Endothelial transport is much
different than epithelial transport.
It also though has
The tight junctions
though can be very tight
such as in the blood-brain barrier
when there’s no movement of fluid,
or in capillaries they sometimes have
fenestrations, pores, or sometimes even clefts
that will allow things
to travel through.
So, endothelial transport is going to
rely less on specific transporters
than epithelial transport does.
You’re going to utilize
things like pressure and
osmolality to move various
solutes and solvents around.
It is primarily a variable associated
with the flux of the substance.
So you will filter
hydrostatic pressure is highly involved,
and osmotic and oncotic pressures.
Remember, osmotic pressures have
to do with the ion differences
and oncotic pressures have to do with
protein differences to draw fluid.
This is an example of how pressure can
move fluid out of a capillary bed.
The higher the amount of pressure,
the more fluid travels through.
Other examples determine about how much the
fenestrations are in terms of their width.
Some will allow more fluid to travel
out and some will allow less.
The other issue that we need to think
about with endothelium versus epithelium
is that we sometimes turn the
membranes a little bit differently.
The terminology used for
the inside surface is
the luminal surface rather
than the apical membrane.
In terms of the outside
is termed the basal surface rather
than the basolateral membrane.
But if you keep those linked
together, you’ll be better off
and able to think about the differences
between epithelial and endothelial surfaces.
Let’s look at how fenestrations
can be regulated
because normally you think of a pore
either being open or being closed,
but you can modulate
this in certain tissues.
The lymphatic is a
great example of this.
So you can have some constriction
and have the pores closed.
And then, after constricting
these, you can open them up.
This then will allow fluid to transport
between the lymphatic circulation.
Then when you have smooth muscle
constriction, you close them up.
Good examples of how you can
modulate these fenestrations.
Now, many tissues you don’t
have the modulatory ability
because either they
are open or closed,
but you do have some regulation
of fenestration widths.
Let’s summarize now the different
ways you’re going to move a solution,
a substance, or a gas across
the endothelial wall.
The first thing you could do is
use something called pinocytosis,
which is the actual pinching
off of a small vesicle
that contains a
solute and solvent.
It moves from the luminal surface to the
basal surface and then releases it out.
Another way you could get fluid
through is by fenestrations,
either fluid travels through or sometimes
solutes travel along with the fluid.
The fenestration width will be dependent
upon what molecules can make it through.
Water will always be
able to make it through,
but sometimes larger
substances like big proteins,
maybe like albumin, have a harder time in
moving through these fenestration slits
so they get stuck on
one side or the other.
A primary variable is
the pressure at which
is in the hydrostatic, which is inside
the vessel to push fluid through.
And that helps with the bulk transport
driven by the pressure change.
You also have diffusion
that is capable of moving
a solute through these
So this is based upon a
The bigger the concentration gradient, the
more solute is allowed to travel through.
Other items, such as gases or other
things that are very soluble
might be able to make it through the
endothelial cell all on its own,
without a fenestration
slit, without pinching off
the particular portion of the membrane
and having it travel through.
These substances are usually
more lipophilic in nature
and therefore can travel through
the membrane on their own.