00:01
Endothelial transport is much
different than epithelial transport.
00:08
It also though has
tight junctions.
00:12
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.
00:33
So, endothelial transport is going to
rely less on specific transporters
than epithelial transport does.
00:43
You’re going to utilize
things like pressure and
osmolality to move various
solutes and solvents around.
00:53
It is primarily a variable associated
with the flux of the substance.
01:00
So you will filter
certain things,
hydrostatic pressure is highly involved,
and osmotic and oncotic pressures.
01:10
Remember, osmotic pressures have
to do with the ion differences
and oncotic pressures have to do with
protein differences to draw fluid.
01:23
This is an example of how pressure can
move fluid out of a capillary bed.
01:30
The higher the amount of pressure,
the more fluid travels through.
01:36
Other examples determine about how much the
fenestrations are in terms of their width.
01:43
Some will allow more fluid to travel
out and some will allow less.
01:47
The other issue that we need to think
about with endothelium versus epithelium
is that we sometimes turn the
membranes a little bit differently.
01:58
The terminology used for
the inside surface is
the luminal surface rather
than the apical membrane.
02:07
In terms of the outside
is termed the basal surface rather
than the basolateral membrane.
02:15
But if you keep those linked
together, you’ll be better off
and able to think about the differences
between epithelial and endothelial surfaces.
02:26
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.
02:38
The lymphatic is a
great example of this.
02:42
So you can have some constriction
and have the pores closed.
02:47
And then, after constricting
these, you can open them up.
02:54
This then will allow fluid to transport
between the lymphatic circulation.
03:01
Then when you have smooth muscle
constriction, you close them up.
03:06
Good examples of how you can
modulate these fenestrations.
03:11
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.
03:22
Let’s summarize now the different
ways you’re going to move a solution,
a substance, or a gas across
the endothelial wall.
03:35
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.
03:47
It moves from the luminal surface to the
basal surface and then releases it out.
03:55
Another way you could get fluid
through is by fenestrations,
either fluid travels through or sometimes
solutes travel along with the fluid.
04:07
The fenestration width will be dependent
upon what molecules can make it through.
04:12
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.
04:27
A primary variable is
the pressure at which
is in the hydrostatic, which is inside
the vessel to push fluid through.
04:39
And that helps with the bulk transport
driven by the pressure change.
04:46
You also have diffusion
that is capable of moving
a solute through these
fenestration slits.
04:53
So this is based upon a
concentration gradient.
04:57
The bigger the concentration gradient, the
more solute is allowed to travel through.
05:03
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.
05:21
These substances are usually
more lipophilic in nature
and therefore can travel through
the membrane on their own.