1. Inspiration:
External intercostals muscles
contract during inspiration
Diaphragm contracts (downwards
and flattens)
This pulls the rib cage upwards and
outwards
These actions cause the thoracic
cavity size to increase
This decreases the pressure inside
the thoracic cavity
Gases move from areas of high
pressure to low pressure areas
Therefore oxygen moves from the
atmosphere (higher pressure) into
the lungs (now low in pressure)
2. Expiration
Usually a passive process
As the intercostals muscles relax
the rib cage moves downwards
The diaphragm relaxes and
returns to its dome shape
This decreases the size of the
thoracic cavity
This causes the pressure to
increase in the thoracic cavity
(smaller volume)
Therefore gases move out of the
lungs (high pressure) into the
atmosphere (lower pressure)
This pulls the rib cage down
more quickly and with greater
force
