Breathing is a means of maximizing the process of gaseous exchange. The
movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere. Breathing involves two stages such as inspiration and expiration. Inspiration can occur if the pressure within the lungs (Intra-pulmonary pressure) is less than the atmospheric pressure, that means there is a negative pressure in the lungs with respect to atmospheric pressure. Similarly the expiration is takes place when the intra-pulmonary pressure is higher than the atmospheric pressure. The muscular diaphragm and a specialized set of muscles, the external and internal inter-costal muscles help in generating such gradients.
i. Inspiration: Intake of atmospheric air into the lungs is called inspiration. It is an active process , as it takes place by the contraction of the muscles of the diaphragm and the external inter-costal muscles, which extend in between the ribs. The contraction of the diaphragm (phrenic muscles) increases the volume of the thoracic chamber in the antero-posterior axis. The contraction of the external inter-costal muscles lifts up the ribs and sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis. The overall increase in thoracic
volume causes a similar increase in the 'pulmonary volume'. An increase in the pulmonary volume decreases the intra-pulmonary pressure to less than that of the atmosphere, which forces the air from the outside to move into the lungs, that means inspiration.
ii. Expiration: Release of alveolar air to the exterior is called expiration. It is a passive process. Relaxation of the diaphragm and the external inter-costal muscles returns the diaphragm and sternum to their normal positions, and reduces the thoracic volume and thereby the pulmonary volume. This leads to an increase in the intra-pulmonary pressure to slightly above that of the atmospheric pressure is causing the expulsion of the air from the lungs, that means expiration.
NOTE : The contraction of the internal intercostal muscles and the lateral abdominal muscles help in 'forced 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 breaths 12-16 times/minute. The volume of the air involved in breathing movements can be estimated by using the spirometer, which helps in clinical assessment of the pulmonary functions.