Diffusion is the process by which particles spread out from areas of higher concentration to lower concentration until evenly distributed. Osmosis is a type of diffusion where the solvent (usually water) moves through a semi-permeable membrane from an area of higher solvent concentration to lower concentration. The rate of diffusion or osmosis depends on factors like concentration gradient, temperature, surface area, and properties of the substances involved. In the body, osmotic pressure and concentration differences across cell membranes and between bodily fluids are important for fluid balance and cell volume regulation.

1. Diffusion
Diffusion is process by witch a gas or substance
in solution expanded, because of motion of its
particles to fill all of the available volume.
The particles (molecules or atoms) of a substance
dissolved in solvent are in continuous random
movement.
There is a net flux of particles from area of high to
area of low concentration.

2. The time required to equilibrium by diffusion is
proportionate to the square of the diffusion
distance.
The value is directly proportionate to cross
sectional area of diffusion and
concentration gradient or chemical gradient
(is the difference in concentration of the diffusing
substance divided by the thickness of the
boundary)

3. .(Fick,s low of diffusion )
Thus
J= -DA ∆c
∆x
J is net rate of diffusion
D is the diffusion coefficient
A is the area
∆c/∆x concentration gradient
The minus sign indicate the direction of diffusion
when movement from high to lower concentration
∆c/∆x is negative so x-DA gives positive
value. Q

4. Osmosis
When substance is dissolved in water, the
concentration of water molecules in the solution
is less than that in pure water
The solution occupies a greater volume than
dose the water alone.
If the solution is placed on one side of
membrane that is permeable to water but not to
solute, and equal volume of water is placed on
the other.

5. water molecules diffuse down their concentration
gradient into the solution.
Osmosis is process of diffusion of solvent
molecules into a region in which there is a higher
concentration of solute to which the membrane is
impermeable.
osmotic pressure of solution is pressure necessary
to prevent solvent migration.

8. Osmotic pressure lowering freezing point
depression and boiling point elevation, depends on
the number rather than the type of particles in
solution.
In ideal solution , osmotic pressure is related to
temperature and the volume.
P=nRT/V
n number of particles
R gas constant
T is absolute temperature
V volume

9. If T is held constant, the osmotic pressure is
proportionate to the number of particles in solution
per unit volume of solution.
The concentration of osmotically active particles is
expressed in osmoles
.
One osmole (osm) equals the gram- molecules
weight of a substance divided by the number of free
moving particles in solution.
Milliosmole (mosm) is 11000 of 1 osm.

10. If a solute is nonionizing compound such as
glucose , the osmotic pressure is a function of the
number of glucose molecules present.
If a solute is ionizes and forms an ideal solution,
each ion is an osmotically active particle. –
NaCl ……… Na+ and Cl- ions so that each mole
in solution would supply 2 osm
One mole of Na2so4 dissociate into …………………..

11. The body fluids are not ideal solutions and
although the dissociation of electrolyte is
complete the number of particles is reduced to
interactions between the ions
1mmol of NaCl per liter in the body fluid
contribute less than 2mosm of osmotically active
particles .

12. The osmolartiy is the number of osmoles per liter
of solution ( plasma)
The osmolality the number of osmoles per
kilogram of solvent
The osmolartiy is affected by the volume of various
solutes in the solution and the temperature while the
osmolality is not .
Osmolality is expressed in milliosmol per liter .

13. Osmolal concentration of plasma : Tonicity
The freezing point of normal human of plasma
averages -0.54 C0 -corresponds to an Osmolal
concentration of 290 mosmL.
All fluid compartment of the body in or nearly in
osmotic equilibrium.
.
The term tonicity is used to described the osmolality
of solution relative to plasma solution
That have the same osmolality as plasma are isotonic
, those with greater osmolality are hypertonic
and with lesser osmolality are hypotonic.

14. 0.9% saline solution is isotonic
5% glucose solution is isotonic when initially
infused intravenously but is metabolized so the net
effect is hypotonic solution.
270mosm in each liter of plasma are contributed by
Na+ and its accompanying anions - Cl- and
HCO-
3
plasma protein less than 2 mosmL

15. Glucose and urea about 5 mosmL but become large
in hyperglycemia or uremia.
The total plasma osmolality is important in
assessing dehydration and overhydaration
Hyperosmolality can cause coma- hyperosmolar
coma.

16. Regulation of cell volume
Cell membrane is flexible , swell when exposed to
extracellular hypotonicity and shrink when exposed to
hypertoncity.
Cell swelling activates channels in cell membrane
increased efflux of K+,, Cl- and small organic solutes.
water follows these osmotically active particles out
of the cell and cell volume return to normal .

17. Nonionic diffusion
For weak acids and bases which pass the cell membranes
in undissociated form , whereas they cross membranes
with difficulty in the ionic form.
If molecules of undissociated substances diffuse from one
side to other of membrane and then dissociate.
there is net movement of the undissociated substances
from one side of membrane to the other.
This occurs in gastrointestinal tract and kidney.