MECHANISM OF BREATHING NEET

17.2 MECHANISM OF BREATHING (NCERT NEET/PMT- 2016,17,05, 11)

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

Breathing involves two stages : 

Inspiration: during which atmospheric air is drawn in. 
  • The diaphragm and a specialised set of muscles – external and internal intercostals between the ribs, help in generation of such gradients. 
  • 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.
  • 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
                                          


Expiration: by which the alveolar air is released out. 
  • Expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure (positive).  
  • 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.
                                             
 On the basis of above text and diagram try to answer the following question

1. What is Intra-Pulmonary Pressure (IPP)? what do you mean by positive and negative IPP?
2. What happens if intra-pulmonary pressure is negative, likewise what will happen if it is positive?
3. What initiates inspiration?
4. If diaphragm contracts then what would be its shape?
5. What is the difference between external and internal inter-costal muscle? which among them contract during inspiration?
6. Contraction in which muscle or structure causes an increase in (a) antero-posterior axis and (b) dorso-ventral axis
7. Have a close look at the above diagram and the labellings.



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