy human breathes 12-16 times/minute. The volume of air involved in breathing movements can be estimated by using a spirometer which helps in clinical assessment of pulmonary functions.