Download Oncotic pressure of blood plasma - Lectures For UG-5

Biological importance of
Osmotic pressure
29.11.12
Osmosis takes place due to difference in hydrostatic pressure of water
and solution separated by a semi permeable membrane (permeable
only to water)
Measurement of Osmotic
Pressure
 Apply an external pressure to the
side containing solution
 The hydrostatic pressure which
just stops osmosis is the osmotic
pressure of the solution
 Osmotic pressure is the excess of
the pressure required to equalize
water activities in the two
compartments
Osmosis when two solutions with different
hydrostatic and osmotic pressure are separated by
a semi permeable membrane
The
difference in net hydrostatic pressures of two solutions
is equal to difference in osmotic pressures of two solutions
 Water is pulled into the solution with
relatively higher pulling tendency
(concentrated solution). Osmosis takes
place
Reverse Osmosis
 Reverse osmosis is a membrane based filtration method that removes
many types of large molecules and ions from solutions by applying
pressure to the solution when it is on one side of a selective membrane.
 If an external pressure is applied on a concentrated solution, this
pressure is distributed evenly throughout the solution
 If the applied pressure is higher than the osmotic pressure water will
flow towards the other side of the membrane leaving solute behind
 This technique is used for purification of water
Reverse Osmosis
Importance of Osmosis and
Osmotic Pressure
 Oncotic pressure of blood plasma
 Formation of tissue fluid
 Regulation of cell volume
Oncotic pressure of blood plasma
 Some 90% by weight of plasma is water and about 8% is
plasma proteins (albumin, globulins, fibrinogens)
 Blood plasma is an aqueous solution containing different
ions (Na+, K+, Ca2+…), small non dissociated molecules
(glucose, amino acids) and proteins- macromolecules
(albumin, globulin etc)
 Each type of molecules contributes with its own osmotic
pressure, the sum representing the
colloid-osmotic
pressure or oncotic pressure of plasma
πplasma = ∑ πmol + ∑ πions+ ∑ πproteins
Oncotic pressure of blood plasma
 Albumin is the major contributor to oncotic
pressure of plasma because it has the lowest
molecular weight of the major plasma proteins and
its concentration is almost double that of globulin
 The total oncotic pressure of an average capillary is
about 28 mmHg with albumin contributing
approximately 22 mmHg of this oncotic pressure.
 Throughout the body, dissolved compounds have
an osmotic pressure.
 Because large plasma proteins cannot easily cross
through the capillary walls, their effect on the
osmotic pressure of the capillary interiors will, to
some extent, balance out the tendency for fluid to
leak out of the capillaries.
 In other words, the oncotic pressure tends to pull
fluid into the capillaries.
Tissue Fluid Formation
 Filtration takes place at the arterial end of capillary
because hydrostatic pressure of blood overcomes
the oncotic pressure of plasma proteins
 Reabsorption takes place at the venous end of
capillary because hydrostatic pressure of blood falls
below the oncotic pressure of plasma proteins
 Net result of this filtration/ultrafiltration and
reabsorption/osmosis at arterial and venous end of
capillary is the tissue fluid formation
Starling Equation
 The Starling equation is an equation
that illustrates the role of hydrostatic
and oncotic forces (the so-called
Starling forces) in the movement of
fluid across capillary membranes
Tissue Fluid Formation
Tissue Fluid Formation
 Removal of tissue fluid
To prevent a build up of tissue fluid
surrounding the cells in the tissue, the
lymphatic system plays a part in the transport of
tissue fluid. Tissue fluid can pass into the
surrounding lymph vessels
Edema
 If the ultrafiltration is excessive, the volume of interstitial
fluid increases. When it becomes clinically detectable, it is
called edema
 Venous obstruction or plasma protein deficiency can lead to
edema
 In conditions where plasma proteins are reduced, e.g. from
being lost in the urine (proteinuria) or from malnutrition,
there will be a reduction in oncotic pressure and an increase
in filtration across the capillary, resulting in excess fluid
buildup in the tissues
Maintenance of Cell Volume
 The determinants of cell volume are
the total number of osmotically active
particles within the cell and the
osmolarity of the extracellular fluid.
 The cell has a considerable quantity
of impermeant solutes i. e. proteins
and organic phosphates whereas the
interstitial fluid is relatively devoid of
these.
 Hence there exists a colloid osmotic
gradient across the cell and this
would draw fluid into the cell. This
effect of cell macromolecules is offset
by the Na+-K+ pump
Maintenance of Cell Volume
 3 positive ions (Na+) are pumped out of the cell (towards
ECF) for every 2 positive ions (K+) pumped into the cell
(towards ICF). This means that there is more positive charges
leaving the cell than entering it.
 As a result, positive charge builds up outside the cell
compared to inside the cell. The difference in charge between
the outside and inside of the cell limits the fluid flow into the
cell. About 90% of the osmotic pressre of extracellular fluid is
due to sodium ions
Maintenance of body fluid osmolality by
Kidney
 Kidney maintains the optimum osmolality of body fluid by regulating
the volume of body fluids
 When water intake is low or when water is lost through diarrhea or
perspiration, the kidney conserves water by producing a small volume
of urine which is hypertonic
 When water intake is high, the kidney excretes a large volume of
hypotonic urine.
 Kidney maintains normal osmolality by regulating excretion of water
and sodium chloride within a narrow range
Surface Tension
 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
Surface Tension
 Water striders use surface tension to walk
on the surface of pond. The surface of the
water behaves like an elastic film: the
insect's feet cause indentations in the
water's surface. Its tiny mass and geometry
of its legs allow it to be supported by the
high surface tension of water
 Formation of drops occurs when a mass of
liquid is stretched. Water adhering to the tap
gains mass until it is stretched to a point
where the surface tension can no longer bind
it to the tap. It then separates and surface
tension forms the drop into a sphere. If a
stream of water were running from the tap,
the stream would break up into drops during
its fall. Gravity stretches the stream, then
surface tension pinches it into spheres
Surface tension at interfaces
 Surface tension at liquid-air interface:
A soap bubble is a thin film of soapy water
enclosing air that forms a hollow sphere. Surface
tension causes a bubble to assume the smallest
surface area to contain a given volume -- resulting
in the spherical shape
 Liquid-solid interface
Beading of rain water on the surface of a waxy
surface, such as a leaf. Water adheres weakly to
wax and strongly to itself, so water clusters into
drops. Surface tension gives them their nearspherical shape, because a sphere has the smallest
possible surface area to volume ratio
 Liquid-Liquid interface
Separation of oil and water (in this case, water and
liquid wax) is caused by a tension in the surface
between dissimilar liquids.
Reference book
 Chapter 3: Human Physiology: From
Cells to Systems By Lauralee Sherwood
 Chapter 1 :An Introduction to Med.
Biophysics by Prakash
 Chapter 10: Biophysics by P. S. Mishra