Surface Tension

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
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
Lipids and proteins effective in lowering surface tension are
found concentrated in the cell wall
Soaps and bile salts reduce the surface tension of water while
sodium chloride and most inorganic salts increase the surface
Laplace’s law states that the pressure inside an inflated
elastic container with a curved surface, e.g., balloon is
inversely proportional to the radius as long as the wall (of
balloon) tension is presumed to change little.
A common illustration of this phenomenon is that the effort
required to blow up a balloon is greatest when the diameter
of the balloon is least
Surface tension of plasma
Bile salts
Hay’s test
Surface tension of plasma is slightly less than water
Emulsifying action of Bile salts
Bile salts are surface active. They facilitate action of
pancreatic lipase and hence absorption of free fatty acids. Bile
salts lowers surface tension of fat droplets
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
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
Surface tension at the airwater interface prevents
expansion of alveoli
Air taken in by respiration
pushes on the water lining
the alveoli
The normal force due to
surface tension of water
pushes the air out reducing
the size of the alveoli
On the basis of Laplace law, air would be displaced from
the smaller alveolus into the larger one and the smaller
alveolus would thus become still smaller
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
Instability is prevented by surfactant. Amount of
surfactant per alveolus is constant
As an alveolus becomes smaller the surfactant
becomes more concentrated at the surface of
alveolar lining fluid. Hence surface tension becomes
progressively more less. This helps alveolus expand
during inhalation
On the other hand as an alveolus becomes bigger,
the surfactant is spread more thinly on the fluid
surface. This increases the 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.
The absence of surfactant in the alveolar
membrane of some premature infants
causes the respiratory stress syndrome in
Factors reducing production of surfactant or
increasing its rate of destruction may
contribute to adult respiratory distresss
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.
Measurements of lung volume obtained during
the controlled inflation/deflation of a normal
lung show that the volumes obtained during
deflation exceed those during inflation, at a
given pressure.
This difference in inflation and deflation volumes
at a given pressure is called hysteresis and is due
to the air-water surface tension that occurs at
the beginning of inflation.
However, surfactant decreases the alveolar
surface tension to very low, near-zero levels
At the end of inhalation, surfactant
distributes thinly on the surface limiting the
volume of the alveoli
Pulmonary surfactant thus greatly reduces
allowing the lung to inflate much more easily,
thereby reducing the work of breathing. It
reduces the pressure difference needed to
allow the lung to inflate.
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
If velocity of layers is same then relative velocity is
zero and the liquid is said to be non viscous
Viscosity of a liquid varies directly with its density
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 and also for breaking
intermolecular H-bonds of associated liquid
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
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
Blood is a suspension of cells in plasma. Plasma is
a water solution of salts and heavily hydrated
The viscosity of blood thus depends on the
viscosity of the plasma, in combination with the
hematocrit (The hematocrit or packed cell volume
(PCV) or erythrocyte volume fraction (EVF) is the
volume percentage (%) of red blood cells in blood.
It is normally about 45% for men and 40% for
Blood cells behave as suspended particles and
increase the viscosity of blood. Fibrinogen due to
its asymmetry and high density imparts maximum
viscosity to the blood
Globulins have greater influence than albumin
Deformability of Erythrocytes
Erythrocyte deformability refers to the cellular
properties of erythrocytes which determine the
degree of shape change under a given level of
applied force
Erythrocytes change their shape extensively under
the influence of applied forces in fluid flow or while
passing through microcirculation.
The extent and geometry of this shape change is
determined by both the mechanical properties of
erythrocytes, the magnitude of the applied forces
and the orientation of erythrocytes with the applied
Erythrocyte deformability is an important
determinant of blood viscosity, hence blood
flow resistance in the vascular system
Deformability of erythrocytes affects viscosity
of blood in small vessels where erythrocytes
are forced to pass through blood vessels with
diameters smaller than their size
In sickle cell anaemia, deformability of
erythrocytes is reduced thereby increasing
viscosity of blood
suspended assumes biconcave
discoid shape indicative of large
excess of its surface area over its
Shape change of erythrocytes
under applied forces is reversible
and the biconcave-discoid shape
is maintained after the removal
of the deforming forces
In other words, erythrocytes
behave like elastic bodies, while
they also resist to shape change
under deforming forces
The viscoelasticity of blood is traceable to the
elastic red blood cells, which occupy about
half the volume.
When the red cells are at rest they tend to
aggregate and stack together
In order for blood to flow freely, the size of
these aggregates must be reduced, which in
turn provides some freedom of internal
motion. The forces that disaggregate the cells
also produce elastic deformation and
orientation of the cells
In Region 1, the cells are in large aggregates
In Region 2, the cells are disaggregated and the applied
forces are forcing the cells to orient. As the shear rate
increases, the applied forces deform the cells
In Region 3, increasing stress deforms the cells, and if
the cells have normal deformability they will form layers
that slide on layers of plasma.
Viscosity of blood also varies with changing haematocrit
When the haematocrit rises, the friction between the
successive layers of blood increases. Hence with increasing
haematocrit the viscosity of the blood rises drastically
This is due to the presence of thermoproteins,
which show physical change s at temperatures
below or above 37 °C
Cryglobulin operates at temperatures below 37 °C
forming reversible or irreversible precipitates,
gels or crystals
Pyroglobulin gets precipitated at 56 °C
Bence-Jones proteins gets precipitated at 40 °C
At 0 °C, the viscosity of blood is increased up to
three times. This reduces the circulation in the
tissues exposed to cold
loss of plasma into the
tissues. This leads to
slowing of blood flow,
disturbance in the axial
formation and increase
in viscosity
increased in diabetes
vomiting and diarrhea
Typical mammalian erythrocytes: (a) seen
from surface; (b) in profile, forming rouleaux
Synovial fluid is highly viscous. Its viscosity
varies from 50 to 200 times that of water. Its
viscosity is due to the presence of hyaluronic
Viscosity of Synovial fluid reflects its hyaluronic
acid content
In inflammatory effusions such as rheumatoid
arthritis etc , the viscosity of synovial fluid is
markedly decreased (down to water like)
indicating the presence of a thin watery fluid
hyaluronidase particles

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