Inspiration and expiration

1. external intercostal muscles contract then ribs and sternum move up and out. Then the width of thorax increases front to back and side to side causing the diaphragm to contract and move down, flattening in the process. 

2. depth of thorax increases top to bottom so the...

   • volume of thorax increases.
   • pressure between the pleural surfaces decreases.
   • lungs expand to fill thoracic cavity.
   • air pressure in alveoli is less than atmospheric pressure.
   • air is forced in by the higher external atmospheric pressure.

As the lungs fill with air the stretch receptors send impulses to the expiratory part of the respiration centre to end breathing in.

1. External intercostal muscles relax so the ribs and sternum move down and in. So the width of thorax decreases front to back and side to side. The diaphragm relaxes and moves up

2. depth of thorax decreases top to bottom. So the ...

   • volume of thorax decreases.
   • pressure between the pleural surfaces increases.
   • lung tissue recoils from sides of thoracic cavity
   • air pressure in alveoli is more than atmospheric pressure.
   • air is forced out.

As the air leaves, the stretch receptors are no longer stimulated. The inhibition of breathing in (via the expiratory part of the centre) stops so breathing in can start again