Breathing in by changing the chest's size.

Diaphragm contraction lengthens the chest front-to-back, external intercostals lift ribs and sternum; thoracic volume rises, intra-pulmonary pressure drops below atmospheric and air flows in.

Draw in 500 mL by making intra-pulmonary pressure about 1–2 mm Hg below atmospheric, then expire without contracting any muscle.

The diaphragm relaxes during inspiration; diaphragm and intercostal axes swapped.

• 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.

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