Mechanism of breathing

There are two phases of breathing: inspiration draws atmospheric air in and expiration releases alveolar air. Air flows in and out along a pressure gradient set up between the lungs and the atmosphere.

Inspiration happens when the pressure inside the lungs (intra-pulmonary pressure) falls below atmospheric pressure; expiration happens when intra-pulmonary pressure rises above atmospheric pressure.

These gradients are produced by the diaphragm together with a specialised set of muscles between the ribs, the external and internal intercostals.

Inspiration starts with contraction of the diaphragm, which increases the volume of the thoracic chamber along the antero-posterior axis.

Contraction of the external intercostal muscles lifts the ribs and sternum, increasing thoracic volume along the dorso-ventral axis.

The rise in thoracic volume increases pulmonary volume, which lowers intra-pulmonary pressure below atmospheric pressure, so air rushes into the lungs.

Expiration: relaxing the diaphragm and intercostals brings the diaphragm and sternum back to their resting positions, thoracic and then pulmonary volume shrink, intra-pulmonary pressure climbs slightly above atmospheric, and air is driven out.

Additional muscles in the abdomen can increase the strength of both inspiration and expiration.

A healthy person breathes about 12–16 times a minute. A spirometer can estimate the volumes of air involved and is useful clinically for assessing lung function.