Answer» - 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 (intrapulmonary 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.
- Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the anteroposterior 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.
- 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 .
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