- Hello everyone and a warm welcome.
Today we're going to learn together
a very important physiological concept
which is capillary exchange.
We are here looking at a picture of where
the capillaries are located.
We know the capillaries are called exchange vessels.
Capillaries coming out from arteries
will allow fluid to move from the vessels
into the interstitial space,
while capillaries that are taking blood back to the veins
will pull fluid back into the venous system.
So how does this occur?
How does the fluid flow out in this direction
and in in that direction?
So this is what we're going to learn together.
The concept of filtration versus reabsorption.
If fluid is leaving the capillaries on the arterial side
and moving to the tissues, we call this filtration.
If on the other hand, fluid is leaving
the interstitial space and moving back into the capillaries
and this occurs on the venous side,
we call this reabsorption.
So, who controls this?
Some pressures.
And we're going to learn the names of those pressures.
So, hydrostatic pressure and osmotic pressure.
Hydrostatic pressure is the pressure that belongs
to a volume of blood and or fluid that will always
push the fluid out to the opposite direction.
So this is hydrostatic pressure.
On the other hand, osmotic pressure belongs to
solutes within the fluid that will pull the fluid in.
And these will always oppose each other.
If we particularly are talking about proteins as solute,
we call this not only osmotic pressure,
we can call it colloid osmotic pressure or oncotic pressure.
So we're going to look at a capillary
and we're going to look at the interstitial fluid
surrounding it and we're going to look
at all of those pressures.
The hydrostatic pressure and the osmotic pressure.
So if I'm looking at the capillaries here,
this is the blood hydrostatic pressure,
it will try to push the fluid out towards the tissues.
I also have the hydrostatic pressure
of the interstitial fluid and it wants to
push fluid into the opposite direction.
So these will work against each other.
If I look at the osmotic pressure, the blood here,
contains a high volume of proteins
and it wants to pull the fluid towards it.
If we look at the tissues,
tissues have very little proteins,
so although they will want to pull the fluid out,
they won't be able to do a lot of that.
So, I have four pressures and you can see
they're working against each other.
What is the net result?
This is our important formula here.
Net filtration pressure, I'm going to add the two pressures
that work towards filtration, pushing out
and my blood hydrostatic pressure,
my interstitial fluid oncotic pressure,
I'm going to add these together, subtract the two pressures
that are working towards reabsorption.
And this is going to be our big formula here
that will determine the actual direction of flow of fluid.
So, we're going to apply this formula twice.
I'm going to take you back here.
I'm going to look at those capillaries over here
on the arterial side and I'm going to look
at those four pressures and how they work.
So, on the arterial side,
this is my blood hydrostatic pressure, it's 30.
The interstitial fluid osmotic pressure
is very little, it's almost zero.
I'm looking at those two pressures.
The blood colloid osmotic pressure is 20
and the interstitial hydrostatic pressure is very low.
So when I put my numbers together
I end up with a positive number.
So if I have a positive number,
fluid is going to move from the capillaries
towards the interstitial fluid to the cells
and this is why my net result is filtration.
Fluid is going to leave the capillaries
and take oxygen and nutrients to the tissues.
I'm going to apply this same formula
and the same concept to my venous side.
So I'm going to look at those same factors here
on the venous capillaries.
So, on the venous side, what do we notice?
I'm using the same pressures
and this is the biggest difference here.
So the blood hydrostatic pressure in veins
is much lower than arteries.
It's only 15.
So when I plug in all other numbers
I end up with a negative net filtration pressure.
If I have a negative number, means negative here,
things, fluid is going to be heavy absorbed back
into the venular capillaries
and it's going to pick up with it
the wastes and carbon dioxide.
So, to emphasize, the biggest and most important factor here
is the blood hydrostatic pressure.
It's much higher in arteries.
It's much lower in the veins.
So when we look over here again,
much higher blood hydrostatic pressure,
much lower blood hydrostatic pressure.
Alright.
We're going to have a little bit
of a clinical application here.
So how does this actually make a difference
with people clinically?
When situations of disease will change those pressures,
things will start functioning differently.
So let's talk about a situation
where the blood hydrostatic pressure falls below normal.
So what does blood hydrostatic pressure actually represent?
It represents a person's blood pressure.
So when a person has low blood pressure
the blood hydrostatic pressure is going to full
and filtration or moving of blood from the capillaries
to the tissues will decrease.
And this is common when people have dehydration
or in situations of bleeding,
there's going to be less fluid moving
from the blood vessels into the tissues
and this person will have abnormal metabolism,
they will not have enough oxygen,
and they will show symptoms.
Another important pressure here to look at
is the blood oncotic pressure.
Where does it come from?
It really comes from the amount of proteins in the blood.
And those proteins will pull the fluids
back into the capillaries.
In situations where this pressure becomes low,
and this is common in situations where people have
very low blood proteins.
Where does this come from?
This is one of the examples here.
We see people who, young children here with starvation.
As you can see they are very thin,
but what can you see in the abdomen?
There is swelling, there is fluid in the tissues.
Why does fluid leave the blood and go to the tissues?
Because they have very low concentration
of proteins in the blood.
This concludes our discussion of capillary exchange.
Thank you.