So when we talk about the
passive transport of water,
this is actually separated
and given a different name.
This process is referred to as
osmosis or the net movement of solvent
through a selectively permeable membrane.
Just like with the other
we're moving from an area of high water
concentration to low water concentration.
But be mindful that when we
use the word concentration,
by definition, is the percent
of solute inside of the solvent.
So, a 95% concentration would be 95
particles of solute per 5 units of water.
So, a lot of times it appears that
osmosis is happening backwards
because it looks like it's going from
low concentration to high, but it's not
because a high concentration
is a low water concentration
and a low concentration is
actually a high water concentration,
so let that sink in for a
second before we move on.
So in our blood vessels, we often
refer to the term tonicity.
Tonicity relates to how a solution
influences the shape of a body's cells.
Specifically, the movement of water in and out of
cells based on the solution that those cells are in.
In an ideal situation, you want your solution in
which the cells are sitting in to be isotonic.
This means that the concentration of
the solution that the cells are in
is equal to the concentration of
the intracellular environment.
This means there is no net movement in or out
of the cell and the cell maintains its shape.
If however, we were to
put a cell inside of a solution
that has a much higher concentration
than the intracellular environment of the cell,
that solution would be referred
to as a hypertonic solution
and what would happen is, all of the water
would move down the water concentration gradient
or up the concentration gradient
and cause the cells to shrink.
This is referred to as crenation.
Opposite to that, if we were to take
cells and put them into a solution
where the concentration of
that solution is much lower
than the concentration of the
intracellular environment of that cell,
this would cause water to rush into the
cell and this would lead to hemolysis
and specifically in our red blood cells,
the blood cells will actually pop.
They don't have a cell wall, so just like a
balloon, there's nothing stoping it from popping
and you can kill a whole bunch of
red blood cells just that fast.
This is especially important
when we're thinking about IVs
and putting any type
of fluid into the blood.
And so, again, if I put the cells in
a hypertonic solution,
that means that the outside of the cell is
higher concentration than the inside of the cell
and water goes out of the cell.
The best way to remember
this is, water follows salt.
So, wherever there's a higher concentration,
that is where the water is going to go
in order to try to reach equilibrium.