Nursing the Nurse

Diffusion 
a. Diffusion is the process whereby a solute
(substance that is dissolved) may spread through a
solution or solvent (solution in which the solute is
dissolved).

b. Diffusion of a solute will spread the molecules
from an area of higher concentration to an area of
lower concentration.

c. A permeable membrane will allow substances to
pass through it without restriction.

d. A selectively permeable membrane will allow
some solutes to pass through without restriction
but will prevent other solutes from passing freely.

e. Diffusion occurs within fluid compartments and
from one compartment to another if the barrier
between the compartments is permeable to the
diffusing substances.



Osmosis 
a. Osmotic pressure is the force that draws the
solvent from a less concentrated solute through a
selectively permeable membrane into a more
concentrated solute, thus tending to equalize the
concentration of the solvent.

b. If a membrane is permeable to water but not to all
the solutes present, the membrane is a selective or
semipermeable membrane.

c. Osmosis is the movement of solvent molecules
across a membrane in response to a concentration
gradient, usually from a solution of lower to one of
higher solute concentration.

d. When a more concentrated solution is on one side
of a selectively permeable membrane and a less
concentrated solution is on the other side, a pull
called osmotic pressure draws the water through
the membrane to the more concentrated side or the
side with more solute.



Filtration 
a. Filtration is the movement of solutes and solvents
by hydrostatic pressure.

b. The movement is from an area of higher pressure
to an area of lower pressure

.

Hydrostatic pressure 
a. Hydrostatic pressure is the force exerted by the
weight of a solution.

b. When a difference exists in the hydrostatic
pressure on two sides of a membrane, water and

diffusible solutes move out of the solution that has
the higher hydrostatic pressure by the process of
filtration.

c. At the arterial end of the capillary, the hydrostatic
pressure is higher than the osmotic pressure;
therefore, fluids and diffusible solutes move out of
the capillary.

d. At the venous end, the osmotic pressure or pull is
higher than the hydrostatic pressure, and fluids and
some solutes move into the capillary.

e. The excess fluid and solutes remaining in the
interstitial spaces are returned to the intravascular
compartment by the lymph channels



Osmolality 
a. Osmolality refers to the number of osmotically
active particles/kilogram of water; it is the
concentration of a solution.

b. In the body, osmotic pressure is measured in
milliosmoles (mOsm).

c. The normal osmolality of plasma is 270 to 300
milliosmoles/kilogram (mOsm/kg) water.