ventilation

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