Importance of Osmosis and
Osmotic Pressure
LECTURE 1
12-3-2012
Osmotic pressure
 The pressure of a solution which
just stops osmosis is the osmotic
pressure
 Osmotic pressure is the excess of
the pressure required to equalize
water activities in the two
compartments
 The term osmotic pressure is used
for a solution and not for a solvent
 All non penetrable solutes in a
solution exerts oncotic pressure
Factors on which osmotic pressure depends
Van’t Hoffs Equation
 According to Van’t Hoff, osmotic pressure (π) depends on the molar
concentration (C) of the solution and the temperature T
 Where
and R is the gas constant
 The osmotic pressure doesn’t depend on the nature of the dissolved
substance; it depends only on the concentration, temperature and
molecule weight.
 Osmotic pressure is higher when concentration difference is higher or
temperature is higher and the molecular weight is lower
Osmolarity/Osmolality
 Osmotic pressure depends mainly on the molar concentration or
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molarity of a solution
To describe the total number of osmotically active particles per litre of
solution term osmolarity is used
Molecules like glucose do not dissociate in water, and only contribute
one particle per molecule. NaCl does dissociate in water (into Na+ and
Cl-), so there are two particles per molecule in solution.
OsM of 1 M glucose =1 OsM
OsM of 1 M NaCl = 2 OsM
So, two solutions can have the same molarity but different osmolarities.
The higher the osmolarity, the greater the osmotic pressure of the
solution.
The osmotic pressure of an ionic solution is
where
I is the numberof ions formed by dissociation per molecule
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
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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 plasma usually tends to pull water into the
circulatory system
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
 In addition albumin binds with Cl- ions which causes attraction and
retention of cations in the vascular compartments and repulsion of
diffusible anions
 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 and reabsorption process is the tissue fluid
formation.
The
balancing
of
filtration/ultrafiltration
and
reabsorption/osmosis at arterial and venous end of capillary is referred
to as the Starling Equilibrium
 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, and eventually ends
up rejoining the blood
Tissue Fluid Formation
Tissue Fluid Formation
Edema
 If the ultrafiltration is excessive, the volume of interstitial fluid
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increases. When it becomes clinically detectable, it is called edema
Venous obstruction, erect posture and 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
Fluid reabsorption into capillaries increases after haemorrhage. Such
response helps to restore blood volume
Excessive reabsorption also occurs during decreased venous pressure
and dehydration
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
ans 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 if 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
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