expiration

• • The external intercostal and diaphragm muscles relax.
  • The ribcage moves downwards and inwards and the diaphragm becomes curved again.
  • The thorax volume decreases, causing the air pressure to increase (to above atmospheric pressure)
  • Air is forced out of the lungs
• Expiration can be forced during this the internal intercostal muscles contract, to pull the ribcage down and in.
• Normal expiration is a passive process- it doesn't require energy