The mechanism of breathing

• Inspiration:

  • external intercostal muscles contract
  • ribs pulled outward and upward
  • diaphragm flattens
  • all of these increase the volume of the thorax which reduces pressure in the lungs
  • atmospheric pressure is now greater than that of the lungs, so air moves out of the lungs
• Expiration:

  • internal intercostal muscles contract, external intercostal muscles relax
  • ribs move downward and inward
  • diaphragm is pushed up
  • decreased volume of the thorax means increase of pressure in the lungs
  • pulmonary pressure if now greater than atmospheric pressure, so air is forced out the lungs