MECHANICS OF RESPIRATION
• The inferior surfaces of the lungs are attached
to the superior surface of the diaphragm
• When the diaphragm contracts, it increases
the volume of the lungs and causes air to
enter the lungs (inspiration)
• When the diaphragm relaxes, its dome moves
superiorly, reducing the volume of the lungs &
expelling the air out (expiration)
Inspiration
• Requires an increase in the size of the thoracic
cavity (which reduces the pressure inside the
lungs)
• Three dimensions of the cavity can be
modified:
● Vertical
● Transverse (lateral)
● Anteroposte
• Vertical expansion produces the greatest
change in capacity & is brought about by the
contraction of the diaphragm
• When the diaphragm contracts
it drops downward
increasing the vertical dimension of the
thoracic cavity
• In quite breathing: inspiration is brought
about mainly by the action of the diaphragm
• During forced inspiration: many other
muscles that are attached to the ribs may
assist in raising the ribs (e.g., scalenus,
sternocleidomast
• The pectoral muscles can be used if the arms
are fixed
• The function of the intercostals is
controversia
Expiration
• Can be a passive process
• When the diaphragm contracts during
inspiration, it compresses the abdominal
organs, increasing pressure within the
abdominal cavity
• This increased pressure helps push the
