Download If we regard the alveoli as spherical bubles, then - Lectures For UG-5

Role of Surfactant in Respiration,
Viscosity and Viscous force
6.12.12
Alveolus
If the alveoli were lined with water alone the
surface tension would be too strong so the alveoli
would collapse
If we regard the alveoli as spherical
bubles, then:
P = inward directed collapsing pressure
T = Surface Tension
r = radius of the buble
According to the law of
LaPlace: the smaller alveoli
(with smaller radius - r) have
a higher tendency to collapse
Importance of Surfactant
• Reduces the tendency of alveoli to
collapse by reducing surface tension
• Reduces work of breathing by increasing
lung compliance
• Prevents pulmonary oedema
• Responsible for alveolar stabilization
Surfactant Increases Lung Compliance
• Compliance is the ability of lungs and
thorax to expand. Lung compliance is
defined as the volume change per unit
of pressure change across the lung.
• Surfactant increases lung compliance
Prevention of Pulmonary Oedema
• Surface tension not only pulls alveolar
wall to the center of alveolus but also
pulls fluid from capillaries into the
interstitial space surrounding the alveolus
and into the alveoli leading to pulmonary
oedema
• Surfactant prevents this phenomenon by
lowering the surface tension
Alveolar Stabilization
Alveolar Stabilization
Surfactant prevent this happening
Alveolar Stability
• In the presence of surfactant, the surface tension
developed in the alveoli is inversely proportional to the
concentration of surfactant per unit area
• Amount of surfactant per alveolus is constant. In the
smaller
alveoli,
surfactant
molecules
become
concentrated at the surface while in a large alveolus
surfactant molecules are scattered over the surface
• Thus in a smaller alveolus the tendency to develop more
pressure due to smaller radius is neutralized by tendency
to develop less pressure due to more concentration of
surfactant per unit area (causing lower surface tension)
• This property of surfactant stabilizes the sizes of the
alveoli, causing the larger alveoli to contract more and
smaller ones to contract less.
Alveolar Stability
Respiratory distress syndrome (RDS)
• Occurs in newborns (especially premature) due
to inadequate formation of surfactant resulting
in elevated alveolar surface tension, increased
work of breathing and inadequate exchange of
gases.
Viscosity
• Viscosity is a property of flowing fluid (liquid/gas) by
virtue of which relative motion between layers in contact
is opposed.
• It is the internal resistance against the free flow of liquid
due to frictional forces between the fluid layers moving
over each other at different velocities
• If velocity of layers is same then relative velocity is zero
and the liquid is said to be non viscous
Factors affecting Viscosity
• Density
Viscosity of a liquid varies directly with its density
• Temperature:
An increase in temperature decreases the viscosity of
liquid. This is due to an increase in the kinetic energy of
molecules for overcoming the resistance due to
intermolecular attractions
• Solute concentration:
Viscosity of a solution is directly proportional to the
solute concentration. Suspended particles cause an
increase in viscosity in proportion to the volume of
suspended material relative to the total volume
Types of Fluid Flow
• Laminar flow:
In Laminar flow, the central axis of flow moves with the highest
velocityand the successive cylindrical laminae moves progressively
more slow as one moves from central axis to the wall
Laminar flow has a parabolic front
• Turbulent flow:
A flow in which the elements of the fluid move irregularly in axial,
radial and circumferential directions. It occurs at different velocities
which fluctuates randomly. The velocity difference across the
container changes erratically
Classification of fluids showing
viscous properties
• Newtonian fluids:
An ideal or Newtonian fluid is one in which the viscosity is
constant and is independent of how rapidly the fluid flows.
Plasma is a Newtonian fluid even at very high protein
concentrations
• Non- Newtonian fluids:
Non-Newtonian fluids change viscosity at variant flow
velocities. Blood is a Non-Newtonian fluid
Viscoelastic fluids:
The fluids which have elastic as well as viscous properties
are called viscoelastic. They have a memory of their
original state i. e. they return to their original state after the
deformation is removed
Viscous force for Newtonian Fluids
• Viscosity is the property of flowing fluid
(liquid/gas) by virtue of which relative motion
between layers in contact is opposed
• Relative motion causes internal friction between
layers in contact. This internal friction is called
viscous force
• For Newtonian fluids, the viscous force is
proportional to the surface area (A) of the layer
and the velocity gradient between layers (in the
direction perpendicular to the layer)
Viscous force and viscosity of Newtonian
Fluids
• η is the constant of proportionality and is the viscosity.
• Negative sign shows that F acts in a direction opposite to
the one in which the layers move
Viscous Force of a Newtonian Fluid
• Relative to the wall, layer closest to it will experience
maximum viscous force and moves with slowest velocity
compared to other layers of fluid.
• Farther the layer of fluid is from wall, faster it moves and
minimum viscous force it experiences
• This implies that F « 1/Δx
• In tubes of larger diameter, role of viscous force is not
significant and only layers closer to the walls will
experience it
• Role of viscous force in fluid flow becomes significant in
tubes with smaller diameter
• Viscous force therefore depends on the radius of the tube
Viscous Force of a Newtonian Fluid
Shear rate of Newtonian Fluids
• When fluid moves along a tube, layers in a
fluid slip over one another and move at
different speeds thereby creating a velocity
gradient in a direction perpendicular to the
wall of the tube. This velocity gradient is
termed the shear rate
• Shear rate is expressed as the change of
velocity between two neighboring fluid layers
divided
by
their
distance
measured
perpendicularly
Reference books
• Chapter 3: Human Physiology: From
Cells to Systems By Lauralee Sherwood
• Chapter 1 :An Introduction to Med.
Biophysics by Prakash
• Chapter 5: Textbook Of Medical
Physiology by Khurana