Ventilation of the Lungs - revision notes

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Ventilation of the Lungs
Basically, the guide to breathing.
Air is d rawn into the alv eoli when t he a ir press ure i n the
lungs is l ower than t he a tmospheric pre ssure, a nd i s forced
out when p ressure i s h igher t han the a tm ospheric p ressure.
Since the i nside of y our b ody i s more or less a n a irtight
chamber, pressure chang es in the lungs wh en the volu me of the
thorax (your innards, bas ically) increase and decreas e.
Volume o f th e tho rax is inc reased when the rib s move upwards
and ou twards a nd t he d iaph ragm l owers . T hese m ovemen ts are
brought t o you by the c ontrac tion of the di aphragm mu scles and
the exter nal in tercosta l musc les. The volu me is decr eased when
the d iaphra gm r elaxes a nd t he e xter nal i nterc ostal m uscles
relax, mo ve dow nwards a nd inw ards, so you b reath out. Air flow
within th e lung s is tid al, so air goe s in a nd out via the same
route. Conseq uently, t her e is a r esidu al v olume of a ir that
cannot b e e xpel led i n s uch a c ul de s ac s ystem. T his means
that so metime s th e fre sh a ir can be
mixed w ith t he a lread y used a ir a nd
dilute it, oppos ed to re placin g it.
Gaseous e xch ange t ak es place in t he
alveoli, w h ich b asi cally l ooks l ike
little b unc hes o f g rapes o r s ports
bags f ille d w ith f ootballs. O xygen
and Carbo n Diox ide d iffuse in and out
of h ere, w hi ch i f p u lled a part i s
equivalent to t he le ngth of on e whole
tennis court (70 M2
). Oxygen and CO2
diffuse a lo ng a g ra dient h ere. C O2
will move to area s whe re there is less CO2,
Oxygen will move to
places whe re th ere i s less Oxy gen.
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