Inspiration & Expiration:

) Inspiration is an active process involving the contraction of:
○ The external intercostal muscles; and
○ The diaphragm muscles.
Contraction of the external intercostal muscles pulls the rib cage upwards
and outwards, while contraction of the diaphragm muscles result in the
flattening of the diaphragm.
B)
The net result is an increase in the volume of the thorax (the part of the
body between the neck and the abdomen where the heart and lungs lie).
C)
D) Pressure in the thorax is thus reduced.
Air is then sucked into the lungs, inflating the alveoli until the internal
pressure equals that of the atmosphere.
E)
F) Expiration is a passive process.
During expiration, only the internal intercostal muscles contract. The rib
cage drops, mainly due to its own weight.
G)
The diaphragm relaxes and is forced into a dome shape by the falling rib
cage.
H)
I) The volume of the thorax is decreased.
J) Pressure is then exerted on the air in the lungs, forcing it out.
During forced breathing, such as during exercise or sneezing, expiration
becomes a much more active and forceful process as the ribs are moved
more vigorously downwards and the diaphragm is moved upwards.