Transcript Slide 1 - Lectures For UG-5

4.12.12
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When thorax expands during inhalation due
to muscular activity in diaphragm, so do the
lungs and when thorax contracts so do the
lungs during exhalation
This is because of the intrapleural fluid
cohesiveness and transmural pressure
gradient
The water molecules in the intrapleural fluid
resist being pulled apart because they are
polar. This tends to bind the pleural surfaces
together
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The intraalveolar pressure of 760mmHg is
greater than intrapleural pressure of
756mmHg
So a greater pressure is pushing outward
than is pushing inward across the lung wall
Because of this transmural pressure gradient,
the lungs are always forced to follow the
movement of chest wall
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Flow of air into or out of the lungs occurs
because of cyclic changes in intra alveolar
pressure
For air to flow in, intraalveolar pressure must
be less than atmospheric pressure during
inhalation and vice versa for exhalation
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The force with which surface
molecules are held is called
the surface tension of the
liquid
It
is
the
force
acting
perpendicularly inward on the
surface layer of a liquid to pull
its surface molecules towards
the interior of the fluid
It keeps the surface like a
stretched
membrane,
and
hence keeps the contact area
minimum
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Temperature
Solute
Solutes can have different effects on surface
tension depending on their structure:
Little or no effect, for example sugar
Increase surface tension, inorganic salts
Decrease surface tension progressively, alcohols
Decrease surface tension and, once a minimum is
reached, no more effect: surfactants
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According to this principle substances which lower the
surface tension becomes concentrated in the surface layer
whereas substances which increase surface tension are
distributed in the interior of the liquid
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Soaps and bile salts reduce the surface tension of water while
sodium chloride and most inorganic salts increase the surface
tension
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Surface tension of plasma is slightly less than water
Emulsifying action of Bile salts
Bile salts are surface active.. Bile salts lowers surface tension
of fat droplets breaking them into smaller droplets, thereby
increasing surface area. This facilitates action of pancreatic
lipase
Hay’s test
This test is based on the surface tension and is employed for
detecting the presence of bile salts in urine (an indication of
jaundice). If urine contains bile salts, the fine sulfur powder
sprinkled on its surface settles down due to lowering of
surface tension. Fine sulfur continues to float on the surface
if urine does not contain bile salts
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Alveoli can be compared to gas in water, as the alveoli are wet and
surround a central air space. The surface tension acts at the airwater interface and tends to make the alveolus smaller (by
decreasing the surface area of the interface).
Surfactant is a lipoprotein mixture secreted by special surfactants
secreting cells i. e. type II granular pneumocytes present in
alveolar epithelium
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Surface tension at the airwater interface prevents
expansion of alveoli
Air taken in by respiration
pushes on the water lining
the alveoli
The inward directed
collapsing force due to
surface tension of water
pushes the air out reducing
the size of the alveoli
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For an alveolus, Laplace law can be stated as
“The magnitude of inward directed collapsing pressure inside
a bubble is inversely proportional to the radius of the bubble
and directly proportional to the surface tension at the airwater interface”
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Consider two alveoli with same surface tension connected
by the same terminal airway
On the basis of Laplace law, the smaller alveoli has the
higher collapsing pressure and air would be displaced
from the smaller alveolus into the larger one
This process would continue until the smaller alveolus
would collapse entirely while displacing all of its air into
the larger one
This process would lead to instability of alveoli