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.