Osmosis
Dr Asma Jabeen
Assistant Professor
Physiology
Learning Objectives
 Define osmosis, osmolarity and osmotic pressure
 Discuss the calculation of osmolarity and its relation
to molarity
 Discuss the mechanism of colloidal osmotic pressure
of plasma
 Describe the clinical significance of plasma colloidal
osmotic pressure
Osmosis

The process of net movement of water
(solvent) caused by concentration
difference of water is called OSMOSIS
OSMOSIS
Osmotic Pressure
When pressure is applied to the sodium chloride
solution, osmosis of water into this solution would be
slowed, stopped, or even reversed. The exact amount
of pressure required to stop osmosis is called the
Osmotic pressure of the sodium chloride solution.
Osmotic Pressure.....cont’d
Osmosis of water from
chamber B into A causes
the level of fluid columns
to become farther and
farther apart, until
eventually a pressure
difference develops
between the two sides
becomes great enough to
oppose the osmotic effect
Osmotic Pressure depends onConcentration
 Temperature
 Molecule weight.
 Osmotic pressure is higher when
concentration difference is higher or
temperature is higher & the molecular
weight is lower.

Importance of Osmosis & Osmotic
Pressure.
Oncotic pressure of blood plasma.
 Formation of tissue fluid.
 Regulation of cell volume.

Conditions-Osmosis is Effective
Isotonic solution-The concentration of
dissolved substances in the solution is the
same as the concentration of dissolved
substances inside the cell.
 Hypotonic solution-The concentration is
less outside than inside the cell of
dissolved substances-so water will enter a
cell –causing the cell to swell-Prone for
lysis.


Hypertonic solution-The concentration
is more outside than inside the cell of the
dissolved substances-so water leaves a
cell by osmosis causing the cell to shrinkLysis.
The osmole:
To express the concentration of a solution in terms
of number of particles, the unit called osmole is used in
place of grams.
1 osmole is one gram molecular weight of osmotically
active solute.
1 gram MW of glucose
-- 1 osmole
1 gram MW of sodium chloride -- 2 osmoles
Osmolarity is osmolar concentration of solution
expressed in osmoles per liter of solution.
Osmolality is osmolar concentration of solution
expressed in osmoles per kg of solvent.
 Although , it is osmole/kg of water that determines
osmotic pressure , for dilute solutions such as in body
The quantitative differences between osmolarity and
osmolality are less than 1 percent.
Calculation of osmolarity
According to the international SI unit use the following
equation :
Calculated osmolarity = 2 Na + Glucose + Urea ( all in
mmol/L).
or
Calculated osmolarity = 2 Na + 2 K + Glucose + Urea
( all in mmol/L).
Calculation of osmolarity and
Its relation with molarity
Molarity is a number of moles that are dissolved in 1 L
of solution.
Where is Osmolarity is the molarity multyplied by the
number of particles that are produced in the solution.
1 g of NaCl is dissolved in 1L of water
What is molarity and osmolarity
Molarity of Na (23)+Cl (34.5)= 57.5g
1g/57.5g=0.017 moles/L
gm molecular wt of
NaCl-salt
Osmolarity=o.017X 2 (number of particles )
=0.034 osmoles/L
Effects of Isotonic, Hypertonic and Hypotonic solutions
Plasma colloid Osmotic Pressure
Proteins are the only dissolved constituents in the
plasma and interstitial fluids that do not readily pass
through the capillary pores. They are responsible for
the osmotic pressure on two sides of the capillary
membrane. This pressure is called Colloid osmotic
Pressure OR Oncotic pressure
The colloid osmotic pressure of normal human
Plasma is 28 mmHg.
Plasma colloid Osmotic Pressure…….cont’d
Albumin
4.5g/dl
21.8 mmHg
Globulin
2.5g/dl
6 mmHg
Fibrinogen 0.3g/dl
0.2 mmHg
Total
7.3 g/dl
28mmHg
 80% of this pressure is exerted by serum albumin.
Clinical significance of plasma Colloid
Osmotic pressure
 Normal concentration of proteins in the plasma
maintain normal plasma colloid osmotic pressure .
 If plasma colloid pressure is less than normal, fluid
will leak out of the capillary membrane resulting
in the condition called Edema.