• Diaphragm contracts and becomes flatter (typically dome-shaped)
  • Digestive organs pushed down
  • (External) intercostal muscles contract
  • Ribcage rises
  • Volume of thorax increases
  • Pressure inside thorax decreases below atmospheric pressure
  • Air is forced into the lungs


  • Diaphragm relaxes
  • Diaphragm pushed up by displaced organs underneath
  • (External) intercostal muscles relax
  • Ribcage falls
  • Volume of thorax decreases
  • Pressure inside thorax increases above atmospheric pressure
  • Air is forced out of the lungs

DiaphragmContracts + flattensRelaxes, becomes dome shaped
Digestive OrgansPushed DownPush Diaphragm up
Intercostal MusclesContractRelax
Chest CavityVolume increasesVolume decreases
ThoraxPressure decreasesPressure increases
AirForced INForced OUT

Exchange Surfaces