breathing

• inspiration:

  • external intercostal muscles contract, internal intercostal muscles relax
  • ribcage moves up and outward, increasing volume of the thorax
  • diaphragm contracts, causing it to flatten, also increasing volume of the thorax
  • increased volume in thoracic cavity so reduced pressure
  • higher pressure outside the thorax than inside
  • air forced into the lungs down a pressure gradient
• expiration:

  • internal intercostal muscles contract
  • external intercostal muscles relax
  • ribcage move downward and inward, reducing volume of the thorax
  • diaphragm relaxes, so it is pushed upward, also reducing volume of the thorax
  • decreased volume in thoracic cavity, so increased pressure
  • higher pressure inside the thorax than outside and so air is forced out of the lungs down a pressure gradient